This single-center study is a randomized, double-blind, active-controlled, multiple-dose, parallel group study to evaluate the potential withdrawal effects, safety and tolerability, and pharmacokinetic and pharmacodynamic effects of multiple doses of oral intact Oxycodone/Naltrexone capsules compared to oxycodone in methadone-maintained opioid-dependent subjects. The study will consist of a Screening Phase, an inpatient Treatment Phase (including a methadone stabilization period, a placebo administration period, a treatment administration period and a safety/washout period), and a Follow-up Phase.

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.

Inclusion Criteria:

1. Healthy male or female subjects 18 to 55 years of age, inclusive.
2. Body mass index (BMI) within the range of 18.0 to 35.0 kg/m2, inclusive, and a minimum weight of 50.0 kg.
3. Maintained on a stable methadone regimen. Subjects must be receiving a methadone dose of 50 to 150 mg/day at Screening, with dosage variation ≤20% for at least 14 days prior to the Screening visit and who are willing to remain on their dose at Screening and for the duration of the study.
4. The study volunteer has been previously exposed to opioids while on methadone maintenance without relapse occurring, or in the opinion of the methadone maintenance program Director or Physician, is not at undue risk of relapse from participation in the clinical study.

Exclusion Criteria:

1. History or presence of clinically significant abnormality as assessed by physical examination, medical history, 12-lead ECG, vital signs, or laboratory values, which in the opinion of the investigator would jeopardize the safety of the subject or the validity of the study results.
2. Clinically significant infection/injury/illness within 1 month prior to Screening.
3. Has been diagnosed with cancer within 5 years prior to screening (excluding squamous or basal cell carcinoma of the skin), or has an active malignancy of any type (including squamous or basal cell carcinoma of the skin).
4. History of major mental illness that in the opinion of the investigator may affect the ability of the subject to participate in the study. Institutionalized subjects will not be eligible for participation.
5. Documented history of a medical condition that, in the opinion of the investigator, would compromise the subject's ability to swallow, absorb, metabolize, or excrete test product, including (but not limited to) intractable nausea and/or vomiting and/or severe gastrointestinal narrowing (pathologic or iatrogenic).
6. Documented history of, or currently active, seizure disorder or history of clinically significant head injury or syncope of unknown origin.
7. History of severe allergic reaction (including anaphylaxis) to any substance, or previous status asthmaticus, or food allergies/intolerances/restrictions, or special dietary needs which, in the judgment of the investigator, contraindicates the subject's participation in the study.
8. History of allergy or hypersensitivity to oxycodone or related drugs (e.g., other opioids) or naltrexone or related drugs (e.g., other antagonists).
9. 13. An employee of the sponsor or research site personnel directly affiliated with this study or their immediate family member defined as a spouse, parent, child or sibling, whether biological or legally adopted.
10. A subject who, in the opinion of the investigator or designee, is considered unsuitable or unlikely to comply with the study protocol for any reason.