Evidence for the Presence of Three Distinct Binding Sites for the Thioflavin T Class of Alzheimer's Disease PET Imaging Agents on β-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 β-amyloid peptide (Aβ)-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 Aβ-(1–40) polymers. All of the compounds were derived from the benzothiazole compound thioflavin T and include 2-4′-(methylamino)phenylbenzothiazole and 2-(4′-dimethylamino-)phenyl-imidazo1,2-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 Aβ fibrils. All of the compounds bound with very high affinity (low nmKd) to a low capacity site (BS3) (1 ligand-binding site per ∼300 Aβ-(1–40) monomers) consistent with the previously recognized binding site for these compounds on the fibrils. However, the compounds also bound with high affinity (Kd ∼100 nm) to either one of two additional binding sites on the Aβ-(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 ∼35 or every ∼4 monomers of Aβ-(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. 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 β-amyloid peptide (Aβ)-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 Aβ-(1–40) polymers. All of the compounds were derived from the benzothiazole compound thioflavin T and include 2-4′-(methylamino)phenylbenzothiazole and 2-(4′-dimethylamino-)phenyl-imidazo1,2-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 Aβ fibrils. All of the compounds bound with very high affinity (low nmKd) to a low capacity site (BS3) (1 ligand-binding site per ∼300 Aβ-(1–40) monomers) consistent with the previously recognized binding site for these compounds on the fibrils. However, the compounds also bound with high affinity (Kd ∼100 nm) to either one of two additional binding sites on the Aβ-(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 ∼35 or every ∼4 monomers of Aβ-(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. Alzheimer's disease (AD) 1The abbreviations used are: AD, Alzheimer's disease; Aβ, amyloid β-peptide; SP, senile plaque; NFT, neurofibrillary tangle; PET, positron emission tomography; FLINT, fluorescence intensity; FRET, fluorescence energy transfer; BP, binding potential; Thio T, thioflavin T; BTA-1, 2-4′-(methylamino)phenylbenzothiazole; 6-Me-BTA-1, 2-4′-(methylamino)phenyl-6-methylbenzothiazole; IMPY-H, 2-(4′-dimethylamino-)phenyl-imidazo1,2-a-pyridine; IMPY-Me, 6-methyl-2-(4′-dimethylamino)phenyl-imidazo1,2-a-pyridine; TZDM, 2-4′-(dimethylamino)phenyl-6-iodobenzothiazole; TZPI, 2-4′-(4″-methylpiperazin-1-yl)phenyl-6-iodobenzothiazole; BF1, 5-bromo-2-(4-dimethylaminophenyl)benzofuran; PIB, Pittsburgh compound B. is a common neurological disease of chronic dementia, memory loss, and cognitive impairment. Central to the neuropathology of AD is the deposition in specific brain regions of polymeric peptide/protein deposits, composed of β-amyloid peptide (Aβ) and tau protein, termed senile plaques (SPs) and neurofibrillary tangles (NFTs), respectively (reviewed in Ref. 1Vickers J.C. Dickson T.C. Adlard P.A. Saunders H.L. King C.E. McCormack G. Prog. Neurobiol. 2000; 60: 139-165Crossref PubMed Scopus (220) Google Scholar). The precise role of these deposits in the pathogenesis of AD is still unclear. However, individuals with mutations leading to increased Aβ production are at a high risk of developing AD (2Selkoe D.J. Schenk D. Annu. Rev. Pharmacol. Toxicol. 2003; 43: 545-584Crossref PubMed Scopus (754) Google Scholar), and individuals with mutations in the tau gene are linked to a class of neurodegenerative diseases including frontotemporal dementia and parkinsonism linked to chromosome 17 (3Tolnay M. Probst A. IUBMB. Life. 2003; 55: 299-305Crossref PubMed Scopus (48) Google Scholar). Moreover, the accumulation of SPs and NFTs follows a predictable temporal and spatial sequence in individuals affected with AD (4Braak H. Braak E. Acta Neuropathol. 