Prasugrel and clopidogrel are antiplatelet prodrugs that are converted to their respective active metabolites through thiolactone intermediates. Prasugrel is rapidly hydrolysed by esterases to its thiolactone intermediate, while clopidogrel is oxidized by cytochrome P450 (CYP) isoforms to its thiolactone. The conversion of both thiolactones to the active metabolites is CYP mediated. This study compared the efficiency, in vivo, of the formation of prasugrel and clopidogrel thiolactones and their active metabolites. The areas under the plasma concentration versus time curve (AUC) of the thiolactone intermediates in the portal vein plasma after an oral dose of prasugrel (1 mg kg(-1)) and clopidogrel (0.77 mg kg(-1)) were 15.8 +/- 15.9 ng h ml(-1) and 0.113 +/- 0.226 ng h ml(-1), respectively, in rats, and 454 +/- 104 ng h ml(-1) and 23.3 +/- 4.3 ng h ml(-1), respectively, in dogs, indicating efficient hydrolysis of prasugrel and little metabolism of clopidogrel to their thiolactones in the intestine. The relative bioavailability of the active metabolites of prasugrel and clopidogrel calculated by the ratio of active metabolite AUC (prodrug oral administration/active metabolite intravenous administration) were 25% and 7%, respectively, in rats, and 25% and 10%, respectively, in dogs. Single intraduodenal administration of prasugrel showed complete conversion of prasugrel, resulting in high concentrations of the thiolactone and active metabolite of prasugrel in rat portal vein plasma, which demonstrates that these products are generated in the intestine during the absorption process. In conclusion, the extent of in vivo formation of the thiolactone and the active metabolite of prasugrel was greater than for clopidogrel's thiolactone and active metabolite.

ABSTRACT:The aim of the current study was to evaluate the accuracy of allometric scaling methods for drugs metabolized by UDP-glucuronosyltransferases (UGTs), such as ketoprofen, imipramine, lorazepam, levofloxacin, zidovudine, diclofenac, furosemide, raloxifene, gemfibrozil, mycophenolic acid, indomethacin, and telmisartan. Human plasma clearance (CL) predictions were conducted from preclinical in vivo data by using multiple-species allometry with the rule of exponents and single-species allometric scaling (SSS) of mice, rats, monkeys, or dogs. Distribution volume at a steady state (V ss ) was predicted by multiple-species allometry or SSS of V ss . Oral plasma clearance (CL po ) was calculated under the assumption that F a ؋ F g was equivalent across species. Each of the results was compared with the observed parameter calculated from the clinical data after intravenous or oral administration. Multiple-species allometry and SSS of mice, rats, and dogs resulted in a similar accuracy of CL and CL po predictions. Monkeys tended to provide the most accurate predictions of human CL and CL po . The ability to predict the half-life, which was determined from CL and V ss predictions, was more accurate in SSS of rats and monkeys. The in vivo fraction metabolized by glucuronidation (f m,UGT ) in bile duct-cannulated monkeys was relatively similar to that of humans compared with other animal species, which likely contributed to the highest accuracy of SSS prediction of monkeys. On the basis of the current results, monkeys would be more reliable than other animal species in predicting human pharmacokinetics and f m,UGT for drugs metabolized by UGTs.

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1. Our previous in vitro studies suggest that inhibition of the acylpeptide hydrolase (APEH) activity as valproic acid glucuronide (VPA-G) hydrolase by carbapenems in human liver cytosol is a key process for clinical drug-drug interaction (DDI) of valproic acid (VPA) with carbapenems. Here, we investigated whether in vivo DDI of VPA with meropenem (MEPM) was caused via inhibition of APEH in dogs. 2. More rapid decrease of plasma VPA levels and increased urinary excretion of VPA-G were observed after co-administration with MEPM compared with those after without co-administration, whereas the plasma level and bile excretion of VPA-G showed no change. 3. Dog VPA-G hydrolase activity, inhibited by carbapenems, was mainly located in cytosol from both the liver and kidney. APEH-immunodepleted cytosols lacked VPA-G hydrolase activity. Hepatic and renal APEH activity was negligible even at 24 h after dosing of MEPM to a dog. 4. In conclusion, DDI of VPA with carbapenems in dogs is caused by long-lasting inhibition of APEH-mediated VPA-G hydrolysis by carbapenems, which could explain the delayed recovery of plasma VPA levels to the therapeutic window even after discontinuation of carbapenems in humans.

