Chemotherapy of Viral Infection (PHCL 402)

1. Chemotherapy of Viral Infection (PHCL 402) ANAFI, Sherifat Bola, PhD Department of Pharmacology & Therapeutics Ahmadu Bello University, Zaria PHCL 402

2. Topics to be Discussed • Antiviral agents • Antiretroviral Agents • Fluoroquinolones • Macrolides • Miscellaneous Agents PHCL 402

3. Drug Treatment of Viral Infections PHCL 402

4. Introduction • Viruses are obligate intracellular parasites; • Their replication depends primarily on synthetic processes of the host cell. • For an antiviral agent to be effective, it must either block viral entry into or exist from the cell or be active inside the host cell. PHCL 402

5. Introduction…. • Progress in antiviral chemotherapy began in the early 1950s • When the search for anticancer drugs generated several new compounds capable of inhibiting viral DNA synthesis. • The two first-generation antivirals, 5-iododeoxyuridine and trifluorothymidine, had poor specificity (i.e. they inhibited host cell DNA as well as viral DNA) • that rendered them too toxic for systemic use • However, both agents are effective when used topically for the treatment of herpes keratitis. PHCL 402

6. Introduction…. • Recent research has focused on the identification of agents with greater selectivity, in vivo stability and lack of toxicity. • Chemoprophylaxis or early initiation of therapy may be key because replication of the virus peaks at or before the manifestation of clinical symptoms in many viral infections. PHCL 402

7. Steps in Viral replication • Attachment of the virus to the host cell • Entry of the virus through the host cell membrane • Uncoating of the viral nucleic acid • Synthesis of early regulatory proteins e.g. nucleic acid polymerases • Synthesis of RNA or DNA • Synthesis of late, structural proteins • Assembly (maturation) of viral particles - & - • Release from the cell Note: Antiviral agents can potentially target any of these steps. • In summary: Adsorption – Penetration – uncoating – Synthesis of materials – Maturation – Release PHCL 402

8. Antiviral Agents • Agents to Treat Herpes Simplex Virus (HSV) and Varicella-Zoster Virus (VZV) Infections • Agents to Treat Cytomegalovirus (CMV) Infections • Antiretroviral Agents • Antihepatitis Agents • Anti-influenza Agents • Other antiviral Agents PHCL 402

9. The Major Sites of Antiviral Drug Action PHCL 402

10. Agents to Treat Herpes Simplex Virus (HSV) and Varicella-Zoster Virus (VZV) Infections • Three oral nucleoside analogs licensed for the treatment of HSV and VZV infections are: acyclovir, valacyclovir & famciclovir • They have similar mechanism of action and similar indications for clinical use • All are well tolerated PHCL 402

11. Acyclovir (Nucleoside Analogue) • Acyclovir is an acyclic guanosine derivative with clinical activity against HSV-1, HSV-2 and VZV Mechanism of Action • Acyclovir requires three phosphorylation steps for activation • It is converted first to the monophosphate derivative by the virus-specified thymidinekinase and then to the di- and triphosphate compounds by host cell enzymes. • Because it requires the viral kinase for initial phosphorylation, acyclovir is selectively activated, and the active metabolite accumulates, only in infected cells. PHCL 402

12. Acyclovir (Nucleoside Analogue) • Acyclovir triphosphate inhibits viral DNA synthesis by two mechanisms: • Competition with deoxyGTP for the viral DNA polymerase, resulting in binding to the DNA template as an irreversible complex • Chain termination following incorporation into the viral DNA PHCL 402

13. Figure I: Mechanism of Action of Antiherpes Agents Virus specified enzymes (e.g. thymidine kinase, UL97) • Nucleoside monophosphate Acyclovir, Penciclovir Ganciclovir, Trifluridine Host kinases Cidofovir FoscarnetDiphosphate Incorporation into viral DNA Triphosphate  Chain termination Competitive inhibition of viral DNA polymerase Inhibition of viral DNA synthesis PHCL 402

14. Pharmacokinetics of Acyclovir • The bioavailability of oral acyclovir is 15-20% • It is unaffected by food i.e. after administration of a standard dose of 200 mg dose orally & may be dose dependent. • An intravenous formulation is available, topical formulation produce high concentrations in herpetic lesions, but systemic concentrations are undetectable by this route • Acyclovir is cleared primarily by glomerular filtration and tubular secretion PHCL 402

