Regular paperPET imaging with [18F]AV-45 in an APP/PS1-21 murine model of amyloid plaque deposition
Introduction
Currently, the methods used to definitively diagnose Alzheimer's disease (AD) requires the presence of progressive dementia and postmortem neuropathology to demonstrate senile plaques composed of β-amyloid (Aβ) aggregates and neurofibrillary tangles formed from hyperphosphorylated tau protein (Dubois et al., 2010, Hemming and Selkoe, 2005).
To noninvasively visualize amyloid deposition in the human brain by positron emission tomography (PET), imaging agents capable of the selective labeling of amyloid fibrils have been developed and tested such as: [11C]-Pittsburgh Compound B (PIB) (Klunk et al., 2005); [11C]4-N-methylamino-4-hydroxystilbene ([11C]SB-13) (Verhoeff et al., 2004); [11C]2-(2-[2-dimethylaminothiazol-5-yl]ethenyl)-6-(2-[fluoro]ethoxy)benzoxazole ([11C]BF-227) (Kudo et al., 2007); and [S-methyl-11C]N,N-Dimethyl-4-(6-(methylthio)imidazo[1,2-a]pyridine-2-yl)aniline ([11C]MeS IMPY) (Seneca et al., 2007). N-[11C]methyl-2-(4′-methylaminophenyl)-6-hydroxybenzothiazole ([11C]-PIB) is the most widely evaluated in PET-based clinical investigations. The ability of [11C]-PIB to detect amyloid deposits in patients with mild cognitive impairment (MCI) has been shown. Although amyloid plaques are found in cognitively normal subjects, it is their rate of progression that is prognostic for developing AD. Overall, the literature has underlined the potential of this probe to identify the Alzheimer's pathology prior to its clinical onset (Morris et al., 2009, Resnick et al., 2010). A pilot trial has recently underscored the fact that PET imaging with [11C]-PIB can detect a decrease of brain amyloid in AD patients treated with bapineuzumab, a potential immunotherapeutic agent for AD (Rinne et al., 2010). However, some instances of unexpectedly high uptake in nondemented cases (Rowe et al., 2007) or poor enhancement in clinically diagnosed AD cases (Edison et al., 2007, Li et al., 2008) have been reported. Furthermore, some Aβ lesions appear to be refractory to PIB binding (Svedberg et al., 2009). The short half-life of [11C] (t1/2 = 20.4 minutes) limits the use of [11C]-PIB and emphasizes the need for [18F]-labeled selective amyloid tracers to envisage large-scale imaging studies.
[18F]-Labeled PET tracers such as [18F]-2-(1-{6-[(2-[18F]fluoroethyl)(methyl)amino]-2-naphtyl}ethylidene) malonitrile (FDDNP) (Small et al., 2006), [18F]-(E)-4-(2-(6-(2-(2-(2-18F-fluoroethoxy)ethoxy)ethoxy)pyridin-3-yl)vinyl)-N-methylbenzamine (AV1) (BAY94-9172, florbetaben; Rowe et al., 2008), [18F]-GE067 (flutemetamol; Koole et al., 2009) and [18F]AV-45 (florbetapir; Kung et al., 2010) have been developed to map the burden of Aβ plaques in the living human brain. The aminonaphthalene derivative, [18F]-FDDNP, is an Aβ tracer which also binds to neurofibrillary tangles (Lockhart, 2006). In PET studies in AD patients, a weakly increased uptake of [18F]-FDDNP has been found in Aβ- and tau-containing cortical brain areas, relative to nondemented subjects (Shin et al., 2008).
Currently in wide use as a research biomarker in the Alzheimer disease neuroimaging Initiative (Jagust et al., 2010) and in Phase III clinical trials of investigational AD drugs, [18F]AV-45, has generated increasing interest as a novel imaging agent to detect Aβ plaques. An earlier in vitro characterization of [18F]AV-45 reported an excellent affinity and specificity for Aβ plaques (Choi et al., 2009, Liu et al., 2010). In the first human study, [18F]AV-45 demonstrated a greater uptake in cortical target regions of AD patients than seen in cognitively healthy subjects (Wong et al., 2010). Histopathological data have described a near-perfect correlation between PET imaging with [18F]AV-45 and amyloid load measured postmortem in the same patients (Clark et al., 2011).
