Mechanism-based inhibition of cytochrome P450 (CYP)2A6 by chalepensin in recombinant systems, in human liver microsomes and in mice in vivo

Yune Fang Ueng, Chien Chih Chen, Yu Ting Chung, Tsung Yun Liu, Yu Ping Chang, Wei Sheng Lo, Norie Murayama, Hiroshi Yamazaki, Pavel Souček, Gar Yang Chau, Chin Wen Chi, Ruei Ming Chen, Ding Tzai Li

Research output: Contribution to journalArticlepeer-review

21 Citations (Scopus)


BACKGROUND AND PURPOSE Chalepensin is a pharmacologically active furanocoumarin compound found in rue, a medicinal herb. Here we have investigated the inhibitory effects of chalepensin on cytochrome P450 (CYP) 2A6 in vitro and in vivo. EXPERIMENTAL APPROACH Mechanism-based inhibition was studied in vitro using human liver microsomes and bacterial membranes expressing genetic variants of human CYP2A6. Effects in vivo were studied in C57BL/6J mice. CYP2A6 activity was assayed as coumarin 7-hydroxylation (CH) using HPLC and fluorescence measurements. Metabolism of chalepensin was assessed with liquid chromatography/mass spectrometry (LC/MS). KEY RESULTS CYP2A6.1, without pre-incubation with NADPH, was competitively inhibited by chalepensin. After pre-incubation with NADPH, inhibition by chalepensin was increased (IC 50 value decreased by 98%). This time-dependent inactivation (k inact 0.044 min -1; K I 2.64 μM) caused the loss of spectrally detectable P450 content and was diminished by known inhibitors of CYP2A6, pilocarpine or tranylcypromine, and by glutathione conjugation. LC/MS analysis of chalepensin metabolites suggested an unstable epoxide intermediate was formed, identified as the corresponding dihydrodiol, which was then conjugated with glutathione. Compared with the wild-type CYP2A6.1, the isoforms CYP2A6.7 and CYP2A6.10 were less inhibited. In mouse liver microsomes, pre-incubation enhanced inhibition of CH activity. Oral administration of chalepensin to mice reduced hepatic CH activity ex vivo. CONCLUSIONS AND IMPLICATIONS Chalepensin was a substrate and a mechanism-based inhibitor of human CYP2A6. Formation of an epoxide could be a key step in this inactivation. 'Poor metabolizers' carrying CYP2A6*7 or*10 may be less susceptible to inhibition by chalepensin. Given in vivo, chalepensin decreased CYP2A activity in mice.

Original languageEnglish
Pages (from-to)1250-1262
Number of pages13
JournalBritish Journal of Pharmacology
Issue number6
Publication statusPublished - Jul 2011


  • CYP2A6
  • chalepensin
  • genotype
  • inhibition
  • mechanism-based

ASJC Scopus subject areas

  • Pharmacology


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