The document discusses anti-viral chemotherapy and considerations for developing effective anti-viral drugs. It notes that ideal drugs are selectively toxic to virus-infected cells, inhibit specific viral enzymes or functions, and have a high therapeutic index. Several classes of anti-viral drugs are described including nucleoside analogs that target viral polymerases, protease inhibitors for HIV, and neuraminidase inhibitors for influenza. Developing drugs that can interfere with viral replication without harming host cells remains an ongoing challenge.

1. Anti-Viral Chemotherapy Bacteria Many antibiotics Highly selective Viruses Use host cell metabolism Selectivity difficult Toxicity www.freelivedoctor.com

2. Anti-Viral Chemotherapy Key is selectivity Other problems Toxicity Rapid excretion Rapid metabolism Poor absorption www.freelivedoctor.com

3. Anti-Viral Chemotherapy Ideal Drug Water soluble Chemically and metabolically stable Easily absorbed (apolar) NOT Toxic Carcinogenic Allergenic Mutagenic Teratogenic www.freelivedoctor.com

4. Anti-Viral Chemotherapy Therapeutic index (T.I.) Minimum dose toxic to cell Minimum dose toxic to virus Effective drug: T.I. = 100-1000 at least www.freelivedoctor.com

5. Anti-Viral Chemotherapy Another consideration: Disease severity Rhinovirus v. Symptomatic rabies or Lassa fever www.freelivedoctor.com

6. Anti-Viral Chemotherapy Reasons for continuing search for anti-virals versus vaccines: For many established diseases there is still no effective vaccine Rapid mutation (retroviruses) Or there are problems with the current vaccine Reassortment (influenza) New and emerging diseases - no vaccine available Vaccine development takes many years Disease that involve immunosuppression (AIDS, cancer, transplantation) At present no drug completely suppresses viral replication (with possible exception of anti-HIV protease inhibitors) www.freelivedoctor.com

7. Anti-Viral Chemotherapy A successful drug must interfere with: A specific viral function e.g. enzyme necessary for viral life cycle A cellular function that the virus needs in order to replicate If interfere with cellular function either: It must be crucial to virus but not the cell or Only the virus-infected cell must be killed (activation of drug in the infected cell only?) www.freelivedoctor.com

8. Anti-Viral Chemotherapy Viral enzymes Nucleic acid polymerases DNA-dependent DNA polymerase - DNA viruses RNA-dependent RNA polymerase - RNA viruses RNA dependent DNA polymerase (RT) - Retroviruses Protease (retrovirus) Integrase (retrovirus) Neuraminidase (orthomyxovirus) www.freelivedoctor.com

9. Anti-Viral Chemotherapy 1962 Idoxuridine Pyrimidine analog Toxic Topical - Epithelial herpetic keratitis (cornea) 1983 Acyclovir Purine analog Sugar modification Chain terminator Anti-herpes Selective to virus-infected cells 1990’s Protease inhibitors www.freelivedoctor.com

10. www.freelivedoctor.com Binding Fusion Reverse transcription Nuclear localization Uncoating Integration Transcription Splicing RNA export Genomic RNA mRNA Translation Modification Budding Assembly Maturation Endocytosis Lysosome

11. Anti-Viral Chemotherapy Binding to surface receptor At present no effective drugs in this class CD4-Ig2 (PRO542) - A more stable version of soluble CD4 is a tetrameric fusion protein of immunoglobulin G and CD4. It can reduce levels of virus in vivo AMD3100 - appears to bind to CXCR4 (fusin) RFI-641 (biphenyl triazine) active against RSV fusion Soluble CD4 - May make HIV more infective as results in chemokine receptor being the only necessary receptor www.freelivedoctor.com

12. Anti-Viral Chemotherapy Peptides derived from gp41 can inhibit infection Probably block interaction of gp41 with cell membrane proteins during fusion T-20: In clinical trials, a nearly two log reduction in plasma HIV levels achieved T-20 FUZEON (enfuvirtide) T-1249 Next generation: Different site from T-20 Membrane Fusion www.freelivedoctor.com

13. Anti-Viral Chemotherapy Endosome low pH often necessary for membrane fusion Lysosomotropic agents Membrane Fusion www.freelivedoctor.com

