Leung Kam
National Center for Biotechnology Information, NLM, NIH, Bethesda, MD
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). VEGF, VEGF, and VEGF are the forms secreted by most cell types and are active as homodimers linked by disulfide bonds. VEGF 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 the free sulfhydryl group of Cys. Backer et al. (9) prepared a Cys-tagged vector of VEGF by cloning two single-chain 3–112 amino acid fragments of VEGF joining head-to-tail to express as scVEGF, which was labeled as Cu-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-polyethylene glycol (PEG)-scVEGF (Cu-DOTA-PEG-scVEGF), Tc-hydrazinonicotinic acid (HYNIC)-scVEGF (Tc-HYNIC-scVEGF), and Cy5.5-scVEGF for imaging VEGFR expression to study tumor angiogenesis. In this chapter, Ga--bis[2-Hydroxy-5-(carboxyethyl)benzyl]ethylenediamine--diacetic acid-PEG-scVEGF (Ga-HBED-CC-PEG-scVEGF) is being developed for positron emission tomography (PET) imaging of VEGFR-2 in tumor vasculature (10).
血管内皮生长因子（VEGF）至少由六种通过可变剪接产生的具有不同氨基酸数量（121、145、165、183、189和206个氨基酸）的异构体组成（1）。VEGF、VEGF和VEGF是大多数细胞类型分泌的形式，以通过二硫键连接的同二聚体形式具有活性。VEGF不像其他VEGF种类那样与肝素结合（2）。VEGF是一种强大的血管生成因子，可诱导内皮细胞的增殖、芽生、迁移和管形成。内皮细胞上有三种高亲和力酪氨酸激酶VEGF受体（VEGFRs）（VEGFR-1，Flt-1；VEGFR-2，KDR/Flt-1；和VEGFR-3，Flt-4）。几种类型的非内皮细胞，如造血干细胞、黑色素瘤细胞、单核细胞、成骨细胞和胰腺β细胞，也表达VEGFRs（1）。已发现VEGFRs在各种肿瘤细胞和肿瘤相关内皮细胞中过表达，但在静止的内皮细胞中无法检测到（3）。VEGFR功能的抑制已被证明可抑制病理性血管生成以及肿瘤生长和转移（4,5）。放射性标记的VEGF已被开发用作成像人类实体瘤和血管生成的示踪剂（6-8）。一种15个氨基酸的融合标签（Cys标签）被开发用于与Cys的游离巯基进行位点特异性缀合。Backer等人（9）通过克隆VEGF的两个单链3-112个氨基酸片段并头对尾连接以表达为scVEGF，制备了一种Cys标记的VEGF载体，其被标记为铜- [1,4,7,10-四氮杂环十二烷-1,4,7,10-四乙酸（DOTA）]-聚乙二醇（PEG）-scVEGF（铜-DOTA-PEG-scVEGF）、锝-肼基烟酸（HYNIC）-scVEGF（锝-HYNIC-scVEGF）和Cy5.5-scVEGF，用于成像VEGFR表达以研究肿瘤血管生成。在本章中，正开发镓-双[2-羟基-5-（羧乙基）苄基]乙二胺-N,N'-二乙酸-PEG-scVEGF（镓-HBED-CC-PEG-scVEGF）用于肿瘤脉管系统中VEGFR-2的正电子发射断层扫描（PET）成像（10）。
