PET imaging to assess blood-brain barrier function in Alzheimer´s disease

A major hallmark of Alzheimers disease (AD) is the accumulation of senile plaques containing -amyloid (Aß) in the brain. Several lines of evidence suggest that reduced Aß clearance from the brain underlies Aß accumulation. Adenosine triphosphate (ATP)-binding cassette (ABC) transporters that are expressed in endothelial cells of the blood-brain barrier (BBB) may play an important role in excreting Aß from brain into the blood. A number of studies suggest that P-glycoprotein (ABCB1) function at the BBB may be impaired in AD patients as compared with age-matched control subjects. Our own data obtained in amyloid precursor protein (APP) transgenic mice (APPtg) that lack the multidrug resistance protein 1 (ABCC1; APPtg x ABCC1-/-) suggest that ABCC1 may play a more important role than ABCB1 in mediating Aß clearance from the brain as reflected by up to 14-fold increased Aß levels in brains of APPtg x ABCC1-/- mice as compared with control mice expressing ABCC1. Moreover, chronic treatment of APPtg mice with thiethylperazine, an antiemetic drug which activates ABCC1 transport activity in vitro, resulted in significant reductions in Aß load as compared with untreated control animals. However, so far it is not known if ABCC1 function at the BBB is indeed altered during the progression of AD and if treatment with thiethylperazine indeed increases the function of ABCC1 at the BBB. To answer these important questions we will directly measure the functional activity of ABCC1 in vivo at the BBB of AD mouse models using positron emission tomography (PET) and the newly developed radiotracer 6-bromo-7-(2-[18F]fluoroethyl)purine ([18F]BFEP). In addition, Aß load in mouse brains will be assessed with [11C]Pittsburgh compound B ([11C]PIB) to establish a correlation between regional ABCC1 function and Aß load in brains of APPtg mice. Using this experimental approach we will investigate the time course of ABCC1 function decline in AD mouse brain and study the effect of thiethylperazine treatment on ABCC1 function. In vivo PET experiments will be complemented with comprehensive immunohistochemistry (IHC) analyses of pathological markers, ABCC1 and Aß as well as Western blot analysis. We hypothesize that ABCC1 function is decreased in AD mice and that there is an inverse relationship between regional brain Aß load and ABCC1 function. Pharmacological induction of ABCC1 activity at the BBB may represent an interesting future therapeutic strategy in AD and PET imaging of cerebral ABCC1 function may be a very useful imaging biomarker in the development of such new therapeutics as well as in early diagnosis of AD. Keywords: positron emission tomography (PET); radiotracer; functional imaging; blood-brain barrier; ABC transporters; ABCC1; Alzheimers disease

There is currently a scarcity of effective treatment options for Alzheimers disease (AD). A major hallmark of AD is the accumulation of senile plaques containing ?-amyloid (A?) in the brain. Several lines of evidence suggest that reduced A? clearance from the brain underlies A? accumulation. Membrane transport proteins, such as P-glycoprotein (ABCB1) and multidrug resistance-associated protein 1 (ABCC1), which are expressed in endothelial cells of the blood-brain barrier (BBB), may contribute to excreting A? from brain into the blood. In this Austrian-German collaborative project, we used the non-invasive nuclear imaging method positron emission tomography (PET) to measure the function of ABCC1 and ABCB1 over the time course of disease progression in a commonly used mouse AD model (APPPS1). Moreover, we tested a pharmacological approach to induce cerebral ABCC1 function as a potential therapeutic approach to reduce A? in the brain. PET imaging with the ABCC1 radiotracer 6-bromo-7-[11C]methylpurine failed to reveal alterations in ABCC1 function in the brains of APPPS1 mice as compared with age-matched control mice. Moreover, treatment with the antiemetic drug and potential ABCC1 inducer thiethylperazine (Torecan) did not alter cerebral ABCC1 function in the employed mouse model. PET with the ABCB1 substrate (R)- [11C]verapamil revealed significant differences in cerebral ABCB1 function between AD and control mice at an early age of 50 days. Both in AD and control mice, there was a pronounced functional decline in cerebral ABCB1 function with increasing age, reaching a reduction of approximately 50% at the age of one year. Our data suggest that impaired ABCB1 function is present before substantial deposition of A? occurs indicating that ABCB1 may play a causative role in the progression of AD and that pharmacological strategies to enhance ABCB1 function at the BBB may be a useful approach to treat AD.In summary, we validated non-invasive imaging approaches to measure the function of cerebral efflux transporters, which have been implicated in brain A? clearance. We found significant alterations in cerebral ABCB1 function and no changes in cerebral ABCC1 function. Our imaging approaches can be readily translated to AD patients to test novel therapeutic approaches to induce transporter function at the human BBB. Moreover, PET imaging of transporter function at the BBB may prove suitable as a tool for early diagnosis of AD.