Comparative Study of Analgesic Therapeutic Index and Addiction Potential of o- and p-Fluorofentanyl

As fluorinated fentanyl derivatives, p-fluorofentanyl and o-fluorofentanyl have caused many abuse-related deaths in recent years. However, their pharmacological properties in the body are still unclear, resulting in a lack of targeted control measures for the prevention of abuse and emergency treatment of acute poisoning. This study comprehensively assessed their therapeutic indices and addiction potential using acute toxicity, hot-plate, conditioned place preference (CPP), and naloxone-precipitated withdrawal tests. Results showed that the subcutaneous median lethal dose (95% confidence interval; LD₅₀ [95% CI]) values of o-fluorofentanyl were 101 (76.26-135) mg/kg (female) and 133 (108-264) mg/kg (male), while the LD₅₀ (95% CI) values of p-fluorofentanyl were 76 (54.2-118) mg/kg (female) and 58 (49.2-81.8) mg/kg (male). The acute toxicity of o-fluorofentanyl and p-fluorofentanyl was greatest when administered intravenously, followed by intraperitoneal, subcutaneous, and intragastric gavage. In the hot-plate test, the median effective dose (ED₅₀) values were 0.076 mg/kg (95% CI: 0.061-0.093 mg/kg) for fentanyl, 0.014 mg/kg (95% CI: 0.013-0.017 mg/kg) for o-fluorofentanyl, and 0.088 mg/kg (95% CI: 0.076-0.095 mg/kg) for p-fluorofentanyl. The therapeutic index (LD₅₀/ED₅₀) of o-fluorofentanyl reached 7214 (95% CI: 5866-7941), surpassing the therapeutic index of fentanyl (1000 [95% CI: 693-1786]) and the therapeutic index of p-fluorofentanyl (863 [95% CI: 713-1242]). In CPP tests, both compounds induced significant effects at 0.1 mg/kg, compared with the effect of fentanyl at 0.3 mg/kg. In naloxone-precipitated withdrawal tests, both compounds triggered pronounced jumping and weight loss, indicating that both substances can produce physical dependence similar to that of fentanyl. In conclusion, compared with fentanyl, o-fluorofentanyl exhibits higher analgesic potency (ED₅₀ = 0.014 mg/kg) and a superior therapeutic index (7214). However, its strong rewarding effects and physical dependence suggest that its addictive potential is significantly higher. The therapeutic index of p-fluorofentanyl (863) is comparable to that of fentanyl, but it also exhibits significant dependence and reward effects. The benzene ring's fluorine position can thus regulate drug effects and dependence risks, offering crucial insights for opioid drug design and regulation.