Pharmacokinetics and pharmacodynamics of cyclopropylfentanyl in male rats

Abstract

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 Blood specimens were withdrawn over an 8-h time period, along with measurements of pharmacodynamic endpoints.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 (C_max) after 15-28 min, whereas metabolite Cmax occurred later at 45-90 min. Cyclopropylfentanyl C_max 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.

Keywords: Cyclopropylfentanyl; Fentanyl analog; Pharmacodynamics; Pharmacokinetics; Rat; UHPLC-MS/MS.

Fig. 1

Chemical structures and mass to charge ratios (m/z) of cyclopropylfentanyl and cyclopropylnorfentanyl

Fig. 2

MRM chromatograms of a rat plasma sample obtained 4 h after injection of 300 µg/kg cyclopropylfentanyl analyzed with the presented UHPLC-MS/MS method. Abbreviations: CPF, cyclopropylfentanyl; CPNF, cyclopropylnorfentanyl

Fig. 3

Time-course of pharmacodynamic effects induced by s.c. cyclopropylfentanyl administration (30, 100, and 300 µg/kg) in male rats. Body temperature, catalepsy score, and hot plate latency were measured at 0, 15, 30, 60, 120, 240, and 480 min after injection. Control animals received s.c. saline vehicle (1 mL/kg). Data are expressed as the mean ± SEM for N = 6 rats/group. Filled symbols represent significant differences when compared to saline-treated animals at a given time point (Tukey’s multiple comparison test, p < 0.05)

Fig. 4

Concentration–time profiles for cyclopropylfentanyl and cyclopropylnorfentanyl in rats after s.c. administration of 30, 100, or 300 µg/kg cyclopropylfentanyl. Rats fitted with indwelling jugular catheters received cyclopropylfentanyl at time zero. Blood samples were withdrawn via the catheters immediately prior to and 15, 30, 60, 120, 240, and 480 min after cyclopropylfentanyl injection. Plasma samples were assayed for analytes using UHPLC-MS/MS. Data are expressed as the mean ± SEM for N = 6 rats/group. Filled symbols represent significant differences when compared to the low-dose group (30 µg/kg) at a given time point (Tukey’s multiple comparison test, p < 0.05)

Fig. 5

Comparison of observed versus predicted area-under-the-curve (AUC) values for cyclopropylfentanyl after s.c. administration of 30, 100, or 300 µg/kg cyclopropylfentanyl in rats. Observed AUC values were obtained from time-concentration profiles depicted in Fig. 4, whereas predicted values at 100 and 300 μg/kg doses were calculated by multiplying the observed AUC values at 30 μg/kg by 3.33 and 10, respectively. Data are expressed as the mean ± SEM for N = 6 rats/group

Fig. 6

Correlations between AUC values for cyclopropylfentanyl versus body temperatures, catalepsy scores, and hot plate latency. Raw data from Figs. 3 and 4 were used to construct the correlation matrices which plot mean temperature, summed catalepsy scores, and mean hot plate latency for each rat versus corresponding AUC values for cyclopropylfentanyl in the same subjects over the 8-h session. Pearson’s r and p values are shown