P-Glycoprotein Is a Major Determinant of Norbuprenorphine Brain Exposure and Antinociception

Abstract

Norbuprenorphine is a major metabolite of buprenorphine and potent agonist of , ␦, and opioid receptors. Compared with buprenorphine, norbuprenorphine causes minimal antinociception but greater respiratory depression. It is unknown whether the limited antinociception is caused by low efficacy or limited brain exposure. Norbuprenorphine is an in vitro substrate of the efflux transporter P-glycoprotein (Mdr1), but the role of P-glycoprotein in norbuprenorphine transport in vivo is unknown. This investigation tested the hypothesis that limited norbuprenorphine antinociception results from P-glycoprotein-mediated efflux and limited brain access. Human P-glycoprotein-mediated transport in vitro of buprenorphine, norbuprenorphine, and their respective glucuronide conjugates was assessed by using transfected cells. P-glycoprotein-mediated norbuprenorphine transport and consequences in vivo were assessed by using mdr1a(ϩ/ϩ) and mdr1a(Ϫ/Ϫ) mice. Antinociception was determined by hotwater tail-flick assay, and respiratory effects were determined by unrestrained whole-body plethysmography. Brain and plasma norbuprenorphine and norbuprenorphine-3-glucuronide were quantified by mass spectrometry. In vitro, the net P-glycoproteinmediated efflux ratio for norbuprenorphine was nine, indicating significant efflux. In contrast, the efflux ratio for buprenorphine and the two glucuronide conjugates was unity, indicating absent transport. The norbuprenorphine brain/plasma concentration ratio was significantly greater in mdr1a(Ϫ/Ϫ) than mdr1a(ϩ/ϩ) mice. The magnitude and duration of norbuprenorphine antinociception were significantly increased in mdr1a(Ϫ/Ϫ) compared with mdr1a(ϩ/ϩ) mice, whereas the reduction in respiratory rate was similar. Results show that norbuprenorphine is an in vitro and in vivo substrate of P-glycoprotein. P-glycoprotein-mediated efflux influences brain access and antinociceptive, but not the respiratory, effects of norbuprenorphine.

Transporter-Mediated Disposition of Opioids: Implications for Clinical Drug Interactions

“…In vitro studies appear in agreement that buprenorphine is most likely not a P-gp substrate (111,166,167), while nobuprenorphine is (36,166). The apparent P-gp mediated efflux of norbuprenorphine at the BBB may explain its minimal antinociceptive effects, despite its opioid receptor affinities.…”

“…Another P-gp knock-out mice study demonstrated that the brain/plasma ratio of norbuprenorphine was significantly greater, as was the magnitude and duration of its antinociceptive effects, relative to wild-types. These results suggest that P-gp plays a major role in limiting the BBB access of norbuprenorphine, potentially explaining its minimal antinociceptive effects (166). Administration of the P-gp inhibitor PSC833 in mice was found to significantly increase the respiratory depressive effects of buprenorphine and norbuprenorphine, as well as significantly increase plasma concentrations and reduce brain efflux of norbuprenorphine.…”

“…MDCK cells stably transfected with human P-gp demonstrated significantly increased net efflux of norbuprenorphine, but not of buprenorphine nor two of its glucuronidated metabolites, relative to untransfected cells (166). Buprenorphine did not undergo P-gp mediated transport in human placental lobules (167) or Caco-2 cells, nor did it stimulate P-gp ATPase activity (111).…”

Role of drug transporters: an overview based on knockout animal model studies

“…After these reports, for various compounds, their brain distribution across the BBB has been compared using the Mdr1a knockout or Mdr1a/1b double knockout mouse. These compounds included the following: fluorescent dye rhodamine-123 (de Lange et al 1998); anti-cancer agents such as vinblastine (van Asperen et al 1999); the cardiovascular agent, digoxin (Kawahara et al 1999); the immunosuppressant, cyclosporin A (Kwei et al 1999); the corticosteroid, dexamethasone (Meijer et al 1998); the glucocorticoid, prednisolone (Karssen et al 2002); the antifungal agent, itraconazole (Miyama et al 1998); anti-HIV1 protease inhibitors (Kim et al 1998); opioid analgesics (Chen and Pollack 1998;Dagenais et al 2002); neurokinin-1 receptor antagonists (Smith et al 2001); and anticonvulsants such as, enaminone (Cox et al 2001) and norbuprenorphine (Brown et al 2012). Interestingly, it has been revealed that adverse effects or pharmacodynamic effects of various molecules in the brain are also related to the function of P-gp (Muller et al 2003;Tahara et al 2005;Morimoto et al 2008).…”

Randomised Comparison of a Novel Buprenorphine Oral Lyophilisate versus Existing Buprenorphine Sublingual Tablets in Opioid-Dependent Patients: A First-in-Patient Phase II Randomised Open Label Safety Study

“…Buprenorphine and also the active metabolite norbuprenorphine were assayed by LC-MS-MS (Liquid Chromatography Tandem Mass Spectrometry, Simbec Research Ltd., Merthyr Tydfil, UK), since animal studies identify norbuprenorphine as a potent respiratory depressant [18,19,20,21]. …”