Characterization of novel nitazene recreational drugs: Insights into their risk potential from in vitro µ-opioid receptor assays and in vivo behavioral studies in mice

Pharmacology of newly identified nitazene variants reveals structural determinants of affinity, potency, selectivity for mu opioid receptors

Nitazenes, a group of synthetic benzimidazole opioids, are a growing public health threat that have been linked to hundreds of deaths within the last five years. New nitazenes are discovered each year in drug samples and toxicology specimens, necessitating rapid pharmacological characterization. We characterized thirteen nitazenes identified by DEA as chemicals of concern, some that have not been previously characterized. We found that most were very high affinity and potency agonists at the mu opioid receptor (MOR) with very high selectivity for MOR versus other opioid receptors. While bulky benzyl substitutions and a lengthier linker reduced affinity and potency for MOR, the majority of nitazenes tested nonetheless exhibited high-to-very high MOR affinity, potency, and selectivity - often greater than that of fentanyl. Three of the nitazenes exhibited a novel pharmacological pattern, with lower selectivity for MOR versus the kappa opioid receptor (KOR), and with a pharmacological profile that more closely resembles morphine than fentanyl. These findings further delineate the chemical determinants of nitazene pharmacology and identify three of the least MOR-selective nitazenes to date.

Navigating nitazenes: A pharmacological and toxicological overview of new synthetic opioids with a 2-benzylbenzimidazole core

Since the first identification of isotonitazene on the recreational drug market in 2019, new synthetic opioids (NSOs) with a 2-benzylbenzimidazole core (colloquially known as 'nitazene' opioids) have increased in prevalence. At the end of 2024, 22 different analogues had been identified in Europe, and worrying trends indicate their increasing presence at the street level. Nitazene analogues originate from a series of research articles from the 1950-60s, but were never marketed as analgesics. Recent pharmacological research has shown that different analogues are highly active, with several being more potent than fentanyl in their ability to activate the μ-opioid receptor and produce opioid effects. This high potency, combined with their unpredictability on the recreational drug market, legal status in some regions, and economic appeal to drug producers, has contributed to a growing number of intoxications and fatalities involving nitazene analogues worldwide. This literature review focuses on the pharmaco-toxicology of nitazene opioids and the characteristics of their emergence on the NSO and broader recreational drug markets from 2019 onwards. Aspects that are covered include (a) a systematic approach to the naming of nitazene analogues, (b) trends, prevalence and identifications on the recreational drug market, (c) structure-activity relationships derived from recent in vitro, in vivo, and in silico research, (d) strategies for detection in biological samples and drug material, and (e) the current legal framework. Finally, innovative approaches to navigate this complex landscape are discussed, together with an outlook for the future.

The synthetic opioid isotonitazene induces locomotor activity and reward effects through modulation of the central dopaminergic system in mice

Synthetic opioids, which differ from fentanyl, have recently emerged as new psychoactive substances and pose public health concerns. The pharmacological effects and drug dependency of these benzimidazole-based opioids, also known as nitazenes, remain unclear. In this study, we examined the selectivity of opioid receptors, effects on motor activity, and expression of reward effects for isotonitazene, which has been detected in many poisonings and fatalities since 2019. Isotonitazene was most selective for the μ-receptor and exhibited more potent agonist effects, with an EC50 of 0.02 nM, than morphine (EC50 = 34 nM) and fentanyl (EC50 = 4.0 nM). In ICR mice, isotonitazene (up to 0.05 mg/kg) increased the locomotor activity in a dose-dependent manner. This effect was significantly suppressed by pretreatment with the opioid receptor antagonists naloxone (3 mg/kg) and β-FNA (1 mg/kg), the dopamine D1 receptor antagonist SCH23390 (0.5 mg/kg), and dopamine D2 receptor antagonist raclopride (6 mg/kg). The reward effects of isotonitazene, evaluated using conditioned place preference (CPP) in mice, showed that conditioning with isotonitazene produced significant dose-dependent CPP scores. Microdialysis analysis also confirmed that the isotonitazene dose that induced CPP (0.05 mg/kg) significantly increased dopamine levels in the nucleus accumbens of mice. These results suggest that isotonitazene, similar to fentanyl and morphine, is a compound with a high risk of forming drug dependence and reward effects via the dopaminergic nervous system. This study provides foundational data for biological evaluation of other nitazene compounds.