ABSTRACT
Semi-quantitative analysis of the drug-related components in biological samples collected during definitive metabolism studies using radiolabelled drug candidates is commonly achieved by HPLC profiling, using either on-line radiochemical detection or off-line liquid scintillation counting (LSC) following collection of the HPLC eluent into vials. However, although the use of LSC with vials has high sensitivity, the approach is time-consuming, laborious and destructive, whilst on-line detection methods are inappropriate for samples with low-levels of radioactivity (commonly the case with plasma samples). The use of 96-well microtitre plates (Scintiplates) for fraction collection during HPLC profiling provides a sensitive, effective and efficient alternative method for the semi-quantitative analysis of radiolabelled components in biological samples. Furthermore, the approach is non-destructive, such that subsequent identification of the isolated components can be achieved. Although the Scintiplate methodology is not appropriate for the analysis of excreta samples, where quenching of the radiochemical signal by endogenous components was observed, the approach was demonstrated to be valid for the relative quantification of [14C]-labelled material in plasma samples for all species investigated. In addition, good sensitivity was observed, with a counting efficiency of 79% for [14C], such that a drug-related component accounting for 10-15 dpm is quantifiable. The utility of the methodology for profiling circulating metabolites was demonstrated by the analysis of a rat plasma sample following oral administration of [14C]-UK-349,862. The Scintiplate approach and subsequent mass spectrometric analysis resulted in the relative quantitation and specific characterisation of circulating metabolites accounting for 93% of the total plasma radioactivity.
