Saquinavir-d9 is primarily used in research and analytical settings as an internal standard or reference compound for the quantification and analysis of saquinavir, a protease inhibitor used in the treatment of HIV. Its deuterated form provides a stable and isotopically labeled counterpart that aids in the precise measurement of saquinavir levels in biological samples, such as blood or plasma, using techniques like liquid chromatography-mass spectrometry (LC-MS). This application is crucial for pharmacokinetic studies, therapeutic drug monitoring, and ensuring accurate dosing in clinical settings. Additionally, it helps in understanding drug metabolism and interactions, contributing to the optimization of HIV treatment regimens.