Antivirals Lect 7,8 Sadia Anjum

Targets for Anti-viral therapy 1. viral attachment to cell and fusion (fusion inhibitors) 2. protein translation in infected cells (interferon) 3. protein processing (specific protease inhibitors) 4.DNA synthetic enzymes (reverse transcriptase inhibitors, DNA polymerase inhibitors) 5. DNA integrase 6. Immune system (effective vaccines, restore immune surveillance)

Targets for Anti-viral therapy

Interferons Interferons are glycoproteins that come in 3 varieties: ά (made in leukocytes) β (made in fibroblasts) γ (made in T cells) activated T cells produce g interferon to modulate the immune response induced by viral infection, IL1, IL2, TNF

Cytokine signaling pathway

Multiple Pathways of Interferon Action

Clinical use of Interferon alpha interferon FDA approved: ◦ genital warts (papillomavirus)  note: resiquimod has also been shown to be effective ◦ hepatitis B and C ◦ Kaposi’s sarcoma (HSV VIII) Toxicity ◦ fever, fatigue, marrow suppression, depression, acute influenza like symptoms ◦ about 10-20% discontinue therapy due to toxicity

Hepatitis C Chronic hepatitis C virus (HCV) infection affects 2.7 million people in the United States. Cirrhosis of the liver resulting from chronic HCV infection is the leading reason for liver transplantation in the U.S. Drug treatments such as interferon and ribavirin are not very effective HCV protease inhibitors are a promising new class of antivirals for this disease  BILN 2061 (Boehringer) looked good but appears to have cardiovascular toxicity and is on hold  VX-950 (Vertex Pharmaceuticals) is now in phase II trials and looks promising

Ribavirin A guanosine analog phosphorylated intracellularly by host enzymes inhibits capping of viral messenger RNA inhibits the viral RNA-dependent RNA polymerase inhibits replication of DNA and RNA viruses

Ribavirin antiviral HCV/Dengue

Telaprevir (VX-950) with peg-IFN looks promising in clinical trials

Anti-influenza virus drugs Amantadine, Rimantadine ◦ cyclic amines ◦ inhibit the uncoating of viral RNA therefore inhibiting replication ◦ only active against influenza A ◦ blocks the influenza M2 ion channel on endosomes and prevents passage of H+ ions required for acidification and viral uncoating ◦ mild CNS effects Zanamivir, Oseltamivir ◦ active against both influenza A and B ◦ inhibits influenza viral neuraminidase. ◦ Neuraminidase must cleave terminal sialic acid residues on receptors recognized by viral hemagglutinin. ◦ Without this cleavage, virus remains trapped on infected cells --no release of infectious particles ◦ treats uncomplicated influenza infections ◦ administered intranasally

Anti-influenza

Targets for treatment of HIV (anti-retroviral drugs

Antiretroviral Agents 1) Nucleoside Reverse Transcriptase Inhibitors (NRTIs) 2) Nonnucleoside Reverse Transcriptase Inhibitors (NNRTIs) 3)Protease inhibitors

Antivirals: Purine Nucleosides AgentAntiviral Activity guanines acyclovirHSV 1 & 2, VZV ganciclovir (DHPG)CMV retinitis and systemic CMV infection ribavirin (RTCD)Influenza types A and B, RSV, LV, HV adenosines didanosine (ddl)HIV vidarabine (Ara-A)HSV, herpes zoster

Antivirals: Pyrimidine Nucleosides AgentAntiviral Activity cytosines lamivudine (3TC)HIV zalcitabine (ddC)HIV thymine idoxuridine (IDU)HSV stavudine (d4T)HIV trifluridineHSV zidovudine (AZT)HIV

Zidovudine (AZT) RTase inhibitor A deoxythymidine analog enters the cell via passive diffusion must be converted to the triphosphate form by mammalian thymidine kinase competitively inhibits deoxythymidine triphosphate for the reverse transcriptase enzyme causes chain termination

mechanism ◦ selective reverse transcriptase inhibitor Zidovudine (AZT) RTase inhibitor

Mechanism of selective toxicity of AZT AZT concentration (uM) DNA synthesis

Zidovudine mechanism ◦ selective reverse transcriptase inhibitor pharmacokinetics: ◦ orally active, penetrates CSF toxicity: ◦ bone marrow depression (anemia, leukopenia) ◦ headache, nausea, myopathy, anorexia, fatigue therapeutic effects: ◦ increase CD4+ T cells partially restoring immune system ◦ reverses AIDS dementia resistance: major problem ◦ RTase mutations

