Antimicrobial Agents: Antifungal Agents

Amphotericin B

History and Source

Amphotericin B was discovered in 1956 by Gold and coworkers, who were studying a strain of Streptomyces nodosus, an aerobic actinomycete, obtained from the Orinoco River Valley of Venezuela.

Chemistry

Amphotericin B is one of a family of some 200 polyene macrolide antibiotics. Those studied to date share the characteristics of four to seven conjugated double bonds, an internal cyclic ester, poor aqueous solubility, substantial toxicity on parenteral administration, and a common mechanism of antifungal action. Amphotericin B (see below for structure) is a heptaene macrolide containing seven conjugated double bonds in the trans position and 3-amino-3,6-dideoxymannose (mycosamine) connected to the main ring by a glycosidic bond. The amphoteric behavior for which the drug is named derives from the presence of a carboxyl group on the main ring and a primary amino group on mycosamine; these groups confer aqueous solubility at extremes of pH. X-ray crystallography has shown the molecule to be rigid and rod-shaped, with the hydrophilic hydroxyl groups of the macrolide ring forming an opposing face to the lipophilic polyenic portion.

Drug Formulations

Amphotericin B is insoluble in water but was formulated for intravenous infusion by complexing it with the bile salt deoxycholate. The complex is marketed as a lyophilized powder (FUNGIZONE) containing 50 mg of amphotericin B, 41 mg of deoxycholate, and a small amount of sodium phosphate buffer. The amphotericin Bdeoxycholate complex (DOC) forms a colloid in water, with particles largely below 0.4 m in diameter. Filters in intravenous infusion lines that filter out particles above 0.22 m in diameter will remove significant amounts of drug. Addition of electrolyte to infusion solutions causes the colloid to aggregate. The resulting cloudy solution should not be infused, because the blood levels achieved are likely to be very low.

N-acyl and O-acyl derivatives of amphotericin B form water-soluble salts, but none is available commercially. The amphipathic nature of amphotericin B has made it possible to create lipid formulations for intravenous infusion. Three such formulations of amphotericin B are marketed in the United States (Wong-Beringer et al., 1998). Amphotericin B colloidal dispersion (ABCD, AMPHOTEC AMPHOCIL) is a colloidal dispersion containing roughly equimolar amounts of amphotericin B and cholesteryl sulfate. Like DOC, ABCD forms a colloidal solution when dispersed in aqueous solution. ABCD particles are disk-shaped; they are 115 nm wide by 4 nm thick. ABCD provides much lower blood levels than DOC in mice and human beings. In mice, 41% to 80% of 14 daily doses can be recovered in the liver. In a randomized, double-blind study in patients with neutropenic fever comparing ABCD (4 mg/kg daily) with DOC (0.8 mg/kg), chills and hypoxia were significantly more common with ABCD (79.8% and 12%, respectively) than with DOC (65.4% and 2.9%), requiring withdrawal of 4.6% of the study patients receiving ABCD, compared with 0.9% of those receiving DOC (White et al., 1998). Hypoxia was associated with severe febrile reactions. Administration of the recommended ABCD dose of 3 to 4 mg/kg over 3 to 4 hours and use of premedication to reduce febrile reactions is advised, particularly with initial infusions. Unlike the situation with DOC, nephrotoxicity rarely leads to dose reductions of ABCD, even in bone marrow transplant recipients (Noskin et al., 1999). Efficacy has been difficult to evaluate in all the published studies to date. ABCD is approved only for patients with invasive aspergillosis who are not responding to or are unable to tolerate DOC. This approval is based on open, noncomparative trials.

A small unilamellar vesicle formulation of amphotericin B ( AMBISOME ) also is available. Amphotericin B (50 mg) is combined with 350 mg of lipid in an approximately 10% molar ratio. The lipid contains hydrogenated soy lecithin (phosphatidylcholine), cholesterol, and distearoylphosphatidylglycerol in a 10:5:4 molar ratio. The drug is supplied as a lyophilized powder, which is reconstituted with sterile water for injection and then the dose diluted with 5% dextrose solution. With complete dispersion, particle size is about 80 nm. Blood levels following intravenous infusion are almost equivalent to those obtained with DOC, and, because AMBISOME can be given at higher doses, blood levels have been achieved that exceed those obtained with DOC. Amphotericin B accumulation in the liver and spleen is higher with AMBISOME than with DOC (de Marie et al., 1994). In a series of 23 patients receiving 3 mg/kg daily for an average of 27 days, the average serum creatinine rise was only 34%, with nephrotoxicity leading to dose reduction in only one patient (Coker et al., 1993). Nephrotoxicity, hypokalemia, and infusion-related reactionssuch as fever, chills, hypoxia, hypotension, and hypertensionare less with AMBISOME than with either DOC or ABLC (see below), but they still occur (Walsh et al., 1999). Infusion-related pain in the back, abdomen, or chest occurs in occasional patients, usually with the first few doses (Johnson et al., 1998). Anaphylaxis has been reported. Most of the information about the efficacy of AMBISOME comes from open, noncomparative studies. Blinded, randomized comparisons of DOC and AMBISOME as empirical therapy or prophylaxis in febrile neutropenic patients are very helpful in comparing toxicity of the two drugs (Walsh et al., 1999; Prentice et al., 1997; Kelsey et al., 1999). By the primary endpoints of these studies, both preparations have been comparable. However, because the efficacy of DOC in these patients cannot be quantitated from historical data, the relative efficacy of AMBISOME is impossible to ascertain. Secondary analysis of mycoses emerging despite the use of AMBISOME or DOC in these trials has been limited by the rarity of this event and the tendency of investigators to reduce the DOC doses to subtherapeutic levels. AMBISOME is approved for empiric therapy of fever in the neutropenic host not responding to appropriate antibacterial agents as well as for salvage therapy of aspergillosis, cryptococcosis, and candidiasis. The recommended dose for empiric therapy is 3 mg/kg daily, and that for treatment of mycoses is 3 to 5 mg/kg intravenously. AMBISOME also is effective in visceral leishmaniasis at doses of 3 to 4 mg/kg daily. The drug is administered in 5% dextrose in water, with initial doses being infused over 2 hours. If well tolerated, infusion duration can be shortened to one hour. Although doses up to 10 mg/kg have been used in a few patients without causing death, 8 mg/kg is highly toxic in dogs (Bekersky et al., 1999).

