Pharmacologic Characterization of Substituted Nitazenes at μ, κ, and Δ Opioid Receptors Suggests High Potential for Toxicity
Open accessAbstract
The benzimidazole opioids (substituted nitazenes) are highly potent μ opiod receptor (MOR) agonists with heroin- or fentanyl-like effects. These compounds have caused hospitalizations and fatal overdoses. We characterized the in vitro pharmacology and structure-activity relationships of 19 nitazenes with substitutions at three positions of the benzimidazole core. Affinities were assessed using agonist radioligand binding assays at human μ, κ, and Δ opioid receptors (MOR, KOR, and DOR, respectively) heterologously expressed in CHO cells. Notably, for MOR binding, nine substituted nitazenes had significantly higher affinities than fentanyl including N-pyrrolidino etonitazene, N-pyrrilidino isonitazene, and N-desethyl isotonitazene; 13 had subnanomolar affinities. Only metodesnitazene and flunitazene had significantly lower affinities than fentanyl. Affinities for the substituted nitazenes at KOR and DOR relative to MOR were 46- to 2580-fold and 180- to 1280-fold lower, respectively. Functional activities were assessed using [35 S]GTP γ S binding assays. Four nitazenes had subnanomolar potencies at MOR: N-pyrrolidino etonitazene, N-pyrrilidino isonitazene, N-pyrrilidino protonitazene and N-desethyl isotonitazene. Ten substituted nitazenes had significantly higher potencies than fentanyl. All tested nitazenes were full MOR agonists. Potencies at KOR and DOR relative to MOR were 7.3- to 7920-fold and 24- to 9400-fold lower, respectively. Thus, many of these compounds are high affinity/high potency MOR agonists with elevated potential to elicit toxicity and overdose at low doses. SIGNIFICANCE STATEMENT: Substituted nitazenes are a growing public health threat. Although the 19 nitazenes tested vary in their opioid receptor pharmacology, a number are very high affinity, high potency, and high efficacy compounds- higher than fentanyl. Their pharmacology suggests high potential for harm.
The claims and statistics of this paper haven’t been checked yet.
Run the check — free, no account needed. It reads the open-access text and checks the sources it cites.
Integrity signals
Registry
Cites
Statistics
Claims the paper makes
The claims and statistics of this paper haven’t been checked yet.
Run the check — free, no account needed. It reads the open-access text and checks the sources it cites.
Claims other papers make about this one
Each is checked against this paper’s own text: does it say what the citing paper says it does?
Isotonitazene's high receptor potency and potential for active metabolites help explain why intoxications present with profound central nervous system depression, miosis, and respiratory depression even when the exposure history may be unclear.
“From a clinical standpoint, isotonitazene’s high receptor potency and potential for active metabolites help explain why intoxications present with profound central nervous system depression, miosis, and respiratory depression even when the exposure history may be unclear [6].”
From Novel 2-Benzylbenzimidazole Opioids: Emerging Drugs of Abuse and Pharmacological Considerations with Nitazene Analogs (2026) · cites this paper as [6]
The Discussion (page unavailable) identifies N-desethyl isotonitazene as an isotonitazene metabolite with similar μ-opioid receptor affinity and higher potency, and reports respiratory depression associated with it. The Introduction describes respiratory distress, confusion, and possible coma as signs of nitazene toxicity. The copy does not establish miosis or an unclear exposure history as features of isotonitazene intoxication. (LLM, conf=0.88)
Experimental — accuracy not yet measuredEtonitazene use can cause very high opioid toxicity, especially respiratory depression, even when the drug is present in very small amounts.
“These findings support the clinical expectation that etonitazene use can cause very high opioid toxicity, especially the common side effect of opioids being respiratory depression, even when the drug is present in very small amounts, highlighting its high potency [6].”
From Novel 2-Benzylbenzimidazole Opioids: Emerging Drugs of Abuse and Pharmacological Considerations with Nitazene Analogs (2026) · cites this paper as [6]
The Results (section 2.1; page unavailable) report very high μ-opioid receptor affinity for etonitazene, supporting its high potency at low concentrations. The Introduction identifies respiratory distress as a sign of nitazene toxicity, but the paper does not establish that very small amounts of etonitazene specifically cause respiratory depression in people. (LLM, conf=0.85)
Experimental — accuracy not yet measuredMetonitazene can produce opioid activation at low concentrations, so unexpected exposure through counterfeit tablets or mixed drug supplies creates an increased risk of overdose.
