Nitroxyl Radicals for Labeling of Conventional Therapeutics and Noninvasive Magnetic Resonance Imaging of Their Permeability for Blood-Brain Barrier: Relationship between Structure, Blood Clearance, and MRI Signal Dynamic in the Brain

The present study describes a novel nonradioactive methodology for in vivo noninvasive, real-time imaging of blood-brain barrier (BBB) permeability for conventional drugs, using nitroxyl radicals as spin-labels and magnetic resonance imaging (MRI). Two TEMPO-labeled analogues (SLENU and SLCNUgly) of the anticancer drug lomustine [1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea] were synthesized, using a substitution of the cyclohexyl part with nitroxyl radical. Nonmodified nitroxyl radical TEMPOL was used for comparison. The nitroxyl derivatives were injected intravenously in healthy mice via the tail vein, and MR imaging of the brain was performed on a 7.0 T MRI. The MRI signal dynamic of SLENU and SLCNUgly followed the same kinetics as nonmodified TEMPO radical. SLENU and SLCNUgly were rapidly transported and randomly distributed in the brain tissue, which indicated that the exchange of cyclohexyl part of lomustine with TEMPO radical did not suppress the permeability of the anticancer drug for BBB. The selected nitroxyl derivatives possessed different hydrophobicity, cell permeabilization ability, and blood clearance. Based on these differences, we investigated the relationship between the structure of nitroxyl derivatives, their half-life in the circulation, and their MRI signal dynamic in the brain. This information was important for estimation of the merits and demerits of the described methodology and finding pathways for overcoming the restrictions.