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

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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 with 111In 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.

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