Antifungals, antivirals, antiprotozoals, and anthelmintics

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1 Antifungals, antivirals, antiprotozoals, and anthelmintics Joseph K. Ritter, PhD Asst. Prof Department of Pharmacology and Toxicology MSB Room 530

2 Difficulties associated with treatment of fungal infections Infections increasingly common High similarity to animal cells (resistant to antibacterial agents) Presence of cell wall Slow growth rate Infections often occur in poorly vascularized tissues

3 Classes of fungal infections Systemic infections (meningitis, pneumonia) Aspergillus Candida albicans Cryptococcus neoformans Blastomyces dermatitidis Histoplasma capsulatum Coccidioises immitis Superficial infections Candida albicans Deeper superficial (dermatophytic) infections Epidermophyton Microsporum Trichopyton

4 Overview of classes of antifungal drugs Polyenes Amphotericin B Nystatin Imidazoles/triazoles Ketoconazole, Fluconazole, Itraconazole Miconazole, Clotrimazole Nucleosides Flucytosine Griseofulvin

5 Amphotericin B (Fungizone ) HO H 3 C O O CH 3 H O H CO H 3 C Amphotericin B O NH 2 O CH 3 polyene = conjugated double bonds

6 Mechanism of polyene antifungals P-lipid Membrane Sterol Pore Amph B HO H O O C NH 2 H 2 N HO O C H O HO HO HO HO HO HO HO O C H O NH 2 HO H 2 N C O O H

7 Effect of sterol on the ability of a particulate fraction from Neurospora to bind nystatin Particle treatment None Extracted Reconstitution None None Nystatin bound (ug/0.4 ml particulate) Extracted Extracted Extract Ergosterol

8 Characteristics of amphotericin B Spectrum: Broadest of all antifungals Kinetics: o High molecular weight, very insoluble in water, IV formulations only o Crosses membranes poorly o Inject where needed o Eliminated unchanged in urine Toxicity: Extremely nephrotoxic Uses: Serious systemic infections

9 Imidazole/azole antifungal Ketoconazole Fluconazole Itraconazole Miconazole Clotrimazole

10 Antifungal mechanism of imidazoles and triazoles Acetate Squalene CYP51 14 alphasterol demethylase 14alpha methyl sterol (lanosterol) Ergosterol Ergosterol required for close packing of P-lipid fatty acid chains

11 Ketoconazole (Nizoral) N H 3 C O C N N O C H 2 O O N Cl CH 2 Cl

12 Ketoconazole (Nizoral) Spectrum: Broad, includes Candida Kinetics: o Crosses membranes, oral absorption good but variable o Good tissue penetration except CNS o Elimination by CYP3A4 metabolism in liver o Dose-dependent half life (6-12 hrs depending on dose) Toxicities/drug interactions o Common o Gastrointestinal upset o Gynecomastia o Menstrual irregularities o Drug interactions!!!! o Rare o Hepatotoxicity Uses-systemic antifungal for oral and vaginal candidiasis, adrenal hypercorticism

13 Ketoconazole inhibits P450s involved in human steroidogenesis Acetate Squalene Androgens Estrogens CYP51 Lanosterol 14 alpha- methyl 14alphademethylase sterol Androstene dione Corticosterone, cortisol, aldosterone Cholesterol P450 SCC P450 17,20-lyase DHEA Pregnenolone Progesterone

14 Itraconazole (Sporanox) Mechanism: same as ketoconazole, higher potency Kinetics Variable absorption, increased by meals Wide distribution-high and prolonged tissue levels Hepatic metabolism Serum half life hr Side effects Few Uses: Systemic antifungal for oral candiasis

15 Fluconazole (Diflucan) Spectrum: Broad Mechanism: same as ketoconazole but more potent Kinetics Oral absorption (less affected by acid) Distributes (high concentrations in CNS) Elimination (90% by kidney) Side effects (less P450 inhibition) Uses: Systemic agent for Cryptococcal meningitis, systemic and mucocutaneous Candida, prophylaxis in IC)

16 Flucytosine ( 5-fluorocytosine) O N NH 2 N H F 5-fluorocytosine Cytosine deaminase O N N H F 5-fluorouracil 5-FU Extremely toxic Anticancer drug 5-FdUTP Blocks DNA synthesis 5-FUTP Blocks RNA synthesis

