S/GSK1349572 is a novel integrase inhibitor with potent in vitro anti-HIV activity, a different in vitro resistance profile than other integrase inhibitors, and a favorable preclinical safety and pharmacokinetics (PK). Randomized, double-blind, placebo-controlled single-dose and multiple-dose, dose escalation studies evaluated PK, safety, and tolerability in healthy subjects. In the single-dose study, 2 cohorts of 10 subjects (8 active, 2 placebo) received suspension doses of 2, 5, 10, 25, 50 and 100 mg in an alternating panel design. In the multiple-dose study, 3 cohorts of 10 subjects (8 active, 2 placebo) received suspension doses of 10, 25 and 50 mg once daily for 10 days. A CYP3A sub-study with midazolam was conducted with the 25mg dose. Laboratory testing, vital signs, ECGs and PK sampling were performed at regular intervals. S/GSK1349572 was well tolerated. Most adverse events (AEs) were mild, with few moderate AEs reported. Headache was the most common AE. No clinically significant laboratory trends or ECG changes were noted. PK was linear over dosage range studied. Steady state geometric mean (CV%) AUC(0-tau) and Cmax ranged from 16.7 (15) mug*h/mL and 1.5 (24) mug/mL at 10 mg to 76.8 (19) mug*h/mL and 6.2 (15) mug/mL at 50 mg, respectively. Geometric mean steady-state Ctau at 50 mg was 1.6 mug/mL, approximately 25-fold higher than the protein-adjusted IC90 (0.064 mug/mL). The half-life was approximately 15 hours. S/GSK1349572 had no impact on midazolam exposure indicating it does not modulate CYP3A activity. The PK profile suggests once daily, low milligram doses will achieve therapeutic concentrations.