EVALUATION OF CELL-SURFACE ANTIGENS AS POTENTIAL TARGETS FOR RECOMBINANT TUMOR TOXINS

Tumor toxins are recombinant polypeptides derived from the fusion of a single-chain antibody domain or a ligand specific for a cell surface receptor with the enzymatic domain of a bacterial or plant toxin. If t hese receptors are preferentially expressed on the surface of tumor ce lls, the tumor toxins can serve as efficient therapeutics. Examples in which growth factors, cytokines, hormones or antibody domains have be en fused to toxins have been described and the bacterially expressed m olecules have shown anti-tumor effects in vitro and in vivo. Since man y tumors are not sensitive to the existing tumor toxins, the search co ntinues for new antigens with tumor specific or tumor enhanced express ion. We explored three antigenic structures for their possible use as targets for tumor toxins, i.e. (st) a glycosphingolipid with an Le(a) antigenic structure which is overexpressed in gastrointestinal carcino mas, e.g, colon carcinoma, (b) the epithelial glycoprotein episialin w hich shows altered expression in various tumor tissues such as breast, ovary and pancreas and (c) the neurotensin receptor which is found on NSLC, pancreatic and colonic cancer cells. For targeting the tumor to xin to the carbohydrate structures, we constructed recombinant single- chain antibody domains (scFv) fused to a truncated form of the exotoxi n A from Pseudomonas aeruginosa (ETA). In the case of the neurotensin receptor, the ligand neurotensin was exploited to construct a recombin ant fusion protein with exotoxin A as the effector domain. Binding exp eriments showed that the recombinant proteins are able to specifically recognize their respective target structures. In vitro cytotoxicity a ssays with neurotensin-ETA showed specific killing activity for recept or positive tumor cell lines. For the scFv-ETA constructs we could not observe in vitro tumor toxicity. We conclude that the antigenic targe ts for tumor toxins have to be carefully chosen and that specific cell binding activity is not a sufficient criterion for tumor cell killing . The neurotensin-ETA, a new anti-tumor agent, shows that seven transm embrane spanning receptors are potential targets which allow toxin bin ding and promote cell killing through the appropriate internalization. (C) 1997 Elsevier Science Ireland Ltd.