Used as a labeled internal standard in mass spectrometry-based assays for quantifying metabolites of certain antiviral or chemotherapeutic agents. Its deuterated structure allows for accurate differentiation from non-labeled compounds in biological matrices, enhancing measurement precision. Commonly applied in pharmacokinetic studies to monitor drug metabolism and clearance, especially in research involving pyrimidine analogs. The aldehyde and acetyl functional groups enable derivatization for improved detection sensitivity.