The third lipid formulation is amphotericin B lipid complex (ABLC, ABELCET ). This preparation of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol in a 7:3 mixture with approximately 35 mol% amphotericin B forms ribbon-like sheets which range in size from 1.6 to 11 m. Blood levels of amphotericin B are much lower with ABLC than with the same dose of DOC. In unpublished studies, blood levels have been comparable when ABLC is given at five times the dose of DOC. In open, noncomparative trials, ABLC has seemed to be effective in a variety of mycoses with the possible exception of cryptococcal meningitis (Walsh et al., 1998; Wingard, 1997; Sharkey et al., 1996). Nephrotoxicity and infusion-related reactions of fever, chills, hypoxia, hypertension, and hypotension appear to be intermediate between DOC and AMBISOME . ABLC is given as 5 mg/kg in 5% dextrose in water, infused once daily over two hours. The drug is approved for salvage therapy of deep mycoses.

DOC has been mixed with a 20% lipid emulsion (INTRALIPID) and infused intravenously. Whether amphotericin B aggregates or is bound to lipid in the infusate is unknown. Nephrotoxicity appears to be less than with DOC at 1 mg/kg, but there is a suggestion that both blood levels and efficacy are also lower (Chavanet et al., 1992). Intravenous infusion of DOC with lipid emulsion is not recommended.

Rational use of the lipid formulations has been problematic for hospital pharmacies and physicians. Cost of the formulations is 20 to 50 times that of DOC, raising formidable cost-benefit issues. The person making the choice must weigh the uncertainties about relative efficacy of the lipid formulations against the gravity of potential nephrotoxicity of the formulation in individual patients. In some patients, the additive burden of amphotericin B nephrotoxicity can help precipitate advanced renal failure, with attendant morbidity and financial burden (Wingard et al., 1999).

Antifungal Activity

Amphotericin B has useful clinical activity against Candida spp., Cryptococcus neoformans, Blastomyces dermatitidis, Histoplasma capsulatum, Sporothrix schenckii, Coccidioides immitis, Paracoccidioides braziliensis, Aspergillus spp., Penicillium marneffei, and the agents of mucormycosis. Some isolates of Candida lusitaniae have appeared to be relatively resistant to amphotericin B.

Amphotericin B has limited activity against the protozoa Leishmania braziliensis and Naegleria fowleri. The drug has no antibacterial activity.

Mechanism of Action

The antifungal activity of amphotericin B depends at least in part on its binding to a sterol moiety, primarily ergosterol, that is present in the membrane of sensitive fungi. By virtue of their interaction with the sterols of cell membranes, polyenes appear to form pores or channels. The result is an increase in the permeability of the membrane, allowing leakage of a variety of small molecules. Additional mechanisms of action may include oxidative damage to fungal cells, at least in vitro.

Fungal Resistance

Mutants selected in vitro for nystatin or amphotericin B resistance replace ergosterol with certain precursor sterols. The rarity of significant amphotericin B resistance arising during therapy has left it unclear whether or not ergosterol-deficient mutants retain sufficient pathogenicity to survive in deep tissue. Failure of amphotericin B to penetrate the fungal cell wall of some resistant species has been suggested by the greater susceptibility of protoplasts.

Absorption, Distribution, and Excretion

Absorption of all amphotericin B formulations from the gastrointestinal tract is negligible. Repeated daily intravenous infusions to adults of 0.5 mg/kg of DOC result in concentrations in plasma of about 1.0 to 1.5 g/ml at the end of the infusion, which fall to about 0.5 to 1.0 g/ml by 24 hours later. The drug is released from its complex with deoxycholate in the bloodstream, and the amphotericin B that remains in plasma is more than 90% bound to proteins, largely -lipoprotein. Approximately 2% to 5% of each dose appears in the urine when patients are on daily therapy. Elimination of the drug appears to be unchanged in anephric patients and in patients receiving hemodialysis. Hepatic or biliary disease has no known effect on metabolism of the drug in human beings. At least one-third of an injected dose can be recovered unchanged by methanolic extraction of tissue at autopsy; the highest concentrations are found in liver and spleen, with lesser amounts in kidney and lung. Concentrations of amphotericin B (DOC) in fluids from inflamed pleura, peritoneum, synovium, and aqueous humor are approximately two-thirds of trough concentrations in plasma. Little amphotericin B penetrates into cerebrospinal fluid (CSF), vitreous humor, or normal amniotic fluid. Because of extensive binding to tissues, there is a terminal phase of elimination with a half-time of about 15 days.

Therapeutic Uses

The usual therapeutic dose of amphotericin B DOC is 0.5 to 0.6 mg/kg, administered in 5% glucose over 4 hours. Candida esophagitis in adults responds to 0.15 to 0.2 mg/kg daily. Rapidly progressive mucormycosis or invasive aspergillosis is treated with doses of 1.0 to 1.2 mg/kg daily until progression is arrested. Double-dose alternate day therapy may be more convenient but is not less toxic and is therefore rarely indicated. The infusion bottle need not be protected from light, as once recommended. Infusion intervals as short as one hour have been used, but, during the first 5 to 7 days of therapy, they lead to more febrile reactions.

Intrathecal infusion of amphotericin B DOC is useful in patients with meningitis caused by Coccidioides. The drug can be injected into the CSF of the lumbar spine, cisterna magna, or lateral cerebral ventricle. Regardless of the site of injection, the treatment is begun with 0.05 to 0.1 mg and increased on a three-times-a-week schedule to 0.5 mg, as tolerance permits. Therapy is then continued on a twice-a-week schedule. Fever and headache are common reactions and may be decreased by intrathecal administration of 10 to 15 mg of hydrocortisone. Less common but more serious problems attend the use of intrathecal injections; the nature of the problem depends on the injection site chosen. Local injections of amphotericin B into a joint or peritoneal dialysate fluid commonly produce irritation and pain. Intraocular injection following pars plana vitrectomy has been used successfully for fungal endophthalmitis.

Intravenous administration of amphotericin B is the treatment of choice for mucormycosis, invasive aspergillosis, extracutaneous sporotrichosis, cryptococcosis, fusariosis, alternariosis, trichosporonosis, and penicilliosis marneffei. Although imidazoles or triazoles are useful in many patients with blastomycosis, histoplasmosis, coccidioidomycosis, and paracoccidioidomycosis, amphotericin B is preferred when these mycoses are rapidly progressive, occur in an immunosuppressed host, or involve the central nervous system. Amphotericin B (DOC or AMBISOME ) also can be useful in selected patients with profound neutropenia and fever that is unresponsive to broad-spectrum antibacterial agents. Amphotericin B given once weekly has been used to prevent relapse in patients with AIDS who have been treated successfully for cryptococcosis or histoplasmosis.

