Magnetic resonance imaging (MRI) signal intensities in tissues are usually enhanced by the injection of metal-based contrast agents (CAs) in patients. Some of these metal compounds have been linked to incurable nephrogenic systemic fibrosis and other neurological disorders, leading to their discontinuation. Herein, we report the design, synthesis, and characterization of a self-assembled metal-free CA which is based on an organic nitroxide radical (4-carboxy-TEMPO) compartmentalized in the biocompatible, biodegradable natural polymer, chitosan (CS), alongside folate targeting for in vivo tumor MRI. This extension conceivably heralds a new application for CS-TEMPO, which hitherto was used in macro-sizes as initiators for grafting CS to various polymers. Significantly, the folic acid (FA)-modified metal-free CA (CS-TEMPO-FA; average size ∼113 nm) is substantially benign, with no evidence of acute toxicity; it exhibits desirable pharmacokinetic profile, enhanced stability in highly reducing environments, high nitroxide loading (molecular relaxivity ∼18.56 mM⁻¹s⁻¹), and effective tumor targeting. This latter characteristic ameliorates the non-specific uptake of CAs by normal cells. CS-TEMPO-FA nanoparticles demonstrate commensurate contrast enhancement of the tumor region up to 1 h post-injection when injected into 4T1 tumor-bearing mice, a contrast enhancement capability hitherto unattained by TEMPO-based CAs. This work points to CS-TEMPO-FA as a potential alternative to metal-based CAs as well as an active folate-targeted metal-free CA for long-term longitudinal tumor MRI which ameliorates the otherwise short in vivo lifetimes of typical organic radicals-based CAs.

通常通过向患者注射金属基造影剂（CA）来增强组织中的磁共振成像（MRI）信号强度。这些金属化合物中的一些已与无法治愈的肾原性系统性纤维化和其他神经系统疾病有关，导致其停用。本文中，我们报告了一种基于有机氮氧化物自由基（4-羧基-TEMPO）的自组装无金属CA的设计，合成和表征，该CA在生物相容性，可生物降解的天然聚合物，壳聚糖（CS）中间隔开叶酸靶向体内肿瘤MRI。可以想象，这一扩展预示着CS-TEMPO的新应用，迄今为止，CS-TEMPO已在宏观尺寸中用作将CS接枝到各种聚合物上的引发剂。值得注意的是，叶酸（FA）改性的无金属CA（CS-TEMPO-FA；平均粒径约为113 nm）基本上是良性的，没有急性毒性的证据；它显示出理想的药代动力学特征，在高度还原的环境中增强的稳定性，高的一氧化氮负载量（分子弛豫度〜18.56 mM ^-1 s ^-1），并有效地靶向肿瘤。后一个特征改善了正常细胞对CA的非特异性吸收。CS-TEMPO-FA纳米粒子在注射到4T1荷瘤小鼠中时，在注射后1小时内显示出相应的肿瘤区域对比度增强，这是迄今为止基于TEMPO的CA所没有的对比度增强能力。这项工作指出CS-TEMPO-FA可作为金属基CA的潜在替代物，以及可用于长期纵向肿瘤MRI的以叶酸为靶点的无金属CA的活性替代物，可改善典型的有机自由基在体内的短暂生命周期，基于CA。