Relative Activity Factor Research Articles

Bisphenol-A glucuronidation in human liver and breast: identification of UDP-glucuronosyltransferases (UGTs) and influence of genetic polymorphisms

1. Bisphenol-A is a ubiquitous environmental contaminant that is primarily metabolized by glucuronidation and associated with various human diseases including breast cancer. Here we identified UDP-glucuronosyltransferases (UGTs) and genetic polymorphisms responsible for interindividual variability in bisphenol-A glucuronidation in human liver and breast.2. Hepatic UGTs showing the highest bisphenol-A glucuronidation activity included UGT2B15 and UGT1A9. Relative activity factor normalization indicated that UGT2B15 contributes >80% of activity at bisphenol-A concentrations under 5 μM, while UGT1A9 contributes up to 50% of activity at higher concentrations.3. Bisphenol-A glucuronidation by liver microsomes (46 donors) ranged from 0.25 to 4.3 nmoles/min/mg protein. Two-fold higher glucuronidation (p = 0.018) was observed in UGT1A9 *22/*22 livers compared with *1/*1 and *1/*22 livers. However, no associations were observed for UGT2B15*2 or UGT1A1*28 genotypes.4. Bisphenol-A glucuronidation by breast microsomes (15 donors) ranged from <0.2 to 56 fmoles/min/mg protein. Breast mRNA expression of UGTs capable of glucuronidating bisphenol-A was highest for UGT1A1, followed by UGT2B4, UGT1A9, UGT1A10, UGT2B7 and UGT2B15. Bisphenol-A glucuronidation was over 10-fold lower in breast tissues with the UGT1A1*28 allele compared with tissues without this allele (p = 0.006).5. UGT2B15 and UGT1A9 contribute to glucuronidation variability in liver, while UGT1A1 is important in breast.

Esterase phenotyping in human liver in vitro: specificity of carboxylesterase inhibitors

1. Esterases may play a major role in the clearance of drugs with functional groups amenable to hydrolysis, particularly in the case of ester prodrugs. To understand the processes involved in the elimination of such drugs, it is necessary to determine the esterases involved. However, the tools currently available for this enzyme phenotyping are relatively scarce.2. The work was aimed at summarizing the selectivity of esterase inhibitors for carboxylesterases 1 and 2 (CES1 and CES2) in the human liver to clarify their suitability for esterase phenotyping. Eserine, at around 10 μM, was found to be a highly specific CES2 inhibitor, whereas other esterase inhibitors turned out less selective. When used together with tacrine, which inhibits cholinesterases but not CES, and ethylenediaminetetraacetic acid (inhibitor of paraoxonases), the involvement of the hydrolyzing esterases in the hepatic clearance of a drug can be elucidated.3. The second approach to esterase phenotyping is based on data from recombinant or isolated esterases, together with relative activity factors, which relate their activities to those of the same enzymes in subcellular fractions.4. These two approaches will help to characterize the hydrolytic metabolism of drug candidates in a similar manner as practiced routinely for the oxidative metabolism by cytochrome P450 enzymes.

Clearance Prediction of HIV Protease Inhibitors in Man: Role of Hepatic Uptake

The aim of this work was to explore the contribution of the organic anion transporting polypeptide-1B (OATP1B) drug transporters in the hepatic clearance (Cl) of all marketed HIV protease inhibitors (PI) in humans. HIV PI uptake rates in OATP1B1/1B3-transfected Chinese hamster ovary cells were converted to uptake Cl values in human hepatocytes via a relative activity factor, which was determined by comparing uptake of known substrates between OATP1B1/3-transfected cells and human hepatocytes. Metabolic Cl values were determined in human liver microsomes. In vivo hepatic Cl values were calculated either by combining drug uptake and metabolism or based on one of these individual Cl processes and compared with published in vivo hepatic Cl values. Excellent in vitro-in vivo correlation (R2 = 0.85) was observed when only uptake Cl values were used, but not when only metabolic Cl was used (R2 = 0.40). The correlation did not improve when both processes were taken into account (R2 = 0.85). PBPK models confirmed the remarkable sensitivity of predicted exposure to hepatic drug uptake, indicating a key role for OATP1B1/3 in hepatic disposition of several HIV PI in man. This may contribute to the interindividual variability in systemic and hepatic exposure to these drugs in the clinic.

