樋坂 章博

Serum concentrations of valproic acid (VPA) are markedly decreased by coadministration of carbapenem antibiotics (CBPMs). Although inhibition of deconjugation of VPA-glucuronide (VPA-G) to VPA by CBPMs has been proposed as one of the mechanisms to account for this drug-drug interaction, little information is available on the mode of inhibition. In the present study, we characterized the enzyme involved in the deconjugation of VPA-G by using human and rat liver cytosol. It is suggested that 1) deconjugation activity inhibited by CBPMs may be selective for VPA-G, 2) deconjugation of VPA-G may be mediated by enzyme(s) other than β-glucuronidase, and 3) the irreversible inactivation may be responsible for the inhibition of deconjugation of VPA-G by CBPMs. Finally, the kinetic parameters for inactivation (K'(app) and k(inact)) were determined for four CBPMs of diverse structure from in vitro experiments. Based on the results of simulation analyses with these parameters and the degradation rate constant of the putative VPA-G deconjugation enzyme obtained from experiments using rats, it is probable that the deconjugation enzyme for VPA-G in the liver is rapidly and mostly inactivated by these CBPMs under clinical situations.

The aim of the present paper was to present an overview of the current status of the methods used to predict the magnitude of pharmacokinetic drug-drug interactions (DDIs) which are caused by apparent changes in cytochrome P450 (CYP) activity with an emphasis on a method using in vivo information. In addition, more than a hundred representative CYP substrates, inhibitor and inducer drugs involved in significant pharmacokinetic DDIs were selected from the literature and are listed. Although the magnitude of DDIs has been conventionally predicted based on in vitro experiments, their predictability is restricted occasionally due to several difficulties, including a precise determination of the unbound inhibitor concentrations at the enzyme site and a reliable in vitro measurement of the inhibition constant (K(i)). Alternatively, a simple method has been recently proposed for the prediction of the magnitude of DDIs based on information fully available from in vivo clinical studies. The new in vivo-based method would be applicable to the adjustment of dose regimens in actual pharmacotherapy situations although it requires a prior clinical study for the prediction. In this review, theoretical and quantitative relationships between the in vivo- and the in vitro-based prediction methods are considered. One of the interesting outcomes of the consideration is that the K(i)-normalized dose (dose/in vitro K(i)) of larger than approximately 20L (2-200L, when variability is considered) may be a pragmatic index which predicts significant in vivo DDIs. In the last part of the article, the relevance of the inclusion of the in vivo-based method into the process of new drug development is discussed for good prediction of in vivo DDIs.

The prediction of drug-drug interactions (DDIs) associated with change in clearance for metabolism is reviewed, particularly focusing on pharmacokinetic theories for prediction based on in vitro and in vivo observation. First, there is discussion about how quantitative determination of the contribution of major clearance pathways is fundamental for the accurate prediction of DDIs. Secondly, the concentrations of causative drugs at sites of interactions are discussed. Although DDIs have been predicted from in vitro pharmacokinetic parameters based on predicted hepatic unbound concentrations of inhibitors and inducers, there are noticeable discrepancies between predicted and observed magnitudes of these DDIs. To solve these issues, a method for the prediction of unbound hepatic concentration is proposed based on theoretical considerations. Finally, a pharmacokinetic model to describe the intestinal first pass metabolism is considered, particularly focusing on the importance of the Q(gut) model. Although this Q(gut) model was proposed as an empirical model, theoretical considerations suggest that the model is regarded as a physiologically-based pharmacokinetic model that can predict significance of intestinal DDIs. Theoretical considerations proposed in the present article may be helpful for future analysis of DDIs.

In optimizing oral pharmacotherapy for patients with renal failure, information on actual urinary excretion ratio of the unchanged drug, which is obtained by dividing a urinary excretion ratio by a bioavailability after oral dose, is quite helpful. In addition, urinary excretion ratio of the active species is sometimes equally important where metabolites have a pharmacological potency. In the present study, we conducted a survey of Japanese package inserts and interview forms of drugs, which is being prescribed at the University of Tokyo Hospital, on pharmacokinetic data that enables an estimation of actual urinary excretion ratio. The total urinary excretion of a drug was documented in 70.1% of package inserts and 84.5% of interview forms, respectively. The total urinary excretion is often measured by radioactivity and thus includes its metabolites and degradation products. However, inclusion of degradation products/metabolites was described explicitly for 43.7% and 66.2%, and the absolute fraction of the unchanged drug or degradation product/metabolite was given only for 29.0% and 48.9% in package inserts and interview forms, respectively. The pharmacological activity of metabolite(s) was documented for 19.8% and 54.3%, and the oral bioavailability was described only for 5.7% and 30.6% in respective documents. For some drugs, the time period for the urine collection was too short to evaluate the urinary excretion ratio. With regard to 65 drugs (38.7%), more detailed information on urinary excretion was found in published books, but not provided in the package inserts or interview forms. It is hoped that more distinct and sufficient descriptions on the urinary excretion and bioavailability will be associated to the package inserts and the interview forms in future, for safe and efficient use of prescription drugs.

