COMBINED RADIOIMMUNOTHERAPY AND RADIOTHERAPY OF HUMAN COLON-CARCINOMAGRAFTED IN NUDE-MICE

The effect of combined radioimmunotherapy (RIT) and fractionated exter nal beam radiotherapy (RT) was assessed in two human colon cancer xeno grafts, Co112 and LS174T in nude mice, These tumors were selected for being resistant to RIT alone, as is usually the case in the clinical s ituation, Tumor-bearing mice were treated with a combination of five X -ray fractions over 5 days followed by RIT with two doses of 1.5 mCi I -131-labeled anticarcinoembryonic antigen monoclonal antibody F(ab')(2 ). In Co112 and LS174T, RIT alone achieved a regrowth delay similar to that of fractionated RT with total doses of 28 and 26 Gy, respectivel y, In both tumor types, an additive therapeutic effect, measured as in creased regrowth delay or local control, was observed when combining R T of different dose levels with RIT, Normal tissue responses were asse ssed by monitoring acute peak skin reactions and blood cell count. Bon e marrow depression for the combination treatment was similar to that of RIT alone; relative to skin, at equitoxic levels, no mice bearing C o112 tumors were locally controlled with a 32 Gy RT dose alone, while this RT combined with RIT gave a local control of 100%. These studies show a therapeutic benefit when external beam RT is combined with RIT.