Papers of the Week

Illicitly manufactured fentanyl is fueling the current overdose epidemic, and non-fentanyl mu-opioid receptor (MOR) agonists are emerging in street drug markets worldwide. The etonitazene analog, isotonitazene, is one example of a non-fentanyl MOR agonist linked to overdose deaths. Little is known about the biological effects of isotonitazene in humans or animal models. To this end, we examined the pharmacodynamics, pharmacokinetics, and metabolism of isotonitazene in rats. Male rats were fitted with surgically implanted intravenous (i.v.) catheters and subcutaneous (s.c.) temperature transponders under ketamine/xylazine anesthesia. One week later, rats received s.c. isotonitazene (3, 10, or 30 μg/kg) or its vehicle, and blood samples (0.3 mL) were collected via catheters at 15, 30, 60, 120, 240 min post-injection. Plasma was assayed for isotonitazene and its metabolites by liquid chromatography tandem mass spectrometry. Pharmacodynamic effects – including hot plate latency, catalepsy score, and body temperature – were assessed at each blood withdrawal. Isotonitazene produced dose-dependent increases in hot plate latency (ED50=4.2 μg/kg) and catalepsy (ED50=8.7 μg/kg), while 30 μg/kg produced marked hypothermia. Isotonitazene concentrations in plasma rose linearly with increasing dose, Cmax (0.5 to 6.6 ng/mL) was achieved within 15 min, and drug half-life ranged from 40 to 60 min. Isotonitazene metabolites were detectable but below the level of quantification. Analgesia, catalepsy, and hypothermia were correlated with mean isotonitazene concentrations. Radioligand binding assays revealed that N-desethyl isotonitazene displays higher affinity at MOR (Ki=2.2 nM) than the parent compound (Ki=15.8 nM). Our findings reveal that isotonitazene is a MOR agonist that is ~1000-fold more potent than morphine (ED50=4.2 mg/kg) as an analgesic agent. Plasma concentrations of isotonitazene are in the low ng/mL range, whereas metabolites are found in even lesser amounts. Although the N-desethyl metabolite of isotonitazene displays high affinity at MOR, extremely low levels are formed in vivo. The ultra-high potency of isotonitazene presents challenges for forensic detection and likely poses grave risk to users who are inadvertently exposed to the drug.