Chiral metabolites

J. A. Chiar otto and I. W. Wainer, Determination of metyrapone and the enantiomers of its chiral metabolite metyrapol in human plasma and urine using coupled achhal-chhal liquid cliromatogruphy , J. Chromatogr. 665 147-154 (1995). 

TABLE 14.1 Analysis of Chiral Drugs, Chiral Metabolites, and Related Compounds... 

Turning to enzymatic hydration, we see from the data in Table 10.1 that phenanthrene 9,10-oxide Fig. 10.10, 10.29) is an excellent substrate for epoxide hydrolase. Comparison of enzymatic hydration of the three isomeric phenanthrene oxides shows that the Vmax with the 9,10-oxide is greater than with the 1,2- or the 3,4-oxide the affinity was higher as well, as assessed by the tenfold lower Km value [90]. Furthermore, phenanthrene 9,10-oxide has a plane of symmetry and is, thus, an achiral molecule, but hydration gives rise to a chiral metabolite with high product enantioselectivity. Indeed, nucleophilic attack by epoxide hydrolase occurs at C(9) with inversion of configuration i.e., from below the oxirane ring as shown in Fig. 10.10) to yield the C-H9.S, 10.S )-9,10-dihydro-9,10-diol (10.30) [91],... 

Figure 11.4 Chromatograms of plasma samples on a silica-chiralcel OJ coupled column system (a) plasma spiked with oxprenolol (internal standard) (b) plasma spiked with 0.40 jxg/ml metyrapone and 0.39 p,g/ml metyrapol (racemate) (c) plasma sample obtained after oral administration of 750 mg metarypone. Peaks are as follows 1, metyrapone 2, metyrapol enantiomers 3, oxprenolol. Reprinted from Journal of Chromatography, 665, J. A. Chiarotto and I. W. Wainer, Determination of metyrapone and the enantiomers of its chiral metabolite metyrapol in human plasma and urine using coupled achiral-chiral liquid chromatography, pp. 147-154, copyright 1995, with permission from Elsevier Science.

Capillary gas chromatographic investigation of diastereoisomeric derivatives revealed that in analogy to results obtained without precursors the chiral metabolites are present as mixtures of enantiomers. However for only a few of these compounds the ratios of enantiomers are identical with those determined in pineapple without precursors. The enantiomeric compositions of ethyl 3-hydroxyhexanoate and ethyl 3-acetoxyhexanoate are almost opposite to those determined for the naturally occurring methyl esters. 6-Octalactone obtained after addition of 5-oxooctanoic acid to pineapple tissue is almost optically pure (92% S) on the other hand -octalactone is naturally present in pineapple tissue as nearly racemic mixture (Table 1,8). 

The potency of enantioselective gas chromatography became apparent in the pharmacokinetic studies of flecainide [3] and two of its chiral metabolites as carried out by Fischer et al. (40). Trace amounts of these compounds in plasma samples were screened for enantiomeric discrimination. 

Species differences in the generation of the chiral sulfoxide metabolite of albendazole in sheep and rats" (37). The chiral metabolites of the prochiral anthelmintic drug albendazole were determined on an (S)-N-(3,5-dinitrobenzoyl)-tyrosine-0- 2-propen l-yl) n-butylamide chiral stationary phase using a mobile phase composed of hexane ethanol. 

For other anti-inflammation drugs such as Naproxen59 and Fenoprofen,60 only the S-isomer is biologically active. Since tedious resolutions are necessary to prepare these compounds, it would be desirable to develop a general asymmetric synthesis of the S(+) arylpropionic acids by microbial oxidation of pro-chiral substrates. This concept has been successfully applied in the preparation of a chiral metabolite of 29. Oxidation of one of the enan-tiotopic methyl groups in 30 with sulfurescens furnished the chiral diol 31.61 A similar oxidation of isobutyric acid (32)62 to S-(+)-3-hydroxy-2-mdthylpropionic acid (33) has been recently employed in the synthesis of the a-tocopherol synthon (34).63... 

