Two randomized, Phase 2b dose-finding studies were conducted in HIV-1 infected adults to compare GSK2248761 100 mg and 200 mg given once daily as part of an antiretroviral treatment regimen. One of the studies (SGN113399) was in subjects with prior exposure to antiretroviral therapy where GSK2248761 100 mg and 200 mg once daily were compared to determine the best dose in this population. A contemporary control arm receiving etravirine 200 mg twice daily was also included, and all arms included a twice-daily background therapy consisting of darunavir/ritonavir 600 mg/100 mg plus raltegravir 400 mg. The other study (SGN113404) was in treatment-naïve subjects, comparing GSK2248761 100 mg and 200 mg once daily to determine the best dose in this population. A contemporary control arm receiving efavirenz 600 mg once daily was also included, and all arms were given background therapy selected by investigators from either once-daily abacavir/lamivudine 600 mg/300 mg or tenofovir/emtricitabine 300 mg/200 mg. Of a planned total population in both studies of 300 subjects, 35 were enrolled before the studies were terminated because of the occurrence of seizures in five subjects. All of the subjects who experienced seizures were enrolled into SGN113399, four randomized to receive 200 mg GSK2248761 and one randomized to receive 100 mg GSK2248761. There were no seizures in the subjects receiving GSK2248761 in study SGN113404. At the time of study termination, subjects had been enrolled and received GSK2248761 at 19 sites in four countries: France, Romania, United States, and Germany. Although potential contributory conditions have been identified in some cases, definitive causative factors for the seizure occurrence have not been established.