The use of cryopreserved and fresh human hepatocytes and human liver microsomes for the in vitro evaluation of Boswellia serrata extract (BSE) as an inhibitor of CYP450

As the use of dietary supplements increases, herb-drug interaction (HDI) potential should be evaluated. Our in vitro strategy to assess HDI uses an integrated liver system yielding physiologic intracellular concentrations of drugs and herbs. BSE is traditionally used for inflammatory diseases but has no reported clinical HDI. Potent in vitro inhibition (≥65%) across major drug metabolizing enzymes using pooled human liver microsomes (PHLM) have been reported in the literature for levels of BSE as low as 1 µg/mL. This potent level of inhibition led us to question the relevance of the results using PHLM. In PHLM, BSE was a direct inhibitor of CYP2C9 and CYP3A4/5 with IC₅₀= 11 µg/mL and 1.4 µg/mL, respectively; no evidence of time-dependent inhibition of either enzyme. We next evaluated the ability of BSE to inhibit CYP2C9 and CYP3A4/5 in pooled suspensions of human hepatocytes, which have robust metabolism and uptake activities but no biliary efflux capabilities. In this system, considerably less direct inhibition of CYP2C9 was observed (IC₅₀ > 50 µg/mL) for BSE. Finally, to represent the in vivo situation with both transporter and metabolic capabilities integrated together, a sandwich-cultured human hepatocyte (SCHH) system was used to assess potential HDI which would yield physiologic intracellular concentrations. In SCHH, direct inhibition of CYP3A4/5 by BSE was observed resulting in an IC₅₀= 17.2 µg/mL. A maximum 30% decrease in 3-OH-ibuprofen formation (CYP2C9) was observed in SCHH treated with BSE (IC₅₀ > 75 µg/mL). These data suggest that PHLM may provide a worst-case scenario highlighting the need to utilize a more physiologically relevant model that integrates transporter and metabolism to assess HDI potential.