Received for publication December 20, 2007.
Revised March 25, 2008.
Accepted for publication March 26, 2008.

The Biotransformation of Prasugrel, a New Thienopyridine Prodrug, by the Human Carboxylesterases 1 and 2

Abstract

Prasugrel is a novel thienopyridine prodrug with demonstrated inhibition of platelet aggregation and activation. The biotransformation of prasugrel to its active metabolite, R-138727, requires ester bond hydrolysis, forming the thiolactone 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 demonstrated Michaelis-Menten kinetics yielding an apparent K_m of 9.25 µM and an apparent V_max of 0.725 nmol of product/min/µg of 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 µM, an apparent V_max of 19.0 nmol/min/µg, and an apparent Hill coefficient of 1.42; while at high concentrations, an apparent IC₅₀ of 76.5 µM was obtained. In humans, no in vivo evidence of inhibition exists. In vitro transport studies using the intestinal Caco-2 epithelial cell model demonstrated 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 hour after a prasugrel oral dose, and the rapid onset of action of prasugrel.

Key words: bioactivation, carboxylesterases, drug absorption, drug disposition, enzyme kinetics, first-pass metabolism, intestinal bioavailability, permeability, prodrugs