Makoto Inada

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.

The use of 13C-erythromycin as an in vivo probe to evaluate CYP3A-mediated drug interactions in rats.

(14)C-erythromycin breath test has been utilized to evaluate the extent of CYP3A activity in vivo. However, its radioactivity sometimes impedes its clinical application. In this study, we employed erythromycin labeled with (13)C ((13)C-EM), a nonradioactive stable isotope, as an in vivo probe of breath test to evaluate CYP3A-mediated drug interactions in rats. A physiologically based pharmacokinetic (PBPK) model to describe (13)CO(2) exhalation altered by drug interactions was newly constructed. Rats received an intravenous or oral administration of (13)C-EM with or without a CYP3A inhibitor or inducer, that is, ketoconazole (KCZ) or dexamethasone (DEX), respectively. Breath samples were taken at designated times, measured with an infrared spectrophotometer, and the Δ(13) CO(2) value (‰) in each sample was obtained. The C(max) and AUC(0-t) of Δ(13) CO(2) were significantly decreased with KCZ and increased with DEX. The PBPK model in this study successfully described the (13)CO(2) exhalation after (13)C-EM administration in the absence and presence of drug interactions. In conclusion, this study proposed a simple and rapid in vivo methodology to utilize (13)C-EM for the quantitative analysis of CYP3A inhibition and induction. This method using small animals may be useful in early drug development processes.

Calcium [13C]carbonate breath test for quantitative measurement of total gastric acid in rats

Abstract Objective. A traditional measurement of gastric acid, involving nasogastric intubation of stomach and acid suction, has been suggested as a gold standard. However, this causes the patient discomfort and cost increase, and is ‘time-consuming’. Material and methods. A calcium [13C]carbonate (Ca13CO3) breath test was carried out in rats without or with concomitant drugs omeprazole (OMP) and pentagastrin (PG) known as an inhibitor and an inducer of acid, respectively. This test was aimed at evaluating a correlation between the breath response and the total amount of gastric acid. To search for an absorption pathway of 13CO2 gas produced by the reaction of Ca13CO3 with hydrochloric acid in the stomach of rats, we compared the breath responses after intra-gastric administration of 13CO2 gas and sodium [13C]bicarbonate (NaH13CO3). Results. A linear relationship of the breath parameter (breath-Cmax) with the dose of Ca13CO3 was obtained in the range of 4–200 µmol/kg. However, theses parameters were saturated at >200 µmol/kg. The direct correlation between the breath-Cmax and the total amount of gastric acid in rats with or without OMPs or PG (r = 0.994) demonstrated that the change in breath response is an accurate or sensitive indicator of the total amount of gastric acid. 13CO2 gas generated in the rat stomach was likely to diffuse across the stomach wall as 13CO2 gas directly into the blood plasma. Conclusions. The present study showed that Ca13CO3 breath test is a good tool to accurately predict the total amount of gastric acid.

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.

Estimation of gastric pH in cynomolgus monkeys, rats, and dogs using [13C]-calcium carbonate breath test

BACKGROUND The determination of gastric pH is important for the confirmation of efficacy of anti-secretory drugs. However, current methods for measurement of gastric pH provide significant stress to animals and humans. AIM The objective of this study is to establish an easy and reliable gastric pH measurement method by determining (13)CO2 concentration in expired air of monkeys, dogs, and rats after oral administration of Ca(13)CO3. METHODS A correlation of (13)CO2 concentration determined by a Ca(13)CO3 breath test with gastric pH just before Ca(13)CO3 administration was analyzed in the 3 animal species. The equations and contribution ratios of regression line were calculated from logarithmic (13)CO2 concentrations at 15min after administration of Ca(13)CO3 using the linear regression analysis. RESULTS The (13)CO2 concentration in the Ca(13)CO3 breath test was well correlated with the gastric pH just before Ca(13)CO3 administration in the 3 animal species (r=-0.977 to -0.952). The equations of regression line between the (13)CO2 concentration and the gastric pH in each animal species showed good contribution ratios (R(2)≥0.89). CONCLUSIONS The Ca(13)CO3 breath test is an informative tool to estimate gastric pH in animals and will be applicable as a new noninvasive tool for patients with GERD/PPI-resistant symptoms.