Yukihiro Hirao

The Uracil Breath Test in the Assessment of Dihydropyrimidine Dehydrogenase Activity: Pharmacokinetic Relationship between Expired 13CO2 and Plasma [2-13C]Dihydrouracil

Purpose: Dihydropyrimidine dehydrogenase (DPD) deficiency is critical in the predisposition to 5-fluorouracil dose-related toxicity. We recently characterized the phenotypic [2-13C]uracil breath test (UraBT) with 96% specificity and 100% sensitivity for identification of DPD deficiency. In the present study, we characterize the relationships among UraBT-associated breath 13CO2 metabolite formation, plasma [2-13C]dihydrouracil formation, [2-13C]uracil clearance, and DPD activity. Experimental Design: An aqueous solution of [2-13C]uracil (6 mg/kg) was orally administered to 23 healthy volunteers and 8 cancer patients. Subsequently, breath 13CO2 concentrations and plasma [2-13C]dihydrouracil and [2-13C]uracil concentrations were determined over 180 minutes using IR spectroscopy and liquid chromatography-tandem mass spectrometry, respectively. Pharmacokinetic variables were determined using noncompartmental methods. Peripheral blood mononuclear cell (PBMC) DPD activity was measured using the DPD radioassay. Results: The UraBT identified 19 subjects with normal activity, 11 subjects with partial DPD deficiency, and 1 subject with profound DPD deficiency with PBMC DPD activity within the corresponding previously established ranges. UraBT breath 13CO2 DOB50 significantly correlated with PBMC DPD activity (rp = 0.78), plasma [2-13C]uracil area under the curve (rp = −0.73), [2-13C]dihydrouracil appearance rate (rp = 0.76), and proportion of [2-13C]uracil metabolized to [2-13C]dihydrouracil (rp = 0.77; all Ps < 0.05). Conclusions: UraBT breath 13CO2 pharmacokinetics parallel plasma [2-13C]uracil and [2-13C]dihydrouracil pharmacokinetics and are an accurate measure of interindividual variation in DPD activity. These pharmacokinetic data further support the future use of the UraBT as a screening test to identify DPD deficiency before 5-fluorouracil-based therapy.

Pharmacokinetics and Metabolism of Delamanid, a Novel Anti-Tuberculosis Drug, in Animals and Humans: Importance of Albumin Metabolism In Vivo.

Delamanid, a new anti-tuberculosis drug, is metabolized to M1, a unique metabolite formed by cleavage of the 6-nitro-2,3-dihydroimidazo[2,1-b] oxazole moiety, in plasma albumin in vitro. The metabolic activities in dogs and humans are higher than those in rodents. In this study, we characterized the pharmacokinetics and metabolism of delamanid in animals and humans. Eight metabolites (M1–M8) produced by cleavage of the imidazooxazole moiety of delamanid were identified in the plasma after repeated oral administration by liquid chromatography–mass spectrometry analysis. Delamanid was initially catalyzed to M1 and subsequently metabolized by three separate pathways, which suggested that M1 is a crucial starting point. The major pathway in humans was hydroxylation of the oxazole moiety of M1 to form M2 and then successive oxidation to the ketone form (M3) mainly by CYP3A4. M1 had the highest exposure among the eight metabolites after repeated oral dosing in humans, which indicated that M1 was the major metabolite. The overall metabolism of delamanid was qualitatively similar across nonclinical species and humans but was quantitatively different among the species. After repeated administration, the metabolites had much higher concentrations in dogs and humans than in rodents. The in vitro metabolic activity of albumin on delamanid probably caused the species differences observed. We determined that albumin metabolism is a key component of the pharmacokinetics and metabolism of delamanid. Nonhepatic formation of M1 and multiple separate pathways for metabolism of M1 suggest that clinically significant drug–drug interactions with delamanid and M1 are limited.

Local Gastric and Serum Concentrations of Rebamipide Following Oral Ingestion in Healthy Volunteers

The purpose of this study was to investigate whether sufficient concentrations of rebamipide (COR) are actually present in the stomach after its oral ingestion at an ordinary clinical dose. Twenty healthy volunteers (total 42 man-days) participated in the study. After ingestion of 100, 200, or 300 mg of rebamipide, endoscopy was performed at 1, 2, 4, and 6 hr, and gastric mucosa or gastric mucus was taken from the antrum. Venous blood samples were taken simultaneously. Samples were analyzed by high-performance liquid chromatography. The COR in the gastric mucosa and gastric mucus did not depend on the original amount ingested. After ingestion of rebamipide, each COR was higher than 10−4 M(37 μg/g tissue) at 1 or 2 hr. On the other hand, the COR in serum did depend on the amount ingested and was lower than 10−6 M(0.37 μg/ml) at every time tested. These results suggest that the COR in the stomach exceeds the levels that are needed for various antiulcer actions and that the rebamipide levels present in the gastric mucosa and gastric mucus result from local penetration.

