18th International HIV Drug Resistance Workshop Basic Principles & Clinical Implications June 9–13 2009, Fort Myers, Florida, USA

THE HIV-1 RT MUTANT Q151L SHOWS DECREASED REPLICATION CAPACITY, SELECTIVE HIGH-LEVEL RESISTANCE TO GS-9148 AND HYPERSUSCEPTIBILITY TO TENOFOVIR AND ZIDOVUDINE

Antivir Ther 2009; 14 Suppl 1:A20 (abstract no. 18)

R Kulkarni¹, NA Margot¹, F. Myrick², J Svarovskaia², J Chen¹, E Lansdon¹, AS Ray¹, T Cihlar¹, S Swaminathan¹, MD Miller¹ and KL White^1
1Gilead Sciences, Inc., Foster City, CA, USA; ²Gilead Sciences, Inc., Durham, NC, USA

BACKGROUND: The investigational nucleotide reverse transcriptase inhibitor (NRTI) GS-9148 maintains potent activity against a majority of clinically observed resistance mutations. GS-9148 resistance selection with HIV type-1 (HIV-1) IIIB previously resulted in a K70E RT mutation and low-level resistance. Here, a Q151L RT mutation was selected by GS-9148 in a clonal LAI HIV-1 strain. Q151L is thought to be an intermediate to development of Q151M.

METHODS: In vitro GS-9148 dose escalation experiments using a clonal HIV-1 strain xxLAI were performed in MT2 cells. Phenotypic profiling of site-directed mutant viruses containing Q151L was performed using a 5-day assay in MT2 cells and PhenoSense and replication capacity assays (Monogram Biosciences). Molecular modeling of Q151L bound to GS-9148 diphosphate (GS-9148-DP) was based on ternary crystal structures of wild-type HIV-1 RT bound to GS-9148-DP or dATP.

RESULTS: Viruses with Q151L were selected after >100 days of GS-9148 treatment. Q151L was the first mutation to develop and showed approximately 20-fold reduced susceptibility to GS-9148 and its prodrug GS-9131. Resistance increased as additional mutations at L74I, K70N/E and L187F/M developed. No viruses containing Q151M developed after prolonged exposure to GS-9148. Q151L viruses showed low-level cross-resistance to multiple NRTIs, but >threefold hypersusceptibility to zidovudine (ZDV) and tenofovir (TFV). In selection experiments of Q151L+S68N virus with ZDV or TFV, Q151L reverted to wild-type, whereas Q151M developed in the absence of drug. Q151L had poor replication capacity (1.4% of wild-type) that improved with compensatory mutations or when converted to Q151M. The crystal structure of wild- type RT bound to GS-9148-DP shows that the 2´fluoro moiety of GS-9148-DP projects toward the Q151 residue. Modeling predicted steric hindrance for GS-9148-DP, but not dATP or other NRTIs when bound to Q151L RT. Virus susceptibility assays confirmed that loss of GS-9148 potency against Q151L is due to the 2´fluoro moiety in the inhibitor.

CONCLUSIONS: Resistance selection revealed Q151L as a novel resistance pathway for GS-9148 in vitro. The Q151M complex shows lower resistance to GS-9148, potentially explaining the lack of its appearance during in vitro selection. The hypersusceptibility of Q151L to TFV and ZDV suggest potential utility of combination therapy of GS-9131 with these agents.

2009-06-09
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