The MUC1 oncoprotein as a functional target: Immunotoxin binding to α/β junction mediates cell killing

MUC1, a heavily glycosylated mucin, has generated considerable interest as a target for tumor killing because of its overexpression in malignancies. Full‐length MUC1 (MUC1/TM) is proteolytically cleaved after synthesis generating α and β subunits, which specifically bind in a noncovalent interaction. Although the β chain remains on the cell surface, the α chain binds in an on‐and‐off interaction. Most anti‐MUC1 antibodies (Abs) described to date recognize epitopes within the highly immunogenic α‐chain tandem repeat. Because the α‐chain is shed, such Abs are sequestered and fail to reach MUC1‐expressing cells. Immunizing with cDNA encoding MUC1/TM and the spliced MUC1/X isoform from which the tandem repeat has been deleted yielded antibodies to the MUC1 α/β junction. Pseudomonas toxin PE38 linked to polyclonal anti‐MUC1 α/β junction Abs both bound and killed MUC1‐positive malignant cells. Monoclonal DMC209 binds the MUC1 α/β junction in both MUC1/X and MUC1/TM. When injected into SCID mice xenotransplanted with human breast cancer MDA‐MB‐231, monoclonal DMC209 showed significant in vivo tumor‐suppressive activity. The MUC1/X α/β junction presents a biologically‐significant target in MUC1‐expressing malignancies because (i) antibodies directed against cell‐bound α/β junction epitopes reach the intended cellular target, (ii) antibodies to junction epitope are internalized into cells, (iii) anti α/β junction antibodies can effectively kill high MUC1‐expressing cancer cells as antibody‐toxin conjugates and (iv) antibodies targeting the MUC1 cell‐bound α/β junction results in tumor suppression in vivo. Our results indicate that cell‐bound MUC1 α/β junction, unlike shed alpha chain, represents a highly effective moiety for targeting and killing MUC1‐expressing malignancies. © 2008 Wiley‐Liss, Inc.