Biphenyls labeled with technetium 99m for imaging β-amyloid plaques in the brain

Formation and accumulation of excess aggregates of β-amyloid (Aβ) plaques in the brain are critical factors contributing to the development and progression of Alzheimer's disease (AD). There is an urgent need for in vivo imaging agents that can specifically demonstrate the location and density of Aβ plaques in the brain. The aim of this study was to develop potential technetium 99m (99mTc)-labeled diagnostic imaging agents specific for the detection of Aβ plaques. Based on previously obtained Aβ plaque-specific biphenyls containing a p-N, N-dimethylaminophenyl group, a series of 99mTc and Re-N2S2-biphenyl derivatives was prepared. The stable neutral and lipophilic 99mTc complexes, [99mTc]19 and [99mTc]23, A+B, were successfully obtained. As surrogates for the 99mTc complexes, the corresponding surrogates, Re complexes of 23, were also prepared. Surprisingly, it was found that the Re complexes showed distinctively different retention profiles as compared with the corresponding 99mTc complexes. Biodistribution studies indicated that [99mTc]23A readily passed through the blood-brain barrier (1.18% dose/brain at 2 min) in contrast to the low brain penetration of [99mTc]19 (0.29% dose/brain at 2 min). Initial results suggested that [99mTc]23A showed selective binding to the Aβ plaque-like structures in the brain sections from transgenic mice but not in the postmortem human brain tissue of patients with confirmed AD. The results provide encouraging evidence that development of a 99mTc-labeled agent for imaging Aβ plaques in the brain may be feasible. Caution should be taken when comparing these 99mTc complexes with the corresponding surrogates - the Re complexes.