ANTIVIRALS, INTERFERON AND VACCINES EDWARD-BENGIE L. MAGSOMBOL, MD, FPCP, FPCC, DASNC Associate Professor in Microbiology

Antivirals APPROACH TO ANTIVIRAL CHEMOTHERAPY 1. Adsorption, Penetration and Uncoating = little is known about the specific reactions involved = only amantadine, rimantadine used vs influenza A = HIV, rhino, EBV now being researched on 2. Replication of Viral Nucleic Acids = attack enzymes which catalyze replication = not present in uninfected cells = all RNA viruses, pox, herpes and adenovirus

Antivirals APPROACH TO ANTIVIRAL CHEMOTHERAPY 3. Integration of Viral Genomes into Cellular Genomes = as part of multiplication cycle (retrovirus and its integrase) = tumorigenesis (papovavirus, herpes) 4. Synthesis of Viral Messenger RNA’s = virus-encoded RNA polymerases, capping enzymes

Antivirals APPROACH TO ANTIVIRAL CHEMOTHERAPY 5. Synthesis of Viral Proteins = viral mRNA translation different from host mRNA 6. Viral Morphogenesis = enzymes which cleave precursors for viral capsid CHONs = ex. viral proteases

Antivirals CLASSES OF ANTIVIRAL AGENTS Synthetic Antiviral Agents I. Analogues of Ribonucleosides and Deoxyribonucleosides = nucleic acids base or derivatives = included into nucleic acid, usually DNA = interfere with nucleic acid function = selectively inhibit viral polymerases A. Idoxuridine and Trifluorothymidine = analogues of thymidine, inhibits viral DNA formation = inhibit multiplication of herpesviruses = used for topical treatment of herpes simplex keratitis = not used for systemic use because of toxicity

Herpes keratoconjunctivitis

Herpes simplex

Herpes simplex

Herpes simplex

Antivirals B. Vidarabine (Adenosine arabinoside, Ara-A) = inhibits HSV and VZV multiplication = act as chain terminators; inhibit viral DNA polymerase more than host DNA polymerase = herpes simplex keratitis; herpes simplex encephalitis (IV route) C. Acyclovir = guanine linked to an open ring analogue of ribose, deoxyribose = thymine or cytosine derivative = phosphorylated by HSV and VZV TKinases = topical or IV in mucocutaneous herpes simplex in immunocompromised hosts and also in genital herpes simplex infections

CHICKENPOX (VARICELLA)

Antivirals D. Ganciclovir = close relative of acyclovir; inhibits HSV multiplication = better substrate for HSV TK than acyclovir = best inhibitor of CMV multiplication in use = probably not a strict chain terminator unlike acyclovir E. Zidovudine (Azidothymidine, AZT, Retrovir) = inhibits retrovirus reverse transcriptase = chain terminator because it does not possess a 3’-OH group = demonstrated clinical efficacy in HIV

CYTOMEGALOVIRUS

AIDS

Antivirals F. Ribavirin (Virazole) = analogue of purine precursor of 5-aminoimidazole 4-carboxamide = wide spectrum: good vs RNA and DNA viruses = target: virus-encoded nucleic acid polymerases = affects elongation and initiation (less extent) = for severe RSV infection (aerosol) in children = reduce mortality on patients with Lassa fever

RSV infection (bronchiolitis)

RESPIRATORY SYNCYTIAL VIRUS

Antivirals Others = analogues of thymidine (BVdU) and cytosine (FIAC) - good vs herpesvirus DNA polymerases with low toxicity = 2’, 3’-dideoxynucleosides act as chain terminators in retrovirus infections including HIV = phosphonoformic acid (foscarnet) and phosphonoacetic acid (PAA) – potent highly specific inhibitors of HSV DNA. = toxic to bones and kidney

Antivirals Others methyl phosphonate derivative (s)-HPMPA = inhibits DNA viruses ex. herpes, pox, adeno and retro PMEA- for retrovirus, HIV and tumor formation

Antivirals Amantadine and Rimantadine = effective inhibitors of influenza A multiplication = affects penetration and uncoating = also inhibits budding and virus particle release = FDA approved for prophylaxis vs influenza A = CNS side effects worse for amantadine than rimantadine = useful for elderlies, immunocompromised, allergies and in epidemics

Antivirals Other Antiviral Agents Isatin-B-thiosemicarbazone = very potent inhibitor of Poxvirus = at 3 mg/L – inhibits vaccinia multiplication (90%) = inhibits translation of late mRNA –> no viral capsid and CHON synthesis -> no progeny Marburan (n-methyl-IBT) – a derivative of IBT = beneficial effects for smallpox contacts

