Blocking effect of human serum but not of cerebrospinal fluid on ricin A chain immunotoxin potentiation by monensin or carrier protein-monensin conjugates.
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 (e.g., 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 Mr 40,000-90,000 protein fraction. Serum components of low molecular weight (less than 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 micrograms/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.
Candiani
C
C
Istituto di Scienze Immunologiche, University of Verona, Italy.
Franceschi
A
A
Chignola
R
R
Pasti
M
M
Anselmi
C
C
Benoni
G
G
Tridente
G
G
Colombatti
M
M
D000643:Ammonium Chloride / Q000494:pharmacology; D000818:Animals; D001790:Blood Physiological Phenomena; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects*; D002555:Cerebrospinal Fluid / Q000502:physiology*; D003432:Cross-Linking Reagents; D004220:Disulfides / Q000378:metabolism; D004229:Dithiothreitol / Q000494:pharmacology; D004357:Drug Synergism; D006801:Humans; D000922:Immunotoxins / Q000633:toxicity*; D007700:Kinetics; D008985:Monensin / Q000494:pharmacology* / Q000633:toxicity; D051381:Rats; D012276:Ricin / Q000633:toxicity*; D012709:Serum Albumin / Q000494:pharmacology* / Q000633:toxicity; D013388:Succinimides
D003432:Cross-Linking Reagents; D004220:Disulfides; D000922:Immunotoxins; D012709:Serum Albumin; D013388:Succinimides; D000643:Ammonium Chloride; C018151:N-succinimidyl 3-(2-pyridyldithio)propionate; D012276:Ricin; D008985:Monensin; D004229:Dithiothreitol