In vitro suppression of K65R reverse transcriptase-mediated tenofovir- and adefovir-5'-diphosphate resistance conferred by the boranophosphonate derivatives.

9-[2-(Boranophosphonomethoxy)ethyl]adenine diphosphate (BH(3)-PMEApp) and (R)-9-[2-(boranophosphonomethoxy)propyl]adenine diphosphate (BH(3)-PMPApp), described here, represent the first nucleoside phosphonates modified on their alpha-phosphates that act as efficient substrates for the human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT). These analogues were synthesized and evaluated for their in vitro activity against wild-type (WT), K65R, and R72A RTs. BH(3)-PMEApp and BH(3)-PMPApp exhibit the same inhibition properties as their nonborane analogues on WT RT. However, K65R RT was found hypersensitive to BH(3)-PMEApp and as sensitive as WT RT to BH(3)-PMPApp. Moreover, the presence of the borane group restores incorporation of the analogue by R72A HIV RT, the latter being nearly inactive with regular nucleotides. The BH(3)-mediated suppression of HIV-1 RT resistance, formerly described with nucleoside 5'-(alpha-p-borano)-triphosphate analogues, is thus also conserved at the phosphonate level. The present results show that an alpha-phosphate modification is also possible and interesting for phosphonate analogues, a result that might find application in the search for a means to control HIV RT-mediated drug resistance.