Relative Human Adult Pharmacokinetics of Opioid Analogues to Guide Design and Clinical Evaluation of Opioid Vaccines and Protective Antibody Therapeutics

1. Introduction

More than 70,000 Americans still die from drug overdoses each year, and since 2000, over 850,000 people have fatally overdosed on opioids [1]. The only Food and Drug Administration (FDA)-approved rescue medications to acutely treat respiratory depression from opioid overdose are nalmefene and naloxone. Both must be given within minutes of an opioid overdose and require availability of a prepared and willing bystander who can administer them. Harm reduction strategies such as programs for education of the public and distribution of fentanyl test strips have limited utility during impulsive and/or opportunistic opioid exposure [2]. Given the limitations of existing strategies to prevent opioid overdose deaths, new approaches for preventing fatal overdoses are needed. Protective immune therapeutics, such as monoclonal antibodies (Ab) and vaccines have shown promise in animal models [3]. Translational development of vaccines is currently underway [4,5]. The diverse clinical uses of opioids—including anesthesia, pain management, treatment of opioid use disorder (OUD) (i.e., methadone, buprenorphine, naltrexone), and overdose reversal (i.e., naloxone, nalmefene)—complicate the development of immunotherapeutic strategies. A thorough understanding of the pharmacokinetics (PK) of individual opioids, including both the primary vaccine target and potential cross-reactive opioids, is essential for the development of safe and effective opioid vaccines.

Clinically relevant peak blood concentrations (C_max) associated with lethal opioid overdose are difficult to define. Overdose events typically occur outside healthcare settings, precluding prospective PK studies with standardized blood sampling. Even when blood specimens are obtained, the timing and circumstances of sample collection relative to drug administration is highly variable, and the risk of overdose is influenced by factors beyond blood opioid concentration, including multi-substance exposure, individual tolerance, genetic influences on metabolism and other co-occurring disorders. Consequently, no universal C_max threshold reliably defines safe opioid exposure. While not a perfect surrogate, serum or plasma concentrations following well-characterized therapeutic opioid dosing regimens provide reproducible benchmarks that can serve as practical target concentrations for the development and evaluation of Ab-inducing opioid vaccines and immunotherapeutics [6,7,8], with the eventual goal of establishing Ab correlates of protection for the target opioids.

Assessing Ab cross-reactivity is a critical step in developing immune therapeutics. Cross-reactivity may confer broader protection against overdose or, conversely, reduce the pharmacologic efficacy of therapeutic opioids, including analgesics and medications used to treat acute pain and OUD. Estimated serum or plasma C_max values are needed to interpret assay data, as they define the clinically relevant concentration range used to generate inhibitory concentration curves for individual opioids [9,10] (Figure 1). This would include matching the putative, yet currently unknown, protective anti-opioid Ab levels with definable plasma/serum C_max with the maximum clinically observed dose of known opioid in a specific clinical setting. As a first step, here we collected and summarized existing knowledge on select opioid-specific PK variables in humans and, where not available, laboratory animals.

2. Methods

We conducted a literature search in PubMed to compile PK and select pharmacodynamic (PD) and chemical information for ten opioid compounds (fentanyl, buprenorphine, morphine, oxycodone, methadone, naloxone, naltrexone, remifentanil, sufentanil, alfentanil). We excluded combination products (i.e., suboxone) and formulations used almost exclusively in combination products (i.e., hydrocodone). Information of interest included typical opioid dosing for adults, serum and plasma C_max, mean dose response for analgesia and anesthesia (respiratory suppression) in humans under various conditions and by study participant pre-exposure status (opioid exposed vs naïve), and LD₅₀, the lethal dose required to kill 50% of a test rodent population. This is not an exhaustive systematic review, but we included the most commonly used opioid medications for which there is available published PK data.

3. Results

The pharmacological properties of the opioids of interest are presented in Table 1. These properties are influenced by product-specific characteristics such as drug structure, formulation (i.e., immediate vs. extended release), number of doses administered, and routes of administration (subcutaneous, oral, IV) [5], which in turn drive effective drug concentrations in serum/plasma. Peak plasma concentrations of unbound drug (C_max) were derived from the most commonly prescribed doses in adults with normal liver and kidney function. Drug intake patterns and dosing depend on context and vary widely based on the recipient’s opioid exposure history, dosing regimen, and the clinical indication for opioid administration. For example, methadone is typically prescribed at lower doses for pain management than for the treatment of OUD, where substantially higher doses are often required. Similarly, buprenorphine dosing varies considerably by indication, formulation, and route of administration. For analgesia with fentanyl in opioid naïve awake adult patients, typical intravenous dosing is in the range of 50 – 100 mcg. Patients undergoing general anesthesia, patients receiving prolonged infusions during mechanical ventilation, and patients with advanced cancer and opioid tolerance often receive much higher doses. Opioid PK also differ between single doses and repeated doses or infusions. This is due in part to accumulation of opioids in body compartments. With a prolonged infusion, the term context-sensitive half-life refers to the time for blood/serum concentration to decrease by 50% if the infusion is suddenly stopped. This half-life varies as a function of the duration of the infusion [11]. Therefore, information from this table should be used as a rough guide acknowledging that opioid vaccine researchers and vaccine developers should anticipate and account for broad opioid dose ranges used by different populations. The results shown primarily focus on analgesic doses/concentrations in opioid naïve adults and where applicable or known, doses/concentrations for acute use, anesthesia or OUD were added.

