A Bifunctional Nucleoside Probe for the Inhibition of the Human Immunodeficiency Virus-Type 1 Reverse Transcriptase.

Nucleoside analogs have proven effective for the inhibition of viral polymerases and are the foundation of many antiviral therapies. In this work, the antiretroviral potential of 6-azauracil analogs was assessed using activity-based protein profiling techniques and functional assays. Probes based on the 6-azauracil scaffold were examined and found to bind to HCV polymerase and HIV-1 reverse transcriptase through covalent modification of residues near the active site. The modified sites on the HIV-1 RT were examined using a mass spectrometry approach, and it was discovered that the azauracil moieties modified the enzyme in proximity to its active site. However, these scaffolds gave little or no inhibition of enzyme activity. Instead, a bifunctional inhibitor was prepared using click chemistry to link the 6-azauracil moiety to azidothymidine (AzT) and the corresponding triphosphate (AzTTP). These bifunctional inhibitors were found to have potent inhibitory function through a mode of action that includes both alkylation and chain termination. An in vitro assay demonstrated that the bifunctional inhibitor was 23-fold more effective in inhibiting HIV-1 RT activity than the parent AzTTP. The bifunctional inhibitor was also tested in HIV-1 permissive T cells where it decreased Gag expression similarly to the front-line drug Efavirenz with no evidence of cytotoxicity. This new bifunctional scaffold represents an interesting tool for inhibiting HIV-1 by covalently anchoring a chain-terminating nucleoside analog in the active site of the reverse transcriptase, preventing its removal and abolishing enzymatic activity, and represents a novel mode of action for inhibiting polymerases including reverse transcriptases.