SMALLPOX

Antivirals 2-Hydroxylbenzylbenzimidazole (HBB) and Guanidine = PICORNAVIRUSES (polio, echo, coxsackie and FMD/enteroviruses) = interfere with replication of viral RNA = prevent the initiation of the synthesis of progeny (+) strands by inhibiting protein 2C

Antivirals Rifampicin and Rifamycin derivatives = binds to bacterial RNA polymerase = prevent initiation of transcription = no binding to animal RNA polymerase = inhibit multiplication of pox and adeno = both early and late mRNAs are transcribed normally (viral polymerase not inhibited) = accumulation of immature virus particles that lack the normal dense spicule layer

Antivirals Arildone, Rhodanine, and WIN 51711 = inhibit uncoating of Picornaviruses by making the virus more stable = does not affect absorption or penetration

Antivirals Inhibitors of Proteases = precursors do not become the functional proteins = HIV protease: essential role in production of a functional virion = Saquinavir, indinavir, ritonavir, nelfinavir, amprenavir – slip into the hydrophobic active site of the enzyme = combine with AZT and a 2nd nucleoside analogue in tx of AIDS

Antivirals Promising New Approaches Inhibition of Adsorption = many viral receptors have been identified Targeted Introduction of Toxins into Infected Cells = directed against infected cells = ricin or the Pseudomonas exotoxin to CD4--- attach to gp120 --- internalized into infected cell

Antivirals Introduction into Cells of Specific Anti-Sense RNA Sequences = many mRNA splice junctions have been sequenced Preventing Interactions Among Protein Molecules = add excess oligopeptides with the same sequence as that of the interacting sequence

Interferons natural antiviral compounds substances that have antiviral properties in adjacent, noninfected cells Types of Interferons Type I: (1) Interferon alpha = maximal antiviral activity (2) Interferon Beta = intermediate Type II: Interferon Gamma = more lymphokine than antiviral

Interferons Regulation of Interferon Expression = not expressed in a normal resting cell = labile repressors bind to promoter elements, block transcription = production of labile suppressors drop in viral infection and allows interferon synthesis to occur

Interferons Mechanism of action = synthesis, secretion, diffusion and binding to cellular receptors = taken up by uninfected cells = viral replication (-) via cellular enzymes Type I = (-) viral protein synthesis (very specific) = 2 enzymes activated: 1. oligo-A synthetase  adenine nucleotide  viral mRNA digestion 2. protein kinase ->phosphorylates EF-2 -> blocks CHON synthesis = block other stages of replication including budding

Interferons Type II : = antiviral effects mediated by: 1. nitric oxide synthetase—increased intracellular nitric oxide levels 2. upregulation of MHC I and II expression 3. activation of monocytes, macrophages and NK cells

Interferon

Interferons Clinical Uses: IFN-A : = treatment of viral infections: condylomata acuminata and chronic hepa B and C = prophylactic or therapeutic agent in immunocomp. hosts (VZV, HSV 1 and 2) = prophylaxis vs CMV in renal transplant = treatment of AIDS-associated Kaposi’s sarcoma and hairy cell leukemia IFN-G: immunostimulant in oncologic and immunedeficiency disorders

Vaccines TYPES OF VACCINES: 1. Inactivated Virus Vaccines = complete inactivation of infectivity with minimum loss of antigenicity = ex. a. UV irradiation b. photodynamic inactivation and white light irradiation c. beta-propiolactone d. formaldehyde (most effective)

Vaccines 2. Attenuated Active Virus Vaccines = Jenner’s smallpox , Theiler’s yellow fever virus, Sabin poliovirus, MMR, adenovirus = repeated passage of human pathogens in other host species = effective in small amounts: amplification effect = recombinant DNA technology has improved attenuation

POLIO VACCINE

MEASLES

Vaccines 3. Subunit Vaccines = viral proteins that elicit formation of neutralizing Ab’s = smaller range of Ab’s (IgA, IgM) produced = genes of these CHONs now can be cloned

Vaccines 4. Viral Vectors = genes of viral CHONs inserted into avirulent viral vectors = thymidine kinase gene of Vaccinia virus = genes are expressed without disease and Ab’s are produced = HA gene of influenza, glycoprotein B gene of herpesvirus, surface Ag of HBV = major limitation is the infectivity of vaccinia itself

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