Bladder irrigation with 50 g/ml of amphotericin B in sterile water is effective for Candida cystitis. Relapse is common if the catheter remains in the bladder or there is significant postvoiding residual urine. Inhalational administration of amphotericin B has not been successful in treatment of pulmonary mycoses, nor has intranasal instillation proved useful in preventing aspergillosis in neutropenic patients. Topical amphotericin B is useful only in cutaneous candidiasis (see below).

Untoward Effects

The major acute reaction to intravenous amphotericin B DOC is fever and chills. Sometimes hyperpnea and respiratory stridor or modest hypotension may occur, but true bronchospasm or anaphylaxis is rare. Patients with preexisting cardiac or pulmonary disease may tolerate the metabolic demands of the reaction poorly and develop hypoxia or hypotension. A test dose of 1 mg may be considered for unstable patients; the patient should be observed for two hours prior to infusing the usual therapeutic dose. Although the reaction ends spontaneously in 30 to 45 minutes, meperidine may shorten it. Pretreatment with oral acetaminophen or use of intravenous hydrocortisone hemisuccinate, 0.7 mg/kg, at the start of the infusion decreases reactions. Febrile reactions abate with subsequent infusions. Infants, children, and patients receiving therapeutic doses of corticosteroids are less prone to reactions.

Azotemia occurs in 80% of patients who receive amphotericin B for deep mycoses (Carlson and Condon, 1994). Toxicity is dose-dependent and transient and is increased by concurrent therapy with other nephrotoxic agents, such as aminoglycosides or cyclosporine. Although permanent histological damage to renal tubules occurs even during short courses, permanent functional deficits are uncommon in patients whose renal function was normal prior to treatment unless a total dose in excess of 3 to 4 g is given (to an adult). Renal tubular acidosis and renal wasting of K⁺ and Mg²⁺ also may be seen during and for several weeks after therapy. Supplemental K⁺ is required in one-third of patients on prolonged therapy. An increase in intrarenal vascular resistance is the major cause of nephrotoxicity in amphotericin Btreated rats (Tolins and Raij, 1988). In patients and experimental animals, loading with sodium chloride has decreased nephrotoxicity, even in the absence of water or salt deprivation. Administration of 1 liter of saline intravenously on the day that amphotericin B is to be given has been recommended for adults who are able to tolerate the Na⁺ load and who are not already receiving that amount in intravenous fluids (Branch, 1988).

Hypochromic, normocytic anemia is usual; the average hematocrit declined to 27% in one study. Decreased production of erythropoietin is the probable mechanism. Patients with low plasma erythropoietin may respond to administration of recombinant erythropoietin. Anemia reverses slowly following therapy. Headache, nausea, vomiting, malaise, weight loss, and phlebitis at peripheral infusion sites are common side effects. Encephalopathy also has been attributed to amphotericin B (Balmaceda and Walker, 1994). Thrombocytopenia or mild leukopenia is observed rarely. Hepatotoxicity is not firmly established.

Flucytosine

Chemistry

Flucytosine is a fluorinated pyrimidine related to fluorouracil and floxuridine. It is 5-fluorocytosine, the formula of which is as follows:

Antifungal Activity

Flucytosine has clinically useful activity against Cryptococcus neoformans, Candida spp., and the agents of chromomycosis. Within these species, determination of susceptibility in vitro has been extremely dependent on the method employed, and susceptibility testing performed on isolates obtained prior to treatment has not correlated with clinical outcome.

Mechanism of Action

All susceptible fungi are capable of deaminating flucytosine to 5-fluorouracil, a potent antimetabolite (seeFigure 491). Fluorouracil is metabolized first to 5-fluorouridylic acid by the enzyme uridine monophosphate (UMP) pyrophosphorylase. It can then either be incorporated into RNA (via synthesis of 5-fluorouridine triphosphate) or be metabolized to 5-fluorodeoxyuridylic acid, a potent inhibitor of thymidylate synthetase. DNA synthesis is impaired as the ultimate result of this latter reaction. Mammalian cells do not convert flucytosine to fluorouracil. This fact is crucial for the selective action of this compound.

Fungal Resistance

Drug resistance that arises during therapy (secondary resistance) is an important cause of therapeutic failure when flucytosine is used alone for cryptococcosis and candidiasis. In chromomycosis, resurgence of lesions after an initial response has led to the presumption of secondary drug resistance. In isolates of Cryptococcus and Candida species, secondary drug resistance has been accompanied by a change in the minimal inhibitory concentration from less than 2.5 g/ml to more than 360 g/ml. The mechanism for this resistance can be loss of the permease necessary for cytosine transport or decreased activity of either UMP pyrophosphorylase or cytosine deaminase (seeFigure 491). In Candida albicans, a diploid fungus, partial resistance can occur because of heterozygous deficiency of UMP pyrophosphorylase. The clinical significance of partial resistance is unknown.

Absorption, Distribution, and Excretion

Flucytosine is absorbed rapidly and well from the gastrointestinal tract. It is widely distributed in the body, with a volume of distribution that approximates total body water. The drug is minimally bound to plasma proteins. The peak plasma concentration in patients with normal renal function is approximately 70 to 80 g/ml, achieved 1 to 2 hours after a dose of 37.5 mg/kg. Approximately 80% of a given dose is excreted unchanged in the urine; concentrations in the urine range from 200 to 500 g/ml. The half-life of the drug is 3 to 6 hours in normal individuals. In renal failure, the half-life may be as long as 200 hours. The clearance of flucytosine is approximately equivalent to that of creatinine. Because of the obligate renal excretion of the drug, modification of dosage is necessary in patients with decreased renal function. It is recommended that concentrations of drug in plasma be measured periodically in patients with renal insufficiency. Peak concentrations should range between 50 and 100 g/ml. Flucytosine is cleared by hemodialysis, and patients undergoing such treatment should receive a single dose of 37.5 mg/kg after dialysis; the drug also is removed by peritoneal dialysis.

