Synthesis and evaluation of Tc-99m labeled peptide conjugates for imaging amyloid-β in the brain

Objectives Amyloid-β (Aβ) imaging agents could provide a noninvasive diagnostic tool to assess plaque burden and monitoring efficacy of anti-amyloid therapeutics for patients with Alzheimer’s disease (AD). To exploit the accessibility of the large scale clinically-installed infrastructure for SPECT imaging cameras and a generator produced radionuclide, we have synthesized and analytically characterized a Tc-99m labeled peptide that selectively binds to Aβ.

Methods ^99mTc-peptide binding to Aβ fibrils and its stability in human serum were both measured in vitro. Fluorescein(FM)-labeled peptide binding to Aβ plaques was determined with both APPsw^+/- mouse brain sections and post-mortem AD human brain sections (CDR=3) ex vivo. ^99mTc-peptide accumulation into the brains of APPsw^+/- mice was assessed 2 h post tail-vein injection via phosphorimaging ex vivo and NanoSPECT imaging in vivo.

Results ^99mTc-peptide binds to Aβ fibrils in a concentration dependent manner, displaying a nearly saturable binding profile. ^99mTc-peptide is stable in human serum for 3 hours at 37^oC. FM-peptide stains Aβ plaques in both APPsw^+/- mouse and human hippocampal (CDR=3) brain tissue sections, but not their age-matched controls. Phosphorimaging of brain sections extracted from mice 2 h post IV-injection shows accumulation of ^99mTc-peptide in the brains APPsw^+/- mice, but none in control mice, providing evidence that ^99mTc-peptide crosses the blood brain barrier (BBB). Preliminary live animal imaging shows greater signal in the brains of APPsw^+/- mice compared to controls.

Conclusions ^99mTc-peptide binds to Aβ fibrils in vitro and Aβ plaques in vivo. It is stable in human serum and traverses the BBB. Upon further validation and optimization, this peptide scaffold may provide a promising SPECT template for assessing Aβ plaque burden in vivo.

Research Support NIH AG030498 (VS), AG03328(VS), P50CA94056 (DPW)