14. Anti-Viral Chemotherapy Amantadine: 1966 Rimantadine: 1993 Only effective against ‘flu A (200mg/day) Marginally effective therapeutically Prophylaxis: Reduce flu by 90% Since disease usually mild and avoidable not used much here Good alternative to vaccine for: Elderly Immunocompromized Allergic Where causative strain not the vaccine strain Affects M2 protein involved in activation of virus before release from cell Amantadine Rimantadine www.freelivedoctor.com

15. ADAMANTANES Rapid emergence to amantadine and rimantadine because of M2 point mutation CDC Because of circulating resistant influenza virus strains, the use of Amantadine and Rimantadine is currently (2007) NOT recommended In the 2005-2006 influenza season, 92% of H3N2 influenza isolates had M2 mutation that would make them resistant www.freelivedoctor.com

16. Uncoating of core of membrane and non-membrane viruses Uncoating of picornavirus e.g. polio, echo, rhino Stabilize coated virus? Anti-Viral Chemotherapy Arildone www.freelivedoctor.com

17. Anti-Viral Chemotherapy N O O O N CH3 WIN 71711 (Disoxaril) Stabilizes picornaviruses - coated virus remains in cytoplasm 3-methylisoxazole group inserts in capsid VP1 and covers ion channel Similar mechanism: Pleconaril Good bioavailability – readily absorbed Crosses blood-brain barrier Old formulation worked poorly (oral) New formulation (nasal spray) Marketed (2004) by Schering-Plough www.freelivedoctor.com 3-methyl isoxazole group

18. Anti-Viral Chemotherapy Human rhinovirus with WIN drug – embeds in VP1 VP1 VP1 www.freelivedoctor.com

19. Anti-Viral Chemotherapy Nucleic Acid Synthesis Polymerases are often virally encoded Other enzymes in nucleic acid synthesis e.g. THYMIDINE KINASE in Herpes Simplex www.freelivedoctor.com

20. Thymidine Kinase Deoxy-thymidine Deoxy-thymidine triphosphate Viral or cellular thymidine kinase adds first phosphate PO 4 PO 4 PO 4 Cellular kinases add two more phosphates to form TTP Anti-Viral Chemotherapy www.freelivedoctor.com

21. Anti-Viral Chemotherapy Why does Herpes simplex code for its own thymidine kinase? TK- virus cannot grow in neural cells because they are not proliferating (not making DNA) Although purine/pyrimidines are present, levels of phosphorylated nucleosides are low Allows virus to grow in cells that are not making DNA “ Thymidine kinase” is a misnomer Deoxynucleoside kinase NON-SPECIFIC www.freelivedoctor.com

22. Anti-Viral Chemotherapy Herpes thymidine kinase will phosphorylate any deoxynucleoside including drugs – as a result of its necessary non-specificity Nucleoside analog may be given in non-phosphorylated form Gets drugs across membrane Allows selectivity as only infected cell has enzyme to phosphorylate the drug ACG P P P Cellular TK (where expressed) does not phosphorylate (activate) the drug www.freelivedoctor.com

23. Anti-Viral Chemotherapy Need for activation restricts drug to: Viruses such as HSV that code for own thymidine kinase Virus such as cytomegalovirus and Epstein-Barr virus that induce cells to overproduce their own thymidine kinase In either case it is the VIRUS-INFECTED cell that activates the drug www.freelivedoctor.com

24. Anti-Viral Chemotherapy Thymidine kinase activates drug but phosphorylated drug inhibits the polymerase Nucleotide analogs Sugar modifications Base modifications Selectivity Viral thymidine kinase better activator Cellular enzyme may not be present in non-proliferating cells Activated drug is more active against viral DNA polymerase that against cell polymerase www.freelivedoctor.com

25. Anti-Viral Chemotherapy Guanine analogs Acyclovir = acycloguanosine = Zovirax Ganciclovir = Cytovene Activated by viral TK Activated ACV is better (10x) inhibitor of viral DNA polymerase than inhibitor of cell DNA polymerase Excellent anti-herpes drug Acyclovir Ganciclovir www.freelivedoctor.com

26. Anti-Viral Chemotherapy Acyclovir: Chain terminator Good anti-herpes drug Normal DNA synthesis www.freelivedoctor.com T P P G P C P A

