Ado-trastuzumab emtansine (T-DM1) is an antibody-drug conjugate comprising trastuzumab conjugated via a stable thioether linker to DM1, a highly potent cytotoxic agent. A population pharmacokinetics (PK) analysis was performed to characterize T-DM1 PK and evaluate the impact of patient characteristics on T-DM1 PK in previously treated patients with HER2-positive advanced gastric cancer (AGC).