Insights into the molecular mechanism of inhibition and drug resistance for HIV-1 RT with carbovir triphosphate.

Abacavir (1592U89, or Ziagen) is a powerful and selective inhibitor of HIV-1 viral replication that has been approved by the FDA for treatment of acquired immunodeficiency syndrome. Abacavir is metabolized to the active compound carbovir triphosphate (CBVTP). This compound is a guanosine analogue containing a 2',3'-unsaturation in its planar carbocyclic deoxyribose ring that acts on HIV-1 reverse transcriptase (RT(WT)) as a molecular target, resulting in chain termination of DNA synthesis. A single amino acid change from methionine 184 to valine in HIV-1 RT (RT(M184V)) has been observed clinically in response to abacavir treatment. The ability of the natural substrate, dGTP, or CBVTP to be utilized during DNA- and RNA-directed polymerization by RT(WT) and RT(M184V) was defined by pre-steady-state kinetic parameters. In the case of RT(WT), CBVTP was found to be a surprisingly poor substrate relative to dGTP. In both DNA- and RNA-directed polymerization, a decrease in the efficiency of CBVTP utilization with respect to dGTP was found with RT(M184V), suggesting that this mutation confers resistance at the level of CBVMP incorporation. The relatively low incorporation efficiency for RT(WT) was unanticipated considering earlier studies showing that the triphosphate form of a thymidine nucleoside analogue containing a planar 2',3'-unsaturated ribose ring, D4TTP, was incorporated with high efficiency relative to the natural substrate, dTTP. The difference may be related to the isosteric replacement of oxygen in the deoxyribose ring with carbon. This hypothesis was tested by synthesizing and evaluating D4GTP (the planar 2',3'-unsaturated deoxyribose guanosine analogue that is complementary to D4TTP). In contrast to CBVTP, D4GTP was found to be an excellent substrate for RT(WT) and no resistance was conferred by the M184V mutation, thus providing novel insight into structure-activity relationships for nucleoside-based inhibitors. In this work, we illustrate how an understanding of the molecular mechanism of inhibition and drug resistance led to the discovery of a novel prodrug of D4G. This compound shows promise as a potent antiviral especially with the drug resistant M184V HIV-1 RT that is so often encountered in a clinical setting.