Erlotinib-d6 is primarily used in research and development as an internal standard in analytical studies. It is particularly valuable in mass spectrometry and chromatography for the precise quantification of erlotinib in biological samples. By serving as a labeled reference compound, it helps ensure accuracy and reliability in pharmacokinetic and metabolic studies. This aids in understanding the drug's behavior in the body, optimizing dosing regimens, and assessing its efficacy and safety in clinical trials. Its application is critical in advancing cancer research, particularly in targeted therapies for non-small cell lung cancer and pancreatic cancer.