Antimicrobial Agents: Penicillins, Cephalosporins, and Other -Lactam Antibiotics

The penicillins constitute one of the most important groups of antibiotics. Although numerous other antimicrobial agents have been produced since the first penicillin became available, these still are widely used, major antibiotics, and new derivatives of the basic penicillin nucleus still are being produced. Many of these have unique advantages, such that members of this group of antibiotics are presently the drugs of choice for a large number of infectious diseases.

History

The history of the brilliant research that led to the discovery and development of penicillin has been recorded by the chief participants. (SeeFleming, 1946; Florey, 1946, 1949; Abraham, 1949; Chain, 1954.) In 1928, while studying Staphylococcus variants in the laboratory at St. Mary's Hospital in London, Alexander Fleming observed that a mold contaminating one of his cultures caused the bacteria in its vicinity to undergo lysis. Broth in which the fungus was grown was markedly inhibitory for many microorganisms. Because the mold belonged to the genus Penicillium, Fleming named the antibacterial substance penicillin.

A decade later, penicillin was developed as a systemic therapeutic agent by the concerted research of a group of investigators at Oxford University headed by Florey, Chain, and Abraham. By May 1940, the crude material then available was found to produce dramatic therapeutic effects when administered parenterally to mice with experimentally produced streptococcal infections. Despite great obstacles to its laboratory production, enough penicillin was accumulated by 1941 to conduct therapeutic trials in several patients desperately ill with staphylococcal and streptococcal infections refractory to all other therapy. At this stage, the crude, amorphous penicillin was only about 10% pure, and it required nearly 100 liters of the broth in which the mold had been grown to obtain enough of the antibiotic to treat one patient for 24 hours. Herrell (1945) records that bedpans actually were used by the Oxford group for growing cultures of Penicillium notatum. Case 1 in the 1941 report from Oxford was that of a policeman who was suffering from a severe mixed staphylococcal and streptococcal infection. He was treated with penicillin, some of which had been recovered from the urine of other patients who had been given the drug. It is said that an Oxford professor referred to penicillin as a remarkable substance, grown in bedpans and purified by passage through the Oxford Police Force.

A vast research program soon was initiated in the United States. During 1942, 122 million units of penicillin were made available, and the first clinical trials were conducted at Yale University and the Mayo Clinic, with dramatic results. By the spring of 1943, 200 patients had been treated with the drug. The results were so impressive that the surgeon general of the U.S. Army authorized trial of the antibiotic in a military hospital. Soon thereafter, penicillin was adopted throughout the medical services of the U.S. Armed Forces.

The deep-fermentation procedure for the biosynthesis of penicillin marked a crucial advance in the large-scale production of the antibiotic. From a total production of a few hundred million units a month in the early days, the quantity manufactured rose to over 200 trillion units (nearly 150 tons) by 1950. The first marketable penicillin cost several dollars per 100,000 units; today, the same dose costs only a few cents.

Chemistry

The basic structure of the penicillins, as shown in Figure 451, consists of a thiazolidine ring (A) connected to a -lactam ring (B), to which is attached a side chain (R). The penicillin nucleus itself is the chief structural requirement for biological activity; metabolic transformation or chemical alteration of this portion of the molecule causes loss of all significant antibacterial activity. The side chain (seeTable 451) determines many of the antibacterial and pharmacological characteristics of a particular type of penicillin. Several natural penicillins can be produced, depending on the chemical composition of the fermentation medium used to culture Penicillium. Penicillin G (benzylpenicillin) has the greatest antimicrobial activity of these and is the only natural penicillin used clinically.

Semisynthetic Penicillins

The discovery that 6-aminopenicil-break lanic acid could be obtained from cultures of Penicillium chrysogenum that were depleted of side-chain precursors led to the development of the semisynthetic penicillins. Side chains can be added that alter the susceptibility of the resultant compounds to inactivating enzymes (-lactamases) and that change the antibacterial activity and the pharmacological properties of the drug. 6-Aminopenicillanic acid is now produced in large quantities with the aid of an amidase from P. chrysogenum (Figure 451). This enzyme splits the peptide linkage by which the side chain of penicillin is joined to 6-aminopenicillanic acid.

Unitage of Penicillin

The international unit of penicillin is the specific penicillin activity contained in 0.6 g of the crystalline sodium salt of penicillin G. One milligram of pure penicillin G sodium thus equals 1667 U; 1.0 mg of pure penicillin G potassium represents 1595 U. The dosage and the antibacterial potency of the semisynthetic penicillins are expressed in terms of weight.

Mechanism of Action of the Penicillins and Cephalosporins

The -lactam antibiotics can kill susceptible bacteria. Although knowledge of the mechanism of this action is incomplete, numerous researchers have supplied information that allows understanding of the basic phenomenon (seeTomasz, 1986; Ghuysen, 1991; Bayles, 2000).

The cell walls of bacteria are essential for their normal growth and development. Peptidoglycan is a heteropolymeric component of the cell wall that provides rigid mechanical stability by virtue of its highly cross-linked latticework structure (Figure 452). In gram-positive microorganisms, the cell wall is 50 to 100 molecules thick, but it is only 1 or 2 molecules thick in gram-negative bacteria (Figure 453). The peptidoglycan is composed of glycan chains, which are linear strands of two alternating amino sugars (N-acetylglucosamine and N-acetylmuramic acid) that are cross-linked by peptide chains.

The biosynthesis of the peptidoglycan involves about 30 bacterial enzymes and may be considered in three stages. The first stage, precursor formation, takes place in the cytoplasm. The product, uridine diphosphate (UDP)-acetylmuramyl-pentapeptide, called a 'Park nucleotide' after its discoverer (Park and Strominger, 1957), accumulates in cells when subsequent synthetic stages are inhibited. The last reaction in the synthesis of this compound is the addition of a dipeptide, D-alanyl-D-alanine. Synthesis of the dipeptide involves prior racemization of L-alanine and condensation catalyzed by D-alanyl-D-alanine synthetase. D-Cycloserine is a structural analog of D-alanine and acts as a competitive inhibitor of both the racemase and the synthetase (seeChapter 48: Antimicrobial Agents: Drugs Used in the Chemotherapy of Tuberculosis, Mycobacterium avium Complex Disease, and Leprosy).

During reactions of the second stage, UDP-acetylmuramyl-pentapeptide and UDP-acetylglucosamine are linked (with the release of the uridine nucleotides) to form a long polymer.

The third and final stage involves the completion of the cross-link. This is accomplished by a transpeptidation reaction that occurs outside the cell membrane. The transpeptidase itself is membrane-bound. The terminal glycine residue of the pentaglycine bridge is linked to the fourth residue of the pentapeptide (D-alanine), releasing the fifth residue (also D-alanine) (Figure 452). It is this last step in peptidoglycan synthesis that is inhibited by the -lactam antibiotics and glycopeptide antibiotics such as vancomycin (by a different mechanism than the -lactams; seeChapter 46: Antimicrobial Agents: The Aminoglycosides). Stereomodels reveal that the conformation of penicillin is very similar to that of D-alanyl-D-alanine (Waxman et al., 1980; Kelly et al., 1982). The transpeptidase probably is acylated by penicillin; that is, penicilloyl enzyme apparently is formed, with cleavage of the CON bond of the -lactam ring.

Although inhibition of the transpeptidase described above is demonstrably important, there are additional, related targets for the actions of penicillins and cephalosporins; these are collectively termed penicillin-binding proteins (PBPs; Spratt, 1980; Ghuysen, 1991). All bacteria have several such entities; for example, S. aureus has four PBPs, while Escherichia coli has at least seven. The PBPs vary in their affinities for different -lactam antibiotics, although the interactions eventually become covalent. The higher-molecular-weight PBPs of E. coli (PBP 1a and 1b) include the transpeptidases responsible for synthesis of the peptidoglycan. Other PBPs in E. coli include those that are necessary for maintenance of the rodlike shape of the bacterium and for septum formation at division. Inhibition of the transpeptidases causes spheroplast formation and rapid lysis. However, inhibition of the activities of other PBPs may cause delayed lysis (PBP 2) or the production of long, filamentous forms of the bacterium (PBP 3). The lethality of penicillin for bacteria appears to involve both lytic and nonlytic mechanisms. Penicillin's disruption of the balance between PBP-mediated peptidoglycan assembly and murein hydrolase activity results in autolysis. Nonlytic killing by penicillin may involve holin-like proteins in the bacterial membrane that collapse the membrane potential (Bayles, 2000).

Mechanisms of Bacterial Resistance to Penicillins and Cephalosporins

Although most or all bacteria contain PBPs, -lactam antibiotics cannot kill or even inhibit all bacteria, and various mechanisms of bacterial resistance to these agents are operative. The microorganism may be intrinsically resistant because of structural differences in the PBPs that are the targets of these drugs. Furthermore, it is possible for a sensitive strain to acquire resistance of this type by the development of high-molecular-weight PBPs that have decreased affinity for the antibiotic. Because the -lactam antibiotics inhibit many different PBPs in a single bacterium, the affinity for -lactam antibiotics of several PBPs must decrease for the organism to be resistant. Altered PBPs with decreased affinity for -lactam antibiotics are acquired by homologous recombination between PBP genes of different bacterial species. Four of the five high-molecular-weight PBPs of the most highly penicillin-resistant Streptococcus pneumoniae isolates have decreased affinity for -lactam antibiotics as a result of interspecies homologous recombination events (Figure 454). In contrast, isolates with high-level resistance to third-generation cephalosporins contain alterations of only two of the five high-molecular-weight PBPs, as the other PBPs have inherently low affinity for the third-generation cephalosporins. Penicillin resistance in Streptococcus sanguis and other viridans streptococci apparently emerged as a result of replacement of its PBPs with resistant PBPs from S. pneumoniae (Carratalet al., 1995). Methicillin-resistant S. aureus are resistant via acquisition of an additional high-molecular-weight PBP (via a transposon from an unknown organism) with a very low affinity for all -lactam antibiotics. The gene encoding this new PBP also is present in and responsible for methicillin resistance in the coagulase-negative staphylococci (Spratt, 1994).

Other instances of bacterial resistance to the -lactam antibiotics are caused by the inability of the agent to penetrate to its site of action or by energy-dependent efflux systems for pumping the antibiotic out of the bacteria (Jacoby, 1994; Nikaido, 1998) (Figure 455). In gram-positive bacteria, the peptidoglycan polymer is very near the cell surface (Figure 453). Only surface macromolecules (capsule) are external to the peptidoglycan. The small -lactam antibiotic molecules can penetrate easily to the outer layer of the cytoplasmic membrane and the PBPs, where the final stages of the synthesis of the peptidoglycan take place. The situation is different with gram-negative bacteria. Their surface structure is more complex, and the inner membrane (which is analogous to the cytoplasmic membrane of gram-positive bacteria) is covered by the outer membrane, lipopolysaccharide, and capsule (Figure 453). The outer membrane functions as an impenetrable barrier for some antibiotics (seeNakae, 1986). However, some small, hydrophilic antibiotics diffuse through aqueous channels in the outer membrane that are formed by proteins called porins. Broader-spectrum penicillins, such as ampicillin and amoxicillin, and most of the cephalosporins diffuse through the pores in the E. coli outer membrane significantly more rapidly than can penicillin G. The number and size of pores in the outer membrane are variable among different gram-negative bacteria. An extreme example is P. aeruginosa, which is intrinsically resistant to a wide variety of antibiotics by virtue of lacking the classical high-permeability porins (Nikaido, 1994). Active efflux pumps serve as another mechanism of resistance, removing the antibiotic from its site of action before it can act (Nikaido, 1998). This is an important mechanism of -lactam resistance in P. aeruginosa, E. coli, and Neisseria gonorrhoeae.

Bacteria can destroy -lactam antibiotics enzymatically. While amidohydrolases may be present, these enzymes are relatively inactive and do not protect the bacteria. -Lactamases, however, are capable of inactivating certain of these antibiotics and may be present in large quantities (seeFigures 451 and 453). Different microorganisms elaborate a number of distinct -lactamases, although most bacteria produce only one form of the enzyme. The substrate specificities of some of these enzymes are relatively narrow, and these often are described as either penicillinases or cephalosporinases. Other 'broad-spectrum' enzymes are less discriminant and can hydrolyze a variety of -lactam antibiotics. Individual penicillins and cephalosporins vary in their susceptibility to these enzymes.

