Beyond Buprenorphine: Emerging Research Compounds in Medication-Assisted Treatment Studies

Buprenorphine has transformed the treatment of opioid use disorder. It reduces withdrawal and craving, blocks many of the effects of other opioids, and can be prescribed in several daily and extended-release formulations.

It is not, however, a perfect fit for every patient. Some people experience difficulty during induction, inadequate symptom control, side effects, limited treatment access, or challenges discontinuing the medication. These limitations have encouraged researchers to investigate new pharmacological strategies.

“Beyond buprenorphine” does not mean “better than buprenorphine.” It means exploring whether future medications could complement today’s options, serve patients who do not respond to them, or address needs that existing treatments leave unmet.

That distinction is important because the experimental compounds discussed here do not have anything approaching the clinical evidence supporting approved medications for opioid use disorder.

The established standard: three FDA-approved medications

The term medications for opioid use disorder, or MOUD, is increasingly preferred over medication-assisted treatment. The newer language recognizes that medication is itself an evidence-based treatment rather than merely an addition to counseling.

The FDA currently recognizes three medications for OUD: methadone, buprenorphine, and naltrexone.

Methadone

Methadone is a long-acting full MOR agonist. When appropriately administered, it prevents withdrawal, reduces craving, and limits the repeated intoxication-and-withdrawal cycle associated with short-acting opioids.

Its major advantage is strong receptor activation and high treatment retention, particularly for people with substantial opioid tolerance. Its disadvantages include respiratory-depression risk, clinically important drug interactions, QT-interval concerns, physical dependence, and—in the United States—significant restrictions on how methadone for OUD is dispensed.

Buprenorphine

Buprenorphine is a high-affinity partial MOR agonist. It can suppress withdrawal and craving while producing a ceiling effect on some opioid effects, including respiratory depression.

It is available in transmucosal and extended-release injectable formulations. Compared with methadone, it can generally be prescribed in a broader range of medical settings. Its high receptor affinity can nevertheless precipitate withdrawal if treatment is initiated while a full agonist remains strongly active. It can also produce physical dependence, and combining it with other respiratory depressants remains dangerous.

Naltrexone

Naltrexone is an opioid receptor antagonist rather than an agonist. Extended-release naltrexone blocks opioid effects without producing opioid intoxication or physical opioid dependence.

Its principal challenge is initiation: a patient must first complete opioid withdrawal and reach an opioid-free state. That requirement creates an induction barrier that does not exist in the same form with agonist treatments. Once naltrexone is stopped, reduced tolerance can also increase the danger of overdose if opioid use resumes.

Methadone and buprenorphine have been associated with substantial reductions in all-cause and overdose mortality while patients remain in treatment. A major meta-analysis found that retention in either treatment was associated with markedly lower mortality. Any proposed replacement must therefore be compared with an effective—and potentially lifesaving—standard, not with an imaginary risk-free baseline.

SR-17018: an atypical MOR research compound

SR-17018 is an experimental MOR agonist originally characterized as strongly biased toward G-protein signaling and away from β-arrestin2 recruitment.

In mice, repeated SR-17018 exposure produced less tolerance than morphine or oxycodone in several—but not all—antinociception models. When substituted after chronic morphine exposure, it suppressed observable withdrawal and restored morphine sensitivity in a hot-plate assay. These substitution findings were reported in Neuropsychopharmacology.

That profile raises an intriguing possibility: perhaps a transition medication could provide enough MOR signaling to prevent abrupt withdrawal while allowing selected adaptations associated with a previous opioid to diminish.

SR-17018 also illustrates why animal results must be interpreted cautiously.

Repeated SR-17018 produced withdrawal after discontinuation in mice. A later study detected tolerance in a different pain assay. Rat self-administration research found reduced but apparent reinforcing effects. Studies in nonhuman primates reported low in-vivo efficacy, emphasizing the difficulty of translating promising rodent pharmacology across species. The primate findings were published in Drug and Alcohol Dependence.

