ABSTRACT:The absorption, metabolism, and excretion of the oral direct thrombin inhibitor, ximelagatran, and its active form, melagatran, were separately investigated in rats, dogs, and healthy male human subjects after administration of oral and intravenous (i.v.) single doses. Ximelagatran was rapidly absorbed and metabolized following oral administration, with melagatran as the predominant compound in plasma. Two intermediates (ethyl-melagatran and OH-melagatran) that were subsequently metabolized to melagatran were also identified in plasma and were rapidly eliminated. Melagatran given i.v. had relatively low plasma clearance, small volume of distribution, and short elimination half-life. The oral absorption of melagatran was low and highly variable. It was primarily renally cleared, and the renal clearance agreed well with the glomerular filtration rate. Ximelagatran was extensively metabolized, and only trace amounts were renally excreted. Melagatran was the major compound in urine and feces after administration of ximelagatran. Appreciable quantities of ethyl-melagatran were also recovered in rat, dog, and human feces after oral administration, suggesting reduction of the hydroxyamidine group of ximelagatran in the gastrointestinal tract, as demonstrated when ximelagatran was incubated with feces homogenate. Polar metabolites in urine and feces (all species) accounted for a relatively small fraction of the dose. The bioavailability of melagatran following oral administration of ximelagatran was 5 to 10% in rats, 10 to 50% in dogs, and about 20% in humans, with low between-subject variation. The fraction of ximelagatran absorbed was at least 40 to 70% in all species. First-pass metabolism of ximelagatran with subsequent biliary excretion of the formed metabolites account for the lower bioavailability of melagatran.

After oral administration of ximelagatran to healthy male subjects, the pharmacokinetic and pharmacodynamic profile of melagatran is predictable and reproducible.

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Capillary microsampling (CMS) has recently been introduced as a response to the demands for more ethical use of laboratory animals according to the 3R principles. In CMS, an exact volume of the blood, plasma or other biofluid is collected in a capillary from which it is washed out, resulting in a diluted sample that can be handled using the existing equipment in the bioanalytical laboratory. CMS differs from traditional large volume sampling as the microsample is diluted before further handling and analysis, and reanalysis is performed using the diluted sample. This has some implications for the validation and this report is an attempt to clarify how to validate and use CMS methods in a regulatory environment. CMS also shows some distinct new opportunities: labile analytes can be immediately stabilized at sample collection and the addition of the internal standard to the whole sample can improve analytical performance. The experiences from 5 years use of CMS of plasma and blood for determination of drug exposure in animal studies are reviewed.

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Background-Sample stability is critical for accurate analysis of drug compounds in biosamples. The use of additives to eradicate the enzymatic activity causing loss of these analytes has its limitations.