Novel Bivalent (99m)Tc-Complex with N-Methyl-Substituted Hydroxamamide as Probe for Imaging of Cerebral Amyloid Angiopathy

Cerebral amyloid angiopathy (CAA) is characterized by the deposition of amyloid aggregates in the walls of the cerebral vasculature. Recently, the development of molecular imaging probes targeting CAA has been attracting much attention. We previously reported the (99m)Tc-hydroxamamide ((99m)Tc-Ham) complex with a bivalent benzothiazole scaffold as a binding moiety for amyloid aggregates ([(99m)Tc]BT2) and its utility for CAA-specific imaging. However, the simultaneous generation of two radiolabeled complexes derived from the geometric isomers was observed in the (99m)Tc-labeling reaction. It was recently reported that the complexation reaction of (99)Tc with N-methyl-substituted Ham provided a single (99)Tc-Ham complex consisting of two N-methylated Ham ligands with marked stability. In this article, we designed and synthesized a novel N-methylated bivalent (99m)Tc-Ham complex ([(99m)Tc]MBT2) and evaluated its utility for CAA-specific imaging. N-Methyl substitution of [(99m)Tc]BT2 prevented the generation of its isomer in the (99m)Tc-labeling reaction. Enhanced in vitro stability of [(99m)Tc]MBT2 as compared with [(99m)Tc]BT2 was observed. [(99m)Tc]MBT2 showed very low brain uptake, which is favorable for CAA-specific imaging. An in vitro inhibition assay using β-amyloid aggregates and in vitro autoradiographic examination of brain sections from a Tg2576 mouse and a CAA patient showed a decline in the binding affinity for amyloid aggregates due to N-methylation of the (99m)Tc-Ham complex. These results suggest that the scaffold of the (99m)Tc-Ham complex may play important roles in the in vitro stability and the binding affinity for amyloid aggregates.