Potential research advantages

• Suppression of morphine withdrawal in a rodent substitution model
• An atypical pattern of tolerance and receptor adaptation
• Lower respiratory effects than conventional opioids in some animal experiments
• Oral activity and central nervous system penetration in rodents
• A pharmacological profile that may help researchers separate different MOR functions

Major disadvantages and unknowns

• No established human efficacy for withdrawal or OUD
• No validated human safety, pharmacokinetic, or interaction profile
• Evidence of physical dependence in animals
• Apparent reinforcement in animal self-administration research
• Conflicting findings across assays and species
• No standardized pharmaceutical manufacturing or approved clinical formulation
• No evidence-based induction, maintenance, or discontinuation protocol

SR-17018 is therefore better described as a promising research tool than as an alternative treatment.

Other biased and low-efficacy MOR agonists

SR-17018 belongs to a broader effort to create MOR agonists with a greater separation between desired and dangerous effects.

Oliceridine, PZM21, and related compounds were developed partly around G-protein-biased or low-efficacy agonism. Oliceridine ultimately received FDA approval for intravenous management of acute pain—not for opioid use disorder. Its development demonstrated that signaling-selective compounds can reach clinical testing, but it did not prove that biased agonism eliminates the fundamental risks of MOR activation.

Other members of the SR series, including SR-14968, have shown antinociceptive activity with reduced respiratory effects in some animal experiments. In a 2024 rat study, both SR-14968 and SR-17018 functioned as reinforcers, although less effectively than fentanyl and oxycodone. The study’s authors concluded that the compounds retained apparent abuse potential.

The lesson for OUD research is clear: a wider respiratory safety window would be valuable, but it is not enough. A viable medication must also be evaluated for craving, reinforcement, compulsive use, physical dependence, withdrawal, real-world retention, and overdose outcomes.

Combined NOP/MOR agonists

Another strategy involves activating both MOR and the nociceptin/orphanin FQ peptide receptor, known as NOP.

Compounds such as AT-121, BU08028, and BU10038 were designed to balance MOR-mediated effects with NOP signaling. In nonhuman primates, AT-121 produced antinociception and reduced the reinforcing effects of oxycodone without producing several typical opioid adverse effects in the tested conditions. The foundational AT-121 study was published in Science Translational Medicine.

This class is particularly interesting because it may influence both pain and drug-reinforcement circuitry. Unlike a pure MOR substitution strategy, NOP/MOR coactivation could theoretically reduce the motivation to seek other opioids while still controlling withdrawal-related distress.

The disadvantages remain substantial. These compounds are largely preclinical, their effects can vary depending on the balance of NOP and MOR activity, and favorable nonhuman-primate results do not establish long-term human safety.

Cebranopadol, another mixed NOP/opioid receptor agonist, has progressed further in pain research than AT-121. However, it can still produce opioid-like adverse effects and has not been established as an OUD medication.

Mitragynine and pharmaceutical kratom research

Mitragynine is the principal alkaloid associated with Mitragyna speciosa, commonly known as kratom. It interacts with opioid and non-opioid systems, while its metabolites—including 7-hydroxymitragynine—may contribute importantly to its pharmacology.

In rats, mitragynine reduced behavioral signs of morphine withdrawal in a comparison that also included methadone and buprenorphine. The animal study was published in Frontiers in Psychiatry.

Human research is beginning to move from loosely standardized botanical products toward defined pharmaceutical formulations. A registered first-in-human study is evaluating MG001, an oral mitragynine formulation. The Phase 1 study is listed on ClinicalTrials.gov.

This approach has several theoretical advantages:

• A nontraditional opioid-receptor profile
• Longstanding reports of self-directed use during opioid withdrawal
• Potential to isolate and standardize a specific active compound
• The possibility of separating mitragynine from variable botanical mixtures

The concerns are equally important. Kratom products have been associated with dependence and withdrawal, contamination, variable alkaloid concentrations, drug interactions, and adverse events. Mitragynine is metabolized into compounds with their own opioid activity, making its human pharmacology more complicated than the phrase “natural alternative” implies.