Arylacetamide Deacetylase is Responsible for Activation of Prasugrel in Human and Dog.

Prasugrel, a thienopyridine anti-platelet agent, is pharmacologically activated by hydrolysis and hydroxylation. It is efficiently hydrolyzed in the intestine after oral administration, and the enzyme responsible for the hydrolysis in humans was demonstrated to be carboxylesterase (CES)2. Prasugrel hydrolase activity is detected in dog intestines, where CES enzymes are absent; therefore, this prompted us to investigate the involvement of an enzyme(s) other than CES. Human arylacetamide deacetylase (AADAC) is highly expressed in the small intestine, catalyzing the hydrolysis of several clinical drugs containing small acyl moieties. In the present study, we investigated whether AADAC catalyzes prasugrel hydrolysis. Recombinant human AADAC was shown to catalyze prasugrel hydrolysis with a CLint value of 50.0 ± 1.2 ml/min/mg protein with a similar Km value to human intestinal and liver microsomes, whereas the CLint values of human CES1 and CES2 were 4.6 ± 0.1 and 6.6 ± 0.3 ml/min/mg protein, respectively. Inhibition studies using various chemical inhibitors and the relative activity factor approach suggested that the contribution of AADAC to prasugrel hydrolysis in human intestine is comparable to that of CES2. In dog intestine, the expression of AADAC, but not CES1 and CES2, was confirmed by measuring the marker hydrolase activities of each human esterase. The similar Km values and inhibition profiles between recombinant dog AADAC and small intestinal microsomes suggest that AADAC is a major enzyme responsible for prasugrel hydrolysis in dog intestine. Collectively, we found that AADAC largely contributes to prasugrel hydrolysis in both human and dog intestine.

In Vitro Evaluation of the Effect of 7-Methyl Substitution on Glucuronidation of Daphnetin: Metabolic Stability, Isoform Selectivity, and Bioactivity Analysis

The C-8 phenol group is essential to exert the bioactivities of daphnetin, but it is readily conjugated with glucuronic acid prior to excretion. In this study, daphnetin-7-methylether (7M-DNP) was used to investigate the effect of 7-methyl substitution on daphnetin glucuronidation in human/rat liver (HLM/RLM) and intestine (HIM/RIM) microsomes, and recombinant UDP-glucuronosyltransferases (UGTs). Compared with daphnetin, the Vmax /Km values of 7M-DNP via 8-O-glucuronidation were 2.1-fold lower in HLM, 1.7-fold lower in HIM, and 2.4-fold lower in RLM, suggesting an improvement in metabolic stability. Different from daphnetin 8-O-glucuronidation exclusively catalyzed by UGT1A6 and UGT1A9, UGT1A1, -1A3, -1A7, -1A8, and -1A9 showed glucuronidation activity toward 7M-DNP. Kinetics studies, chemical inhibition, and the relative activity factor approach were used to demonstrate that UGT1A9 was mainly responsible for the reaction in HLM, whereas UGT1A1 was a primary contributor in HIM. The Vmax /Km values of 7M-DNP glucuronidation in HLM and HIM were 0.61-0.74-fold lower than those of rat, suggesting the differences between the two species. The bioactivity analysis demonstrated that 7M-DNP had an anti-inflammatory activity comparable to that of daphnetin. These findings indicated that the outcomes of 7-methyl substitution on daphnetin might be positive, but this should be confirmed in future in vivo studies.

In vitro identification of human cytochrome P450 isoforms involved in the metabolism of Geissoschizine methyl ether, an active component of the traditional Japanese medicine Yokukansan

1. Yokukansan (YKS) is a traditional Japanese medicine also called kampo, which has been used to treat neurosis, insomnia, and night crying and peevishness in children. Geissoschizine methyl ether (GM), a major indole alkaloid found in Uncaria hook, has been identified as a major active component of YKS with psychotropic effects. Recently, GM was reported to have a partial agonistic effect on serotonin 5-HT1A receptors. However, there is little published information on GM metabolism in humans, although several studies reported the blood kinetics of GM in rats and humans. In this study, we investigated the GM metabolic pathways and metabolizing enzymes in humans.2. Using recombinant human cytochrome P450 (CYP) isoforms and polyclonal antibodies to CYP isoforms, we found that GM was metabolized into hydroxylated, dehydrogenated, hydroxylated+dehydrogenated, demethylated and water adduct forms by some CYP isoforms.3. The relative activity factors in human liver microsomes were calculated to determine the relative contributions of individual CYP isoforms to GM metabolism in human liver microsomes (HLMs). We identified CYP3A4 as the CYP isoform primarily responsible for GM metabolism in human liver microsomes.4. These findings provide an important basis for understanding the pharmacokinetics and pharmacodynamics of GM and YKS.

