Vol. 289, Issue 2, 1084-1089, May 1999

Active Transport of Fentanyl by the Blood-Brain Barrier¹

Thomas K. Henthorn, Yang Liu, Mrinal Mahapatro and Ka-yun Ng

University of Colorado Health Sciences Center, Department of Pharmaceutical Sciences, School of Pharmacy and Departments of Anesthesiology and Neurosurgery, School of Medicine, Denver, Colorado

Previous studies have shown that uptake of the lipophilic opioid, fentanyl, by pulmonary endothelial cells occurs by both passive diffusion and carrier-mediated processes. To evaluate if the latter mechanism also exists in brain endothelium, transport of [³H]fentanyl was examined in primary cultured bovine brain microvessel endothelial cell (BBMEC) monolayers. Uptake of fentanyl appears to occur via a carrier-mediated process as uptake of [³H]fentanyl by BBMECs was significantly inhibited in a dose-dependent manner by unlabeled fentanyl. Fentanyl uptake was also significantly inhibited by either 4°C or sodium azide/2-deoxyglucose, suggesting that carrier-mediated uptake of fentanyl was an active process. Fentanyl was also tested to determine whether it might be a substrate of the endogenous blood-brain barrier efflux transport system, P-glycoprotein (P-gp). Release of [³H]fentanyl or rhodamine 123, a known substrate of P-gp, previously loaded in the BBMECs was studied in the presence or absence of either fentanyl or verapamil, a known competitive inhibitor of P-gp. Both fentanyl (10 µM) and verapamil (100 µM) decreased release of rhodamine 123 from BBMECs, indicating that fentanyl is a substrate of P-gp in the BBMECs. This was further supported by the observation that uptake of [³H]fentanyl was significantly increased in Mg²⁺-free medium, a condition known to reduce P-gp activity. However, release of [³H]fentanyl was significantly increased when incubated with either unlabeled fentanyl or verapamil. These results suggest that the active P-gp-mediated extrusion of fentanyl in these cells is overshadowed by an active inward transport process, mediated by an as yet unidentified transporter. In addition, verapamil was shown to be a substrate of both P-gp and the fentanyl uptake transporter.