15. Pharmacokinetics of Acyclovir… • The mean elimination half-life is approximately 3 hours in patients with normal renal function & 20 hours in patients with anuria • It crosses the blood brain barrier producing a CSF concentration that is approximately 50% of that in plasma • Clearance is largely renal and includes an element of tubular secretion; renal impairment requires dose schedule adjustment. PHCL 402

16. Uses of Acyclovir • Acyclovir is effective against herpes (simplex & zoster), but much less active against cytomegalovirus (also herpes virus). • Acyclovir accelerates healing in herpetic keratitis • The efficacy of topical acyclovir in genital and labial herpes simplex has been unimpressive • Acyclovir given orally accelerates healing in genital herpes. PHCL 402

17. Uses of Acyclovir…. • It does not eliminate vaginal carriage, so caesarian section is indicated to avoid neonatal herpes. • Treatment of shingles (herpes zoster) should be started within 72 hours of the onset and is useful for patients with severe pain, although it shortens the illness only modestly • In generalized herpes simplex or herpatic meningoencephalitis, acyclovir is given intravenously PHCL 402

18. Uses of Acyclovir…. • Used in chickenpox: oral acyclovir 400 mg 4 times a day for 7 days given during the incubation period may abort chickenpox in susceptible contacts PHCL 402

19. Adverse Effects ofAcyclovir • A reversible rise in plasma urea and creatinine • Neurological disturbance e.g. tremors, delirium, seizures • Rash • Nausea and vomiting • Hepatitis • Headache • Stinging and burning sensation after each application when used topically PHCL 402

20. Agents to Treat Cytomegalovirus (CMV) Infections • CMV infections occur primarily in the setting of advanced immunosuppression and are typically due to reactivation of latent infection • E.g. Foscarnet PHCL 402

21. Foscarnet • Foscarnet is active against several important viruses notably HIV-1 and all human herpes viruses including acyclovir-resistant herpes viruses and cytomegalovirus (CMV). • It is used to treat CMV infections (retinitis, pneumonitis, colitis & oesophagitis) and acyclovir-resistant herpes simplex virus (HSV) infections in immunocompetent and immunosuppressed hosts. • It is given intravenously as loading dose followed by infusions. PHCL 402

22. Mechanism of Action • Foscarnet (phosphonoformic acid) is an inorganic pyrophosphate compound and a nucleotide analogue that acts as a non-competitive inhibitor of viral DNA polymerase and inhibits the reverse transcriptase from several retroviruses. • It is inactive against eukaryotic DNA polymerases at concentrations that inhibit viral DNA replication. PHCL 402

23. Pharmacokinetics of Foscarnet • Foscarnet is poorly absorbed (2-5%) after oral administration • Plasma concentrations decay in a triphasic manner and the terminal t1/2 is 18 hours • Excreted renaly by glomerular filtration & tubular excretion • 20% remains in the body bound in bone or 30% of the drug may be deposited in bone with a half life of several months PHCL 402

24. Adverse Effects • Nephrotoxicity • CNS effects e.g. irritability, anxiety, fits • Nausea, vomiting & headache • Thrombophlebitis • Hypocalcaemia & hypomagnasemia • Hypoglycaemia (rare) PHCL 402

25. (2) Ganciclovir • Ganciclovir is used to treat sight or life threatening CMV infections in immunocompromised hosts. • It also has potent activity against herpes viruses 1 & 2 and is used to treat acyclovir-resistant herpes. • Valgancyclovir is the L-valy ester prodrug of gancyclovir used orally twice daily for initial control & suppression of CMV retinitis. PHCL 402

26. Mechanism of Action • Ganciclovir is an acyclic guanosine analog that requires activation by triphosphorylation before inhibiting the viral DNA polymerase. • Initial phosphorylation is catalyzed by the virus-specified protein kinase phosphotransferase UL 97 in CMV-infected cells. • The activated compound competitively inhibits viral DNA polymerase & causes termination of viral DNA elongation PHCL 402

27. Adverse Effects of Ganciclovir • Neutropenia and bone marrow suppression (cell counts usually return to normal within two to five days of discontinuing the drug) • Temporary or possibly permanent inhibition of spermatogenesis or oogenesis • Phlebitis & pain at intravenous infusion site • Rashes & fever • GIT upsets • Transient increase in liver enzymes & serum creatinine in under-hydrated patients PHCL 402

