Four-dimensional multiphoton imaging of brain entry, amyloid binding, and clearance of an amyloid-β ligand in transgenic mice

Abstract

The lack of a specific biomarker makes preclinical diagnosis of Alzheimer's disease (AD) impossible, and it precludes assessment of therapies aimed at preventing or reversing the course of the disease. The development of a tool that enables direct, quantitative detection of the amyloid-␤ deposits found in the disease would provide an excellent biomarker. This article demonstrates the real-time biodistribution kinetics of an imaging agent in transgenic mouse models of AD. Using multiphoton microscopy, Pittsburgh compound B (PIB) was imaged with sub-m resolution in the brains of living transgenic mice during peripheral administration. PIB entered the brain quickly and labeled amyloid deposits within minutes. The nonspecific binding was cleared rapidly, whereas specific labeling was prolonged. WT mice showed rapid brain entry and clearance of PIB without any binding. These results demonstrate that the compound PIB has the properties required for a good amyloid-imaging agent in humans with or at risk for AD.positron emission tomography ͉ senile plaques ͉ Alzheimer's disease T he development of radioligands for in vivo imaging in the brain demands the demonstration of specificity for the target with good brain entry and exit kinetics. A technique that would allow direct assessment of the biodistribution kinetics with ligand-receptor specificity simultaneously would hasten development of new radioligands and confirm the utility of existing agents. Multiphoton microscopy is an in vivo imaging technique with very high spatial and temporal resolution that allows the precise determination of ligand binding specificity with sub-m resolution that can be used to characterize imaging probes in small animal models. In the context of Alzheimer's disease (AD), the ''receptors'' are amyloid-␤ plaques and neurofibrillary tangles (1, 2). AD is only diagnosed with certainty after death. The definitive diagnosis of this disease in living patients will depend on the detection of one or both of these neuropathological lesions either directly, by means of brain imaging, or indirectly, through a suitable biomarker that parallels their development (3). Progress with therapeutics aimed at removing these lesions will also be accelerated with the use of an end point to evaluate their efficacy (4). Significant progress has been achieved in developing imaging agents that enter the brain and target plaques and tangles specifically to allow direct imaging in humans (5-9). One promising compound, 2-(4Ј-methylaminophenyl)-6-hydroxy-benzothiazole (referred to as Pittsburgh compound B or PIB), a derivative of thioflavin T, labels plaques and cerebral amyloid angiopathy (CAA) in tissue sections from AD patients (6, 10, 11). The compound also crosses the blood-brain barrier (BBB), allowing peripheral administration of the amyloid-targeting reagent. This compound is readily labeled with carbon-11 for positron emission tomography (PET) scanning (10). Because the half-life of carbon-11 is Ϸ20 min, successful imaging will depend on very rapid brain entry,...

Near-infrared fluorescence molecular imaging of amyloid beta species and monitoring therapy in animal models of Alzheimer’s disease

“…Two-photon microscopic imaging has been an important tool for studying amyloidosis of AD (57,(72)(73)(74). However, most of the fluorescent probes have short emission, which can limit imaging depth significantly.…”

Quantitative magnetic resonance techniques as surrogate markers of Alzheimer’s disease

“…The most promising candidates in neuroimaging are amyloid-labeling agents used in PET imaging. 1,2 However, human validation studies with nuclear medicine amyloid-labeling agents are incomplete at this time. It is our belief that indirect measures of AD with quantitative MR techniques can be valid biomarkers as well, particularly of disease progression.…”

Molecular Brain Imaging in the Multimodality Era

“…Bacskai et al (2003) used multiphoton microscopy to image PiB in PDAPP and Tg2576 mice (both 18 to 20 months old) and PSAPP mice (12 months old). PiB was imaged in real time as it appeared in vessels, cleared across the blood-brain barrier and first labeled amyloid angiopathy, followed by parenchymal dense-core plaques (first labeled at periphery then gradually filled in to the core), and eventually cleared from the brain (3D volume: 615 Â 615 mm 2 , depth: B150 mm) (Bacskai et al, 2003). This provided early evidence that PiB was suited for the in vivo detection of amyloid deposits in transgenic mice (Bacskai et al, 2003).…”