In general, gram-positive bacteria produce a large amount of -lactamase, which is secreted extracellularly (Figure 453). Most of these enzymes are penicillinases. The information for staphylococcal penicillinase is encoded in a plasmid, and this may be transferred by bacteriophage to other bacteria. The enzyme is inducible by substrates, and 1% of the dry weight of the bacterium can be penicillinase. In gram-negative bacteria, -lactamases are found in relatively small amounts, but are located in the periplasmic space between the inner and outer cell membranes (Figure 453). Since the enzymes of cell-wall synthesis are on the outer surface of the inner membrane, these -lactamases are strategically located for maximal protection of the microbe. -Lactamases of gram-negative bacteria are encoded either in chromosomes or in plasmids, and they may be constitutive or inducible. The plasmids can be transferred between bacteria by conjugation. These enzymes may hydrolyze penicillins, cephalosporins, or both (seeDavies, 1994). However, there is an inconsistent correlation between the susceptibility of an antibiotic to inactivation by -lactamase and the ability of that antibiotic to kill the microorganism. For example, penicillins that are hydrolyzed by -lactamase (e.g., carbenicillin) are able to kill certain strains of -lactamaseproducing gram-negative microbes.

Other Factors That Influence the Activity of -Lactam Antibiotics

The density of the bacterial population and the age of an infection influence the activity of -lactam antibiotics. The drugs may be several thousand times more potent when tested against small bacterial inocula than when tested against a dense culture. Many factors are involved. Among these are the greater number of relatively resistant microorganisms in a large population, the amount of -lactamase produced, and the phase of growth of the culture. The clinical significance of this effect of inoculum size is uncertain. The intensity and the duration of penicillin therapy needed to abort or cure experimental infections in animals increase with the duration of the infection. The primary reason is that the bacteria are no longer multiplying as rapidly as they do in a fresh infection. These antibiotics are most active against bacteria in the logarithmic phase of growth and have little effect on microorganisms in the stationary phase, when there is no need to synthesize components of the cell wall.

The presence of proteins and other constituents of pus, low pH, or low oxygen tension does not appreciably decrease the ability of -lactam antibiotics to kill bacteria. However, bacteria that survive inside viable cells of the host are protected from the action of the -lactam antibiotics.

Classification of the Penicillins and Summary of Their Pharmacological Properties

It is useful to classify the penicillins according to their spectra of antimicrobial activity (seeTable 451; Chambers, 2000).

Although the pharmacological properties of the individual drugs are discussed in detail below, certain generalizations are useful. Following absorption of orally administered penicillins, these agents are widely distributed throughout the body. Therapeutic concentrations of penicillins are achieved readily in tissues and in secretions such as joint fluid, pleural fluid, pericardial fluid, and bile. However, only low concentrations of these drugs are found in prostatic secretions, brain tissue, and intraocular fluid, and penicillins do not penetrate living phagocytic cells to a significant extent. Concentrations of penicillins in cerebrospinal fluid (CSF) are variable, but are less than 1% of those in plasma when the meninges are normal. When there is inflammation, concentrations in CSF may increase to as much as 5% of the plasma value. Penicillins are eliminated rapidly, particularly by glomerular filtration and renal tubular secretion, such that their half-lives in the body are short; values of 30 to 90 minutes are typical. Concentrations of these drugs in urine thus are high.

Penicillin G and Penicillin V

Antimicrobial Activity

The antimicrobial spectra of penicillin G(benzylpenicillin) and penicillin V (the phenoxymethyl derivative) are very similar for aerobic gram-positive microorganisms. However, penicillin G is five to ten times more active against Neisseria species sensitive to penicillins and against certain anaerobes.

Penicillin G has activity against a variety of species of gram-positive and gram-negative cocci, although many bacteria previously sensitive to the agent are now resistant. Most streptococci (but not enterococci) are very susceptible to the drug; concentrations of less than 0.01 g/ml usually are effective. However, penicillin-resistant viridans streptococci (Carratalet al., 1995) and S. pneumoniae are becoming more common. During 1997, 13.6% of S. pneumoniae sterile-site isolates had high-level [minimal inhibitory concentration (MIC) g/ml)] and 11.4% of isolates had low-level (MIC g/ml) penicillin resistance, for a total of 25% of isolates (Centers for Disease Control and Prevention, 1999). Penicillin-resistant pneumococci are especially common in pediatric populations, such as children attending day care centers. Many penicillin-resistant pneumococci also are resistant to third-generation cephalosporins.

Whereas most strains of S. aureus were highly sensitive to similar concentrations of penicillin G when this agent was first employed therapeutically, more than 90% of strains of staphylococci isolated from individuals inside or outside of hospitals are now resistant to penicillin G. Most strains of Staphylococcus epidermidis also are resistant to penicillin. Unfortunately, penicillinase-producing strains of gonococci that are highly resistant to penicillin G have become widespread. With rare exceptions, meningococci are quite sensitive to penicillin G.

Although the vast majority of strains of Corynebacterium diphtheriae are sensitive to penicillin G, some are highly resistant. This also is true for Bacillus anthracis. Most anaerobic microorganisms, including Clostridium species, are highly sensitive. Bacteroides fragilis is an exception; many strains are now resistant because of elaboration of -lactamase. Some strains of Bacteroides melaninogenicus also have acquired this trait. Actinomyces israelii, Streptobacillus moniliformis, Pasteurella multocida, and L. monocytogenes are inhibited by penicillin G. Most species of Leptospira are moderately susceptible to the drug. One of the most exquisitely sensitive microorganisms is Treponema pallidum. Borrelia burgdorferi, the organism responsible for Lyme disease, also is susceptible. None of the penicillins is effective against amoebae, plasmodia, rickettsiae, fungi, or viruses.

Absorption

Oral Administration of Penicillin G

About one-third of an orally administered dose of penicillin G is absorbed from the intestinal tract under favorable conditions. Gastric juice at pH 2 rapidly destroys the antibiotic. The decrease in gastric acid production with aging accounts for better absorption of penicillin G from the gastrointestinal tract of older individuals. Absorption is rapid, and maximal concentrations in blood are attained in 30 to 60 minutes. The peak value is approximately 0.5 U/ml (0.3 g/ml) after an oral dose of 400,000 U (about 250 mg) in an adult. Ingestion of food may interfere with enteric absorption of all penicillins, perhaps by adsorption of the antibiotic onto food particles. Thus, oral penicillin G should be administered at least 30 minutes before a meal or 2 hours after. Despite the convenience of oral administration of penicillin G, this route should be used only in infections in which clinical experience has proven its efficacy.

Oral Administration of Penicillin V

The virtue of penicillin V in comparison with penicillin G is that it is more stable in an acidic medium, and therefore is better absorbed from the gastrointestinal tract. On an equivalent oral-dose basis, penicillin V (K⁺ salt PEN-VEE K, V-CILLIN K, others) yields plasma concentrations two to five times greater than those provided by penicillin G. The peak concentration in the blood of an adult after an oral dose of 500 mg is nearly 3 g/ml. Once absorbed, penicillin V is distributed in the body and excreted by the kidney in a manner similar to that of penicillin G.

Parenteral Administration of Penicillin G

After intramuscular injection, peak concentrations in plasma are reached within 15 to 30 minutes. This value declines rapidly, since the half-life of penicillin G is 30 minutes.

Many means for prolonging the sojourn of the antibiotic in the body and thereby reducing the frequency of injections have been explored. Probenecid blocks renal tubular secretion of penicillin, but it is rarely used for this purpose (see below). More commonly, repository preparations of penicillin G are employed. The two such compounds currently favored are penicillin G procaine (DURACILLIN, A.S., WYCILLIN, others) and penicillin G benzathine (BICILLIN L-A, PERMAPEN). Such agents release penicillin G slowly from the area in which they are injected and produce relatively low but persistent concentrations of antibiotic in the blood.

Penicillin Gprocaine suspension is an aqueous preparation of the crystalline salt that is only 0.4% soluble in water. Procaine combines with penicillin mol for mol; a dose of 300,000 U thus contains approximately 120 mg of procaine. When large doses of penicillin G procaine are given (e.g., 4.8 million units), procaine may reach toxic concentrations in the plasma. If the patient is believed to be hypersensitive to procaine, 0.1 ml of 1% solution of procaine should be injected intradermally as a test. The anesthetic effect of the procaine accounts in part for the fact that injections of penicillin G procaine are virtually painless.

The injection of 300,000 U of penicillin Gprocaine produces a peak concentration in plasma of about 0.9 g/ml within 1 to 3 hours; after 24 hours the concentration is reduced to 0.1 g/ml, and by 48 hours it has fallen to 0.03 g/ml. A larger dose (600,000 U) yields somewhat higher values that are maintained for as long as 4 to 5 days.

Penicillin G benzathine suspension is the aqueous suspension of the salt obtained by the combination of 1 mol of an ammonium base and 2 mol of penicillin G to yield N,N^'-dibenzylethylenediamine dipenicillin G. The salt itself is only 0.02% soluble in water. The long persistence of penicillin in the blood after a suitable intramuscular dose reduces cost, need for repeated injections, and local trauma. The local anesthetic effect of penicillin G benzathine is comparable to that of penicillin G procaine.

Penicillin G benzathine is absorbed very slowly from intramuscular depots and produces the longest duration of detectable antibiotic of all the available repository penicillins. For example, in adults, a dose of 1.2 million units given intramuscularly produces a concentration in plasma of 0.09 g/ml on the first, 0.02 g/ml on the fourteenth, and 0.002 g/ml on the thirty-second day after injection. The average duration of demonstrable antimicrobial activity in the plasma is about 26 days.

Distribution

Penicillin G is distributed widely throughout the body, but the concentrations in various fluids and tissues differ widely. Its apparent volume of distribution is about 0.35 liter/kg. Approximately 60% of the penicillin G in plasma is reversibly bound to albumin. Significant amounts appear in liver, bile, kidney, semen, joint fluid, lymph, and intestine.

While probenecid markedly decreases the tubular secretion of the penicillins, this is not the only factor responsible for the elevated plasma concentrations of the antibiotic that follow its administration. Probenecid also produces a significant decrease in the apparent volume of distribution of the penicillins.

Cerebrospinal Fluid

Penicillin does not readily enter the CSF when the meninges are normal. However, when the meninges are acutely inflamed, penicillin penetrates into the CSF more easily. Although the concentrations attained vary and are unpredictable, they are usually in the range of 5% of the value in plasma and are therapeutically effective against susceptible microorganisms.

Penicillin and other organic acids are secreted rapidly from the CSF into the bloodstream by an active transport process. Probenecid competitively inhibits this transport and thus elevates the concentration of penicillin in CSF. In uremia, other organic acids accumulate in the CSF and compete with penicillin for secretion; the drug occasionally reaches toxic concentrations in the brain and can produce convulsions.

Excretion

Under normal conditions, penicillin G is rapidly eliminated from the body, mainly by the kidney but in small part in the bile and by other routes. Approximately 60% to 90% of an intramuscular dose of penicillin G in aqueous solution is eliminated in the urine, largely within the first hour after injection. The remainder is metabolized to penicilloic acid. The half-time for elimination of penicillin G is about 30 minutes in normal adults. Approximately 10% of the drug is eliminated by glomerular filtration and 90% by tubular secretion. Renal clearance approximates the total renal plasma flow. The maximal tubular secretory capacity for penicillin in the normal male adult is about 3 million units (1.8 g) per hour.

Clearance values are considerably lower in neonates and infants because of incomplete development of renal function; as a result, after doses proportionate to surface area, the persistence of penicillin in the blood is several times as long in premature infants as in children and adults. The half-life of the antibiotic in children less than 1 week old is 3 hours; by 14 days of age it is 1.4 hours. After renal function is fully established in young children, the rate of renal excretion of penicillin G is considerably more rapid than in adults.

Anuria increases the half-life of penicillin G from a normal value of 0.5 hour to about 10 hours. When renal function is impaired, 7% to 10% of the antibiotic may be inactivated each hour by the liver. Patients with renal shutdown who require high-dose therapy with penicillin can be treated adequately with 3 million units of aqueous penicillin G followed by 1.5 million units every 8 to 12 hours. The dose of the drug must be readjusted during dialysis and the period of progressive recovery of renal function. If, in addition to renal failure, hepatic insufficiency also is present, the half-life will be prolonged even further.

