Stability and cytotoxicity of Fab-ricin A immunotoxins prepared with watersoluble long chain heterobifunctional crosslinking agents

The effects of the hindered and non-hindered water soluble long-chain disul fide bonds on the stability and cytotoxicity of the ricin A chain (RTA) imm unotoxin were examined. The RTA immunotoxins were prepared with the Fab fra gments of anti-common acute lymphoblastic leukemia antigen (CALLA) monoclon al antibody (Fab-RTA) using sulfosuccinimidyl-6-[(-methyl(-(2-pyridyldithio )toluamido]hexanoate (S-LC-SMPT) and sulfosuccinimidyl-6-[3-(2-pyridyldithi o)-propionamido]hexanoate (S-LC-SPDP). The prepared Fab-RTA immunotoxins we re evaluated for their conjugation yield, immunoreactivity, thermal and dis ulfide bond stability and cytotoxicity. The conjugation yield of the Fab-RT A immunotoxin from the water soluble long chain crosslinking agents, S-LC-S MPT and S-LC-SPDP: were comparable. Both Fab-RTA immunotoxins exhibited a s imilar immunoreactivity and thermal stability in aqueous solution. However, S-LC-SMPT -mediated Fab-RTA, sterically hindered, showed an enhanced disul fide bond stability in vitro over S-LC-SPDP mediated one. In the cytotoxici ty against antigenic cell Daudi, the S-LC-SMPT -mediated RTA immunotoxin ma intained a comparable cytotoxicity, compared with S-LC-SPDP mediated Fab-RT A immunotoxin.