Effects of Substituents on the Properties of Metal-Free MRI Contrast Agents.

Materials possessing electron spin can shorten the T 1 relaxation times in magnetic resonance imaging (MRI). For example, gadolinium (Gd) complexes with seven f-orbital electrons are widely used as contrast agents in clinical applications. However, Gd has severe potential side effects, and thus metal-free alternatives are needed. Toward this end, we synthesized seven NO radicals consisting of a dioxa-azaspiro[4.5]decane framework having various substituents, DAD-X (X = methyl, ethyl, n-propyl, c-propyl, vinyl, phenyl, and 2-pyridyl), that functioned as metal-free MRI contrast agents. The relationship between (i) water-proton relaxivity and log P and (ii) reactivity for ascorbic acid and the spin density of the NO oxygen atom were established, which provided a basis for the rational design of practical metal-free contrast agents.

Water-Proton Relaxivities of Radical Nanoparticles Self-Assembled via Hydration or Dehydration Processes.

Nanoparticles capable of accumulating in tumor tissues are promising materials for tumor imaging and therapy. In this study, two radical nanoparticles (RNPs), denoted as 1 and 2, composed of self-assembled ureabenzene derivatives possessing one or two amphiphilic side chains were demonstrated to be candidates for metal-free functional magnetic resonance imaging (MRI) contrast agents (CAs). Because of the self-assembly behavior of 1 and 2 in a saline solution, spherical RNPs of sizes ∼50-90 and ∼30-100 nm were detected. In a highly concentrated solution, RNP 1 showed considerably small water-proton relaxivity values (r1 and r2), whereas RNP 2 showed an r1 value that was around 5 times larger than that of RNP 1. These distinct r1 values might be caused by differences in the self-assembly behavior by a hydration or dehydration process. In vivo studies with RNP 2 demonstrated a slightly enhanced T1-weighted image in mice, suggesting that the RNPs can potentially be used as metal-free functional MRI CAs for T1-weighted imaging.

Thermal- and pH-Dependent Size Variable Radical Nanoparticles and Its Water Proton Relaxivity for Metal-Free MRI Functional Contrast Agents.

For development of the metal-free MRI contrast agents, we prepared the supra-molecular organic radical, TEMPO-UBD, carrying TEMPO radical, as well as the urea, alkyl group, and phenyl ring, which demonstrate self-assembly behaviors using noncovalent bonds in an aqueous solution. In addition, TEMPO-UBD has the tertiary amine and the oligoethylene glycol chains (OEGs) for the function of pH and thermal responsiveness. By dynamic light scattering and transmission electron microscopy imaging, the resulting self-assembly was seen to form the spherical nanoparticles 10-150 nm in size. On heating, interestingly, the nanoparticles showed a lower critical solution temperature (LCST) behavior having two-step variation. This double-LCST behavior is the first such example among the supra-molecules. To evaluate of the ability as MRI contrast agents, the values of proton ((1)H) longitudinal relaxivity (r1) were determined using MRI apparatus. In conditions below and above CAC at pH 7.0, the distinguishable r1 values were estimated to be 0.17 and 0.21 mM(-1) s(1), indicating the suppression of fast tumbling motion of TEMPO moiety in a nanoparticle. Furthermore, r1 values became larger in the order of pH 7.0 > 9.0 > 5.0. Those thermal and pH dependencies indicated the possibility of metal-fee MRI functional contrast agents in the future.