A Study to Evaluate Withdrawal Effects Following Dosing of Oxycodone/Naltrexone in Methadone-Maintained Subjects

Approximately 34 methadone-maintained opioid-dependent subjects will be enrolled in the study. All subjects will be admitted to the clinical study unit (CSU; Visit 2) for a 15-day (with 14 overnight stays) inpatient Treatment Phase visit. Subjects will first undergo a 3-day methadone stabilization period (Days 1 to 3) in which all subjects' methadone administration time will be gradually shifted to the time of dosing that will be used throughout the Treatment Phase (approximately 8 am daily). Following the methadone dosing-time stabilization period, subjects will begin a 2-day oxycodone placebo administration period. On Day 4 and Day 5, all subjects will receive a total of 3 oxycodone placebo doses, separated by 6 hours each, starting from approximately one hour following methadone dose administration. Placebo will be administered in a single-blind manner. Starting on Day 6, subjects will be randomized in a 2:1 (Oxycodone/Naltrexone or Oxycodone) fashion to receive one of the following treatments in a double-blinded manner: * Oxycodone/Naltrexone, multiple oral daily dosing for 5 days * Oxycodone, multiple oral daily dosing for 5 days Subjects will receive one of the above-listed treatments three times daily, beginning approximately 1 hour following methadone dosing and every 6 hours until 3 doses have been administered. Subjects will be dosed in this manner for 5 consecutive days (Days 6 to 10). All subjects will remain confined in the CSU for 4 days after the first of three doses of study drug is administered on Day 10 for the safety/washout period, during which they will continue to be assessed and monitored for withdrawal effects. Subjects will be discharged approximately 96 hours after the first dose of study drug (oxycodone/naltrexone or oxycodone) administered on the morning of Day 10, if medically acceptable according to the investigator or designee. All subjects will receive daily phone calls until the Follow-up Phase during which subjects will be asked about the presence of withdrawal symptoms and any other adverse events (AEs). On Days 4 to 9, safety, withdrawal, pharmacokinetic and pharmacodynamic assessments will be performed up to 13 hours after the first of the three daily study drug doses is administered. Following Day 10 study drug administration, assessments will be completed until 96 hours after the first of the three daily study drug doses is administered on the morning of Day 10. The Follow-up Phase will be conducted approximately 10 to 14 days after the last drug administration on Day 10 or after early withdrawal from the study and will include standard safety assessments.