Simple high-performance liquid chromatographic method for the determination of tolbutamide and its metabolites in human plasma and urine using photodiode-array detection

Comparison of metabolite profiles generated in Aroclor-induced rat liver and human liver subcellular fractions: considerations for in vitro genotoxicity hazard assessment

Because it is well known that metabolites of chemicals and drugs are frequently the ultimate species responsible for genotoxicity and carcinogenicity, in vitro testing to identify the human genotoxicity hazard potential of new chemicals and drugs routinely utilizes liver S-9 fraction from rats treated with Aroclor 1254 as a system that can generate metabolites. However, it is frequently questioned as to whether such an in vitro metabolite generation system is the most relevant for human risk, or whether the assay would be better served by using a human-derived in vitro system. To address this, 16 common drugs have been examined for profiles of metabolites in Aroclor-induced rat liver S-9 and pooled human liver S-9. Metabolite profiles were compared using high pressure liquid chromatography coupled with ion trap mass spectrometry, in line with ultraviolet or radiometric detection to help make semiquantitative comparisons. Results showed that, with few exceptions, metabolites generated in the human system were also generated in the rat system. Also, in several cases the rat system generated considerably more metabolites, suggesting that there is a potential that positive genotoxicity findings could be caused by metabolites that have no relevance to humans. These findings suggest that when conducting in vitro genotoxicity testing using the Aroclor-induced rat liver S-9 system, knowledge of the metabolite profile in the system is important, and a comparison to the profile generated in human liver S-9 could be of value when interpreting the genotoxicity results.

Effects of Ginkgo biloba extract on pharmacokinetics and pharmacodynamics of tolbutamide and midazolam in healthy volunteers

This study was undertaken to clarify the influence of repeated oral administration of Ginkgo biloba extract (GBE) on CYP2C9 and CYP3A4. CYP2C9 probe (tolbutamide, 125 mg) and CYP3A4 probe (midazolam, 8 mg) were orally administered to 10 male healthy volunteers before and after GBE intake (360 mg/d) for 28 days, and they received 75 g glucose after the dosing of tolbutamide. Plasma drug concentrations and blood glucose levels were measured. The area under concentration versus time curve (AUC0-infinity) for tolbutamide after GBE intake was slightly but significantly (16%) lower than that before GBE intake. Concomitantly, GBE tended to attenuate AUC0-2 of blood glucose-lowering effect of tolbutamide. AUC0-infinity for midazolam was significantly (25%) increased by GBE intake and oral clearance was significantly (26%) decreased. Thus, it is suggested that the combination of GBE and drugs should be cautious in terms of the potential interactions, especially in elderly patients or patients treated with drugs exerting relatively narrow therapeutic windows.

Chlorpropamide 2-hydroxylation is catalysed by CYP2C9 and CYP2C19 in vitro: chlorpropamide disposition is influenced by CYP2C9, but not by CYP2C19 genetic polymorphism

AIMS

We evaluated the involvement of cytochrome P450 (CYP) isoforms 2C9 and 2C19 in chlorpropamide 2-hydroxylation in vitro and in chlorpropamide disposition in vivo.

METHODS

To identify CYP isoforms(s) that catalyse 2-hydroxylation of chlorpropamide, the incubation studies were conducted using human liver microsomes and recombinant CYP isoforms. To evaluate whether genetic polymorphisms of CYP2C9 and/or CYP2C19 influence the disposition of chlorpropamide, a single oral dose of 250 mg chlorpropamide was administered to 21 healthy subjects pregenotyped for CYP2C9 and CYP2C19.

RESULTS

In human liver microsomal incubation studies, the formation of 2-hydroxychlorpropamide (2-OH-chlorpropamide), a major chlorpropamide metabolite in human, has been best described by a one-enzyme model with estimated K(m) and V(max) of 121.7 +/- 19.9 microm and 16.1 +/- 5.0 pmol min(-1) mg(-1) protein, respectively. In incubation studies using human recombinant CYP isoforms, however, 2-OH-chlorpropamide was formed by both CYP2C9 and CYP2C19 with similar intrinsic clearances (CYP2C9 vs. CYP2C19: 0.26 vs. 0.22 microl min(-1) nmol(-1) protein). Formation of 2-OH-chlorpropamide in human liver microsomes was significantly inhibited by sulfaphenazole, but not by S-mephenytoin, ketoconazole, quinidine, or furafylline. In in vivo clinical trials, eight subjects with the CYP2C9*1/*3 genotype exhibited significantly lower nonrenal clearance [*1/*3 vs.*1/*1: 1.8 +/- 0.2 vs. 2.4 +/- 0.1 ml h(-1) kg(-1), P < 0.05; 95% confidence interval (CI) on the difference 0.2, 1.0] and higher metabolic ratios (of chlorpropamide/2-OH-chlorpropamide in urine: *1/*3 vs.*1/*1: 1.01 +/- 0.19 vs. 0.56 +/- 0.08, P < 0.05; 95% CI on the difference - 0.9, - 0.1) than did 13 subjects with CYP2C9*1/*1 genotype. In contrast, no differences in chlorpropamide pharmacokinetics were observed for subjects with the CYP2C19 extensive metabolizer vs. poor metabolizer genotypes.

CONCLUSIONS

These results suggest that chlorpropamide disposition is principally determined by CYP2C9 activity in vivo, although both CYP2C9 and CYP2C19 have a catalysing activity of chlorpropamide 2-hydroxylation pathway.