Therapeutic Uses

Pneumococcal Infections

Penicillin G remains the agent of choice for the management of infections caused by sensitive strains of S. pneumoniae. However, strains of pneumococci resistant to usual doses of penicillin G are being more frequently isolated in several countries, including the United States (seeCenters for Disease Control and Prevention, 1999; Fiore et al., 2000).

Pneumococcal Pneumonia

Until it is highly likely or established that the infecting isolate of pneumococcus is penicillin-sensitive, pneumococcal pneumonia should be treated with a third-generation cephalosporin or with vancomycin. For parenteral therapy of sensitive isolates of pneumococci, penicillin G or penicillin G procaine is favored. Although oral treatment with 500 mg of penicillin V given every 6 hours for treatment of pneumonia due to penicillin-sensitive isolates has been used with success in this disease, it cannot be recommended for routine initial use because of the existence of resistance. Therapy should be continued for 7 to 10 days, including 3 to 5 days after the patient's temperature has returned to normal.

Pneumococcal Meningitis

Until it is established that the infecting pneumococcus is penicillin-sensitive, pneumococcal meningitis should be treated with a combination of vancomycin and a third-generation cephalosporin (John, 1994; Catalan et al., 1994). Some authorities advocate a third-generation cephalosporin plus rifampin. Prior to the appearance of penicillin resistance, penicillin treatment reduced the death rate in this disease from nearly 100% to about 25%. The recommended therapy is 20 million to 24 million units of penicillin G daily by constant intravenous infusion or divided into boluses given every 2 to 3 hours. The usual duration of therapy is 14 days.

Streptococcal Infections

Streptococcal Pharyngitis (Including Scarlet Fever)

This is the most common disease produced by Streptococcus pyogenes (group A -hemolytic streptococcus). Penicillin-resistant isolates have yet to be observed for S. pyogenes (Tomasz, 1994). The preferred oral therapy is with penicillin V, 500 mg every 6 hours for 10 days. Equally good results are produced by the administration of 600,000 U of penicillin Gprocaine intramuscularly, once daily for 10 days, or by a single injection of 1.2 million units of penicillin G benzathine. Parenteral therapy is preferred if there are questions of patient compliance. Penicillin therapy of streptococcal pharyngitis reduces the risk of subsequent acute rheumatic fever; however, current evidence suggests that the incidence of glomerulonephritis that follows streptococcal infections is not reduced to a significant degree by treatment with penicillin.

Streptococcal Pneumonia, Arthritis, Meningitis, and Endocarditis

While uncommon, these conditions should be treated with penicillin G when they are caused by S. pyogenes; daily doses of 12 million to 20 million units are administered intravenously for 2 to 4 weeks. Such treatment of endocarditis should be continued for a full 4 weeks.

Infections Caused by Other Streptococci

The viridans streptococci are the most common cause of infectious endocarditis. These are nongroupable -hemolytic microorganisms that are increasingly resistant to penicillin G (MIC < 0.1 g/ml). Since enterococci may also be -hemolytic and certain other -hemolytic strains may be relatively resistant to penicillin, it is important to determine quantitative microbial sensitivities to penicillin G in patients with endocarditis. Patients with penicillin-sensitive viridans group streptococcal endocarditis can be treated successfully with 1.2 million units of procaine penicillin G, given four times daily for 2 weeks, or with daily doses of 12 million to 20 million units of intravenous penicillin G for 2 weeks, both regimens in combination with streptomycin, 500 mg intramuscularly every 12 hours or gentamicin (1 mg/kg every 8 hours). Some physicians prefer a 4-week course of treatment with penicillin G alone.

Enterococcal endocarditis is one of the few diseases that is optimally treated with two antibiotics. The recommended therapy for penicillin- and aminoglycoside-sensitive enterococcal endocarditis is 20 million units of penicillin G or 12 grams of ampicillin daily, administered intravenously in combination with an aminoglycoside. Therapy usually should be continued for 6 weeks, but selected patients with a short duration of illness (less than 3 months) have been treated successfully in 4 weeks (Wilson et al., 1984).

Infections with Anaerobes

Many anaerobic infections are caused by mixtures of microorganisms. The majority are sensitive to penicillin G. An exception is the B. fragilis group, in which up to 75% of strains may be resistant to high concentrations of this antibiotic. Pulmonary and periodontal infections (with the exception of -lactamase-producing Prevotella melaninogenica) usually respond well to penicillin G, although a multicenter study indicated that clindamycin is more effective than penicillin for therapy of lung abscess (Levison et al., 1983). Mild-to-moderate infections at these sites may be treated with oral medication (either penicillin G or penicillin V, 400,000 U four times daily). More severe infections should be treated with 12 million to 20 million units of penicillin G intravenously. Brain abscesses also frequently contain several species of anaerobes, and most authorities prefer to treat such disease with high doses of penicillin G (20 million units per day) plus metronidazole or chloramphenicol. Some physicians add a third-generation cephalosporin for activity against aerobic gram-negative bacilli.

Staphylococcal Infections

The vast majority of staphylococcal infections are caused by microorganisms that produce penicillinase. A patient with a staphylococcal infection who requires treatment with an antibiotic should receive one of the penicillinase-resistant penicillinsfor example, nafcillin, oxacillin, or methicillin.

So-called methicillin-resistant staphylococci are resistant to penicillin G, all of the penicillinase-resistant penicillins, and the cephalosporins. Isolates occasionally may appear to be sensitive to various cephalosporins in vitro, but resistant populations arise during therapy and lead to failure (Chambers et al., 1984). Vancomycin is the drug of choice for infections caused by these bacteria, although reduced susceptibility to vancomycin has now been observed (Centers for Disease Control and Prevention, 1997). Ciprofloxacin also may be effective, although prolonged therapy often leads to the emergence of ciprofloxacin-resistant S. aureus.

Meningococcal Infections

Penicillin G remains the drug of choice for meningococcal disease. Patients should be treated with high doses of penicillin given intravenously, as described for pneumococcal meningitis. Penicillin-resistant strains of N. meningitides have been reported in Britain and Spain but are infrequent at present. In 1997, 97% of N. meningitides isolates analyzed from the United States were penicillin-sensitive (Rosenstein et al., 2000). The occurrence of penicillin-resistant strains should be considered in patients who are slow to respond to treatment (Sprott et al., 1988, Mendelman et al., 1988). It should be remembered that penicillin G does not eliminate the meningococcal carrier state, and its administration thus is ineffective as a prophylactic measure.

Gonococcal Infections

Gonococci gradually have become more resistant to penicillin G, and penicillins are no longer the therapy of choice, unless it is known that gonococcal strains in a particular geographical area are susceptible. Uncomplicated gonococcal urethritis is the most common infection, and a single intramuscular injection of 250 mg of ceftriaxone is the recommended treatment (Sparling and Handsfield, 2000).

Gonococcal arthritis, disseminated gonococcal infections with skin lesions, and gonococcemia should be treated with ceftriaxone, 1 g daily given either intramuscularly or intravenously for 7 to 10 days. Ophthalmia neonatorum also should be treated with ceftriaxone for 7 to 10 days (25 to 50 mg/kg per day intramuscularly or intravenously).

Syphilis

Therapy of syphilis with penicillin G is highly effective. Primary, secondary, and latent syphilis of less than 1 year's duration may be treated with penicillin G procaine (2.4 million units per day intramuscularly) plus probenecid (1.0 g per day orally) for 10 days or with 1 to 3 weekly intramuscular doses of 2.4 million units of penicillin G benzathine (3 doses in patients with HIV). Patients with late latent syphilis, neurosyphilis, or cardiovascular syphilis may be treated with a variety of regimens. Since these diseases are potentially lethal and their progression can be halted (but not reversed), intensive therapy with 20 million units of penicillin G daily for 10 days is recommended.

Infants with congenital syphilis discovered at birth or during the postnatal period should be treated for at least 10 days with 50,000 U/kg daily of aqueous penicillin G in two divided doses or 50,000 U/kg of procaine penicillin G in a single daily dose (seeTramont, 2000).

The majority (70% to 90%) of patients with secondary syphilis develop the Jarisch-Herxheimer reaction. This also may be seen in patients with other forms of syphilis. Several hours after the first injection of penicillin, chills, fever, headache, myalgias, and arthralgias may develop. The syphilitic cutaneous lesions may become more prominent, edematous, and brilliant in color. Manifestations usually persist for a few hours, and the rash begins to fade within 48 hours. It does not recur with the second or subsequent injections of penicillin. This reaction is thought to be due to release of spirochetal antigens, with subsequent host reactions to the products. Aspirin gives symptomatic relief, and therapy with penicillin should not be discontinued.

Actinomycosis

Penicillin G is the agent of choice for the treatment of all forms of actinomycosis. The dose should be 12 million to 20 million units of penicillin G intravenously per day for 6 weeks. Some physicians continue therapy for 2 to 3 months with oral penicillin V (500 mg four times daily). Surgical drainage or excision of the lesion may be necessary before cure is accomplished.

Diphtheria

There is no evidence that penicillin or any other antibiotic alters the incidence of complications or the outcome of diphtheria; specific antitoxin is the only effective treatment. However, penicillin G eliminates the carrier state. The parenteral administration of 2 to 3 million units per day in divided doses for 10 to 12 days eliminates the diphtheria bacilli from the pharynx and other sites in practically 100% of cases. A single daily injection of penicillin G procaine for the same period produces about the same results.

Anthrax

Penicillin G is the agent of choice in the treatment of all clinical forms of anthrax. However, strains of Bacillus anthracis resistant to this antibiotic have been recovered from human infections. When penicillin G is used, the dose should be 12 million to 20 million units per day.

Clostridial Infections

Penicillin G is the agent of choice for gas gangrene; the dose is in the range of 12 million to 20 million units per day, given parenterally. Adequate debridement of the infected areas is essential. Antimicrobial drugs probably have no effect on the ultimate outcome of tetanus. Debridement and administration of human tetanus immune globulin may be indicated. Penicillin is administered, however, to eradicate the vegetative forms of the bacteria that may persist.

Fusospirochetal Infections

Gingivostomatitis, produced by the synergistic action of Leptotrichia buccalis and spirochetes that are present in the mouth, is readily treatable with penicillin. For simple 'trench mouth,' 500 mg of penicillin V given every 6 hours for several days is usually sufficient to clear the disease.

Rat-Bite Fever

The two microorganisms responsible for this infection, Spirillum minor in the Orient and Streptobacillus moniliformis in America and Europe, are sensitive to penicillin G, the therapeutic agent of choice. Since most cases due to Streptobacillus are complicated by bacteremia and, in many instances, by metastatic infections especially of the synovia and endocardium, the dose should be large; a daily dose of 12 million to 15 million units given parenterally for 3 to 4 weeks has been recommended.

Listeria Infections

Penicillin G or ampicillin with or without gentamicin are regarded as the drugs of choice in the management of infections due to L. monocytogenes. The recommended dose of penicillin G is 15 million to 20 million units parenterally per day for at least 2 weeks. When endocarditis is the problem, the dose is the same, but the duration of treatment should be no less than 4 weeks.

Lyme Disease

Although a tetracycline is the usual drug of choice for early disease, amoxicillin is effective; the dose is 500 mg three times daily for 21 days. Severe disease is treated with a third-generation cephalosporin or 20 million units of intravenous penicillin G daily for 14 days.

Erysipeloid

The causative agent of this disease, Erysipelothrix rhusiopathiae, is sensitive to penicillin. The uncomplicated infection responds well to a single injection of 1.2 million units of penicillin G benzathine. When endocarditis is present, penicillin G, 12 million to 20 million units per day, has been found to be effective; therapy should be continued for 4 to 6 weeks.

Prophylactic Uses of the Penicillins

Demonstration of the effectiveness of penicillin in eradicating microorganisms was quickly, and quite naturally, followed by attempts to prove that it also was effective in preventing infection in susceptible hosts. As a result, the antibiotic has been administered in almost every situation in which a risk of bacterial invasion has been present. As prophylaxis has been investigated under controlled conditions, it has become clear that penicillin is highly effective in some situations, useless and potentially dangerous in others, and of questionable value in still others (seeChapter 43: Antimicrobial Agents: General Considerations).

