Process for preparing purified drug conjugates

A process for preparing a cell-binding agent-cytotoxic agent conjugate comprising the steps of:

• (a) contacting a cell-binding agent with a bifunctional crosslinking reagent to covalently attach a linker to the cell-binding agent and thereby prepare a first mixture comprising cell-binding agents having linkers bound thereto,
• (b) subjecting the first mixture to tangential flow filtration, selective precipitation, adsorptive filtration, or adsorptive chromatography and thereby prepare a purified first mixture of cell-binding agents having linkers bound thereto,
• (c) conjugating a cytotoxic agent to the cell-binding agents having linkers bound thereto in the purified first mixture by reacting the cell-binding agents having linkers bound thereto with a cytotoxic agent in a solution having a pH of about 6.5 to prepare a second mixture comprising (i) cell-binding agent chemically coupled through the linker to the cytotoxic agent, (ii) free cytotoxic agent, and (iii) reaction by-products, and
• (d) subjecting the second mixture to selective precipitation, adsorptive filtration, or adsorptive chromatography to purify the cell-binding agents chemically coupled through the linkers to the cytotoxic agent from the other components of the second mixture and thereby prepare a purified second mixture of cell-binding agents chemically coupled through the linkers to the cytotoxic agent,
• with the proviso that if the first mixture is subjected to tangential flow filtration in step (b), the second mixture is not subjected to adsorptive chromatography in step (d).