Inhibitory effects of the essential oil of chamomile (Matricaria recutita L.) and its major constituents on human cytochrome P450 enzymes

Abstract

Chamomile extracts and tea are widely used herbal preparations for the treatment of minor illnesses (e.g. indigestion, inflammation). In this study the inhibitory effect of chamomile essential oil and its major constituents on four selected human cytochrome P450 enzymes (CYP1A2, CYP2C9, CYP2D6 and CYP3A4) was investigated. Increasing concentrations of the test compounds were incubated with individual, recombinant CYP isoforms and their effect on the conversion of surrogate substances was measured fluorometrically in 96-well plates; enzyme inhibition was expressed as IC₅₀ and K_i value in relation to positive controls.

Crude essential oil demonstrated inhibition of each of the enzymes, with CYP1A2 being more sensitive than the other isoforms. Three constituents of the oil, namely chamazulene (IC₅₀ = 4.41 μM), cis-spiroether (IC₅₀ = 2.01 μM) and trans-spiroether (IC₅₀ = 0.47 μM) showed to be potent inhibitors of this enzyme, also being active towards CYP3A4. CYP2C9 and CYP2D6 were less inhibited, only chamazulene (IC₅₀ = 1.06 μM) and α-bisabolol (IC₅₀ = 2.18 μM) revealed a significant inhibition of the latter. As indicated by these in vitro data, chamomile preparations contain constituents inhibiting the activities of major human drug metabolizing enzymes; interactions with drugs whose route of elimination is mainly via cytochromes (especially CYP1A2) are therefore possible.

Introduction

Cytochromes P450, a group of more than 50 different proteins, are the principal enzymes for the oxidative metabolism of drugs and other xenobiotics. Several isoforms, such as CYP1A2, CYP2C9, CYP2D6 and CYP3A4 appear to be most relevant for the metabolism of clinically significant drugs (Wiseman, 2005, Crespi et al., 1997). Inhibition of one of them often results in unexpected and sometimes severe adverse drug interactions, as the metabolic clearance of co-administered drugs can be altered dramatically (Goshman et al., 1999). Many of these interactions have been reported so far, for example the withdrawal of mibefradil (Prosicor®) from the market because of life threatening interactions with a number of drugs, e.g. cyclosporine and imipramine (Welker et al., 1998, Bapiro et al., 2001). These interactions are not limited to synthetic drugs, natural products may also act as triggers for changes in the CYP activity; for example CYP-modulating effects are described for grapefruit juice or St. John's Wort (Ameer and Weintraub, 1997, Dürr et al., 2000, Zou et al., 2002a). Nevertheless, being considered as “natural” (thus harmless), the impact of natural products on drug interactions is still underestimated (Obach, 2000).

One of the most popular herbs for the treatment of indigestion, cramps, inflammations and other kinds of minor illnesses is chamomile. Matricaria recutita L. (syn. M. chamomilla, Chamomilla recutita), Asteraceae, is an annual, 20–50 cm high, white flowering plant, indigenous to the eastern Mediterranean (Schilcher, 1987, Hänsel and Sticher, 2004). Preparations (e.g. ointments, inhalations, tinctures, teas) of chamomile flowers are easily available over-the-counter, without the recommendation of a physician.

Most of the pharmaceutical value of the plant lies in its characteristically blue coloured volatile oil (content in M. recutita up to 1.5%). Other activity related compounds are flavonoids (spasmolytic properties) and polysaccharides (Robbers and Tyler, 2000, Avallone et al., 2000). The oil comprises a complex mixture of sesquiterpenes (α-bisabolol, bisabololoxides A and B, farnesene), sesquiterpenelactones (chamazulene, with intense blue colour) and acetylene-derivatives (spiroethers; for structures and respective molecular masses see Fig. 1). These compounds are responsible for the anti-inflammatory, anti-bacterial and anti-fungal actions of chamomile (Ammon et al., 1996, Rekka et al., 1996, Amirghofran et al., 2000, Lee and Shibamoto, 2002).

M. recutita essential oil and its constituents (e.g. α-bisabolol) are membrane permeable in vitro (Szentmihalyi et al., 2001). Maliakal and Wanwimolruk (2001) reported a decreased CYP1A2 activity in rats by 39% after four weeks of chamomile tea consumption, thus bioavailability via this route can be assumed (but exact pharmacological data are still missing). To our knowledge, the only comparable data refer to the pharmacokinetics of thymol. This monoterpene found in thyme essential oil shows a good oral bioavailability (maximum plasma concentration of 90 ng/ml, after a single oral dose of 1 mg thymol; Kohlert et al., 2002). Chamomile tea contains 10–15% of the essential oil present in the plant, whole extracts and preparations certainly more (Foster and Tyler, 1999). Adverse effects or drug interactions are reported for chamomile extracts or tea, e.g. anaphylactic reactions (Subiza et al., 1989), inhibition of morphine dependence (Gomaa et al., 2003) or potentiated effects of warfarin (Heck et al., 2000); reasons for these interactions are mainly unknown. Therefore, it was the aim of this study to investigate inhibitory properties of M. recutita volatile oil and its major bioactive constituents against selected CYP enzymes. The compounds were isolated by chromatographic methods and subjected to a high-throughput assay, which is based on a flourometric determination of enzyme inhibitory substances.