Metabolic Activation of
Prasugrel: Nature of the Two
Competitive Pathways Resulting in the Opening of Its Thiophene Ring
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Abstract
The mechanism generally admitted for the bioactivation
of the antithrombotic
prodrug, prasugrel, <b>1c</b>, is its two-step enzymatic conversion
into a biologically active thiol metabolite. The first step is an
esterase-catalyzed hydrolysis of its acetate function leading to a
thiolactone metabolite <b>2c</b>. The second step was described
as a cytochrome P450 (P450)-dependent oxidative opening of the thiolactone
ring of <b>2c</b>, with intermediate formation of a reactive
sulfenic acid metabolite that is eventually reduced to the corresponding
active thiol <b>3c</b>. This article describes a detailed study
of the metabolism of <b>1c</b> by human liver microsomes and
human sera, with an analysis by HPLC-MS under conditions allowing
a complete separation of the thiol metabolite isomers, after derivatization
with 3′-methoxy phenacyl bromide. It shows that there are two
competing metabolic pathways for the opening of the <b>2c</b> thiolactone ring. The major one, which was previously described,
results from a P450- and NADPH-dependent redox bioactivation of <b>2c</b> and leads to <b>3c</b>, two previously reported thiol
diastereomers bearing an exocyclic double bond. It occurs with NADPH-supplemented
human liver microsomes but not with human sera. The second one results
from a hydrolysis of <b>2c</b> and leads to an isomer of <b>3c</b>, <b>3c endo</b>, in which the double bond has migrated
from an exocyclic to an endocyclic position in the piperidine ring.
It occurs both with human liver microsomes and human sera, and does
not require NADPH. However, it requires Ca<sup>2+</sup> and is inhibited
by paraoxon, which suggests that it is catalyzed by a thioesterase
such as PON-1. Chemical experiments have confirmed that hydrolytic
opening of thiolactone <b>2c</b> exclusively leads to derivatives
of the endo thiol isomer <b>3c endo</b>