Antitumour activity of a melanoma-specific immunotoxin, ME20-LysPE40.

An immunotoxin conjugate has been prepared by linking an internalizing antibody with melanoma selectivity, ME20, with a binding-defective form of Pseudomonas exotoxin A, LysPE40. ME20-LysPE40 binds to a 105,000 Da cell-surface antigen present on melanoma cells (ME20-M) within twofold of unmodified ME20 and was cytotoxic to two human melanoma cell lines, H3606 and MALME-3M, with EC50 values of 100 and 200 pM, respectively. Immunotoxin treatment, initiated 1 day following subcutaneous implantation of H3606 melanoma cells into mice, prevented outgrowth of tumour xenografts in > 50% of the mice. In contrast, only a modest inhibition in tumour growth was observed if the immunotoxin was administered 5 days after implantation of in vivo passaged H3606 tumour fragments in mice. This study shows that the internalizing monoclonal antibody ME20 IgG can be used for targeting a toxin toward melanoma cells displaying the ME20-M antigen.

In vivo activities of acidic fibroblast growth factor-Pseudomonas exotoxin fusion proteins.

Fibroblast growth factor receptors are highly expressed in a variety of cancer cells and activated vasculature. Using chimeric toxins targeted to cell-surface a FGF receptors, we have demonstrated specific cytotoxic activity to these cell types. These molecules, aFGF-PE40 and aFGF-PE40 KDEL, are fusion proteins containing acidic FGF and either a 40- or a 66-kDa binding defective form of Pseudomonas exotoxin, respectively. Both aFGF-toxin fusion proteins were able to inhibit protein synthesis in vitro in a variety of carcinoma cell lines. The half-life of aFGF-PE40 in serum was found to be 41 min when coadministered with heparin. Administration of aFGF-PE40 or aFGF-PE4E KDEL with heparin inhibits the growth of established KB and preestablished A431 epidermoid carcinoma xenografts in athymic mice. The antitumor activities of the two aFGF-toxin fusion proteins were equivalent against the KB tumor xenografts. While we were able to slow the growth of the KB tumor xenografts, we were unable to cause tumor regressions. Histochemical analysis of treated versus untreated tumor tissue revealed a difference in tumor size but not of vascularity. We conclude that aFGF-PE40 and aFGF-PE4E KDEL have in vivo antitumor activity that targets the tumor cell mass rather than vascular structures in mice xenografted with human epidermoid carcinoma.

Antitumor activity of the single-chain immunotoxin BR96 sFv-PE40 against established breast and lung tumor xenografts.

We have constructed a single-chain immunotoxin consisting of the variable H and L chains of the carcinoma-reactive mAb BR96, fused to the binding defective protein toxin, PE40. This molecule, BR96 sFv-PE40, has been shown to be extremely cytotoxic toward a variety of BR96 Ag-expressing tumor cell lines. When administered i.v. into athymic mice carrying L2987 tumor xenografts, BR96 sFv-PE40 was cleared rapidly from the blood with a half-life of approximately 30 min. This is in comparison to a chemical conjugate, chiBR96-LysPE40, that remained in the blood for almost 2 h. In addition, the smaller single-chain immunotoxin (67 kDa) penetrates the tumor faster than the larger chemical conjugate (190 kDa). Using a variety of administration schedules and doses, we treated established human tumor xenografts in athymic mice with both the single-chain immunotoxin BR96 sFv-PE40 and the chemical conjugate chiBR96-LysPE40. In both L2987 lung carcinoma and MCF-7 breast carcinoma models, we found that BR96 sFv-PE40 completely regressed the tumor xenografts. With an administration schedule of q4dx5, the tumors were totally regressed and did not reappear. The chiBR96-LysPE40 conjugate produced partial tumor regressions, although at near maximum tolerated dose. These results show that the single-chain immunotoxin, BR96 sFv-PE40, is a potent antitumor agent.