Assay of ezlopitant, a substance P receptor antagonist, and metabolites in biological matrices by gas chromatography with mass spectrometric detection: simultaneous analysis of a benzyl alcohol and alkene

A method for the analysis of the substance P antagonist ezlopitant and two active metabolites in serum using solid-phase extraction followed by GC–MS analysis is described. The linear dynamic range was 1.0 to 100 ng/ml and precision and accuracy over this range were within 15%. Upon injection of reconstituted sample extracts into the hot injector port of the gas chromatograph, the benzyl alcohol metabolite undergoes a small amount of spontaneous dehydration to the alkene metabolite. We have incorporated an additional hexadeuterated internal standard of the benzyl alcohol into the assay to permit measurement of the extent of dehydration in each sample. This novel approach should be generally applicable to the simultaneous determination of benzyl alcohols and corresponding alkenes by GC–MS when the possibility exists that the alcohol can undergo spontaneous dehydration to the alkene in the injector port of GC instrumentation.