Amphotericin B

Amphotericin B is an antifungal agent obtained from Streptomyces nodosus, an actinomyces found in the soil. It is a member of the polyene family of antibiotics, so called because their structure contains a large lactone (macrolide) ring with numerous conjugated double bonds (Figure 40-1). The polar hydroxylated portion and the nonpolar hydrocarbon sequence lend an amphophilic character to the molecule. Polyenes are unstable in solution because of the unsaturated chromophore region, which is easily photo-oxidized. Amphotericin B exerts either fungistatic or fungicidal activity depending on the concentration of the drug, the pH, and the fungus involved. Peak activity occurs at a pH between 6.0 and 7.5. Amphotericin B has a broad spectrum of antifungal activity and is effective against Candida species, Histoplasma capsulatum, Cryptococcus neoformans, and Coccidioides immitis.

FIGURE 40-1 Structural formulas of polyene antifungal agents. Nystatin A₁ is one of three compounds found in the commercial nystatin preparation.

Amphotericin B is not absorbed from the skin or mucous membranes and is poorly and inconsistently absorbed from the gastrointestinal tract. Because of its insolubility in an aqueous medium, the drug is reconstituted in a solution of the bile salt deoxycholate immediately before use. For systemic infections, amphotericin B is administered by slow intravenous infusion (over 2 to 6 hours each day). The drug is bound in plasma to various lipoproteins and in tissues to cholesterol-containing membranes. More recent studies showed that amphotericin B lipid complex or liposomal amphotericin B preparations could be used for systemic infections, particularly in premature infants and other immunocompromised patients.⁵⁰ Amphotericin B can also be prepared in colloidal dispersion with sodium cholesteryl sulfate in a 1 : 1 discoidal complex. Colloidal amphotericin B showed reduced peak plasma levels, prolonged residence time, and reduced renal toxicity and hepatotoxicity compared with conventional amphotericin B preparations.⁴¹

The exact metabolic pathway of amphotericin B is unknown, but most of the drug is biotransformed and slowly excreted by the kidney over the next 2 months. The plasma concentration of amphotericin B is unaffected by renal disease; no dosage adjustment needs to be made in patients with compromised renal function. Amphotericin B applied topically as a 3% cream, ointment, or lotion is useful in the treatment of superficial Candida infections. Because C. albicans infection can readily occur in patients receiving broad-spectrum antibiotics, these agents are sometimes administered with amphotericin B or nystatin. The efficacy of fixed-ratio combinations has not been proven, however, and does not reflect sound therapy.

The only adverse effects accompanying the topical application or oral administration of amphotericin B are local irritation and mild gastrointestinal disturbances. As an intravenous agent, however, amphotericin B is the most toxic antibiotic in current use. Intravenous amphotericin B causes many side effects, including hypotension, delirium, fever, nausea, vomiting, abdominal pain, anorexia, headache, and thrombophlebitis. Hypochromic, normocytic anemia is induced by amphotericin B, and leukopenia and thrombocytopenia occur rarely. Allergic reactions of all types have been reported, including anaphylaxis. All patients receiving intravenous amphotericin B show some degree of nephrotoxicity, which may lead to discontinuation of therapy. Permanent damage of the kidneys does not occur, however, in patients receiving a cumulative dosage of less than 4 g during a normal therapeutic interval of several weeks. Great caution should be exercised when amphotericin B is used with other nephrotoxic drugs. Because amphotericin B can cause hypokalemia, it can increase digitalis toxicity. The toxic effects of cyclosporine may also be increased.

Nystatin

Nystatin is a polyene antibiotic obtained from Streptomyces noursei. Its structure is similar to the structure of amphotericin B (see Figure 40-1). Nystatin is relatively insoluble in water and unstable except as a dry powder.

