Towards improved tracers for in vivo imaging of amyloid-beta plaque

Background/Aim: Amyloid-plaque formation is one of the earliest and most relevant physiological processes in the development of Alzheimer's disease (AD). In order to be useful for disease staging and therapy monitoring, a quantification of the amyloid plaque load is required. Despite the promising initial Results: from clinical trials, the regional brain uptake is not correlating completely to the established regional pattern of plaque load as measured by post-mortem quantification. We present a report on our efforts to develop tracers with enhanced selectivity for AB40 and AB42, respectively, and possessing improved in vivo properties. Methods: Apart from altered substitution pattern on the aromatic rings of BTAs and IMPYs the extent and nature of N-anilino alkylation was varied. In addition, structures have been selected allowing for C-11 and radiohalogenation. Affinity was determined using competition binding assays (Ki) on synthetic AB40 and AB42 amyloid fibrils. Together with structural overlays and modeling calculations, the affinity and selectivity data were used for the identification and assessment of relevant substructures and their influence on AB-binding. Results: For IMPYs, the binding experiments to fibrils of synthetic AB42- and AB40-peptides showed up to 92 % inhibition of binding of the 3H-labeled reference at the 100 nM level to AB42 and up to 83 % inhibition to AB40 resulting in AB42-to-AB40-selectivity ratios of up to 41. The lipophilicity (logPoctanol/PBS) of the compounds was in the range of 0.5 to 3.5. From our library of BTAs, 6 were selected and showed Ki-values in the range of 1.3-12.4 on AB40, with selectivity ratios of 2-10 over AB42. The uptake of BTAs in the brain of mice (wild type) at 2 min was in the range of 6.1 to 19.3% ID/g (PIB: 10.3). The 2 min-to-30 min uptake ratios were in the range of 1.9-9.8 (PIB: 10.7). By overlays and trend analyses, four distinct substructures with a differentially, pronounced impact on the binding were identified (Fig.1). These substructures apparently independently determine binding and thus allow subtle adjustment of molecular properties. In addition, as recently demonstrated for a series of BTA-analogues, this model enables tailoring of lipophilicity, brain uptake, plasma half-life and in vivo stability. Conclusions: Twenty-five novel IMPYs and 15 novel BTAs substituted with radiohalogens or C-11 in different positions have been synthesized and evaluated as AB-ligands. Changes of the substitution pattern can be used to improve the selectivity for the two relevant AB-proteins. Work is in progress to further explore the identified key molecular descriptors for further optimization of the combination of radiolabel and the identified lead structures. Verification of the in vitro data is currently being performed in a double transgenic mice model ex vivo and in vivo and compared with aged-matched controls.