Agents in the therapy of DNA and RNA Viruses Dr. Thomas Abraham Spring 2004

Overview of Viral Biochemistry Viruses are intracellular parasites composed of nucleic acids (RNA or DNA) and enclosed in a protein coat (capsid, antigenic) and sometimes lipoprotein envelop (antigenic). They depend on the metabolic processes of the host cell for energy and the biochemical substrates required for replication and protein synthesis. Can be classified as DNA or RNA viruses HSV-1

DNA Viruses Examples: smallpox, chicken pox herpesvirus, adenovirus, hepadnavirus (HBV), papillomavirus. Enter into host cell, uncoat and transfer viral DNA molecule to host nucleus. Viral DNA transcribed to mRNA by host RNA polymerase and mRNA is translated in host ribosomes to viral proteins. Virally encoded proteins participate in further gene transcription, or can be assembled into virions along with copies of viral DNA which can be shed from host.

DNA Viruses Herpes Zoster Herpetic Lesions Varicella Zoster

DNA Viruses Hepatitis B Virus: Adenoviral Conjunctivitis Epstein-Barr Virus Lesion: Papilloma Warts: Hepatitis B Virus:

RNA Viruses Examples include: Rubella virus, rhabdovirus, polio virus, influenza virus, Yellow fever virus, Picornaviruses and measles virus. RNA viruses store genetic material in the form of RNA (viral RNA) that is encased in a viral particle that is often taken into mammalian cells by an avid endocytic process. The vRNA has to transcribe into the appropriate messenger RNA by a viral RNA polymerase (RNAp). The resultant mRNA leads to the production of viral structural and non-structural proteins that are translated by the mammalian ribosomal complex to produce components of the viral particle. The viral particle and nascent vRNA are packaged into the complete virion and released from the infected cell by a shedding process (budding).

RNA Viruses Measles Eruptions: Ebola Virus: Hepatitis A virus:

RNA Viruses Influenza Virus:

Structure of Nucleic Acids - Review Most antiviral agents in clinical use mimic the various nucleic acids required for replication of viral genetic material (either DNA or RNA). The production of false nucleotides by the administered drugs results in decreased propensity for viral replication and decreased infectivity. In general antiviral agents have a static activity that requires an active immune system to clear viral particles from the host.

Agents used against DNA viruses II. Purine nucleoside analogs: Guanosine/deoxyguanosine analogs

Agents used against DNA viruses II. Purine nucleoside analogs: Guanosine/deoxyguanosine analogs Acyclovir (Zovirax®), ganciclovir (Cytovene®), famciclovir (Famvir®), valacylovir (Valtrex®). Are the acyclic sugar derivatives of guanosine or deoxyguanosine (the ribose sugar is lacking). The acyclic deoxyguanosine/guanosine derivatives are converted to monophosphorylated form by action of viral thymidine kinase followed by di- and triphosphorylation by the action of mammalian kinases.

Agents used against DNA viruses (Guanine nucleotides) DNA pol DNA pol DNA pol Acyclovir DNA pol DNA polymerase Phosphodiester bond Nucleotides Hydrogen bond

Agents used against DNA viruses: Purine analogs Preferential sequestration of these agents is achieved in virus infected cells due to the higher affinity of the drug for viral rather than mammalian thymidine kinase. The resultant acyclovir triphosphate competes with dGTP for incorporation into the DNA primer strand resulting in premature chain termination and inactivation of DNA polymerase. Acyclovir may be administered orally although the bioavailability is relatively low. The valyl ester of acyclovir (valcyclovir, Valtrex®) has improved oral bioavailability (> 50%) and is rapidly converted to acyclovir by hepatic and serum esterases. Famciclovir (Famvir®) which is a diacetyl ester of penciclovir (Penvir®) also has improved bioavailability. Acyclovir shows wide distribution to body fluids such as viral vesicles, aqueous humor, breast milk, amniotic fluid and CSF. Primary route of elimination for acyclovir ganciclovir and penciclovir is renal (glom. filtration and tubular secretion).

