Combined radiotherapy and radioimmotherapy of human colon carcinoma grafted in nude mice.

The effect of combined radioimmunotherapy (RIT) and fractionated
external beam radiotherapy (RT) was assessed in two human colon cancer
xenografts, Col 12 and LS174T in nude mice. These tumors were selected
for being resistant to RIT alone, as is usually the case in the clinical
situation. 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
131I-labeled anticarcinoembryonic antigen monoclonal antibody F(ab')2.
In Col 12 and LS174T, RIT alone achieved a regrowth delay similar to
that of fractionated RT with total doses of 28 and 26 Gy, respectively. In
both tumor types, an additive therapeutic effect, measured as increased
regrowth delay or local control, was observed when combining RT of
different dose levels with RIT. Normal tissue responses were assessed by
monitoring acute peak skin reactions and blood cell count. Bone marrow
depression for the combination treatment was similar to that of RIT
alone; relative to skin, at equitoxic levels, no mice bearing Col 12 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.