Lamivudine (3TC) similar to zidovudine resistance develops quickly: selects for met184val mutation in RTase lamivudine + zidovudine combination dramatically slows resistance development

NNRTIs(non-nucleoside reverse transcriptase inhibitors) Nevirapine Delavirdine Efavirenz

NNRTIs Bind to site on viral reverse transcriptase, different from NRTIs results in blockade of RNA and DNA dependent DNA polymerase activity do not compete with nucleoside triphosphates do not require phosphorylation these drugs can not be given alone Nevirapine- prevents transmission of HIV from mother to newborn when given at onset of labor and to the neonate at delivery

Protease Inhibitors The protease enzyme cleaves precursor molecules to produce mature, infectious virions these agents inhibit protease and prevent the spread of infection These agents cause a syndrome of altered body fat distribution, insulin resistance, and hyperlipidemia

Indinavir and Ritonavir Specific inhibitors of the HIV-1 protease enzyme mediated by expression of multiple and variable protease amino acid substitutions Side Effects :hyperbilirubinemia Contraindications :inhibitor/substrate for CPY3A4, do not give with antifungal azoles Saquinavir A synthetic peptide-like substrate analog inhibits HIV-1 protease prevents cleavage of viral polyproteins

CCR5 and CXCR4 antagonists Vicriviroc (Schering): Oct, 2005 discontinued Phase II trial due to therapeutic failure in some pts Aplaviroc (GlaxoSmithKline): Oct, 2005 Phase III trials halted due to hepatotoxicity Pfizer still has a compound in trials

Anti-viral drugs Anti-herpes virus agents Acyclovir / Valacyclovir Famciclovir / Penciclovir Ganciclovir / Cidofovir Foscarnet Trifluridine / Idoxuridine / Vidarabine 30

Anti-viral drugs Mechanism of action of Acyclovir and congeners : All drugs are phosphorylated by a viral thymidine- kinase, then metabolized by host cell kinases to nucleotide analogs. The analog inhibits viral DNA-polymerase Only actively replicating viruses are inhibited Acyclovir is thus selectively activated in cells infected with herpes virus. Uninfected cells do not phosphorylate acyclovir. 31

Acyclovir FDA approved in 1982 structural analog of deoxyguanosine mechanism of selective toxicity  phosphorylated by viral thymidine kinase (TK)  preferential incorporation by viral DNA polymerase  causes chain termination ~20% oral bioavailability (valacyclovir is 3-5X better) penetrates CSF effective in treating ◦ primary Herpes infections (genital, encephalitis, neonatal) ◦ chronic Herpes (does not cure but can reduce recurrence)

Acyclovir mechanism of action

Resistance to acyclovir chronic therapy with acyclovir, valacyclovir, famciclovir mutations in viral TK gene ◦ alter affinity for drug or just completely inactivate the gene viral DNA polymerase mutations ◦ reduce recognition of phosphorylated drug as substrate for DNA synthesis

Ganciclovir for CMV, varicella zoster (chicken pox/shingles) similar to acyclovir 6-9% oral bioavailability penetrates CSF more active than acyclovir against CMV toxicity: bone marrow suppression, CNS (headaches, convulsion, psychosis) ◦ some toxic effects seen in about 40% of pts

Foscarnet binds to pyrophosphate site of viral polymerase (also RTase) 100 fold greater selective inhibitor or viral versus human polymerase poor oral bioavailability nephrotoxicity is high (~50%) but reversible hypocalcemia and CNS toxicity is also significant (25% pts) useful in acyclovir/ganciclovir resistant HSV or CMV

Virus Diseases Drug(s) of choice Alternative drugs FLU A Influenza Amantadine Rimantadine RSV Pneumonia, bronchiolitis Ribavirin (aerosol) HSV Genital herpes Acyclovir Foscarnet Keratitis Conjunctivitis Trifluridine Idoxuridine Vidarabine Encephalitis Acyclovir Neonatal HSV infection Acyclovir Vidarabine Herpes infections in immuno- compromised host Acyclovir Foscarnet 37

VZV In normal host No therapy In immunocompro-mised host, or during pregnancy Acyclovir Foscarnet CMV Retinitis Ganciclovir Foscarnet HIV AIDS HIV antibody positive with CD4 count < 500/mm 3 Zidovudine ± protease inhibitors Didanosine, Stavudine HBV HCV Hepatitis B, C Interferons 38