1991; 82: 239-259Crossref PubMed Scopus (12153) Google Scholar, 5Gold G. Kovari E. Corte G. Herrmann F.R. Canuto A. Bussiere T. Hof P.R. Bouras C. Giannakopoulos P. J. Neuropathol. Exp. Neurol. 2001; 60: 946-952Crossref PubMed Scopus (60) Google Scholar). Detection of these deposits may therefore provide a clinically useful tool for monitoring disease progression and treatment effects. Positron emission tomography (PET) uses positron emitting radioligands to image and measure ligand-binding sites in vivo and is currently being developed as a sensitive way to track SPs and NFTs in patients (reviewed in Ref. 6Nordberg A. Lancet Neurol. 2004; 3: 519-527Abstract Full Text Full Text PDF PubMed Scopus (386) Google Scholar). Three broad categories of PET ligands of AD-associated polymers are currently under investigation. One class is derived from the dye thioflavin T (Thio T) and includes BTA/PIB (7Klunk W.E. Wang Y. Huang G.F. Debnath M.L. Holt D.P. Mathis C.A. Life Sci. 2001; 69: 1471-1484Crossref PubMed Scopus (405) Google Scholar, 8Mathis C.A. Wang Y. Holt D.P. Huang G.F. Debnath M.L. Klunk W.E. J. Med. Chem. 2003; 46: 2740-2754Crossref PubMed Scopus (876) Google Scholar), IMPY (9Kung M.P. Hou C. Zhuang Z.P. Zhang B. Skovronsky D. Trojanowski J.Q. Lee V.M. Kung H.F. Brain Res. 2002; 956: 202-210Crossref PubMed Scopus (201) Google Scholar), TZDM, and TZPI (10Zhuang Z.P. Kung M.P. Hou C. Skovronsky D.M. Gur T.L. Plossl K. Trojanowski J.Q. Lee V.M. Kung H.F. J. Med. Chem. 2001; 44: 1905-1914Crossref PubMed Scopus (205) Google Scholar). A second class is characterized by a styrylbenzene backbone and includes X34 (11Styren S.D. Hamilton R.L. Styren G.C. Klunk W.E. J. Histochem. Cytochem. 2000; 48: 1223-1232Crossref PubMed Scopus (236) Google Scholar), X04 (12Klunk W.E. Bacskai B.J. Mathis C.A. 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The binding of these compounds to Aβ-(1–40) fibrils generated in vitro has been investigated in a number of laboratories, and the results of these studies so far are consistent with three independent binding sites, one for each class of and with high binding However, the binding for these classes of ligand has been to with ligand-binding sites occurring every C.A. Wang Y. Holt D.P. Huang G.F. Debnath M.L. Klunk W.E. J. Med. Chem. 2003; 46: 2740-2754Crossref PubMed Scopus (876) Google or (14Agdeppa E.D. Kepe V. Liu J. Flores-Torres S. Satyamurthy N. Petric A. Cole G.M. Small G.W. Huang S.C. Barrio J.R. J. Neurosci. 2001; 21: 189Crossref Google of Aβ-(1–40) and a of the to from development of these radioligands clinically of the ligands and FDDNP have so far to in vivo studies W.E. H. A. Wang Y. G. Holt D.P. M. Huang G.F. S. B. Debnath M.L. J. J.C. J. B.J. A. P. G. Mathis C.A. B. Neurol. 2004; 55: PubMed Scopus Google Scholar, G.W. E.D. Kepe V. Satyamurthy N. Huang S.C. Barrio J.R. J. Neurosci. 2002; PubMed Google Scholar), of and AD The results increased of in of the brain to affected by However, the precise of the in these studies the low of ligand-binding sites on Aβ fibrils. The is by vivo of binding to brain from patients with AD C.A. Wang Y. Holt D.P. Huang G.F. Debnath M.L. Klunk W.E. J. Med. Chem. 2003; 46: 2740-2754Crossref PubMed Scopus (876) Google Scholar). Aβ were in were from a by J. V. P. P. J. Am. Med. 2000; PubMed Scopus Google Scholar). The a binding higher than based on the of Aβ fibrils in vitro C.A. Wang Y. Holt D.P. Huang G.F. Debnath M.L. Klunk W.E. J. Med. Chem. 2003; 46: 2740-2754Crossref PubMed Scopus (876) Google Scholar). The binding properties of the ligands to Aβ fibrils have been characterized by using binding either of radioligand to (10Zhuang Z.P. Kung M.P. Hou C. Skovronsky D.M. Gur T.L. Plossl