The potentiation of monoclonal antibody/ligand toxin (immunotoxin) cytotoxicity by the ionophore monensin (Mo) or by human serum albumin-monensin (HSA-Mo) conjugates was investigated. Since disulfide cross-linked HSA-Mo (HSA-SPDP-Mo) is rapidly inactivated by human serum (M. Colombatti et al., Cancer Res., 50:1385-1391, 1990), we synthesized thioether cross-linked HSA-Mo conjugates (HSA-SIA-Mo). HSA-SIA-Mo is resistant to treatment with reducing agents (eg., glutathione, dithiothreitol) and shows potentiating activity identical to that of Mo or of HSA-SPDP-Mo, enhancing immunotoxin (IT) cytotoxicity 45-35,000-fold. Human leukemic and tumor cell lines are highly sensitive to treatment with IT in combination with Mo, HSA-SPDP-Mo, or HSA-SIA-Mo (concentration required to inhibit protein synthesis by 50%, 10(-10)-2.5 x 10(-13) M). IT potentiation by both types of HSA-Mo conjugates, however, is inhibited by whole human serum. In contrast, human cerebrospinal fluid has no effect on the potentiation of IT by Mo or HSA-Mo conjugates. The serum blocking factors reside mostly in a M(r) 40,000-90,000 protein fraction. Serum components of low molecular weight (< 10,000) show no detectable effect upon the stability of HSA-Mo conjugates. The toxicity of HSA-SIA-Mo in vivo was investigated by intrathecal injections in rats. Concentrations of up to 60-mu-g/kg can be injected into the brain with only transient neurological sequelae. We therefore conclude that if the systemic delivery of HSA-Mo conjugates for the potentiation of ricin A chain-IT presents some limitations due to the blocking effect of serum, the application of HSA-Mo conjugates in combination with ricin A chain-IT for regional tumor therapy in the brain appears more promising.