27. Termination Also inhibits: Epstein Barr Cytomegalovirus Acyclovir: Chain terminator Selective: Virus phosphorylates drug Polymerase more sensitive Anti-Viral Chemotherapy www.freelivedoctor.com T P P G P C P A P A ACG P-P-P

28. Anti-Viral Chemotherapy Acyclovir very effective against: Herpes simplex keratitis (topical) Latent HSV (iv) Fever blisters – Herpes labialis (topical) Genital herpes (topical, oral, iv) Resistant mutants in thymidine kinase or DNA polymerase Appears not to be teratogenic or carcinogenic Ganciclovir very effective against cytomegalovirus – viral DNA polymerase is very sensitive to drug activated by cell TK www.freelivedoctor.com

29. Anti-Viral Chemotherapy Adenine arabinoside (Ara-A) Problems : Severe side effects Resistant mutants (altered polymerase) Chromosome breaks (mutagenic) Tumorigenic in rats Teratogenic in rabbits Insoluble Use: topical applications in ocular herpes simplex Competitive inhibitor of virus DNA polymerase which is much more sensitive than host polymerase www.freelivedoctor.com

30. Anti-Viral Chemotherapy Adenine arabinoside HSV encephalitis Neonatal herpes Disseminated herpes zoster Hepatitis B Poor in vivo efficacy: DEAMINATION www.freelivedoctor.com

31. Anti-Viral Chemotherapy Other sugar modifications: AZT azidothymidine DDI dideoxyinosine DDC dideoxycytidine www.freelivedoctor.com

32. Anti-Viral Chemotherapy Base change analogs Altered base pairing Mutant DNA Resistant mutants Trifluorouridine Viroptic anti-HSV Idoxuridine www.freelivedoctor.com

33. Anti-Viral Chemotherapy Fluoroiodo aracytosine has both a base and a sugar alteration www.freelivedoctor.com

34. Anti-Viral Chemotherapy Cidofovir Cidofovir: DNA chain terminator DNA polymerase inhibitor A acyclic nucleoside phosphonate (not a phosphate) - C-O-P bond in a nucleoside monophosphate replaced by a phosphonate (C-P) More stable www.freelivedoctor.com

35. Anti-Viral Chemotherapy Cidofovir Indicated: CMV retinitis May be useful: Pox: molluscum contagiosum virus Polyoma: JC - progressive multifocal leukoencephalopathy Adenovirus - gastroenteritis Vaccinia in immunocompromisation? Used in on case: Eczema vaccinatum www.freelivedoctor.com

36. Anti-Viral Chemotherapy Cidofovir www.freelivedoctor.com

37. Prodrugs e.g. Famciclovir Taken orally Converted by patient’s metabolism HSV thymidine kinase P Host kinase P P Penciclovir: Available as topical cream Glaxo-SmithKlein www.freelivedoctor.com

38. Anti-Viral Chemotherapy Non-nucleoside Non-competitive RT inhibitors Combination therapy with AZT Resistance mutations will be at different sites The most potent and selective RT inhibitors Nanomolar range Minimal toxicity (T.I. 10,000-100,000) Synergistic with nucleoside analogs (AZT) Good bio-availability Resistant mutants - little use in monotherapy www.freelivedoctor.com

39. Anti-Viral Chemotherapy Sustiva (S) -6- chloro-4-(cyclopropylethynyl)-1,4-dihydro-4-(trifluoromethyl)-2H-3, 1-benzoxazin-2-one. DuPont Nevirapine www.freelivedoctor.com

40. Anti-Viral Chemotherapy Nevirapine: Approved for AIDS patients Good blocker of mother to child transmission peri-natal - breast feeding Single dose at delivery reduced HIV transmission by 50% Single dose to baby by 72 hours Efavirenz (Sustiva, DMP266) In combination therapy will suppress viral load as well as HAART and may be better – Approved for AIDS patients www.freelivedoctor.com

41. Anti-Viral Chemotherapy www.freelivedoctor.com Phosphono acetic acid (PAA) Phosphono formic acid O O HO P C OH Binds pyrophosphate site of polymerase Competitive inhibitor 10 -100x greater inhibition of herpes polymerase Toxic: accumulates in bones, nephrotoxicity Rapid resistance Clinical trial: CMV in AIDS patients

