Simultaneous LC–MS/MS Quantification of four KRAS Inhibitors (RMC-9805, RMC-7977, RMC-6236, and MRTX1133) in Human Plasma: Towards Therapeutic Drug Monitoring in Pancreatic Ductal Adenocarcinoma

Purpose: Pancreatic ductal adenocarcinoma (PDAC) remains a highly lethal malignancy, with KRASG12D mutations present in approximately 40–45% of cases. Emerging RAS(ON) inhibitors, including RMC-6236, RMC-9805, RMC-7977, and MRTX1133, offer a promising precision medicine alternative to non-specific chemotherapy. However, except for MRTX1133, published methods for quantifying these compounds in human plasma or blood remain unavailable. Methods: We developed a liquid chromatography–tandem mass spectrometry (LC–MS/MS) method for simultaneous quantification of MRTX1133, RMC-6236, RMC-9805, and RMC-7977 in human plasma. Plasma proteins were precipitated with ethanol using reserpine, verapamil, and labetalol as internal standards. Before LC–MS/MS analysis, 4,4′-methylenebis(2,6-diethylaniline) was added as a volumetric standard. Volumetric and internal-standard quantification produced comparable results. Due to matrix effects, analytes should be quantified using calibration curves prepared in spiked plasma. Results: The method showed excellent linearity, with r2 > 0.991, across 0.1–50 ng per 0.1 mL plasma. The lower limit of quantification was 0.1 ng for RMC-6236, RMC-9805, and RMC-7977, and 0.02 ng for MRTX1133. Conclusions: This first simultaneous assay for multiple KRASG12D inhibitors in human plasma supports pharmacokinetic studies and biomarker-driven PDAC clinical trials.