TARGETING TUMOR-CELLS VIA EGF RECEPTORS - SELECTIVE TOXICITY OF AN HBEGF-TOXIN FUSION PROTEIN

Over-expression of the epidermal growth factor receptor (EGFR) is a ha llmark of numerous solid tumors, thus providing a means of selectively targeting therapeutic agents, Heparin-binding epidermal growth factor (HBEGF) binds to EGFRs with high affinity and to heparan sulfate prot eoglycans, resulting in increased mitogenic potential compared to othe r EGF family members. We have investigated the feasibility of using HB EGF to selectively deliver a cytotoxic protein into EGFR-expressing tu mor cells. Recombinant fusion proteins consisting of mature human HBEG F fused to the plant ribosome-inactivating protein saporin (SAP) were expressed in Escherichia coil. Purified HBEGF-SAP chimeras inhibited p rotein synthesis in a cell-free assay and competed with EGF for bindin g to receptors on intact cells. A construct with a 22 amino-acid flexi ble linker (L-22) between the HBEGF and SAP moieties exhibited an affi nity for the EGFR that was comparable to that of HBEGF. The sensitivit y to HBEGF-L-22-SAP was determined for a variety of human tumor cell l ines, including the 60 cell lines comprising the National Cancer Insti tute Anticancer Drug Screen. HBEGF-L-22-SAP was cytotoxic in vitro to a variety of EGFR-bearing cell lines and inhibited growth of EGFR-over -expressing human breast carcinoma cells in vivo. In contrast, the fus ion protein had no effect on small-cell lung carcinoma cells, which ar e EGFR-deficient. Our results demonstrate that fusion proteins compose d of HBEGF and SAP exhibit targeting specificity and cytotoxicity that may be of therapeutic value in treating a variety of EGFR bearing mal ignancies. (C) 1998 Wiley-Liss, Inc.