Simultaneous determinations of tolbutamide and its hydroxy and carboxy metabolites in serum and urine: application to pharmacokinetic studies of tolbutamide in the rat

Development of a fast LC-MS/MS method for quantification of rilmenidine in human serum: elucidation of fragmentation pathways by HRMS

Rilmenidine is an alpha 2 adrenoreceptor agonist used in the treatment of mild and moderate hypertension. In this study, a fast and accurate liquid chromatographic method with tandem mass spectrometric detection has been validated in order to assure quantification of rilmenidine in human serum. The fragmentation pathway of protonated rilmenidine was studied using high-resolution mass spectrometry (HRMS). This study compared selectivity, linearity, accuracy, precision, extraction efficiency, matrix effect and sensitivity using common liquid-liquid extraction (LLE) and solid-phase extraction (SPE) procedures. The limit of quantitation for both extraction techniques was 0.1 ng/ml. Several differences between the LLE and SPE have been observed in terms of linearity, accuracy, precision and matrix effect. Additionally, the advantages of SPE included less manual work load and increased recovery of rilmenidine in human serum to approximately 80% (LLE, 57%). The developed method involving SPE was found to be accurate (relative error (RE) < 5%), reproducible (relative standard deviation, RSD < 7%), robust and suitable for quantitative analysis of rilmenidine in serum samples obtained from patients under antihypertensive treatment.

Identification of oral antidiabetics and their metabolites in human urine by liquid chromatography/tandem mass spectrometry--a matter for doping control analysis

Since 1999, insulin belongs to the list of prohibited substances of the International Olympic Committee and the World Anti-Doping Agency. Except for patients suffering from insulin-dependent diabetes mellitus, the administration of insulin is not allowed. Therapeutics developed to treat non-insulin-dependent diabetes mellitus act as releasing factors of endogenously produced insulin or improve its efficiency mediating the glucose uptake into insulin-dependent tissues. Hence, these compounds are also relevant for sports drug testing, and a fast, robust, and sensitive assay was developed to identify 12 oral antidiabetic agents or respective hydroxylated metabolites in human urine. Urine specimens are enzymatically hydrolyzed; target analytes are extracted by liquid-liquid extraction and identified by means of liquid chromatography interfaced to tandem mass spectrometry by electrospray ionization. Detection limits of respective drugs ranged between 10 and 30 ng/mL, metabolites of therapeutics were characterized by diagnostic fragmentation pathways upon collisionally activated dissociation of protonated molecules, and general fragmentation routes were proposed.

A convenient five-drug cocktail for the assessment of major drug metabolizing enzymes: a pilot study

AIMS

To assess the feasibility of administering at the same time low doses of five probe drugs, metoprolol (25 mg), chlorzoxazone (250 mg), tolbutamide (250 mg), dapsone (100 mg) and caffeine (100 mg) to determine simultaneously the activities of CYP2D6, CYP2E1, CYP2C9, CYP3A4, CYP1A2, N-acetyltransferase-2 and xanthine oxidase.

METHODS

Ten healthy young non-smoking males received the following drugs or combinations of drugs over a 5-week period: week 1) metoprolol; 2) tolbutamide; 3) caffeine, chlorzoxazone and dapsone; 4) caffeine, chlorzoxazone, dapsone and metoprolol; 5) caffeine, chlorzoxazone, dapsone, metoprolol and tolbutamide. The drugs were self-administered at bedtime and urine was collected for the following 8 h.

RESULTS

Mean molar phenotypic ratios obtained after administering metoprolol (mean change of -11%) or tolbutamide (mean change of -0.3%) alone, were not significantly different from those obtained when other drugs were co-administered (P > 0.05). The mean within-subject coefficients of variation were 33%, 18%, 22%, 13%, 16%, 13% and 5% for CYP3A4, CYP2D6, CYP2C9, CYP2E1, CYP1A2, N-acetyltransferase 2 and xanthine oxidase metabolic ratios, respectively. No significant interactions (P > 0.5) were observed during the simultaneous administration of various combinations of the five probe drugs.

CONCLUSIONS

We propose that this cocktail, composed of five widely available drugs, constitutes a promising means of simultaneously determining the activities of the major CYP enzymes in large populations.

