Glycol chitosan/heparin-immobilized gold-deposited iron oxide nanoparticles

Glycol chitosan/heparin-immobilized gold-deposited iron oxide (IO) nanoparticles (NPs), abbreviated as composite NPs, are a NP-based contrast agent developed by Yuk et al. for tumor stroma-targeted magnetic resonance imaging (MRI) (1). Tumor tissue is different from normal tissue in the content and function of its stroma. At the initial stage of tumor development, tumor cells secrete growth factors, including vascular permeability factor that renders the local microvasculature hyperpermeable to fibrinogen and other plasma proteins. Extravasated fibrinogen crosslinks with fibrin and other proteins, forming a meshwork in the tumor stroma (2, 3). This sequence also occurs in the process of wound-healing and in the progression of many other diseases, but the tumor stroma contains increased amounts of fibrinogen, collagen, fibronectin, fibrinogen-derived products, and other proteins (2, 4). These proteins either come from the leaky blood vessels or are inappropriately synthesized by the tumor cells. Functionally, normal stroma generates an antiproliferative environment, whereas tumor stroma promotes tumor cell proliferation, migration, and lymphovascular invasion by providing adhesion proteins, proteases, and growth factors (5). Abnormal formation of clotted plasma protein meshwork in the tumor stroma and on the tumor blood vessel walls has prompted investigators to generate tumor stroma-targeted imaging and therapeutic agents. Pilch et al. screened a phage library on plasma clots and obtained two cyclic decapeptides (CLT; CLT1 and CLT2) (3). The fluorescence-labeled CLT peptides specifically accumulated in the fibrillar meshwork that existed in tumor stroma, but they were not detectable in other tissues in animal models of human tumors. Ye et al. conjugated the CLT1 peptide with gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA), and the generated agent Gd-DTPA-CLT1 exhibited high specificity to the fibronectin-fibrin complexes in the tumor stroma (6). Simberg et al. also reported the clotted plasma protein-targeted multimodal agent CREKA-SPIO-Cy7, which was synthesized by conjugation of the peptide CREKA with superparamagnetic IO and cyanine 7 (7). On the basis of the interaction between the fibrinogen-derived products and heparin, Yuk et al. synthesized the MRI contrast agent composite NPs with heparin as the specific ligand (1). Heparin is a highly-sulfated glycosaminoglycan with a high negative charge density. Heparin has two commercial types: unfractionated heparin (UFH, 12–15 kDa) and low molecular weight heparin (LMWH, 2–9 kDa) (8). UFH is a heterogeneous mixture of glycosaminoglycans that bind to the enzyme inhibitor antithrombin III via the pentasaccharide. UFH also binds many other proteins, endothelial cells, platelets, and other circulating cells. LMWH is produced by the depolymerization of heparin and acts by targeting factor Xa activity, rather than by activating anti-thrombin activity. Heparin also exhibits anti-tumor effects by inhibiting tumor growth and spread. Composite NPs have been shown to selectively distribute in tumors in an animal model of murine squamous cell carcinoma (SCC). This chapter summarizes the data obtained with the composite NPs (1).