Background:  GS-9137 (JTK-303) is a strand-transfer inhibitor of HIV-1 integrase (IN) and is being developed for the treatment of HIV-1 infection. The resistance profile of GS-9137 was investigated by culturing HIV-1 in increasing concentrations of GS-9137 in vitro and characterizing the resulting mutants.

Methods:  HIV-1 IIIb was cultured in MT-2 cells. Selection was commenced at a GS-9137 concentration of 2X EC₅₀; virus growth was monitored by cytopathic effect. Virus was passaged to the next drug concentration (2-fold increase) when extensive cytopathic effect was observed; selection continued until the CC₅₀ was achieved. Viral pools were phenotyped against GS-9137 and control ART drugs, including other integrase inhibitors. Population and clonal sequencing of the HIV-1 IN coding region were used to identify potential GS-9137 resistance mutations. Site-directed mutant viruses were constructed and characterized phenotypically against GS-9137 and control ART compounds.

Results:  Selection of HIV-1 with GS-9137 resulted in the initial emergence of a T66I mutation in the IN catalytic core; an additional mutation, R263K, located in the C-terminal DNA binding domain, was also selected. A second independent GS-9137 selection experiment resulted in initial selection of T66I, followed by S153Y or F121Y mutations. Emergence of these mutations was associated with phenotypic resistance to GS-9137. Site-directed mutant viruses carrying T66I, R263K, or T66I+R263K had 15.1-, 5.2-, and 98-fold reduced susceptibility to GS-9137, respectively. In contrast, susceptibility of these mutants to other IN inhibitors (L-870,810 and MK-0518) and ART drugs of other classes was equivalent to wild type. Site-directed mutant viruses encoding the E92Q IN mutation, selected by GS-9137 in independent experiments, had 36-fold reduced susceptibility to GS-9137 and ~7-fold reduced susceptibility to MK-0518, but remained fully susceptible to other ART drug classes.

Conclusions:  Selection of HIV-1 in vitro with GS-9137 resulted in the emergence of a T66I IN mutation and subsequently additional IN mutations. Site-directed mutant viruses carrying the T66I mutation showed reduced susceptibility to GS-9137, but remained susceptible to other IN inhibitors, including MK-0518. In contrast, site-directed mutant viruses with E92Q demonstrated both resistance to GS-9137 and evidence of cross-resistance to MK-0518. All the IN inhibitor mutants remained fully susceptible to ART drugs of other classes.