Streptococcal Infections

The administration of penicillin to individuals exposed to Streptococcus pyogenes affords protection from infection. The oral ingestion of 200,000 units of penicillin G or penicillin V twice a day or a single injection of 1.2 million units of penicillin G benzathine is effective. Indications for this type of prophylaxis include outbreaks of streptococcal disease in closed populations, such as boarding schools or military bases. Patients with extensive deep burns are at high risk of severe wound infections with S. pyogenes; several days of 'low-dose' prophylaxis appears to be effective in reducing the incidence of this complication.

Recurrences of Rheumatic Fever

The oral administration of 200,000 units of penicillin G or penicillin V every 12 hours produces a striking decrease in the incidence of recurrences of rheumatic fever in susceptible individuals. Because of the difficulties of compliance, parenteral administration is preferable, especially in children. The intramuscular injection of 1.2 million units of penicillin G benzathine once a month yields excellent results. In cases of hypersensitivity to penicillin, sulfisoxazole or sulfadiazine, 1 g twice a day for adults, also is effective; for children weighing under 27 kg, the dose is halved. Prophylaxis must be continued throughout the year. The duration of such treatment is an unsettled question. It has been suggested that prophylaxis should be continued for life, because instances of acute rheumatic fever have been observed in the fifth and sixth decades. However, the necessity for such prolonged prophylaxis has not been established and may be unnecessary for certain young adults judged to be low risk for recurrence (Berrios et al., 1993).

Syphilis

Prophylaxis for a contact with syphilis consists of a course of therapy as described for primary syphilis. A serological test for syphilis should be performed at monthly intervals for at least 4 months thereafter.

Surgical Procedures in Patients with Valvular Heart Disease

About 25% of cases of subacute bacterial endocarditis follow dental extractions. This observation, together with the fact that up to 80% of persons who have teeth removed experience a transient bacteremia, emphasizes the potential importance of chemoprophylaxis for those who have congenital or acquired valvular heart disease of any type and need to undergo dental procedures. Since transient bacterial invasion of the bloodstream occurs occasionally after surgical procedures (e.g., tonsillectomy and genitourinary and gastrointestinal procedures) and during childbirth, these too are indications for prophylaxis in patients with valvular heart disease. Whether the incidence of bacterial endocarditis actually is altered by this type of chemoprophylaxis remains to be determined.

Detailed recommendations for both adults and children with valvular heart disease have been formulated (seeDajani et al., 1990; Durack, 2000).

The Penicillinase-Resistant Penicillins

The penicillins described in this section are resistant to hydrolysis by staphylococcal penicillinase. Their appropriate use should be restricted to the treatment of infections that are known or suspected to be caused by staphylococci that elaborate the enzymethe vast majority of strains of this bacterium that are encountered in the hospital or in the general community. These drugs are much less active than is penicillin G against other penicillin-sensitive microorganisms, including non-penicillinase-producing staphylococci.

The role of the penicillinase-resistant penicillins as the agents of choice for most staphylococcal disease may be changing with the increasing incidence of isolates of so-called methicillin-resistant microorganisms. As commonly used, this latter term denotes resistance of these bacteria to all of the penicillinase-resistant penicillins and cephalosporins. Such strains are usually resistant as well to the aminoglycosides, tetracyclines, erythromycin, and clindamycin. Vancomycin is considered to be the drug of choice for such infections. Some physicians use a combination of vancomycin and rifampin, especially for life-threatening infections and those involving foreign bodies. Methicillin-resistant S. aureus contains an additional high molecular weight PBP with a very low affinity for -lactam antibiotics (Spratt, 1994). From 40% to 60% of strains of S. epidermidis also are resistant to the penicil-linase-resistant penicillins by the same mechanism. As with methicillin-resistant S. aureus, these strains may appear to be susceptible to cephalosporins on disc sensitivity testing, but there is usually a significant population of microbes that is resistant to cephalosporins and that emerges during such therapy. Vancomycin also is the drug of choice for serious infection caused by methicillin-resistant S. epidermidis; rifampin is given concurrently when a foreign body is involved.

The Isoxazolyl Penicillins: Oxacillin, Cloxacillin, and Dicloxacillin

These three congeneric semisynthetic penicillins are similar pharmacologically and are thus conveniently considered together. Their structural formulas are shown in Table 451. All are relatively stable in an acidic medium and are adequately absorbed after oral administration. All are markedly resistant to cleavage by penicillinase. These drugs are not substitutes for penicillin G in the treatment of diseases amenable to it. Furthermore, because of variability in intestinal absorption, oral administration is not a substitute for the parenteral route in the treatment of serious staphylococcal infections that require a penicillin unaffected by penicillinase.

Pharmacological Properties

The isoxazolyl penicillins are potent inhibitors of the growth of most penicillinase-producing staphylococci. This is their valid clinical use. Dicloxacillin (PATHOCIL, others) is the most active, and many strains of S. aureus are inhibited by concentrations of 0.05 to 0.8 g/ml. Comparable values for cloxacillin and oxacillin are 0.1 to 3 g/ml and 0.4 to 6 g/ml, respectively. These differences may have little practical significance, however, since dosages (see below) are adjusted accordingly. These agents are, in general, less effective against microorganisms susceptible to penicillin G, and they are not useful against gram-negative bacteria.

These agents are rapidly but incompletely (30% to 80%) absorbed from the gastrointestinal tract. Absorption of the drugs is more efficient when they are taken on an empty stomach; preferably they are administered one hour before or two hours after meals to ensure better absorption. Peak concentrations in plasma are attained by 1 hour and approximate 5 to 10 g/ml after the ingestion of 1 g of oxacillin. Slightly higher concentrations are achieved after the administration of 1 g of cloxacillin, while the same oral dose of dicloxacillin yields peak plasma concentrations of 15 g/ml. There is little evidence that these differences are of clinical significance. All these congeners are bound to plasma albumin to a great extent (approximately 90% to 95%); none is removed from the circulation to a significant degree by hemodialysis.

The isoxazolyl penicillins are rapidly excreted by the kidney. Normally, about one-half of any of these drugs is excreted in the urine in the first 6 hours after a conventional oral dose. There also is significant hepatic elimination of these agents in the bile. The half-lives for all are between 30 and 60 minutes. Intervals between doses of oxacillin, cloxacillin, and dicloxacillin do not have to be altered for patients with renal failure. The above-noted differences in plasma concentrations produced by the isoxazolyl penicillins are related mainly to differences in rate of urinary excretion and degree of resistance to degradation in the liver.

Nafcillin

This semisynthetic penicillin is highly resistant to penicillinase and has proven effective against infections caused by penicillinase-producing strains of S. aureus. Its structural formula is shown in Table 451.

Pharmacological Properties

Nafcillin (UNIPEN, NALLPEN, others) is slightly more active than oxacillin against penicillin Gresistant S. aureus (most strains are inhibited by 0.06 to 2 g/ml). While it is the most active of the penicillinase-resistant penicillins against other microorganisms, it is not as potent as penicillin G.

Nafcillin is inactivated to a variable degree in the acidic medium of the gastric contents. Its absorption after oral administration is irregular, regardless of whether the drug is taken with meals or on an empty stomach. Consequently, although oral preparations are available, injectable preparations should be used because of the variable absorption of nafcillin from the gastrointestinal tract. The peak plasma concentration is about 8 g/ml 60 minutes after a 1-g intramuscular dose. Nafcillin is about 90% bound to plasma protein. Peak concentrations of nafcillin in bile are well above those found in plasma. Concentrations of the drug in CSF appear to be adequate for therapy of staphylococcal meningitis.

The Aminopenicillins: Ampicillin, Amoxicillin, and Their Congeners

These agents have similar antibacterial activity and a spectrum that is broader than the antibiotics heretofore discussed. They are all destroyed by -lactamase (from both gram-positive and gram-negative bacteria).

Antimicrobial Activity

Ampicillin and the related aminopenicillins are bactericidal for both gram-positive and gram-negative bacteria. The meningococci and L. monocytogenes are sensitive to the drug. Many pneumococcal isolates have varying levels of resistance to ampicillin. Penicillin-resistant strains should be considered ampicillin/amoxicillin-resistant. H. influenzae and the viridans group of streptococci exhibit varying degrees of resistance. Enterococci are about twice as sensitive to ampicillin, on a weight basis, as they are to penicillin G (MIC for ampicillin averages 1.5 mg/ml). Although most strains of N. gonorrhoeae, E. coli, P. mirabilis, Salmonella, and Shigella were highly susceptible when ampicillin was first used in the early 1960s, an increasing percentage of these species is now resistant. From 30% to 50% of E. coli, a significant number of P. mirabilis, and practically all species of Enterobacter are presently insensitive. Resistant strains of Salmonella (plasmid mediated) have been recovered with increasing frequency in various parts of the world. Most strains of Shigella are now resistant. Most strains of Pseudomonas, Klebsiella, Serratia, Acinetobacter, and indole-positive Proteus also are resistant to this group of penicillins; these antibiotics are less active against B. fragilis than is penicillin G. However, concurrent administration of a -lactamase inhibitor such as clavulanate or sulbactam markedly expands the spectrum of activity of these drugs (see below).

Ampicillin

This drug is the prototypical agent of the group. Its structural formula is shown in Table 451.

Pharmacological Properties

Ampicillin (OMNIPEN, POLYCILLIN, others) is stable in acid and is well absorbed after oral administration. An oral dose of 0.5 g produces peak concentrations in plasma of about 3 g/ml at 2 hours. Intake of food prior to ingestion of ampicillin results in less complete absorption. Intramuscular injection of 0.5 or 1 g of sodium ampicillin yields peak plasma concentrations of about 7 or 10 g/ml, respectively, at 1 hour; these decline exponentially, with a half-time of approximately 80 minutes. Severe renal impairment markedly prolongs the persistence of ampicillin in the plasma. Peritoneal dialysis is ineffective in removing the drug from the blood, but hemodialysis removes about 40% of the body store in about 7 hours. Adjustment of the dose of ampicillin is required in the presence of renal dysfunction. Ampicillin appears in the bile, undergoes enterohepatic circulation, and is excreted in appreciable quantities in the feces.

Amoxicillin

This drug, a penicillinase-susceptible semi-synthetic penicillin, is a close chemical and pharmacological relative of ampicillin (seeTable 451). The drug is stable in acid and is designed for oral use. It is more rapidly and completely absorbed from the gastrointestinal tract than is ampicillin, which is the major difference between the two. The antimicrobial spectrum of amoxicillin is essentially identical to that of ampicillin, with the important exception that amoxicillin appears to be less effective than ampicillin for shigellosis.

Peak concentrations of amoxicillin (AMOXIL, others) in plasma are two to two and one-half times greater for amoxicillin than for ampicillin after oral administration of the same dose; they are reached at 2 hours and average about 4 g/ml when 250 mg is administered. Food does not interfere with absorption. Perhaps because of more complete absorption of this congener, the incidence of diarrhea with amoxicillin is less than that following administration of ampicillin. The incidence of other adverse effects appears to be similar. While the half-life of amoxicillin is similar to that for ampicillin, effective concentrations of orally administered amoxicillin are detectable in the plasma for twice as long as with ampicillin, again because of the more complete absorption. About 20% of amoxicillin is protein-bound in plasma, a value similar to that for ampicillin. Most of a dose of the antibiotic is excreted in an active form in the urine. Probenecid delays excretion of the drug.

Therapeutic Indications for the Aminopenicillins

Upper Respiratory Infections

Ampicillin and amoxicillin are active against S. pyogenes and many strains of S. pneumoniae and H. influenzae, which are major upper respiratory bacterial pathogens. The drugs constitute effective therapy for sinusitis, otitis media, acute exacerbations of chronic bronchitis, and epiglottitis caused by sensitive strains of these organisms. Amoxicillin is the most active of all the oral -lactam antibiotics against both penicillin-sensitive and penicillin-resistant S. pneumoniae (Friedland and McCracken, 1994). Based on the increasing prevalence of pneumococcal resistance to penicillin, an increase in dose of oral amoxicillin (from 40 to 45 up to 80 to 90 mg/kg per day) for empiric treatment of acute otitis media in children is recommended (Dowell et al., 1999). Ampicillin-resistant H. influenzae may be a problem in many areas. The addition of a -lactamase inhibitor (amoxicillin-clavulanate or ampicillin-sulbactam) extends the spectrum to -lactamase-producing H. influenzae and Enterobacteriaceae. Bacterial pharyngitis should be treated with penicillin G or penicillin V, since S. pyogenes is the major pathogen.

