Metabolism and disposition of eltrombopag, an oral, nonpeptide thrombopoietin receptor agonist, in healthy human subjects.

The metabolism and disposition of eltrombopag, the first-in-class small molecule human thrombopoietin receptor agonist, were studied in six healthy men after a single oral administration of a solution dose of [(14)C]eltrombopag (75 mg, 100 µCi). Eltrombopag was well tolerated. The drug was quickly absorbed and was the predominant circulating component in plasma (accounting for 63% of the total plasma radioactivity). A mono-oxygenation metabolite (M1) and acyl glucuronides (M2) of eltrombopag were minor circulating components. The predominant route of elimination of radioactivity was fecal (58.9%). Feces contained approximately 20% of dose as glutathione-related conjugates (M5, M6, and M7) and another 20% as unchanged eltrombopag. The glutathione conjugates were probably detoxification products of a p-imine methide intermediate formed by metabolism of M1, which arises through cytochrome P450-dependent processes. Low levels of covalently bound drug-related intermediates to plasma proteins, which could result from the reaction of the imine methide or acyl glucuronide conjugates with proteins, were detected. The bound material contributes to the longer plasma elimination half-life of radioactivity. Renal elimination of conjugates of hydrazine cleavage metabolites (mostly as M3 and M4) accounted for 31% of the radiodose, with no unchanged eltrombopag detected in urine.

A rapid method for quantitatively estimating metabolites in human plasma in the absence of synthetic standards using a combination of liquid chromatography/mass spectrometry and radiometric detection.

An approach to estimating the levels of drug-related metabolites in human plasma in the absence of synthesized chemical standards has been developed. High-performance liquid chromatography/mass spectrometry (LC/MS) in combination with radiometric detection was used in this method. Biologically derived [(14)C] metabolites from preclinical in vitro and in vivo matrices are used as [(14)C] metabolite standards and their concentrations in matrices are calculated based on the corresponding radioactivity. The amount of drug-related metabolites in human plasma samples can be estimated by determining relative MS responses of metabolites between plasma and [(14)C] metabolite standards, and using the calculated concentrations of metabolite standards as calibrants. An example for the estimation of metabolites in human plasma was used to demonstrate the utility of this methodology.