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CYP3A4, the major form of cytochrome P450 (P450) expressed in the adult human liver, is involved in the metabolism of approximately 50% of commonly prescribed drugs. Several genetic polymorphisms in CYP3A4 are known to affect its catalytic activity and to contribute in part to interindividual differences in the pharmacokinetics and pharmacodynamics of CYP3A4 substrate drugs. In this study, catalytic activities of the two alleles found in East Asians, CYP3A4*16 (T185S) and CYP3A4*18 (L293P), were assessed using the following seven substrates: midazolam, carbamazepine, atorvastatin, paclitaxel, docetaxel, irinotecan, and terfenadine. The holoprotein levels of CYP3A4.16 and CYP3A4.18 were significantly higher and lower, respectively, than that of CYP3A4.1 when expressed in Sf21 insect cell microsomes together with human NADPH-P450 reductase. CYP3A4.16 exhibited intrinsic clearances (V(max)/K(m)) that were lowered considerably (by 84-60%) for metabolism of midazolam, carbamazepine, atorvastatin, paclitaxel, and irinotecan compared with CYP3A4.1 due to increased K(m) with or without decreased V(max) values, whereas no apparent decrease in intrinsic clearance was observed for docetaxel. On the other hand, K(m) values for CYP3A4.18 were comparable to those for CYP3A4.1 for all substrates except terfenadine; but V(max) values were lower for midazolam, paclitaxel, docetaxel, and irinotecan, resulting in partially reduced intrinsic clearance values (by 34-52%). These results demonstrated that the impacts of both alleles on CYP3A4 catalytic activities depend on the substrates used. Thus, to evaluate the influences of both alleles on the pharmacokinetics of CYP3A4-metabolized drugs and their drug-drug interactions, substrate drug-dependent characteristics should be considered for each drug.

The anticancer prodrug, irinotecan, is converted to its active form 7-ethyl-10-hydroxycamptothecin (SN-38) by carboxylesterases, and SN-38 is inactivated by UDP-glucuronosyltransferase (UGT)1A1-mediated glucuronidation. UGT1A9 also mediates this reaction. In a recent study, it was reported that the UGT1A9 IVS1+399 (I399)CT polymorphism is associated with increased SN-38 glucuronidation both in vitro and in vivo. However, its role in UGT1A9 expression levels and activity is controversial. Thus, we evaluated the role of I399CT in SN-38 glucuronidation using 177 Japanese cancer patients administered irinotecan. I399CT was detected at a 0.636 allele frequency. This polymorphism was in strong linkage disequilibrium (LD) with UGT1A9*1b (-126_-118T(9)T(10), |D'| = 0.99) and UGT1A1*6 (211GA, 0.86), in moderate LD with UGT1A1*60 (-3279TG, 0.55), but weakly associated with UGT1A1*28 (-54_-39A(TA)(6)TAAA(TA)(7)TAA, 0.25). Haplotype analysis showed that 98% of the I399C alleles were linked with low-activity haplotypes, either UGT1A1*6, *28, or *60. On the other hand, 85% of the T alleles were linked with the UGT1A1 wild-type haplotype *1. Although I399T-dependent increases in SN-38 glucuronide/SN-38 area under concentration-time curve (AUC) ratio (an in vivo marker for UGT1A activity) and decreases in SN-38 AUC/dose were apparent (P0.0001), these effects were no longer observed after stratified patients by UGT1A1*6, *28, or *60 haplotype. Thus, at least in Japanese populations, influence of I399CT on SN-38 glucuronidation is attributable to its close association with either UGT1A1*6, *28, or *60.

