Methadone is a clinically used opioid agonist that is oxidatively metabolized by cytochrome P450 (CYP) isoforms to a stable metabolite, EDDP. Methadone is a chiral drug administered as the racemic mixture of (R)-(-)- and (S)-(+)-methadone, but (R)-methadone is the active isomer. The cytochrome P450 (CYP) isoform involved in methadone's metabolism is thought to be CYP3A4, but human drug-drug interaction studies are not consistent with this. The ability of the common human drug-metabolizing CYPs (obtained from baculovirus-infected insect cell supersomes) to generate 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrilidine (EDDP) from racemic methadone was examined and then determined if the CYP isoforms metabolized methadone stereoselectively. Only CYP2B6, 2C19, and 3A4 generated measurable EDDP from 1 microg/ml of racemic methadone. The hierarchy of EDDP generation was CYP2B6> CYP2C19 >/= CYP3A4. At 10 microg/ml of methadone, CYP2C9 and CYP2D6 also generated EDDP, but in at least 10-fold lower quantities than CYP2B6. Michaelis-Menten kinetic data demonstrated that CYP2B6 had the highest V(max) (44 ng/min/10pmol) and the lowest K(m) (12.6 microg/ml) for EDDP formation of all the CYP isoforms. In human liver microsomes with high and low CYP2B6 expression but equivalent CYP3A4 expression, high CYP2B6 expression microsomes generated twice the amount of EDDP from 10 microg/ml of methadone than low CYP2B6 expression microsomes. When stereoselective metabolism of racemic methadone by CYP2B6, 2C19, and 3A4 was examined using an enantiospecific methadone assay, CYP2B6 preferentially metabolized (S)-methadone, CYP2C19 preferentially metabolized (R)-methadone, and CYP3A4 showed no preference. These data suggest that multiple CYPs metabolized methadone but CYP2B6 had the highest V(max)/K(m). In addition, only CYP2B6 and 2C19 showed stereoselective metabolism. Our data could explain why the plasma concentration ratio of R/S methadone is variable and why drugs that induce CYP2B6 such as nevirapine and efavirenz also induce methadone metabolism, while the CYP3A4 inducer rifabutin has no effect on methadone pharmacokinetics.

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Very few antagonists have been identified for the human pregnane X receptor (PXR). These molecules may be of use for modulating the effects of therapeutic drugs, which are potent agonists for this receptor (e.g., some anticancer compounds and macrolide antibiotics), with subsequent effects on transcriptional regulation of xenobiotic metabolism and transporter genes. A recent novel pharmacophore for PXR antagonists was developed using three azoles and consisted of two hydrogen bond acceptor regions and two hydrophobic features. This pharmacophore also suggested an overall small binding site that was identified on the outer surface of the receptor at the AF-2 site and validated by docking studies. Using computational approaches to search libraries of known drugs or commercially available molecules is preferred over random screening. We have now described several new smaller antagonists of PXR discovered with the antagonist pharmacophore with in vitro activity in the low micromolar range [S-p-tolyl 3Ј,5-dimethyl-3,5Ј-biisoxazole-4Ј-carbothioate (SPB03255) (IC 50 , 6.3 M) and 4-(3-chlorophenyl)-5-(2,4-dichlorobenzylthio)-4H-1,2,4-triazol-3-ol (SPB00574) (IC 50 , 24.8 M)]. We have also used our computational pharmacophore and docking tools to suggest that most of the known PXR antagonists, such as coumestrol and sulforaphane, could also interact on the outer surface of PXR at the AF-2 domain. The involvement of this domain was also suggested by further site-directed mutagenesis work. We have additionally described an FDA approved prodrug, leflunomide (IC 50 , 6.8 M), that seems to be a PXR antagonist in vitro. These observations are important for predicting whether further molecules may interact with PXR as antagonists in vivo with potential therapeutic applications.Our knowledge of ligand-protein interactions for some of the nuclear hormone receptors is in the nascent stages. This has downstream implications for understanding, predicting and modulating the potential xenobiotic and environmental molecule effects on transcription of key genes in human. For example, the pregnane X receptor (PXR; NR1I2; also known as SXR or PAR) regulates multiple genes, including the enzymes CYP3A4 (Bertilsson et al., 1998;Blumberg et al., 1998;Kliewer et al., 1998), CYP2B6 (Goodwin et al., 2001, and CYP2C9 as well as the transporter P-glycoprotein (ABCB1) (Synold et al., 2001) and others. There is a very broad structural diversity in the molecules that bind to human PXR from bile salts (Schuetz and Strom, 2001;Krasowski et al., 2005) to anticancer compounds (Mani et al., 2005;Ekins et al., 2007). Several X-ray crystal structures of the ligand binding domain (LBD) of PXR (Watkins et al., 2001(Watkins et al., , 2002(Watkins et al., , 2003aXue et al., 2007b) have determined that S.E., N.A., V. K., and W.J.W. gratefully acknowledge the support for this work provided by the USEPA-funded Environmental Bioinformatics and Computational Toxicology Center (ebCTC), under STAR Grant number GAD R 832721-010. This work was supported in pa...