Urinary Tract Infections

Most uncomplicated urinary tract infections are caused by Enterobacteriaceae, and E. coli is the most common species; ampicillin often is an effective agent, although resistance is increasingly common. Enterococcal urinary tract infections are treated effectively with ampicillin alone.

Meningitis

Acute bacterial meningitis in children is most frequently due to S. pneumoniae or N. meningitidis. Since 20% to 30% of strains of S. pneumoniae now may be resistant to this antibiotic, ampicillin is not indicated for single-agent treatment of meningitis. Ampicillin has excellent activity against L. monocytogenes, a cause of meningitis in immunocompromised persons. Thus, the combination of ampicillin and vancomycin plus a third-generation cephalosporin is a rational regimen for empiric treatment of suspected bacterial meningitis.

Salmonella Infections

Disease associated with bacteremia, disease with metastatic foci, and the enteric fever syndrome (including typhoid fever) respond favorably to antibiotics. A fluoroquinolone or ceftriaxone is considered by some to be the drug of choice, but the administration of trimethoprimsulfamethoxazole or high doses of ampicillin (12 g per day for adults) also is effective. In some geographical areas, resistance to ampicillin is common. The typhoid carrier state has been eliminated successfully in patients without gallbladder disease with ampicillin, trimethoprimsulfamethoxazole, or ciprofloxacin.

Antipseudomonal Penicillins: The Carboxypenicillins and the Ureidopenicillins

The carboxypenicillins, carbenicillin and ticarcillin and their close relatives, are active against some isolates of P. aeruginosa and certain indole-positive Proteus species that are resistant to ampicillin and its congeners. They are ineffective against most strains of S. aureus, Enterococcus faecalis, Klebsiella, and L. monocytogenes.B. fragilis is susceptible to high concentrations of these drugs, but penicillin G is actually more active on the basis of weight. The ureidopenicillins, mezlocillin and piperacillin, have superior activity against P. aeruginosa compared to carbenicillin and ticarcillin. In addition, mezlocillin and piperacillin are useful for treatment of infections with Klebsiella. The carboxypenicillins and the ureidopenicillins are sensitive to destruction by -lactamases.

Carbenicillin and Carbenicillin Indanyl

Carbenicillin

This drug is a penicillinase-susceptible derivative of 6-aminopenicillanic acid. Its structural formula is shown in Table 451. Carbenicillin was the first penicillin with activity against P. aeruginosa and some Proteus strains that are resistant to ampicillin. It has been superseded by ticarcillin or piperacillin for most uses (see below).

Preparations of carbenicillin may cause adverse effects in addition to those that follow use of the other penicillins (see below). Congestive heart failure may result from the administration of excessive Na⁺. Hypokalemia may occur because of obligatory excretion of cation with the large amount of nonreabsorbable anion (carbenicillin) presented to the distal renal tubule. The drug interferes with platelet function, and bleeding may occur because of abnormal aggregation of platelets.

Carbenicillin Indanyl Sodium (GEOCILLIN

This congener is the indanyl ester of carbenicillin; it is acid-stable and is suitable for oral administration. After absorption, the ester is rapidly converted to carbenicillin by hydrolysis of the ester linkage. The antimicrobial spectrum of the drug is therefore that of carbenicillin. Although relatively low concentrations of carbenicillin are achieved in plasma, the active moiety is excreted rapidly in the urine. Thus, the only use of this drug is for the management of urinary tract infections caused by Proteus species other than P. mirabilis and by P. aeruginosa.

Ticarcillin (TICAR

This semisynthetic penicillin (Table 451) is very similar to carbenicillin, but it is two to four times more active against P. aeruginosa. Ticarcillin is inferior to piperacillin and mezlocillin for the treatment of serious infections caused by Pseudomonas.

Mezlocillin

This ureidopenicillin is more active against Klebsiella than is carbenicillin; its activity against Pseudomonasin vitro is similar to that of ticarcillin. It is more active than ticarcillin against Enterococcus faecalis. Mezlocillin sodium (MEZLIN) is available as a powder to be dissolved for injection and contains about 2 mEq of Na⁺ per gram. The usual dose for adults is 6 to 18 g per day, divided into four to six portions. Mezlocillin and piperacillin (below) are excreted in bile to a significant degree. In the absence of biliary tract obstruction, high concentrations of mezlocillin in bile are achieved by intravenous administration.

Piperacillin

Piperacillin (PIPRACIL) extends the spectrum of ampicillin to include most strains of P. aeruginosa, Enterobacteriaceae (non--lactamse-producing), and many Bacteroides species. In combination with a -lactamase inhibitor (piperacillin-tazobactam, ZOSYN ) it has the broadest antibacterial spectrum of the penicillins. Pharmacokinetic properties are reminiscent of the other ureidopenicillins. High biliary concentrations are achieved, as with mezlocillin.

Therapeutic Indications

These penicillins are important agents for the treatment of patients with serious infections caused by gram-negative bacteria. Such patients frequently have impaired immunological defenses, and their infections often are acquired in the hospital. Many authorities feel that a -lactam agent, often in combination with an aminoglycoside, should be employed for all such infections. Therefore, these penicillins find their greatest use in treating bacteremias, pneumonias, infections following burns, and urinary tract infections due to microorganisms resistant to penicillin G and ampicillin; the bacteria especially responsible include P. aeruginosa, indole-positive strains of Proteus, and Enterobacter species. Since Pseudomonas infections are common in neutropenic patients, therapy for severe bacterial infections in such individuals should include a -lactam antibiotic with good activity against these microorganisms.

Untoward Reactions to Penicillins

Hypersensitivity Reactions

Hypersensitivity reactions are by far the most common adverse effects noted with the penicillins, and these agents probably are the most common cause of drug allergy. Allergic reactions complicate between 0.7% and 4% of all treatment courses. There is no convincing evidence that any single penicillin differs from the group in its potential for causing true allergic reactions. In approximate order of decreasing frequency, manifestations of allergy to penicillins include maculopapular rash, urticarial rash, fever, bronchospasm, vasculitis, serum sickness, exfoliative dermatitis, StevensJohnson syndrome, and anaphylaxis (Anonymous, 1988; Weiss and Adkinson, 2000). The overall incidence of such reactions to the penicillins varies from 0.7% to 10% in different studies.

Hypersensitivity reactions may occur with any dosage form of penicillin; allergy to one penicillin exposes the patient to a greater risk of reaction if another is given. On the other hand, the occurrence of an untoward effect does not necessarily imply repetition on subsequent exposures. Hypersensitivity reactions may appear in the absence of a previous known exposure to the drug. This may be caused by unrecognized prior exposure to penicillin in the environment (e.g., in foods of animal origin or from the fungus producing penicillin). Although elimination of the antibiotic usually results in rapid clearing of the allergic manifestations, they may persist for 1 or 2 weeks or longer after therapy has been stopped. In some cases, the reaction is mild and disappears even while the use of penicillin is continued; in others, it necessitates immediate cessation of penicillin treatment. In a few instances, it is necessary to interdict the future use of penicillin because of the risk of death, and the patient should be so warned. It must be stressed that fatal episodes of anaphylaxis have followed the ingestion of very small doses of this antibiotic or skin testing with minute quantities of the drug.

Penicillins and breakdown products of penicillins act as haptens after their covalent reaction with proteins. The most important antigenic intermediate of penicillin appears to be the penicilloyl moiety, which is formed when the -lactam ring is opened. This is considered to be the major (predominant) determinant of penicillin allergy. In addition, minor determinants of allergy to penicillins are present. These include the intact molecule itself and penicilloate. These products are formed in vivo and also can be found in solutions of penicillin prepared for administration. The terms major determinant and minor determinant refer to the frequency with which antibodies to these haptens appear to be formed. They do not describe the severity of the reaction that may result. In fact, anaphylactic reactions to penicillin are usually mediated by IgE antibodies against the minor determinants.

Antipenicillin antibodies are detectable in virtually all patients who have received the drug and in many who have never knowingly been exposed to it (Klaus and Fellner, 1973). Recent treatment with the antibiotic induces an increase in major-determinant-specific antibodies that are skin sensitizing. The incidence of positive skin reactors is three to four times higher in atopic than in nonatopic individuals. Clinical and immunological studies suggest that immediate allergic reactions are mediated by skin-sensitizing or IgE antibodies, usually of minor-determinant specificities. Accelerated and late urticarial reactions usually are mediated by major-determinantspecific skin-sensitizing antibodies. The recurrent-arthralgia syndrome appears to be related to the presence of skin-sensitizing antibodies of minor-determinant specificities. Some maculopapular and erythematous reactions may be due to toxic antigen-antibody complexes of major determinantspecific IgM antibodies. Accelerated and late urticarial reactions to penicillin may terminate spontaneously because of the development of blocking antibodies.

Skin rashes of all types may be caused by allergy to penicillin. Scarlatiniform, morbilliform, urticarial, vesicular, and bullous eruptions may develop. Purpuric lesions are uncommon and are usually the result of a vasculitis; thrombocytopenic purpura may occur very rarely. HenochSchnlein purpura with renal involvement has been a rare complication. Contact dermatitis is observed occasionally in pharmacists, nurses, and physicians who prepare penicillin solutions. Fixed-drug reactions also have occurred.More severe reactions involving the skin are exfoliative dermatitis and exudative erythema multiforme of either the erythematopapular or vesiculobullous type; these lesions may be very severe and atypical in distribution and constitute the characteristic StevensJohnson syndrome. The incidence of skin rashes appears to be highest following the use of ampicillin, being about 9%; rashes follow the administration of ampicillin in nearly all patients with infectious mononucleosis. When allopurinol and ampicillin are administered concurrently, the incidence of rash also increases. Ampicillin-induced skin eruptions in such patients may represent a 'toxic' rather than a truly allergic reaction. Positive skin reactions to the major and minor determinants of penicillin sensitization may be absent. The rash may clear even while administration of the drug is continued.

The most serious hypersensitivity reactions produced by the penicillins are angioedema and anaphylaxis. Angioedema with marked swelling of the lips, tongue, face, and periorbital tissues, frequently accompanied by asthmatic breathing and 'giant hives,' has been observed after topical, oral, or systemic administration of penicillins of various types.

Acute anaphylactic or anaphylactoid reactions induced by various preparations of penicillin constitute the most important immediate danger connected with their use. Among all drugs, the penicillins are most often responsible for this type of untoward effect. Anaphylactoid reactions may occur at any age. Their incidence is thought to be 0.004% to 0.04% in persons treated with penicillins (Kucers and Bennett, 1987). About 0.001% of patients treated with these agents die from anaphylaxis. It has been estimated that there are at least 300 deaths per year due to this complication of therapy. About 70% have had penicillin previously, and one-third of these reacted to it on a prior occasion. Anaphylaxis has most often followed the injection of penicillin, although it also has been observed after oral ingestion of the drug and even has resulted from the intradermal instillation of a very small quantity for the purpose of testing for the presence of hypersensitivity. The clinical pictures that develop vary in severity. The most dramatic is sudden, severe hypotension and rapid death. In other instances, bronchoconstriction with severe asthma; abdominal pain, nausea, and vomiting; extreme weakness and a fall in blood pressure; or diarrhea and purpuric skin eruptions have characterized the anaphylactic episodes.

Serum sickness varies from mild fever, rash, and leukopenia to severe arthralgia or arthritis, purpura, lymphadenopathy, splenomegaly, mental changes, electrocardiographic abnormalities suggestive of myocarditis, generalized edema, albuminuria, and hematuria. It is mediated by IgG antibodies. This reaction usually appears after penicillin treatment has been continued for 1 week or more; it may be delayed, however, until 1 or 2 weeks after the drug has been stopped. Serum sickness caused by penicillin may persist for a week or longer.

Vasculitis of the skin or other organs may be related to hypersensitivity to penicillin. The Coombs reaction frequently becomes positive during prolonged therapy with a penicillin or cephalosporin, but hemolytic anemia is rare. Reversible neutropenia may occur. It is not known if this is truly a hypersensitivity reaction; it has been noted with all of the penicillins and has been seen in up to 30% of patients treated with 8 to 12 g of nafcillin for longer than 21 days. The bone marrow shows an arrest of maturation.

