Labeling anti-HER2/neu Monoclonal Antibodies with 111 In and 90 Y Using a Bifunctional DTPA Chelating Agent

Abstract

The goal of this investigation was to develop stable radioimmunoconjugates (RICs) of anti-HER2/neu monoclonal antibodies (MoAbs) for imaging and therapy in an animal model bearing human breast tumor xenografts that express normal (MCF-7 cells) and increased amounts of HER2/neu receptors (HCC-1954, BT-474, SKBR-3 cells) on their cell surface membranes. Pharmacy-grade Herceptin^®, a murine anti-HER2/neu MoAb, and nonspecific mouse IgG protein were conjugated with the recently developed DTPA linker known as CHX-A″-DTPA. These immunoconjugates were labeled with ¹¹¹InCl₃ and ⁹⁰YCl₃. Using a molar excess of 10:1 CHX-A″-DTPA to immunoglobulin, average specific activities of 1.87 μCi ¹¹¹In/μg RIC and 2.71 μCi ⁹⁰Y/μg RIC were obtained. The purity of RICs was 96%+ for ¹¹¹In and 99%+ for ⁹⁰Y. Stability in human plasma at 37°C for both RICs ranged from 98% at 24 h to 85% at 96 h. Binding capacity of the RICs was tested with human cancer cell lines MCF-7, HCC-1954, BT-474, and SKBR-3. Using ¹¹¹In-labeled nonspecific IgG protein as a control, ¹¹¹In-Herceptin^® RIC was found to bind to MCF-7 cells with a ratio of 2.5:1 and to SKBR-3 cells with a ratio of 85:1 after 3 h of incubation. ¹¹¹In anti-HER2/neu RIC bound to MCF-7 cells with a ratio of 6:1 and to SKBR-3 cells with a ratio of 115:1 after 3 h of incubation. ⁹⁰Y-anti-HER2/neu RIC bound 10-times greater to BT-474 cells than to MCF-7 cells. Thus, these MoAbs can be labeled with¹¹¹In and ⁹⁰Y using the CHX-A″-DTPA linker. The resulting RICs (¹¹¹In- and ⁹⁰Y-anti HER2/neu antibodies) are stable and bind significantly to HER2 overexpressing tumor cell lines.