Disulfur bridge linkers for conjugation of a cell-binding molecule

A compound of Formula (III):

• wherein:
• Cb represents an antibody;
• n is 1-20;
• Z1 and Z2 are the same or different and represent a thiol;
• disulfide;
• amino;
• carboxy;
• aldehyde;
• ketone;
• maleimido;
• haloacetyl;
• hydrazine;
• alkoxyamino;
• hydroxy group;
• carboxy halide;
• N-hydroxysuccinimide ester;
• p-nitrophenyl ester;
• dinitrophenyl ester;
• pentafluorophenyl ester;
• ester formed with tetrafluorophenol, difluorophenol, monofluorophenol, pentachlorophenol, triflate, imidazole, dichlorophenol, tetrachlorophenol, 1-hydroxybenzotriazole, tosylate, mesylate, or 2-ethyl-5-phenylisoxazolium-3′-sulfonate;
• or anhydride;
• R1, R2, R3 and R4 are the same or different, and are absent, a linear alkyl having from 1 to 8 carbon atoms, branched or cyclic alkyl having from 3 to 8 carbon atoms, linear, branched or cyclic alkenyl or alkynyl having from 2 to 8 carbon atoms, an ester, ether or amide having 2 to 8 carbon atoms, a polyethyleneoxy unit of formula (OCH2CH2)p, wherein p is an integer from 1 to about 1000, 6-maleimidocaproyl, carbonyl, sulfonyl, thioether, azide, amine, imine, polyamine, hydrazine, hydrazone, urea, semicarbazide, carbazide, alkoxylamine, urethane, amino acid, peptide having 1 to 20 natural or unnatural amino acids, acyloxylamine, hydroxamic acid, or combination thereof, or a combination thereof with a disulfide, hydrozone, triazole, or alkoxime unit.