“Because metonitazene can produce opioid activation at low concentrations, unexpected exposure through counterfeit tablets or mixed drug supplies create an increased risk of someone having a overdose [6].”
From Novel 2-Benzylbenzimidazole Opioids: Emerging Drugs of Abuse and Pharmacological Considerations with Nitazene Analogs (2026) · cites this paper as [6]
The Results (section 2.3; page unavailable) report that metonitazene activates μ-opioid receptors at low nanomolar concentrations. The Discussion links nitazenes to overdose potential, but the passages do not address unexpected exposure through counterfeit tablets or mixed drug supplies, so that specific risk scenario is not supported. (LLM, conf=0.87)
Experimental — accuracy not yet measuredActivation of the mu opioid receptor by 2-benzylbenzimidazole opioids inhibits ascending nociceptive signaling but also suppresses brainstem respiratory drive, explaining why severe opioid toxidromes can occur after a small exposure.
“The 2-benzylbenzimidazole opioids primarily exert their effects through the agonism of the mu opioid receptor. Activation of this receptor inhibits ascending nociceptive signaling but also suppresses brainstem respiratory drive, which explains why severe opioid toxidromes can occur after a small exposure [6, 16].”
From Novel 2-Benzylbenzimidazole Opioids: Emerging Drugs of Abuse and Pharmacological Considerations with Nitazene Analogs (2026) · cites this paper as [6]
The Abstract and Discussion (page not provided) describe substituted nitazenes as potent, selective mu-opioid receptor agonists and report antinociception, respiratory depression, and overdose risk. They do not identify inhibition of ascending nociceptive signaling or suppression of brainstem respiratory drive as the mechanisms, or establish that a small exposure causes a severe toxidrome. (LLM, conf=0.86)
Experimental — accuracy not yet measuredIn vitro studies show that many nitazene analogs demonstrate high mu opioid receptor affinity, high functional potency and agonist activity, with multiple agents producing opioid receptor activation comparable to or greater than fentanyl.
“In vitro studies show that many nitazene analogs demonstrate high mu opioid receptor affinity, high functional potency and agonist activity, with multiple agents that produce opioid receptor activation comparable to or greater than fentanyl [6].”
From Novel 2-Benzylbenzimidazole Opioids: Emerging Drugs of Abuse and Pharmacological Considerations with Nitazene Analogs (2026) · cites this paper as [6]
Nitazene analogs (including isotonitazene, metonitazene, protonitazene, and etonitazene) are highly potent mu opioid receptor agonists.
“Nitazene analogs include those such as isotonitazene, metonitazene, protonitazene, and etonitazene are highly potent mu opioid receptor agonists and are increasingly detected in counterfeit and illicit pharmaceutical products with exposure in other substances including other drugs.”
From Novel 2-Benzylbenzimidazole Opioids: Emerging Drugs of Abuse and Pharmacological Considerations with Nitazene Analogs (2026) · cites this paper as [6]
Experimental pharmacology studies demonstrate that nitazene analogs function as highly effective mu opioid receptor agonists, often producing receptor activation at concentrations substantially lower than those required for traditional opioids.
“Experimental pharmacology studies have demonstrated that nitazene analogs function as highly effective mu opioid receptor agonists, often producing receptor activation at concentrations substantially lower than those required for traditional opioids [6, 25].”
From Novel 2-Benzylbenzimidazole Opioids: Emerging Drugs of Abuse and Pharmacological Considerations with Nitazene Analogs (2026) · cites this paper as [6]
Self-check
Percentages, sums and reported differences re-computed from the paper’s own numbers.
The self-check needs the paper’s full text.
Cites
Yung-Chi, C., & Prusoff, W. H. (1973) · Biochemical Pharmacology · doi:10.1016/0006-2952(73)90196-2 · Cited by 12,922 · see citing works
Drug Overdose Deaths in the United States, 1999-2017.
