Preclinical PET as Translational Tool for Imaging Alzheimer's Disease

Alzheimer's disease (AD) causes unprecedented ethical, social and economical implications. The disease concept remains unclear and, hence, comprehensible therapeutic targets do not exist. This requires new and reliable approaches to investigate the disease concept further and to create hard evidence on postulated phenomena. Excessive production of amyloid-β (Aβ) via proteolysis of the amyloid precursor protein (APP) and the subsequent oligomerization and aggregation to Aβ plaques in the brain are considered to be initial pathological events and a major early and causal hallmark of AD. Therefore, non-invasive and quantitative Aβ imaging with positron emission tomography (PET) in humans is by now considered a helpful technique in this quest. Preclinical Aβ imaging in rodent disease models has been limited to very few studies and is still challenging. Recent improvements of small animal PET (μPET) performance have facilitated in vivo imaging of rats and mice and together with the abundance of available transgenic mouse (Tg) models of AD, should provide a valuable method for translational research such as developing specific imaging markers and monitoring new therapeutic approaches. However, intrinsic methodological constraints such as image resolution of PET and applicability of available research models have so far limited the feasibility of PET imaging in transgenic mouse models of AD. The development and evaluation of a feasible protocol for small-animal PET imaging of Aβ plaques in mouse brain is presented as a first major contribution of this thesis. For this, the established 'gold-reference' radiotracer [11C]PiB was used with specific activities commonly used in human studies. In vivo mouse brain MR images for anatomical reference were acquired with a clinical 1.5 T system. An initially unpublished, homozygous APP/PS1 transgenic mouse line (ARTE10) with low inter-animal variability was characterized for its potential applicability to in vivo imaging and employed to measure Aβ burden at different disease stages in homozygous and hemizygous animals. Extensive multi-modal cross-validations for the PET results in this mouse model with ex vivo and in vitro methodologies, including regional brain biodistribution, dual-label digital autoradiography, protein quantification with ELISA, fluorescence microscopy and semi-automated histological quantification are described as a second major contribution. This includes an additional all-in-one experimental design as a toolbox for translational applications. Application of the established combined methodology to the evaluation and ranking of two novel lead compounds for AD imaging, as examples, 2-(p-[11C]Methylaminophenyl)-7-methoxyimidazo[2,1-b]benzothiazole ([11C]IBT) and 2-(4'-bromophenyl)-6-iodoimidazo[1,2-a]pyridine ([124I]BrIMPY), is shown as a third major contribution. Specific in vivo [ 11C]PiB uptake in individual brain regions with Aβ plaque deposition was demonstrated and validated in all animals of a large study cohort including homozygous AD animals as young as nine months. Corresponding to the extent of Aβ plaque pathology, old homozygous AD animals (21 months) showed the highest uptake followed by old hemizygous (23 months) and young homozygous mice (9 months). In all AD age groups the cerebellum was shown to be suitable as an intracerebral reference region. PET results were cross-validated and consistent with all applied ex vivo and in vitro methodologies. This also verified the validity and applicability of the animal model for PET imaging. The reliability and validity of the all-in-one design was verified. Specific in vivo uptake of ([11C]IBT and ([124I]BrIMPY in Aβ-containing telencephalic brain regions with comparable ex vivo binding patterns to PiB and excellent correlation with Aβ plaque pathology was shown by applying the combined experimental concept. The results confirm that the experimental setup for non-invasive ([11C]PiB imaging of Aβ plaques in a novel APP/PS1 mouse model of AD provides a feasible, reproducible and robust protocol for small-animal Aβ plaque imaging. The experimental framework with this model allows for longitudinal imaging studies with follow-up periods of approximately one and a half years. The representative and successful evaluation of new PET and SPECT imaging agents with high sensitivities for in vivo Aβ detection confirm that the developed experimental design provides a toolbox for AD research in transgenic mice and, hence, the foundation for translational Alzheimer neuroimaging.