A very important example is the case of 2-(l-naphthyl)propane-l,2-diol 42, which was isolated as a chiral metabolite of 1-isopropylnaphthalene in rabbits. 

Figure 13.35. Disopyramide (40), mianserin (41), and (5)-para-hydroxyphen3doin (S-42), the chiral metabolite of phenytoin (25)shown in Fig. 13.22.

Most often chiral metabolites are characterized by optical activity because of their origin in enzyme mediated reaction sequences. In fact, this rule is of such generality that the first example of a marine secondary metabolite of low enantiomeric purity (8-9) appeared as recently as in 1984. 

Another paper describes the synthesis of two chiral metabolites of ifosfamide (31 R = C2H4CI), namely the inactive 4-oxoifosfamide and the 2-amino-tetrahydro-2iy-l,3,2-oxazaphosphorine 2-oxide (31 R = H), enantiomers of each being obtained through the use of 1-phenylethylamine. Reduction of (31 R = COCHgCl) with borane gave ifosfamide with unchanged chirality. In addition, two achiral metabolites, viz, the inactive (32) and the active (33), were also prepared. 

Some chiral metabolites of antimalarial drugs possess significant levels of antimalarial activity in comparison to parent drug. For example, the main circulating metabolite of halofantrine, ( )-desbutylhalofantrine, possesses an in vitro level of antimalarial activity similar to that of parent drug [42]. The enantiomers of this metabolite also share similar antimalarial activities in vitro. Stereoselectivity in pharmacokinetic properties of these chiral metabolites could influence an assessment of antimalarial activities. On the other hand, neither of the enantiomers of the major 4-carboxylic acid metabolite of mefloquine possesses antimalarial activity [43]. 

Many metabolites both ionizable and nonionizable can be measured by different forms of CE such as CZE, MEKC, and chirality. Metabolites with strong UV absorption such as nucleotides, phenolic amino acids, and their metabolites are easy to measure by CE. However, some of these require concentration and clean up before the CE step [5]. This can be achieved by traditional concentrating methods, such as solid phase and solvent extraction or by concentration on the capillary (stacking). Examples of small molecules that have been analyzed by CE are nucleotides [79,80], amino acids, catecholamines [81-83], and sugars [84,85]. Below is a more detailed discussion of some of these compounds. 

Ketamine, originally developed as an anesthetic, was recently investigated as a potential drug that would reverse the problem of protein metabolism in AIDS patients. Analysis of the distribution of the drug in various body fluids was complicated by the presence of two chiral metabolites. A 30 m long,... 

Methoxychlor is a prochiral compound, and the mono-demethylation reaction would yield chiral metabolites, (Ry and (5)-mono-OH-MXC (Figure 4). Understanding the nature of the P4S0 enzymes contributing for the demethylation reaction of methoxychlor in different animal species, stereoselectivity of the initial demethylation step, which is the conunon metabolic reaction for all animal species tested, was further investigated. 

Synthesis of chiral metabolites transketolase-catalyzed reaction of... 

The reports mentioned above provide a systematic coverage of the nonimmobi-lized enzymatic reactors used in biocatalytic reactions under continuous flow operation. Results from microreactor experiments were comparatively higher than conventionally mixed batch reactors in terms of conversion rate and improvement of product yield as demonstrated for hydrolysis [140], dehalogenation [141], oxidation [142], esteriflcation [143], synthesis of isoamyl acetate [144,145], synthesis of cyanohydrins [147,148], synthesis of chiral metabolites [153], reduction [151], and bioluminescent reaction [149]. The small volumes involved and the favorable mass transfer inherent to these devices make them particularly useful for the screening of biocatalysts and rapid characterization of bioconversion systems. The remarkable results of such studies revealed that the product yield could be enhanced significantly in comparison with the conventional batch runs. 

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