Toxicokinetics of lefetamine and derived diphenylethylamine designer drugs-Contribution of human cytochrome P450 isozymes to their main phase I metabolic steps.

Lefetamine was first marketed in the 1940s as an opioid analgesic. Since withdrawal symptoms were observed during treatment, it became a controlled substance. Its N-ethyl and N-iso-propyl derivatives appeared on the illicit drug market in 2008. Metabolism studies for lefetamine and these derivatives showed that N-dealkylation was the initial step for all three substances in rats. The involvement of the ten most important human cytochrome P450 (CYP) isozymes in this N-dealkylation should be studied now. Studies showed the involvement of CYP1A2, CYP2B6, CYP2C19, and CYP3A4 in N-dealkylation of all three compounds and additionally CYP2D6 for lefetamine and NEDPA. All kinetic profiles followed classic Michaelis-Menten kinetics. Using the relative activity factor approach, the following net clearances were calculated: for lefetamine 8% by CYP1A2, 72% by CYP2B6, 2% by CYP2C19, 1% by CYP2D6, and 17% by CYP3A4; for NEDPA 27% by CYP1A2, 30% by CYP2B6, 23% by CYP2C19, 4% by CYP2D6, and 17% by CYP3A4; for NPDPA 18% by CYP1A2, 24% by CYP2B6, 28% by CYP2C19, and 30% by CYP3A4. In addition, calculated data were compared to chemical inhibition studies in human liver microsomes. Due to the involvement of at least four enzymes in the initial metabolic steps, the risk of CYP-related drug-drug or drug-food interactions should be low.

PBPK modeling of irbesartan: incorporation of hepatic uptake.

Physiological based pharmacokinetic (PBPK) modeling is now commonly used in drug development to integrate human or animal physiological data in order to predict pharmacokinetic profiles. The aim of this work was to construct and refine a PBPK model of irbesartan taking into account its active uptake via OATP1B1/B3 in order to predict more accurately its pharmacokinetic profile using Simcyp(®). The activity and expression of the human hepatocyte transporters OATP1B1 and OATP1B3 were studied. The relative activity factors (RAFs) for OATP1B1 and OATP1B3 transporters were calculated from intrinsic clearances obtained by concentration dependent uptake experiments in human hepatocytes and HEK overexpressing cells: RAF1B1 using estrone-3-sulfate and pitavastatine clearances, and RAF1B3 using cholecystokinine octapeptide (CCK-8) clearances. The relative expression factor (REF) was calculated by comparing immunoblotting of hepatocytes (REFHH ) or tissues (REFtissue) with those of overexpressing HEK cells for each transporter. These scaling factors were applied in a PBPK model of irbesartan using the Simcyp® simulator. Pharmacokinetic simulation using REFHH (1.82 for OATP1B1, 8.03 for OATP1B3) as an extrapolation factor was the closest to the human clinical pharmacokinetic profile of irbesartan. These investigations show the importance of integrating the contribution of the active uptake of a drug in the liver to improve PBPK modeling.

