In vitro evaluation of Schisandra extracts (SE) herb-drug interaction potential utilizing B-Clear® sandwich-culture human hepatocytes (SCHH)

Herb-drug interaction (HDI) potential should be evaluated given the dramatic increase in the sale and use of alternative medicines. A fully integrated hepatic cell system that includes drug metabolism, drug transport, and key regulatory pathways was utilized to assess SE HDI potential. We investigated SE for hepatocyte cytotoxicity using ATP and LDH assays following 24 or 72 hrs. exposure. After 24 hrs. exposure with S. chinensis extract (SCE) or S. sphenanthera extract (SSE), no marked changes were observed in either assay at the concentrations examined. However, ATP pools were modestly reduced in hepatocytes following 72 hrs. of exposure at the highest concentrations. Next, we examined the SE potential to inhibit CYP3A4/5 enzyme activity and P-gP function in our system. Significant inhibition of OH-midazolam formation was observed in SCHH by SCE (IC₅₀ = 4.4 µg/mL) and SPE (IC₅₀ = 0.8 µg/mL). Pre-incubation of SCE in SCHH prior to midazolam exposure yielded a 3-fold decrease in the CYP3A4/5 IC₅₀ providing evidence of time-dependent inhibition; an effect not observed with SPE. Significant inhibition of digoxin efflux was observed with increasing concentrations of both SE, reducing the digoxin hepatobiliary clearance significantly. Finally, we examined the potential of SE to modulate the expression of P450 or drug transporter proteins in SCHH following 72 hr. of exposure to SCE or SPE. St. John's Wort (20 µg/mL), a positive control inducer, induced CYP2B6 and CYP3A4 mRNA and enzyme activity demonstrating that key regulatory pathways in SCHH were functional. Changes were observed in gene expression and functional assays in response to SE. Our results of SPE inhibition of CYP3A4/5 and P-gP are consistent with clinical data that demonstrated significant SPE drug interactions resulting in > 60% increase in AUC of tacrolimus. In summary, in vitro results demonstrated that SCE and SPE both had similar inhibition profiles of CYP3A4/5 and P-gP but SCE has a higher potential to cause clinically relevant HDIs as compared to the SPE varietal because of time-dependent inhibition.