Validation of an LC-MS/MS method for the quantitative determination of the orexin receptor antagonist almorexant and its four primary metabolites in human plasma.

A sensitive and selective LC-MS/MS method has been developed to quantify almorexant and its four primary metabolites M3, M5, M6, and M8 in human plasma samples. The method involved protein precipitation with acetonitrile in the high calibration range and liquid/liquid extraction with ethyl acetate in the low calibration range. Labeled internal standards were available for four analytes.