Insulin-like growth factor I receptor antagonism augments response to chemoradiation therapy in colon cancer cells

Background. Colorectal cancer remains one of the most prevalent malignancies in the United States. Improvement in local disease control is seen when 5-fluorouracil (5-FU) is used in combination with pelvic irradiation for rectal adenocarcinoma. The frequent overexpression of insulin-like growth factor I receptor (IGF-I-R) in rectal adenocarcinoma suggests that inhibition of the signal transduction pathway may be a novel approach to enhance tumor response. This investigation seeks to define the role of IGF-I-R antagonism, using monoclonal antibody alpha -IR3, in augmenting cytotoxicity to adjuvant chemoradiation therapy for adenocarcinoma of the rectum. Materials and methods. SW 480 colon cancer cells were cultured to semiconfluent conditions with dose titrations performed for 5-FU to determine that the IC50 (inhibitory concentration of 50% of the cells) was 0.5 mug/ml The IC50 for 5-FU was reassessed in the presence of IGF-I, Experimental groups included colon cancer cells combined with 5-FU; 6-MeV external beam radiation (100-500 cGy); and alpha -IR-3. Results. The addition of 100 ng/ml IGF-I Ih prior to 5-FU or radiation significantly blunted the expected cytotoxicity, resulting in a 10-fold increase in the IC50 (from 0.5 to 5 mug/ml). Receptor antagonism using the monoclonal antibody alpha -IR-3 (100-400 ng/ml) produced a dose-dependent increase in cytotoxicity compared with 5-FU alone. The addition of radiation produced synergistic amplification of this response. Conclusions. IGF-I-R activation blocks the expected cytotoxic effects of 5-FU and external beam radiation. Receptor antagonism increased the cytotoxic response of chemoradiation therapy. These data suggest the utility of inhibiting IGF-I-R signal transduction in the treatment of rectal adenocarcinoma. (C) 2000 Academic Press.