Practical strategies when using a stable isotope labeled microtracer for absolute bioavailability assessment: A case study of a high oral dose clinical candidate GDC-0810.

The pH labile metabolite, hydrophobicity, high oral dose and systematic exposure of GDC-0810 posed tremendous challenges to develop a LC-MS method for a stable isotope labeled aBA study. In this study, we explored practical solutions to balance stability and sensitivity and to cope with the impact of high Cp.o. to Ci.v. ratio on the labeling selection and assay dynamic range. A [13 C9 ] GDC-0810 was synthesized to minimize the isotopic interference between PO dose, internal standard and I.V. microtracer. A highly sensitive LC-MS assay was validated for quantitation of [13 C9 ] GDC-0810 from 5 to 1250 pg/mL. The optimized method was applied to a proof of concept cynomolgus monkey aBA study and the bioavailability calculated using microtracer dosing and regular dosing were similar to each other.