ADCs may undergo structural changes in circulation or in tissues. The antibody portion may experience proteolysis or amino acid modifications such as deamidation and oxidation, which is common to other protein therapeutics. The linker and payload moieties may undergo changes that directly affect safety and efficacy. For example, non-specific payload release may result in undesired toxicity. The released payload components from ADC are subject to small molecule metabolism by enzymes in the liver such as CYP450s. Another scenario is when linker-payload is still conjugated to the antibody, but has been modified. Typically, conjugated linkerpayload remain quite stable in circulation because ADCs reside in the extracellular environment and have limited exposure to metabolizing enzymes. However, biotransformation can still occur on the linker-payload. One reported example is maleimide ring hydrolysis commonly observed on cysteine conjugated ADCs. Once maleimide is hydrolyzed, the linkerpayload is no longer subject to elimination through retro-Michael reaction and thus prevents nonspecific deconjugation. Another reported biotransformation is hydrolysis on the payload structure. Other observed changes to linker-payload are cleavages or partial loss, such as hydrazone cleavage, peptide bond cleavage, carbamate cleavage, and disulfide cleavage, etc. In summary, the biotransformation of conjugated payload is very different from released payload or small molecules.