Abstract
An isotope-dilution bioanalytical assay for abemaciclib and its metabolites in combination with olaparib was developed and validated in human plasma K2 EDTA. For the quantitative assay, human plasma samples (or human plasma QC samples) were spiked with internal standard solution before a simple protein precipitation with methanol. The extract was injected onto a liquid chromatography-tandem mass spectrometry (LC-MS/MS) instrument where it was chromatographically separated by a polar end-capped reversed phase column and guard using gradient elution with water and methanol both modified with 0.2 % formic acid (v/v) as the mobile phases. The analytes and internal standards were measured by heated electrospray ionization (HESI) in positive polarity using selected reaction monitoring (SRM) on a triple quadrupole mass spectrometer. The assay was validated for linear ranges as follows: 0.4 - 1000 nM abemaciclib, 0.35 - 1000 nM M2 and M18, 0.5 - 1000 nM M20, and 0.75 - 1000 nM olaparib. The inter-day or between day precision for the quality controls (n = 18) was < 13 % and the accuracy was ± 12 %, for all analytes, including the lower limit of quantification (LLOQ). The intra-day or within day precision for the quality controls (n = 6) was ≤ 11 % and the accuracy was ± 12 % for low, mid, and high and < 19 % at LLOQ. The recovery in human plasma was determined to be between 92 % and 102 % for all analytes spanning the linear range. The validated, bioanalytical quantitative assay was designed to measure abemaciclib, its metabolites, and olaparib for pharmacokinetic evaluation of patients in clinical trials for breast, brain, and ovarian cancers.