Used primarily as an internal standard in mass spectrometry-based bioanalytical assays to accurately quantify N-Acetyl Mesalazine in biological samples such as plasma, serum, or urine. Its deuterated structure allows for reliable differentiation from the non-labeled compound during chromatographic separation, ensuring high precision in pharmacokinetic and metabolic studies. Commonly applied in clinical research and drug development for inflammatory bowel disease (IBD), where Mesalazine (5-aminosalicylic acid) is a key therapeutic agent. Enables consistent measurement of metabolite levels, supporting dose optimization and therapeutic drug monitoring.