Hedegaard, H., Miniño, A., & Warner, M. (2018) · PubMed · Cited by 313 · see citing works
Toll, L., Berzetei-Gurske, I. P., Polgar, W. E., Brandt, S. R., Adapa, I. D., Rodriguez, L., Rw, S., Haggart, D., O’Brien, A. D., White, A. E., et al. (1998) · PubMed · doi:10.7270/q23j3bh2 · Cited by 243 · see citing works
Emmerson, P. J., Clark, M. J., Mansour, A., Akil, H., Woods, J. H., & Medzihradsky, F. (1996) · Journal of Pharmacology and Experimental Therapeutics · doi:10.1016/s0022-3565(25)20761-3 · Cited by 182 · see citing works
Vandeputte, M. M., Van Uytfanghe, K., Layle, N. K., St. Germaine, D. M., Iula, D. M., & Stove, C. P. (2021) · ACS Chemical Neuroscience · doi:10.1021/acschemneuro.1c00064 · Cited by 161 · see citing works
Isotonitazene Quantitation and Metabolite Discovery in Authentic Forensic Casework
Krotulski, A. J., Papsun, D. M., Kacinko, S. L., & Logan, B. K. (2020) · Journal of Analytical Toxicology · doi:10.1093/jat/bkaa016 · Cited by 146 · see citing works
Treatment of overdose in the synthetic opioid era
Skolnick, P. (2021) · Pharmacology & Therapeutics · doi:10.1016/j.pharmthera.2021.108019 · Cited by 123 · see citing works
Agonist binding, agonist affinity and agonist efficacy at G protein‐coupled receptors
Strange, P. G. (2008) · British Journal of Pharmacology · doi:10.1038/sj.bjp.0707672 · Cited by 120 · see citing works
Synthese basisch substituierter, analgetisch wirksamer Benzimidazol-Derivate
Hunger, A., Kebrle, J., Rossi, A., & Hoffmann, K. J. (1957) · Cellular and Molecular Life Sciences · doi:10.1007/bf02161116 · Cited by 103 · see citing works
Montanari, E., Madeo, G., Pichini, S., Busardò, F. P., & Carlier, J. (2022) · Therapeutic Drug Monitoring · doi:10.1097/ftd.0000000000000970 · Cited by 89 · see citing works
Naloxone Use in Novel Potent Opioid and Fentanyl Overdoses in Emergency Department Patients
Amaducci, A., Aldy, K., Campleman, S. L., Li, S., Meyn, A., Abston, S., Culbreth, R. E., Krotulski, A., Logan, B., Wax, P., et al. (2023) · JAMA Network Open · doi:10.1001/jamanetworkopen.2023.31264 · Cited by 84 · see citing works
Vandeputte, M. M., Krotulski, A. J., Walther, D., Glatfelter, G. C., Papsun, D., Walton, S. E., Logan, B. K., Baumann, M. H., & Stove, C. P. (2022) · Archives of Toxicology · doi:10.1007/s00204-022-03276-4 · Cited by 80 · see citing works
Torralva, R., Eshleman, A. J., Swanson, T. L., Schmachtenberg, J. L., Schutzer, W. E., Bloom, S. H., Wolfrum, K. M., Reed, J. F., & Janowsky, A. J. (2020) · Journal of Pharmacology and Experimental Therapeutics · doi:10.1124/jpet.120.265561 · Cited by 74 · see citing works
Self-citationEshleman, A. J., Nagarajan, S., Wolfrum, K. M., Reed, J. F., Nilsen, A., Torralva, R., & Janowsky, A. J. (2020) · Biochemical Pharmacology · doi:10.1016/j.bcp.2020.114293 · Cited by 64 · see citing works
Self-citationBjörkman, S., Wada, D., Stanski, D. R., & Ebling, W. F. (1994) · Journal of Pharmacokinetics and Biopharmaceutics · doi:10.1007/bf02353862 · Cited by 61 · see citing works
In vitro functional characterization of a panel of non-fentanyl opioid new psychoactive substances
Vandeputte, M. M., Cannaert, A., & Stove, C. P. (2020) · Archives of Toxicology · doi:10.1007/s00204-020-02855-7 · Cited by 58 · see citing works
Mu-Opioid Receptor Selective Superagonists Produce Prolonged Respiratory Depression