Fever may be the only evidence of a hypersensitivity reaction to the penicillins. It may reach high levels and be maintained, remittent, or intermittent; chills occasionally occur. The febrile reaction usually disappears within 24 to 36 hours after administration of the drug is stopped but may persist for days.

Eosinophilia is an occasional accompaniment of other allergic reactions to penicillin. At times, it may be the sole abnormality, and eosinophils may reach levels of 10% to 20% or more of the total number of circulating white blood cells.

Interstitial nephritis rarely may be produced by the penicillins; methicillin has been implicated most frequently. Hematuria, albuminuria, pyuria, renal-cell and other casts in the urine, elevation of serum creatinine, and even oliguria have been noted. Biopsy shows a mononuclear infiltrate with eosinophilia and tubular damage. IgG is present in the interstitium. This reaction usually is reversible.

Management of the Patient Potentially Allergic to Penicillin

Evaluation of the patient's history is the most practical way to avoid the use of penicillin in patients who are at the greatest risk of adverse reaction. The majority of patients who give a history of allergy to penicillin should be treated with a different type of antibiotic. Unfortunately there is no available means to confirm a history of penicillin allergy. Skin testing for IgE-mediated immediate-type responses is compromised by the lack of a commercially available minor determinant mixture and the inability of skin tests using major and minor penicillin determinants to predict confidently allergic reactions to synthetic penicillins. Radioallergosorbent tests (RAST) for IgE antipenicilloyl determinants suffer from the same limitations as skin tests (Weiss and Adkinson, 2000).

Desensitization occasionally is recommended for patients who are allergic to penicillin and who must receive the drug. This procedure consists of administering gradually increasing doses of penicillin in the hope of avoiding a severe reaction and should be performed only in an intensive-care setting. This may result in a subclinical anaphylactic discharge and the binding of all IgE before full doses are administered. Penicillin may be given in doses of 1, 5, 10, 100, and 1000 U intradermally in the lower arm, with 60-minute intervals between doses. If this is well tolerated, then 10,000 U and 50,000 U may be given subcutaneously. Desensitization also may be accomplished by the oral administration of penicillin (Sullivan et al., 1982). When full doses are reached, penicillin should not be discontinued and then restarted, since immediate reactions may recur (seeWeiss and Adkinson, 2000, for details). The patient should be observed constantly during the desensitizing procedure, an intravenous line must be in place, and epinephrine and equipment and expertise for artificial ventilation must be on hand. It must be emphasized that this procedure may be dangerous, and its efficacy is unproven.

Patients with life-threatening infections (e.g., endocarditis or meningitis) may be continued on penicillin despite the development of a maculopapular rash, although alternative antimicrobial agents should be used whenever possible. The rash often clears as therapy is continued. This is thought to be due to the development of blocking antibodies of the IgG class. The rash may be treated with antihistamines or adrenocorticosteroids, although there is no evidence that this therapy is efficacious. Rarely, exfoliative dermatitis with or without vasculitis develops in these patients if therapy with penicillin is continued.

Other Adverse Reactions

The penicillins have minimal direct toxicity. Apparent toxic effects that have been reported include bone marrow depression, granulocytopenia, and hepatitis. The last-named effect is rare but is most commonly seen following the administration of oxacillin and nafcillin (Kirkwood et al., 1983). The administration of penicillin G, carbenicillin, piperacillin, or ticarcillin has been associated with a potentially significant defect of hemostasis that appears to be due to an impairment of platelet aggregation; this may be caused by interference with the binding of aggregating agents to platelet receptors (Fass et al., 1987).

Most common among the irritative responses to penicillin are pain and sterile inflammatory reactions at the sites of intramuscular injectionsreactions that are related to concentration. Serum transaminases and lactic dehydrogenase may be elevated as a result of local damage to muscle. In some individuals who receive penicillin intravenously, phlebitis or thrombophlebitis develops. Many persons who take various penicillin preparations by mouth experience nausea, with or without vomiting, and some have mild-to-severe diarrhea. These manifestations often are related to the dose of the drug.

When penicillin is injected accidentally into the sciatic nerve, severe pain occurs and dysfunction in the area of distribution of this nerve develops and persists for weeks. Intrathecal injection of penicillin G may produce arachnoiditis or severe and fatal encephalopathy. Because of this, intrathecal or intraventricular administration of penicillins should be avoided. The parenteral administration of large doses of penicillin G (greater than 20 million units per day, or less with renal insufficiency) may produce lethargy, confusion, twitching, multifocal myoclonus, or localized or generalized epileptiform seizures. These are most apt to occur in the presence of renal insufficiency, localized lesions of the central nervous system (CNS), or hyponatremia. When the concentration of penicillin G in CSF exceeds 10 g/ml, significant dysfunction of the CNS is frequent. The injection of 20 million units of penicillin G potassium, which contains 34 mEq of K⁺, may lead to severe or even fatal hyperkalemia in persons with renal dysfunction.

Injection of penicillin Gprocaine may result in an immediate reaction, characterized by dizziness, tinnitus, headache, hallucinations, and sometimes seizures. This is due to the rapid liberation of toxic concentrations of procaine. It has been reported to occur in 1 of 200 patients receiving 4.8 million units of penicillin G procaine to treat their venereal disease.

Reactions Unrelated to Hypersensitivity or Toxicity

Regardless of the route by which the drug is administered, but most strikingly when it is given by mouth, penicillin changes the composition of the microflora by eliminating sensitive microorganisms. This phenomenon is usually of no clinical significance, and the normal microflora is reestablished shortly after therapy is stopped. In some persons, however, superinfection results from the changes in flora. Pseudomembranous colitis, related to overgrowth and production of a toxin by Clostridium difficile, has followed oral and, less commonly, parenteral administration of penicillins.

Aminoglycosides contain amino sugars linked to an aminocyclitol ring by glycosidic bonds. They are polycations, and their polarity is in part responsible for pharmacokinetic properties shared by all members of the group. For example, none is absorbed adequately after oral administration, inadequate concentrations are found in cerebrospinal fluid, and all are excreted relatively rapidly by the normal kidney.

The aminoglycosides are used primarily to treat infections caused by aerobic gram-negative bacteria; they interfere with protein synthesis in susceptible microorganisms. In contrast to most inhibitors of microbial protein synthesis, which are bacteriostatic, the aminoglycosides are bactericidal. Mutations affecting proteins in the bacterial ribosome, the target for these drugs, can confer marked resistance to their action. Resistance most commonly results from the acquisition of plasmids or transposon-encoding genes for aminoglycoside-metabolizing enzymes or from impaired transport of drug into the cell. There can be cross-resistance between some members of the class.

Although they are widely used and important agents, serious toxicity is a major limitation to the usefulness of the aminoglycosides. The same spectrum of toxicity is shared by all members of the group. Most notable are nephrotoxicity and ototoxicity, which can involve both the auditory and vestibular functions of the eighth cranial nerve.

History and Source

The development of streptomycin was the result of a well-planned, scientific search for antibacterial substances. Stimulated by the discovery of penicillin, Waksman and coworkers examined a number of soil actinomycetes between 1939 and 1943. In 1943, a strain of Streptomyces griseus was isolated that elaborated a potent antimicrobial substance, streptomycin, which was shown to inhibit the growth of the tubercle bacillus and a number of aerobic gram-positive and gram-negative microorganisms. In less than two years, extensive bacteriological, chemical, and pharmacological investigations of streptomycin had been carried out, and its clinical usefulness was established (Waksman, 1949). However, streptomycin-resistant gram-negative bacilli and gram-positive cocci (enterococci) have emerged, limiting its clinical usefulness. It is now rarely used except for the treatment of certain types of streptococcal or enterococcal endocarditis, tularemia, and plague, and for treatment of tuberculosis.

In 1949, Waksman and Lechevalier isolated a soil organism, Streptomyces fradiae, which produced a group of antibacterial substances that were named neomycin. One component, neomycin B, is still used topically or given orally for its local effect on bowel flora, because it causes severe renal toxicity and ototoxicity when administered parenterally.

Kanamycin, an antibiotic elaborated by Streptomyces kanamyceticus, was first produced and isolated by Umezawa and coworkers at the Japanese National Institutes of Health in 1957. Because of toxicity and the emergence of resistant microorganisms, kanamycin has been replaced almost entirely by the newer aminoglycosides.

Gentamicin and netilmicin are both broad-spectrum antibiotics derived from species of the actinomycete Micromonospora. The difference in spelling (-micin) as compared with that of the other aminoglycoside antibiotics (-mycin) reflects this difference in origin. Gentamicin was first studied and described by Weinstein and coworkers in 1963. It has a broader spectrum of activity than kanamycin and currently is used widely. Tobramycin and amikacin were introduced into clinical practice in the 1970s. Tobramycin is one of several components of an aminoglycoside complex (nebramycin) that is produced by Streptomyces tenebrarius (Higgins and Kastner, 1967). It is most similar in antimicrobial activity and toxicity to gentamicin. In contrast to the other aminoglycosides, amikacin and netilmicin are semisynthetic products. Amikacin, which is a derivative of kanamycin, was described by Kawaguchi and co-workers (1972); netilmicin is a derivative of sisomicin. Other aminoglycoside antibiotics have been developed (e.g., arbekacin, isepamicin, sisomicin). These have not been introduced into clinical practice in the United States, because numerous potent, less toxic alternatives (e.g., broad-spectrum -lactam antibiotics and quinolones) are available.

Chemistry

The aminoglycosides consist of two or more amino sugars joined in glycosidic linkage to a hexose nucleus, which is usually in a central position (seeFigure 461). This hexose, or aminocyclitol, is either streptidine (found in streptomycin) or 2-deoxystreptamine (all other available aminoglycosides). These compounds are thus aminoglycosidic aminocyclitols, although the simpler term aminoglycoside is commonly used to describe them. An additional drug, spectinomycin, is an aminocyclitol that does not contain amino sugars; it is discussed in Chapter 47: Antimicrobial Agents: Protein Synthesis Inhibitors and Miscellaneous Antibacterial Agents.

The aminoglycoside families are distinguished by the amino sugars attached to the aminocyclitol. In the neomycin family, which includes neomycin B and paromomycin, an aminoglycoside used orally for the treatment of intestinal parasitic infections, there are three amino sugars attached to the central 2-deoxystreptamine. The kanamycin and gentamicin families have only two such amino sugars. Neomycin B has the following structural formula:

In the kanamycin family, which includes kanamycins A and B, amikacin, and tobramycin, two amino sugars are linked to a centrally located 2-deoxystreptamine moiety; one of these is a 3-aminohexose (seeFigure 461). The structural formula of kanamycin A, which is the major component of the commercial product, is as follows:

Amikacin is a semisynthetic derivative prepared from kana mycin A by acylation of the 1-amino group of the 2-deoxy- streptamine moiety with 2-hydroxy-4-aminobutyric acid.

The gentamicin family, which includes gentamicin C₁, C_1a, and C₂, sisomicin, and netilmicin (the 1-N-ethyl derivative of sisomicin), contains a different 3-amino sugar (garosamine). Variations in methylation of the other amino sugar result in the different components of gentamicin (Figure 461). These modifications appear to have little effect on biological activity.

Streptomycin and dihydrostreptomycin (the latter is no longer available because of excessive ototoxicity) differ from the other aminoglycoside antibiotics in that they contain streptidine rather than 2-deoxystreptamine, and their aminocyclitol is not in a central position. The structural formula of streptomycin is as follows:

Mechanism of Action

The aminoglycoside antibiotics are rapidly bactericidal. Bacterial killing is concentration-dependent: the higher the concentration, the greater the rate at which bacteria are killed. A postantibiotic effect, that is, residual bactericidal activity persisting after the serum concentration has fallen below the minimum inhibitory concentration, also is characteristic of aminoglycoside antibiotics, and the duration of this effect is concentration-dependent. These properties probably account for the efficacy of once-daily dosing regimens of aminoglycosides. Although much is known about their capacity to inhibit protein synthesis and decrease the fidelity of translation of mRNA at the ribosome (Shannon and Phillips, 1982), the precise mechanism responsible for the rapidly lethal effect of aminoglycosides on bacteria is unknown.