Nearly a decade after discovering molecular chirality in 1848, Louis Pasteur changed research direction and began investigating fermentations. Conflicting explanations have been given for this switch to microbiology, but the evidence strongly suggests that Pasteur's appointment in 1854 to the University of Lille--an agricultural-industrial region where fermentation-based manufacturing was of great importance--and an appeal for help in 1856 by a local manufacturer experiencing problems in his beetroot-fermentation-based alcohol production played a significant role. Thus began, in late 1856, Pasteur's pioneering studies of lactic and alcoholic fermentations. In 1857, reportedly as a result of a laboratory mishap, he found that in incubations of ammonium (+/-)-tartrate with unidentified microorganisms (+)-tartaric acid was consumed with considerable preference over (-)-tartaric acid. In 1860, he demonstrated a similar enantioselectivity in the metabolism of tartaric acid by Penicillium glaucum, a common mold. Chance likely played a significant role both in Pasteur's shift to microbiology and his discovery of enantioselective tartrate fermentations, but he rejected pure serendipity as a significant factor in experimental science and in his own career. Pasteur's milestone discovery of biological enantioselectivity began the process that in the long run established the fundamental importance of molecular chirality in biology.

A variety of reactions can be categorized under the global concept of the "interconversion of stereoisomers." Thus, racemization or epimerization can result from inversion of labile chiral centers. From the examples available, some predictive rules are suggested for a chiral center of the type R"R'RC-H undergoing base-catalyzed inversion and a provisional table of affecting groups is presented. Unimolecular inversion of nonsymmetrical, nonplanar ring systems can also result in racemization or epimerization, but no generalization can yet be offered. Beside these cases of nonenzymatic reactions, a limited variety of enzymatic reactions can operate to interconvert stereoisomers, the outcome rarely being a racemic mixture. An important aspect of stereoisomer interconversion is the time scale in which the phenomenon is observed. Thus, several reactions to nonezymatic racemization or epimerization are fast compared to the duration of action of the drug and therefore have pharmacological significance, while other are slower and are of pharmaceutical relevance only.

Racemic ketoconazole (KTZ) was the first orally active azole antifungal agent used in clinical practice and has become widely used in the treatment of mucosal fungal infections associated with AIDS immunosuppression and cancer chemotherapy. However, the use of KTZ has been limited because of adverse drug-drug interactions. KTZ blocks ergosterol biosynthesis by inhibiting the fungal cytochrome P450 (CYP51). KTZ is also a potent inhibitor of human cytochrome P450 3A4 (CYP3A4) enzyme, the major drug-metabolizing CYP isozyme in the human liver. We examined the enantioselective differences of KTZ in the inhibition of human CYP3A4 and in antifungal action. Dextro- and levo-KTZ exhibited modest enantioselective differences with respect to CYP3A4 inhibition of testosterone and methadone metabolism. For both substrates levo-KTZ was approximately a 2-fold more potent inhibitor. We examined the enantioselective differences in the in vitro activity of KTZ against medically relevant species of Candida and Aspergillus, as well as Cryptococcus neoformans. Overall, levo-KTZ was 2-4-fold more active than dextro-KTZ. Therefore, levo-KTZ is a more potent inhibitor of CYP3A4 and has stronger in vitro antifungal activity. Chirality 16:79-85, 2004.

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Louis Pasteur resolved sodium ammonium (±)-tartrate in 1848, thereby discovering molecular chirality. Although hindered by the primitive state of organic chemistry, he introduced new terminology and nomenclature for his new science of molecular and crystal chirality. He was well prepared for this task by his rigorous education and innate abilities, and his linguistic achievements eventually earned him membership in the supreme institution for the French language, the Académie française. Dissymmetry had been in use in French from the early 1820s for disruption or absence of symmetry or for dissimilarity or difference in appearance between two objects, and Pasteur initially used it in the latter connotation, without any reference to handedness or enantiomorphism. Soon, however, he adopted it in the meaning of chirality. Asymmetry had been in use in French since 1691 but Pasteur ignored it in favor of dissymmetry. The two terms are not synonymous but it is not clear whether Pasteur recognized this difference in choosing the former over the latter. However, much of the literature mistranslates his dissymmetry as asymmetry. Twenty years before Pasteur the British polymath John Herschel proposed that optical rotation in the noncrystalline state is due to the "unsymmetrical" [his term] nature of the molecules and later used dissymmetrical for handed. Chirality, coined by Lord Kelvin in 1894 and introduced into chemistry by Mislow in 1962, has nearly completely replaced dissymmetry in the meaning of handedness, but the use of dissymmetry continues today in other contexts for lack of symmetry, reduction of symmetry, or dissimilarity.