Description SMPT is a heterobifunctional sulfhydryl- and amine-reactive cross-linker. SMPT is used often for conjugating a toxin molecule to a monoclonal antibody directed against a cell-surface antigen. These immunotoxins produced with SMPT are highly potent as the cleavable disulfide imparts increased cytotoxicity compared to conjugates without a cleavable bond, such as those produced with SPDP. Also, next to the pyridine-2-thione in the spacer arm are a benzene ring and a methyl group adjacent to a carbon that hinders the disulfide bond, allowing exceptional conjugate stability in vivo. SMPT has the added benefit of being more stable to hydrolysis in aqueous solutions than other NHS cross-linkers.