Flucytosine is present in CSF at a concentration about 65% to 90% of that simultaneously present in the plasma. The drug also appears to penetrate into the aqueous humor.

Therapeutic Uses

Flucytosine ( ANCOBON ) is given orally in amounts of 100 to 150 mg/kg per day, divided into 4 doses at 6-hour intervals. Dosage must be adjusted for decreased renal function. Flucytosine is used predominantly in combination with amphotericin B. Flucytosine had a possible, though not statistically significant, benefit and no added toxicity when added to 0.7 mg/kg of amphoterin B for the initial two weeks of therapy of cryptococcal meningitis in AIDS patients (van der Horst et al., 1997). An all-oral regimen of flucytosine plus fluconazole also has been advocated for therapy of AIDS patients with cyrptococcosis, but there is substantial gastrointestinal toxicity with the combination and no evidence that flucytosine adds benefit to the regimen. In cryptococcal meningitis of non-AIDS patients, in whom the goal is cure and not suppression, the role of flucytosine is more conjectural. Addition of flucytosine to six weeks or more of therapy with amphotericin B runs the risk of substantial bone marrow suppression or colitis if the flucytosine dose is not promptly adjusted downward as amphotericin Binduced azotemia occurs. More rapid culture conversion has been shown in cryptococcal meningitis of non-AIDS patients when flucytosine is added to an amphotericin B dose of 0.3 mg/kg daily but not when added to the more commonly used dosage of 0.7 mg/kg. Use of flucytosine in deep candidiasis has all but disappeared because of toxicity, absence of an intravenous formulation, and availability of other agents.

Untoward Effects

Flucytosine may depress the function of bone marrow and lead to the development of leukopenia and thrombocytopenia; patients are more prone to this complication if they have an underlying hematological disorder, are being treated with radiation or drugs that injure the bone marrow, or have a history of treatment with such agents. Other untoward effectsincluding rash, nausea, vomiting, diarrhea, and severe enterocolitishave been noted. In approximately 5% of patients, plasma levels of hepatic enzymes are elevated, but this effect reverses when therapy is stopped. Toxicity is more frequent in patients with AIDS or azotemia (including those who are receiving amphotericin B concurrently) and when concentrations of the drug in plasma exceed 100 g/ml (Stamm et al., 1987). Toxicity may be the result of conversion of flucytosine to 5-fluorouracil by the microbial flora in the intestinal tract of the host.

Imidazoles and Triazoles

The azole antifungals include two broad classes, imidazoles and triazoles. Both classes share the same antifungal spectrum and mechanism of action. The systemic triazoles are more slowly metabolized and have less effect on human sterol synthesis than do the imidazoles. Because of these advantages, new congeners under development are mostly triazoles, not imidazoles. Of the drugs now on the market in the United States, clotrimazole, miconazole, ketoconazole, econazole, butoconazole, oxiconazole, and sulconazole are imidazoles; terconazole, itraconazole, fluconazole, and new azoles in clinical trials are triazoles. The topical use of azole antifungals is described in the second section of this chapter. The structure of triazole is as follows:

Antifungal Activity

Susceptibility testing with azole antifungals has not been useful in predicting which fungal species will respond to therapy. Although individual drugs have their own useful spectrum, azoles as a group have clinically useful activity against C. albicans, Candida tropicalis, Candida glabrata, C. neoformans, B. dermatitidis, H. capsulatum, C. immitis, Paracoccidioides brasiliensis, and ringworm fungi (dermatophytes). Aspergillus spp. and S. schenckii are intermediate in susceptibility. Candida krusei and the agents of mucormycosis appear to be resistant. These drugs do not appear to have any useful antibacterial or antiparasitic activity, with the possible exception of antiprotozoal effects against Leishmania major.

Mechanism of Action

At concentrations achieved during systemic use, the major effect of imidazoles and triazoles on fungi is inhibition of sterol 14--demethylase, a microsomal cytochrome P450dependent enzyme system. Imidazoles and triazoles thus impair the biosynthesis of ergosterol for the cytoplasmic membrane and lead to the accumulation of 14--methylsterols. These methylsterols may disrupt the close packing of acyl chains of phospholipids, impairing the functions of certain membrane-bound enzyme systems such as ATPase and enzymes of the electron transport system and thus inhibiting growth of the fungi.

Some azoles, such as clotrimazole, directly increase permeability of the fungal cytoplasmic membrane, but the concentrations required are likely only obtained with topical use.

Azole resistance has emerged gradually during prolonged azole therapy and has caused clinical failure in patients with far-advanced HIV infection and oropharyngeal or esophageal candidiasis. The primary mechanism of resistance in C. albicans is accumulation of mutations in ERG11, the gene coding for the C14--sterol demethylase (Marichal et al., 1999). These mutations appear to protect heme in the enzyme pocket from binding to azole but allow access of the natural substrate for the enzyme, lanosterol. Cross resistance is conferred to all azoles. Increased azole efflux by both ATP-binding cassette (ABC) and major facilitator superfamily (MFS) transporters can add to fluconazole resistance in C. albicans and C. glabrata. Increased production of C14--sterol demethylase is another potential cause of resistance.

Ketoconazole

Ketoconazole, administered orally, has been replaced by itraconazole for the treatment of all mycoses except when the lower cost of ketoconazole outweighs the advantage of itraconazole. Itraconazole lacks ketoconazole's hepatotoxicity and corticosteroid suppression, while retaining most of ketoconazole's pharmacological properties and expanding the antifungal spectrum. Introduction of even newer triazoles likely will reduce further the usefulness of ketoconazole. The structural formula of ketoconazole is as follows:

Absorption, Distribution, and Excretion

Oral absorption of ketoconazole varies among individuals. Since an acidic environment is required for the dissolution of ketoconazole, bioavailability is markedly depressed in patients taking H₂-histamine receptor blocking agents such as cimetidine or proton pump inhibitors. Simultaneous administration of antacids also may impair absorption, as will didanosine (an anti-HIV agent) products, which contain a buffer to neutralize gastric acid and increase the absorption of didanosine. Ingestion of food has no significant effect on the maximal concentration of the drug achieved in plasma. After oral doses of 200, 400, and 800 mg, peak plasma concentrations of ketoconazole are approximately 4, 8, and 20 g/ml. The half-life of the drug increases with dose, and it may be as long as 7 to 8 hours when the dose is 800 mg. Ketoconazole is metabolized extensively, and the inactive products appear in the feces. Concentrations of active drug in urine are very low. In blood, 84% of ketoconazole is bound to plasma proteins, largely albumin; 15% is bound to erythrocytes; and 1% is free. Metabolism of the drug is unchanged by azotemia, hemodialysis, or peritoneal dialysis. Moderate hepatic dysfunction has no effect on the concentration of ketoconazole in blood.

