Comparison of ventilatory depression by carfentanil, fentanyl, and heroin, and reversal by opioid receptor antagonists in rats

Fentanyl and its analogs continue to drive the overdose crisis in the United States. It is unclear whether there are unique properties of fentanyls that increase the risk of opioid overdose compared with other opioid receptor agonists (e.g., heroin). This study compared the ventilatory depressant effects of the opioid receptor agonists heroin, fentanyl, and carfentanil, and reversal of those effects by opioid receptor antagonists. This study used whole-body plethysmography in rats to determine the ventilatory depressant effects of heroin (178–1780 μg/kg), fentanyl (5.6–56 μg/kg), and carfentanil (0.56–5.6 μg/kg) when administered alone and in mixtures, to compare the profile of effects across drugs and determine whether there are significant interactions between the drugs when co-administered. The potencies of the opioid receptor antagonists naloxone and diprenorphine to reverse opioid-induced ventilatory depression were compared across opioid agonists. All three agonists reduced minute volume with carfentanil being ∼50- and 100-fold more potent than fentanyl and heroin, respectively. The profile of effects on ventilation did not differ across agonists and no significant drug-drug interactions were detected. Naloxone and diprenorphine dose-dependently reversed the ventilatory depressant effects of each opioid agonist. Both antagonists were less potent at reversing the effects of carfentanil compared with reversing a functionally equivalent dose of heroin. These findings suggest that fentanyl might not produce unique effects on breathing that increase risk of overdose. That naloxone and diprenorphine were less potent to reverse the effects of carfentanil compared with an equivalent dose of heroin is consistent with existing literature and emphasizes the need for continued evaluation of potential differences in pharmacological properties across opioid receptor agonists.