Aminoglycosides diffuse through aqueous channels formed by porin proteins in the outer membrane of gram-negative bacteria to enter the periplasmic space (Nakae and Nakae, 1982). Transport of aminoglycosides across the cytoplasmic (inner) membrane depends on electron transport, in part because of a requirement for a membrane electrical potential (interior negative) to drive permeation of these antibiotics (Bryan and Kwan, 1983). This phase of transport has been termed energy-dependent phase I. It is rate-limiting and can be blocked or inhibited by divalent cations (e.g., Ca²⁺ and Mg²⁺), hyperosmolarity, a reduction in pH, and anaerobiasis. The last two of these conditions impair the ability of the bacteria to maintain the membrane potential, which is the driving force necessary for transport. Thus, the antimicrobial activity of aminoglycosides is reduced markedly in the anaerobic environment of an abscess, in hyperosmolar acidic urine, and so forth. Once inside the cell, aminoglycosides bind to polysomes and interfere with protein synthesis by causing misreading and premature termination of translation of mRNA (seeFigure 462). The aberrant proteins produced may be inserted into the cell membrane, leading to altered permeability and further stimulation of aminoglycoside transport (Busse et al., 1992). This phase of aminoglycoside transport, termed energy-dependent phase II (EDP₂), is poorly understood; however, it has been suggested that EDP₂ is in some way linked with disruption of the structure of the cytoplasmic membrane, perhaps by the aberrant proteins. This concept is consistent with the observed progression of the leakage of small ions, followed by larger molecules and, eventually, by proteins from the bacterial cell prior to aminoglycoside-induced death. This progressive disruption of the cell envelope, as well as other vital cell processes, may help to explain the lethal action of aminoglycosides (Bryan, 1989).

The primary intracellular site of action of the aminoglycosides is the 30 S ribosomal subunit, which consists of 21 proteins and a single 16 S molecule of RNA. At least three of these proteins and perhaps the 16 S ribosomal RNA as well contribute to the streptomycin binding site, and alterations of these molecules markedly affect the binding and subsequent action of streptomycin. For example, a single amino acid substitution of asparagine for lysine at position 42 of one ribosomal protein (S₁₂) prevents binding of the drug; the resultant mutant is totally resistant to streptomycin. Another mutant, in which glutamine is the amino acid at this position, is dependent on streptomycin, which is actually required for survival. The other aminoglycosides also bind to the 30 S ribosomal subunit; however, they also appear to bind to several sites on the 50 S ribosomal subunit (Davis, 1988).

Aminoglycosides disrupt the normal cycle of ribosomal function by interfering, at least in part, with the initiation of protein synthesis, leading to the accumulation of abnormal initiation complexes or 'streptomycin monosomes,' shown schematically in Figure 462B (Luzzatto et al., 1969). Another effect of the aminoglycosides is their capacity to induce misreading of the mRNA template, causing incorrect amino acids to be incorporated into the growing polypeptide chains (seeTai et al., 1978). The aminoglycosides vary in their capacity to cause misreading, and this property presumably depends on differences in their affinities for specific ribosomal proteins. Although there appears to be a strong correlation between bactericidal activity and the ability to induce misreading (Hummel and Bck, 1989), it remains to be established that this is the primary mechanism of aminoglycoside-induced cell death.

Microbial Resistance to the Aminoglycosides

Bacteria may be resistant to the antimicrobial activity of the aminoglycosides because of failure of permeation of the antibiotic, low affinity of the drug for the bacterial ribosome, or inactivation of the drug by microbial enzymes. Drug inactivation is by far the most important explanation for the acquired microbial resistance to aminoglycosides that is encountered in clinical practice.

Penetration of drug through the pores in the outer membrane of gram-negative microorganisms into the periplasmic space may be retarded; resistance of this type is unimportant clinically. Once the aminoglycoside does reach the periplasmic space, it may be altered by microbial enzymes that phosphorylate, adenylate, or acetylate specific hydroxyl or amino groups (Figure 461). The genes for these enzymes are acquired primarily by conjugation and the transfer of DNA as plasmids and resistance transfer factors (Davies, 1994; seeChapter 43: Antimicrobial Agents: General Considerations). These plasmids have become widespread in nature (especially in hospital environments), and they code for a large number of enzymes (more than 20) that have markedly reduced the clinical usefulness of aminoglysides. Amikacin is less vulnerable to these inactivating enzymes because of protective molecular side chains (Figure 461); thus, this drug has a particularly important role in certain hospital settings. The metabolites of the aminoglycosides may compete with the unaltered drug for intracellular transport, but they are incapable of binding effectively to ribosomes and interfering with protein synthesis.

Acquisition of aminoglycoside-inactivating enzymes by enterococci has become a source of concern. In several centers, a significant percentage of clinical isolates of these organisms (both Enterococcus faecalis and Enterococcus faecium) are highly resistant to all aminoglycosides because of this mechanism (Spera and Farber, 1992; Vemuri and Zervos, 1993). Because different enzymes are responsible for inactivation of gentamicin and streptomycin, a smal proportion of gentamicin-resistant strains of enterococci will be susceptible to streptomycin. Resistance to gentamicin indicates resistance to tobramycin, amikacin, kanamycin, and netilmicin, because the inactivating enzyme is bifunctional and modifies all of these aminoglycosides (Murray, 1991). The synergistic bactericidal effect of penicillin or vancomycin and an aminoglycoside on enterococci is lost. Enterococci also have acquired plasmids that code for -lactamases (Murray and Mederski-Samaroj, 1983) and vancomycin resistance (Leclercq et al., 1988). These factors could make serious enterococcal infections extremely difficult to treat. Strains of E. faecium that are resistant to virtually all clinically important antibiotics have emerged as pathogens in intensive care units in hospitals across the United States.

Another common form of natural resistance to aminoglycosides is caused by failure of the drug to penetrate the cytoplasmic (inner) membrane. As mentioned above, the transport of aminoglycosides across the cytoplasmic membrane is an oxygen-dependent, active process. Strictly anaerobic bacteria are thus resistant to these drugs, since they lack the necessary transport system. Similarly, facultative bacteria become resistant when they are grown under anaerobic conditions (Mates et al., 1983). The significance of this transport defect for resistance to aminoglycosides among aerobic gram-negative bacilli is not known. Natural resistance to amikacin by Pseudomonas maltophilia and certain other microorganisms appears to have a similar basis, as does the low-level resistance of some gram-positive cocci to aminoglycosides.

Resistance that results from alterations in ribosomal structure is relatively uncommon for aminoglycosides other than streptomycin. Single-step mutations in Escherichia coli that result in the substitution of an amino acid in a crucial ribosomal protein may prevent binding of the drug. Although such strains of E. coli are highly resistant to streptomycin, they are not widespread in nature. Similarly, only 5% of strains of Pseudomonas aeruginosa exhibit such ribosomal resistance to streptomycin. It has been estimated that approximately half of the streptomycin-resistant strains are ribosomally resistant (Eliopoulos et al., 1984). There is no synergistic effect of penicillin and streptomycin against these strains demonstrable in vitro. Because ribosomal resistance often is specific for streptomycin, these strains of enterococci generally are sensitive to a combination of penicillin and gentamicinin vitro.

Antibacterial Activity of the Aminoglycosides

The antibacterial activity of gentamicin, tobramycin, kanamycin, netilmicin, and amikacin is primarily directed against aerobic, gram-negative bacilli. Kanamycin, like streptomycin, has a more limited spectrum compared with other aminoglycosides, and in particular it should not be used to treat infections caused by Serratia or P. aeruginosa. As noted above, these antibiotics have little activity against anaerobic microorganisms or facultative bacteria under anaerobic conditions. Their action against most gram-positive bacteria is limited. Streptococcus pneumoniae and Streptococcus pyogenes are resistant, and, in fact, gentamicin has been added to blood-agar plates to aid in the isolation of these microorganisms from sputum and pharyngeal secretions. Although not active when used alone, either streptomycin or gentamicin in combination with a cell wallactive agent, such as a penicillin or vancomycin, is active against 'sensitive' strains of enterococci and streptococci at concentrations that can be achieved clinically. Such combinations result in a more rapid bactericidal effect than is produced by either drug alone (i.e., they are synergistic). Both gentamicin and tobramycin are active in vitro against more than 90% of strains of Staphylococcus aureus and 75% of strains of Staphylococcus epidermidis. The clinical efficacy of aminoglycosides alone in the treatment of serious staphylococcal infections has not been documented, and they should not be used. Gentamicin-resistant mutant strains of staphylococci emerge rapidly during exposure to the drug. Moreover, staphylococcal resistance that is mediated by conjugative plasmids that code for aminoglycoside-modifying enzymes is common among methicillin-resistant strains of staphylococci.

The aerobic gram-negative bacilli vary in their susceptibility to the aminoglycosides as shown in Table 461. 'Sensitive' microorganisms are defined as those inhibited by concentrations that can be achieved clinically in plasma without a high incidence of toxicity; when given at 8- or 12-hour intervals, these therapeutic peak values range from 4 to 12 g/ml for gentamicin, tobramycin, and netilmicin and 20 to 35 g/ml for amikacin and kanamycin. Tobramycin and gentamicin exhibit similar activity against most gram-negative bacilli, although tobramycin usually is more active against P. aeruginosa and against some strains of Proteus species. Many gram-negative bacilli that are resistant to gentamicin because of plasmid-mediated inactivating enzymes also will inactivate tobramycin. Nosocomial flora have shown a gradual increase in resistance to gentamicin and tobramycin over the last 20 to 30 years. The relative frequency of these changes varies dramaticallyeven in different units within a single hospital (Cross et al., 1983). Fortunately, amikacin and, in some instances, netilmicin have retained their activity in this setting, probably due to resistance of the drugs to many of the aminoglycoside-inactivating enzymes. These agents thus have a broad spectrum of activity and are particularly valuable in treating nosocomial infections.

Absorption

The aminoglycosides are highly polar cations and therefore are very poorly absorbed from the gastrointestinal tract. Less than 1% of a dose is absorbed following either oral or rectal administration. The drugs are not inactivated in the intestine, and they are eliminated quantitatively in the feces. However, long-term oral or rectal administration may result in accumulation of aminoglycosides to toxic concentrations in patients with renal impairment. Absorption of gentamicin from the gastrointestinal tract may be increased by gastrointestinal disease (ulcers, inflammatory bowel disease; Breen et al., 1972). Instillation of these drugs into body cavities with serosal surfaces may result in rapid absorption and unexpected toxicity, i.e., neuromuscular blockade. Similarly, intoxication may occur when aminoglycosides are applied topically for long periods to large wounds, burns, or cutaneous ulcers, particularly if there is renal insufficiency.

All of the aminoglycosides are absorbed rapidly from intramuscular sites of injection. Peak concentrations in plasma occur after 30 to 90 minutes and are similar to those observed 30 minutes after completion of an intravenous infusion of an equal dose over a 30-minute period. In critically ill patients, especially those in shock, absorption of drug may be reduced from intramuscular sites because of poor perfusion.

Distribution

Because of their polar nature, the aminoglycosides largely are excluded from most cells, from the central nervous system, and from the eye. Except for streptomycin, there is negligible binding of aminoglycosides to plasma albumin. The apparent volume of distribution of these drugs is 25% of lean body weight and approximates the volume of extracellular fluid (Barza et al., 1975).

Concentrations of aminoglycosides in secretions and tissues are low. High concentrations are found only in the renal cortex and in the endolymph and perilymph of the inner ear; this may contribute to the nephrotoxicity and ototoxicity caused by these drugs. Concentrations in bile approach 30% of those found in plasma as a result of active hepatic secretion, but this represents a very minor excretory route for the aminoglycosides. Penetration into respiratory secretions is poor (Levy, 1986). Diffusion into pleural and synovial fluid is relatively slow, but concentrations that approximate those in the plasma may be achieved after repeated administration. Inflammation increases the penetration of aminoglycosides into peritoneal and pericardial cavities.

Concentrations of aminoglycosides in cerebrospinal fluid (CSF) that are achievable with parenteral administration of drug usually are subtherapeutic. In experimental animals and human beings, concentrations in CSF are less than 10% of those in plasma in the absence of inflammation; this value may approach 25% when there is meningitis (Strausbaugh et al., 1977). The concentrations achieved are therefore inadequate for the treatment of gram-negative bacillary meningitis in adults. Intrathecal or intraventricular administration of aminoglycosides has been used to achieve therapeutic levels, but the availability of third-generation cephalosporins has now made this unnecessary in most cases. There is no proven benefit of either intrathecal or intraventricular injection of aminoglycosides to neonates with meningitis, perhaps because of the immaturity of the blood-brain barrier (McCracken et al., 1980). Penetration of aminoglycosides into ocular fluids is so poor that effective therapy of bacterial endophthalmitis requires periocular and intraocular injections of the drugs (Barza, 1978).

