Vol. 29, Issue 8, 1110-1113, August 2001

Inhibition and Inactivation of Human Cytochrome P450 Isoforms by Phenethyl Isothiocyanate

Miki Nakajima, Ryoko Yoshida, Noriaki Shimada, Hiroshi Yamazaki, and Tsuyoshi Yokoi

Division of Drug Metabolism, Faculty of Pharmaceutical Sciences, Kanazawa University, Kanazawa, Japan (M.N., R.Y., H.Y., T.Y.); and Daiichi Pure Chemicals Co., Ltd., Ibaraki, Japan (N.S.)

The inhibition and mechanism-based inactivation potencies of phenethyl isothiocyanate (PEITC) for human cytochrome P450 (CYP) activities were investigated using microsomes from baculovirus-infected insect cells expressing specific human CYP isoforms. PEITC competitively inhibited phenacetin O-deethylase activity catalyzed by CYP1A2 (K_i = 4.5 ± 1.0 µM) and coumarin 7-hydroxylase activity catalyzed by CYP2A6 (K_i = 18.2 ± 2.5 µM). Benzyloxyresorufin O-dealkylase activity catalyzed by CYP2B6 was most strongly and noncompetitively inhibited (K_i = 1.5 ± 0.0 µM). Paclitaxel 6-hydroxylase activity catalyzed by CYP2C8 was not affected by PEITC up to 100 µM. PEITC noncompetitively inhibited S-warfarin 7-hydroxylase activity catalyzed by CYP2C9 (K_i = 6.5 ± 0.9 µM), S-mephenytoin 4'-hydroxylase activity catalyzed by CYP2C19 (K_i = 12.0 ± 3.2 µM), bufuralol 1'-hydroxylase activity catalyzed by CYP2D6 (K_i = 28.4 ± 7.9 µM), and chlorzoxazone 6-hydroxylase activity catalyzed by CYP2E1 (K_i = 21.5 ± 3.4 µM). The inhibition for testosterone 6-hydroxylase activity catalyzed by CYP3A4 was a mixed-type of competitive (K_i = 34.0 ± 6.5 µM) and noncompetitive (K_i = 63.8 ± 12.5 µM) inhibition. Furthermore, PEITC is a mechanism-based inactivator of human CYP2E1. The k_inact value was 0.339 min¹ and K_i was 9.98 µM. Human CYP1A2, CYP2A6, CYP2B6, CYP2D6, and CYP3A4 were not inactivated. The present study directly proved that the chemopreventive effects of PEITC for nitrosamine-induced carcinogenesis are due to the inhibition of CYP by an in vitro study. The possibility that PEITC would affect the pharmacokinetics of clinically used drugs that are metabolized by these CYP isoforms was also suggested.