Ketoconazole reaches keratinocytes efficiently, and its concentration in vaginal fluid approaches that in plasma. The concentration of ketoconazole in the CSF of patients with fungal meningitis is less than 1% of the total drug concentration in plasma.

Induction of hepatic microsomal enzymes by rifampin, isoniazid, and possibly by phenytoin accelerates the metabolic clearance of ketoconazole, and concentrations of the antifungal agent may be reduced by more than 50%. Ketoconazole raises plasma concentrations of cyclosporine, midazolam, triazolam, indinavir, and phenytoin because the drugs are metabolized by the cytochrome P450 enzyme CYP3A4. The anticoagulant effect of warfarin also may be enhanced.

Therapeutic Uses

Ketoconazole ( NIZORAL ) is effective in blastomycosis, histoplasmosis, coccidioidomycosis, pseudallescheriasis, paracoccidioidomycosis, ringworm, tinea versicolor, chronic mucocutaneous candidiasis, Candida vulvovaginitis, and oral and esophageal candidiasis. Efficacy is poor in immunosuppressed patients and in meningitis. The usual adult dose is 400 mg taken once daily. Children are given 3.3 to 6.6 mg/kg daily. Duration of therapy is 5 days for Candida vulvovaginitis, 2 weeks for Candida esophagitis, and 6 to 12 months for deep mycoses. The slow response to therapy has made ketoconazole inappropriate for patients with severe or rapidly progressive mycoses. For all of the above indications, itraconazole has replaced ketoconazole for patients who can afford the more expensive, newer product.

Untoward Effects

The most common side effects of ketoconazole are dose-dependent nausea, anorexia, and vomiting, which occur in about 20% of patients receiving 400 mg daily. Administration of the drug with food, at bedtime, or in divided doses may improve tolerance. An allergic rash occurs in about 4% of ketoconazole-treated patients and pruritus without rash in about 2%. Hair loss has also been reported.

Ketoconazole inhibits steroid biosynthesis in patients, as it does in fungi, by inhibition of cytochrome P450dependent enzyme systems. Several endocrinologic abnormalities thus may be evident. Approximately 10% of females report menstrual irregularities. A variable number of males experience gynecomastia and decreased libido and potency. At high doses, azoospermia has been reported, but sterility has not been permanent. Doses of ketoconazole as low as 400 mg can cause a transient drop in the plasma concentrations of free testosterone and estradiol C-17. Similar doses also can cause a transient decrease in the ACTH-stimulated plasma cortisol response and can suppress androgen production in women with polycystic ovary syndrome. Daily doses of 800 to 1200 mg of ketoconazole have been used to suppress plasma cortisol in patients with Cushing's disease. Similar doses were evaluated in patients with prostatic carcinoma. Hypertension and fluid retention have been reported and are associated with elevated concentrations of deoxycorticosterone, corticosterone, and 11-deoxycortisol. Although reports of Addison's disease due to ketoconazole are not convincing, it would seem prudent to discontinue the drug before major surgical procedures and to avoid using high doses in patients with trauma, severe burns, or other stressful conditions.

Mild, asymptomatic elevation of aminotransferase activity in plasma is common, occurring in 5% to 10% of patients; these values revert to normal spontaneously. Symptomatic drug-induced hepatitis is rare but is potentially fatal. Hepatitis may occur after a few days of treatment, or it may be delayed for many months. The earliest symptoms are anorexia, malaise, nausea, and vomiting, with or without dull abdominal pain. Liver function tests usually mimic the pattern seen with hepatitis A, but a cholestatic or mixed picture can occur. Patients should be alerted to the symptoms and asked to return for liver function tests should this toxicity be suspected. Ketoconazole is teratogenic in animals, causing syndactyly in rats. Its use during pregnancy is not recommended, and because of secretion of the drug into breast milk, its use in nursing mothers also is unwise.

Itraconazole

This synthetic triazole is a 1:1:1:1 racemic mixture of four diastereoisomers (two enantiomeric pairs), each possessing three chiral centers. The structural formula is closely related to the imidazole, ketoconazole, as shown below:

Absorption, Distribution, and Excretion

Itraconazole SPORONOX) is available as a capsule and two solution formulations, one for oral and one for intravenous administration. The capsule form of the drug is best absorbed in the fed state, but the oral solution is better absorbed in the fasting state and provides, under that condition, peak plasma concentrations that are more than 150% of those obtained with the capsule. Both the oral solution and intravenous formulation are solubilized in a 40:1 weight ratio of hydroxypropyl--cyclodextrin, so that administration of 200 mg of itraconazole provides 8 g of this excipient. Itraconazole is metabolized in the liver, primarily by the cytochrome CYP3A4 isoenzyme system, and it inhibits the metabolism of other drugs by CYP3A4. Itraconazole is present in plasma with an approximately equal concentration of a biologically active metabolite, hydroxyitraconazole. Bioassays may report up to approximately 3.3 times as much itraconazole in plasma as do physical methods such as HPLC, depending on the susceptibility of the bioassay organism to hydroxyitraconazole. Some laboratories using HPLC will also report the levels of hydroxyitraconazole as well as native drug, so the two may be added together for total antifungal drug concentration. The native drug and metabolite are 99.8% and 99.5% bound to plasma proteins, respectively. Neither appears in urine or CSF. The half-life of itraconazole in the steady state is approximately 30 to 40 hours. Steady-state levels of itraconazole are not reached for 4 days and those of hydroxyitraconazole for 7 days, for which reason loading doses are recommended when treating deep mycoses. Far advanced liver disease will increase itraconazole plasma concentrations, but azotemia and hemodialysis have no effect. Some 80% to 90% of intravenously administered hydroxypropyl--cyclodextrin is excreted in the urine. Intravenously administered hydroxypropyl--cyclodextrin accumulates is the presence of azotemia. Intravenous administration of itraconazole is contraindicated in patients with a creatinine clearance below 30 ml/min because of concern about hydroxypropyl--cyclodextrin concentrations. Itraconazole is not carcinogenic but is teratogenic in rats, classified as pregnancy category C, and is contraindicated during pregnancy. Hydroxypropyl--cyclodextrin causes pancreatic adenocarcinoma in rats but not mice. Relevance of this finding to clinical use of the oral solution is unclear.