K. Trojanowski J.Q. Lee V.M. Kung H.F. J. Med. Chem. 2001; 44: 1905-1914Crossref PubMed Scopus (205) Google Scholar, H.F. Lee C.W. Zhuang Z.P. Kung M.P. Hou C. Plossl K. J. Am. Chem. Soc. 2001; 123: 12740-12741Crossref PubMed Scopus (180) Google or a low of radioligand and of (7Klunk W.E. Wang Y. Huang G.F. Debnath M.L. Holt D.P. Mathis C.A. Life Sci. 2001; 69: 1471-1484Crossref PubMed Scopus (405) Google Scholar, H.F. Kung M.P. Zhuang Z.P. Hou C. Lee C.W. Plossl K. Zhuang B. Skovronsky D.M. Lee V.M. Trojanowski J.Q. Imaging 2003; PubMed Scopus Google Scholar). The very high affinity binding as affinity the the is a and sensitive to ligand-binding The results of these have been by ligand-binding have these by developing a of binding that of the fluorescence of Thio T and a number of compounds to measure their binding properties a wide range of ligand and Aβ The results of these were with radioligand binding and are consistent with the presence of the previously described low ligand-binding site also the presence of additional higher ligand-binding sites on the fibrils. The selectivity of these additional sites and their potential for the of PET radioligands for the of AD and are and of the compounds in are in and are as thioflavin T (Thio 2-4′-(methylamino)phenylbenzothiazole C.A. Bacskai B.J. Kajdasz S.T. McLellan M.E. Frosch M.P. Hyman B.T. Holt D.P. Wang Y. Huang G.F. Debnath M.L. Klunk W.E. Med. Chem. 2002; PubMed Scopus Google (7Klunk W.E. Wang Y. Huang G.F. Debnath M.L. Holt D.P. Mathis C.A. Life Sci. 2001; 69: 1471-1484Crossref PubMed Scopus (405) Google (9Kung M.P. Hou C. Zhuang Z.P. Zhang B. Skovronsky D. Trojanowski J.Q. Lee V.M. Kung H.F. Brain Res. 2002; 956: 202-210Crossref PubMed Scopus (201) Google (9Kung M.P. Hou C. Zhuang Z.P. Zhang B. Skovronsky D. Trojanowski J.Q. Lee V.M. Kung H.F. Brain Res. 2002; 956: 202-210Crossref PubMed Scopus (201) Google 2-4′-(dimethylamino)phenyl-6-iodobenzothiazole (10Zhuang Z.P. Kung M.P. Hou C. Skovronsky D.M. Gur T.L. Plossl K. Trojanowski J.Q. Lee V.M. Kung H.F. J. Med. Chem. 2001; 44: 1905-1914Crossref PubMed Scopus (205) Google (10Zhuang Z.P. Kung M.P. Hou C. Skovronsky D.M. Gur T.L. Plossl K. Trojanowski J.Q. Lee V.M. Kung H.F. J. Med. Chem. 2001; 44: 1905-1914Crossref PubMed Scopus (205) Google and M. Kung M.P. Hou C. Kung H.F. Med. 2002; PubMed Scopus Google Scholar). Thio T from The of the compounds and for by high and by All were from with the of and that were from and of Aβ-(1–40) number from at in at for in an at The of fibrils by binding 2004; and Thio T binding 2004; with by using were used production or and at in ligand binding and as also by (7Klunk W.E. Wang Y. Huang G.F. Debnath M.L. Holt D.P. Mathis C.A. Life Sci. 2001; 69: 1471-1484Crossref PubMed Scopus (405) Google Scholar). compounds were as The final of in the of the low of the ligands the of Thio T) in were in with and at a of fluorescence with ligand binding to Aβ-(1–40) were in a using a the binding of Thio T, a were used for the using a of ligand or nm) and of Aβ-(1–40) polymer. using a of Aβ-(1–40) nm) and of ligand to as under were in a using a and emission The a of nm) and of Aβ-(1–40) All fluorescent were in in a final of and were for at All were at in and were using to using the site ligand-binding a of Aβ-(1–40) and Thio T (1 and used of ligands to were in in a final of and were for at in a using a All were in and the fluorescent in the of a binding of were using to using were derived from the Y. Pharmacol. PubMed Scopus Google Scholar), were the nm) and nm) of Thio T used in the of Aβ-(1–40) and Thio T (1 and either or in were in a nm) were at using and emission of and respectively. of Aβ-(1–40) nm) a range of nm) in with for at The final binding in the presence of Thio T. The bound and were by using an by with