Used as a stable isotope-labeled internal standard in mass spectrometry and bioanalytical assays for quantifying N-Acetyl-L-leucine-N-methylamide and related metabolites in biological samples. Its deuterated structure (d16) ensures accurate measurement by minimizing matrix effects and ion suppression. Commonly applied in pharmacokinetic and metabolic studies, especially in research involving neurodegenerative diseases where N-Acetyl-L-leucine derivatives have therapeutic potential. The labeled compound allows precise tracking and differentiation from endogenous non-labeled analytes in complex matrices such as plasma, cerebrospinal fluid, and tissue extracts.