Agents used against DNA viruses: Purine analogs Acyclovir and penciclovir are used for HSV infections (genital lesions, herpetic gingivostomatitis, encephalitis and keratoconjunctivitis), Varicella-Zoster virus infections, Epstein-Barr virus infections and CMV prophylaxis. Toxicities of acyclovir: Adverse effects: N, V, D, renal toxicity (crystalline nephropathy), tremors, delirium, seizures, rash, diaphoresis, hypotension. Additive nephrotoxicity with cyclosporine, decreased clearance by probenicid; decreases the clearance of methotrexate. Resistance to acyclovir analogs due to: decreased affinity of the drug for the viral thymidine kinase; downregulation of the vTK ; alteration in viral DNA polymerase

Agents used against DNA viruses: Purine analogs Ganciclovir and valaganciclovir (valyl ester of ganciclovir) have similar mechanism of action as acyclovir derivatives requiring conversion to triphosphate form for inhibition of vDNA polymerase and premature chain termination. Cellular ganciclovir triphosphate levels are ~10-fold higher in infected versus uninfected cells due to the actions of CMV phosphokinase. Adverse effects of ganciclovir: Bone marrow suppression with neutropenia, thrombocytopenia is dose-limiting in up to 30% of pts.. Headaches, convulsions, coma from CNS actions result less often. Resistance to ganciclovir primarily due to CMV strains with altered phosphokinase selectivity for guanine analogs

Agents used against DNA viruses: purine analogs Purine nucleoside Analog: Vidarabine (Ara-A) Adenosine analog that is a steric bioisostere in the sugar residue (ribose to arabinose). Mammalian kinases convert drug to triphosphorylated form to inhibit vDNA polymerase and produce chain termination. Ara-A is used in viral infections due to HSV but poor selectivity for virally-infected cells versus normal cells limits systemic use. Systemic adverse effects are limited due to primarily topical application for HSV outbreak (keratitis, conjunctivitis, genital).

Agents used against DNA viruses: pyrimidine analogs Cidofovir (Vistide®) Cidofovir is phosphorylated by mammalian kinases to the diphosphate form and appears to have an extended intracellular half-life. The diphosphate form appears to inhibit the DNA polymerase activity of various viruses but clinical use is primarily for CMV retinitis and HSV infections resistant to acyclovir. Phosphonate structure allows drug to by-pass need for HSV, CMV phosphokinases.

Agents used against DNA viruses: pyrimidine analogs Cidofovir (cont.) HSV or CMV resistant to acyclovir due to vDNA polymerase mutations will show cross-resistance to cidofovir. Highly polar nature requires IV administration for adequate bioavailability with drug distribution in total body water. Cidofovir is eliminated unchanged in kidneys. Nephrotoxicity is dose-limiting effect and is evidenced by proteinuria, azotemia, glycosuria, metabolic acidosis.

Agents used against DNA viruses: pyrimidine analogs Deoxythymine derivatives Idoxuridine (Herplex®) and Trifluridine (Viroptic®) are phosphorylated by cellular nucleotide kinases and may have multiple sites of action. Triphosphorylated form competes with deoxthymine for binding to vDNA polymerase and incorporation into DNA chain results in strand breaks. Has activity against HSV1, HSV2, vaccinia and some adenoviruses Topical application minimizes systemic toxicity Used in HSV keratitis, conjunctivitis

Agents used against DNA viruses Pyrophosphate analog: Foscarnet (Foscavir®) Does not require bioactivation and appears to inhibit viral DNA polymerase by mimicking the triphosphate portion of nucleotides. Thus competes with nucleotides for binding to DNA polymerase, RNA polymerase or HIV reverse transcriptase. Low oral absorption necessitates IV administration; up to 20% of administered dose may sequester in bone and elimination from the body is primarily by renal route. Used in treatment of CMV infections (retinitis), especially those resistant to acyclovir or ganciclovir; also used to treat acyclovir resistant HSV and VZV infections. Adverse effects include: nephrotoxicity (additive with other nephrotoxic agents), hypocalcemia and related arrythmias, genital ulcers, paresthesias, tetany, seizures, CNS effects (headaches, tremors, irritability, hallucinations).

