Patent ID: 7989598

Claim:
A process of making a cell-binding agent maytansinoid conjugate, the process comprising: (a) modifying the cell-binding agent with a cross-linking agent, and (b) conjugating the modified cell-binding agent with a thiol-containing maytansinoid thereby providing a non-cleavable linker between the cell-binding agent and the thiol-containing maytansinoid to produce a conjugate wherein the non-cleavable linker is a linker that is substantially resistant to acid-induced cleavage, light-induced cleavage, peptidase-induced cleavage, esterase-induced cleavage, and disulfide bond cleavage, and wherein said cell binding agent is an antibody, a single chain antibody or an antibody fragment that specifically binds to a target cell, wherein the antibody, single chain antibody or antibody fragment comprises a human constant region, provided that the cell-binding agent is not an anti-erbB antibody.