Used as a stable isotope-labeled internal standard in mass spectrometry-based bioanalytical studies to accurately quantify erlotinib in biological samples such as plasma, serum, or tissue extracts. Its primary application is in pharmacokinetic and metabolism studies during drug development and clinical research. The deuterium labeling ensures minimal interference with the native compound while allowing precise measurement due to distinct mass detection. Commonly employed in LC-MS/MS workflows for therapeutic drug monitoring and drug interaction studies.