Agents used against DNA viruses Fomivirisen (Vitravene®) A 21-base phosphorothiorate oligonucleotide designed to inhibit progression of CMV retinitis in AIDS pts. The antisense oligonucleotide is complementary to the IE-2 mRNA which it binds to prevent synthesis of specific CMV proteins Administered as intravitreal injection into the posterior eye to treat CMV esp. in imunocompromised pts., or disease due to resistant strains Docosanol (Abreva®) Long-chain saturated alcohol in 10% cream (OTC) is applied to affected areas of mouth to shorten the duration of herpes outbreak. May decrease the replication of HSV by blocking viral fusion and entry into host cells

Agents used against RNA viruses Tricyclic amines: amantadine (Symmetrel®) and rimantidine(Flumadine®) Both drugs affect viral uncoating step to inhibit replication but rimantadine may also have additional effects on viral RNA synthesis. Both have good oral absorption rates with very large volumes of distribution; good penetration into nasal mucosal and saliva and also found in breast milk. Major route of elimination is renal. Primary use in the treatment of Influenza A and C virus infections. Adverse effects are GI (anorexia, nausea) and CNS (lightheadedness, tremors, headaches, insomnia, poor concentration) related.

Agents used against RNA viruses Purine analog: Ribavirin (Virazole®) A purine (guanosine) analog that has a modified base rather than the sugar.   Cellular kinases convert drug to the appropriate triphosphate to inhibit viral RNA polymerase, mRNA synthesis and nucleic acid synthesis. Also has activity against GTP-dependent capping of viral mRNA. Shows activity against RSV bronchiolitis, Influenza virus A and B, HIV-1, HCV and some hemorrhagic fevers. Aerosolized administration can cause bronchial and conjuctival irritation; high intravenous doses may produce bone marrow suppression and anemia; potential teratogen (Category X).

Agents used against RNA viruses Interferon alpha (Roferon®, Intron-A®) Produced naturally by almost all cells when exposed to viral particles or during viral antigen exposure. Has effects to inhibit virally directed transcription, translation, and protein processing. Is a 17000 - 20,000 molecular weight protein that is produced commercially by recombinant DNA technology and administered by a SC or IM injection. Used in the treatment of chronic hepatitis B and C infection, Kaposi’s sarcoma, myeloid leukemias and Condylomas (due to HPV). Adverse effects include: flu-like symptoms (fever, headache, myalgia, arthralgia), depression and granulocytopenia and leukopenia at higher doses.

Agents used against RNA viruses Production of Recombinant Interferon alpha

Agents used against RNA viruses Neuraminidase Inhibitors: Agents in this class have a novel antiviral action and include zanamivir (Relenza®) and oseltamivir (Tamiflu®). Zanamivir is a neuraminidase (sialidase) inhibitor. Zanamivir selectively inhibits the neuraminidases of influenza A and B (unlike amantidine that is only effective against Influenza A). It has been found to effective even on strains that are resistant to amantadine. Influenza virus neuraminidase is a surface glycoprotein that catalyzes the cleavage of the linkage between a terminal sialic acid and adjacent sugar residue: Viral neuraminidase promotes the release of virions from infected cells; promotes the penetration of virus into respiratory epithelial cells;

Agents used against RNA viruses This action of viral neuraminidase promotes the spread of virus in the respiratory tract by multiple mechanisms: prevents the formation of viral aggregates; prevents inactivation of virus by respiratory mucus; Neuraminidase inhibitors act extracellularly and bind to an unoccupied area of influenza neuraminidase that results in competitive inhibition of the enzyme and prevention of sialic acid-sugar bond hydrolysis.

Agents used against RNA viruses   For zanamivir topical application via inhalation of the powder into the lungs provides a high drug concentration at the site of infection and may improve its antiviral effects and reduce the risk of resistance. Oseltamivir is well absorbed orally and the ester is hydrolysed to the carboxylate that is the active form. The primary elimination route is renal and dose adjustments need to be made in renal failure. Adverse effects usually observed are: N,V,D, headache, vertigo, insomnia, bronchitis and abdominal pain. Zanamivir also has propensity to cause bronchospasms, probably due to high local concentrations.

Agents used against RNA viruses Agents against Picornaviruses: Pleconaril (Picovir®) Primarily used against rhinoviruses and enterovirus infections especially in immunocompromised pts. Pleconaril binds to a viral coat protein involved in adhesion to and entry of the virus into mammalian cells. Generally found to have few adverse effects but clinical experience is limited.