Application of the PKCYP-test to predict the amount of in vivo CYP2C11 using tolbutamide as a probe

Previous reports have shown that the determination of drug metabolism capacity can be made by the pharmacokinetic estimation of the quantity of cytochrome P450 (CYP) in vivo (PKCYP-test), in which an apparent liver-to-blood free concentration gradient in vivo (qg) is introduced, which is useful for evaluating fluctuations of CYPIA2 in rats. The aim of the present study was to examine the application of the PKCYP-test to evaluate the quantity of in vivo CYP2C11 by using tolbutamide as a probe, to confirm its validity using a physiologically-based pharmacokinetic rat model. Rats treated with carbon tetrachloride (CCl4-treated rats) were used as a model for low levels of CYP2C11 in the liver. In CCl4-treated rats, the total body clearance (CLtot) of tolbutamide and the amount of CYP2C11 fell to about a quarter and a third of that in control rats, respectively. The time-course of tolbutamide concentrations in serum in control rats could be simulated by a physiologically-based pharmacokinetic model. In CCl4-treated rats, take into consideration the qg value of control rats, the level of CYP2C11 was accurately predicted by the PKCYP-test, and the time-course of tolbutamide concentrations in serum could be predicted by the same physiologically-based pharmacokinetic model. In conclusion, we have shown that the PKCYP-test can be used to predict levels of CYP2C11. It was also demonstrated that the qg and amount of CYP are useful parameters in the PKCYP-test by constructing a physiologically-based pharmacokinetic model which was applied to the PKCYP-test.

Quantitative determination of tolbutamide and its metabolites in human plasma and urine by high-performance liquid chromatography and UV detection

An isocratic, high-performance liquid chromatography method has been developed for simultaneous determination of the oral antidiabetic tolbutamide and two of its metabolites, 4-hydroxytolbutamide and carboxytolbutamide, in human plasma and urine. The method was based on simple one-step liquid-liquid extraction with tertiary-butyl methyl ether as extraction solvent. The chromatographic eluent was 23:77 (v/v) methanol: 0.01 M aqueous sodium acetate buffer pH 3.0, and the UV detection was performed at a wavelength of 230 nm. The limit of detection was 0.1 microM for tolbutamide in plasma and 1.5 microM, 0.5 microM, and 0.75 microM for carboxytolbutamide, 4-hydroxytolbutamide, and tolbutamide, respectively, in urine. The limit of quantitation was 0.5 micro for tolbutamide in plasma and 2 microM, 0.75 microM, and 1.25 microM for carboxytolbutamide, 4-hydroxytolbutamide, and tolbutamide, respectively, in urine. The overall mean recoveries ranged from 91% to 109% for tolbutamide in plasma and from 80% to 98% in urine for all three compounds.

Inhibitory effect of nicotine and its metabolites on tolbutamide hydroxylation in rat liver microsomes

A simple HPLC/fluorescence method to detect hydroxytolbutamide (a major metabolite of the anti-diabetic drug tolbutamide) has been developed. The effects of nicotine and some of its metabolites on tolbutamide hydroxylation is described. An extraction procedure with diethyl ether was followed by isocratic HPLC analysis of tolbutamide hydroxylation with a binary mobile phase composed of 10 mM monobasic sodium phosphate in methanol (45:55, v/v, apparent pH 2.28). A detection limit of sub-nanogram amounts (0.353 ng) of hydroxytolbutamide was obtained with fluorescence detection at 226 nm for excitation and 318 nm for emission. Overall precision values for hydroxytolbutamide was determined with coefficients of variation of 1.4-4.6% when nanogram levels of the metabolite were analyzed. Differential inhibitory responses were demonstrated for tolbutamide hydroxylation to nicotine and its metabolites. Tolbutamide hydroxylation was apparently inhibited by cotinine and relatively less inhibited by nicotine. Nornicotine, however, caused very little inhibition of tolbutamide hydroxylation. The implication is that nornicotine may not share similar affinity for the substrate binding site for tolbutamide. The results also suggest that heavy smokers may experience reduction in tolbutamide metabolism. The assay system itself will be useful for future studies of tolbutamide, and possibly related sulfonylureas.

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.

Evaluation of hepatic dysfunction in endotoxin pretreated rats using tolbutamide as a marker

The pharmacokinetics of tolbutamide (TB) have been studied in endotoxin pretreated rats with the aim of evaluating TB as a marker for endotoxin effects. Endotoxin dose of 10 mg/kg resulted in a 50% rate of mortality. TB was i.v. administered 24 h. after endotoxin dosing. Clearance (Cl) decreased by approximately 2/3 of its value, area under the curve (AUC) and half-life (t1/2) in the pretreated animals were an average 1.5 times the values for the respective controls. Volume of distribution (Vd) increased by 10% approximately. These findings suggest that endotoxin pretreatment may cause hepatic damage by producing a decrease in Cl and an increase in the t1/2 of TB. But, SGOT levels in pretreated animals were not significantly different. This phenomenon may be explained by the increase in plasma protein binding of TB during endotoxin pretreatment, which decreases the free fraction of the drug in plasma available for metabolism. Endotoxin increased tmax of hydroxy-TB, while no change in Cmax was observed. Since tmax is inversely related to the formation and elimination rates of hydroxy-TB, an increase in tmax may be due to the decrease in both elimination rates. No change in Cmax may be due to the decrease in the rate of formation which is equivalent to the decrease in the rate of elimination of hydroxy-TB.