Process for the preparation of pegylated drug-linkers and intermediates thereof

A composition comprising Antibody Drug Conjugates represented by Formula 12 and Formula 12A, optionally in pharmaceutically acceptable salt form, having the structures of:

• respectively, wherein
• Ab is an antibody;
• S is a sulfur atom from the antibody;
• the Ab-S— moeity is attached to the carbon atom a or R to the carboxylic acid functional group;
• D is an auristatin Drug Unit;
• L1, L2 and L3, independently are selected from the group consisting of optionally substituted C1-C20 alkylene, optionally substituted C4-C20 heteroalkylene, optionally substituted C3-C8 carbocyclo, optionally substituted C6-C10 arylene, optionally substituted C5-C10 heteroarylene and optionally substituted C3-C8 heterocyclo;
• RC is a PEG Capping Unit;
• subscript n ranges from 2 to 24;
• and
• subscript p ranges from about 1 to about 16,
• wherein the composition contains no more than 10 wt. % Formula 12A Antibody Drug Conjugate, or
• the Formula 12 and Formula 12A Antibody drug Conjugates, optionally in pharmaceutically acceptable salt form, have the structures of:
• respectively, wherein the variable groups have their previous meanings and wherein the composition contains no more than 10 wt. % of Formula 12A Antibody Drug Conjugate.