Although in PK terms the relationship between drug concentration and drug-specific Ab binding is not linear, using mathematical models accounting for hillslope and top/bottom optical density (=strength of the binding between drug and inhibitor compound representing no/complete Ab inhibition) (Figure 1B), researchers could use the half-maximal inhibitory concentration (IC₅₀) to infer the amount of the target drug needed to inhibit half of the available vaccine-induced Abs in serum of vaccinated subjects, as well as the amount of any non-vaccine targeted opioids required to maintain their efficacy in presence of opioid-specific vaccination.

4. Discussion

Characterization of Ab specificity is essential to ensure that opioid vaccines achieve their intended effects while minimizing unintended interference with opioid pharmacotherapy. Incorporation of human serology modeling in human clinical studies requires reliable estimates of key PK parameters including C_max and maximum clinically recommended dose for the drugs of interest. Vaccine developers must evaluate Ab activity against different opioids to assess the potential for clinically meaningful interference with opioid therapy. Cross-reacting Ab might be expected in vaccinated individuals and their quantitative (concentration) and qualitative (binding affinity, avidity, specificity) properties are critical parameters to study in PK/PD preclinical studies. In fact, Ab affinity is critical for prediction of potential opioid vaccine interference. For example, an individual receiving low-dose methadone for pain management may tolerate vaccine-induced Ab binding without significant impact on analgesia if affinity of the vaccine-induced Ab to methadone is low, whereas the same Ab concentrations could compromise treatment efficacy in an individual receiving higher-dose methadone for OUD if Ab affinity to methadone is high. Competitive ELISA assays provide an in vitro approach for evaluating the ability of off target opioids to interfere with vaccine-induced Ab binding (Figure 1).

Currently, two active human trials of opioid vaccines are underway. A Phase 1a/1b multi-site (New York State Psychiatric Institute and the Clinilabs clinical research unit in Eatontown, New Jersey) triple blind randomized clinical trial of an oxycodone vaccine (Oxy(Gly)4-sKLH Conjugate Vaccine, adsorbed to the aluminum adjuvant Alhydrogel) is testing safety, tolerability, Ab production and efficacy which is operationally defined as self-reported drug “liking” following opioid administration measured using a visual analog scale (0-100) (NCT04458545) [52]. This trial was initiated in 2023 with a recruitment goal of forty-five healthy adults, aged 18 to 59 years, who meet Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) criteria for OUD but are not seeking treatment to reduce their drug use. Participants are randomized to placebo, low dose experimental vaccine or high dose experimental vaccine delivered at weeks 0, 3, 6 and 18 and followed for 42 weeks. Ab titers, concentrations, affinity for oxycodone, cross-reactivity to off-target opioid agonists and antagonists (including endogenous opioid neuropeptides), and subclasses of the serum IgG Ab are measured at baseline and repeatedly over the course of the study period to characterize the magnitude and duration of the Ab response to vaccination.

The second trial is a phase 1/2 clinical trial of an adjuvanted anti-fentanyl vaccine (ARMR-100) to test its safety, tolerability and immunogenicity, defined as its ability to induce an Ab that binds to fentanyl and prevents its penetration of the blood-brain-barrier. Initiated in early 2026, this study will enroll 40-60 healthy adults at the Center for Human Drug Research in the Netherlands and include a human fentanyl challenge phase. As of June 17, 2026, the vaccine has shown favorable preliminary safety results among 24 study participants along with encouraging early immunogenicity results, with further efficacy data being forthcoming [53,54].

Inimmune in collaboration with the University of Montana, University of Minnesota, and Columbia University have an Investigational New Drug (IND) approved for an anti-heroin vaccine including the Inimmune / University of Montana Opioid Vaccine custom toll-like receptor adjuvant (INI-4001). A phase 1 clinical trial funded by the National Institutes of Health (NIH) Helping to End Addiction Longterm (HEAL) Initiative is currently undergoing site initiation activities (personal communication, Jay Evans). There is also an INI-4001 adjuvanted anti-fentanyl vaccine that has demonstrated pre-clinical efficacy in fentanyl challenge models in a number of different species and has advanced to IND preparations and current Good Manufacturing Practice (cGMP) manufacturing with the first Phase I trial expected to begin in 2027 [55,56].

As the field of opioid vaccine development matures and precision formulations with sophisticated antigen-adjuvant combinations advance through the FDA IND processes, we anticipate initiation of additional human clinical trials of mono- and multi-valent opioid vaccines and immunotherapeutics soon. We believe the information in this review will help opioid vaccine researchers and vaccine developers in assessing and de-risking their formulations ahead of use in safety and efficacy clinical trials. Of note, PK and PD parameters for potential antibody transmission to neonates from vaccinated mothers and those who receive blood or organ donations from vaccinated individuals are not covered in this review and will require further research.

5. Conclusion

We present PK information about molecules that bind to the µ-opioid receptor that help guide development of a vaccine to prevent opioid overdose. The data collated here may assist investigators in establishing PK parameters for reliable in vitro/ex vivo testing to measure opioid vaccine effectiveness, safety and cross-reactivity, and ultimately help guide efforts towards defining opioid vaccine correlates of protection. We hope this work will inform future discovery and development of opioid vaccines and preventative therapeutics, as well as vaccine or drug compound dosing decisions in future human challenge studies.