The objective of this study is to estimate the effect of lersivirine on the pharmacokinetics of R-methadone and S-methadone and to investigate the safety and tolerability of lersivirine when co-administered with methadone. Symptoms of methadone withdrawal will also be assessed.

Inclusion Criteria:

• Healthy male and/or female subjects (Healthy is defined as no clinically relevant abnormalities identified by a detailed medical history except drug abuse), full physical examination, including blood pressure and pulse rate measurement, 12-lead ECG and clinical laboratory tests).
• Body Mass Index (BMI) of 17.5 to 35.5 kg/m2; and a total body weight >50 kg (110 lbs).
• Receiving stable methadone maintenance treatment for at least 3 months (dose range 50-150 mg QD).

Exclusion Criteria:

• Subjects on therapy for Hepatitis B and/or Hepatitis C; Evidence of impaired liver function (ie, AST and/or ALT greater than 3 times the upper limit of normal, total bilirubin >1.5 times the upper limit of normal, albumin <3.5 g/dL).
• 12-lead ECG demonstrating QTc >450 msec for males and QTc >470 msec for females or any other clinically significant abnormalities at screening.