Regio- and Isoform-Specific Glucuronidation of Psoralidin: Evaluation of 3-O-Glucuronidation as a Functional Marker for UGT1A9

In this study, we aimed to determine the glucuronidation potential of psoralidin in humans and to perform validation on use of psoralidin-3-O-glucuronidation as a functional marker for UGT1A9. Glucuronidation kinetics was determined using human liver microsomes (HLMs), human intestine microsomes (HIM), and expressed UDP-glucuronosyltransferase (UGT) enzymes. The chemical structures of metabolites were determined by liquid chromatography-mass spectrometry and nuclear magnetic resonance spectroscopy analyses. Validation of psoralidin-3-O-glucuronidation as a UGT1A9 marker was performed using combined approaches including reaction phenotyping, chemical inhibition, activity correlation analysis, and determination of relative activity factor (RAF). HLM and UGT1A9 generated two monoglucuronides (9-O-glucuronide and 3-O-glucuronide) from psoralidin, whereas HIM, UGT1A1, UGT1A7, and UGT1A8 generated one only (9-O-glucuronide). Formation of 3-O-glucuronide in HLM was markedly inhibited by the UGT1A9-selective inhibitors magnolol and niflumic acid. Further, psoralidin-3-O-glucuronidation was strongly correlated with propofol-glucuronidation in a group of nine individual HLMs (r = 0.978, p < 0.001). Strong correlation was also observed between psoralidin-3-O-glucuronidation and the UGT1A9 protein levels measured by Western blotting (r = 0.944, p < 0.001). Moreover, UGT1A9 was responsible for 99.6% of psoralidin-3-O-glucuronidation in HLM based on the RAF approach. In conclusion, psoralidin was subjected to efficient glucuronidation, generating one or two monoglucuronides depending on UGT isozymes. Also, psoralidin-3-O-glucuronidation was an excellent in vitro marker for UGT1A9.

Relative contributions of the major human CYP450 to the metabolism of icotinib and its implication in prediction of drug–drug interaction between icotinib and CYP3A4 inhibitors/inducers using physiologically based pharmacokinetic modeling

Objective: Icotinib is an anticancer drug, but relative contributions of CYP450 have not been identified. This study was carried out to identify the contribution percentage of CYP450 to icotinib and use the results to develop a physiologically based pharmacokinetic (PBPK) model, which can help to predict drug–drug interaction (DDI).Methods: Human liver microsome (HLM) and supersome using relative activity factor (RAF) were employed to determine the relative contributions of the major human P450 to the net hepatic metabolism of icotinib. These values were introduced to develop a PBPK model using SimCYP. The model was validated by the observed data in a Phase I clinical trial in Chinese healthy subjects. Finally, the model was used to simulate the DDI with ketoconazole or rifampin.Results: Final contribution of CYP450 isoforms determined by HLM showed that CYP3A4 provided major contributions to the metabolism of icotinib. The percentage contributions of the P450 to the net hepatic metabolism of icotinib were determined by HLM inhibition assay and RAF. The AUC ratio under concomitant use of ketoconazole and rifampin was 3.22 and 0.55, respectively.Conclusion: Percentage of contribution of CYP450 to icotinib metabolism was calculated by RAF. The model has been proven to fit the observed data and is used in predicting icotinib–ketoconazole/rifampin interaction.

Abstract 4337: In vitro biotransformation of and inhibitory effects of CZ48 in human liver microsomes

