68Ga-1,4,7-Triazacyclononane-1,4,7-triacetic acid-polyethylene glycol-single-chain Cys-tagged vascular endothelial growth factor-121

Excerpt

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 do (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 (VEGFRs) 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 VEGFRs (1).

VEGFRs have been found to be overexpressed in various tumor cells and tumor-associated endothelial cells but are not detectable in quiescent endothelial cells (3). Inhibition of VEGFR 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). A 15-amino-acid fusion tag (Cys-tag) 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 3–112 amino acid fragments of VEGF₁₂₁ joining head-to-tail to express as scVEGF, which was labeled as ⁶⁴Cu-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-polyethylene glycol (PEG)-scVEGF (⁶⁴Cu-DOTA-PEG-scVEGF), ^99mTc-hydrazinonicotinic acid (HYNIC)-scVEGF (^99mTc-HYNIC-scVEGF), and Cy5.5-scVEGF for imaging VEGFR expression to study tumor angiogenesis. In this chapter, ⁶⁸Ga-1,4,7-Triazacyclononane-1,4,7-triacetic acid-PEG-scVEGF (⁶⁸Ga-NOTA-PEG-scVEGF) is being developed for positron emission tomography (PET) imaging of VEGFR-2 in tumor vasculature (10).