Labeling anti-HER2/neu monoclonal antibodies with 111In and 90Y using a bifunctional DTPA chelating agent

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 (InCl3)-In-111 and (YCl3)-Y-90. Using a molar excess of 10:1 CHX-A"-DTPA to immunoglobulin, average specific activities of 1.87 muCi In-111/mug RIC and 2.71 muCi(90)Y/mug RIC were obtained. The purity of RICs was 96%+ for In-111 and 99%+ for Y-90. Stability in human plasma at 37degreesC 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 SKBR3. Using In-111-labeled nonspecific IgG protein as a control, In-111-Herceptin(R) 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. In-111-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. 9 Y-anti-HER2/neu RIC bound 10-times greater to BT-474 cells than to MCF-7 cells. Thus, these MoAbs can be labeled with In-111 and Y-90 using the CHX-A"-DTPA linker. The resulting RICs (In-111- and Y-90-anti HER2/neu antibodies) are stable and bind significantly to HER2 overexpressing tumor cell lines.