Inhibition of human recombinant cytochrome P450s by curcumin and curcumin decomposition products
Introduction
Multiple drug therapy is a common therapeutic practice especially in patients with multiple complications (Nadler et al., 2003, Hemaiswarya and Doble, 2006). If two or more drugs with affinity for the same cytochrome P450 (CYP) enzyme are co-administered, their biotransformation may be compromised, leading to undesirable accumulation of the drugs with toxic side effects as possible consequence. Drug–drug interactions involving CYPs have been identified as an important cause of adverse drug reactions and therapeutic failure (Honig et al., 1993, Pea and Furlanut, 2001). Drug–drug interactions may be due to enzyme induction or inhibition, the latter being more common (Hemaiswarya and Doble, 2006, Zafar and Sharif, 2003). Next to drugs, several natural compounds have also been shown to cause significant interactions at the level of drug-metabolizing enzymes (Ioannides, 2002, Obach, 2000).
Curcumin, a polyphenolic component of turmeric (Curcuma longa), is a yellow pigment widely used for coloring of foods. It has been shown previously to exhibit anticancer, antioxidant, anti-inflammatory, antiparasitic and anti-HIV properties (Leu and Maa, 2002, Cole et al., 2004, Vajragupta et al., 2005, Reddy et al., 2005). Curcumin has also been shown to have chemoprotective, chemopreventive and immuno-modulating properties (Cole et al., 2004, Donatus et al., 1990, Cheng et al., 2001). Curcumin can be considered as a safe compound, because oral doses as high as 8 g/day administered to humans did not result in overt side effects (Cheng et al., 2001). Clinical trials for the use of curcumin as an anticancer agent are currently ongoing (Sharma et al., 2004).
Because relatively high doses of curcumin are evaluated in human studies, it might be anticipated that curcumin might cause drug–drug interactions at the level of intestinal and/or liver drug metabolism. Several in vivo and in vitro animal studies have shown that curcumin can significantly modulate the activity of several drug-metabolizing enzymes by down-regulation, induction or by direct inhibition. Oetari et al. (1996) and Thapliyal and Maru (2001) reported potent inhibition of rat liver microsomal CYP1A1, CYP1A2 and CYP2B1 enzymes by curcumin. In in vivo studies repetitive administration of curcumin to rats resulted in down-regulation of intestinal CYP3A-enzymes, whereas hepatic and renal CYP3A-levels were significantly induced (Zhang et al., 2006). Also, down-regulation of esophagal CYP2B1 and CYP2E1 was reported after intragastric treatment of rats, which might partially explain the chemopreventive activity of curcumin against carcinogenic N-nitrosamines (Mori et al., 2006).
As yet the effects of curcumin on the major human drug-metabolizing CYPs have not been studied. Due to the large species differences in the properties of metabolic enzymes and metabolic profiles of drugs, the animal studies described above are poorly predictive for the human situation (Eagling et al., 1998, Guengerich, 1997). The present in vitro investigation therefore was designed to assess the potential of curcumin to cause drug–drug interactions via inhibition of the five most important human drug-metabolizing CYPs. Major human CYP isoforms responsible for the metabolism and disposition of about 90% of the therapeutic drugs on the market include CYP1A2, CYP2D6, CYP2B6, CYP2E1, CYP2C9 and CYP3A4 (Shimada et al., 1994). Because curcumin has been shown to be chemically unstable under neutral and alkaline conditions (Wang et al., 1997), the inhibitory properties of these decomposition products were also studied as mixture and individually, if available. Degradation products which have been identified include trans-6-(4′-hydroxy-3′-methoxyphenyl)-2,4-dioxo-5-hexenal, and minor products being vanillin, vanillic acid, ferulic aldehyde and ferulic acid (Fig. 1) (Wang et al., 1997). Apart from reversible inhibition, mechanism-based inhibition was also taken into consideration.
Section snippets
Materials
Methoxyresorufin and benzyloxyresorufin were synthesized by the method of Burke et al. (1985) and the purity was determined by high performance liquid chromatography (HPLC), mass spectrometry and 1H NMR. The plasmid, pSP19T7LT_2D6 containing human CYP2D6 bicistronically co-expressed with human cytochrome P450 NADPH reductase was kindly provided by Prof. M. Ingelman-Sundberg (Stockholm, Sweden). The plasmids, BMX100/h1A2 and pCWh3A4 with human cytochrome P450 NADPH reductase were kindly donated
Decomposition of curcumin
Degradation of curcumin under various pH conditions, and the stability of curcumin in physiological matrices have been previously reported (Wang et al., 1997). To identify the inhibitory potentials of the decomposition products of curcumin towards human CYPs, decomposition experiments were also performed in the present study. Curcumin was treated with 0.1 M phosphate buffer of pH 7.4 at 37 °C for 1 h, according to the procedure described by Wang et al. (1997). Fig. 2 shows the HPLC chromatogram of
Discussion
The purpose of this study was to evaluate the inhibitory potential of curcumin and its decomposition products on the five important human drug-metabolizing CYPs, namely CYP1A2, CYP3A4, CYP2B6, CYP2C9 and CYP2D6. Earlier reports on the inhibition of rat liver microsomal CYPs by curcumin showed that curcumin is a strong inhibitor of CYP1A and CYP2B (Oetari et al., 1996, Thapliyal and Maru, 2001). In the present study, curcumin and its decomposition products were first screened at a high
Acknowledgements
We thank Ed Groot of the Molecular Toxicology Section and Ben Bruyneel of the Analytical Chemistry and Spectroscopy Section of the Vrije Universiteit, for their technical assistance.
Funds for this project were provided by the government of the Republic of Ghana, through the Government of Ghana (GOG) and Getfund scholarship schemes.
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