28. Pharmacokinetics • May be administered intravenously, orally or via intraocular implant • Cerebrospinal fluid concentration are approximately 50% of those in serum • The elimination half life is 4 hours with normal renal function & intracellular half life is 18 hours • Clearance of the drug is linearly related to creatinine clearance • Readily cleared by hemodialysis • Its oral bioavailability is poor PHCL 402

29. Antiretroviral Agents • Substantial advances have been made in antiretroviral therapy since the introduction of the first agent, zidovudine in 1987 • Many ARVs (combination therapies) with maximally efficacious & potent agents that will reduce viral replication to the lowest possible level & decrease the likelihood of emergence of resistance are now available. PHCL 402

30. Antiretroviral Agents • Administration of highly active antiretroviral therapy (HAART) typically comprising a combination of 3-4 ARV has become the standard of care • Such regimens may be composed of the 4 classes of antiretroviral agents: 1. Nucleoside/nucleotide reverse transcriptase inhibitors 2. Nonnucleoside reverse transcriptase inhibitors 3. Protease inhibitors 4. Fusion inhibitors 5. Integrase inhibitors: Raltegravir PHCL 402

31. 1. Nucleoside/nucleotide Reverse Transcriptase Inhibitors • The NRTIs act by competitive inhibition of HIV-1 reverse transcriptase & can also be incorporated into the growing viral load DNA chair to cause termination. • Each requires intracytoplasmic activation via phosphorylation by cellular enzymes to the triphosphate form. • Most have activity against HIV-2 as well as HIV-1. • Nucleoside analogs may be associated with mitochondrial toxicity probably owing to inhibition of mitochondrial DNA polymerase gamma, PHCL 402

32. Nucleoside/nucleotide Reverse Transcriptase Inhibitors…. • They can increase the risk of lactic acidosis with hepatic steatosis, which may be fatal, as well as disorders of lipid metabolism. • NRTI treatment should be suspended in the setting of rapidly rising aminotransferase levels, progressive hepatomegally or metabolic acidosis of unknown cause, e.g.: • Zidovudine, didanosine, abacavir, stavudine, emtricitabine, lamivudine, zalcitabine and tenofovir PHCL 402

33. Zidovudine (AZT) • This is a thymidine analogue • After phosphorylation in host cell, zidovudine triphosphate selectively inhibits viral reverse transcriptase in preference to cellular DNA polymerase • Resistance to this drug occurs by point mutations which alter reverse transcriptase enzyme • Oral absorption is rapid but bioavailability is 65% • It crosses placental and is found in milk PHCL 402

34. Zidovudine (AZT)…. • Anaemia and neutropenia are the most important and dose-related adverse effects • Paracetamol increases AZT toxicity probably by competing for glucuronidation • Azole antifungals also inhibit AZT metabolism • Stavudine and zidovudine exhibit mutual antagonism by competing for the same activation pathway • AZT along with 2 other ARV drugs is the standard choice for post-exposure prophylaxis of HIV, as well as for mother to offspring transmission PHCL 402

35. Abacavir (ABC): • Abacavir is a guanosine analogs that is well absorbed following oral administration (83%) & unaffected by food. • The elimination half life is 1.5 hours, • The intracellular half life ranges from 12 to 26 hours PHCL 402

36. Didanosine (ddl) • Didanosine is a synthetic analog of deoxyadenosine. • Oral bioavailability is 30-40%, dosing on an empty stomach is required. • Elimination half life is 1.5 hours but intracellular half life is 20-24 hours. • The major clinical toxicity associated with didanosine therapy is dose-dependent pancreatitis. • It decreases absorption of drugs requiring acid Ph e.g. itraconazole PHCL 402

37. Emtricitabine • Emtricitabine is a fluorinated analog of lamivudine with a long intracellular half life >39 hours, allowing for once daily dosing. • It is one of the least toxic of NRTIs: skin pigmentation, hepatitis, pancreatitis. • It is well absorbed PHCL 402

38. Non nucleoside Reverse Transcriptase Inhibitors (NNRTIs) • E.g. Delavirdine, Nevirapine and Efavirenz • The NRTIs bind directly to HIV-1 reverse transcriptase, resulting in blockade of RNA and DNA-dependent DNA polymerase. • The binding site of NNRTIs is near to but distinct from that of NRTIs • Unlike the NRTI agents, NNRTIs neither compete with nucleoside triphosphates nor require phosphorylation to be active • They are non-competitive inhibitors PHCL 402