Drug Interactions

Table 492 lists the known interactions of itraconazole with other drugs, but this list is still expanding. Many of the interactions can cause serious toxicity of the companion drug, as potentially fatal cardiac arrhythmias with cisapride, quinidine, or astemizole. Other interactions may decrease the itraconazole concentrations below therapeutic levels.

Therapeutic Uses

Itraconazole given as a capsule is the drug of choice for patients with indolent, nonmeningeal infections due to B. dermatitidis, H. capsulatum, P. brasiliensis, and C. immitis. This dosage form also is useful in therapy of indolent invasive aspergillosis outside the central nervous system, particularly after the infection has been stabilized with amphotericin B. The intravenous formulation is approved for the initial two weeks of therapy with blastomycosis, histoplasmosis, and indolent aspergillosis. The intravenous route would be most appropriate for patients unable to tolerate the oral formulation or unable to absorb it because of decreased gastric acid. Approximately half the patients with distal subungual onychomycosis respond well to itraconazole (Evans and Sigurgeirsson, 1999). Although not an approved use, itraconazole is often the best choice for treatment of pseudallescheriasis, an infection not responding to amphotericin B therapy, as well as cutaneous and extracutaneous sporotrichosis, tinea corporis, and extensive tinea versicolor. HIV-infected patients with disseminated histoplasmosis or P. marneffei infections have a decreased incidence of relapse if given prolonged itraconazole 'maintenance' therapy (Wheat et al., 1993; Supparatpinyo et al., 1998). It is as yet unclear whether patients responding to highly active antiretroviral therapy (HAART) will require less than lifelong therapy (seeChapter 51: Antiretroviral Agents: Antiretroviral Agents). Itraconazole is not recommended for maintenance therapy of cryptococcal meningitis in HIV-infected patients because of a high incidence of relapse. However, itraconazole prophylaxis in patients with advanced HIV infection yielded a decreased incidence of cryptococcosis as well as histoplasmosis. The incidence of mucosal candidiasis was not reduced (McKinsey et al., 1999). This study was done in the pre-HAART era, and itraconazole prophylaxis is not recommended. Long-term therapy has been used in non-HIV-infected patients with allergic bronchopulmonary aspergillosis to decrease the dose of corticosteroids and reduce attacks of acute bronchospasm (Salez et al., 1999).

Itraconazole solution is effective and approved for use in oropharyngeal and esophageal candidiasis. Because the preparation has more gastrointestinal side effects than do fluconazole tablets, itraconazole solution usually is reserved for patients not responding to fluconazole (Saag et al., 1999). Unfortunately, these patients are often receiving protease inhibitors and other drugs that make itraconazole contraindicated. Itraconazole capsules and oral solution are not bioequivalent and should not be used interchangeably.

Dosage

In treating deep mycoses, two 100-mg capsules are given twice daily with food. Divided doses are said to increase the area under the curve (AUC) (seeChapter 1: Pharmacokinetics: The Dynamics of Drug Absorption, Distribution, and Elimination) compared to once-daily dosing, even though the half-life is about 30 hours. For the first three days, 200 mg three times daily is used as a loading dose. For maintenance therapy of HIV-infected patients with disseminated histoplasmosis, 200 mg once daily is used. Onychomycosis can be treated with either 200 mg once daily for 12 weeks or as 200 mg twice daily for one week out of each month, so-called pulse therapy (Evans and Sigurgeirsson, 1999). Retention of active drug in the nail keratin permits intermittent treatment. Daily therapy is preferred by some authorities for infections likely to be more refractory but is twice the cost of pulse therapy. Once-daily terbinafine (250 mg) is slightly superior to pulse therapy with itraconazole (see below). Treatment is usually continued for three months. Intravenous itraconazole is reserved for seriously ill patients; it is given as an infusion over one hour of 200 mg twice daily for two days followed by 200 mg once daily for 12 days. Safety and efficacy of the intravenous regimen beyond 14 days is currently unknown. Itraconazole oral solution should be taken fasting in a dose of 100 mg in 10 ml once daily and swished vigorously in the mouth before swallowing to optimize any topical effect. Patients with fluconazole-resistant oropharyngeal or esophageal thrush are given 100 mg twice a day for 2 to 4 weeks.

Untoward Effects

Adverse effects of itraconazole therapy can occur as a result of interactions with many other drugs (see below). Itraconazole capsules, in the absence of interacting drugs, are well tolerated at 200 mg daily. Gastrointestinal distress occasionally prevents use of 400 mg per day. In a series of 189 patients receiving 50 to 400 mg per day, nausea and vomiting were recorded in 10%, hypertriglyceridemia in 9%, hypokalemia in 6%, increased serum aminotransferase in 5%, rash in 2%, and at least one side effect in 39%. Occasionally, hepatotoxicity or rash leads to drug discontinuation, but most adverse effects can be handled with dose reduction. Profound hypokalemia has been seen in patients receiving 600 mg or more daily (Sharkey et al., 1991) and in those who recently have received prolonged amphotericin B therapy. Doses of 300 mg twice daily have led to other side effects, including adrenal insufficiency, lower limb edema, hypertension, and, in one patient, rhabdomyolysis (Sharkey et al., 1991). Doses above 400 mg per day are not recommended for long-term use.

Intravenous itraconazole has been well tolerated except for chemical phlebitis. A dedicated catheter port is required. If other medications are administered through the same port at a later time, the catheter should be thoroughly flushed with normal saline. Although the volume of infusion is small, 25 ml of itraconazole in excipient plus 50 ml of saline, infusion durations less than one hour are not recommended. Toxicity of high plasma levels of hydroxypropyl--cyclodextrin in azotemic patients is unknown, but until this issue is clarified, the intravenous formulation is contraindicated in patients with a creatinine clearance below 30 ml/min.

The oral solution of itraconazole is well tolerated but has all the adverse effects of itraconazole capsules. Anaphylaxis has been observed rarely, as well as severe rash, including StevensJohnson syndrome. Some patients complain of the taste, and gastrointestinal side effects are common, though compliance is generally unimpaired. Diarrhea, abdominal cramps, anorexia, and nausea are more common than with the capsules.

