Interaction of the amyloid imaging tracer FDDNP with hallmark Alzheimer's disease pathologies.

The distinctive cortical uptake of the tracer (18)F-FDDNP (2-(1-{6-[(2-fluoroethyl(methyl)amino]-2-naphthyl}ethylidene)malononitrile) in Alzheimer's disease (AD) is believed to be because of its binding to both neurofibrillary tangles (NFTs) and highly fibrillar senile plaques. We therefore investigated the binding of a tracer concentration of (3)H-FDDNP to brain sections containing AD hallmark pathologies. Semi-adjacent sections were labelled with (3)H-PIB (Pittsburgh compound-B, 2-[4'-(methylamino)phenyl]-6-hydroxybenzothiazole) and (14)C-SB13 (4-N-methylamino-4'-hydroxystilbene) for comparison. Neocortical sections containing widespread senile plaques and cerebrovascular amyloid angiopathy, produced a sparse and weak labelling following incubation with (3)H-FDDNP. Furthermore, in sections containing NFTs, there was no overt labelling of the pathology by (3)H-FDDNP. In contrast, sections labelled with (3)H-PIB displayed extensive labelling of diffuse plaques, classical plaques, cerebrovascular amyloid angiopathy and NFTs. (14)C-SB13 produced a broadly similar binding pattern to PIB. Radioligand binding assays employing in vitro generated amyloid-beta peptide fibrils demonstrated a approximately 10-fold reduced affinity for (3)H-FDDNP (85.0 +/- 2.0 nM) compared with (3)H-PIB (8.5 +/- 1.3 nM). These data provide an alternative mechanistic explanation for the observed low cortical uptake of (18)F-FDDNP in AD; in that the ligand is only weakly retained by the hallmark neuropathology because of its low affinity for amyloid structures.

Characterisation of the binding of amyloid imaging tracers to rodent Abeta fibrils and rodent-human Abeta co-polymers.

Despite the application of amyloid imaging agents such as PIB, SB13, and FDDNP in Alzheimer's disease (AD) patients, the successful use of these agents in transgenic mice models of AD has not been reported to date. As a first step in understanding the behaviour of these ligands in transgenic models of AD, we have investigated in a series of in vitro ligand binding assays the interaction of selected agents, including PIB, FDDNP, SB13, and BSB, with amyloid fibrils produced from rodent Abeta(1-40) (roAbeta) peptide. The data indicate that the ligand binding affinities together with the pattern and number of binding sites on the roAbeta fibrils are broadly conserved with that reported previously for human Abeta(1-40) (huAbeta) fibrils. However, characterisation of huAbeta fibrils formed in the presence of increasing amounts of roAbeta (1, 5, 10% w/w) demonstrated a dose-dependent reduction in the number of high affinity [(3)H]Me-BTA-1 binding sites such that at the highest amount of roAbeta the specific signal was reduced by approximately 95%. These studies suggest that (i) the presence of small amounts of roAbeta in huAbeta fibrils has the potential to cause subtle ultrastructural alterations in the polymers and (ii) the weak binding signal observed in vivo in the transgenic mouse models of AD may in part be due to the decreased number of high affinity binding sites on the Abeta fibrils.

Delineation of positron emission tomography imaging agent binding sites on beta-amyloid peptide fibrils.

A range of imaging agents for use in the positron emission tomography of Alzheimer's disease is currently under development. Each of the main compound classes, derived from thioflavin T (PIB), Congo Red (BSB), and aminonaphthalene (FDDNP) are believed to bind to mutually exclusive sites on the beta-amyloid (Abeta) peptide fibrils. We recently reported the presence of three classes of binding sites (BS1, BS2, BS3) on the Abeta fibrils for thioflavin T derivatives and now extend these findings to demonstrate that these sites are also able to accommodate ligands from the other chemotype classes. The results from competition assays using [3H]Me-BTA-1 (BS3 probe) indicated that both PIB and FDDNP were able to displace the radioligand with Ki values of 25 and 42 nM, respectively. BSB was unable to displace the radioligand tracer from the Abeta fibrils. In contrast, each of the compounds examined were able to displace thioflavin T (BS1 probe) from the Abeta fibrils when evaluated in a fluorescence competition assay with Ki values for PIB, FDDNP, and BSB of 1865, 335, and 600 nM, respectively. Finally, the Kd values for FDDNP and BSB binding to Abeta fibrils were directly determined by monitoring the increases in the ligand intrinsic fluorescence, which were 290 and 104 nM, respectively. The results from these assays indicate that (i) the three classes of thioflavin T binding sites are able to accommodate a wide range of chemotype structures, (ii) BSB binds to two sites on the Abeta fibrils, one of which is BS2, and the other is distinct from the thioflavin T derivative binding sites, and (iii) there is no independent binding site on the fibrils for FDDNP, and the ligand binds to both the BS1 and BS3 sites with significantly lower affinities than previously reported.

Evidence for the presence of three distinct binding sites for the thioflavin T class of Alzheimer's disease PET imaging agents on beta-amyloid peptide fibrils.

Imaging the progression of Alzheimer's disease would greatly facilitate the discovery of therapeutics, and a wide range of ligands are currently under development for the detection of beta-amyloid peptide (Abeta)-containing plaques by using positron emission tomography. Here we report an in-depth characterization of the binding of seven previously described ligands to in vitro generated Abeta-(1-40) polymers. All of the compounds were derived from the benzothiazole compound thioflavin T and include 2-[4'-(methylamino)phenyl]benzothiazole and 2-(4'-dimethylamino-)phenyl-imidazo[1,2-a]-pyridine derivatives, 2-[4'-(dimethylamino)phenyl]-6-iodobenzothiazole and 2-[4'-(4''-methylpiperazin-1-yl)phenyl]-6-iodobenzothiazole, and a benzofuran compound (5-bromo-2-(4-dimethylaminophenyl)benzofuran). By using a range of fluorescent and radioligand binding assays, we find that these compounds display a more complex binding pattern than described previously and are consistent with three classes of binding sites on the Abeta fibrils. All of the compounds bound with very high affinity (low nm K(d)) to a low capacity site (BS3) (1 ligand-binding site per approximately 300 Abeta-(1-40) monomers) consistent with the previously recognized binding site for these compounds on the fibrils. However, the compounds also bound with high affinity (K(d) approximately 100 nm) to either one of two additional binding sites on the Abeta-(1-40) polymer. The properties of these sites, BS1 and BS2, suggest they are adjacent or partially overlapping and have a higher capacity than BS3, occurring every approximately 35 or every approximately 4 monomers of Abeta-(1-40)-peptide, respectively. Compounds appear to display selectivity for BS2 based on the presence of a halogen substitution (2-[4'-(dimethylamino)phenyl]-6-iodobenzothiazole, 2-[4'-(4''-methylpiperazin-1-yl)phenyl]-6-iodobenzothiazole, and 5-bromo-2-(4-dimethylaminophenyl)benzofuran) on their aromatic ring system. The presence of additional ligand-binding sites presents potential new targets for ligand development and may allow a more complete modeling of the current positron emission tomography data.