Nystatin has a spectrum of activity slightly narrower than that of amphotericin B, but is nevertheless active against many species of Candida; Histoplasma; Cryptococcus; Blastomyces; and the dermatophytes Epidermophyton, Trichophyton, and Microsporum. Similar to amphotericin B, nystatin is either fungistatic or fungicidal depending on the concentration of the drug present, the pH of the surrounding medium, and the nature of the infecting organism. The mechanism of action of nystatin is also similar to that of amphotericin B. In vitro, some species of Candida, such as Candida tropicalis, can develop resistance to nystatin, but resistance is rarely observed clinically.

Nystatin is not appreciably absorbed from the skin, mucous membranes, or gastrointestinal tract. After oral administration, the bulk of the administered dose appears unchanged in the feces. Because of unacceptable systemic toxicity, nystatin is never given parenterally. A newer form of nystatin encapsulated in liposomes showed reduced systemic cytotoxicity, however, making it an active systemic antifungal agent.³⁸ Also, liposomal nystatin has been suggested to target Candida species that are resistant to amphotericin B.¹⁰

Nystatin is used primarily to treat candidal infections of the mucosa, skin, intestinal tract, and vagina. Although the efficacy of oral nystatin for enteric candidiasis has been questioned, topical nystatin remains a drug of choice for the treatment of candidal infections of the oral cavity (oral moniliasis, thrush, denture stomatitis). It has also been used prophylactically in immunocompromised patients.⁶⁴ To treat oral candidiasis, 2 to 3 mL of a suspension containing 100,000 U/mL of nystatin is placed in the mouth, swished, and held for at least 5 minutes before swallowing. This regimen is repeated every 6 hours for at least 10 days or for 48 hours after remission of symptoms. Alternatively, 1 to 2 lozenges (200,000 U per each) may be used four to five times per day. For denture stomatitis, nystatin ointment (100,000 U/g) can be applied topically every 6 hours to the tissue surface of the denture.

Nystatin is well tolerated. Only mild and transient gastrointestinal disturbances, such as nausea, vomiting, and diarrhea, have occurred after oral ingestion. The major complaint associated with nystatin is its bitter, foul taste.

Ketoconazole

Ketoconazole (Figure 40-2) is rarely used because of its toxicity and the availability of other azoles. Ketoconazole was the first oral antifungal agent to be approved for the treatment of deep systemic mycoses. It is well absorbed from the gastrointestinal tract, provided that the stomach content is acidic. Drugs that increase gastric pH, such as antacids and H₂ antihistamines, markedly reduce its absorption.⁵⁴ It should be reserved for cases refractory to other therapy.^11,66,80

FIGURE 40-2 Structural formulas of several imidazole and triazole antifungal agents.

The drug should be used cautiously because severe hepatotoxicity occurs in approximately 0.01% of individuals. Ketoconazole markedly inhibits the synthesis of testosterone and estradiol, which may lead to gynecomastia and menstrual irregularities. Alterations of adrenal steroid synthesis may also occur. Ketoconazole inhibits the metabolism of cyclosporine, phenytoin, sulfonylureas, warfarin, and several other drugs. Isoniazid increases the metabolism of ketoconazole.

Miconazole

Miconazole (see Figure 40-2) was the first imidazole antifungal drug to be approved for topical and parenteral use. It is no longer used systemically. Cutaneous candidiasis and vulvovaginitis caused by C. albicans respond rapidly and reliably to 2% miconazole nitrate cream. Oral candidiasis is also effectively treated; however, a specific formulation for intraoral use is unavailable. Other topical uses of miconazole are treatment of cutaneous dermatophyte infections caused by Epidermophyton, Microsporum, and Trichophyton. Untoward effects after topical administration of miconazole are rare, but burning, skin maceration, itching, and redness can develop.

Clotrimazole

Clotrimazole is an imidazole antifungal drug used for various mucosal and cutaneous infections. The antifungal spectrum and mechanism of action are similar to other azoles. Clotrimazole is restricted to topical use. A preparation specifically suited for intraoral application is marketed.