Fluconazole

Fluconazole is a fluorinated bistriazole. Its structure is as follows:

Absorption, Distribution, and Excretion

Fluconazole is almost completely absorbed from the gastrointestinal tract. Concentrations in plasma are essentially the same whether the drug is given orally or intravenously, and bioavailability is not altered by food or gastric acidity. Peak plasma concentrations are 4 to 8 g/ml after repetitive doses of 100 mg. Renal excretion accounts for over 90% of elimination, and the elimination half-time is 25 to 30 hours. Fluconazole diffuses readily into body fluids, including sputum and saliva; concentrations in CSF are 50% to 90% of the simultaneous values in plasma. The dosage interval should be increased from 24 to 48 hours with a creatinine clearance of 21 to 40 ml/min and to 72 hours at 10 to 20 ml/min. A dose of 100 to 200 mg should be given after each hemodialysis. About 11% to 12% of plasma drug is protein bound.

Interactions

Fluconazole significantly increases plasma concentrations of astemizole, cisapride, cyclosporine, rifampin, rifabutin, sulfonylureas (glipizide, tolbutamide, others), theophylline, tacrolimus, and warfarin. Patients who receive more than 400 mg daily or azotemic patients who have elevated fluconazole blood levels may experience drug interactions not otherwise seen. Rifampin decreases the fluconazole AUC by about 25%, an amount that ordinarily would not be significant. Drugs that decrease gastric acidity do not significantly lower fluconazole blood levels.

Therapeutic Uses

Candidiasis

Fluconazole, 200 mg on the first day and then 100 mg daily for at least 2 weeks, is effective in oropharyngeal candidiasis. Esophageal candidiasis responds to 100 to 200 mg daily. This dose also has been used to decrease candiduria in high-risk patients (Sobel et al., 2000). A single dose of 150 mg is effective in vaginal candidiasis. A dose of 400 mg daily decreases the incidence of deep candidiasis in allogeneic bone marrow transplant recipients and is useful in treating candidemia of nonimmunosuppressed patients. Fluconazole is not known to be effective treatment for deep candidiasis in profoundly neutropenic patients. Based on resistance in vitro, Candida krusei would not be expected to respond to fluconazole or other azoles.

Cryptococcosis

Fluconazole, 400 mg daily, is used for the initial 8 weeks in cryptococcal meningitis of AIDS after the patient's clinical condition has been stabilized with intravenous amphotericin B. After the 8 weeks, the dose is decreased to 200 mg daily for life. It is still unclear whether or not patients with a sustained response to HAART can stop lifelong maintenance therapy. For AIDS patients with cryptococcal meningitis who are alert and oriented and have other favorable prognostic signs, initial therapy with 400 mg daily may be considered. The role of fluconazole for non-AIDS patients with cryptococcosis is not defined.

Other Mycoses

Fluconazole has become the drug of choice for treatment of coccidioidal meningitis because of much less morbidity than with intrathecal amphotericin B. In other forms of coccidioidomycosis, fluconazole appears roughly comparable to itraconazole. Fluconazole has activity against histoplasmosis, blastomycosis, sporotrichosis, and ringworm, but response is less than with equivalent doses of itraconazole. Fluconazole is not effective in the prevention or treatment of aspergillosis. As with other azoles, there is no activity in mucormycosis.

Fluconazole ( DIFLUCAN ) is marketed in the United States as tablets of 50, 100, and 200 mg for oral administration, powder for oral suspension providing 10 and 40 mg/ml, and intravenous solutions containing 2 mg/ml in saline and in dextrose solution. Dosage is 50 to 400 mg once daily and is identical for oral and intravenous administration. Children are treated with 3 to 6 mg/kg once daily.

Untoward Effects

Nausea and vomiting may be seen at doses above 200 mg daily. Patients receiving 800 mg daily may require antiemetics and may need to be treated intravenously to prevent emesis, which reduces drug availability. Data provided in the package insert states that side effects in patients receiving more than 7 days of drug, irrespective of dose, include the following: nausea 3.7%, headache 1.9%, skin rash 1.8%, vomiting 1.7%, abdominal pain 1.7%, and diarrhea 1.5%. Reversible alopecia may occur with prolonged therapy at 400 mg daily. Rare cases of deaths due to hepatic failure (Jacobson et al., 1994) or StevensJohnson syndrome have been reported. Although the relationship between these deaths and fluconazole treatment is not completely clear in these cases, it is prudent to be alert to early symptoms of these disorders and to stop fluconazole if they occur. Fluconazole is teratogenic in rodents and has been associated with skeletal and cardiac deformities in three infants born to two women taking high doses during pregnancy (Pursley et al., 1996). The drug should be avoided during pregnancy.

Pneumocandins

Caspofungin acetate (CANCIDASE, MK-0991) is a water-soluble, semisynthetic lipopeptide derivative of pneumocandin B_o. Drugs of this class, also referred to as echinocandins, inhibit the formation of D-glucans in the fungal cell wall. Resistance is conferred by mutations in the FKS1 gene, which codes for a large subunit of (1,3)-glucan synthase. Azole-resistant isolates of C. albicans remain susceptible to caspofungin. The drug is active in experimental animal infection with C. albicans, Aspergillus fumigatus, P. carinii, and H. capsulatum. Clinical trials are in progress with intravenous formulations of caspofungin and a similar lipopeptide, FK463, in patients with deep candidiasis, and with neutropenia and fever not responding to antibacterial therapy.

Griseofulvin

Chemistry

The structural formula of griseofulvin is as follows:

The drug is practically insoluble in water.

Antifungal Activity

Griseofulvin is fungistatic in vitro for various species of the dermatophytes Microsporum, Epidermophyton, and Trichophyton. The drug has no effect on bacteria or on other fungi.

Resistance

Although failure of ringworm lesions to improve is not rare, isolates from these patients usually are still susceptible to griseofulvinin vitro.

Mechanism of Action

A prominent morphological manifestation of the action of griseofulvin is the production of multinucleate cells as the drug inhibits fungal mitosis. An explanation for this phenomenon appears to come from studies of the effects on mammalian cells of higher concentrations of the antibiotic. Griseofulvin causes disruption of the mitotic spindle by interacting with polymerized microtubles. Although the effects of the drug are thus similar to those of colchicine and the vinca alkaloids, its binding sites on the microtubular protein are distinct. There is evidence that griseofulvin binds to a microtubule-associated protein in addition to its binding to tubulin.

