Novel swine model of severe respiratory depression induced by controlled fentanyl and heroin administration

Background: This study characterized a swine model of fentanyl- and heroin-induced non-lethal apnea to support development of translational strategies for opioid overdose interventions. Methods: Spontaneously breathing, isoflurane-anesthetized Hanford swine (n = 18; 4-25 kg) received intravenous fentanyl (n = 12) or heroin (n = 6) via constant rate infusion (fentanyl: 30 μg/kg/h; heroin: 1 mg/kg/h) until production of respiratory arrest (i.e., apnea), defined as the absence of spontaneous breaths for two consecutive minutes. Fentanyl-treated pigs were either sexually immature (~ 2 months) or mature (~ 4 months); heroin-treated pigs were immature; both groups included equal numbers of males and females. Results: Results are presented as median (IQR) or mean ± SEM. Immature pigs required higher fentanyl doses to induce apnea compared with mature pigs, 17 (15-36) μg/kg vs 8 (7-10) μg/kg respectively, despite similar serum fentanyl concentration at apnea, which were 4 (3-7) and 6 (4-8) ng/mL respectively, consistent with a significantly faster clearance in immature pigs. No significant age-related differences were observed in norfentanyl concentrations or in its pharmacokinetic parameters. Heroin infusion resulted in apnea at 360 (290-502) μg/kg. Although serum heroin concentrations remained below the LLOQ throughout all time points, its metabolites 6-acetylmorphine (6-AM) and morphine serum concentration at apnea were 57 (44-91) and 34 (29-61) ng/mL, with 6-AM displaying faster clearance and a shorter half-life than morphine. Latency to resume spontaneous breathing was longer in immature than mature pigs that received fentanyl, and pigs exposed to heroin exhibited more severe respiratory depression, characterized by prolonged apnea and delayed or incomplete recovery, with a reduced response to naloxone compared with fentanyl. No statistically significant differences were detected between male and female pigs. Conclusion: In conclusion, this reproducible model enables evaluation of opioid-induced respiratory depression and its pathophysiological consequences, supporting targeted therapeutic development.