Effects of CYP2C19 and CYP2C9 genetic polymorphisms on the disposition of and blood glucose lowering response to tolbutamide in humans

Several recent in-vitro data have revealed that CYP2C19, in addition to CYP2C9, is also involved in the 4-methylhydroxylation of tolbutamide. We evaluated the relative contribution of CYP2C9 and CYP2C19 genetic polymorphisms on the disposition of blood glucose lowering response to tolbutamide in normal healthy Korean subjects in order to reappraise tolbutamide as a selective in-vivo probe substrate of CYP2C9 activity. A single oral dose of tolbutamide (500 mg) or placebo was administered to 18 subjects in a single-blind, randomized, crossover study with a 2-week washout period. Twelve subjects (of whom six were CYP2C19 extensive metabolizer (EM) and six were CYP2C19 poor metabolizer (PM) genotype) were of the homozygous wild-type CYP2C9*1 genotype; the other six subjects were of the CYP2C9*1/*3 and CYP2C19 EM genotype. Pharmacokinetic parameters were estimated from plasma and urine concentrations of tolbutamide and 4-hydroxytolbutamide. Serum glucose concentrations were measured before and after oral intake of 100 g dextrose. In subjects heterozygous for the CYP2C9*3 allele, C(max) and AUC of tolbutamide were significantly greater and the plasma half-life significantly longer than those in homozygous CYP2C9*1 subjects. No pharmacokinetic differences were found between CYP2C19 EM and PM genotype subjects. The estimated AUC of the increase in serum glucose after oral intake of 100 g dextrose was 2.7-fold higher in subjects with the wild-type CYP2C9 genotype than in those with CYP2C9*1/*3, but CYP2C19 genetic polymorphism did not alter the blood glucose lowering effect of tolbutamide. The plasma AUC of 4-hydroxytolbutamide and the ratio of 4-hydroxytolbutamide/tolbutamide did not differ significantly between CYP2C19 PM and EM genotype subjects, while these parameters were about twice as high in subjects with the wild-type CYP2C9 genotype than in heterozygous CYP2C9*3 subjects (P < 0.05). Our results strongly suggest that the disposition and hypoglycemic effect of tolbutamide are affected mainly by CYP2C9 genetic polymorphism, but not by CYP2C19 polymorphism. The in-vivo contribution of CYP2C19 to tolbutamide 4-methylhydroxylation appears to be minor in humans. This suggests that, at least in vivo, tolbutamide remains a selective probe for measuring CYP2C9 activity in humans.

Pharmacokinetic interaction between diltiazem and tolbutamide

The effect of co-administered tolbutamide and diltiazem on each drug's pharmacokinetics was studied in eight healthy male volunteers aged 21-25 years, with a 3 x 3 randomised crossover design. Each subject received orally 60 mg of diltiazem hydrochloride or 500 mg of tolbutamide, or both drugs. The washout period between each treatment was 7 days. Serum levels of diltiazem and tolbutamide were determined by HPLC. Serum profiles were analysed using a non-compartmental model. There was no change in the pharmacokinetics of diltiazem in the presence of tolbutamide. There was approximately 10% increase in AUC0-24 and Cmax for tolbutamide in the presence of diltiazem.

Comparative placental transport of oral hypoglycemic agents in humans: a model of human placental drug transfer

OBJECTIVE

This study compares the human placental transport of glyburide, glipizide, chlorpropamide, and tolbutamide.

STUDY DESIGN

The recirculating single cotyledon human placenta model tested maternal-to-fetal transport in term placentas perfused immediately after delivery. Drug levels were measured by high-performance liquid chromatography and liquid scintillation spectrometry, and transport rates were calculated by comparing maternal and fetal concentrations.

RESULTS

The transport of these substances differed significantly over a tenfold range (analysis of variance, p < 0.0008). A significant association exists by multiple linear regression between drug transfer and molecular weight, dissociation constant, and the octanol-water partition coefficient (R2 = 0.91, p < 0.0001).

CONCLUSIONS

There is significant variability in human placental transfer rates of the oral hypoglycemics, which strongly correlates with molecular properties. These data suggest that less fetal exposure may occur with second-generation sulfonylureas and anticipate that regression models may be useful in selecting agents that minimize placental transport to the fetus.

Determination of tolbutamide hydroxylation in rat liver microsomes by high-performance liquid chromatography: effect of psychoactive drugs on in vitro activity

A simplified HPLC method for tolbutamide metabolism to hydroxytolbutamide has been used to screen sixty psychoactive drugs for their ability to inhibit rat liver microsomal tolbutamide hydroxylation. One-step extraction with diethyl ether was followed by reconstitution and isocratic HPLC analysis with a binary mobile phase (ammonium phosphate:methanol, 45:55, v/v). Nanogram amounts of hydroxytolbutamide formation were estimated with UV detection at 240 nm. Hydroxytolbutamide formation was linear with incubation times of 40-120 min, but specific activity increased with increases in microsomal protein (0.15-1.10 mg). A differential inhibitory response was demonstrated for tolbutamide and debrisoquine hydroxylation to 5 psychoactive drugs, suggesting that tolbutamide hydroxylation is not dependent on P4502D1. Sixty psychoactive drugs, or drug metabolites, (at 33 microM) were then co-incubated with tolbutamide (at 2.5 and 10.2 microM). Tolbutamide hydroxylation was refractory (< 25% inhibition) to twenty-four of the drugs and only mildly inhibited (25-50% inhibition) by twenty-eight. Two compounds, trans-3-methylfentanyl and flurazepam, produced > 50% inhibition that was independent of tolbutamide concentration. Five of the drugs (methadone, chlorpheniramine, meperidine, 6-monoacetylmorphine and methylphenidate), however, caused greater than 50% inhibition in a competitive manner which suggests these drugs may share an affinity for the substrate binding site for tolbutamide.