Push–pull benzothiazole derivatives as probes for detecting β-amyloid plaques in Alzheimer’s brains
Graphical abstract
Introduction
The formation of β-amyloid (Aβ) plaques is a key neurodegenerative event in Alzheimer’s disease (AD).1, 2 Since the imaging of these plaques in vivo may lead to the presymptomatic diagnosis of AD, many molecular probes for this purpose, including PET/SPECT and MRI tracers, have been developed.3, 4, 5, 6, 7, 8, 9, 10, 11, 12 The PET ligand [11C]-2-(4-(methylamino)phenyl)-6-hydroxybenzothiazole (6-OH-BTA-1 or PIB) with a benzothiazole backbone (Fig. 1) has shown particular promise in early clinical trials and is currently being used in a number of human studies.13, 14, 15 In addition to PET/SPECT and MRI probes, much attention has focused on the development of near-infrared fluorescent (NIRF) probes targeting Aβ plaques.16, 17, 18 NIRF probes are typically small molecule fluorescent dyes designed to absorb and emit light in the near-infrared region, where tissue scattering and absorption is lowest. The simple synthesis, low-cost, and long shelf-life of NIRF probes, together with the low-cost of optical imaging devices, present an attractive alternative to MRI and PET/SPECT techniques.
Among NIRF probes reported, to date, NIAD crosses the blood–brain barrier, selectively binds Aβ with high affinity, clears quickly from the brain, and absorbs and emits within the near-infrared region (650–900 nm), often called the ‘optical window’ (Fig. 1).17 A series of NIAD derivatives have been designed and synthesized based on a classical push–pull architecture with terminal donor (hydroxy or dimethylamino group) and acceptor (dicyanomethylene group) moieties that are interconnected by a highly polarized bridge (dithienylethenyl group), because various donor and acceptor groups can be used to manipulate the relative energies of HOMO and LUMO and obtain the desired long wavelength of absorption/emission bands.17
On the basis of this approach to the molecular design, we planned to develop novel push–pull dyes for detecting Aβ plaques in the brain. We selected benzothiazole or styrylbenzothiazole as the highly polarized bridge, a dimethylamino group as the donor, and a dicyanomethylene group as the acceptor. In the present study, we designed and synthesized two benzothiazole-derived push–pull dyes (PP-BTA-1 and PP-BTA-2 in Fig. 2), and evaluated their biological potential as probes for detecting Aβ plaques in the brain. To our knowledge, this is the first time push–pull benzothiazole derivatives have been proposed as Aβ imaging probes for detecting AD.
Section snippets
Results and discussion
The target benzothiazole derivatives were prepared as shown in Scheme 1, Scheme 2. PP-BTA-1 (4) was successfully synthesized in a yield of 21.4% according to methods reported previously (Scheme 1).19 The formation of styrylbenzothiazole in the synthesis of PP-BTA-2 (7) (Scheme 2) was achieved by a Wadsworth–Emmons reaction between diethyl (4-cyanobenzyl)phosphonate and 6-dimethylaminobenzothiazole-2-carbaldehyde. The desired (E)-styrylbenzothiazole derivative was prepared in a yield of 23.0%.
Conclusion
In conclusion, we successfully designed and synthesized benzothiazole-derived push–pull dyes for imaging Aβ plaques in the brain. In binding experiments in vitro, these benzothiazole compounds showed high affinity for Aβ(1-42) aggregates. PP-BTA-1 and PP-BTA-2 clearly stained Aβ plaques in both mouse brain and human brain, reflecting their affinity for Aβ aggregates in vitro. These findings suggest that additional structural changes on the benzothiazole backbone may be applied to potential Aβ
Experimental
1H NMR spectra were obtained on a JEOL JNM-LM400 with TMS as an internal standard. Coupling constants are reported in hertz. Multiplicity was defined by s (singlet), d (doublet), t (triplet), br (broad) and m (multiplet). Mass spectra were obtained on a SHIMADZU LCMS-2010 EV. PIB was purchased from ABX (Radeberg, Germany). Other reagents were of reagent grade and used without further purification unless otherwise indicated.
Acknowledgements
This study was supported by the Program for Promotion of Fundamental Studies in Health Sciences of the National Institute of Biomedical Innovation (NIBIO), a Health Labour Sciences Research Grant, and a Grant-in-Aid for Young Scientists (A) and Exploratory Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References and notes (21)
Neurobiol. Aging
(1998)- et al.
Am. J. Geriatr. Psychiat.
(2002) - et al.
Nucl. Med. Biol.
(2003) - et al.
Lancet. Neurol.
(2008) Physiol. Rev.
(2001)- et al.
J. Med. Chem.
(2003) - et al.
Ann. Neurol.
(2004) - et al.
Am. J. Geriatr. Psychiat.
(2004) - et al.
N. Eng. J. Med.
(2006) - et al.
J. Nucl. Med.
(2007)
Cited by (0)
- †
These authors contributed equally to this work.