Antifungal drugs Lec 19-20 31 -1-2017 Dr. Naza M. Ali

2. Flucytosine (5-FC) Is fungistatic Is a synthetic pyrimidine antimetabolite Is often used in combination with amphotericin B The combination used in the treatment of systemic mycoses and for meningitis caused by Cryptococcus neoformans and Candida albicans Amphotericin B increases cell permeability, allowing more 5-FC to penetrate the cell / synergistic.

Mechanism of action: 5-FC enters fungal cells via a cytosine-specific permease an enzyme not found in mammalian cells. 5-FC is then converted by a series of steps to a false nucleotide (5-FdUMP) this false nucleotide inhibits thymidylate synthase.

Thus depriving the organism of thymidylic acid ( an essential DNA component). The unnatural mononucleotide is further metabolized to a trinucleotide & is incorporated into fungal RNA, where it disrupts nucleic acid and protein synthesis.

Pharmacokinetics: well absorbed by oral route Toxicity: 1. Reversible bone marrow depression, 2. Alopecia 3. Liver dysfunction

3. Azole antifungal agents -Ketoconazole -Fluconazole -Itraconazole -Voriconazole -Posaconazole (new)

Mechanism of action: Azoles are fungistatic. The azoles interfere with fungal cell membrane permeability by inhibiting the synthesis of ergosterol. These drugs act at the step of 14 alpha demethylation of lanosterol ( catalyzed by CYP 450) . This inhibition disrupts membrane structure and function so inhibits fungal cell growth.

Ketoconazole Orally is effective against dermatophytes. Narrow antifungal spectrum it is now rarely used for systemic mycoses. Used for chronic mucocutaneous candidiasis Orally is effective against dermatophytes. Although penetration into tissues is limited, it is effective in the treatment of histoplasmosis in lung, bone, skin, and soft tissues.

Topical ketoconazole is used to treat tinea corporis, tinea cruris, and tinea pedis in the treatment of seborrheic dermatitis and dandruff.

Ketoconazole Pharmacokinetics When taken orally it requires gastric acid for dissolution and is absorbed through the gastric mucosa The absorption of ketoconazole is impair when take with: antacids H2-histamine receptor blockers proton-pump inhibitors

Adverse effects: Allergies, dose-dependent GIT disturbances, Endocrine effects ( gynecomastia, decreased libido, impotence, menstrual irregularities) in addition to blocking fungal ergosterol synthesis, also inhibits human gonadal and adrenal steroid synthesis, leading to decreased testosterone and cortisol production. ketoconazole inhibits CYP450 enzymes.

Drug interactions and contraindications: By inhibiting cytochrome P450, ketoconazole potentiate the toxicities phenytoin, tolbutamide & warfarin. Rifampin is an inducer of the cytochrome P450 system shorten the duration of action of ketoconazole.

Fluconazole Clinically important Why? because of its lack of the endocrine side effects of ketoconazole, its excellent penetrability into the CSF of both normal and inflamed meninges.

Clinical uses Drug of choice in esophageal & oropharyngeal candidiasis Most infection caused by Coccidioides. A single oral dose eradicates vaginal candidiasis Drug of choice for initial & secondary prophylaxis against cryptococcal meningitis.

Itraconazole Is a broad antifungal spectrum Itraconazole is well absorbed orally, but it requires acid for dissolution.

Is used in treatment of onychomycosis. Drug of choice for systemic infection In esophageal candidiasis the drug is active against some strains resistant to fluconazole. In acquired immunodeficiency syndrome–associated histoplasmosis.

Voriconazole is a broad spectrum antifungal agents, it is a codrug of choice for treatment of invasive Aspergillosis.

Posaconazole is a new oral, broad spectrum antifungal agents in treatment of fungal infections caused by Mucor species. liver metabolism is responsible for elimination of Ketoconazole Itraconazole Voriconazole Except Fluconazole is eliminated by kidney.

Ketoconazole Fluconazole Vorioconazole Posaconazole Spectrum Narrow Expanded Route Oral Oral, IV Oral ,IV T ½ (hours) 6-9 30 24 66 CSF Penetration No Yes Elimination Hepatic Renal

4. Echinocandins include: Caspofungin It is unique antifungal action inhibiting the synthesis of β 1-2 glucan, a critical component of fungal cell walls leading to lysis and cell death. It is limited use to Aspergillus It is used IV once daily. Half –life is 9-11 hours

Caspofungin: is Licensed for empiric antifungal therapy during febrile neutropenia Micafungin is licensed for prophylaxis of candida infections in bone marrow transplant patients

Cutaneous Mycotic Infections Fungi that cause superficial skin infections are called dermatophytes. These tinea infections are classified by the site of their infection, such as tinea pedis, which refers to an infection of the feet.

The three different fungi that cause the majority of cutaneous infections are Trichophyton, Microsporum, Epidermophyton These pathogens have a high affinity for keratinized tissue such as skin, hair, and nails.

Drugs for Cutaneous Mycotic Infections 1. Squalene epoxidase inhibitor: Terbinafine 2. Griseofulvin 3. Nystatin 4. Imidazoles 5. Ciclopirox 6. Tolnaftate

1. Squalene epoxidase inhibitors Act by inhibiting squalene epoxidase, resulting in the blocking of the biosynthesis of ergosterol which is an essential component of fungal cell membrane.

Terbinafine (fungicidal) Inhibits a fungal enzyme, squalene epoxidase. It cause accumulation of toxic levels of squalene, which interfere with ergosterol synthesis. It is accumulated in keratin, is much more effective than griseofulvin in onychomycosis and require shorter duration of therapy (3 months).

Pharmacokinetics: Terbinafine / oral and topical It is deposited in the skin, nails, and fat. A prolonged terminal half-life of 200 - 400 hours may reflect the slow release from these tissues. Patients with either moderate renal impairment or hepatic cirrhosis have reduced clearance.

Adverse effects GIT disturbances headache, and rash hepatotoxicity neutropenia.

2. Griseofulvin (fungistatic) Oral absorption is enhanced by high-fat food. In treatment of dermatophytic infections of nails Griseofulvin requires 6-12 months duration of therapy, and is dependent on the rate of replacement of healthy skin or nails. It accumulates in newly synthesized, keratin-containing tissue where it causes disruption of the mitotic spindle and inhibition of fungal mitosis.

3. Nystatin Is a polyene antibiotic, and its structure, chemistry, mechanism of action, and resistance resemble those of amphotericin B. Use in topical treatment of Candida infections because of its systemic toxicity. The drug is negligibly absorbed from the GIT Used orally for the treatment of oral candidiasis.

4. Imidazoles are azole derivatives include Miconazole, Clotrimazole, Terconazole, Econazole Are topically active drugs for vulvovaginal candidiasis , contact dermatitis.

5.Ciclopirox inhibits the transport of essential elements in the fungal cell, disrupting the synthesis of DNA, RNA, and protein. Ciclopirox 1% shampoo for treatment of seborrheic dermatitis.

6. Tolnaftate It is used to treat tinea pedis, tinea cruris, tinea corporis It is available as a 1% solution, cream, and powder