A pathology-based biomarker could be useful in the diagnosis of AD patients, both to identify early individuals at risk (Thal et al., 2006) and to facilitate the testing of investigational trials that target Aβ-related disease (Mathis et al., 2007). The discovery of novel drugs against AD is largely based on the use of transgenic mouse models of amyloidosis which express mutant forms of human Aβ precursor protein (APP) and presenilin-1 (PS1) (Duyckaerts et al., 2008). The use of these models in preclinical trials is required for the validation of translational biomarkers. Ideally, such biomarkers should rely on the same technology in animal models and in patients. The biomarker should allow a quantification of differences in biological end points after the administration of an investigational drug. Such, the use of a translational biomarker should allow a better extrapolation of preclinical research to clinical studies.
Different AD biomarkers and their associated techniques were tested in animal models of AD to detect amyloid plaques (Delatour et al., 2010). In transgenic mice, in vivo multiphoton microscopy is a brain imaging modality that allows one to image amyloid deposits and associated lesions in small tissue volumes at high resolution (Bacskai et al., 2003). However, this technique is invasive and probes only small fields-of-view. Near-infrared fluorescence (NIRF) imaging is a promising tool for the noninvasive imaging of target-specific interactions in animals but remains limited by poor resolution, and lack of quantification and three-dimensional information (Hintersteiner et al., 2005, Okamura et al., 2011). Magnetic resonance imaging (MRI)-based approaches theoretically provide the spatial resolution needed to detect amyloid plaques. Although currently limited for clinical applications due to unfavorably long acquisition times and low blood-brain barrier penetration of their bulky ligands, MRI has been used to visualize amyloid plaques in AD mouse models (Chamberlain et al., 2011, Dhenain et al., 2009, Petiet et al., 2011, Sigurdsson et al., 2008). However, optical and MRI tracers need to be administered in substantial quantities to achieve approximately 1∼μM masses, which is much higher than that required for PET tracers (approximately 1 nM). Non-PET tracers thus might influence the course of amyloid pathogenesis particularly in longitudinal, multiscan experiments. PET is limited by the short half-life of positron-emitting nuclei and the limited availability of the technology. Nevertheless, with respect to clinical applications, PET seems currently to be the most promising approach as well as being the “gold standard” for quantification.
Advances in PET technology have now enabled accurate in vivo imaging in small animals (Matsumura et al., 2003). However, few studies have been performed with amyloid-targeted PET agents in murine models of AD; these studies are open to criticism. In APP transgenic mice (13–15 months), an attempt to visualize Aβ with microPET and [18F]-FDDNP was unsuccessful (Kuntner et al., 2009). In APP transgenic mice (Toyama et al., 2005) and APP/PS1 double transgenic mice (Klunk et al., 2005), no significantly enhanced uptake of [11C]-PIB was detected, even in aged animals. Specifically, the density of high-affinity PIB binding sites per molecule of Aβ is much lesser in transgenic mice, which overexpress mutations similar to those of familial AD, when compared with AD patients (Higuchi et al., 2010). With high-specific radioactivity [11C]-PIB, (Maeda et al. 2007) were the first to provide evidence for the capability of PET imaging to quantitatively map amyloid accumulation in an appropriate murine model (17-, 21-, and 29-month-old APP23 transgenic mice).
To date, no longitudinal [18F]AV-45 microPET study has evaluated in vivo the evolution of senile plaques in murine models of AD. The first published ex vivo autoradiography of 25-month-old APPswe/PSEN1 transgenic mice with [18F]AV-45 showed a dense labeling of the plaques in the cortical regions and hippocampus, as confirmed by colocalization with thioflavin S, a dye commonly used for staining the amyloid fibrils (Choi et al., 2009).
