Victor E. Marquez

Conformational Studies and Anti-HIV Activity of Mono- and Difluorodideoxy Nucleosides

The enormous disparity in anti-HIV activity that is evident for a large number of 2′,3′-dideoxynucleoside analogues belies their apparent structural similarity. Biochemically, the differences observed could result mainly from differences in the ability of the individual dideoxynucleosides to generate the corresponding 5′-triphosphates, combined with differences in the ability of the resulting anabolites to inhibit the target enzyme, reverse transcriptase (RT). However, knowledge that irrespective of the nature of the aglycon, the 5′-triphosphates of the most common dideoxynucleosides (U, C, T, A, G) and AZT inhibit HIV-RT within a similar range of submicromolar concentrations,1,2 helps to establish the identity of the most critical step. Clearly, if the ability of these 5′-triphosphate metabolites to block RT is very similar, the crucial parameter has to be the efficiency with which cellular enzymes are capable of generating adequate amounts of these 5′-triphosphates. Indeed, for the same group of dideoxynucleosides, the great disparity in measured levels of 5′-triphosphates agrees well with the more than 1000-fold range in potency observed for these compounds as anti-HIV agents, 1 Achieving an effective intracellular concentration of the triphosphate metabolite depends, in turn, on the ease of transport of the drug as well as its particular affinity for the appropriate cellular nucleoside and nucleotide kinases. Of the three kinases that are necessary for full activation of dideoxynucleosides, the first kinase appears to be the most selective, and it is this particular phosphorylation step that is recognized as the most critical.3 This first kinase, however, is not a universal enzyme. In fact, several different enzymes perform this first phosphorylation step depending on the nature of the aglycon. AZT, for example, is converted to the 5′-monophosphate by thymidine kinase,4 ddC is phosphorylated by deoxycytidine kinase,5 which can also phosphorylate ddA,6 ddA is phosphorylated by adenosine kinase,6 and ddI, ddG, and carbovir are phosphorylated by yet another enzyme, a phosphoribosyltransferase/5′-nucleotidase that utilizes IMP as a phosphate donor.7 Based on the existence of this array of phosphorylating enzymes, it is important that structure-activity comparisons (SAR) be performed on groups of compounds expected to be phosphorylated by the same enzyme.