Malcolm, N. J., Palkovic, B., Sprague, D. J., Calkins, M. M., Lanham, J. K., Halberstadt, A. L., Stucke, A. G., & McCorvy, J. D. (2023) · iScience · doi:10.1016/j.isci.2023.107121 · Cited by 53 · see citing works
Glatfelter, G. C., Vandeputte, M. M., Chen, L., Walther, D., Tsai, M.-H. M., Shi, L., Stove, C. P., & Baumann, M. H. (2023) · Psychopharmacology · doi:10.1007/s00213-023-06451-2 · Cited by 52 · see citing works
Papsun, D. M., Krotulski, A. J., & Logan, B. K. (2022) · American Journal of Forensic Medicine & Pathology · doi:10.1097/paf.0000000000000787 · Cited by 51 · see citing works
De Luca, M. A., Tocco, G., Mostallino, R., Laus, A., Caria, F., Musa, A., Pintori, N., Ucha, M., Poza, C., Ambrosio, E., et al. (2022) · Neuropharmacology · doi:10.1016/j.neuropharm.2022.109263 · Cited by 48 · see citing works
Emerging Characteristics of Isotonitazene-Involved Overdose Deaths: A Case-Control Study
Shover, C. L., Falasinnu, T., Freedman, R. B., & Humphreys, K. N. (2020) · Journal of Addiction Medicine · doi:10.1097/adm.0000000000000775 · Cited by 39 · see citing works
Vandeputte, M. M., Tsai, M.-H. M., Chen, L., Glatfelter, G. C., Walther, D., Stove, C. P., Shi, L., & Baumann, M. H. (2023) · Drug and Alcohol Dependence · doi:10.1016/j.drugalcdep.2023.109939 · Cited by 39 · see citing works
Kozell, L. B., Eshleman, A. J., Swanson, T. L., Bloom, S. H., Wolfrum, K. M., Schmachtenberg, J. L., Olson, R. J., Janowsky, A. J., & Abbas, A. I. (2023) · Journal of Pharmacology and Experimental Therapeutics · doi:10.1124/jpet.122.001454 · Cited by 37 · see citing works
Self-citationWalton, S. E., Krotulski, A. J., Glatfelter, G. C., Walther, D. M., Logan, B. K., & Baumann, M. H. (2022) · Psychopharmacology · doi:10.1007/s00213-022-06292-5 · Cited by 33 · see citing works
Calello, D. P., Aldy, K., Jefri, M., Nguyen, T.-A. T., Krotulski, A., Logan, B., Brent, J., Wax, P., Walton, S., Manini, A. F., et al. (2022) · Clinical Toxicology · doi:10.1080/15563650.2022.2084406 · Cited by 26 · see citing works
Lutz, P. (2012) · PubMed · Cited by 16 · see citing works
Flagged first, then by citations
Cited by
Sorted by citations
De Vrieze, L. M., Walton, S. E., Pottie, E., Papsun, D., Logan, B. K., Krotulski, A. J., Stove, C. P., & Vandeputte, M. M. (2024) · Archives of Toxicology · doi:10.1007/s00204-024-03774-7 · Cited by 58 · see citing works
Vandeputte, M. M., & Stove, C. P. (2025) · Neuropharmacology · doi:10.1016/j.neuropharm.2025.110470 · Cited by 35 · see citing works
Vandeputte, M. M., Glatfelter, G. C., Walther, D., Layle, N. K., St. Germaine, D. M., Ujváry, I., Iula, D. M., Baumann, M. H., & Stove, C. P. (2024) · Pharmacological Research · doi:10.1016/j.phrs.2024.107503 · Cited by 35 · see citing works
Bade, R., Nadarajan, D., Hall, W., Brown, J. A., NPS Wastewater Consortium, & Schumann, J. (2025) · Addiction · doi:10.1111/add.70027 · Cited by 28 · see citing works
Curtis, B., Lawes, D. J., Caldicott, D., & McLeod, M. D. (2025) · Drug Testing and Analysis · doi:10.1002/dta.3910 · Cited by 19 · see citing works
Monti, M. C., De Vrieze, L. M., Vandeputte, M. M., Persson, M., Gréen, H., Stove, C. P., & Schlotterbeck, G. (2024) · Journal of Pharmaceutical and Biomedical Analysis · doi:10.1016/j.jpba.2024.116453 · Cited by 18 · see citing works
The emergence of nitazenes: a new chapter in the synthetic opioid crisis
