Pharmacokinetics and pharmacodynamics of cyclopropylfentanyl in male rats.

BACKGROUND: Illicitly manufactured fentanyl and its analogs are a major driving force behind the ongoing opioid crisis. Cyclopropylfentanyl is a fentanyl analog associated with many overdose deaths, but limited knowledge is available about its pharmacology. In the present study, we developed a bioanalytical method for the determination of cyclopropylfentanyl and its main metabolite cyclopropylnorfentanyl and evaluated pharmacokinetic-pharmacodynamic relationships in rats. METHOD: An ultra-high performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) method was developed and validated for determination of cyclopropylfentanyl and cyclopropylnorfentanyl in rat plasma. Male Sprague-Dawley rats fitted with jugular catheters and temperature transponders received cyclopropylfentanyl (30, 100, and 300 µg/kg) or saline subcutaneously. Blood specimens were withdrawn over an 8-h time period, along with measurements of pharmacodynamic endpoints. RESULTS: The analytical method was validated, and both analytes exhibited a low limit of quantification (15 pg/mL). Cyclopropylfentanyl caused dose-related increases in hot plate latency (ED50 = 48 µg/kg) and catalepsy (ED50 = 87 µg/kg) and produced long-lasting hypothermia at the highest dose. Plasma cyclopropylfentanyl rose rapidly in a dose-related fashion, reaching maximal concentration (Cmax) after 15-28 min, whereas metabolite Cmax occurred later at 45-90 min. Cyclopropylfentanyl Cmax values were similar to concentrations measured in non-fatal intoxications in humans; however, differences in parent drug: metabolite ratio indicated possible interspecies variance in metabolism. CONCLUSION: Our study shows that cyclopropylfentanyl produces typical opioid-like effects in male rats. Cyclopropylfentanyl displays much greater analgesic potency when compared to morphine, suggesting that cyclopropylfentanyl poses increased overdose risk for unsuspecting users.