Study Evaluating MOA-728 in Subjects on Stable Methadone Maintenance

Inclusion Criteria: * History of methadone treatment for at least 1 month before day -1 at a dose of ≥ 30 mg/day, and a positive drug test result for methadone. * Healthy as determined by the investigator on the basis of screening evaluations. Exclusion Criteria: * Allergy to opioids (eg, codeine, morphine, or oxymorphone) or opioid antagonists (eg, naloxone or naltrexone). * Any significant cardiovascular, hepatic, renal, respiratory, gastrointestinal, endocrine, immunologic, dermatologic, hematologic, neurologic, or psychiatric disease.