Does Kappa Agonism Improve Reversal of 'Tranq-Dope' Overdose? Evidence from a Rodent Model

Background/Objectives: The recreational use of fentanyl (FT) combined with xylazine (XZ), known as "tranq-dope," poses a growing public health threat due to its high toxicity and mortality. This study evaluated the effectiveness of naloxone (NX), its lipophilic prodrug NX90, and their combinations with the mixed κ-agonist/µ-antagonist nalbuphine (NB) in reversing overdose and restoring respiratory function in a rat model. Methods: Male and female Wistar rats received intramuscular FT (0.104 mg/kg) + XZ (1 mg/kg) to induce overdose, followed by intranasal administration of NX, NX90, or combinations with NB. Physiological parameters, reflex recovery, time to overdose, and reversal outcomes were assessed during individualized clinical monitoring. Results: At the low FT dose (0.052 mg/kg), adding XZ (1 mg/kg) shortened time to overdose by ~2600 s compared with FT alone, whereas onset times were similar at medium and high FT doses. In the dose-finding cohort, FT + XZ co-administration was associated with a higher respiratory rate than FT alone at the highest fentanyl dose tested, an exploratory finding warranting confirmation in larger studies. Most interventions did not significantly shorten time to reversal; however, NX + NB (females) and NX90 + NB (both sexes) showed shorter reversal times than NX alone. However, respiratory rate at reversal was significantly improved with NX + NB, ½NX90 + NB and NX90 + NB (90 ± 6, 86 ± 5 and 92 ± 5 breaths/min) compared with naloxone alone (80 ± 6 breaths/min). Interventions containing nalbuphine (κ-agonist/µ-antagonist) yielded higher RR and HR at reversal than NX alone, consistent with an interpretive framework in which κ-µ opioid balance may influence observed physiological recovery patterns. Conclusions: Comparable or improved reversal outcomes could be achieved using half-doses of NX or NX90 with NB-potentially reducing the total dose of naloxone and mitigating the risk of precipitated withdrawal in individuals with opioid use disorder.