Patent ID: 6884874

Claim:
An N-methyl-L-alanine containing maytansinoid conjugated to a cell-binding agent prepared by a process comprising (1) coupling a compound of Formula VII; where R 4 and R 5 are independently H, linear alkyl, branched alkyl, cyclic alkyl, aryl, substituted aryl, heterocyclic, or R 4 and R 5 taken together form a bond L is where R 1 is H, methyl or ethyl; M is a direct bond, CH 2 , a is 0 or an integer of 1 to 9 when M is a direct bond provided that when M is one or more carbon atoms a is 0 or an integer of 1 to 8; d is an integer of 3 to 8; and Q is H, linear alkyl, branched alkyl, cyclic alkyl, simple or substituted heterocyclic with N-methyl-L-alanine to form a racemization-free compound; (2) forming an N-methyl-L-alanine containing maytansinoid with the said racemization-free compound; and (3) linking said N-methyl-L-alanine containing maytansinoid to the cell-binding agent to form a racemization-free N-methyl-L-alanine containing maytansinoid conjugated to a cell-binding agent.