Abstract BACKGROUND AND PURPOSE CZ48 is a novel potent anticancer agent currently in a phase I clinical trial. The absorption of CZ48 in humans is approximately less than 10% based on the excretion ratio of the unchanged form in the feces following oral administration. It was reported that CZ48 acts as a pro-drug and exerts its anticancer activity through the active metabolite CPT in vivo. The specific enzymes involved in its biotransformation have not been identified. To date, it has been very difficult to predict drug-drug interactions with CZ48 in the clinical setting due to a lack of information. Cytochrome P450 enzymes are primarily found in liver cells but are also located in other organ cells throughout the body. Therefore, in vitro studies were conducted to identify and characterize the cytochrome P450 enzymes involved in the formation of the major metabolite CPT. In this study, we conducted in vitro experiments by using in vitro human liver models which have been developed in the past few decades, including isoenzymes, microsomes, cytosol, S9 fraction, statins and inhibitors/inducers to allow us to evaluate the potential for drug-drug interactions. METHODS Using an integrated approach CZ48 biotransformation was studied for CYP-mediated metabolic reaction with human liver microsomes (HLM), cytosol and S9 fraction. Seven major isoenzymes were used to investigate, through relative activity factor approach, the contribution of selective statins or of several inhibitors/inducers in the phase-I metabolism of CZ48. RESULTS Experiments revealed that 97.9% of CZ48 was metabolized in HLM in 2 hours, and 64.8% of CZ48 was biotransformed in the S9 fraction, while cytosol was less responsible (13.4%) for CZ48 biotransformation. The majority (68.5%) of CZ48 was metabolized into CPT when co-incubated with CYP3A4. The statins, mevastatin (lovastatin), atorvastatin and simvastatin, are specific CYP3A4 substrates and inhibited CZ48 biotransformation. In contrast, rosuvastatin showed no inhibitory effect. At CZ48 concentrations of 0.25, 0.5 and 1 uM, verapamil, erythromycin, clarithromycin and CYP3A4 showed the strong inhibitory effects on CZ48 biotransformation. However, based on results with human liver microsomes in the presence of the CYP3A4 inducers phenytoin, rifampicin and carbamazepine, it was suggested that cz48 was partly metabolized by other enzymes in the microsomes. CONCLUSIONS Our results establish the metabolic pathways of CZ48, revealing the significant role of CYPs in its metabolism. Of the major human CYPs, CYP3A4 is responsible for catalyzing the biotransformation of CZ48. Indeed, CYP3A4 has extremely broad substrate specificity and is responsible for the metabolism of 50% of drugs in current use. Our results provide insight into the underlying biochemical mechanisms of CZ48, which would help to predict clinical outcomes. Note: This abstract was not presented at the meeting. Citation Format: Xing Liu, Albert DeJesus, Dana Vardeman, Zhisong Cao, Beppino Giovanella. In vitro biotransformation of and inhibitory effects of CZ48 in human liver microsomes. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4337. doi:10.1158/1538-7445.AM2014-4337

Glucuronidation of capsaicin by liver microsomes and expressed UGT enzymes: reaction kinetics, contribution of individual enzymes and marked species differences

Objective: The objectives of this study are to characterize capsaicin glucuronidation using liver microsomes and to determine the contribution of individual UDP-glucuronosyltransferase (UGT) enzymes to hepatic glucuronidation of capsaicin.Methods: The rates of glucuronidation were determined by incubating capsaicin with uridine diphosphoglucuronic acid-supplemented microsomes. Kinetic parameters were derived by model fitting. Determination of the relative activity factors, expression-activity correlation and activity correlation analysis were performed to identify the main UGT enzymes contributing to capsaicin metabolism.Results: Capsaicin was efficiently glucuronidated in pooled human liver microsomes (pHLM). UGT1A1, 1A9 and 2B7 (as well as the gastrointestinal enzymes UGT1A7 and 1A8) showed considerable activities. Capsaicin glucuronidation was significantly correlated with 3-O-glucuronidation of β-estradiol (r = 0.637; p = 0.014) and with UGT1A1 protein levels (r = 0.616; p = 0.019) in a bank of individual HLMs (n = 14). Also, capsaicin glucuronidation was strongly correlated with zidovudine glucuronidation (r = 0.765; p < 0.01) and with UGT2B7 protein levels (r = 0.721; p < 0.01). UGT1A1, 1A9 and 2B7 contributed 30.3, 6.0 and 49.0% of total glucuronidation of capsaicin in pHLM, respectively. Further, glucuronidation of capsaicin by liver microsomes showed marked species difference.Conclusion: Capsaicin was subjected to significant hepatic glucuronidation, wherein UGT1A1 and 2B7 were the main contributing enzymes.

Glucuronidation of macelignan by human liver microsomes and expressed UGT enzymes: identification of UGT1A1 and 2B7 as the main contributing enzymes.