Administration of aminoglycosides to women late in pregnancy may result in accumulation of drug in fetal plasma and amniotic fluid. Streptomycin can cause hearing loss in children born to women who receive the drug during pregnancy (Warkany, 1979), as can tobramycin. Insufficient data are available regarding the other aminoglycosides; it is thus recommended that they be used with caution in pregnancy and only for strong clinical indications in the absence of suitable alternatives.

Dosing

Recommended doses of individual aminoglycosides in the treatment of specific infections are given in later sections of this chapter. Traditionally, the total daily dose of aminoglycosides is administered as two or three equally divided doses. Administration of the total dose once daily, however, appears to be less toxic and just as effective (Verpooten et al., 1989; Gilbert, 1991; Prins et al., 1993; The International Antimicrobial Therapy Cooperative Group of the European Organization for Research and Treatment of Cancer, 1993; Charnas et al., 1997; Urban and Craig, 1997; Gilbert et al., 1998; Rybak et al., 1999). Toxicity results from accumulation of drug in the inner ear and kidney. The amount of drug that accumulates increases with higher plasma concentrations and longer periods of exposure. Elimination (or washout) of aminoglycoside from these organs occurs more slowly than from plasma and is retarded by high plasma concentrations (Tran Ba Huy et al., 1983), accounting for the association between toxicity and high plasma trough concentrations (Swan, 1997). Toxicity, then, can be considered as a threshold phenomenon, more likely to occur the longer the plasma concentration exceeds a relatively safe upper limit (e.g., above a recommended trough concentration) (Figure 463). A once-daily dosing regimen, despite the higher peak concentration, provides a longer period when concentrations are below the threshold for toxicity than does a multiple-dosing regimen (12 hours versus less than 3 hours total in the example shown in the figure), accounting for its lower toxicity. Aminoglycoside bactericidal activity, on the other hand, is directly related to the concentration achieved, because aminoglycosides have concentration-dependent killing and a concentration-dependent postantibiotic effect. This enhanced activity at higher concentrations probably accounts for the equivalent efficacy of a once-daily regimen compared to a multiple-dosing regimen despite the relatively prolonged periods of time that plasma concentrations are 'subtherapeutic,'i.e., below the minimum inhibitory concentration (MIC).

Numerous studies in a variety of clinical settings employing virtually every commonly used aminoglycoside have demonstrated that once-daily regimens are just as safe as or safer than multiple-dosing regimens and as efficacious (Barza et al., 1996; Deaney and Tate, 1996; Ferriols-Lisart and Alos-Alminana, 1996; Freeman and Strayer, 1996; Ali and Goetz, 1997; Bailey et al., 1997; Charnas et al., 1997; Freeman et al., 1997; Deamer, 1998). Once-daily dosing also costs less and is more easily administered. Administration of aminoglycosides as a single daily dose is for these reasons generally preferred with few exceptions. Exceptions are use in pregnancy, neonatal infections, and low-dose combination therapy of bacterial endocarditis, because data documenting equivalent safety and efficacy are inadequate. Once-daily dosing should also be avoided in patients with creatinine clearances less than 20 to 25 ml/min, because accumulation is likely to occur. Less frequent dosing (e.g., every 48 hours) is more appropriate for these patients.

Whether once-daily or multiple-daily dosing is chosen, the dose must be adjusted for patients with creatinine clearances below 80 to 100 ml/min (Table 462). If it is anticipated that the patient will be treated with an aminoglycoside for more than three to four days, then plasma concentrations should be monitored to avoid accumulation of the drug. In addition, an aminoglycoside in general should not be used as a single agent except for urinary tract infections because of relatively poor tissue penetration and poorer outcome compared to combination regimens or other classes of antibiotics (Bodey et al., 1985; Leibovici et al., 1997).

For twice-daily or three-times-daily dosing regimens, both trough and peak plasma concentrations are determined. The trough sample is obtained just prior to a dose, and the peak sample is obtained thirty minutes following intramuscular injection or thirty minutes after an intravenous infusion given over thirty minutes. The peak concentration is used to document that the dose produces therapeutic concentrations, generally accepted to be 4 to 10 g/ml for gentamicin, netilmicin, and tobramycin and 15 to 30 g/ml for amikacin and streptomycin (Gilbert et al., 1999). The trough concentration is used to avoid toxicity by monitoring for accumulation of drug. Trough concentrations should be less than 1 to 2 g/ml for gentamicin, netilmicin, and tobramycin and 5 to 10 g/ml for amikacin and streptomycin.

Monitoring of aminoglycoside plasma concentrations also is important when using a once-daily dosing regimen, although peak concentrations are not routinely determined (these will be three to four times higher than the peak achieved with a multiple-daily-dosing regimen). Several approaches may be used to determine that drug is being cleared and not accumulating. The simplest method is to obtain a trough sample 24 hours after dosing and adjust the dose to achieve the recommended plasma concentration, e.g., below 1 to 2 g/ml in the case of gentamicin or tobramycin. This approach is probably the least desirable. An undetectable trough concentration could reflect grossly inadequate dosing with prolonged periods (perhaps well over half of the dosing interval), during which concentrations are subtherapeutic in patients who rapidly clear the drug. In contrast, a 24-hour trough concentration target of 1 to 2 g/ml would actually increase aminoglycoside exposure compared to a multiple-daily-dosing regimen (Barclay et al., 1999). This defeats the goal of providing a washout with concentrations of 0 to 1 g/ml between 18 to 24 hours after a dose. A second approach relies on nomograms to target a range of concentration in a sample obtained earlier in the dosing interval. For example, if the plasma concentration from a sample obtained 8 hours after a dose of gentamicin is between 1.5 and 6 g/ml, then the concentration at 18 hours will be <1 g/ml (Chambers et al., 1998). A target range of 1 to 1.5 g/ml for gentamicin at 18 hours for patients with creatinine clearances above 50 ml/min and 1 to 2.5 g/ml for clearances below 50 ml/min also has been used (Gilbert et al., 1998). The most accurate method for monitoring plasma levels for dose adjustment is to measure the concentration in two plasma samples drawn several hours apart (e.g., at 2 and 12 hours after a dose). The clearance then can be calculated and the dose adjusted to achieve the desired target range.

Elimination

The aminoglycosides are excreted almost entirely by glomerular filtration, and concentrations in the urine of 50 to 200 g/ml are achieved. A large fraction of a parenterally administered dose is excreted unchanged during the first 24 hours, with most of this appearing in the first 12 hours. The half-lives of the aminoglycosides in plasma are similar and vary between 2 and 3 hours in patients with normal renal function. Renal clearance of aminoglycosides is approximately two-thirds of the simultaneous creatinine clearance; this observation suggests some tubular reabsorption of these drugs.

Following a single dose of an aminoglycoside, disappearance from the plasma exceeds renal excretion by 10% to 20%; however, after 1 to 2 days of therapy, nearly 100% of subsequent doses is eventually recovered in the urine. This lag period probably represents saturation of binding sites in tissues. The rate of elimination of drug from these sites is considerably longer than from plasma; the half-life for tissue-bound aminoglycoside has been estimated to range from 30 to 700 hours (Schentag and Jusko, 1977). For this reason, small amounts of aminoglycosides can be detected in the urine for 10 to 20 days after drug administration is discontinued. Aminoglycoside bound to renal tissue exhibits antibacterial activity and protects experimental animals against bacterial infections of the kidney, even when the drug no longer can be detected in serum (Bergeron et al., 1982).

The concentration of aminoglycoside in plasma produced by the initial dose is dependent only on the volume of distribution of the drug. Since the elimination of aminoglycosides is almost entirely dependent on the kidney, a linear relationship exists between the concentration of creatinine in plasma and the half-life of all aminoglycosides in patients with moderately compromised renal function. In anephric patients, the half-life varies from 20 to 40 times that determined in normal individuals. Because the incidence of nephrotoxicity and ototoxicity is related to the concentration to which an aminoglycoside accumulates, it is critical to reduce the maintenance dosage of these drugs in patients with impaired renal function. The size of the individual dose, the interval between doses, or both, can be altered. There is no conclusive information on the best approach, and even the currently accepted therapeutic range has been questioned (McCormack and Jewesson, 1992). The most consistent plasma concentrations are achieved when the loading dose is given in milligrams per kilogram of body weight; and since aminoglycosides are minimally distributed in fatty tissue, the lean or expected body weight should be used. Methods for calculation of dosage are described in Appendix II.

There are obvious difficulties in utilizing any of these approaches for ill patients with rapidly changing renal function (Lesar et al., 1982). In addition, even when known factors are taken into consideration, concentrations of aminoglycosides achieved in plasma after a given dose vary widely among patients (Barza et al., 1975). If the extracellular volume is expanded, the volume of distribution is increased and concentrations will be reduced. For unknown reasons, the clearances are increased, and the half-lives of the aminoglycosides are reduced in patients with cystic fibrosis; the volume of distribution is increased in patients with leukemia (Rosenthal et al., 1977; Spyker et al., 1978). Patients with anemia (hematocrit <25%) have a concentration in plasma that is higher than expected, probably because of a reduction in the number of binding sites on red blood cells (Siber et al., 1975).

Determination of the concentration of drug in plasma is an essential guide to the proper administration of aminoglycosides. In patients with life-threatening systemic infections, aminoglycoside concentrations should be determined several times per week (more frequently if renal function is changing) and always should be determined within 24 hours after a change in dosage.

Aminoglycosides are removed from the body by either hemodialysis or peritoneal dialysis. Approximately 50% of the administered dose is removed in 12 hours by hemodialysis, which has been used for the treatment of overdosage. As a general rule, a dose equal to half the loading dose administered after each hemodialysis should maintain the plasma concentration in the desired range; however, a number of variables make this a rough approximation at best. Continuous arteriovenous hemofiltration (CAVH) and continuous venovenous hemofiltration (CVVH) will result in aminoglycoside clearances approximately equivalent to 15 ml/min and 15 to 30 ml/min of creatinine clearance, respectively, depending on the flow rate. The amount of aminoglycoside removed can be replaced by administering approximately 15% to 30% of the maximum daily dose (Table 462) each day. Frequent monitoring of drug concentrations in plasma is again crucial.

Peritoneal dialysis is less effective than hemodialysis in removing aminoglycosides. Clearance rates are approximately 5 to 10 ml per minute for the various drugs, but are highly variable. If a patient who requires dialysis has bacterial peritonitis, a therapeutic concentration of the aminoglycoside probably will not be achieved in the peritoneal fluid, since the ratio of the concentration in plasma to that in peritoneal fluid may be 10 to 1 (Smithivas et al., 1971). It is thus recommended that antibiotic be added to the dialysate to achieve concentrations equal to those desired in plasma. For intermittent dosing via peritoneal dialysate, 2 mg/kg of amikacin is added to the bag once a day. The corresponding dose for gentamicin, netilmicin, or tobramycin is 0.6 mg/kg. For continuous dosing, the dose of amikacin is 12 mg per liter (25 mg/l loading dose in the first bag) and the dose of gentamicin, netilmicin, or tobramycin is 4 mg per liter in each bag (8 mg/l loading dose). This should be preceded by administration of a loading dose, either parenterally or in dialysis fluid.

Although excretion of aminoglycosides is similar in adults and children over 6 months of age, half-lives of the drugs may be significantly prolonged in the newborn. Newborn infants who weigh less than 2 kg have half-lives for aminoglycosides of 8 to 11 hours during the first week of life, while those who weigh over 2 kg eliminate these drugs with half-lives of about 5 hours (Yow, 1977). It is thus critically important to monitor concentrations of aminoglycosides during treatment of neonates (Philips et al., 1982).

Aminoglycosides can be inactivated by various penicillins in vitro (Konishi et al., 1983) and in patients with end-stage renal failure (Blair et al., 1982), thus making dosage recommendations even more difficult. Amikacin appears to be the least affected by this interaction.