Stereoselectivity in drug metabolism can not only influence the pharmacological activities, tolerability, safety, and bioavailability of drugs directly, but also cause different kinds of drug–drug interactions. Thus, assessing stereoselectivity in drug metabolism is of great significance for pharmaceutical research and development (R&D) and rational use in clinic. Although there are various methods available for assessing stereoselectivity in drug metabolism, many of them have shortcomings. The indirect method of chromatographic methods can only be applicable to specific samples with functional groups to be derivatized or form complex with a chiral selector, while the direct method achieved by chiral stationary phases (CSPs) is expensive. As a detector of chromatographic methods, mass spectrometry (MS) is highly sensitive and specific, whereas the matrix interference is still a challenge to overcome. In addition, the use of nuclear magnetic resonance (NMR) and immunoassay in chiral analysis are worth noting. This review presents several typical examples of drug stereoselective metabolism and provides a literature-based evaluation on current chiral analytical techniques to show the significance and challenges of stereoselectivity assessing methods in drug metabolism.

In recent years, finding effective biomarkers for identifying early stage cancer and predicating prognosis is crucial for renal cell carcinoma (RCC) diagnosis and treatment. In this study, a dramatic decrease of the solute carrier family 47 member 2 (SLC47A2) mRNA in RCC comparing with the paired adjacent nontumor tissues from patients at low Tumor Node Metastasis stage was observed. Thus, patients with SLC47A2 transcriptional repression are susceptible to RCC. Little is known about the regulation mechanism of SLC47A2 We found that it was a bivalent gene that was enriched with both histone H3 lysine 4 trimethylation (H3K4me3) and lysine 27 trimethylation (H3K27me3). Loss of mixed lineage leukemia 1 binding at the gene promoter caused decreased H3K4me3 enrichment and H3K4me3/H3K27me3 ratio, and subsequently repressed the expression of SLC47A2 These two epigenetic markers modulated the expression of SLC47A2 simultaneously, suggesting the regulation pattern for bivalent genes. Histone H3 lysine 27 acetylation also contributed to the expression of SLC47A2 An E2F1-histone deacetylase 10 complex catalyzed deacetylation of H3K27, then prevented the enrichment of H3K4me3, and finally reduced SLC47A2 expression. Consequently, the combined effect of all these factors determined SLC47A2 transcriptional repression in RCC tissues.

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We previously showed that anthraquinones (including rhein, emodin, aloe-emodin, chrysophanol and physcion) were inhibitors of human organic anion transporter 1 (hOAT1) and hOAT3, causing transporter-mediated drug-drug interactions in rats. In this study, the time-dependent inhibition (TDI) of hOAT1 and hOAT3 by anthraquinones was investigated. Madin-Darby canine kidney (MDCK)-hOAT1, HEK293-hOAT3 and their parental cells were used. Preincubation with chrysophanol or physcion for 30 min significantly increased the inhibition of hOAT1, but preincubation with rhein, emodin, aloe-emodin or probenecid had no effect on hOAT1 activity. By contrast, preincubation of hOAT3 with emodin, aloe-emodin, chrysophanol or physcion for 30 min significantly increased its inhibition, but preincubation with rhein or probenecid had no effect on activity. As the incubating time lengthened, from 0 to 60 min, both the inhibition of hOAT1 by chrysophanol and physcion and the inhibition of hOAT3 by emodin, aloe-emodin, chrysophanol and physcion were observed to increase in a time-dependent manner. In conclusion, our results suggest that some anthraquinones contribute to the TDI of hOAT1 and hOAT3. An inhibition study without the preincubation procedure may underestimate the inhibitory potential of anthraquinones against hOAT1 and hOAT3. The underlying mechanisms of TDI of hOAT1 and hOAT3 need to be further investigated. Key words organic anion transporter; anthraquinone; timedependent inhibitionIn CYP450 enzyme-mediated metabolism, time-dependent inhibition (TDI) has attracted significant attention. TDI of CYP450 enzymes refers to a change in the inhibitory potency during an in vitro incubation or dosing period in vivo. 1)However, does drug transporter-based TDI exist? Amundsen et al.2) reported that inhibition of organic anion transporting polypeptide 1B1 (OATP1B1) by cyclosporine A occurred in a time-dependent manner. Preincubation of OATP1B1 with cyclosporine A increased the inhibition of this protein; however, the underlying mechanism was not described in that report. Other researchers found that preincubation of OATP2B1 with apple juice or orange juice increased their inhibition effect.3)In general, there are very few articles that report on drug transporter-based TDI.In our previous study, we found that anthraquinones (rhein, emodin, aloe-emodin, chysophanol or physcion) inhibited human organic anion transporter 1 (hOAT1) and hOAT3. 4)Anthraquinones were coincubated with substrates of hOAT1 or hOAT3 in that study. However, we observed that preincubation of hOAT1 and hOAT3 with some of the anthraquinones increased hOAT1 and hOAT3 inhibition. Currently, TDI of the OATs family has not been studied. Therefore, in this study, the inhibitory effect of preincubation anthraquinones with hOAT1 and hOAT3 was investigated. The results provide evidence for the complex inhibitory mechanisms of drug transporters.