Suppression of Multidrug-resistant HIV-1 Reverse Transcriptase Primer Unblocking Activity by α-Phosphate-modified Thymidine Analogues

A dipeptide insertion between codons 69 and 70 together with the amino acid substitution T215Y in the reverse transcriptase (RT)-coding region of human immunodeficiency virus type 1 (HIV-1) strains are known to confer phenotypic resistance to zidovudine (AZT) and stavudine (d4T). Phenotypic resistance correlates with an increased ATP-dependent phosphorolytic activity. Nucleoside α-boranophosphate diastereoisomers derived from AZT and d4T were tested as substrates of a multidrug-resistant HIV-1 RT (designated as SS RT) bearing a Ser-Ser insertion at codons 69–70 and other drug resistance-related mutations, in DNA polymerization assays and ATP-mediated excision reactions. Using pre-steady-state kinetics, we show that SS RT can incorporate both Rp and Sp diastereoisomers, although Rp is the preferred isomer. Chirality at the internucleotidic linkage formed upon incorporation of nucleoside α-boranophosphate did not affect ATP-mediated excision. As reported for AZT and d4T-terminated primers, substituting Thr, Asn or Ser for Tyr215 abrogates the ATP-dependent phosphorolytic activity on primers terminated with α-boranophosphate derivatives of thymidine analogues. However, unlike in the case of AZT, eliminating the dipeptide insertion in SS RT had no effect on the ATP-mediated excision of primers terminated with α-boranophosphate derivatives of d4T. Studies with ATP analogues showed that exchanging a non-bridging oxygen atom at the γ-phosphate group for sulfur causes a significant reduction of the ATP-dependent phosphorolytic activity of SS RT. Interestingly, SS RTʹs excision activity is completely eliminated upon phosphorothioate substitution at the 3′ end of primers terminated with AZT. These results suggest that phosphorothioate derivatives of currently approved drugs could be useful against excision-proficient HIV-1 strains.