Differential Addictive Potential of Cis- and Trans-3-Methylfentanyl Revealed by Pharmacokinetic-Pharmacodynamic Profiling

Rationale 3-Methylfentanyl (3-MF) is a highly potent synthetic opioid linked to a high risk of overdose. Although its analgesic effects are known to be stereoselective, the differences in addictive potential between its cis- and trans-isomers have not been fully elucidated. Objectives This study aimed to compare the abuse liability of cis- and trans-3-MF isomers using behavioral, pharmacokinetic, neurochemical, and transporter-based evaluations in rodents. Methods Conditioned place preference (CPP) was used to assess addiction potential in rodents. Pharmacokinetic profiles were evaluated in rats via intravenous (iv), intramuscular (im), and subcutaneous (sc) routes. Brain microdialysis measured nucleus accumbens (NAc) exposure and monoamine release. In vitro transporter assays assessed interactions with breast cancer resistance protein (BCRP) and P-glycoprotein (P-gp). Results Cis-3-MF induced CPP at 8-fold lower doses than trans-3-MF (10 vs. 80 µg/kg). Pharmacokinetic analyzes revealed that trans-3-MF had higher plasma exposure after intravenous administration, while cis-3-MF showed nearly 100% bioavailability via intramuscular or subcutaneous routes. Microdialysis demonstrated 2.7-fold greater nucleus accumbens exposure of cis-3-MF and a more rapid and pronounced DA release (Cmax: 67 vs. 37 ng/mL for trans-3-MF). In contrast, trans-3-MF induced a slower increase in DA but elevated 5-HT levels. Trans-3-MF was identified as a partial BCRP substrate, potentially limiting brain penetration. Neither isomer interacted significantly with P-gp. Conclusions Stereochemistry critically influences the addictive potential of 3-MF isomers. Cis-3-MF demonstrates greater abuse liability, driven by enhanced brain exposure and dopaminergic activation. These findings contribute to understanding stereoselective risks in synthetic opioid toxicity and support improved safety evaluations for novel analogs.