GS-9131 is an NRTI candidate for treatment of patients with resistance to other NRTIs. HIV reverse transcription is inhibited by GS-9131 by chain termination. In this study, we employed cell culture models to shed light on the ability of escape mutants to emerge under increasing drug pressure.
Cord blood mononuclear cells (CBMC) and MT-2 cells were infected with clinical isolates and passaged in increasing concentrations of GS-9131 and tenofovir (TFV). Virus growth was monitored by weekly determinations of reverse transcriptase (RT) activity. For MT-2 cells, supernatants were collected at the peak of infection by cytopathic effect scoring. In order to identify alterations in the RT region, viral RNA was extracted from tissue culture supernatants and sequenced.
After 40 weeks of sustained drug treatment, none of the CBMC viral cultures tested yielded major resistance mutations. Despite the lack of changes in the RT region, most of the isolates were able to endure moderate to very high concentrations of the drugs, 500-20,000 -fold increase for GS-9131 and 100-20,000 -fold for TDF. Using 3TC as a control, the M184I or V mutations rapidly arose in most viruses. Previous studies with GS-9148, for which GS-9131 is a pro-drug, were done in MT-2 cells, and some resistance patterns were identified. In our experiment using MT-2 cells, no major resistance pathways emerged. One isolate did select for the L187M mutation, which was also identified in the previous study.
Two methods were employed in order to obtain a better picture of the ability of GS-9131, a drug in development, to put pressure on viruses to escape. The lack of emergent variants indicates that GS-9131 is a promising antiretroviral for HIV treatment, which has also been shown to be suitable for individuals harbouring NRTI mutations. Its versatility for use in combination with other drugs may provide more precise and potent options to patients with limitations due to NRTI resistance.