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 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.
Highlights
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)
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 results from this study extend our previous model of ligand binding sites on A fibrils in that it demonstrates that BS1, BS2, and BS3 are able to accommodate a range of chemotype structures
Summary
The development of imaging agents to detect the senile plaques and neurofibrillary tangles associated with Alzheimer disease (AD) is a rapidly emerging and important field for both preclinical and clinical drug development (1). We have recently reported that the interaction of the thioflavin T class of ligands is much more complex than previously appreciated with three classes of binding sites (BSs) present on in vitro generated A-(1– 40) fibrils (12); they are BS1, a medium density site associated with the high fluorescence state of thioflavin T, BS2, a high density site with binding specificity for halogencontaining ligands such as TZPM and TZPI, and BS3, a low density site detected primarily by radioligand assays In light of these findings we have investigated whether the apparently contradictory results of the displacement assays could be rationalized in terms of the three-site binding model we proposed for the thioflavin T ligands. The results suggest that FDDNP does not have a single discrete type of binding site on the fibrils
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