the bound ligand were with of Life and for in a Life All were in The specific binding under these were using to the and the number of binding sites using the site ligand-binding a of Aβ-(1–40) nm) and nm) and used of on the were in and were for at and as described binding in the presence of Thio T. nm) used to binding for Thio T. All were in and the specific binding in the of a binding of were using to using were derived as using the Y. Pharmacol. PubMed Scopus Google Scholar), the nm) and nm) of the radioligand used in the of Aβ were on of each of and of in A of Aβ fibrils in on of the and at for and at in an using an the of the were with a and were for the of the and the of the The were for thioflavin T binding by of each with of thioflavin and the fluorescence of in an using a of each by by as described previously G. A. B. H. S. P. Y. K. P. J. J. 2003; PubMed Scopus Google Scholar). were with the β-amyloid at a by an at a and were developed with of the were using a and using binding by of each with of in The were for at and as described by of compounds used in fluorescent with and emission of and that are from of Thio T nm) the compounds display in their bound to Aβ-(1–40) fibrils with in their and emission consistent with ligand binding a on the fibrils. The to BTA-1, a in bound to the Aβ-(1–40) fibrils. The in ligand binding to Aβ-(1–40) fibrils were in to measure compound were and assays, compound binding by using a of ligand or nm) and of Aβ-(1–40) A and A of Aβ-(1–40) nm) and of ligand were used in the and The binding and from the and are in of fluorescent and radioligand binding is that the were of with the of that the were of with the of is that the were of with the of in a new The binding of to Aβ-(1–40) fibrils by fluorescent using a of ligand nm) and of The from The of the compound at of is the for a PubMed Scopus Google and the of the Aβ-(1–40) polymers. The binding for of the ligands in were consistent with a of binding sites for each of the The from the were to on the with the the of binding for the compounds the two of the with the previously that fluorescence are higher the of binding is The to Thio T to that using a by H. K. M. T. PubMed Scopus Google and H. Sci. PubMed Scopus Google Scholar), that the of binding of with of on the Aβ that the binding were on as report the However, an for the number of potential binding sites on the Aβ The binding previously with these ligands that the binding site on the Aβ fibrils site per Aβ peptide C.A. Wang Y. Holt D.P. Huang G.F. Debnath M.L. Klunk W.E. J. Med. Chem. 2003; 46: 2740-2754Crossref PubMed Scopus (876) Google Scholar, E.D. Kepe V. Liu J. Flores-Torres S. Satyamurthy N. Petric A. Cole G.M. Small G.W. Huang S.C. Barrio J.R. J. Neurosci. 2001; 21: 189Crossref Google Scholar). that the of binding sites on the fibrils than the of Aβ peptide monomers to the of the are based on the Aβ peptide to an of the binding by a on the binding site the of is on the ligand and as a the the binding the an of the ligand-binding site on the Aβ-(1–40) fibrils the ligands two Thio T, IMPY-H, and have a of ligand-binding site per ∼35 Aβ-(1–40) TZDM, TZPI, and have a of ligand-binding site per ∼4 Aβ-(1–40) that were at two classes of binding sites on the Aβ fibrils and are by the that the of compounds a halogen on their aromatic the from the fluorescence are as consistent with a affinity binding for each ligand and binding higher than that previously and with the of the binding site and selectivity the that more than one class of binding site on the Aβ a low site ∼35 Aβ monomers) and a higher site ∼4 Aβ monomers) that are termed BS1 and BS2, respectively. of BS1 and binding to Aβ-(1–40) fibrils characterized by using a fluorescent Thio T as a for The from these were higher than in the However, they