Methods: BUP (11 µM) was incubated with and without a wide concentration range of inhibitors in pooled human intestinal (HIM) and liver (HLM) microsomes. The effect of inhibitors on CYP (norbuprenorphine) and UGT (buprenorphine glucuronide) metabolite formation of BUP was monitored separately to determine their inhibitory potency (IC50). Equilibrium GI solubility of the inhibitors (except ginger extract) in fasted-state simulated intestinal fluid (FaSSIF) was determined using a slightly modified ‘shake-flask’ method. Additionally, ADMET predictor™ was used to predict the biorelevant solubilities of these compounds.
Results: Of the 5 inhibitors, pterostilbene was the most potent CYP inhibitor in HIM (IC50= 1.30 ± 0.89 µM) and HLM (IC50=0.79 ± 0.10 µM). α-mangostin in HIM (IC50 = 5.57 ± 1.00 µM) and silybin in HLM (IC50=1.21 ± 0.34 µM) appeared to be most potent in inhibiting the glucuronidation of BUP. Ginger extract & chrysin exhibited apparent IC50 values ranging between 10–27 µM, indicating their moderate to low CYP & UGT inhibitory potency in both the microsomes. The equipotent combination of pterostilbene and ginger extract exhibited an overall additive effect towards CYP and UGT inhibition in HIM, as estimated using the curve-shift analysis method. The biorelevant solubilities (µg/ml) of chrysin, α-mangostin, pterostilbene and silybin were found to be 2.02 ± 0.01, 66.1 ± 0.10, 83.2 ± 0.07 and 0.66 ± 0.01 µg/ml, respectively. The predicted solubilites were in good agreement with the measured solubilites except for chrysin (103 µg/ml) and silybin (160 µg/ml).
Conclusions: The measured and/or predicted biorelevant GI solubilities of the four inhibitors exceeded their intestinal inhibitory potencies towards CYP and/or UGT metabolism of BUP. Amongst the tested inhibitors, pterostilbene appeared to be a potent inhibitor of the major metabolic pathway (CYP) of BUP with ~ 235 fold higher maximum GI solubility (GI volume=250 ml) than the intestinal CYP IC50 value. Pterostilbene has been reported to show dose dependent bioavailability in rats ranging from 12% to 80% [1, 2]. Thus, pterostilbene has fair potential of achieving sufficiently higher local (gut lumen) and systemic concentrations to produce a clinically relevant enhancement in the oral bioavailability and systemic exposures of orally administered BUP.
1. Kapetanovic IM, Muzzio M, Huang Z, Thompson TN and McCormick DL. Pharmacokinetics, oral bioavailability, and metabolic profile of resveratrol and its dimethylether analog, pterostilbene, in rats. Cancer Chemother Pharmacol 2011; 68: 593-601.
2. Lin HS, Yue BD and Ho PC. Determination of pterostilbene in rat plasma by a simple HPLC-UV method and its application in pre-clinical pharmacokinetic study. Biomed Chromatogr 2009; 23: 1308-15.
Funding: Center for Innovative Technology, Matching Funds Grant#MF14S-007-LS.