Macelignan is a natural phenolic compound that possesses many types of health benefits such as antiinflammation. This study aimed to characterize the metabolism of macelignan via the glucuronidation pathway and to identify the main UGT enzymes involved in macelignan glucuronidation. The rates of glucuronidation were determined by incubating macelignan with UDPGA-supplemented microsomes. Kinetic parameters were derived by fitting an appropriate model to the data. Reaction phenotyping, the relative activity factor (RAF) approach and activity correlation analysis were employed to identify the main UGT enzymes contributing to the hepatic metabolism of macelignan. Glucuronidation of macelignan in pooled human liver microsomes (pHLM) was rather efficient with a high CLint (the intrinsic clearance) value of 13.90 ml/min/mg. All UGT enzymes, except UGT1A4, 1A6 and 2B10, showed metabolic activities toward macelignan. UGT1A1 and 2B7 were the enzymes with the highest activities; the CLint values were 4.92 and 2.13 ml/min/mg, respectively. Further, macelignan glucuronidation was significantly correlated with 3-O-glucuronidation of β-estradiol (r = 0.69; p < 0.01) and glucuronidation of zidovudine (r = 0.60; p < 0.05) in a bank of individual HLMs (n = 14). Based on the RAF approach, UGT1A1 and 2B7, respectively, contributed 55.40% and 32.20% of macelignan glucuronidation in pHLM. In conclusion, macelignan was efficiently metabolized via the glucuronidation pathway. It was also shown that UGT1A1 and 2B7 were probably the main contributors to the hepatic glucuronidation of macelignan.

Metabolism of diosbulbin B in vitro and in vivo in rats: formation of reactive metabolites and human enzymes involved.

Diosbulbin B (DB), a major constituent of the furano-norditerpenes in Dioscorea bulbifera Linn, exhibits potential antineoplasmic activity and hepatotoxicity. The metabolism and reactive metabolites of DB in vitro (with human and animal liver microsomes) and in vivo in rats were investigated. The human enzymes involved in DB metabolism were identified. DB was first catalyzed into reactive metabolites of 2-butene-1,4-dial derivatives dependent on NADPH and then trapped by Tris base or oxidized to hemiacetal lactones (M12 and M13) in microsomal incubations. Tris base was used as buffer constituent and as a trapping agent for aldehyde. Methoxylamine and glutathione (GSH) were also used as trapping agents. DB metabolism in vivo in rats after oral administration was consistent with that in vitro. The structures of M12 and M13, as well as mono-GSH conjugates of DB (M31), were confirmed by nuclear magnetic resonance spectroscopy of the chemically synthesized products. The bioactivation enzymes of DB were identified as CYP3A4/5, 2C9, and 2C19. CYP3A4 was found to be the primary enzyme using human recombinant cytochrome P450 enzymes, specific inhibitory studies, and a relative activity factor approach for pooled human liver microsomes. Michaelis-Menten constants K(m) and V(max) were determined by the formation of M31. The reactive metabolites may be related to the hepatotoxicity of DB. The gender difference in CYP3A expression in mice and rats contributed to the gender-related liver injury and pharmacokinetics in mice and rats, respectively.

Contribution of CNT1 and ENT1 to ribavirin uptake in human hepatocytes.

The objective of this study was to investigate the contributions of a sodium-dependent concentrative nucleoside transporter (CNT) 1 and an equilibrative nucleoside transporter (ENT) 1 to ribavirin uptake in human hepatocytes. The initial studies in oocytes expressing CNT1 and ENT1 showed increases in ribavirin uptake, indicating that ribavirin was a substrate for both CNT1 and ENT1. The CNT1- and ENT1-mediated ribavirin uptake showed concentration dependency with the following kinetics parameters: Km 26.3 μM and Vmax 426.2 fmol/min/oocyte for CNT1; Km 70.5 μM and Vmax 134.3 fmol/min/oocyte for ENT1. Ribavirin uptake clearance in six human hepatocytes ranged from 21.3 to 300.7 μL/min. Estimation of the contributions of CNT1 and ENT1 to the hepatic uptake of ribavirin by using a relative activity factor method indicated that the relative contribution of ENT1 to the ribavirin uptake was 82.8 ± 3.9%. Real-time polymerase chain reaction analysis of CNT1 and ENT1 expressions in the hepatocytes showed that ENT1 mRNA expression was closely correlated with ribavirin uptake (R = 0.95, P = 0.003) while CNT1 was not. The findings indicated that ENT1 was the major transporter controlling the hepatic uptake of ribavirin.

Prediction of organic anion-transporting polypeptide 1B1- and 1B3-mediated hepatic uptake of statins based on transporter protein expression and activity data.

