Experimental radioimmunotherapy with Re-186-MAG3-A7 anti-colorectal cancermonoclonal antibody: Comparison with I-131-counterpart

A murine IEG(1) against a Mr 45 kD tumor-associated glycoprotein in human c olorectal cancer, A7, was radiolabeled with Re-186 by a chelating method wi th a mercaptoacetyltriglycine (MAG3). Its specific activity was 119 MBq/mg, which would be high enough for a therapeutic purpose, and its immunoreacti vity was preserved well as was I-131-A7 labeled by the chloramine-T method. Growth of human colon cancer xenografts, 9.14 +/- 0.44 mm in diameter, in nude mice was significantly suppressed by an intravenous dose of 4.48 MBq o f Re-186-A7. The therapeutic outcome with (186)ReA7 was better than that wi th 4.63 MBq of I-131-A7. Toxicity of treatments assessed by body weight cha nge was similar with both conjugates. These results are likely caused by th e tumor size and more favorable physical properties of Re-186 than those of I-131.