Respiratory Depressant Effects of Fentanyl Analogs

Abstract

Aim Opioid-related fatalities involving synthetic narcotics have reached unprecedented levels. This study evaluated the respiratory depressant effects of seven fentanyl analogs that have either emerged in the recreational drug marketplace or been identified in toxicological analyses following fatal or non-fatal intoxications and for which their effects on ventilation had not been previously characterized. Methods Adult male Swiss Webster mice (N = 8/group) were administered fentanyl analogs (isobutyrylfentanyl, crotonylfentanyl, para-methoxyfentanyl, para-methoxybutyrylfentanyl, 3-furanylfentanyl, thiophenefentanyl, benzodioxolefentanyl) and their effects on respiratory rate, tidal volume, and minute volume as compared to mu-opioid-receptor (MOR) agonist standards (fentanyl, morphine, and buprenorphine) were measured using whole body plethysmography (WBP). Results All compounds elicited significant (p ≤ 0.05) hypoventilation relative to vehicle at least at one dose tested: morphine (1, 3.2, 10, 32 mg/kg), buprenorphine, (0.032, 0.1, 0.32, 1, 3.2 mg/kg), fentanyl (0.01, 0.032, 0.1, 1, 32 mg/kg), isobutyrylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), crotonylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), para-methoxyfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), para-methoxybutyrylfentanyl (0.32, 1, 3.2, 10 mg/kg), 3-furanylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), thiophenefentanyl (1, 3.2, 10, 32, 100 mg/kg), and benzodioxolefentanyl (3.2, 10, 32, 100 mg/kg). The ED50 values for hypoventilation showed a rank order of potency as follows: fentanyl (ED50 = 0.96 mg/kg) > 3-furanylfentanyl (ED50 = 2.60 mg/kg) > crotonylfentanyl (ED50 = 2.72 mg/kg) > para-methoxyfentanyl (ED50 = 3.31 mg/kg) > buprenorphine (ED50 = 10.8 mg/kg) > isobutyrylfentanyl (ED50 = 13.5 mg/kg) > para-methoxybutyrylfentanyl (ED50 = 16.1 mg/kg) > thiophenefentanyl (ED50 = 18.0 mg/kg) > morphine (ED50 = 55.3 mg/kg) > benzodioxolefentanyl (ED50 = 10168 mg/kg). A naloxone pretreatment (10 mg/kg) attenuated the hypoventilatory effects of all drugs. Conclusions These results establish that the respiratory depressant effects of these fentanyl analogs are at least in part mediated by the MOR.