Antitumor effect of anti-epidermal growth factor receptor monoclonal antibodies plus cis-diamminedichloroplatinum on well established A431 cell xenografts.

We have explored the therapeutic effects of anti-epidermal growth factor receptor monoclonal antibodies (MAbs) 225 and 528 on well established A431 epidermoid carcinoma xenografts, approximately 400 mm3 (1 cm in diameter) at the start of treatment. In previous reports we demonstrated that MAbs 225 and 528 prevented the growth of A431 cell xenografts in nude mice when treatment was begun on the day of tumor cell inoculation. Since anti-epidermal growth factor receptor MAb therapy of well established tumors was unable to retard growth, we explored combination therapy with MAb plus the chemotherapeutic agent cis-diamminedichloroplatinum (cis-DDP). Additive and concentration-dependent growth-inhibitory effects of MAb with cis-DDP were observed in cultures of A431 cells. Neither intensive treatment with 225 MAb (1 mg/mouse, i.p. on day 8 after tumor inoculation, and twice weekly for 4 weeks) nor a maximally tolerated single dose of cis-DDP [150 micrograms/25 g (6 mg/kg) mouse weight, i.p. on day 8] had significant effects on tumor growth. However, the two treatments in combination resulted in substantial xenograft growth inhibition, compared with both an untreated control group and animals treated with a single modality. When a second dose of cis-DDP (150 micrograms/25 g) was added after 10 days, combination therapy with 225 MAb produced striking antitumor effects. At the end of 1 month tumor xenografts had disappeared in all but one mouse, and no tumor relapses occurred during 6 months of observation. Identical results were obtained with anti-epidermal growth factor receptor MAb 528 in combination with cis-DDP. The results of these studies provide a novel approach to the treatment of well established tumor xenografts, which may have application in the therapy of human malignancies.