Human carboxylesterase 2 (hCE-2) is a member of the serine esterase superfamily and is responsible for hydrolysis of a wide variety of xenobiotic and endogenous esters. hCE-2 also activates an anticancer drug, irinotecan (7-ethyl-10-[4-(1-piperidino)-1-piperidino]-carbonyloxycamptothecin, CPT-11), into its active metabolite, 7-ethyl-10-hydroxycamptothecin (SN-38). In this study, a comprehensive haplotype analysis of the CES2 gene, which encodes hCE-2, in a Japanese population was conducted. Using 21 single nucleotide polymorphisms (SNPs), including 4 nonsynonymous SNPs, 100C>T (Arg(34)Trp, *2), 424G>A (Val(142)Met, *3), 1A>T (Met(1)Leu, *5), and 617G>A (Arg(206)His, *6), and a SNP at the splice acceptor site of intron 8 (IVS8-2A>G, *4), 20 haplotypes were identified in 262 Japanese subjects. In 176 Japanese cancer patients who received irinotecan, associations of CES2 haplotypes and changes in a pharmacokinetic parameter, (SN-38 + SN-38G)/CPT-11 area under the plasma concentration curve (AUC) ratio, were analyzed. No significant association was found among the major haplotypes of the *1 group lacking nonsynonymous or defective SNPs. However, patients with nonsynonymous SNPs, 100C>T (Arg(34)Trp) or 1A>T (Met(1)Leu), showed substantially reduced AUC ratios. In vitro functional characterization of the SNPs was conducted and showed that the 1A>T SNP affected translational but not transcriptional efficiency. These findings are useful for further pharmacogenetic studies on CES2-activated prodrugs.

Aryl hydrocarbon receptor (AhR) is an important transcriptional regulator involved in the induction of CYP1A1, CYP1A2, CYP1B1, UGT1A1, and UGT1A6. In this study, functional properties of four novel naturally occurring human AhR variants (K401R, N487D, I514T, and K17T/R554K) were examined along with the single variants K17T and R554K. The luciferase reporter assay using the CYP1A1 promoter reporter in HeLa cells treated with beta-naphthoflavone or 3-methylcholanthrene, which are known as typical agonists for AhR, showed that reporter activities of the K401R and N487D variants were reduced to 40 to 58% of those of wild-type (WT) but not of the other variants. Similarly, the K401R and N487D variants also reduced the omeprazole-induced reporter activities to approximately 56 and 74% of those of the WT, respectively. The reduced activities of the two variants were probably caused by the reduced protein expression levels, since the protein levels of the K401R and N487D variants were approximately 52 and 47% of the WT, respectively, without any changes in their mRNA levels. The reduced protein levels were recovered by treatment with a proteasome inhibitor MG132 [N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal], suggesting that the reduced protein levels were caused by the accelerated proteasomal degradation by a proteasome. Together, the current data demonstrate that the K401R and N487D variants reduce their apparent transcriptional activities, both ligand-induced and omeprazole-induced activation, probably through reduced protein expression. Thus, these two variants may influence drug metabolism through reduced induction of CYP1A1 and other target enzymes.

Cytochrome P450 2C8 is one of the primary enzymes responsible for the metabolism of a wide range of drugs such as paclitaxel, cerivastatin, and amiodarone. We have sequenced the CYP2C8 gene from 201 Japanese subjects and found five novel nonsynonymous single nucleotide polymorphisms (SNPs): 511GA (G171S), 556CT (R186X; X represents the translational stop codon), 556CG (R186G), 740AG (K247R), and 1149GT (K383N), with the allele frequency of 0.0025. The CYP2C8 variants were heterologously expressed in COS-1 cells and functionally characterized in terms of expression level, paclitaxel 6alpha-hydroxylase activity, and intracellular localization. The prematurely terminated R186X variant was undetectable by Western blotting and inactive toward paclitaxel 6alpha-hydroxylation. The G171S, K247R, and K383N variants exhibited properties similar to those of the wild-type CYP2C8. Paclitaxel 6alpha-hydroxylase activity of the R186G transfectant was only 10 to 20% that of wild-type CYP2C8. Furthermore, the R186G variant displayed a lower level of protein expression in comparison to the wild type, which was restored by the addition of a proteasome inhibitor (MG-132; Z-Leu-Leu-Leu-aldehyde). The reduced CO-difference spectral analysis using recombinant proteins from an insect cell/baculovirus system revealed that the R186G variant has a minor peak at 420 nm in addition to the characteristic Soret peak at 450 nm, suggesting the existence of improperly folded protein. These results indicate that the novel CYP2C8 SNPs, 556CT (R186X) and 556CG (R186G), could influence the metabolism of CYP2C8 substrates such as paclitaxel and cerivastatin.

