Monoclonal antibody MA5 recognizes a determinant displayed on high molecular weight antigens associated with secretory and malignant breast epithelial cells. MA5 reactivity with >95% of primary and metastatic breast tumors, surface expression of the antigen, as well as its ability to localize within breast tumor xenografts prompted this initial study to determine the efficacy of MA5 to localize breast tumors by radioimmunoscanning. A total of 17 patients was monitored, each receiving 2 mg of purified MA5 labeled with 5 mCi of 111In. Some patients also received 3 or 18 mg of unlabeled carrier antibody (MA5); no serious allergic reactions were noted. Primary tumors, bone lesions, soft tissue recurrences, and lung metastases >3 cm in diameter were detectable, whereas only one lesion (hilar node) <3 cm was localized. Significant antibody accumulation was noted in the liver and less significant uptake in the spleen and bone. The extensive fibrosis and poor vascularization of breast tumors may partly explain the limited sensitivity obtained thus far. The imaging results obtained with MA5 are compared with other antibodies which we show recognize the same antigens.

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Presented at the “Second Conference on Radioimmunodetection and Radioimmunotherapy of Cancer,” September 8–10, 1988, Princeton, NJ. Supported by the National Cancer Institute (Canada), the Cancer Research Society, Fonds de la Recherche en Santé du Quebec and by USPHS Grant CA44628 awarded by the National Cancer Institute, Department of Health and Human Services. P. P. M. is a scholar of the Medical Research Council of Canada.

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