were to the additional that the fluorescent were binding and consistent with that were additional ligand-binding sites on the Aβ fibrils. The binding were consistent with from a of site for each ligand on the fibrils and also that the compounds the two from the that and were to of the bound Thio T TZDM, TZPI, and were to of the bound Thio T Full for the IMPY ligands to compound at higher is that they are of the Thio T and that their are consistent with their The of the ligands TZDM, TZPI, and to partially Thio T may a of the spatial of higher BS2 to BS1 on the Aβ fibrils that they either partially or with BS1 and BS2 on the Aβ for the spatial of BS1 and BS2 from fluorescent energy The fluorescent properties of nm) and Thio T nm) were as a potential By using an of BF1, the emission of Aβ-(1–40) fibrils with either Thio T or Thio were of the the the presence of an emission at to the for the Thio T emission by with BTA-1, has with Thio T for fluorescent at with the that is to energy to Thio T consistent with BS1 and BS2 independent ligand-binding sites on the Aβ fibrils. a of on the Aβ to the in binding the and fluorescent were of the the binding of to Aβ-(1–40) fibrils by using a binding The binding consistent with the presence of a high affinity binding site on the with an of and a binding of at Aβ The is higher than the previously for compound of (7Klunk W.E. Wang Y. Huang G.F. Debnath M.L. Holt D.P. Mathis C.A. Life Sci. 2001; 69: 1471-1484Crossref PubMed Scopus (405) Google and the low to ligand-binding site per Aβ-(1–40) is consistent with that for compounds as and C.A. Wang Y. Holt D.P. Huang G.F. Debnath M.L. Klunk W.E. J. Med. Chem. 2003; 46: 2740-2754Crossref PubMed Scopus (876) Google Scholar). The from of were consistent with the of the previously described very low site on Aβ-(1–40) fibrils. that the ligand-binding of site were from BS1 and BS2, is that a class of binding termed binding assays, of the ligands were to in a consistent with a class of binding sites on the Aβ The derived from these were than the derived from the and Thio T fluorescent However, the of binding the radioligand and fluorescent the radioligand have a under a of of ligand binding to low site on the Aβ-(1–40) fibrils. The from the current of are in with the previously for these compounds in of and of binding The to ligand binding at by using fluorescent may to a number of The of site to a binding or would to the more BS1 and By that a ligand an bound to of the sites, the from would a or a than that at BS1 and BS2, respectively. is that ligand binding to may or in ligand fluorescence to by either or BS2, and on the Aβ the were to that BS1 and BS2 were in on the Aβ the radioligand that is also on the of fibrils. we the Aβ fibrils by using a from J. G. A. J. 2000; PubMed Scopus Google Scholar), and we the for Aβ and thioflavin T and binding on the of the the the two on and The of the from the is in and is consistent with the high of the Aβ fibrils and to that previously using J. G. A. J. 2000; PubMed Scopus Google Scholar). is a on the of Aβ of a second of high in the of than The ligand binding properties of Thio T and to these display an pattern to each and also to the from the Aβ the that the ligand-binding sites are the Aβ and are to of polymers. The development of useful PET ligands for the detection of SPs is being of the would have A. Lancet Neurol. 