Inhibition of P-glycoprotein at the Blood-Brain Barrier by Phytochemicals Derived from Traditional Chinese Medicine

The blood-brain barrier (BBB) is a key determinant for drug transport through brain vessels. It restricts the pharmacological efficacy in numerous neurological diseases, including brain tumors. A major functional constituent of BBB is P-glycoprotein, which is also a major obstacle for effective chemotherapy of brain tumors. An appealing strategy is to selectively modulate BBB function using P-glycoprotein inhibitors. We assessed 57 chemically defined compounds derived from medicinal plants used in traditional Chinese medicine for their potential to inhibit P-glycoprotein. Nine phytochemicals inhibited P- glycoprotein in porcine brain capillary endothelial cells (PBCECs) and multidrug-resistant CEM/ADR5000 cells as shown by a calcein fluorescence assay. The cytotoxicity of the 57 phytochemicals was measured by a growth inhibition assay. Seven compounds inhibiting P-glycoprotein at lower doses were cytotoxic to drug-sensitive parental CCRF-CEM cells at higher doses. Of them, five were not cross-resistant to CEM/ADR5000 cells (baicalein, bufalin, glybomine B, deoxyserofendic acid, and shogaol). Bufalin was chosen as a lead compound. Of a further six bufalin-related compounds, scillarenin showed improved features in comparison to bufalin. It was cytotoxic to cancer cells at a nanomolar range. COMPARE and hierarchical cluster analyses of microarray-based mRNA expression were used to investigate determinants of sensitivity or resistance of the bufalin-related compounds downstream of P-glycoprotein. CEM/ADR5000 cells were not cross-resistant, but were collaterally sensitive towards scillarenin. Finally, scillarenin inhibited P-glycoprotein in PBCECs. Taken together, these data show that scillarenin is a potential novel candidate for P-glycoprotein inhibition at BBB, and, thereby, may improve the efficacy of therapy regimens in treating brain diseases.