Bioanalytical Methods
The concentrations of giredestrant in plasma were measured using validated LC/MS-MS assays. Giredestrant and its internal standard [(13C8, 15N) GDC 9545] were extracted from human plasma by supported liquid extraction (SLE). Giredestrant concentrations were calculated using a standard curve with a 1/Concentrations of 4β-HC were measured using an LC/MS-MS assay that was developed and validated in human plasma. 4β-HC and its internal standard 4β-HC-d7 were extracted from human plasma by SLE. 4β-HC concentrations were calculated using a standard curve with a 1/For plasma, urine, and 4β-HC assays, the mass spectrometer was operated in positive electrospray ionization mode under optimized conditions with multiple reaction monitoring of analytes and internal standards. The precision and accuracy of assays were satisfactory throughout the study.For metabolite identification, plasma samples were extracted by protein precipitation. Parent and metabolites were analyzed by LC and high-resolution MS. The metabolite structures were proposed on the basis of fragmentation patterns and comparison with those of the parent.