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

Opioid use disorder (OUD) remains a major public health challenge, driven in part by relapse and overdose risk after detoxification. Extended-release naltrexone, an opioid receptor antagonist, is approved as a once-monthly intramuscular injection (Vivitrol®) and reduces relapse risk by blocking opioid effects. However, treatment adherence may be limited by the need for repeated clinic visits and injections, and drug levels decline toward the end of each dosing interval, potentially reducing protection during periods of highest relapse vulnerability.

iSTEP-N is an investigational subcutaneous implant designed to provide long-term delivery of naltrexone from a biodegradable polymer matrix. After placement beneath the skin of the thigh, the implant slowly releases medication over many months and gradually resorbs. Sustained medication delivery may improve adherence compared with monthly injections and maintain opioid receptor blockade without requiring repeated dosing visits.

Protocol iSTEP-N 101b is a Phase 1b study in healthy volunteers designed to further characterize pharmacokinetics and safety of extended-release naltrexone delivered via iSTEP-N implants compared with monthly injectable naltrexone. The study evaluates two implant dose levels in parallel with active-comparator treatment to determine whether the implant can maintain plasma naltrexone concentrations comparable to or exceeding those achieved with monthly injections over a one-year period.

The primary focus of the study is characterization of plasma concentrations of naltrexone over time and estimation of pharmacokinetic parameters that describe drug exposure following implant placement. Secondary analyses include evaluation of concentrations of the major metabolite, 6β-naltrexol, and assessment of safety and tolerability across treatment groups.

Because the investigational product is a drug-device combination, the protocol also evaluates implant site outcomes and the duration of implant detectability and drug release. Participants receiving iSTEP-N who continue to have detectable implant material or measurable naltrexone levels after the first year may enter a long-term follow-up phase. This phase allows investigators to determine how long the implant continues to release drug and when the implant is no longer detectable by clinical examination or imaging.

The study design incorporates staged enrollment and independent safety monitoring to ensure early identification of any safety concerns during implant administration and follow-up. Frequent safety assessments are performed throughout participation, including monitoring for implant site reactions and systemic adverse effects associated with naltrexone.

Results from this study are intended to guide dose selection and study design for subsequent clinical development of long-acting naltrexone implants aimed at reducing barriers to sustained opioid relapse prevention therapy.

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.