Biodistribution and Radioimmunotherapy of Human Breast Cancer Xenografts with Radiometal-Labeled DOTA Conjugated Anti-HER2/neu Antibody 4D5

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Abstract
HER2/neu oncogene encodes a 185 kDa trans-membrane protein which is overexpressed in 20−30% of breast and ovarian cancers and portends a poor prognosis. We have studied the targeting and therapy of this oncoprotein with 4D5, a murine monoclonal antibody which recognizes a distinct epitope on the extracelluar domain of HER2/neu. We conjugated the antibody with an active ester of the macrocyclic chelating agent DOTA, radiolabeled the conjugate with either 111In or 90Y, and studied the antibody distribution and therapy, respectively, in athymic mice bearing xenografts of MCF7/HER2/neu, a human breast cancer cell line transfected with the HER2/neu oncogene. For the biodistribution of 111In-labeled DOTA-4D5, a high specificity of tumor localization (30% ID/g) was seen with a tumor-to-blood ratio of greater than 2 at 48 h postinjection. Compared to a previously published study with 125I-labeled 4D5 in beige nude mice bearing NIH3T3/HER2/neu xenografts [De Santes et al. (1992) Cancer Res. 52, 1916−1923], 111In-labeled 4D5 antibody gave superior antibody uptake in tumor (30% ID/g vs 17% ID/g at 48h). In the therapy study, treatment of the nude mice bearing MCF7/HER2/neu xenografts with 100 μCi (3 μg) of 90Y-labeled DOTA-4D5 caused a 3-fold reduction of tumor growth compared to untreated controls (injected with human serum albumin) in 40 days. Treatment of animals with 100 μCi of nonspecific antibody 90Y-labeled DOTA-Leu16 (3 μg) had no tumor growth inhibition. Treatment with unlabeled DOTA-4D5 (3 μg) had a slight effect on tumor growth compared to untreated controls. When analyzed at the level of single animals, no effect was seen in seven of nine animals; however, in two of the animals, tumor growth inhibition was observed. Although a cold antibody therapeutic effect was unexpected at this dose level (3 μg), it may be possible that in some animals that 3 μg of antibody of 90Y-labeled DOTA-4D5 augmented tumor growth reduction. To further explore the effects of cold antibody treatment alone, animals were treated with 100 or 400 μg of unlabeled 4D5 administered in two doses. These animals showed a 1.7−1.8-fold reduction in tumor growth over 28 days, a result less than that obtained with RIT only.