The biotransformation of prasugrel, a new thienopyridine prodrug, by the human carboxylesterases 1 and 2.

2-Acetoxy-5-(alpha-cyclopropylcarbonyl-2-fluorobenzyl)-4,5,6,7-tetrahydrothieno[3,2-c]pyridine (prasugrel) is a novel thienopyridine prodrug with demonstrated inhibition of platelet aggregation and activation. The biotransformation of prasugrel to its active metabolite, 2-[1-[2-cyclopropyl-1-(2-fluorophenyl)-2-oxoethyl]-4-mercapto-3-piperidinylidene]acetic acid (R-138727), requires ester bond hydrolysis, forming the thiolactone 2-[2-oxo-6,7-dihydrothieno[3,2-c]pyridin-5(4H)-yl]-1-cyclopropyl-2-(2-fluorophenyl)ethanone(R-95913), followed by cytochrome P450-mediated metabolism to the active metabolite. The presumed role of the human liver- and intestinal-dominant carboxylesterases, hCE1 and hCE2, respectively, in the conversion of prasugrel to R-95913 was determined using expressed and purified enzymes. The hydrolysis of prasugrel is at least 25 times greater with hCE2 than hCE1. Hydrolysis of prasugrel by hCE1 showed Michaelis-Menten kinetics yielding an apparent K(m) of 9.25 microM and an apparent V(max) of 0.725 nmol product/min/microg protein. Hydrolysis of prasugrel by hCE2 showed a mixture of Hill kinetics at low substrate concentrations and substrate inhibition at high concentrations. At low concentrations, prasugrel hydrolysis by hCE2 yielded an apparent K(s) of 11.1 microM, an apparent V(max) of 19.0 nmol/min/microg, and an apparent Hill coefficient of 1.42, whereas at high concentrations, an apparent IC(50) of 76.5 microM was obtained. In humans, no in vivo evidence of inhibition exists. In vitro transport studies using the intestinal Caco-2 epithelial cell model showed a high in vivo absorption potential for prasugrel and rapid conversion to R-95913. In conclusion, the human carboxylesterases efficiently mediate the conversion of prasugrel to R-95913. These data help explain the rapid appearance of R-138727 in human plasma, where maximum concentrations are observed 0.5 h after a prasugrel p.o. dose, and the rapid onset of action of prasugrel.

Pharmacokinetics and hepatic extraction of recombinant human parathyroid hormone, hPTH (1-34), in rat, dog, and monkey.

The pharmacokinetics (PK) and hepatic extraction (E(H)) of human PTH (1-34), hPTH (1-34), were characterized in rat, dog, and monkey, following intraportal (IPO) and intravenous (IV) bolus administration. hPTH (1-34) was administered to Sprague-Dawley rats (2, 10, 100 microg/kg), beagle dogs (3, 6 microg/kg), and rhesus monkeys (6, 30 microg/kg). Serum concentrations of immunoreactive hPTH (1-34) were used to derive PK parameters. IPO bioavailability (F(IPO)) was determined by comparing dose-normalized serum exposure (i.e., AUC(IPO)/AUC(IV)). E(H) was estimated as 1-F(IPO). In all species, greater than dose-proportional increases in exposure (i.e., C(max) and AUC) were observed for both routes. Dose-dependent disposition (i.e., time-average clearance (CL) and half-life (t(1/2)) were observed in all three species. In rats, E(H) values of 71% (2 microg/kg), 35% (10 microg/kg), and <1% (100 microg/kg) were obtained. In dogs, E(H) values of 90% (3 microg/kg) and 66% (6 microg/kg) were obtained. In monkeys, E(H) values of 25% (6 microg/kg) and <1% (30 microg/kg) were observed. In conclusion, hPTH (1-34) is subject to hepatic first pass extraction in rat, dog, and monkey with evidence of saturation in the rat. Saturable hepatic extraction in dog and monkey is inconclusive due to the limited dose range investigated.