Pharmacokinetic Modelling of [2-13C]Uracil Metabolism in Normal and DPD-Deficient Dogs

A physiologically based pharmacokinetic (PBPK) model to simulate the plasma concentration and 13CO2 exhalation after [2-13C]uracil administration to DPD-suppressed dogs was developed. Simulation using this PBPK model should be useful in clinical situations where DPD-deficient patients at risk are to be detected with [2-13C]uracil as an in vivo probe.

Desirable pharmacokinetic properties of 13C-uracil as a breath test probe of gastric emptying in comparison with 13C-acetate and 13C-octanoate in rats

Objective. To investigate the possible use of a 13C-uracil breath test for gastric emptying by evaluating the pharmacokinetic properties of 13C-uracil in a breath test in rats, in comparison with 13C-acetate and 13C-octanoate, traditional 13C-probes for gastric emptying. Material and methods. Absorption of the 13C-probes from different parts of the gastrointestinal tract was evaluated in fasted rats. 13C-Uracil breath tests for gastric emptying were carried out in conditions where delayed gastric emptying was induced by clonidine, quinpirole, and propantheline, and in a postoperative ileus model. Following oral administration, we measured residual 13C-uracil in the stomach and correlated the amount with the breath response. Results. All the 13C-probes employed were well absorbed from the intestine after intraduodenal administration. After intragastric administration, 13C-uracil was not absorbed from the stomach, but 13C-acetate and 13C-octanoate were partly absorbed from the stomach. The cumulative 14C-uracil recovery (%) at 168 h was 92.3, 6.3, or 0.5%, from expired gases, urine, and feces, respectively. Δ13C values in 13C-uracil breath tests were decreased in conditions characterized by delayed gastric emptying. A highly negative correlation was observed between the breath response and the residual ratio of 13C-uracil in the stomach after oral administration of 13C-uracil, indicating that 13C-uracil can be used as an in vivo probe for evaluating gastric emptying in a quantitative manner. Conclusions. This study showed that 13C-uracil has desirable pharmacokinetic properties as an in vivo probe of gastric emptying. It is thus suggested that the 13C-uracil breath test may be useful for the measurement of gastric emptying in humans.

Enantioselective analysis of tolvaptan in rat and dog sera by high-performance liquid chromatography and application to pharmacokinetic study.

Tolvaptan is a competitive vasopressin V2-receptor antagonist that inhibits water reabsorption in the renal collecting ducts. A selective high-performance liquid chromatography method for determining tolvaptan enantiomers in rat and dog sera was developed and validated. Benzyl salicylate was used as an internal standard. Sample preparation involved liquid-liquid extraction with an n-hexane:diethyl ether (1:1, v/v) solution, followed by solid-phase extraction using a silica-gel cartridge. Chromatographic separation was performed on a cellulose-based chiral stationary phase in the reversed phase mode. The analytes were monitored with UV detection. The calibration curve showed linearity over the concentration range from 0.025 to 2.5 μg/mL for each analyte. Precision as the percentage coefficient of variation did not exceed 14.8%, and accuracy as relative error was within ±14.6% for the analytes. The validated method was successfully applied to evaluate the pharmacokinetics of oral tolvaptan enantiomers in rats and dogs, indicating gender and species differences in the systemic exposure to tolvaptan enantiomers.

Hemodialysis clearance of iosimenol, a novel iso-osmolar radiographic contrast medium.

Background Iodinated contrast media (CM) have molecular and pharmacokinetic properties likely to make them highly dialyzable. Controlled clinical studies allowing for comparisons of hemodialysis clearance between different test substances and in multiple hemodialysis filters are, however, complex and not always practically feasible. A miniaturized in vitro method was therefore developed to evaluate the dialyzability of a new CM. Purpose To evaluate hemodialysis clearance of iosimenol, a novel iso-osmolar contrast medium (CM), in a select variety of hemodialysis filters and in comparison to commercially available CM. Material and Methods Three different high-flux and one low-flux membrane were used in miniaturized dialyzers to evaluate the in vitro blood clearance of iosimenol. Commercially available CM (iodixanol and iohexol) served as control substances. In vitro dialysis parameters were then used to predict clinical hemodialysis clearances. Residual ratios of endogenous substances (inorganic phosphate, urea nitrogen, creatinine, total bilirubin, and albumin) were used as proof of reliability of the in vitro dialysis system. Results Dialyzable small endogenous molecules were readily eliminated in all membranes. The removal ratios of iosimenol were generally similar to that of iodixanol in all membranes except the high-flux polysulfone but were consistently lower than that of iohexol. The blood clearance of iosimenol during clinical hemodialysis was predicted as, on average, approximately 85 mL/min with the high-flux membranes and 47 mL/min with the low-flux membrane. Conclusion The dialyzability of iosimenol was evaluated using a newly developed in vitro dialysis system, and iosimenol was readily cleared from blood with all four tested membranes. And it is suggested that the dialysis parameters can predict clinical hemodialysis clearance of CM.