For treatment of oral candidiasis, clotrimazole is available as a 10-mg troche. Slow dissolution in the mouth results in the binding of clotrimazole to the oral mucosa, from which it is gradually released to maintain at least fungistatic concentrations for several hours. The swallowed drug is variably but poorly absorbed. It is metabolized in the liver and eliminated in the feces along with the unabsorbed drug.

One troche dissolved in the mouth five times a day for 2 weeks is the standard regimen for oropharyngeal candidiasis. Patient compliance is believed to be enhanced by the more pleasant taste of clotrimazole compared with nystatin. Clotrimazole also seems to be useful for the topical treatment of oral candidiasis in patients with AIDS.^80,88,89 For cutaneous candidiasis and dermatophytoses, a 1% cream or lotion is equivalent to topical miconazole.

Irritation associated with topical clotrimazole, although unlikely, is qualitatively similar to the irritation described in association with miconazole. Occasionally, minor gastrointestinal upset may follow oral ingestion of the drug.

Itraconazole

Itraconazole is a water-insoluble triazole compound that shows a broader spectrum of antifungal activity and a faster clinical effect compared with some other azoles. Similar to ketoconazole, itraconazole is well absorbed from the gastrointestinal tract when it is given with meals. It is highly bound to plasma proteins (>99%) and has a long half-life (approximately 20 hours after a single dose, ≤60 hours at steady state). Although the concentrations of itraconazole in saliva and cerebrospinal fluid are negligible, tissue concentrations are two to five times higher than that of plasma. The drug is mostly metabolized in the liver and partially eliminated in the bile.

When given in therapeutic doses, itraconazole exerts effective antifungal activity against paracoccidioidomycosis, blastomycosis, aspergillosis, histoplasmosis, sporotrichosis, candidiasis, and various dermatophytoses. Previous studies show that itraconazole is effective for suppressive therapy and primary treatment of histoplasmosis in patients seropositive for human immunodeficiency virus (HIV).^55,83 Drug interactions are qualitatively similar to those noted for ketoconazole, but occur less frequently. Itraconazole and related triazoles are more specific for fungal 14-α-demethylase, however, and do not affect mammalian steroid metabolism as greatly.²³ Adverse effects include rashes, hepatotoxicity, hypokalemia, hypertension, and heart failure in susceptible patients.

Fluconazole

Fluconazole is a water-insoluble, fluorine-substituted bistriazole with effective antifungal activity in immunocompetent and immunocompromised patients. Fluconazole is significantly less potent as an inhibitor of mammalian steroid synthesis, indicating more specific antifungal actions than ketoconazole. It is well absorbed from the gastrointestinal tract (the drug is also available for intravenous injection), weakly bound to plasma proteins (12%), and well distributed throughout the body. Peak plasma concentrations are reached within 2 hours after oral administration; concentrations in the cerebrospinal fluid are generally more than 50% of the corresponding plasma values. Fluconazole has a long plasma half-life of 20 to 50 hours in adults and approximately 17 hours in children. Fluconazole is excreted largely unchanged in the kidney.

Fluconazole is active in suppressive therapy and primary treatment of cryptococcal meningitis, which may occur in patients with AIDS.⁴⁶ It is effective in the treatment of mucosal candidiasis, including oropharyngeal and esophageal candidiasis.¹¹ Weekly use of fluconazole was suggested to have prophylactic value against mucosal candidiasis in HIV-seropositive patients.⁷¹ It is also used in the primary treatment of coccidioidal meningitis and treatment of blastomycosis and histoplasmosis. In one study, fluconazole was found to be more effective against oral candidiasis than nystatin in immunocompromised children.³⁷ It may also be effective in candidiasis resistant to polyenes and imidazoles.⁵⁷

Nausea, vomiting, gastric pain, headache, and rashes are the most common adverse effects. Increases in serum transaminases have been reported in less than 5% of individuals receiving fluconazole. Seizures, anaphylaxis, and exfoliative dermatitis occur rarely. The drug interactions generally resemble those of itraconazole, but to a lesser degree. Gastric pH has little effect on the oral absorption of fluconazole (antacids and H₂ antihistamines do not interact).

