Zr-Desferrioxamine -isothiocyanatobenzyl-anti-hepatocyte growth factor nanobody 1E2 fused to albumin-binding nanobody Alb8

MET is a proto-oncogene protein, also known as c-Met or hepatocyte growth factor receptor (HGFR) (1). MET is a membrane receptor with tyrosine kinase activity that is essential for embryonic development and wound healing. MET is normally expressed by stem cells, progenitor cells, and cells of epithelial origin, whereas expression of hepatocyte growth factor (HGF), which is the only known ligand of MET, is restricted to cells of mesenchymal origin. Upon HGF stimulation, MET induces several biological responses that lead to invasive growth (2). Abnormal MET activation in cancer correlates with poor prognosis because deregulated MET triggers tumor growth, induces angiogenesis, and activates metastasis (3). MET and HGF are overexpressed in many types of human malignancies, including kidney, liver, stomach, breast, and brain cancers (2, 4, 5). Inhibition of MET function has been shown to inhibit pathological angiogenesis as well as tumor growth and metastasis (6). On the other hand, targeting HGF may also be beneficial to patients by blocking binding of HGF to MET. Nanobodies are the smallest intact antigen-binding fragments (15 kDa) isolated from heavy-chained camelid antibodies, and they exhibit efficient and specific tumor targeting (7, 8). With circulating blood half-lives of <1 h, monospecific nanobodies may not efficiently block binding of growth factors to their receptors (9). In order to extend the half-life of the nanobody, Vosjan et al. (10) linked an anti-HGF 1E2 nanobody with an anti-albumin nanobody (Alb8) to form a bispecific molecule (1E2-Alb8). For use with positron emission tomography, 1E2-Alb8 was conjugated with the bifunctional chelate -isothiocyanatobenzyl-desferrioxamine (Df-Bz-NCS) and labeled with Zr to form Zr-Df-Bz-NCS-1E2-Alb8 for imaging HGF expression in tumors.