Vol. 30, Issue 4, 391-396, April 2002

Interaction of Irinotecan (CPT-11) and Its Active Metabolite 7-Ethyl-10-Hydroxycamptothecin (SN-38) with Human Cytochrome P450 Enzymes

Nobumitsu Hanioka, Shogo Ozawa, Hideto Jinno, Toshiko Tanaka-Kagawa, Tetsuji Nishimura, Masanori Ando, and Jun-ichi Sawada

Project Team for Pharmacogenetics (N.H., S.O., J.S.), Division of Environmental Chemistry (N.H., H.J., T.T.-K., T.N., M.A.), Division of Pharmacology (S.O.), and Division of Biochemistry and Immunochemistry (J.S.), National Institute of Health Sciences, Setagaya-ku, Tokyo, Japan

The inhibition and mechanism-based inactivation potencies of irinotecan (7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin; CPT-11) and its active metabolite (7-ethyl-10-hydroxycamptothecin; SN-38) for human cytochrome P450 (P450) enzymes were investigated to evaluate the potential for drug interactions involving CPT-11 using microsomes from insect cells expressing specific human P450 isoforms. The mechanism and potential for interaction were examined by Lineweaver-Burk analysis, and NADPH-, time- and concentration-dependent effects were observed. CPT-11 and SN-38 competitively inhibited CYP3A4 (testosterone 6-hydroxylation) activity with K_i values of 129 and 121 µM, respectively. CYP2A6 (coumarin 7-hydroxylation) and CYP2C9 (diclofenac 4'-hydroxylation) activities exhibited a mixed type of inhibition comprising competitive and noncompetitive components in response to SN-38, the K_i values being 181 and 156 µM, respectively. On the other hand, CYP1A2 (phenacetin O-deethylation), CYP2B6 (7-ethoxycoumarin O-deethylation), CYP2C8 (paclitaxel 6-hydroxylation), CYP2C19 (S-mephenytoin 4'-hydroxylation), CYP2D6 (bufuralol 1'-hydroxylation), and CYP2E1 (chlorzoxazone 6-hydroxylation) were hardly affected by either compound. Furthermore, CPT-11 and SN-38 were suggested to be mechanism-based inactivators of CYP3A4. The k_inact and K_I values of CPT-11 and SN-38 were 0.06 min¹ and 24 µM and 0.10 min¹ and 26 µM, respectively. However, no inactivation of CYP2A6 and CYP2C9 by SN-38 was observed. These results mean that CPT-11 and SN-38 interact with human P450 isoforms, such as CYP2A6, CYP2C9, and CYP3A4, in vitro and imply that the significant drug interactions involving CPT-11 may be caused by a mechanism-based inactivation of CYP3A4 by SN-38 as an active metabolite of CPT-11 rather than competitive inhibition.