Charged linkers and their uses for conjugation

A compound of formula (III):

• wherein Cb represents a cell-binding agent which is an antibody, or a fragment thereof;
• n is an integer from 1 to 20;
• and
• the compound of formula (III) is formed by reacting a functional group on the cell-binding agent with Y in a compound of formula (I):
• wherein Z is a thiol, disulfide, unsubstituted or substituted pyridyldisulfide, unsubstituted or substituted amino, —ONH2, carboxy, aldehyde, ketone, azide, unsubstituted or substituted maleimido, haloacetyl, halogen, hydrazine, hydrazide or hydroxy group;
• M is H, Na, K, N+R1R2R3 or a pharmaceutical salt thereof;
• R1 and R2 are the same or different and are a linear alkyl having from 1-6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, linear, branched or cyclic alkenyl or alkynyl having from 2 to 6 carbon atoms, polyethyleneoxy unit of formula (OCH2CH2)p, wherein p is an integer from 1 to about 1000, or a combination thereof;
• R3 is absent, a linear alkyl having from 1 to 6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, a linear, branched or cyclic alkenyl or alkynyl having from 2 to 6 carbon atoms, polyethyleneoxy unit of formula (OCH2CH2)p, wherein p is an integer from 1 to about 1000, or a combination thereof;
• or R1, R2 and R3 are respectively a chain of atoms selected from the group consisting of C, N, O, S, Si, and P, and R1, R2 and R3 optionally covalently connect among each other;
• R4, and R5, are the same or different and are absent, H, a linear alkyl having from 1 to 6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, linear, branched or cyclic alkenyl or alkynyl having from 2 to 6 carbon atoms, polyethyleneoxy unit of formula (OCH2CH2)p-1OCH2CH3, wherein p-1 is an integer from 1 to about 1000, or a combination thereof;
• and
• Y is an unsubstituted or substituted N-hydroxysuccinimide ester, hydrazide, p-nitrophenyl ester, dinitrophenyl ester, pentafluorophenyl ester, pyridyldisulfide, nitropyridyldisulfide, unsubstituted or substituted maleimido, β-maleimidopropionamido, haloacetate, or carboxylic acid halide.