17 Flucytosine Mechanism: blocks nucleic acid synthesis Kinetics: >95% oral availability, distribution into CSF, renal elimination (unchanged, glomerular filt) Toxicity: bone marrow suppression Uses: serious systemic infections (meningitis, etc.), combined with amphotericin B

18 Drugs used exclusively for treating superficial fungal infections Nystatin (Nilstat) Spectrum: Extremely Broad Mechanism: see amphotericin B Uses: topical only mucocutaneous Candida infections (oral, vaginal, skin, GI)

19 Drugs used exclusively for treating superficial fungal infections Clotrimazole Imidazole too toxic for systemic use Mechanism: same as ketoconazole Uses: topical only mucocutaneous Candida infections (oral, vaginal, skin, GI Precautions: best to avoid in alcoholics, individuals with liver disease

20 Drugs used exclusively for treating dermatophytic fungal infections Griseofulvin (Fulvicin ) Spectrum: Dermatophytes Mechanism: interferes with fungal mitosis, static effect Kinetics: PO for topical infections Concentrates in skin, hair, nails Eliminated by liver metabolism Uses: Infections of hair, nails, scalp

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22 Problems with treating viral infections Simple structure Dependence on host cell enzymes for replication

23 Typical virus infection cycle Synthesis of non-structural proteins RNA/DNA Synthesis Synthesis and processing of structural proteins Assembly of virions & release

24 O N N HSV thymidine kinase (infected cells) Acyclovir (Zovirax) H 2 N HO H 2 C N H C H 2 O N C H 2 Acycloguanosine-P Host enzyme Acycloguanosine-P-P-P Host enzyme Inhibition of viral DNA synthesis

25 Acyclovir (Zovirax) Mechanism: Incorporates into DNA during DNA synthesis and terminates synthesis Kinetics: PO use Short T1/2 Renal excretion unchanged Resistance: common Toxicity: Few side effects Can be nephrotoxic at high doses Use: Symptomatic relief of oral and genital HSV infection

26 Other agents related to acyclovir available for treatment of HSV Valacyclovir better oral availability Famciclovir even better oral availability much longer intracellular half life than either acyclovir or valacyclovir

27 Ganciclovir More toxic cousin of other cyclovirs Undergoes phosphorylation by CMV kinases to form triphosphate which inhibits CMV DNA polymerase Used primarily for cytomegalovirus (CMV) infections Usually IV infusion (5-9% oral availability) for 2 weeks, urinary excretion Resistance common during therapy Toxicity: Anemia, leukopenia

28 O O Foscarnet HO P C Alternative for acyclovir-resistant herpes and ganciclovir-resistant CMV Inhibits herpes virus and CMV DNA synthesis directly (no phosphorylation required) Nephrotoxicity, Ca and PO4 imbalance

29 Topicals for herpes simplex eye infections Idoxuridine cytarabine trifluorothymidine

30 Vidarabine Alternative to acyclovir, foscarnet for resistant HSV Guanosine analogue, phosphorylated by cellular kinases to inhibit DNA synthesis Topical for herpes keratitis, intravenous for severe systemic infections Low toxicity

31 Ribavirin Guanosine analogue, interferes with guanine monophosphate formation (DNA synthesis) Primary clinical use is for treatment of respiratory syncytial virus infections in neonates Major toxicity: bone marrow suppression

32 Antiretroviral agents Nucleoside analogue Protease inhibitors

33 AZT (Azidothymidine, zidovudine) Anti-HIV drug, acts by inhibiting reverse transcriptase Good oral absorption, elimination by liver metabolism Resistance common when used as monotherapy Use associated with granulocytopenia, anemia

34 Didanosine Dideoxyinosine Dideoxycytidine Mechanism: inhibits reverse transcriptase Resistance common (but no cross resistance to each other) Pancreatitis and polyneuropathy

35 Protease inhibitors HIV polypeptide Protease required for cleavage of precursor protein to release reverse transcriptase, structural proteins, integrase, and the protease itself Ritonavir, saquinavir, indinavir Resistance develops over long term

36 Amantadine, rimantidine Inhibit replication of flu virus Mechanism is through inhibition of viral uncoating following entry into cell Neurotoxicity (hallucinations, seizures)

37 Interferons Recombinant (nonglycosylated) versions of natural cytokines Three forms Mechanisms: inhibition of transcription, translation, protein processing, and virus maturation Uses: genital warts, viral hepatitis (B and C), cancers, multiple sclerosis Limitation: toxicity