Targeted inhibition of tumor-cell growth by recombinant heregulin-toxin fusion proteins.

Fusion of functional domains of proteins by in vitro recombination of gene fragments can be used to generate novel anti-tumor agents. The combination of tumor-cell-recognition functions and toxic functions results in cytotoxic molecules with a high specificity for tumor cells. Human adenocarcinomas are frequently characterized by over-expression of members of the epidermal-growth-factor (EGF) receptor family (ErbB-1, 2, 3 and 4), when compared with normal cells. These tumors are particularly suited to treatment with recombinant toxins. The human heregulins (HRG) and their rat counterparts (neu differentiation factor, NDF) have been identified as ligands for these receptors. Two chimeric heregulin-toxin fusions consisting of the EGF-like receptor recognition domain of the heregulin isoforms HRG alpha and HRG beta I, and the domains II, Ib and III of the Pseudomonas exotoxin A (ETA) were constructed. HRG beta I-ETA is highly cytotoxic for the mammary carcinoma cell lines SK-BR-3 and MDA-MB-453. HRG alpha-ETA was less active than HRG beta I-ETA. The killing activity of the recombinant toxins correlated with the expression levels of ErbB-3 and/or ErbB-4 in the cell lines studied. High expression of ErbB-2 is not sufficient to confer sensitivity towards the HRG-ETA. Treatment of mice with 0.4 mg/kg/day of HRG beta I-ETA caused growth retardation of transplanted human breast tumor cells. Higher levels of HRG beta I-ETA administration resulted in acute hemorrhagic necrosis of the liver.