Carrier Protein-Monensin Conjugates: Enhancement of Immunotoxin Cytotoxicity and Potential in Tumor Treatment1

We have investigated the potentiation of transferrin |Tfn]-toxin |Tfnricin toxin A chain (RTA) and Tfn-So6 saporin toxin) and monoclonal antibody-RTA conjugates by monensin (Mo) and by a human serum albumin (HSA)-monensin conjugate in vitro. The in vivo survival and in vitro and in vivo toxicity of HSA-Mo were also studied; monensin was chemically linked to HSA carrier protein via a disulfide bridge. HSAMo was 2-13-fold less toxic than Mo for cells in vitro. HSA-Mo was active in the same concentration range as Mo in potentiating mAb-RTA and Tfn-toxin conjugates reactive with Tfn receptors expressed by differ ent cell lines in monolayer cell cultures. Multiceli tumor spheroid cultures were used to investigate the target cell killing effect of cytotoxic conju gates and HSA-Mo in three-dimensional structures mimicking the prop erties of nonvascularized micrometastases. Spheroids 300-400 urn were as sensitive to Tfn-RTA and HSA-Mo in combination as monolayer cells. After 24 h incubation at 37°Cin human serum about 2% HSA-Mo molecules remained available for immunotoxin potentiation and about 10% after 24 h incubation in human cerebrospinal fluid. BALB/c mice tolerated injections of 2 mg/kg HSA-Mo i.v. and of 16 mg/kg i.p. The HSA-Mo half-life in the serum of BALB/c mice was 0.5 h. Following i.v. injection about 0.5% of the initial HSA-Mo persisted in the circulation