Caprari, C., Ferri, E., Rossetti, P., Gregori, A., Citti, C., & Cannazza, G. (2025) · Archives of Toxicology · doi:10.1007/s00204-025-04102-3 · Cited by 19 · see citing works
A Putative Binding Model of Nitazene Derivatives at the μ-Opioid Receptor
Clayton, J., Shi, L., Robertson, M. J., Skiniotis, G., Michaelides, M., Stavitskaya, L., & Shen, J. (2025) · Neuropharmacology · doi:10.1016/j.neuropharm.2025.110437 · Cited by 12 · see citing works
Behavioural Effects and Naloxone Effectiveness With New Synthetic Opioids
Nielsen, S., Silva, J. P., Jones, J. D., Krotulski, A., Poovendran, D., Muzangizi, D., Forte, G., White, J., & Comer, S. D. (2026) · Drug and Alcohol Review · doi:10.1111/dar.70040 · Cited by 6 · see citing works
Berardinelli, D., Taoussi, O., Ovat, D. Y., Pichini, S., Pulver, B., Auwärter, V., Busardò, F. P., Basile, G., Laudadio, E., & Carlier, J. (2026) · Archives of Toxicology · doi:10.1007/s00204-025-04163-4 · Cited by 5 · see citing works
Verbeek, J., & Brinkman, D. J. (2025) · Basic & Clinical Pharmacology & Toxicology · doi:10.1111/bcpt.70078 · Cited by 5 · see citing works
Maxwell, M. K., Anwar, A. I., Hasoon, J., & Kaye, A. D. (2026) · Current Pain and Headache Reports · doi:10.1007/s11916-026-01525-0
- Authors
- Kozell, L. B., Eshleman, A. J., Wolfrum, K. M., Swanson, T. L., Bloom, S. H., Benware, S., Schmachtenberg, J. L., Schutzer, K. A., Schutzer, W. E., Janowsky, A., & Abbas, A. I.
- Affiliations
- Portland VA Medical Center (US)
- ORCID
- Venue
- The Journal of Pharmacology and Experimental Therapeutics
- Publisher
- Elsevier BV
- Published
- 2024-05
- Volume
- 389
- Issue
- 2
- Pages
- 219-228
- ISSN
- 0022-3565 (ISSN-L)
- PMC
- PMC11026150
- OpenAlex
- W4392564830
- Cited by
- 42
- References
- 27
- Citations by year
- 2026: 18 · 2025: 18 · 2024: 6
- FWCI
- 9.99
- Type
- journal article
- Language
- en
- Open access
- bronze
- doi.org
- Locations
- Journal of Pharmacology and Experimental Therapeutics (publishedVersion)
- PubMed Central (publishedVersion)
- PubMed (publishedVersion)
- Funders
- U.S. Department of Justice
- U.S. Department of Veterans Affairs (ADA12013, 14S-RCS-006)
- American Society for Pharmacology and Experimental Therapeutics
- Oregon Health and Science University
- Portland VA Research Foundation
- National Institutes of Health (-0001)
- United States Drug Enforcement Administration (D-22-OD-0001)
- Topics
- Life Sciences › Pharmacology, Toxicology and Pharmaceutics › Toxicology › Forensic Toxicology and Drug Analysis
- Health Sciences › Medicine › Public Health, Environmental and Occupational Health › Opioid Use Disorder Treatment
- Life Sciences › Pharmacology, Toxicology and Pharmaceutics › Pharmacology › Alkaloids: synthesis and pharmacology
- Keywords
- mu-opioid receptor, kappa opioid receptor, delta opioid receptor, opioid receptor affinity, structure-activity relationship, opioid overdose, opioid toxicity, etonitazene, GTPgammaS binding
- MeSH terms
- Analgesics, Opioid, Animals, Benzimidazoles, Cricetulus, Fentanyl, Cricetinae, Humans, Receptors, Opioid, mu, Receptors, Opioid, delta, Receptors, Opioid, kappa
About these checks
Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.
Statistics and arithmetic are recomputed, not judged.
Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.
What counts as a claim ›How we measure confidence ›How we check this ›
Are you an author, or do you see a problem with this page? Request a review
