A simple liquid chromatography-tandem mass spectrometry method to determine relative plasma exposures of drug metabolites across species for metabolite safety assessments.

Recent regulatory guidance suggests that metabolites identified in human plasma should be present at equal or greater levels in one of the animal species used in safety assessments. In this report, a high-performance liquid chromatography-tandem mass spectrometry method is described whereby quantitative comparisons of exposures to metabolites between species can be obtained in the absence of authentic standards of the metabolites, calibration curves, and other attributes of standard bioanalytical methods. This novel method was tested using six drug-metabolite combinations. Plasma samples from animals are mixed with control plasma from humans and vice versa to remove possible differential effects of matrices. Through multiple ion monitoring-triggered enhanced product ion (EPI) scans, all metabolites were qualitatively confirmed, and daughter ions were selected for the most sensitive mass transitions to trigger EPI scans. Direct comparisons of metabolites in animal versus human plasma were achieved by calculating the peak area ratios of the metabolites versus an internal standard. Linearity of instrument responses was established by serial dilution. A statistical analysis demonstrated that experimentally measured ratios of the parent and metabolites in rat versus human correlated well with the nominal ratios of concentrations using linear regression with an average slope of 0.99 ± 0.08 (r = 0.994 ± 0.005). This analysis showed that if the experimentally determined ratio of mass spectrometer responses is ≥ 2.0, then the actual exposure ratio is unity or greater (p < 0.01). This method offers time- and resource-sparing advantages to ascertaining metabolite exposure comparisons between humans and laboratory animal species. A strategy for application of this approach within standard drug development processes is described.