Yingjie Guo

Mechanism of CYP2C9 Inhibition by Flavones and Flavonols

This article describes an in vitro investigation of the inhibition of cytochrome P450 (P450) 2C9 by a series of flavonoids made up of flavones (flavone, 6-hydroxyflavone, 7-hydroxyflavone, chrysin, baicalein, apigenin, luteolin, scutellarein, and wogonin) and flavonols (galangin, fisetin, kaempferol, morin, and quercetin). With the exception of flavone, all flavonoids were shown to inhibit CYP2C9-mediated diclofenac 4′-hydroxylation in the CYP2C9 RECO system, with Ki value ≤2.2 μM. In terms of the mechanism of inhibition, 6-hydroxyflavone was found to be a noncompetitive inhibitor of CYP2C9, whereas the other flavonoids were competitive inhibitors. Computer docking simulation and constructed mutants substituted at residue 100 of CYP2C9.1 indicate that the noncompetitive binding site of 6-hydroxyflavone lies beside Phe100, similar to the reported allosteric binding site of warfarin. The other flavonoids exert competitive inhibition through interaction with the substrate binding site of CYP2C9 accessed by flurbiprofen. These results suggest flavonoids can participate in interactions with drugs that act as substrates for CYP2C9 and provide a possible molecular basis for understanding cooperativity in human P450-mediated drug-drug interactions.

Catalytic activities of human cytochrome P450 2C9*1, 2C9*3 and 2C9*13.

Cytochrome P450 2C9 (CYP2C9) is a geneticly polymorphic enzyme responsible for the metabolism of some clinically important drugs. CYP2C9*13 is an allele identified in a Chinese poor metabolizer of lornoxicam which has a Leu90Pro amino acid substitution. This paper reports on a study aimed at comparing the catalytic properties of CYP2C9*13 with those of the wild-type CYP2C9*1 and mutant CYP2C9*3 (Ile359Leu) in the COS-7 expression system using various substrates. CYP2C9*3 and *13 produced far lower luminescence than CYP2C9*1 in luciferin H metabolism. CYP2C9*13 exhibited an 11-fold increase in Km but no change in Vmax with tolbutamide as the substrate, a five-fold increase in Km and an 88.8% reduction in Vmax with diclofenac. These data indicate that CYP2C9*13 exhibits reduced metabolic activity toward all studied CYP2C9 substrates. The magnitude of the CYP2C9*13-associated decrease in intrinsic clearance (Vmax/Km) is greater than that associated with CYP2C9*3.

Liquid chromatographic–tandem mass spectrometric method for the simultaneous quantitation of telmisartan and hydrochlorothiazide in human plasma

A rapid and sensitive method using liquid chromatography-tandem mass spectrometry (LC-MS/MS) has been developed for the simultaneous determination of telmisartan and hydrochlorothiazide in human plasma. Sample preparation involved liquid-liquid extraction with diethyl ether-dichloromethane (60:40, v/v). The analytes and internal standard, probenecid, were separated on a Venusil XBP-C(8) column using gradient elution with acetonitrile-10 mM ammonium acetate-formic acid at a flow rate of 1.2 mL/min. Detection was by electrospray negative ionization mass spectrometry using multiple reaction monitoring of the transitions at m/z 513.0-->469.4 for telmisartan, m/z 295.9-->268.9 for hydrochlorothiazide and m/z 283.9-->239.9 for probenecid. For both analytes, the method was linear in the range 1.00-600 ng/mL with intra- and inter-day precision (as relative standard deviation) <or=10.6% and accuracy (as relative error) <or=4.2%. The assay was successfully applied to a pharmacokinetic study in 9 healthy volunteers given a single oral dose of a combination tablet containing telmisartan 80 mg and hydrochlorothiazide 12.5 mg.

Effects of ABCB1 polymorphisms on plasma carbamazepine concentrations and pharmacoresistance in Chinese patients with epilepsy.

P-glycoprotein may play a role in drug resistance in epilepsy by limiting gastrointestinal absorption and brain access of antiepileptic drugs (AEDs). We sought to investigate the effects of ABCB1 polymorphisms on plasma carbamazepine (CBZ) concentrations and pharmacoresistance in Chinese patients with epilepsy. C1236T, G2677T/A, and C3435T polymorphisms of ABCB1 were genotyped by polymerase chain reaction amplification followed by restriction fragment length polymorphism analysis or direct automated DNA sequencing in 84 patients treated with CBZ monotherapy. Patients with 3435-TT (n=15) had lower adjusted CBZ concentrations than those with 3435-CC (n=30) (P=0.026). However there were no associations between all the studied genotypes, haplotypes, or diplotypes involving ABCB1 C1236T, G2677T/A, and C3435T polymorphisms and pharmacoresistance in the patient cohort. Our results suggest that ABCB1 3435-TT is associated with decreased plasma CBZ levels in Chinese patients with epilepsy. However, whether this contributes to CBZ resistance needs to be further investigated in a larger cohort of patients.