Pharmaceutical mitragynine must therefore be evaluated as a drug, not presumed safe because its source is botanical.

Ibogaine and rapid-transition research

Ibogaine is pharmacologically different from MOR substitution therapies. It interacts with multiple neurotransmitter systems and has been investigated for its reported ability to interrupt withdrawal and craving after a limited number of administrations.

A Phase 1/2a study has evaluated an oral ibogaine formulation in opioid withdrawal. The trial is registered on ClinicalTrials.gov.

Ibogaine research is compelling because it asks whether OUD treatment can target broader neuroplastic and motivational processes instead of maintaining opioid-receptor occupancy. However, ibogaine can produce dangerous cardiac effects, including QT prolongation and arrhythmias. Neurological and psychiatric risks also require careful screening and monitoring.

Any legitimate clinical development would require pharmaceutical-standard material, continuous medical observation, cardiac safeguards, and controlled trials. Unsupervised use cannot be equated with clinical research.

How the experimental approaches compare

What future OUD research must measure

A promising compound cannot be judged solely by whether it reduces withdrawal signs during several hours of an animal experiment.

Future studies should evaluate:

1. Human pharmacokinetics and active metabolites

Researchers must establish absorption, distribution, metabolism, elimination, accumulation, and interaction risks. Rodent half-lives cannot be converted into human treatment schedules by simple arithmetic.

2. Respiratory safety under realistic conditions

Studies must consider alcohol, benzodiazepines, gabapentinoids, sedatives, and illicit opioid exposure. A compound that appears safe by itself may behave very differently in combination.

3. Reinforcement and abuse liability

Reduced euphoria is not the same as an absence of compulsive use. Self-administration, drug-liking, escalation, and diversion must be studied directly.

4. Dependence and discontinuation

Researchers must determine whether a candidate eliminates dependence, creates a milder form of dependence, or merely transfers dependence from one compound to another.

5. Induction in the fentanyl era

Illicit fentanyl and its analogues complicate medication transitions because of their potency, variable exposure, and prolonged redistribution from body tissues. New treatments must be tested in populations that reflect the current drug supply.

6. Retention, overdose, and quality of life

The outcomes that ultimately matter are survival, treatment retention, reduced nonmedical opioid use, improved functioning, and patient-defined recovery—not simply a favorable receptor assay.

7. Individualized treatment

OUD is heterogeneous. Some patients need strong agonist stabilization, some prefer partial agonism, some choose antagonist therapy, and others may benefit from long-acting formulations or future non-opioid interventions. The goal should be a larger evidence-based toolbox, not a single universal replacement.

The future is likely to be complementary

Emerging compounds should not be framed as reasons to abandon buprenorphine or methadone. Those medications already prevent deaths and support long-term recovery.

The more realistic future is complementary. Biased MOR agonists may reveal how to reduce certain opioid liabilities. NOP/MOR compounds may combine withdrawal control with reduced reinforcement. Defined mitragynine formulations may clarify whether kratom-derived pharmacology can be converted into a reproducible medication. Non-opioid approaches may eventually help with craving, stress, and relapse without maintaining MOR dependence.

SR-17018 occupies an important place in this research landscape because it demonstrates that MOR agonists need not all produce identical signaling or tolerance profiles. Its scientific value is real. Its clinical value remains unknown.

The next generation of OUD treatments will require the same standard applied to every serious medicine: controlled human trials, pharmaceutical quality, transparent adverse-event reporting, and direct comparison with treatments already known to save lives.

Research notice: SR-17018 and most compounds discussed in this article are not FDA-approved treatments for opioid withdrawal or opioid use disorder. This article is educational and should not be interpreted as medical, dosing, or tapering guidance.