Received for publication February 4, 2009.
Revised May 11, 2009.
Accepted for publication May 18, 2009.

Further Assessment of 17-Ethinyl Estradiol as an Inhibitor of Different Human Cytochrome P450 Forms in Vitro

Abstract

17-ethinyl estradiol (EE) was systematically evaluated as a reversible and time-dependent inhibitor of eleven human drug-metabolizing P450s (CYP1A1, 1A2, 1B1, 2B6, 2C8, 2C9, 2C19, 2D6, 2J2, 3A4 and 3A5) in vitro. When ranked, the lowest IC₅₀ (concentration of inhibitor required to decrease activity by 50%) values were obtained with recombinant CYP1A1 (rCYP1A1, IC_50(total) = IC_50(free) = 2.7 µM) and CYP2C19 activity in human liver microsomes (HLM, IC_50(total) = 4.4 µM; IC_50(free) = 2.8 µM). For rCYP1A1, formal inhibition studies revealed that EE was a competitive inhibitor (K_i(free) = 1.4 µM). All other IC₅₀ values were greater than 8.0 µM and the weakest inhibition was observed with CYP1A2 activity in HLM (IC_50(free) > 39 µM). In agreement, the IC₅₀ characterizing the inhibition of melatonin (MEL) 6-hydroxylation in human intestine microsomes (CYP1A1 catalyzed) was lower than that of HLM (0.91 µM versus >40 µM). Because EE is known to impact the PK of CYP2C19 probe drugs, this raises the possibility that the concentration of EE during first pass may exceed 1000 nM, sufficient to impact CYP1A1 and CYP2C19, with less impact on CYP3A4 and other P450s. The results implicate intestinal CYP1A1, and possibly CYP2C19, as the loci of EE drug interactions with highly extracted drugs like MEL. Overall, it is very difficult to rationalize drug interactions involving EE based on direct inhibition of CYP2B6 (e.g., selegiline) and hepatic CYP1A2(e.g., MEL, tizanidine, caffeine and theophylline).

Key words: CYP inhibition, CYP1A, cytochrome P450 isoforms, drug disposition, drug-drug interactions, enzyme inhibitors, human CYP enzymes, inhibition