39. Delavirdine • Delavirdine has an oral bioavailability of about 85% but this is reduced by antacids or H2-blockers • It is extensively metabolized to inactive metabolites by the CYP3A & CYP2D6 enzymes & also inhibits CYP3A4 & 2C9. • Therefore, there are numerous potential drug-drug interactions to be considered • Because of this effect, however, co-administration with delarvirdine may allow indinavir or saquinavir to be dosed twice daily instead of the usual 3 times a day. PHCL 402

40. Nevirapine (NVP) • Oral bioavailability is excellent (-90%) & not food dependent • Highly lipophilic & achieves cerebrospinal fluid levels that are 45% of those in plasma. • A single dose of nevirapine (200 mg) has been shown to be effective in the prevention of transmission of HIV from mother to new born when administered to women at the onset of labour & followed by a 2 mg/kg oral dose to the neonate within 3 days after delivery. • However, resistance has been documented after this single dose PHCL 402

41. Nevirapine…. • Severe skin rashes have been reported including steven-Jonhson syndrome & toxic epidermal necrolysis • NVP is hepatotoxic, therefore should not be used in patients with hepatic dysfunction PHCL 402

42. Protease Inhibitors (PIs) • Compounds in this class include amprenavir, ritonavir, indinavir, lopinavir, nelfinavir, saquinavir, atazanavir & tipranavir • They cause a rapid and marked reduction of HIV-1 replication • It acts at a late step in HIV replication, i.e. maturation of the new virus particles when the RNA genome acquires the core proteins and enzymes PHCL 402

43. Protease Inhibitors (PIs)… • They bind to the active site of protease molecule • Interfere with its cleaving function and are more effective viral inhibitors than AZT • Because they act at a late step of viral cycle, they are effective in both newly as well as chronically infected cells • Under their influence, HIV-infected cells produce immature noninfectious viral progeny, hence prevent further rounds of infection PHCL 402

44. Protease Inhibitors (PIs)…. • Their plasma t1/2 ranges from 2-8 hours • All are extensively metabolized mainly by CYP3A4 except nelfinavir • All PIs especially ritonavir and lopinavir are potent inhibitors of CYP3A4 • It interact with many drugs PHCL 402

45. Protease Inhibitors (PIs)…. • Monotherapy with one of these drugs in previously AZT treated patients reduced HIV viral levels, increased CD4 cell count and improved the clinical condition • However, viral resistance developed against the PIs over months due to selection of resistant mutants in a stepwise manner • Combination of NRTIs with PIs have been found more effective than either drug given alone –and- • Triple therapy is more effective than double therapy PHCL 402

46. Protease Inhibitors (PIs)…. • Current recommendations are to use a PI in combination with either two NRTIs or one NRTI plus one NNRTI • PIs are avoided in 1st line regimens because their use in initial regimens restricts second line regimen options (reserved for failure case) PHCL 402

47. Fusion Inhibitors • E.g. Enfuvirtide • Enfuvirtide is the first representative of this class of antiretroviral agents • It blocks the interaction between the HIV-gp41 protein & the host cell membrane by binding to a hydrophobic groove in the N36 region of gp41 (HIV-1 envelop transmembrane glycoprotein gp41 which is involved in fusion of viral and cellular membranes) • Fusion of the two membranes is thus prevented and entry of the virus into the cell is blocked PHCL 402

48. Fusion Inhibitors…. • Resistance to enfuvirtide can arise by mutations in its gp41 binding site • It has no cross resistance with other classes of ARV drugs • The injections are painful and cause local nodules/cysts PHCL 402

49. HIV Therapeutic Regimens • Choice of regimen has to be made on the basis of efficacy, durability, tolerability, convenience, drug interactions, impact on future options and cost • First line Antiretroviral Regimens: 2 NRTIs + 1 NNRTI • Preferred: lamivudine + Zidovudine + Nevirapine • Alternative: • lamivudine + zidovudine + efavirenz • Lamivudine + stavudine + efavirenz • Lamivudine + stavudine + nevirapine PHCL 402

50. HIV Therapeutic Regimens… Second line: NRTI components • tenofovir+ Abacavir • Didnosine + Abacavir • Tenofovir + Zidovudineetc • PI component: • Lopinavir • Atazanavir • Saquinavir • Indinaviretc PHCL 402