No abstract

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 ␤-amyloid (A␤) peptide fibrils. We recently reported the presence of three classes of binding sites (BS1, BS2, BS3) on the A␤ 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 K i values of 25 and 42 nM, respectively. BSB was unable to displace the radioligand tracer from the A␤ fibrils. In contrast, each of the compounds examined were able to displace thioflavin T (BS1 probe) from the A␤ fibrils when evaluated in a fluorescence competition assay with K i values for PIB, FDDNP, and BSB of 1865, 335, and 600 nM, respectively. Finally, the K d values for FDDNP and BSB binding to A␤ 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 A␤ 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.The development of imaging agents to detect the senile plaques and neurofibrillary tangles associated with Alzheimer disease (AD) 1 is a rapidly emerging and important field for both preclinical and clinical drug development (1). Three main structural classes or chemotypes of positron emission tomography imaging agents are currently under development and are derived from thioflavin T (PIB (2), IMPY (3), TZDM (4)), Congo Red/styrylbenzene (ISB (4), BSB (5), X34 (6), stilbene (7)) and aminonaphthalene (FDDNP, FENE (8)) backbones. These ligands are primarily believed to target the polymeric form of ␤-amyloid (A␤) peptide associated with the senile plaques and may in addition bind to neurofibrillary tangles, which are composed of tau protein polymers (1).Because the number of compounds under development has increased, a somewhat confusing picture has emerged with respect to the interaction of members of the different chemotype classes with the A␤ fibrils. Initial findings, which were based on radioligand binding assays, indicated an independent binding site on the A␤ fibrils for each of the three chemotypes. For example, Agdeppa et al. (9) demonstrated that [ 18 F]FDDNP was not displaced from A␤ fibrils by either Congo Red or thioflavin T. In addition, data from Zhuang et al. (4...

The distinctive cortical uptake of the tracer 18F‐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 3H‐FDDNP to brain sections containing AD hallmark pathologies. Semi‐adjacent sections were labelled with 3H‐PIB (Pittsburgh compound‐B, 2‐[4′‐(methylamino)phenyl]‐6‐hydroxybenzothiazole) and 14C‐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 3H‐FDDNP. Furthermore, in sections containing NFTs, there was no overt labelling of the pathology by 3H‐FDDNP. In contrast, sections labelled with 3H‐PIB displayed extensive labelling of diffuse plaques, classical plaques, cerebrovascular amyloid angiopathy and NFTs. 14C‐SB13 produced a broadly similar binding pattern to PIB. Radioligand binding assays employing in vitro generated amyloid‐β peptide fibrils demonstrated a ∼10‐fold reduced affinity for 3H‐FDDNP (85.0 ± 2.0 nM) compared with 3H‐PIB (8.5 ± 1.3 nM). These data provide an alternative mechanistic explanation for the observed low cortical uptake of 18F‐FDDNP in AD; in that the ligand is only weakly retained by the hallmark neuropathology because of its low affinity for amyloid structures.