Vascular endothelial growth factor (VEGF) consists of at least six isoforms with various numbers of amino acids (121, 145, 165, 183, 189, and 206 amino acids) produced through alternative splicing (1). VEGF121, VEGF165, and VEGF189 are the forms secreted by most cell types and are active as homodimers linked by disulfide bonds. VEGF121 does not bind to heparin like the other VEGF species (2). VEGF is a potent angiogenic factor that induces proliferation, sprouting, migration, and tube formation of endothelial cells. There are three high-affinity tyrosine kinase VEGF receptors on endothelial cells (VEGFR-1, Flt-1; VEGFR-2, KDR/Flt-1; and VEGFR-3, Flt-4). Several types of non-endothelial cells such as hematopoietic stem cells, melanoma cells, monocytes, osteoblasts, and pancreatic β cells also express VEGF receptors (1).
VEGF receptors were found to be overexpressed in various tumor cells and tumor-associated endothelial cells (3). Inhibition of VEGF receptor function has been shown to inhibit pathological angiogenesis as well as tumor growth and metastasis (4, 5). Radiolabeled VEGF has been developed as a tracer for imaging solid tumors and angiogenesis in humans (6-8). Cys-tag, a fusion tag comprising 15 amino acids, was developed for site-specific conjugation via the free sulfhydryl group of Cys. Backer et al. (9) prepared a Cys-tagged vector of VEGF121 by cloning two single-chain fragments (amino acid sequence 3–112 ) of VEGF121 joining head-to-tail to express as scVEGF, which can be labeled as 64Cu-1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA)-scVEGF (64Cu-DOTA-scVEGF), 99mTc-hydrazinonicotinic acid (HYNIC)-scVEGF (99mTc-HYNIC-scVEGF), and Cy5.5-scVEGF for imaging VEGFR expression to study tumor angiogenesis (10). 99mTc-HYNIC-scVEGF is being developed for single-photon emission computed tomography (SPECT) imaging of VEGFR-2 in tumor vasculature.