DM4-SPDP is a drug-linker conjugate composed of a potent antitubulin agent DM4 and a linker SMCC to make antibody drug conjugate. SPDP is a short-chain crosslinker for amine-to-sulfhydryl conjugation via NHS-ester and pyridyldithiol reactive groups that form cleavable (reducible) disulfide bonds with cysteine sulfhydryls.