Effects of EPHX1, SCN1A and CYP3A4 genetic polymorphisms on plasma carbamazepine concentrations and pharmacoresistance in Chinese patients with epilepsy

The pharmacokinetics and pharmacodynamics of carbamazepine (CBZ) vary widely among patients with epilepsy. In this study, we sought to investigate the effects of genetic polymorphisms of the microsomal epoxide hydrolase (EPHX1), the sodium channel α subunit type I (SCN1A) and the cytochrome P450 3A4 (CYP3A4) genes on plasma CBZ concentrations and pharmacoresistance in Chinese patients with epilepsy. The EPHX1 c.337T>C, c.416A>G, SCN1A IVS5-91G>A or CYP3A4*1G polymorphisms were detected by polymerase chain reaction-restriction fragment length polymorphism approach or direct DNA sequencing in 83 Chinese patients treated with CBZ monotherapy. Patients with the variant EPHX1 c.416A>G genotypes had higher adjusted plasma CBZ concentrations compared to those with the wild type genotype (P<0.05). In contrast, the SCN1A IVS5-91G>A and CYP3A4*1G variant alleles had no significant effects on CBZ maintenance doses or adjusted CBZ concentrations. There were no associations between all the studied genotypes involving EPHX1 c.337T>C, c.416A>G, SCN1A IVS5-91G>A or CYP3A4*1G polymorphisms and pharmacoresistance in this patient cohort. These results suggest that the EPHX1 c.416A>G polymorphism affects CBZ metabolism in Chinese patients with epilepsy. However, whether it contributes to CBZ resistance needs to be further investigated in a larger cohort of patients.

Trantinterol, a novel β2-adrenoceptor agonist, noncompetitively inhibits P-glycoprotein function in vitro and in vivo.

P-glycoprotein (P-gp)-mediated drug-drug interactions are important factors causing adverse effects of drugs in clinical use. The aim of this study was to determine whether trantinterol (also known as SPFF), a novel β2-adrenoceptor agonist, was a P-gp inhibitor or substrate. The results showed that trantinterol was not a substrate of P-gp but increased rhodamine 123 (Rho 123) uptake by MDCK-MDR1 cells and decreased the efflux transport of both Rho 123 and cyclosporine A (CsA) in bidirectional transport studies across MDCK-MDR1 cell monolayers. This suggested that trantinterol was a P-gp inhibitor but not a P-gp substrate. The mechanism of inhibition was investigated in the P-gp-Glo assay system, where it was found that trantinterol inhibited P-gp ATPase activity in a dose-dependent manner. A subsequent study using the antibody binding assay with the conformation-sensitive P-gp-specific antibody UIC2 confirmed that trantinterol decreased UIC2 binding at 10 μM in contrast to the competitive inhibitor, verapamil. This suggested that trantinterol was a noncompetitive inhibitor of P-gp. Finally, a pharmacokinetic study in rat showed that trantinterol significantly increased the area under the plasma concentration-time curve (AUC) and maximum plasma concentration (Cmax) of digoxin and paclitaxel (PAC), and the Cmax of cyclosporine A (CsA). In summary, trantinterol is a potent noncompetitive P-gp inhibitor which may increase the bioavailability of other P-gp substrate drugs coadministered with it.

A Unique Collision-Induced Dissociation Reaction of Cholamine Derivatives of Certain Prostaglandins

AbstractProstaglandins (PGs) are biologically active metabolites of arachidonic acid containing 20 carbon atoms, a cyclic moiety, and two side chains (A and B) in common. The bioassay of PGs requires high sensitivity because of their low concentration in tissues and blood and has usually been carried out by electrospray ionization tandem mass spectrometry (ESI-MS/MS) in the negative ion mode. Chemical derivatization of PG carboxylic acid groups to introduce positive charge-carrying groups is an established strategy to improve the sensitivity and selectivity of such assays. In this study, we exploited this approach for structural identification of a series of PGs using cholamine derivatization through an amidation reaction. However, we observed that collision-induced dissociation of these derivatives gave rise to unexpected product ions that we postulated were formed by unique long-range intramolecular reactions resulting in dehydration of the B chain accompanied by fragmentation of the A chain through an unusual Hofmann rearrangement. Evidence for the proposed mechanism is presented based on ESI-MS/MS and high resolution mass spectrometry studies of cholamine derivatives of PGE1, PGE2, PGD2, PGI2, and C-17 methyl deuterium-labeled limaprost.n Graphical Abstract