Human pancreatic RNase1-human epidermal growth factor fusion: An entirely human 'immunotoxin analog' with cytotoxic properties against squamous cell carcinomas

The gene encoding human pancreatic ribonuclease 1 (hpRNase1) was fused with a gene encoding human epidermal growth factor (hEGF). The hybrid human protein was isolated from Escherichia coli inclusion bodies, refolded and purified to homogeneity. The fusion protein competed with ¹²⁵I-hEGF for binding to hEGF receptors (EGFR) and had ribonucleolytic activities approaching those of hpRNase1. Several conformations having different enzymatic activities could be detected after reversed-phase high-performance liquid chromatographic analysis, the less hydrophobic molecules being the most active. The hybrid protein was specifically cytotoxic to A431, an EGFR overexpressing squamous carcinoma cell line, with an IC₅₀ of ~10^-7 M. In contrast, recombinant hpRNase1 had an IC₅₀ higher than 10^-4 M. A mixture of free hEGF and free hpRNase1 was not more cytotoxic than hpRNase1 alone and no cytotoxicity was detected in EGFR-deficient control cells. Taken together, these data suggest that this construct might be useful for targeted therapy of esophageal, lung and other squamous cell carcinomas and also breast cancers overexpressing EGFR, which correlate with a poor prognosis and cannot be cured by surgery alone. Engineering hybrid molecules with endogenous human proteins for targeted therapy may alleviate the dose-limiting immunogenicity and toxicity of conventional immunotoxins.