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In Vitro and in Vivo Reactivity of an Internalizing Antibody, RS7, with Human Breast Cancer¹

Garden State Cancer Center at the Center for Molecular Medicine and Immunology, Newark, New Jersey 07103

RS7, a murine IgG1 antibody raised against human lung carcinoma, possesses pancarcinoma reactivity. The antigen defined by this antibody is present in tumors of the lung, stomach, bladder, breast, ovary, uterus, and prostate. Efficient targeting and therapy by radiolabeled RS7 has been demonstrated previously in animals bearing Calu-3 (an adenocarcinoma of the lung) xenografts. In this study, the efficiency of tumor targeting and the efficacy of therapy of this antibody in nude mice bearing the MDA-MB-468 human breast carcinoma were evaluated. The tumor:nontumor ratios of RS7 were 1.9–2.1 times higher than those for Ag8 (the control antibody) on day 14, except for the heart. These values were similar to that of RS7 in the Calu-3 xenograft model. Radioimmunotherapy using 250 µCi of ¹³¹I-labeled RS7 in animals bearing 0.1 cm³ tumors (10 days old) caused the disappearance of tumors in 6 of 10 animals at 2 weeks postinjection. Tumors eventually disappeared from all animals, and animals remained tumor-free until the termination of the study (11 weeks of duration), except for one animal that developed a transient reappearance of tumor. The tumors in animals that received an equal dose of ¹³¹I-labeled Ag8, or unlabeled RS7 or Ag8, either were unchanged or continued to grow. Treatment that used 275 µCi of ¹³¹I-labeled RS7 in animals carrying established tumors (1 month old, 0.2–0.3 cm³) showed that this antibody is effective in controlling the growth of this tumor. Tumors in the treatment group began to disappear between the second and third weeks after the injection of the radiolabeled antibody. Seven of 10 animals remained tumor free at 15 weeks after the injection. Tumors in animals that received an equal dose of control antibody persisted but grew at a slower pace compared to the untreated group. No systemic toxicity was observed.