[7] COMBINING TENOFOVIR TO OTHER NRTIs: IMPACT ON RESISTANCE AND VIRAL FITNESS AT THE MOLECULAR LEVEL

2nd International AIDS Society Conference on HIV Pathogenesis and Treatment

Paris, France - July 13 - 16, 2003

[TITLE:] COMBINING TENOFOVIR TO OTHER NRTIs: IMPACT ON RESISTANCE AND VIRAL FITNESS AT THE MOLECULAR LEVEL

[AUTHOR(S):] B Canard¹, J Deval¹, KL White², MD Miller², J Courcambeck³, B Selmi1 and J Boretto¹
¹CNRS, Marseille, France; ²Gilead, Foster City, USA; and ³Genosciences, Marseille, France

IAS Conf HIV Pathog Treat 2003 Jul 13-16;2nd: Abstract No. 7
Antiviral Therapy 2003; 8(Suppl. 1):S187

[ABSTRACT:] Tenofovir disoproxil fumarate (TDF), the most recently approved antiretroviral drug, does not seem to select for highly resistant mutants. The most significant tenofovir resistance variant to date (K65R) shows only a limited resistance to the active intracellular metabolite of tenofovir, TFV-DP (4.4-fold resistance, as judged by pre-steady state analysis). This resistance is due to a decrease in k_pol(TFV-DP): from 7 s^-1 for WT RT to 0.32 s^-1 for K65R RT. On the other hand, M184V RT, the 3TC-resistant mutant, displays a 2.5-fold increase in susceptibility to TFV-DP. When the two mutations are combined, the susceptibility of K65R/M184V RT to TFV-DP is similar to that of WT RT. Molecular modelling studies show that the methyl group of tenofovir makes a favourable Van der Waals interaction with valine 184, explaining TFV-DP resensitization with M184V. Cell culture susceptibility data corroborate the observation of resensitization as the K65R/M184V HIV double mutant shows only 1.7 fold reduced susceptibility to tenofovir. Interestingly, the decrease in resistance to tenofovir with the double mutant is associated with a 5- fold decrease in the affinity of K65R/M184V RT for the natural nucleotide substrate dATP. The incorporation of all four natural nucleotides was thus investigated. We observed that each of them was poorly incorporated by K65R/M184V RT, with a 27% global incorporation efficiency as compared to WT RT. The M184V substitution was found to contribute to the diminished binding of the natural nucleotides to RT, whereas K65R affects catalysis generally. In vitro replication capacity assays using recombinant HIV also show that K65R/M184V is less fit than WT HIV, in agreement with our enzymatic data. Our data describe at the molecular level both how a resistant virus is unable to resist to two drugs simultaneously, and for the first time, how viral fitness of a resistant virus is directly linked to its decreased ability to use natural nucleotide substrates. All together, these data predict a benefit for the combination of tenofovir DF with 3TC, as well as open new avenues in how to drive resistant virus to reduced viral fitness.

030714
7