Effective Treatment of Established Human Breast Tumor Xenografts in Immunodeficient Mice with a Single Dose of the α-Emitting Radioisotope Astatine-211 Conjugated to Anti-HER2/neu Diabodies

Purpose: Successful radioimmunotherapy strategies depend on selecting radioisotopes with physical properties complementary to the biological properties of the targeting vehicle. Small, engineered antitumor antibody fragments are capable of rapid, highly specific tumor targeting in immunodeficient mouse models. We hypothesized that the C6.5 diabody, a noncovalent anti-HER2 single-chain Fv dimer, would be an ideal radioisotope carrier for the radioimmunotherapy of established tumors using the short-lived α-emitting radioisotope 211At. Experimental Design: Immunodeficient nude mice bearing established HER2/neu–positive MDA-MB-361/DYT2 tumors treated with N-succinimidyl N-(4-[211At]astatophenethyl)succinamate (211At-SAPS)-C6.5 diabody. Additional cohorts of mice were treated with 211At-SAPS T84.66 diabody targeting the carcinoembryonic antigen or 211At-SAPS on a diabody specific for the Müllerian inhibiting substance type II receptor, which is minimally expressed on this tumor cell line. Results: A single i.v. injection of 211At-SAPS C6.5 diabody led to a 30-day delay in tumor growth when a 20 μCi dose was administered and a 57-day delay in tumor growth (60% tumor-free after 1 year) when a 45 μCi dose was used. Treatment of mice bearing the same tumors with 211At-SAPS T84.66 diabody at the same doses led to a delay in tumor growth, but no complete responses, likely due to substantially lower expression of this antigen on the MDA-MB-361/DYT2 tumors. In contrast, a dose of 20 μCi of 211At-SAPS on the anti–Müllerian-inhibiting substance type II receptor diabody did not affect tumor growth rate, demonstrating specificity of the therapeutic effect. Conclusions: These findings indicate that diabody molecules can be effective agents for targeted radioimmunotherapy of solid tumors using powerful, short-lived α-emitting radioisotopes.