Like antibiotics for bacteria, antiviral drugs are a class of antimicrobials used specifically for treating viral infections. They are relatively harmless to host because they inhibit the development of pathogens instead of destroying them. Most of the antiviral agents need to be activated by viral and cellular enzymes before exerting antiviral effect. Hence, activity of enzymes and concentration of substrates will influence the efficacy of these drugs.

In majority of acute infections, viral replication is already at its peak when symptoms appear. To be effective, antiviral therapy has to be started in the incubation period, i.e has to be prophylactic.

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• Uncoating

Amantadine/rimantidine prevents uncoating and release of influenza RNA but resistance arises readily. Pleconaril inhibits uncoating of picornaviruses and is active against enteroviruses and rhinoviruses; it is absorbed orally and clinical trials suggest it shortens clinical symptoms.

 

• Nucleoside analogues

• Chain termination

Aciclovir is selectively converted into acyclo-guanosine monophosphate (acyclo-GMP) by viral enzymes, then into a potent inhibitor of viral DNA polymerase by host enzymes. The acyclo- GMP causes viral DNA chain termination. Resistance occurs through the development of deficient thymidine kinase production or alteration in the viral polymerase gene. The drug can be taken orally and crosses the blood-brain barrier. Other agents (e.g. ganciclovir) work in a similar way.

 

• Reverse transcriptase inhibition

Lamivudine inhibits the reverse transcriptase of hepatitis B and HIV (see below). Nucleoside and nucleotide inhibitors are being developed as alternative treatments for hepatitis B; these include adefovir, entecavir, tenofovir, telbivudine and clevudine. Ribavirin is a guanosine analogue that inhibits several steps in viral replication including capping and elongation of viral mRNA. It is active against respiratory syncytial virus, influenza A and B, parainfluenza virus, Lassa fever, hantavirus and other arenaviruses.

 

• Nucleoside reverse transcriptase inhibitors

Nucleoside reverse transcriptase inhibitors (NRTIs) inhibit reverse transcriptase by being incorporated as faulty nucleotides. Examples include the longest established antiretroviral drug zidovudine (AZT), plus lamivudine (3TC), stavudine (d4T), tenofovir, didanosine (ddI) zalcitabine (ddC) and abacavir (see HIV infection and AIDS ).

 

• Non-nucleoside reverse transcriptase inhibitors

Non-nucleoside reverse transcriptase inhibitors (NNRTIs) inhibit reverse transcriptase directly; examples include nevirapine, efavirenz, delavirdine and etravirine. They have been shown to be effective agents in combination regimens. As resistance occurs after a single mutation, they are used in maximally suppressive regimens only.

 

• Protease inhibitors

Protease inhibitors target the HIV-encoded protease. They are highly effective antiretroviral compounds that cause significant falls in viral load. They include atazanavir, indinavir, lopinavir, ritonavir and saquinavir. Ruprintrivir acts in the same way against human rhinovirus 3C protease. It is administered by nasal spray and appears to have useful activity in rhinovirus infection.

 

• Fusion inhibitors

Enfuvirtide inhibits binding with gp134; maraviroc inhibits binding to CCR5 preventing fusion. Both agents are used for salvage therapy in AIDS (see HIV infection and AIDS ).

 

• Release inhibitors

Neuraminidase inhibitors including zanamivir and oseltamivir inhibit the final stage in the release of virus from the host cell.

 

• Integrase inhibitors

These agents are being developed to block the insertion of the HIV viral genome into the DNA of the host cell.

 

• Other agents

Infections with hepatitis B and hepatitis C can also be treated with α-interferon, a host cytokine.

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Classificaton based on the virus it acts upon:

1. Anti-herpes virus: Idoxuridine, acyclovir, valacyclovir, famciclovir, ganciclovir, forscarnet

Except forscarnet which is an inhibitor of DNA polymerase and reverse transcriptase, others are purine and pyrimidine anlaogues.

 

2. Anti-retrovirus:

a.Nucleoside reverse transcriptase inhibitors (NRTIs): Zidovudine (AZT), Didanosine, Zalcitabine, Stavudine, Lamivudine, Abacavir

b.Non-nucleoside reverse transcriptase inhibitors (NNRTIs):Nevirapine, Efavirenz, Delaviridine

c. Protease inhibitors: Ritonavir, Indinavir, Nelfinavir, Saquinavir, Lopinavir

 

3. Anti-influenza virus: Amatidine, Rimantadine

 

4. Nonselective antiviral drugs: Ribavirin, Lamivudine, Interferon alpha