Antifungal Agents - 1

Antifungal Agents-1 Antifungal Agents Objectives: By the end of lectures all students should know: Available antifungal drugs; Their MOA; Their Pharmacokinetic properties; Their clinical uses Their major side effects and drug interactions ◼ Fungi consist of: - Rigid cell wall composed of chitin ( N – acetylglucosamine ) (bacterial cell wall is composed of peptidoglycan) - Plasma or cell membrane which contains ergosterol (human cell mebmrane is composed of cholesterol) (selectivity to some antifungal agents) ◼ - Fungi have nucleus and well defined nuclear membrane, and chromosomes Fungi are eukaryotic organisms that live as saprobes or parasites They are complex organisms in comparison to bacteria (prokaryotic cells=have no nuclear membranes and no mitochondria) Therefore antibacterial agents are not effective in fungal infections and antifungal agents are ineffective in bacterial infections ◼ ◼ Fungal infections are termed mycoses and can be divided into: (1) Superficial infections: affecting skin, nails, scalp or mucous membranes (2) Systemic infections: affecting deeper tissues and organs Superficial fungal infections can be classified into the dermatomycoses and candidiasis (Candida is a commonly normal flora of mouth, skin, intestines and vagina) ◼ ◼ Dermatomycoses are infections of the skin, hair and nails, caused by dermatophytes. The commonest are due to Tinea organisms which are also known as ringworms In superficial candidiasis, the fungus candida infects the mucous membranes of the mouth (oral thrush), or the vagina (vaginal thrush) or the skin Systemic fungal infections include: - Systemic candidiasis - Cryptoccocal meningitis or endocarditis - Pulmonary aspergillosis - Blastomycosis - Histoplasmosis - Coccidioidomycosis - Paracoccidioidomycosis…etc ◼ Fungal infections whether, superficial or systemic, are common in patients with weak immune system e.g.: - Patients with AIDS - Debilitated patients - Patients underwent organ transplantation and on immunosuppressants - Patients under anticancerous therapy ◼ Antifungal Drugs Classes 1. Polyenes (polyene macrolide antibiotics) Amphotericin B Nystatin Natamycin Polyenes mechanism of action: Bind to ergosterol in fungal plasma membrane leading to formation of pores and hence increased permeability of the membrane. This allows leakage of intracellular ions and enzymes especially loss of intracellular k+ causing death to the fungus They bind selectively to ergosterol in fungus but not to cholesterol in mammalian plasma membranes ◼ ◼ Mechanisms of resistance to polyenes: - Decreased ergosterol content of the fungal membrane - Impaired binding to ergosterol 2. Azoles: ◼ ◼ Azole antifungal agents have added greatly to the therapeutic options for treatment of systemic fungal infections The azole antifungal agents in clinical use contain either two or three nitrogens in the azole ring and are thereby classified as imidazoles (e.g., ketoconazole; miconazole; clotrimazole) or triazoles (e.g., itraconazole; and fluconazole), respectively Azoles mechanism of action: - Azoles are fungistatic - They inhibit cytochrome P450 demethylase enzyme which is important for formation of ergosterol - This inhibition disrupts membrane structure and function and, thereby, inhibits fungal growth ◼ Mechanism of resistance to Azoles: Mutation in the gene encoding for demethylase ◼ 3. Allylamines: Terbinafine Naftifine Butenafine Mechanism of action Allylamines: Inhibit fungal squalene epoxidase, thereby decreasing the synthesis of ergosterol. This plus the accumulation of toxic amounts of squalene result in the death of the fungal cell Significantly higher concentrations of Terbinafine are needed to inhibit human squalene epoxidase, an enzyme required for the cholesterol synthetic pathway (representing some selectivity to fungi) ◼ Echinocandins: Caspofungin Micafungin Anidulafungin ◼ Mechanism of action: Interfere with the synthesis of the fungal cell wall by inhibiting the synthesis of D-glucan, leading to lysis and fungal cell death ◼ ◼ Antifungals that inhibit mitosis: Griseofulvin Mechanism of action: It inhibits fungal mitosis by inhibiting mitotic spindle formation The drug binds to tubulin, interfering with microtubule function, thus inhibiting mitosis ◼ Drugs that inhibit DNA synthesis (antimetabolites): Flucytosine (5FC=5-fluorocytosine) ◼ Mechanism of action: It enters fungal cells by permease (an enzyme not found in mammalian cells) and is then converted by a series of steps to 5-fluorodeoxyuridine 5'monophosphate This false nucleotide inhibits thymidylate synthase, thus depriving the fungus of thymidylic acid an essential DNA component ◼ The mononucleotide is further metabolized to a trinucleotide (5-fluorodeoxyuridine 5'triphosphate) and is incorporated into fungal RNA, thus disrupting nucleic acid and protein synthesis. Amphotericin B increases cell permeability, allowing more Flucytosine to penetrate the cell. Thus, Flucytosine and Amphotericin B are synergistic Amphotericin B - It is a macrolide antibiotic, poorly absorbed orally, useful for fungal infection of gastrointestinal tract - Drug of choice for most systemic infections, given as slow IV infusion - Locally used in corneal ulcers (ophthalmic oint.), arthritis (intra-articular) and bladder irrigation Penetration through BBB is poor but increases in inflamed meninges - Excreted slowly via kidneys, traces found in urine for months after cessation of drug - Half life 15 days Side effects to Amphotericin: - Most serious is renal toxicity, which occurs in 80% of patients - Hypokalaemia in 25% of patients - Hypomagnesaemia - Anemia & thrombocytopenia - Impaired hepatic function ◼ - Anorexia, nausea, vomiting, abdominal, joint and muscle pain, loss of weight, and fever - Anaphylactic shock - To reduce the toxicity of Amphotericin B, several new formulations have been developed in which amphotericin B is packaged in a lipid-associated delivery system (Liposomal preparations) ◼ Such delivery systems have more efficacy , less nephrotoxicity but very expensive Nystatin - It is a polyene macrolide, similar in structure to Amphotericin B and with same MOA - Too toxic for systemic use - Not absorbed from GIT, skin or vagina, therefore administered orally to prevent or treat superficial candidiasis of mouth, esophagus or intestinal tract - Oral suspension of 100,000 U/ml 4 times a day and tablets 500,000 U of Nystatin are used to decrease GIT colonization with Candida - For vaginal candidiasis in form of pessaries used for 2 weeks - In cutaneous infection available in cream, ointment or powder forms and applied 2-3 times a day Natamycin - It is a macrolide polyene antifungal used to treat fungal keratitis, an infection of the eye. It is especially effective against Aspergillus and Fusarium corneal infections - Also effective in Candida, Cephalosporium and Penicillium - Not absorbed when given orally - Available in cream and ophthalmic eye drops Flucytosine - Has useful activity against Candida and Cryptococcus - It is synthetic pyrimidine antimetabolite that is often used in combination with Amphotericin B - It is fungistatic, effective in combination with Itraconazole for treating chromoblastomycosis and with Amphotericin B for treating cryptococosis - Highly effective in cryptococcal meningitis in AIDS patients - Flucytosine is absorbed rapidly and well from GIT - Widely distributed in body and penetrates well into CSF ◼ Side effects to Flucytosine: - Reversible neutropenia, thrombocytopenia and occasional bone marrow depression - Nausea, vomiting, diarrhea, severe enterocolitis - Reversible hepatic enzyme elevation in 5% of patients