ABSTRACT:Prasugrel [2-acetoxy-5-(␣-cyclopropylcarbonyl-2-fluorobenzyl)-4,5,6,7-tetrahydrothieno[3,2-c]pyridine], a thienopyridine antiplatelet agent, undergoes rapid hydrolysis in vivo to a thiolactone intermediate, 2-[2-oxo-6,7-dihydrothieno[3,2-c]pyridin-5(4H)-yl]-1-cyclopropyl-2-(2-fluorophenyl)ethanone (R-95913), which is further converted to a pharmacologically active metabolite, 2-[1-2-cyclopropyl-1-(2-fluorophenyl)-2-oxoethyl]-4-mercapto-3-piperidinylidene acetic acid (R-138727), by oxidation via cytochromes P450. In this study, we investigated how much the intestine and liver contribute to the formation of R-95913 and R-138727 after intraduodenal administration of prasugrel (1 mg/kg) to portal vein-and hepatic vein-cannulated dogs. The areas under the plasma concentration-time curve up to 2 h of R-95913 in the portal, hepatic, and systemic veins were 525, 32, and 17 ng ⅐ h/ml, respectively, and those of R-138727 were 564, 529, and 495 ng ⅐ h/ml, respectively. The dose of prasugrel was absorbed and then converted to R-95913 and R-138727 by 93 and 13%, respectively, in the intestine. In the liver, 23% of the R-95913, which passed through the intestine, was converted to R-138727. In conclusion, this is the first report to directly demonstrate that the conversion of prasugrel to R-138727 in the intestine is comparable to that converted in the liver of dogs. IntroductionPrasugrel [2-acetoxy-5-(␣-cyclopropylcarbonyl-2-fluorobenzyl)-4,5,6,7-tetrahydrothieno[3,2-c]pyridine; Effient in the United States and Efient in the European Union], clopidogrel (Plavix/Iscover), and ticlopidine (Ticlid) are thienopyridine antiplatelet agents. Prasugrel is indicated to reduce the rate of thrombotic cardiovascular events and stent thrombosis in patients with acute coronary syndrome who are undergoing percutaneous coronary intervention (Wiviott et al., 2007; Effient package insert, Eli Lilly and Company, Indianapolis, IN). The thienopyridines are prodrugs that are converted in vivo to their pharmacologically active metabolites possessing a thiol group via a corresponding thiolactone metabolite (Farid et al., 2010). In clinical trials, prasugrel achieved a greater and faster antiplatelet effect than clopidogrel (Payne et al., 2007;Wallentin et al., 2008). Such responses to prasugrel are attributed to higher and faster exposure to its active metabolite R-138727 (2-[1-2-cyclopropyl-1-(2-fluorophenyl)-2-oxoethyl]-4-mercapto-3-piperidinylidene acetic acid) than clopidogrel's response (Sugidachi et al., 2007;Ernest et al., 2008). Prasugrel is rapidly hydrolyzed to a thiolactone intermediate R-95913 (2-[2-oxo-6,7-dihydrothieno[3,2-c]pyridin-5(4H)-yl]-1-cyclopropyl-2-(2-fluorophenyl)ethanone), mainly by human carboxylesterase 2 during absorption through the gastrointestinal tract (Williams et al., 2008). R-95913 is metabolized to R-138727 by cytochrome P450 isoforms, and the main contributors are CYP3A4 and CYP2B6 (38 -70 and 2-36%, respectively), with smaller contributions by CYP2C9 and CYP2C19 (14 -19 and 8 -11%, respectively) (Rehme...

The placental and plasma cystine aminopeptidase (CAP) in pregnant animals was examined on stability after the treatment with L-methionine, ethylene diamine tetra-acetic acid (EDTA) and heat. Inhibitory effects of these treatments on enzyme activities were different among CAPs from the animal species, however, significant correlation in those effects between placental and plasma CAPs was observed. These results suggested that plasma CAP might reflect placental CAP and seemed to be available for estimating maternal gestational conditions.

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Plasma Cystine aminopeptidase (CAP) activities were examined in 40 pregnant and 10 nonpregnant Thoroughbred horses by spectrophotometric analysis using S-benzyl-L-Cystine-p dimethylaminoanilide as a substrate. Plasma CAP activities were remarkably low and no difference was observed between pregnant and non-pregnant horses. No changes of plasma CAP activities related with pregnant conditions, such as placental growth, functions and fetal growth, were observed. These results suggested that the plasma CAP activities seemed to be unavailable for the marker of maternal gestational conditions in horses.