Accurate diagnosis of lymph node metastasis in various cancer patients is very important as it is one of the most important factors for the choice of preoperative chemoradiotherapy, surgical treatment, and patient prognosis . Recently, magnetic resonance (MR) imaging with ultrasmall superparamagnetic iron oxide (USPIO) has gained approval as a noninvasive method for the detection of lymph node metastases in several tumors [2, 3]. MR provides images with excellent anatomical detail but, at the same time, is relatively insensitive to lymph node metastases due to the limited sensitivity of current node size criteria in differentiating benign from malignant nodes. However, the MR results can be improved by using a superparamagnetic contrast agent such as USPIO [4, 5]. USPIO acts as a negative contrast agent, and therefore, normal functioning lymph nodes can be distinguished from lymph node metastases on the basis of magnetic resonance signal characteristics, independent of nodal size [6–9]. USPIO nanoparticles have offered new potential for early detection of lymph nodes and their metastases using magnetic resonance imaging (MRI). These nanoparticles are taken up by macrophages in normal lymph nodes and show signal reduction (e.g., by susceptibility-based relaxation). Because less nanoparticle uptake is seen in metastatic nodes, these nodes may be overlooked as a result of the minimal signal change in the occupying mass. Fortunately, among various MR protocols that show the lymph nodes, susceptibility base sequences are very sensitive to small magnetic changes inside the nodes . Some USPIO particles (mean diameter less than 50 nm) are used as MRI contrast agents (e.g., Sinerem®, European Medicines Agency, Canary Wharf, London; Combidex®, Radboud University Nijmegen Medical Centre, Nijmegen, Netherlands; Clariscan™, Nycomed, Drammensveien, Asker, Norway) to differentiate metastatic nodes from lymph nodes. These USPIOs are composed of iron oxide nanoparticles coated with polymers such as low molecular weight dextran and polyethyleneglycol (PEG).
In the present study, we used an amphiphilic poly(2-hydroxyethyl aspartamide) (PHEA) graft copolymer, and not a block copolymer, to form biocompatible USPIO (less than 30 nm in diameter). PHEA, which is a poly(amino acid) derivative, was used to coat the iron oxide (Fe3O4) nanoparticles for MRI applications. PHEA is a synthetic polymer having a protein-like structure, obtained by the reaction of ethanolamine with polysuccinimide (PSI), which is prepared by thermal polycondensation of d,l-aspartic acid. PHEA has good biopharmaceutical properties as drug carrier such as high water solubility, multi-functionality, absence of toxicity, antigenicity, immunogenicity, and low cost of production [11–17]. Hydrophobic side chain was grafted to the PHEA backbone to aid in good solubility of hydrophobic Fe3O4 nanoparticles in aqueous phases. Hexadecyl alkyl groups permit hydrophobic interaction with ligands on Fe3O4 nanoparticles, and hydrophobic van der Waals interaction affords good stability in aqueous solution .
In this paper, we synthesized amphiphilic graft derivatives of PHEA by the introduction of hydrophobic hexadecylamine (C16) as a linker of iron oxide. We evaluated the feasibility of using PHEA-C16-iron oxide nanoparticles as MRI contrast agent for the detection of lymph nodes and performed in vitro and in vivo toxicology studies.