In Vivo Detection of Amyloid-β Deposits Using Heavy Chain Antibody Fragments in a Transgenic Mouse Model for Alzheimer's Disease

This study investigated the in vivo properties of two heavy chain antibody fragments (VHH), ni3A and pa2H, to differentially detect vascular or parenchymal amyloid-beta deposits characteristic for Alzheimer's disease and cerebral amyloid angiopathy. Blood clearance and biodistribution including brain uptake were assessed by bolus injection of radiolabeled VHH in APP/PS1 mice or wildtype littermates. In addition, in vivo specificity for A beta was examined in more detail with fluorescently labeled VHH by circumventing the blood-brain barrier via direct application or intracarotid co-injection with mannitol. All VHH showed rapid renal clearance (10-20 min). Twenty-four hours post-injection Tc-99m-pa2H resulted in a small yet significant higher cerebral uptake in the APP/PS1 animals. No difference in brain uptake were observed for Tc-99m-ni3A or DTPA(In-111)-pa2H, which lacked additional peptide tags to investigate further clinical applicability. In vivo specificity for A beta was confirmed for both fluorescently labeled VHH, where pa2H remained readily detectable for 24 hours or more after injection. Furthermore, both VHH showed affinity for parenchymal and vascular deposits, this in contrast to human tissue, where ni3A specifically targeted only vascular A beta. Despite a brain uptake that is as yet too low for in vivo imaging, this study provides evidence that VHH detect A beta deposits in vivo, with high selectivity and favorable in vivo characteristics, making them promising tools for further development as diagnostic agents for the distinctive detection of different A beta deposits.