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The effect of radiosensitizers on radio‐immunotherapy, using 131I‐Labelled anti‐cea antibodies in a human colonic xenograft model

Anti‐CEA antibodies were used for radio‐immunotherapy in an established LSI74T colonic xenograft in nude mice. A single IV dose of either l3lI‐PK4S (polyclonal) or ‐A5B7 (monoclonal) antibody produced tumour regression, and significantly delayed subsequent tumour growth. Administration of a clearing second antibody, 24 hr post 1311‐PK4S and at 5 times the dose, significantly reduced the therapeutic effect of radio‐antibody alone. Tumours of mice given non‐specific antibody or unlabelled anti‐CEA antibody grew like those of untreated controls. In an attempt to enhance therapy without increasing dose, radiosensitizers normally employed with external beam radiation were used in combination therapy. When the hypoxic cell radiosensitizer misonidazole was combined with 1311‐A5B7, it significantly prolonged tumour‐growth inhibition over radio‐antibody alone. Conversely, the therapeutic effect of either radio‐antibody was significantly reduced when used in combination with the halogenated pyrimidine radiosensitizer 5‐iododeoxyuridine. Neither sensitizer alone affected tumour growth.