Epidermal growth factor receptor-dependent cytotoxicity for human squamous carcinoma cell lines of a conjugate composed of human EGF and RNase 1

Recombinant human ribonuclease 1 (RNase 1) was chemically linked to recombinant human epidermal growth factor (EGF). The EGF-RNase conjugate showed dose-dependent cytotoxicity for EGF receptor-overexpressing A431 and TE-8 human squamous carcinoma cells with an IC₅₀ of 2 x 10^-7 M and 10^-6 M, respectively, whereas the IC₅₀ of RNase alone was almost 10^-4M. An unconjugated mixture of EGF and RNase had no greater effect than RNase alone. The conjugate showed no detectable cytotoxicity against EGF receptor-deficient small cell lung cancer cells (H69). Addition to excess EGF in the medium protected A431 cells from the EGF-RNase conjugate cytotoxicity. The cytotoxicity of the EGF-RNase conjugate was positively correlated with the EGF receptor numbers of each cell line. The chimeric toxin composed of only human proteins might be a more useful anti-cancer agent with less immunogenicity than the conventional chimeric toxins.