Selective killing of squamous carcinoma cells by an immunotoxin that recognizes the EGF receptor

We have conjugated a murine monoclonal antibody (B4G7) against the human epidermal growth factor (EGF) receptor to gelonin, a 60S ribosome inactivating protein, via N-succinimidyl-3-(2-pyridyldithio)propionate (SPDP) and 2-iminothiolane. The B4G7-gelonin conjugate bound to the cell surface in proportion to the number of EGF receptors and competed with B4G7 antibody for binding to EGF receptors. The conjugate killed EGF receptor-hyperproducing squamous carcinoma cells (A431, NA, Ca9-22, TE5), and to some extent, human fibroblasts (HFO). It did not kill EGF receptor-deficient small-cell lung cancer cells (H69) and mouse fibroblasts (Swiss/3T3). Free B4G7, gelonin or a mixture of B4G7 and gelonin did not kill A431 cells. The number of EGF receptors was correlated to cytotoxicity at 10(-8) M of the conjugate, and the data were fitted to the regression equation: y = -35.83 log x +233.4 (correlation coefficient = -0.9995). These results suggest that the B4G7-gelonin conjugate may be a useful weapon for targeting therapy to squamous-cell carcinomas.