Inhibition of paclitaxel metabolism in vitro in human hepatocytes by Ginkgo biloba preparations.

Since the late 1980s, chemotherapy-induced cognitive impairment, also known as "chemobrain", has been a recognized side effect in patients undergoing cancer treatment ( Matsuda et al., 2005 ). Although products containing Ginkgo biloba may be used by patients undergoing chemotherapy with paclitaxel and other agents, the potential for an herb-drug interaction with this combination has not been adequately explored. This report describes the inhibition of paclitaxel metabolism by Ginkgo preparations in vitro in human hepatocytes. Hydrolyzate of Ginkgo extract (10-100 mM in terpene lactone concentration) caused a dose-dependent inhibition of the 6α -hydroxylation of paclitaxel, the enzymatic activity responsible for the majority of the clearance of that drug in clinical applications; parent extract had no effect. Contrary to the assumed therapeutic benefit of Ginkgo, its concomitant use with paclitaxel could result in elevated blood levels of the chemotherapeutic, with attendant exacerbation of cognitive impairment and other toxic effects associated with cancer therapy.

An in vitro evaluation of cytochrome P450 inhibition and P-glycoprotein interaction with goldenseal, Ginkgo biloba, grape seed, milk thistle, and ginseng extracts and their constituents.

Drug-herb interactions can result from the modulation of the activities of cytochrome P450 (P450) and/or drug transporters. The effect of extracts and individual constituents of goldenseal, Ginkgo biloba (and its hydrolyzate), grape seed, milk thistle, and ginseng on the activities of cytochrome P450 enzymes CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 in human liver microsomes were determined using enzyme-selective probe substrates, and their effect on human P-glycoprotein (Pgp) was determined using a baculovirus expression system by measuring the verapamil-stimulated, vanadate-sensitive ATPase activity. Extracts were analyzed by HPLC to standardize their concentration(s) of constituents associated with the pharmacological activity, and to allow comparison of their effects on P450 and Pgp with literature values. Many of the extracts/constituents exerted > or = 50 % inhibition of P450 activity. These include those from goldenseal (normalized to alkaloid content) inhibiting CYP2C8, CYP2D6, and CYP3A4 at 20 microM, ginkgo inhibiting CYP2C8 at 10 microM, grape seed inhibiting CYP2C9 and CYP3A4 at 10 microM, milk thistle inhibiting CYP2C8 at 10 microM, and ginsenosides F1 and Rh1 (but not ginseng extract) inhibiting CYP3A4 at 10 microM. Goldenseal extracts/constituents (20 microM, particularly hydrastine) and ginsenoside Rh1 stimulated ATPase at about half of the activity of the model substrate, verapamil (20 microM). The data suggest that the clearance of a variety of drugs may be diminished by concomitant use of these herbs via inhibition of P450 enzymes, but less so by Pgp-mediated effects.

Pharmacokinetics and disposition of the kavalactone kawain: interaction with kava extract and kavalactones in vivo and in vitro.

Reported adverse drug interactions with the popular herb kava have spurred investigation of the mechanisms by which kava could mediate these effects. In vivo and in vitro experiments were conducted to examine the effects of kava extract and individual kavalactones on cytochrome P450 (P450) and P-glycoprotein activity. The oral pharmacokinetics of the kavalactone, kawain (100 mg/kg), were determined in rats with and without coadministration of kava extract (256 mg/kg) to study the effect of the extract on drug disposition. Kawain was well absorbed, with >90% of the dose eliminated within 72 h, chiefly in urine. Compared with kawain alone, coadministration with kava extract caused a tripling of kawain AUC(0-8 h) and a doubling of C(max). However, a 7-day pretreatment with kava extract (256 mg /kg/day) had no effect on the pharmacokinetics of kawain administered on day 8. The 7-day pretreatment with kava extract only modestly induced hepatic P450 activities. The human hepatic microsomal P450s most strongly inhibited by kava extract (CYP2C9, CYP2C19, CYP2D6, CYP3A4) were inhibited to the same degree by a "composite" kava formulation composed of the six major kavalactones contained in the extract. K(i) values for the inhibition of CYP2C9 and CYP2C19 activities by methysticin, dihydromethysticin, and desmethoxyyangonin ranged from 5 to 10 microM. Kava extract and kavalactones (< or =9 microM) modestly stimulated P-glycoprotein ATPase activities. Taken together, the data indicate that kava can cause adverse drug reactions via inhibition of drug metabolism.

Inhibition of human cytochrome P450 activities by kava extract and kavalactones.

The herb kava has recently been associated with numerous drug interactions, but its interaction with cytochrome P450 (P450) enzymes has not been investigated. In the present work the inhibition of P450 enzymes by kava extract and individual kavalactones in human liver microsomes (HLMs) was investigated. Whole kava extract (normalized to 100 microM total kavalactones) caused concentration-dependent decreases in P450 activities, with significant inhibition of the activities of CYP1A2 (56% inhibition), 2C9 (92%), 2C19 (86%), 2D6 (73%), 3A4 (78%), and 4A9/11 (65%) following preincubation for 15 min with HLMs and NADPH; CYP2A6, 2C8, and 2E1 activities were unaffected. The activities of CYP2C9, 2C19, 2D6, and 3A4 were also measured after incubation of HLMs with the major kavalactones kawain (K), desmethoxyyangonin (DMY), methysticin (M), dihydromethysticin (DHM) (each at 10 microM), and NADPH. Whereas K did not inhibit these enzymes, there was significant inhibition of CYP2C9 by DMY (42%), M (58%), and DHM (69%); of 2C19 by DHM (76%); of 2D6 by M (44%); and of 3A4 by DMY (40%), M (27%), and DHM (54%). Consistent with their potency as inhibitors, the two major kavalactones bearing a methylenedioxyphenyl moiety (M and DHM) formed "455 nm" metabolic intermediate complexes after incubation with HLMs and NADPH, but K and DMY did not. These data indicate that kava has a high potential for causing drug interactions through inhibition of P450 enzymes responsible for the majority of the metabolism of pharmaceutical agents.