Other imidazoles and triazoles

Terconazole, a member of the triazole antifungal drugs, is supplied in a vaginal suppository for vaginal candidiasis. Butoconazole and tioconazole are imidazoles that are also used topically for vulvovaginitis. Oxiconazole and sulconazole are used topically for infections caused by dermatophytes. Econazole is another imidazole derivative that is used topically for the treatment of dermatophyte and Candida infections.

Voriconazole and posaconazole have been developed more recently and represent a new generation of triazole antifungals with enhanced pharmacologic properties. These drugs show broad-spectrum fungicidal activity against molds and fungistatic activity against Candida and other yeasts.⁴⁸ Voriconazole is a derivative of fluconazole exhibiting increased antifungal activity and specificity. It is the drug of choice for the treatment of invasive aspergillosis caused by Aspergillus terreus, which is increasingly observed as a pathogen in immunocompromised patients.⁷⁴ Voriconazole is also effective against dimorphic fungi (Histoplasma, Coccidioides, and Blastomyces species), yeasts (Candida krusei, Candida glabrata, C. neoformans, and Tricosporon asahii), and pathogenic molds (Fusarium and Scedosporium species).⁴⁰ Adverse side effects of voriconazole include erythematous rash, visual disturbances, hepatotoxicity, and headache.⁴⁷ Voriconazole is considered a safer alternative to other antifungals such as amphotericin B for patients at risk of renal dysfunction or receiving concurrent administration of nephrotoxic drugs.⁸¹

Posaconazole is a newer addition to the antifungal triazoles that structurally resembles itraconazole (see Figure 40-2). The antifungal spectrum of posaconazole is similar to that of voriconazole except that its potential therapeutic efficacy has also been shown against Zygomycetes (e.g., Rhizopus, Absidia, and Mucor species).²⁷ Voriconazole and posaconazole are effective against a wide variety of Candida species. In particular, voriconazole administration led to a higher success rate against C. tropicalis compared with amphotericin B and fluconazole.⁵² Posaconazole was found to be as effective as fluconazole for the treatment of oropharyngeal candidiasis in patients infected with HIV and led to fewer incidences of clinical relapse.⁷⁹ Posaconazole may have extensive uses in dentistry in the future.

Caspofungin

Caspofungin (see Figure 40-3) is derived from the fermented by-product of Glarea lozoyenisi. It is the first echinocandin approved by the FDA for clinical use. It is an echinocandin with antifungal activity against a wide variety of fungal pathogens, including Candida,⁸ Pneumocystis, Aspergillus, and Histoplasma species. Caspofungin disrupts the formation of the fungal cell wall by inhibiting the enzyme 1,3-β-d-glucan synthase, which is necessary for β(1,3)-d-glucan polymerization in filamentous fungi. Because this mechanism of action differs from those of amphotericin B and the azole compounds, combination therapy using caspofungin with other antifungal agents has been suggested and has yielded synergistic effects against cryptococcal species.³⁰ Caspofungin showed higher therapeutic efficacy against candidal infections compared with amphotericin B in immunocompromised patients.^15,44

Caspofungin is of particular importance in patients with life-threatening systemic fungal infection who cannot tolerate amphotericin B or azole therapy; it is generally well tolerated when administered parenterally. The manufacturer recommends intravenous infusion of 70 mg of caspofungin acetate for the first day, followed by 50 mg/day thereafter. Common adverse effects resemble histamine-mediated symptoms, such as rash, facial swelling, pruritus, or sensation of warmth. One case of anaphylaxis was also reported with the initial administration of the drug.