UDP-glucuronosyltransferase (UGT) 1A10 is an isoform of UGT1A, which is expressed in extrahepatic, biliary and aerodigestive/gastrointestinal tissues. We have previously reported two nonsynonymous single nucleotide polymorphisms in exon 1 of human UGT1A10 gene; 177GA and 605CT resulting in amino acid alterations, M59I and T202I, respectively. In the present study, wild-type (WT) and these variant UGT1A10 cDNAs were transiently expressed in COS-1 cells for functional characterization. Glucuronidation activities in these COS-1 membrane fractions were assayed using 7-hydroxy-4-trifluoromethylcoumarin (HTFMC) and 17 beta-estradiol (E2) as substrates. WT and variant UGT1A10s catalyzed HTFMC glucuronidation with similar apparent K(m) values of approximately 5 microM, whereas the V(max) value of T202I normalized by the expressed UGT1A10 protein levels was nearly half of those of WT and M59I. High-performance liquid chromatography analysis of E2 glucuronide revealed that UGT1A10 catalyzed E2 3-O-glucuronidation but not 17-O-glucuronidation. Similarly, the three UGT1A10s catalyzed E2 3-O-glucuronidation with comparable apparent K(m) values (approximately 2 microM), whereas the normalized V(max) value of T202I was almost half that of WT and M59I. These results suggest that the lowered glucuronidation activity of T202I affects the gastrointestinal glucuronidation of orally administrated chemicals and the enterohepatic circulation of biliary excreted metabolites.

7-Ethyl-10-hydroxycamptothecin (SN-38), an active metabolite of antitumor agent irinotecan (CPT-11), is conjugated and detoxified to SN-38-glucuronide by UDP-glucuronosyltransferase (UGT) 1A1. Genetic polymorphisms in UGT1A1 are thought to contribute to severe diarrhea and/or leukopenia caused by CPT-11. In this regard, it has been reported that polymorphisms in the promoter region could affect the CPT-11 pharmacokinetics and interindividual variation of toxicity. However, little information is available on the influence of UGT1A1 polymorphisms in the coding region on the SN-38 glucuronidation activity. In the present study, wild-type (WT) and three variant (G71R, P229Q, and Y486D) cDNAs of human UGT1A1s were transiently expressed in COS-1 cells, and the kinetic parameters of these UGT1A1s were determined for SN-38 glucuronidation. A partially reduced UGT1A1 protein expression was observed in COS-1 cells for G71R and Y486D. WT UGT1A1 catalyzed SN-38 glucuronidation with an apparent K(m) value of 11.5 microM, whereas those of G71R, P229Q, and Y486D were 14.0, 18.0, and 63.5 microM, respectively. The SN-38 glucuronidation efficiency ratio (V(max)/K(m)) normalized for the level of expression was 1.4, 0.66 (47% of WT), 0.73 (52%), and 0.07 (5%) microl/min/mg of protein for WT, G71R, P229Q, and Y486D, respectively. Thus, the SN-38 glucuronidation activity of Y486D was drastically reduced, whereas the reduction in the G71R and P229Q activities was fractional. The decreased SN-38 glucuronidation efficiency ratio of G71R and P229Q could be critical in combination with other polymorphisms in the UGT1A1 gene.

We found nucleotide variability in the 5′-upstream region and exonic sequences of a gene-encoding canalicular multispecific organic anion transporter/multidrug resistance-associated protein 2 (cMOAT/MRP2) by polymerase chain reaction-based sequencing using genomic DNA from 72 established cell lines derived from 72 Japanese individuals. Four single nucleotide polymorphisms (SNPs) were found in the 5′-untranslational region and 21 in the exonic regions. Of them, 14 were nonsynonymous SNPs. One deletion of seven consecutive adenines resulting in a frameshift variant was also found. Four SNPs, c-24t, g1249a (V417I), c2366t (S789F), and c3972t (I1324I), were the same as those recently reported. A strong association was found between c-24t (5′-untranslated region) and c3972t (exon 28), with the promoter activity of the former worth being compared.