P-Glycoprotein on Blood-Brain Barrier Plays a Vital Role in Fentanyl Brain Exposure and Respiratory Toxicity in Rats.

Fentanyl is a rapid-acting, short duration opioid analgesic agent. In recent years, increased prescription and illicit use of fentanyl drugs have led to major safety concerns and a growing death toll. However, the causes of fentanyl-induced fatal adverse effects have not been thoroughly researched. This study investigated P-glycoprotein (P-gp) modulated blood-brain barrier penetration of fentanyl and its resulting toxicity in vitro and in vivo. ATPase assays were performed together with bi-directional transport assays using Caco-2 cells in the presence and absence of tariquidar, a P-gp inhibitor, to confirm the P-gp substrate property of fentanyl. In vivo determinations of brain pharmacokinetic profile, duration of loss of righting reflex, and respiratory function were further conducted following intravenous administration of low and high doses of fentanyl with and without tariquidar in rats. Fentanyl significantly increased the ATPase activity of P-gp membrane. The efflux ratio of fentanyl on Caco-2 cells was >2, which was remarkably reduced when co-incubated with tariquidar. Using concomitant tariquidar with fentanyl (40 μg/kg) in rats, the unbound brain-to-plasma concentration ratio in rats increased 2.9- fold. The duration of loss of righting reflex was significantly extended, and fentanyl-induced respiratory depression was aggravated. At high fentanyl doses (80 µg/kg), inhibition of P-gp resulted in severe respiratory toxicity in the rats and even death. Collectively, these results gave strong evidence that P-gp plays a vital role in fentanyl blood-brain barrier penetration and the resulting toxicity. Fentanyl requires close monitoring in clinic when administered concomitantly with P-gp inhibitors.