To Evaluate the Pharmacokinetics, Safety and Tolerability of Retosiban (GSK221149) Co-administered With EFAVIRENZ

This will be a randomized single sequence open label study. This study is designed to determine if chronic dosing with efavirenz (EFZ) will have an effect on the pharmacokinetics (PK) of intravenously-administered retosiban in healthy volunteers. The study consists of screening (28 days), treatment (1 dosing session) and follow-up (7 to 14 days) period, and the total duration of study participation for each subject will be approximately 8 weeks. During the treatment period, subjects will be admitted to the clinical research unit the day before dosing (Day 1) and will remain until completion of the last assessment on Day 20. All subjects will receive on Day 1, a 6 milligrams (mg) bolus of retosiban for 5 minutes (min), followed by a 6 mg/hour (hr) infusion for 12 hrs. On Day 2, a washout day will occur. On Days 3-17, subjects will receive EFZ 600 mg once daily in the evening. On Day 18, subjects will receive a 6 mg bolus of retosiban for 5 mins, followed by a 6 mg/hr infusion for 12 hrs plus a 600 mg dose of EFZ.