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Vol. 31, Issue 5, 645-651, May 2003
Pharmaceutical Institute, Christian-Albrechts-University of Kiel,
Kiel, Germany
N-Hydroxylated amidines (amidoximes) can be used as
prodrugs of amidines. The prodrug principle was developed in our
laboratory for pentamidine and had been applied to several other drug
candidates. One of these compounds is melagatran, a novel,
synthetic, low molecular weight, direct thrombin inhibitor. To increase
the poor oral bioavailability due to its strong basic amidine
functionality selected to fit the arginine side pocket of thrombin, the
less basic N-hydroxylated amidine was used in addition
to an ethyl ester-protecting residue. The objective of this
investigation was to study the reduction and the hydrolytic metabolism
of ximelagatran via two mono-prodrugs
(N-hydroxy-melagatran and ethyl-melagatran) to
melagatran by in vitro experiments. New high-performance liquid chromatography methods were developed to analyze all four
compounds. The biotransformation of ximelagatran to melagatran
involving the reduction of the amidoxime function and the ester
cleavage could be demonstrated in vitro by microsomes and mitochondria from liver and kidney of pig and human, and the kinetic parameters were
determined. So far, one enzyme system capable of reducing N-hydroxylated structures has been identified in pig
liver microsomes, consisting of cytochrome
b5, NADH-cytochrome
b5 reductase, and a P450 isoenzyme of the
subfamily 2D. This enzyme system also reduces ximelagatran and
N-hydroxy-melagatran. The participation of recombinant
human CYP1A2, 2A6, 2C8, 2C9, 2C19, 2D6, and 3A4 with cytochrome
b5 and b5
reductase in the reduction can be excluded. In summary, ximelagatran
and N-hydroxy-melagatran are easily reduced by several
enzyme systems located in microsomes and mitochondria of different organs.
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