Dr. Oesterheld’s Comment: Although clinicians may be well aware of the potential of grapefruit juice to be an inhibitor of CYP3A4, only a few may be aware of the possibility of other CYP-based food- drug interactions. In this study, fennel, an herb and alternate drug is shown to be a modest CYP3A4 inhibitor. Since its inhibition is mechanism-based, the recovery from inhibition may take a few days since new CYP will need to be manufactured and as a result, it is more likely to cause drug interactions. However, because the Ki is low (15), whether it actually is involved in invivo drug interactions is hard to say.

J Agric Food Chem. 2007 Dec 12;55(25):10162-7. Epub 2007 Nov 8.

Inhibition on human liver cytochrome P450 3A4 by constituents of fennel (Foeniculum vulgare): identification and characterization of a mechanism-based inactivator.

Subehan , Zaidi SF, Kadota S, Tezuka Y.Institute of Natural Medicine, University of Toyama, 2630- Sugitani, Toyama 930-0194, Japan.

Fennel, a seed of Foeniculum vulgare, is used as a culinary spice and traditional medicine. The methanolic extract of fennel showed a characteristic of mechanism-based inactivation on erythromycin N-demethylation mediated by human liver microsomal cytochrome P450 3A4 (CYP3A4). The present study was conducted to identify the fennel constituent having the inhibition. Thirteen compounds have been isolated from a methanol extract of fennel and tested for their inhibition on CYP3A4. Among them, 5-methoxypsoralen (5-MOP) showed the strongest inhibition with an IC50 value of 18.3 microM and a mixed type of inhibition. In addition, with the preincubation time of 20 min only 5-MOP showed preincubation time dependency; the IC50 value decreased from 18.3 microM with a preincubation time of 0 min to 4.6 microM with a preincubation time of 20 min. Further investigation on 5-MOP showed the characteristics of time-dependent inhibition, requirement of NADPH, lack of protecting effect of nucleophiles, and recovery of CYP3A4 activity by the competitive inhibitor. This result suggests that the inhibitory activity of CYP3A4 by 5-MOP was a mechanism-based inactivation. The kinetic parameter for mechanism-based inactivation was characterized by a KI value of 15.0 microM and a kinact value of 0.098 min(-1).  PMID: 17988092