Abstract
PURPOSE: Human B7-H3 (hB7-H3) is a promising molecular imaging target differentially expressed on the neovasculature of breast cancer and has been validated for preclinical ultrasound (US) imaging with anti-B7-H3-antibody-functionalized microbubbles (MB). However, smaller ligands such as affibodies (ABY) are more suitable for the design of clinical-grade targeted MB. EXPERIMENTAL DESIGN: Binding of ABY(B7-H3) was confirmed with soluble and cell-surface B7-H3 by flow cytometry. MB were functionalized with ABY(B7-H3) or anti-B7-H3-antibody (Ab(B7-H3)). Control and targeted MB were tested for binding to hB7-H3-expressing cells (MS1(hB7-H3)) under shear stress conditions. US imaging was performed with MB(ABY-B7-H3) in an orthotopic mouse model of human MDA-MB-231 coimplanted with MS1(hB7-H3) or control MS1(WT) cells and a transgenic mouse model of breast cancer development. RESULTS: ABY(B7-H3) specifically binds to MS1(hB7-H3) and murine-B7-H3-expressing monocytes. MB(ABY-B7-H3) (8.5 ± 1.4 MB/cell) and MB(Ab-B7-H3) (9.8 ± 1.3 MB/cell) showed significantly higher (P < 0.0001) binding to the MS1(hB7-H3) cells compared with control MB(Non-targeted) (0.5 ± 0.1 MB/cell) under shear stress conditions. In vivo, MB(ABY-B7-H3) produced significantly higher (P < 0.04) imaging signal in orthotopic tumors coengrafted with MS1(hB7-H3) (8.4 ± 3.3 a.u.) compared with tumors with MS1(WT) cells (1.4 ± 1.0 a.u.). In the transgenic mouse tumors, MB(ABY-B7-H3) (9.6 ± 2.0 a.u.) produced higher (P < 0.0002) imaging signal compared with MB(Non-targeted) (1.3 ± 0.3 a.u.), whereas MB(ABY-B7-H3) signal in normal mammary glands and tumors with B7-H3 blocking significantly reduced (P < 0.02) imaging signal. CONCLUSIONS: MB(ABY-B7-H3) enhances B7-H3 molecular signal in breast tumors, improving cancer detection, while offering the advantages of a small size ligand and easier production for clinical imaging.