Cyclodextrin-Based Organic Radical Contrast Agents for in vivo Imaging of Gliomas

Cyclodextrins (CDs), a class of cyclic oligosaccharides formed by α-(1,4) linked glucopyranose units, were functionalized with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) radicals to prepare water soluble supramolecular organic radical contrast agents (ORCAs) for the in vivo detection of glioma tumor in animal models. A first set of molecules (CDn1, n=6,7,8 is the number of both TEMPO and glucopyranose units) was studied by superconducting quantum interference devices (SQUID) magnetometry in order to define the role of the CD macrocycle on the effective magnetic moment (μ_eff ). The μ_eff value increased from 3.982 μ_B (CD61) to 4.522 μ_B (CD81) but was limited by intramolecular antiferromagnetic (AF) interactions. A set of water-soluble ORCAs (CDn8, n=6,7,8) was prepared by a sequence of thiol-ene and Cu(I)-catalyzed alkyne-azide "click" reactions. Their ¹ H water relaxivities r₁ of these ORCAs were between 0.739 mM^-1  s^-1 (CD68) to 1.047 mM^-1  s^-1 (CD88) in D₂ O/H₂ O 9 : 1 (v : v) at 300 K. One of them (CD78) was tested on glioma-bearing rats with reduced side effects and good relaxivity in vivo.