Absorption, Distribution, and Excretion

The oral administration of a 0.5-g dose of griseofulvin produces peak plasma concentrations of approximately 1 g/ml in about 4 hours. Blood levels are quite variable. Some studies have shown improved absorption when the drug is taken with a fatty meal. Since the rates of dissolution and disaggregation limit the bioavailability of griseofulvin, microsized and ultramicrosized powders are now used in preparations (FULVICINP G GRISACTIN GRIFULVINV). Although the bioavailability of the ultramicrocrystalline preparation is said to be 50% greater than that of the conventional microsized powder, this may not always be true. Griseofulvin has a half-life in plasma of about 1 day, and approximately 50% of the oral dose can be detected in the urine within 5 days, mostly in the form of metabolites. The primary metabolite is 6-methylgriseofulvin. Barbiturates decrease the absorption of griseofulvin from the gastrointestinal tract.

The drug is deposited in keratin precursor cells. The antibiotic present in such cells when they differentiate is tightly bound to, and persists in, keratin and makes this substance resistant to fungal invasion. For this reason, the new growth of hair or nails is the first to become free of disease. As the fungus-containing keratin is shed, it is replaced by normal tissue. Griseofulvin is detectable in the stratum corneum of the skin within 4 to 8 hours of oral administration. Sweat and transepidermal fluid loss play an important role in the transfer of the drug in the stratum corneum. Only a very small fraction of a dose of the drug is present in body fluids and tissues.

Therapeutic Uses

Mycotic disease of the skin, hair, and nails due to Microsporum, Trichophyton, or Epidermophyton responds to griseofulvin therapy. Infections that are readily treatable with this agent include infections of the hair (tinea capitis) caused by Microsporum canis, Microsporum audouini, Trichophyton schoenleinii, and Trichophyton verrucosum;'ringworm' of the glabrous skin; tinea cruris and tinea corporis caused by M. canis, Trichophyton rubrum, T. verrucosum, and Epidermophyton floccosum; and tinea of the hands (T. rubrum, Trichophyton mentagrophytes) and beard (Trichophyton species). Griseofulvin also is highly effective in 'athlete's foot' or epidermophytosis involving the skin and nails, the vesicular form of which is most commonly due to T. mentagrophytes and the hyperkeratotic type to T. rubrum. However, topical therapy is preferred (see below). T. rubrum and T. mentagrophytes infections may require higher-than-conventional doses. Since very high doses of griseofulvin are carcinogenic and teratogenic in laboratory animals, the drug should not be used to treat trivial infections that respond to topical therapy.

The recommended daily dose of griseofulvin is 5 to 15 mg/kg for children and 500 mg to 1 g for adults. Doses of 1.5 to 2.0 g daily may be used for short periods in severe or extensive infections. Best results are obtained when the daily dose is divided and given at 6-hour intervals, although the drug often is given twice per day. Treatment must be continued until infected tissue is replaced by normal hair, skin, or nails, which requires 1 month for scalp and hair ringworm, 6 to 9 months for fingernails, and at least a year for toenails. Itraconazole or terbinafine is preferred for onychomycosis. Griseofulvin is not effective in treatment of subcutaneous or deep mycoses.

Untoward Effects

The incidence of serious reactions associated with the use of griseofulvin is very low. One of the minor effects is headache; it is sometimes severe and usually disappears as therapy is continued. The incidence of headache may be as high as 15%. Other nervous system manifestations include peripheral neuritis, lethargy, mental confusion, impairment of performance of routine tasks, fatigue, syncope, vertigo, blurred vision, transient macular edema, and augmentation of the effects of alcohol. Among the side effects involving the alimentary tract are nausea, vomiting, diarrhea, heartburn, flatulence, dry mouth, and angular stomatitis. Hepatotoxicity also has been observed. Hematologic effects include leukopenia, neutropenia, punctate basophilia, and monocytosis; these often disappear despite continuation of therapy. Blood studies should be carried out at least once a week during the first month of treatment or longer. Common renal effects include albuminuria and cylindruria without evidence of renal insufficiency. Reactions involving the skin are cold and warm urticaria, photosensitivity, lichen planus, erythema, erythema multiforme-like rashes, and vesicular and morbilliform eruptions. Serum sickness syndromes and severe angioedema develop rarely during treatment with griseofulvin. Estrogen-like effects have been observed in children. A moderate but inconsistent increase of fecal protoporphyrins has been noted when the drug is used for a long time.

Griseofulvin induces hepatic microsomal enzymes, thus increasing the rate of metabolism of warfarin; adjustment of the dosage of the latter agent may be necessary in some patients. The drug may reduce the efficacy of some oral contraceptive agents, probably by a similar mechanism.

Terbinafine

Terbinafine is a synthetic allylamine, structurally similar to the topical agent naftifine. Its structural formula is shown below:

Terbinafine is well absorbed, but bioavailability is decreased to about 40% because of first-pass metabolism in the liver. Proteins bind more than 99% of the drug in plasma. Drug accumulates in skin, nails, and fat. The initial half-life is about 12 hours but extends to 200 to 400 hours at steady state. Drug can be found in plasma for 4 to 8 weeks after prolonged therapy (Balfour and Faulds, 1992). Terbinafine is not recommended in patients with marked azotemia or hepatic failure because terbinafine plasma levels are increased by unpredictable amounts. Rifampin decreases and cimetidine increases plasma terbinafine concentrations. The drug is well tolerated, with a low incidence of gastrointestinal distress, headache, or rash. Rarely, hepatotoxicity, severe neutropenia, StevensJohnson syndrome, or toxic epidermal necrolysis may occur. The drug is in pregnancy category B. Its mechanism of action is probably inhibition of fungal squalene epoxidase, blocking ergosterol biosynthesis.

Terbinafine ( LAMISIL ), given as one 250-mg tablet daily, is at least as effective for onychomycosis of nails as 200 mg daily of itraconazole, and slightly more effective than pulse itraconazole therapy (see above) (Evans, 1999). Duration of treatment varies with the nail being treated but typically is 3 months. Although not approved for this use, terbinafine (250 mg daily) also is effective in ringworm elsewhere on the body. No pediatric formulation is available, so there is little experience with the drug in tinea capitis, usually a disease of children. The topical use of terbinafine is discussed in the following section.