B. Points of action of antiviral molecules

Thousands of compounds can inhibit viral replication in cell culture. In general, the more complex the regulatory mechanisms of a virus, the easier it is to find molecules that can inhibit it. It is often very hard to decide which of the compounds should be investigated further. A broad estimate of the ratios of the activity of antiviral compounds in cell culture, animal models and humans is 1000:10:1.

1. Cell-free virus

Few antiviral compounds inhibit or inactivate extracellular virus in vivo. An exception is the series of WIN compounds which bind to the external proteins of picornaviruses. These compounds bind to and fit within the canyons which exists on the surface of picornavirus virions and thereby stabilizing the particle and prevent uncoating.

2. Virus Adsorption

There is considerable theoretical interest in compounds able to block the adsorption of virus to susceptible cells. In the case of HIV, which binds specifically to CD4 receptors, short peptides have been synthesized to correspond with the sequence of the receptor binding site of CD4 molecule and with the binding protein gp120. These peptides should block the interaction of the receptor region and gp120 without interrupting other receptor functions of the CD4.

3. Virus entry and uncoating

Viruses such as influenza and certain flaviviruses enter by viropexis or engulfment. Immediately afterwards, while in a cytoplasmic endosome (vacuole), the virus catalyses fusion between the viral lipid-containing membrane and the membrane of the intracellular vacuole. The fusion is mediated by a sequence of hydrophobic amino acids or one of the glycoproteins of the virus. A compound that interrupts fusion would block virus replication at this early stage. In the case of influenza A, the fusion sequence on the HA molecule can only act after a structural 3- dimensional rearrangement of the HA molecule. This major change, whereby the HA trimer opens out like the petals of a flower, probably occurs only at a low pH5.5 found in lysosomal vacuoles. Amantidine appear to inhibit influenza A replication in part by raising the pH of the cytoplasmic vacuole, thus preventing virus-induced fusion and hence virus uncoating. Other enveloped viruses such as paramyxoviruses and HIV, enter cells by virus-induced fusion with the plasma membrane of the cell. This "fusion from without" may be susceptible to short peptides which may act on the fusion sequence extracellularly.

4. Transcription and translation of viral nucleic acids and release of virus

Most of the antiviral drugs now known act by inhibiting the replication or transcription of viral nucleic acids.

1. a. Inhibitors of herpes DNA polymerase - by far the most amenable target for antiviral drugs is the herpes simplex DNA polymerase. The most successful antiviral compound yet developed is acyclovir inhibits the function of this enzyme. The ideal antiviral drug should (1) be taken up only into infected cells (2) the actual inhibitory molecule should be generated inside the infected cell by enzymatic activity (3) the inhibitor should have a selective effect on a virus enzyme. Acyclovir demonstrates all of the above characteristics.
   
2. b. Inhibitors of viral reverse transcriptase - AZT and the majority of other compounds act as chain terminators. AZT triphosphate binds to and inhibit virus RT more effectively than normal cellular DNA polymerases and so some antiviral specificity is achieved. However, the compound is certainly not comparable to acyclovir in terms of antiviral specificity. This is reflected in the toxicity of AZT in clinical practice. This cellular toxicity may be partly explained by the fact that normal cellular enzymes phosphorylate AZT and is thus activated in both infected and uninfected cells.  

5. Translation

It may be possible to interfere with the viral mRNA itself. Small anti-sense oligonucleotides can be constructed which are complementary to specific genes, such as the rev gene. Fomivirsen (Vitravene) is a 21-base anti-sense oligonucleotide complementary to the early region 2 mRNA of CMV. It is approved for the local treatment of CMV retinitis in AIDS patients.  

6. Assembly

HIV protease is required for the cleavage of the gag-pol fusion protein. Inhibitors of this enzyme may therefore block the assembly of HIV.