The aim of our present study was 2-fold: firstly, to evaluate ex vivo by autoradiography; secondly, to follow in vivo by microPET, the [18F]AV-45 uptake in a double transgenic mouse model (APP/PS1-21) in parallel with C57Bl6 mice as control animals. In this APP/PS1-21 model, cerebral amyloidosis is detectable at 1.5–2 months of age, and then quickly increases to reach a plateau at 8 months (Radde et al., 2006).
Section snippets
Animals
Animal experiment procedures were performed in strict accordance with the directives of the European Union (86/609/EEC) and validated by the Regional Ethics Committee (approval number: 0310-02).
APP/PS1-21 (line 21) and corresponding (C57Bl6) control mice were used. APP/PS1-21 mice coexpress human APP carrying the K670N/M671L “Swedish” double mutation and hPS1 L166P with a 3-fold overexpression of human APP over endogenous mouse APP. Mice express the transgene under the control of a
Plaque labeling with intravenously injected [18F]-AV45
Ex vivo autoradiography of the brains of 8-month-old APP/PS1-21 mice 30 minutes after the injection of [18F]AV-45 yielded contrast images of putative plaques. Although faint signals were detected in both C57Bl6 and APP/PS1-21 mice, clusters of punctate labeling were found in the cortex, hippocampus, and striatum of the transgenic mice (Fig. 1). The levels of radiotracer binding in the studied regions were quantified by calculating the ratio of regional radioactivity against that of the
Discussion
The present study is the first to indicate that the novel Aβ plaque imaging agent, [18F]AV-45, currently in Phase III clinical trials, is also of interest to monitor amyloid plaque deposition in APP/PS1-21 mice. The validation of a pertinent translational biomarker of AD, both in human as well as in animal models of this disease, as used here with the same in vivo approach based on PET technology, presents considerable interest.
Because PET measurements require trace amounts of an imaging agent,
Disclosure statement
The authors disclose no conflicts of interest.
Animal experiment procedures were performed in strict accordance with the directives of the European Union (86/609/EEC) and validated by the Regional Ethics Committee (approval number: 0310-02).
Acknowledgements
This work was supported by CEA and the Région Basse-Normandie. We thank Olivier Tirel, the cyclotron operator and Dr. Mathias Jucker, Hertie-Institute for Clinical Brain Research, University of Tübingen, for providing the APP/PS1-21 line, and Dr. Eric T. MacKenzie for his helpful comments and discussion on the revision of this manuscript.
References (57)
- et al.
Time sequence of maturation of dystrophic neurites associated with Aβ deposits in APP/PS1 transgenic mice
Exp. Neurol
(2003) - et al.
Reduction of the cerebrovascular volume in a transgenic mouse model of Alzheimer's disease
Neuropharmacology
(2009) - et al.
Characterization of in vivo MRI detectable thalamic amyloid plaques from APP/PS1 mice
Neurobiol. Aging
(2009) - et al.
Revising the definition of Alzheimer's disease: a new lexicon
Lancet Neurol
(2010) - et al.
Impaired neurogenesis, neuronal loss, and brain functional deficits in the APPxPS1-Ki mouse model of Alzheimer's disease
Neurobiol. Aging
(2011) “Absolute” or “relative”: choosing the right outcome measure in neuroimaging
Neuroimage
(2009)- et al.
Amyloid β-protein is degraded by cellular angiotensin-converting enzyme (ACE) and elevated by an ACE inhibitor
J. Biol. Chem
(2005) - et al.
In vivo visualization of key molecular processes involved in Alzheimer's disease pathogenesis: Insights from neuroimaging research in humans and rodent models
Biochim. Biophys. Acta
(2010) - et al.
The Alzheimer's Disease Neuroimaging Initiative positron emission tomography core
Alzheimers Dement
(2010) - et al.
Optimization of Automated Radiosynthesis of [18F]AV-45: A new pET imaging agent for Alzheimer's disease
Nucl. Med. Biol
(2010)