Insights Influencing the Selection of Stable Isotopically Labeled Internal Standards (SIL‐ISs) for LC–MS/MS Quantitative Assays

Validated assays are vital to support the accurate reporting of pharmacokinetics for parent drug/metabolite(s), regardless of the stage of drug development. One of the recent nuances in the field has focused on the selection of an appropriate stable isotopically labeled internal standard (SIL-IS) with a sufficient mass difference from the target analyte as a panacea to render unambiguous quantitation of the target analyte(s) in liquid chromatography-tandem mass spectrometry methods (LC-MS/MS). Although the intent of using SIL-IS is well supported, there are potential issues that may arise because of cross-signal contributions due to various factors, which could potentially lead to nonlinearity in the standard curves and further contribute to wider assay biases. This perspective article presents the available knowledge in the field pertaining to potential issues with the use of SIL-IS and provides insights/commentary on these situations, as a means to both understand and manage such issues during the development of assays.