Organic anion-transporting polypeptides (OATP) 1B1 and OATP1B3 are drug transporters mediating the active hepatic uptake of their substrates. Because they exhibit overlapping substrate specificities, the contribution of each isoform to the net hepatic uptake needs to be considered when predicting drug-drug interactions. The relative contribution of OATP1B1- and OATP1B3-mediated uptake of statins into hepatocytes was estimated based on either relative transporter protein expression data or relative activity data. Therefore, kinetics of eight statins and OATP1B1- and OATP1B3-specific reference substrates was determined in OATP1B1- and OATP1B3-expressing human embryonic kidney 293 cells and in human cryopreserved hepatocytes. Absolute OATP1B1 and OATP1B3 protein abundance was determined by liquid chromatography-tandem mass spectrometry in all expression systems. Transporter activity data generated in recombinant cell lines were extrapolated to hepatocyte values using relative transporter expression factors (REF) or relative activity factors (RAF). Our results showed a pronounced OATP1B1 and comparatively low OATP1B3 protein expression in the investigated hepatocyte lot. Based on REF scaling, we demonstrated that the active hepatic uptake clearances of reference substrates, atorvastatin, pravastatin, rosuvastatin, and simvastatin were well predicted within twofold error, demonstrating that OATP1B1 and OATP1B3 were major contributors. For other statins, the net hepatic uptake clearance was underpredicted, suggesting the involvement of other hepatic uptake transporters. Summarized, we showed that REF- and RAF-based predictions were highly similar, indicating a direct transporter expression-activity relationship. Moreover, we demonstrated that the REF-scaling method provided a powerful tool to quantitatively assess the transporter-specific contributions to the net uptake clearance of statins in hepatocytes.

Hepatic Glucuronidation of Isoneochamaejasmin A from the Traditional Chinese MedicineStellera ChamaejasmeL. Root

Isoneochamaejasmin A (INCA), a biflavonoid, is one of main active ingredients in the dried root of Stellera chamaejasme L., a widely used traditional Chinese medicine. In the present study, we identified the glucuronidation metabolite of INCA and characterized the UDP glucuronosyltransferases (UGTs) responsible for INCA glucuronidation. 7-O-glucuronide (M1) and 4'-O-glucuronide (M2) were identified by incubation of INCA with human liver microsomes (HLMs) in the presence of UDP glucuronic acid, and their structures were confirmed by high-resolution mass spectrometry and nuclear magnetic resonance analyses. Although INCA is a single enantiomer molecule, its M1 metabolite showed two equal-size peaks on a πNAP stationary phase but only one peak on a C(18) stationary phase, indicating that the 7-/7''- and 4'-/4'''-hydroxyl groups of INCA were in different spatial configurations relative to each other. Among the recombinant human UGT isoform test and correlation analysis, UGT1A1, UGT1A3, and UGT1A9 were found to mediate M1 formation, whereas only UGT1A3 mediated M2 formation. Kinetic studies showed obvious species differences between human, mouse, rat, dog, and pig liver microsomes. UGT1A1, HLMs, and human intestinal microsomes, but not human kidney microsomes, exhibited substrate inhibition for the formation of M1. UGT1A1-mediated formation of M1 showed a 6- and 11-fold higher V(max) than did UGT1A3- and UGT1A9-mediated formation of M1, respectively. The results of the relative activity factor assay showed that UGT1A1 contributed approximately 75% in the formation of M1. These findings collectively indicate that UGT1A1 is the major enzyme in the formation of M1, whereas UGT1A3 is the major enzyme in the formation of M2.

Dimethocaine, a synthetic cocaine derivative: Studies on its in vitro metabolism catalyzed by P450s and NAT2

Dimethocaine (DMC), a synthetic derivative of cocaine, is distributed and consumed as “new psychoactive substance” (NPS) without any safety testing at the forefront. It is mainly metabolized by N-acetylation, N-deethylation or hydroxylation. Therefore, the aim of the presented study was to determine the human NAT and P450 isozymes involved in this major metabolic steps, to measure the kinetics of the reactions, and to estimate the contribution on in vivo hepatic clearance. For these studies, cDNA-expressed NATs and P450s were used and formation of metabolites after incubation was measured using LC–MS or LC–MSn. For N-acetylation, NAT2 could be shown to be the only isoform catalyzing the reaction in vitro hence assuming to be the only relevant enzyme for in vivo acetylation. Kinetic profiles of all P450 catalyzed metabolite formations followed classic Michaelis–Menten behavior with enzyme affinities (Km values) between 3.6 and 220μM. Using the relative activity factor approach, the net clearances for deethylation of DMC were calculated to be 3% for P450 1A2, 1% for 2C19, <1% for 2D6, and 96% for 3A4. The net clearances for hydroxylation of DMC were calculated to be 32% for P450 1A2, 5% for 2C19, 51% for 2D6, and 12% for 3A4. Furthermore, these data were confirmed by chemical inhibition tests in human liver microsomes. As DMC is metabolized via two main steps and different P450 isoforms were involved in the hepatic clearance of DMC, a clinically relevant interaction with single P450 inhibitors should not be expected. However, a slow acetylation phenotype or inhibition of NAT2 could lead to decreased N-acetylation and hence leading to an increased risk of side effects caused by this arylamine.