Micafungin

Micafungin is a synthetic derivative of lipopeptides isolated from Coleophoma empetri. It is approved for therapeutic use against esophageal candidiasis and for chemoprophylaxis against candidiasis in neutropenic patients undergoing HSCT.⁶⁰ Fluconazole has been the primary drug of choice for chemoprophylaxis against candidiasis and aspergillosis in HSCT patients. A comparative phase III clinical trial of antifungal prophylactic efficacy showed superior results, however, with micafungin compared with fluconazole.⁷⁷ Among 889 adult and pediatric patients enrolled for HSCT, 50 mg of micafungin administered daily to patients yielded an 80% success rate versus 73.5% in patients who received 400 mg daily dose of fluconazole. Also, the patients receiving micafungin prophylaxis reported fewer side effects and fewer incidences of discontinued therapy compared with patients receiving fluconazole prophylaxis.

Micafungin was found to be as effective as fluconazole against esophageal candidiasis with a similar spectrum of adverse effects in HIV patients.²⁹ A more recent study also showed a comparable level of therapeutic efficacy of micafungin against candidemia and invasive candidiasis as liposomal amphotericin B when both drugs were delivered as an intravenous infusion.⁵³ Compared with amphotericin B, micafungin treatment led to significantly fewer adverse effects, such as hypokalemia, rigors, back pain, infusion-related reactions, and nephrotoxicity. Micafungin is considered a well-tolerated addition to the antifungal armamentarium. Micafungin is given at a daily infusion dose of 150 mg for esophageal candidiasis and 50 mg for antifungal prophylaxis.

Anidulafungin

Anidulafungin is derived from Aspergillus nidulans. It is the newest addition to echinocandin antifungals approved for esophageal candidiasis, candidemia, and invasive candidiasis. Anidulafungin showed potent and broad antifungal activity against Candida and Aspergillus species, including species resistant to fluconazole.⁶⁷ Compared with azole antifungals, anidulafungin was more effective in vitro against C. albicans, C. tropicalis, C. glabrata, and C. krusei, but not Candida famata and Candida parapsilosis.⁹ Anidulafungin was also more effective than caspofungin against Aspergillus.⁶⁸ Large-scale clinical trials confirmed the therapeutic efficacy of anidulafungin against invasive candidiasis compared with fluconazole.⁶⁹ Anidulafungin is given by intravenous infusion as a 100-mg daily maintenance dose for invasive candidiasis and 50-mg daily dose for esophageal candidiasis. A loading dose is also recommended for the first day of treatment.

Flucytosine

Flucytosine, a fluorinated analogue of cytosine (5-fluorocytosine) (Figure 40-4), is a synthetic antimycotic agent orally effective in the treatment of systemic fungal infections, in particular infections caused by yeasts. Flucytosine has a limited antifungal spectrum compared with amphotericin B and is mainly effective against Candida and Cryptococcus. It is also active against some species of Cladosporium and Phialophora, the latter being etiologic agents for chromoblastomycosis.

FIGURE 40-4 Structural formula of flucytosine.

Flucytosine is taken up into sensitive fungal cells by cytosine permease, where it is converted to 5-fluorouracil by cytosine deaminase. The 5-fluorouracil is metabolized further to yield 5-fluorodeoxyuridine monophosphate, a competitive inhibitor of thymidylate synthetase. The formation of thymidine monophosphate from deoxyuridine monophosphate is blocked, and the synthesis of DNA is impaired; 5-fluorouridine triphosphate is also formed in fungal cells, leading to the synthesis of defective RNA. Selective toxicity against fungi is achieved with flucytosine because mammalian cells do not readily take up the drug or convert it to 5-fluorouracil.

Flucytosine is indicated for the treatment of systemic candidiasis and cryptococcosis; however, resistance to flucytosine frequently develops during therapy of these infections. Mechanisms of resistance include decreased flucytosine uptake by fungal cells (altered permease) and decreased synthesis of active nucleotide metabolites (decreased deaminase and other enzyme activities). Flucytosine is normally used in combination with amphotericin B, which seems to increase fungal uptake of flucytosine and to result in synergistic effects against certain fungal diseases. Perhaps more important, coadministration permits reduction in the dose of amphotericin B.

