Oroxylin A, a methylated metabolite of baicalein, exhibits a stronger inhibitory effect than baicalein on the CYP1B1‐mediated carcinogenic estradiol metabolite formation

Human cytochrome P450 1B1 (CYP1B1)‐mediated formation of 4‐hydroxyestradiol (4‐OHE2) from 17β‐estradiol plays an important role in the progression of human breast cancer, while the biotransformation of 17β‐estradiol to 2‐hydroxyestradiol mediated by cytochrome P450 1A1 (CYP1A1) is considered as a less harmful pathway. In this study, inhibitory effects of flavonoids baicalein and oroxylin A, a metabolite of baicalein in human body, on CYP1A1 and 1B1 activities were investigated in vitro. The inhibition intensities of baicalein and oroxylin A towards CYP1B1 were greater than towards CYP1A1 with a mixed mechanism. In addition, oroxylin A showed a stronger inhibitory effect than baicalein towards the CYP1B1‐mediated 17β‐estradiol 4‐hydroxylation, with the IC 50 values of 0.0146 and 2.27 μM, respectively. Docking studies elucidated that oroxylin A had a stronger binding affinity than baicalein for CYP1B1. In MCF‐7 cells, compared with baicalein‐treated groups, oroxylin A with lower doses decreased and increased the formation of 4‐OHE2 and 2‐hydroxyestradiol, respectively, with a preferential induction of mRNA of CYP1A1 over CYP1B1. In conclusion, this study demonstrated that oroxylin A showed a stronger inhibitory effect than baicalein on CYP1B1‐mediated 4‐OHE2 formation in MCF‐7 cells, providing crucial implications for their possibly preventive/therapeutic potential against breast cancer via inhibition of CYP1B1, particularly of oroxylin A.