42. Anti-Viral Chemotherapy Ribavirin Guanosine analog Non-competitive inhibitor of RNA polymerase in vitro Little effect on ‘flu in vitro Often good in animals but poor in humans Aerosol use: respiratory syncytial virus i.v./oral: reduces mortality in Lassa fever, Korean and Argentine hemorrhagic fever www.freelivedoctor.com

43. Anti-Viral Chemotherapy May induce mutations in RNA viruses www.freelivedoctor.com May inhibit RNA cap formation N N N HN H 2 N O CH 3 O CH 2 P P CH 2 O CH 3 P base

44. Anti-Viral Chemotherapy Viral Protein synthesis: No inhibitors www.freelivedoctor.com

45. Anti-Viral Chemotherapy Protein processing: Proteolysis Glycosylation www.freelivedoctor.com

46. Anti-Viral Chemotherapy GAG Integrase Polymerase Protease GAG/POL polyprotein Retrovirus --- HIV www.freelivedoctor.com

47. Anti-Viral Chemotherapy GAG Integrase Polymerase Protease folds and cuts itself free www.freelivedoctor.com

48. Anti-Viral Chemotherapy GAG Integrase Polymerase Protease cuts at a site between the integrase and polymerase www.freelivedoctor.com

49. Anti-Viral Chemotherapy GAG Integrase polymerase www.freelivedoctor.com

50. Anti-Viral Chemotherapy Saquinavir www.freelivedoctor.com

51. Anti-Viral Chemotherapy HIV aspartyl protease inhibitors Indinavir+AZT+3TC www.freelivedoctor.com

52. Anti-Viral Chemotherapy Indinavir (Merke) INDINAVIR + AZT + 3TC (HAART) No detectable HIV by PCR Before: 20,000 - 11,000,000 RNA copies /ml After: < 200-400 copies Lasts several years No replication No resistance www.freelivedoctor.com

53. Anti-Viral Chemotherapy Integrase Inhibitor Isentress Approved for use in adults by the USFDA in October, 2007 www.freelivedoctor.com

54. Anti-Viral Chemotherapy Influenza Requires neuraminidase to escape from cell Requires neuraminidase to penetrate mucus Zanamivir - RELENZA (fall 1997) Neuraminidase inhibitor Active against Influenza A and Influenza B www.freelivedoctor.com

55. Relenza Tamiflu (oseltamivir) www.freelivedoctor.com

56. Anti-Viral Chemotherapy After neuraminidase inhibition, ‘flu hemagglutinin binds to sialic acid on other virus particles: virus clumps OR virus sticks to mucous in respiratory tract www.freelivedoctor.com

57. Neuraminidase of virus removes sialic acid from cell surface thereby releasing virus www.freelivedoctor.com

58. Virus hemagglutinin sticks new virus particle to sialic acid on cell surface Virus cannot escape from infected cell after neuraminidase inhibition www.freelivedoctor.com

59. Anti-Viral Chemotherapy Zanamivir - Relenza Neuraminidase inhibitor Nasal spray Shortens symptoms by a few days Tamiflu: Oral neuraminidase inhibitor www.freelivedoctor.com

60. NEURAMINDIASE INHIBITORS TREATMENT Oseltamivir is approved for treatment of persons aged > 1 year Zanamivir is approved for treatment of persons aged > 7 years PROPHYLAXIS Oseltamivir and zanamivir can be used for chemoprophylaxis of influenza Oseltamivir is licensed for use in persons aged > 1 year Zanamivir is licensed for use in persons aged > 5 years Anti-Viral Chemotherapy www.freelivedoctor.com

61. NEURAMINIDASE INHIBITORS Development of viral resistance to zanamivir and oseltamivir during treatment has been identified No transmission of neuraminidase inhibitor-resistant viruses in humans has been documented to date Data are limited concerning the effectiveness of zanamivir and oseltamivir for treatment of influenza among persons at high risk for serious complications of influenza Anti-Viral Chemotherapy Among influenza virus- infected participants in 10 clinical trials, the risk for pneumonia among those participants receiving oseltamivir was approximately 50% lower than among those persons receiving a placebo www.freelivedoctor.com