Amyloid tracers detect multiple binding sites in Alzheimer's disease brain tissue

Abstract

Imaging fibrillar amyloid-β deposition in the human brain in vivo by positron emission tomography has improved our understanding of the time course of amyloid-β pathology in Alzheimer's disease. The most widely used amyloid-β imaging tracer so far is (11)C-Pittsburgh compound B, a thioflavin derivative but other (11)C- and (18)F-labelled amyloid-β tracers have been studied in patients with Alzheimer's disease and cognitively normal control subjects. However, it has not yet been established whether different amyloid tracers bind to identical sites on amyloid-β fibrils, offering the same ability to detect the regional amyloid-β burden in the brains. In this study, we characterized (3)H-Pittsburgh compound B binding in autopsied brain regions from 23 patients with Alzheimer's disease and 20 control subjects (aged 50 to 88 years). The binding properties of the amyloid tracers FDDNP, AV-45, AV-1 and BF-227 were also compared with those of (3)H-Pittsburgh compound B in the frontal cortices of patients with Alzheimer's disease. Saturation binding studies revealed the presence of high- and low-affinity (3)H-Pittsburgh compound B binding sites in the frontal cortex (K(d1): 3.5 ± 1.6 nM; K(d2): 133 ± 30 nM) and hippocampus (K(d1):5.6 ± 2.2 nM; K(d2): 181 ± 132 nM) of Alzheimer's disease brains. The relative proportion of high-affinity to low-affinity sites was 6:1 in the frontal cortex and 3:1 in the hippocampus. One control showed both high- and low-affinity (3)H-Pittsburgh compound B binding sites (K(d1): 1.6 nM; K(d2): 330 nM) in the cortex while the others only had a low-affinity site (K(d2): 191 ± 70 nM). (3)H-Pittsburgh compound B binding in Alzheimer's disease brains was higher in the frontal and parietal cortices than in the caudate nucleus and hippocampus, and negligible in the cerebellum. Competitive binding studies with (3)H-Pittsburgh compound B in the frontal cortices of Alzheimer's disease brains revealed high- and low-affinity binding sites for BTA-1 (Ki: 0.2 nM, 70 nM), florbetapir (1.8 nM, 53 nM) and florbetaben (1.0 nM, 65 nM). BF-227 displaced 83% of (3)H-Pittsburgh compound B binding, mainly at a low-affinity site (311 nM), whereas FDDNP only partly displaced (40%). We propose a multiple binding site model for the amyloid tracers (binding sites 1, 2 and 3), where AV-45 (florbetapir), AV-1 (florbetaben), and Pittsburgh compound B, all show nanomolar affinity for the high-affinity site (binding site 1), as visualized by positron emission tomography. BF-227 shows mainly binding to site 3 and FDDNP shows only some binding to site 2. Different amyloid tracers may provide new insight into the pathophysiological mechanisms in the progression of Alzheimer's disease.

What amyloid ligands can tell us about molecular polymorphism and disease

“…18 F has the advantages of a longer, more user-friendly radioactive half-life (2 h vs. 20 min for 11 C) as well as lower non-specific uptake than with PIB. These ligands compete for PIB binding in the AD brain (Ni, et al, 2013). Other ligands with different structures also are being evaluated.…”

[18F]Florbetaben: A Review in β-Amyloid PET Imaging in Cognitive Impairment

“…In vitro, [ 18 F]florbetaben showed nanomolar binding affinity to synthetic b-amyloid fibrils [10,11] and to b-amyloid plaques in human AD brain homogenates [inhibition constant (K i ) 6.7 nmol/L] [9]. A high-and a low-affinity binding site (K i 1.0 and 65 nmol/L) have been identified for unlabelled florbetaben in frontal cortex homogenates of human AD brain [12].…”

The role of PET quantification in neurological imaging: FDG and amyloid imaging in dementia

“…While none of the patients with a negative scan progressed to AD in this cohort, the probability of progression did not differ between those who showed high cortical Attempts to interpret quantitative amyloid tracer binding in physiological terms are also compromised by the fact that binding is due to the physicochemical properties of the amyloid beta-sheet formation [47] and, in contrast to neuroreceptor studies, does not relate to a single receptor. A group at Karolinska [48] described at least three binding sites with different properties, and recent data suggest that binding involves a ganglioside-amyloid beta complex [49]. Possibly tracer binding to enzymes, such as oestrogen sulfotransferase [50], also contribute to a non-specific signal background.…”