Characterization of in Vitro Biotransformation of New, Orally Active, Direct Thrombin Inhibitor Ximelagatran, an Amidoxime and Ester Prodrug

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Abstract
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 reducingN-hydroxylated structures has been identified in pig liver microsomes, consisting of cytochromeb5, NADH-cytochromeb5 reductase, and a P450 isoenzyme of the subfamily 2D. This enzyme system also reduces ximelagatran andN-hydroxy-melagatran. The participation of recombinant human CYP1A2, 2A6, 2C8, 2C9, 2C19, 2D6, and 3A4 with cytochromeb5 and b5reductase 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.