iSTEP-N 101b: Pharmacokinetics and Safety Study of Low- and High-Dose Naltrexone Implants vs Monthly Vivitrol in Healthy Volunteers

Inclusion Criteria: Participants must meet all of the following criteria to be eligible for the study: Healthy adult male or female volunteers, ages 18 to 55 years. Able and willing to provide written informed consent prior to any study procedures. In good general health as determined by medical history, physical examination, vital signs, ECG, and laboratory tests (chemistry, hematology, and coagulation). No clinically significant abnormalities in lab results, as determined by the investigator. Negative urine drug screen for opioids, cocaine, amphetamines, benzodiazepines, cannabinoids, and other substances of abuse. Negative breath alcohol test at screening and baseline. Negative naloxone challenge test, indicating no physiological opioid dependence. For females of childbearing potential: Negative serum pregnancy test at screening and Day 0. Agreement to use acceptable contraception (including oral hormonal contraception) for the duration of the study. For males with female partners of childbearing potential: Agreement to use effective contraception throughout the study. Negative infectious disease panel, including HIV, hepatitis B surface antigen, and hepatitis C antibody. No evidence of suicidal ideation or behavior on the Columbia-Suicide Severity Rating Scale (C-SSRS) at screening. Hamilton Depression Rating Scale (HAM-D17) total score of 0 to 10, consistent with no or minimal depressive symptoms. Willing and able to comply with all study visits, procedures, and restrictions, including pharmacokinetic sampling. Willing to refrain from donating blood during the study period. Agree not to use opioid-containing medications for the duration of the study unless medically necessary and approved by the investigator. Exclusion Criteria: Participants who meet any of the following conditions will be excluded: Current or recent history (past 12 months) of opioid use disorder, substance use disorder, or alcohol dependence. Positive urine drug test or breath alcohol test at screening or prior to randomization. Failure of the naloxone challenge test, indicating possible physical opioid dependence. Use of any investigational drug or device within 30 days prior to screening. Use of any opioid-containing medications (prescription or OTC) within 14 days prior to screening. Known hypersensitivity or allergy to naltrexone, polycaprolactone (PCL), polylactic acid (PLA), polymeric implants, or any other component of the study drug. History of chronic pain, neurological or psychiatric disorders, or any condition requiring regular use of medications. Active medical condition or past medical history that, in the opinion of the investigator, could interfere with the study or pose an undue risk, including: Cardiovascular disease Hepatic or renal impairment Gastrointestinal disorders affecting absorption Respiratory disease Seizure disorder Autoimmune or inflammatory disorders Females who are pregnant, breastfeeding, or planning to become pregnant during the study period. Baseline systolic blood pressure \>160 mmHg or diastolic blood pressure \>100 mmHg. History of major depressive disorder, bipolar disorder, psychosis, or other severe psychiatric illness. HAM-D17 score \>10 at screening or Day 0. Any suicidal ideation or behavior as defined by the C-SSRS in the past 6 months. Participation in another clinical trial with investigational medication or device within the past 30 days. Known coagulation disorder or current use of anticoagulants. Inability to comply with study procedures due to geographic, social, or mental limitations. Presence of any implantable medical device that may interfere with study procedures or assessments. History of keloid formation or abnormal wound healing, which may affect implant insertion site. History of HIV, hepatitis B, or hepatitis C, unless determined to be false positive or clinically insignificant by the investigator. Any condition that, in the investigator's opinion, would make the subject unsuitable for the study.