Enantioselectivity in the cytochrome P450-dependent conversion of tegafur to 5-fluorouracil in human liver microsomes.

Tegafur (FT) is a prodrug of 5-fluorouracil (5-FU) used in cancer chemotherapy, and the bioactivation of FT to 5-FU is mainly catalyzed by cytochrome P450 (CYP) in hepatic microsomes. FT has a chiral center and is a racemate consisting of the enantiomers, R- and S-FT. In the present study, we clarified the enantioselectivity in the conversion of FT to 5-FU and identified human CYP isoforms involved in the metabolism of its enantiomers using human hepatic preparations and recombinant CYP isoforms. Although 5-FU was generated from both FT enantiomers, R-FT was a preferred substrate than S-FT, because of the considerably higher intrinsic clearance for 5-FU formation from R-FT in liver. Eadie–Hofstee plots in microsomes showed that the conversions of R- and S-FT to 5-FU followed biphasic and monophasic kinetics, respectively. Based on the evaluation using cDNA-expressed enzymes, CYP2A6 showed the highest activity for 5-FU formation from R-FT with the Km value similar to that of the high-affinity component in microsomes. Also, CYP2A6 was the most effective catalyst for S-FT. Inhibition studies using CYP-selective inhibitors and anti-CYP antibodies demonstrated that CYP2A6 mainly contributed to the enantioselective metabolism of FT, and were almost in accordance with the relative percentage contribution of each CYP isoform to the metabolism of FT estimated using relative activity factor methods. These results suggest that the enantioselectivity in the bioactivation of FT to 5-FU in humans is mainly due to the large difference of the catalytic activity of CYP2A6 between R- and S-FT.

In vitro metabolism of a novel antithrombotic compound, S002-333, and its enantiomers: quantitative cytochrome P450 phenotyping, metabolic profiling and enzyme kinetic studies

1. S002-333, (2-(4′-methoxy-benzenesulfonyl)-2,3,4,9-tetrahydro-1H-pyrido (3,4-b) indole-3-carboxylic acid amide) is a novel potent antithrombotic molecule currently under development phase. It is the racemic mixture of two enantiomers, namely S004-1032 (R-form) and S007-1558 (S-form).2. The contribution of five major isoenzymes, namely CYP2B6, 2C9, 2C19, 2D6 and 3A4 was quantified using recombinant P450s in the phase-I metabolism through relative activity factor approach. CYP2C19 was found to be the major contributor for S002-333 and S007-1558, while CYP3A4 showed greater involvement in S004-1032 metabolism. Chemical inhibition and immunoinhibition studies reconfirmed the results in human liver microsomes (HLM).3. Four major phase-I metabolites of S002-333; M-1 and M-3 (oxidative), M-2 (O-demethylated) and M-4 (dehydrogenated) were characterized in HLM. These metabolites constituted 11.2, 11.3 and 21.5% of the parent in comparison with the net phase-I metabolism of 29.9, 31.4 and 38.3% of S002-333, S004-1032 and S007-1558, respectively.4. Among CYP2C9, 2C19 and 3A4, the relative contribution of CYP2C9 was found to be maximum during M-1 through M-4 formation. Enzyme kinetic analysis for detected metabolites indicated that M-1 to M-3 followed classical hyperbolic kinetics, whereas M-4 showed evidence of autoactivation. In conclusion, the results suggest prominent role of CYP2C9, 2C19 and 3A4 isoforms for enantioselective disposition of S002-333 in vitro.