Flucytosine is well absorbed from the gastrointestinal tract, and the peak plasma concentration is attained within 1 to 2 hours after oral administration. The drug is widely distributed throughout the body; it attains a concentration in cerebrospinal fluid approximately 65% to 90% that of the plasma. Flucytosine has a half-life of 3 to 6 hours and is excreted unchanged in the urine.

The major toxicity of flucytosine is depression of the bone marrow, resulting in anemia, leukopenia, and thrombocytopenia. This effect is dose-related and is reversible. Because flucytosine is excreted mainly through the kidneys, it is advisable to measure the plasma concentration of the drug periodically, especially because it is normally given with the highly nephrotoxic amphotericin B. An elevation of hepatic enzymes in plasma and hepatomegaly occurs in approximately 5% of patients receiving flucytosine. Lastly, flucytosine may cause nausea, vomiting, diarrhea, and (rarely) severe enterocolitis. These toxic effects may result from the formation and release of 5-fluorouracil by fungi and intestinal microbes.

Tolnaftate and allylamine antifungal drugs

Tolnaftate is a thiocarbamate that is commonly used as a topical antifungal agent against mild-moderate superficial fungal infection in skin and toenails, such as tinea pedis, tinea cruris, tinea corporis, tinea manuum, and tinea versicolor. Susceptible dermatophytes include Malassezia furfur, Epidermophyton floccosum, Trichophyton mentagrophytes, Trichophyton tonsurans, and Microsporum canis. However, tolnaftate is generally ineffective against yeasts, however, such as C. albicans.¹² Adverse effects associated with the topical use of tolnaftate are generally mild and could involve allergic contact dermatitis. Possible teratogenic effects of tolnaftate spray use during pregnancy have been suggested²⁶ and need further investigation for conclusive results.

The mechanism of action of tolnaftate involves noncompetitive inhibition of fungal squalene epoxidase, which is a membrane-bound enzyme necessary for conversion of acetate to sterols and biosynthesis of ergosterol. This mechanism of drug action is shared by another class of antifungals known as allylamines, which include naftifine and terbinafine. Terbinafine is effective against dermatophytes (Microsporum, Trichophyton, and Epidermophyton species) and molds (Aspergillus and Scopulariopsis species).³⁹ Terbinafine is highly lipophilic and keratophilic and accumulates in the stratum corneum of skin and nails. Similar to thiocarbamate antifungals, allylamine agents are used effectively for dermatophytosis of skin and nails. The adverse effects of terbinafine include mild and transient forms of gastrointestinal symptoms, rash, urticaria, pruritus, and neutropenia, but the drug is generally well tolerated. Terbinafine is used orally and topically, whereas naftifine is used only topically.

Griseofulvin

Griseofulvin was first isolated from Penicillium griseofulvum dierckx in 1939, but its antifungal activity was unknown until 1946. It exerts a fungistatic effect against Microsporum, Epidermophyton, and Trichophyton species that infect skin, hair, and nails. Griseofulvin interacts with polymerized microtubules, causing the disruption of the mitotic spindle and eventually fungal mitosis.

Griseofulvin is variably absorbed from the gastrointestinal tract; micronization of the primary drug particles (see Chapter 2) and ingestion with a fatty meal improve bioavailability. Although most of the absorbed drug is inactivated in the liver by dealkylation, the plasma half-life is fairly long (approximately 20 hours), and griseofulvin readily reaches the skin, hair, and nails, where it binds avidly to newly synthesized keratin and inhibits fungal invasion through surface keratin. Serious side effects are uncommon, but griseofulvin may induce nausea, vomiting, diarrhea, fatigue, headache, and mental confusion. The drug may also cause hematologic and dermatologic reactions. As an inducer of cytochrome P450 enzymes, griseofulvin is contraindicated in patients with acute intermittent porphyria and may participate many drug interactions, potentially decreasing the effectiveness of drugs such as warfarin and oral contraceptives. Its use has waned as a result of marketing of newer drugs for treating superficial fungal infections.