Antiviral Drugs Viruses are obligate intracellular parasites. lack both a cell wall and a cell membrane. They do not carry out metabolic processes. Viruses use much of the host’s metabolic machinery. Few drugs are selective enough to prevent viral replication without injury to the infected host cells.

Structure of viruses Consist of two basic components: nucleic acid (single- or double-stranded RNA or DNA) - never both Surrounded by protein coat: “capsid” and sometimes an outer lipid “envelope” A fully assembled infectious virus: “virion” Often contain crucial virus-specific enzymes Often visible by electron microscopy

Antiviral Drugs Therapy for viral diseases is further complicated because: The clinical symptoms appear late in the course of the disease, at a time when most of the virus particles have replicated. At this stage of viral infection, administration of drugs that block viral replication has limited effectiveness. Some antiviral agents are useful as prophylactic agents. Antiviral drugs share the common property of being virustatic; they are active only against replicating viruses and do not affect latent virus.

General anti-viral strategies are to inhibit: Viral attachment to host cell, Penetration. Uncoating Viral enzymes: DNA/RNA polymerases, etc. Reverse transcriptases, proteases, etc. Release of virus from cell surface membranes

Treatment of Herpes virus Infections Herpes viruses are associated with a broad spectrum of diseases, for example, cold sores, viral encephalitis, and genital infections. The drugs that are effective against these viruses exert their actions during the acute phase of viral infections and are without effect during the latent phase.

Acyclovir Acyclovir (acycloguanosine) is the prototypic antiherpetic therapeutic agent. Herpes simplex virus (HSV) types 1 and 2, varicella-zoster virus (VZV). It is the treatment of choice in HSV encephalitis. The most common use of acyclovir is in therapy for genital herpes infections. Acyclovir is administered by intravenous (IV), oral, or topical routes. The drug distributes well throughout the body, including the cerebrospinal fluid (CSF). Acyclovir is partially metabolized to an inactive product. Excretion into the urine occurs both by glomerular filtration and tubular secretion Acyclovir accumulates in patients with renal failure.

Acyclovir – Structure Purine Mimic Similarity to 2`-deoxyguanosine (dGTP): lack of 3` hydroxyl

Acyclovir - MOA Step 1: Activation

Acyclovir – MOA (Summary) Acyclovir Monophosphate Acyclovir triphosphate Herpes virus specific thymidine kinase Inhibits herpes virus DNA Polymerase competitively Cellular kinases Gets incorporated in viral DNA and stops lengthening of DNA strands. The terminated DNA Inhibits DNA-polymerase irreversibly

Adverse effects: Side effects of acyclovir treatment depend on the route of administration. Local irritation may occur from topical application. After oral administration. headache, diarrhea, nausea, and vomiting may result. Transient renal dysfunction may occur at high doses or in a dehydrated patient receiving the drug intravenously. Resistance: Altered or deficient thymidine kinase and DNA polymerases have been found in some resistant viral strains and are most commonly isolated from immunocompromised patients. Cross resistanc to the other agents in this family occurs.

Adamantane Antivirals The therapeutic spectrum of the adamantane derivatives: Amantadine Rimantadine Influenza A infections. Due to widespread resistance, the adamantanes are not recommended in the United States for the treatment or prophylaxis of influenza A. Mechanism of action: Amantadine and rimantadine interfere with the function of the viral M2 protein, possibly blocking uncoating of the virus particle and preventing viral release within infected cells.

Amantadine is mainly associated with CNS adverse effects, such as insomnia, dizziness, and ataxia. More serious adverse effects may include hallucinations and seizures. Amantadine should be employed cautiously in patients with psychiatric problems, cerebral atherosclerosis, renal impairment, or epilepsy. Rimantadine causes fewer CNS reactions. Both drugs cause GI intolerance. They should be used with caution in pregnant and nursing mothers. Resistance: Resistance can develop rapidly, and resistant strains can be readily transmitted to close contacts. Resistance has been shown to result from a change in one amino acid of the M2 matrix protein. Cross-resistance occurs between the two drugs.

Neuraminidase Inhibitors Mechanism of action: Influenza viruses employ a specific neuraminidase that is inserted into the host cell membrane for the purpose of releasing newly formed virions. This enzyme is essential for the virus life cycle. Oseltamivir and zanamivir selectively inhibit neuraminidase, thereby preventing the release of new virions and their spread from cell to cell.

Neuraminidase Inhibitors Oseltamivir Zanamivir Both are effective against both type A and type B influenza viruses. They do not interfere with the immune response to influenza vaccine. Administered prior to exposure, neuraminidase inhibitors prevent infection and, when administered within 24 to 48 hours after the onset of symptoms, they modestly decrease the intensity and duration of symptoms.

Adverse Effects Resistance: Oseltamivir: gastrointestinal (GI) discomfort and nausea, which can be alleviated by taking the drug with food. Zanamivir: irritation of the respiratory tract occurs with. It should be used with caution in individuals with asthma or chronic obstructive pulmonary disease, because bronchospasm may occur. Resistance: Mutations of the neuraminidase enzyme have been identified in adults treated with either of the neuraminidase inhibitors. These mutants, however, are often less infective and virulent than the wild type.