$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
Antiviral Nucleoside Inhibitors
Antiviral nucleoside inhibitors are structural analogs of natural nucleosides that interfere with viral DNA or RNA synthesis. These compounds selectively target viral polymerases due to their resemblance to host nucleosides, thereby disrupting viral genome replication.
Mechanism of Acyclovir Action
Acyclovir is a guanosine analog with a three-carbon acyclic side chain. It selectively targets herpes simplex virus type 1 (HSV-1), herpes simplex virus type 2 (HSV-2), and varicella-zoster virus (VZV). Once inside an infected cell, a virus-encoded thymidine kinase phosphorylates acyclovir to form acyclovir monophosphate. Then, host cellular kinases further convert it sequentially into its diphosphate and active triphosphate forms.
Acyclovir triphosphate mimics deoxyguanosine triphosphate. In virus-infected cells, it is mistakenly incorporated by viral DNA polymerase into the growing viral DNA strand. Because it lacks a 3'-hydroxyl group, no additional nucleotides can be added, resulting in premature termination of DNA synthesis and subsequent inhibition of viral replication. Its preferential affinity for viral DNA polymerase over host polymerase enhances its selective toxicity.
Broader Application and Resistance Management
Various nucleoside inhibitors, such as ganciclovir, zidovudine, and ribavirin, block viral replication by interfering with the action of viral enzymes or inducing chain termination. Ganciclovir, another guanosine analog, is commonly used against cytomegalovirus (CMV). Zidovudine, a thymidine analog, is widely used in the treatment of HIV. Ribavirin, a broad-spectrum antiviral, disrupts the replication of several RNA viruses through mechanisms such as inhibition of viral RNA polymerase and induction of lethal mutagenesis.
Combining these drugs makes it more difficult for a virus to develop resistance, as it would need to simultaneously acquire multiple mutations to evade the distinct mechanisms of action. This combination strategy is a cornerstone of antiviral therapy, particularly in managing chronic viral infections like HIV and hepatitis C.
