Received for publication December 26, 2007.
Revised April 3, 2008.
Accepted for publication April 8, 2008.

The mibefradil derivative NNC55-0396, a specific T-type calcium channel antagonist, exhibits less CYP3A4 inhibition than mibefradil

Abstract

A novel mibefradil derivative, NNC55-0396, designed to be hydrolysis resistant was shown to be a selective T-type Ca²⁺ channel inhibitor without L-type Ca²⁺ channel efficacy. However, its effects on cytochrome P450s have not previously been examined. We investigated the inhibitory effects of NNC55-0396 toward 7 major recombinant human cytochrome P450s: CYP3A4, 2D6, 1A2, 2C9, 2C8, C19, and 2E1, and compared its effects with those of mibefradil and its hydrolyzed metabolite, Ro40-5966. Our results show that CYP3A4 and 2D6 are the two P450s most affected by mibefradil, Ro40-5966, and NNC55-0396. Mibefradil (IC₅₀=33±3 nM, K_i=23±0.5 nM) and Ro40-5966 (IC₅₀=30±7.8 nM, K_i=21±2.8 nM) have a nine to ten-fold greater inhibitory activity towards recombinant CYP3A4 benzyloxy-4-trifluoromethylcoumarin O-debenzylation activity than NNC55-0396 (IC₅₀=300±30 nM, K_i=210±6 nM). More dramatically, mibefradil (IC₅₀=566±71 nM, K_i=202±39 nM) shows 19-fold higher inhibition of CYP3A-associated testosterone 6-hydroxylase activity in human liver microsomes compared to NNC55-0396 (IC₅₀=11±1.1 µM, K_i=3.9±0.4 µM). Loss of testosterone 6-hydroxylase activity by recombinant CYP3A4 was shown to be time- and concentration-dependent with both compounds. However, NNC55-0396 (K_I=3.87µM, K_inact=0.061 min^-1) is a much less potent mechanism-based inhibitor than mibefradil (K_inact=83 nM, K_inact=0.048 min^-1). In contrast, NNC55-0396 (IC₅₀= 29±1.2 nM, K_i=2.8±0.3 nM) and Ro40-5966 (IC₅₀=46±11 nM, K_i=4.5±0.02 nM) have a three to four-fold greater inhibitory activity towards recombinant CYP2D6 than mibefradil (IC₅₀=129±21 nM, K_i=12.7±0.9 nM). Our results suggest that NNC55-0396 could be a more favorable T-type Ca²⁺ antagonist than its parent compound, mibefradil, which was withdrawn from the market due to strong inhibition of CYP3A4.

Key words: adverse drug reactions, CYP inhibition, CYP2D, CYP3A, cytochrome P450, drug development, drug-drug interactions, human CYP enzymes, mechanism-based inhibition, microsomes