PURPOSE: Hepatobiliary excretions of drugs from the blood to the bile include two essential transmembrane processes: uptake into hepatocytes and secretion from hepatocytes. The purpose of this study was to clarify the transport mechanisms underlying these processes for a new non-peptide endothelin antagonist, (+)-(5S,6R,7R)-2-butyl-7-[2((2S)-2-carboxypropyl)-4-methoxyphenyl]-5-(3,4-methylenedioxy-phenyl)cyclopentenol[1,2-b]pyridine-6-carboxylic acid (J-104132). METHODS: Biliary excretion of J-104132 was assessed in rats after intravenous injection. To evaluate the hepatic uptake process, J-104132 was incubated with freshly isolated rat hepatocytes and the uptake of J-104132 was calculated. To evaluate the biliary secretion process, the uptake of J-104132 into rat canalicular membrane vesicles that were isolated from normal Sprague-Dawley rats or Eisai hyperbilirubinemic rats was measured. RESULTS: After intravenous injection, J-104132 was recovered from the bile quantitatively (99.7 +/- 1.3%) as its intact form. J-104132 was taken up by isolated rat hepatocytes in a time- and temperature-dependent manner. The uptake was saturable with Km and Vmax of 5.7 microM and 564 pmol/min/10(6) cells, respectively. The uptake was Na+ independent and was reduced in the presence of ATP depleters (rotenone and carbonyl cyanide-p-(trifluoromethoxy)-phenylhydrazone), organic anions (dibromosulfophthalein, indocyanine green, BQ-123, and pravastatin), and bile acids (taurecholate and cholate). In Sprague-Dawley rats, J-104132 was taken up by canalicular membrane vesicle ATP-dependently with Km and Vmax values of 6.1 microM and 552 pmol/min/mg protein, respectively. However, ATP-dependent uptake disappeared in Eisai hyperbilirubinemic rats. CONCLUSIONS: These data suggest that energy-dependent and carrier-mediated transport systems play important roles in hepatobiliary excretion of J-104132 (both uptake and secretion processes), which is the main excretion route in rats. As for the secretion process of J-104132, an involvement of mrp2 was demonstrated.

It is suggested that the bioavailability of CYP3A4 substrates might be low due to first-pass metabolism in the small intestine, and it is possible that P-glycoprotein (P-gp) may influence first-pass metabolism in a co-operative manner. We have collected information of the pharmacokinetics of CYP3A4 substrates to evaluate the fraction absorbed (Fa), intestinal availability (Fg) and hepatic availability (Fh) and have investigated the intestinal first-pass metabolism and the effect of P-gp on this. The pharmacokinetic data involved ten compounds metabolized by CYP3A4 in humans, with and without an inhibitor or inducer. FaFg, which is the product of Fa and Fg, and Fh were calculated using three liver blood flow rates (17.1, 21.4, 25.5 mL/min/kg) in consideration of variations in the liver flow rate. Co-administration with an inhibitor of CYP3A4 and treatment of an inducer of CYP3A4 caused an increase and decrease in the FaFg of CYP3A4 substrates, regardless of the liver blood flow, indicating that CYP3A4 substrates exhibit a first-pass effect in their metabolism. This holds true regardless of whether the compounds are P-gp substrates or not. No relationship was observed between FaFg and Fh, regardless of the hepatic blood flow rate and the P-gp substrates. The FaFg of both P-gp and non P-gp substrates decreased as the hepatic intrinsic clearance increased. FaFg was markedly reduced when the hepatic intrinsic clearance was more than 100 mL/min/kg. This in vivo intrinsic clearance corresponds to an in vitro intrinsic clearance of 78 muL/min/mg human hepatic microsomal protein, equivalent to a half-life of 8.9 min for the substrate in a commonly used metabolic stability test with human microsomes (1 mgMs protein/mL). This phenomenon was not observed in substrates of CYP isoforms other than CYP3A4. In conclusion, it is suggested that CYP3A4 substrates which have a hepatic intrinsic clearance of 100 mL/min/kg exhibit a low bioavailability due to intestinal first-pass metabolism, regardless of whether they are substrates of P-gp or not.

Compounds (2-5) with a 6-carboxy-5,7-diarylcyclopentenopyridine skeleton were designed, synthesized, and identified as a new class of potent non-peptide endothelin receptor antagonists. The regio-isomer 2 was found to show potent inhibitory activity with an IC(50) value of 2.4 nM against (125)I-labeled ET-1 binding to human ET(A) receptors and a 170-fold selectivity for ET(A) over ET(B) receptors. Furthermore, 2 displayed more potent in vivo activity than did the indan-type compound 1 in a mouse ET-1 induced lethality model, suggesting the potential of 2 as a new lead structure. Derivatization on substituted phenyl groups at the 5- and 7-positions of 2 revealed that a 3,4-methylenedioxyphenyl group at the 5-position and a 4-methoxyphenyl group at the 7-position were optimal for binding affinity. Further derivatization of 2 by incorporating a substituent into the 2-position of the 4-methoxyphenyl group led to the identification of a more potent ET(A) selective antagonist 2p with an IC(50) value of 0.87 nM for ET(A) receptors and a 470-fold selectivity. In addition, 2p showed highly potent in vivo efficacy (AD(50): 0.04 mg/kg) in the lethality model.

The hepatobiliary transport of an anionic cyclopentapeptide endothelin antagonist, BQ-123, was studied in rats. Biliary excretion of [H-3]BQ-123 was extensive in vivo (similar to 75% of intravenous infusion rates). Liver-to-plasma and bile-to-liver concentration ratios at steady state were similar to 3 and 200, respectively, suggesting that hepatic uptake and biliary excretion are concentrative processes. The biliary excretion clearance exhibited a saturation at a hepatic concentration of >100 nmol/g liver and was markedly reduced in Eisai hyperbilirubinemic rats, which have a hereditary defect of canalicular multispecific organic anion transporter. An ATP-dependent and saturable uptake of BQ-123 by isolated canalicular membrane vesicles was observed in vitro. Impaired transport of BQ-123 was also confirmed in canalicular membrane vesicles prepared from Eisai hyperbilirubinemic rats. These results demonstrate that the biliary excretion process is ATP-driven primary active transport. It is proposed that a canalicular multispecific organic anion transporter is mainly responsible for the biliary excretion of BQ-123.