2004; 3: 519-527Abstract Full Text Full Text PDF PubMed Scopus (386) Google Scholar). However, the binding properties of these PET ligands to Aβ fibrils have been the of the to have developed and a range of for the have a of the binding of a of Thio T to Aβ-(1–40) fibrils in are consistent with the presence of a previously described binding site (BS3) to of the compounds with high also the presence of two additional high affinity and more binding sites and The of these additional binding sites on the Aβ-(1–40) fibrils may the of the in vivo by PET with these A number of of the presence of ligand-binding sites on the Aβ-(1–40) polymers. the and from two independent are higher than the from and the currently described radioligand binding of additional ligand-binding sites on the Aβ-(1–40) fibrils. the also that the binding site of BS1 and BS2 are higher than that of the selectivity of compounds a halogen substitution on their aromatic ring a for BS1 and The of to and a site that BS1 and BS2 are in complete on the However, the binding for ligands and would that the site is a of the polymer. A the potential of binding sites on the Aβ-(1–40) is in that three of binding sites are on the of Aβ fibrils in the and the the spatial of BS1 and BS2 and the of fluorescence and sites of polymers by A high of amyloid fibrils from Aβ is to the ligand-binding sites with on the polymer. The in the fluorescence of the ligands is of compound binding a on the and that BS1 and BS2 are with the two and on the Aβ peptide (reviewed in Ref. 2000; PubMed Scopus Google Scholar). The presence of classes of binding sites on the Aβ fibrils is also by two studies using the ligand The using a radioligand binding that by Thio T from Aβ-(1–40) fibrils E.D. Kepe V. A. Satyamurthy N. Liu J. Huang S.C. Small G.W. Cole G.M. Barrio J.R. 2003; PubMed Scopus Google Scholar). However, a T. N. T. M. H. H. T. Y. T. Neurosci. Res. 2004; 48: PubMed Scopus Google in a fluorescent using Thio T and Aβ-(1–40) fibrils an to that described FDDNP to Thio T. are consistent with the presence of independent and binding sites on the fibrils. The in the fluorescence of Thio T has been as a dye in the of AD Google Scholar, G. J. Histochem. Cytochem. PubMed Scopus Google Scholar). would suggest that the in the is with with BS1 and would also suggest that the fluorescent with the of these Thio T in the compounds are used at high is from from the in with the higher BS1 and for the of a radioligand in in vivo of the binding potential is the of a for PET studies C.A. Wang Y. Holt D.P. Huang G.F. Debnath M.L. Klunk W.E. J. Med. Chem. 2003; 46: 2740-2754Crossref PubMed Scopus (876) Google Scholar). The in vivo of for the of the has been J. V. P. P. J. Am. Med. 2000; PubMed Scopus Google have the of Aβ-(1–40) and and in of from and AD suggest that of Aβ in the regions of AD By using the of the with the of for a of is However, the in vitro binding of the radioligand binding to is the is to is to a PET in higher have been for and binding to AD brain the of Aβ in were C.A. Wang Y. Holt D.P. Huang G.F. Debnath M.L. Klunk W.E. J. Med. Chem. 2003; 46: 2740-2754Crossref PubMed Scopus (876) Google Scholar). One that from the of studies is as potential of BS1 and BS2 with in the from a PET (Kd and (Kd nm) as compounds for BS2, and and the of these sites and the of Aβ to a of are and respectively. The for of the compounds are at the of the for Aβ suggest that BS2 may a more than BS1 and to the PET also suggest that the of higher affinity compounds BS2 may in PET ligands for the radioligand currently in for compound and may the site on the the low BS3, and that as using Thio T, may compounds with more properties for in vivo a complex for the binding of PET radioligands being for SPs in AD, with the presence of ligand-binding sites on Aβ fibrils. The development of for AD is by the that is sensitive of the progression and of the is affected A. Lancet Neurol. 2004; 3: 519-527Abstract Full Text Full Text PDF PubMed Scopus (386) Google Scholar). the properties of Aβ fibrils in the more of more specific PET radioligands to track the progression of disease and the development of and for with the radioligand binding and and and for