Switching from an analogous to a stable isotopically labeled internal standard for the LC‐MS/MS quantitation of the novel anticancer drug Kahalalide F significantly improves assay performance

The importance of a stable isotopically labeled (SIL) internal standard for the quantitative LC-MS/MS assay for Kahalalide F in human plasma is highlighted. Similar results can be expected for other LC-MS/MS assays. Therefore, we emphasize the need for an SIL internal standard for accurate and precise LC-MS/MS assays of drugs in biological matrices.