Towards the development of radioimaging agents for the diagnosis of Alzheimer’s disease

Alzheimer’s disease (AD) is the most common form of dementia, constituting 60–70% of all dementia cases. Characterised by progressive neuronal damage and the presence of senile plaques (SP) and neurofibrillary tangles (NFT) within brain regions, it is estimated that 35.6 million people world-wide are living with AD, with numbers expected to increase substantially over the next 20 years. As current treatments only target disease progression, a large emphasis has been placed on identifying imaging agents which would allow for earlier diagnosis of AD and monitoring of treatment effectiveness. The central theme of this thesis is the development of SP imaging agents for application in the early diagnosis of AD using positron emission tomography (PET). Towards this end, a synthetic route was devised and implemented which enabled the preparation of a large library of compounds consisting of a SP targeting modality (chalcone or benzothiazole) covalently linked to a metal chelator (TACN or cyclen). These compounds were then subjected to electronic absorption and photoluminescence studies. Following this, a promising benzothiazole derivative BT22’ and it’s corresponding Zn(II) complex Zn(II)-BT22 were applied in preliminary in vitro biological testing. A series of chalcone-TACN and chalcone-cyclen derivatives were prepared, which differed in the position of ring substitution. Coupling of the chalcone and macrocyclic components was attempted using either amide coupling or nucleophilic substitution, with the latter route proving more successful. Electronic absorption studies involving the chalcone-TACN derivatives CH22, CH24 and CH26, identified absorption maxima (π→π*) between 305–340 nm, with the para substituted derivatives (CH22 and CH26) exhibiting λmax at longer wavelengths compared to the meta-substituted CH24. Titration with Cu(II) was found to generate a d→d band centered at ca. 655 nm. All three compounds were shown to exhibit a near 1:1 binding stoichiometry, with the complexes adopting a distorted square pyramidal geometry around the Cu(II) centre. Similar titrations with Zn(II) produced only minimal changes to the UV-visible spectra. A series of benzothiazole-TACN and benzothiazole-cyclen derivatives were prepared using the nucleophilic substitution approach. In addition, N-alkylation of the 4’-amine of 2-(4-aminophenyl)benzothiazole (BT1) was performed to produce methyl-, isopropyl- and benzyl derivatives, to study the effect of substitution on compound stability and Aβ binding ability. Electronic absorption studies identified λmax (π→π*) at 320 nm for the nonalkylated derivatives BT17 and BT18, whilst alkylation (compounds BT19–BT24) caused a hypsochromic shift to ca. 305 nm. Titration with Cu(II) saw variations in the behaviour of the TACN and cyclen compounds, as well as the nonalkylated/alkylated derivatives. The nonalkylated TACN and cyclen complexes exhibited d→d bands centered at ca. 645 and 582 nm, respectively, which were consistent with a distorted square pyramidal Cu(II) geometry. In comparison, the d→d band of the alkylated TACN and cyclen complexes was found at ca. 690 and 575 nm, respectively. This indicated little change in Cu(II) geometry for the cyclen complexes and a significant shift towards trigonal bipyramidal geometry for the TACN complexes. In most cases, a close to 1:1 Cu:L binding stoichiometry was observed. As expected, titration with Zn(II) was found to produce minimal changes in the absorption spectra. The emission spectra displayed π*→π bands centered at ca. 380 (BT18–BT24) and 390 nm (BT17). For all eight compounds, titration with Cu(II) produced dramatic reductions in the emission intensity, likely due to the chelation enhanced quenching (CHEQ) effect. In comparison, titration with Zn(II) caused emission enhanced fluorescence (CHEF) of the TACN derivatives, whilst the cyclen derivatives produced mixed results; varying decreases in fluorescence were observed depending on the derivative used. In addition, BT17 and BT18 underwent a 15–30 nm bathochromic shift upon Zn(II) complexation, possible due to twisted intramolecular charge transfer (TICT). Preliminary in vitro biological testing using BT22’ (free base form) and its corresponding Zn(II) complex, Zn(II)-BT22, appear promising. ThT displacement assays showed significant reductions in ThT-Aβ(1-42) emission intensity at 484 nm upon addition of BT22’, suggesting good competitive binding. The direct binding assay results showed an enhancement in emission intensity at 380 nm upon initial interaction of BT22’ or Zn(II)-BT22 with Aβ(1-42). Over a t = 15 min time frame, Zn(II)-BT22 was found to have a better sustained enhancement, suggesting a potentially higher binding affinity for the Aβ(1-42) fibril.