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A novel positron emission tomography contrast agent targeting cathepsin d shows preferential in vivo retention in an Alzheimer’s disease mouse model

      Background

      Early detection of Alzheimer’s disease (AD) pathology remains a serious challenge for both diagnosis and development of treatment. Cathepsin D (CatD), a lysosomal aspartyl protease, is over-expressed in AD and therefore is a potential biomarker. Previously, we introduced a novel Contrast Agent (CA) that was preferentially taken up by CatD over-expressing cells (in-vitro) and able to transverse the BBB in mice (ex-vivo). We have also found that a Near-Infrared-labeled version of this CA demonstrates prolonged in-vivo retention in the brain of a transgenic (Tg) mouse model of AD at 12 months compared to age matched wild type controls. Here, we present the performance of a CA labeled with 68Ga evaluated by micro Positron Emission Tomography (microPET).

      Methods

      The CA consists of a Cell Penetrating Peptide (CPP; the Tat peptide from HIV-1), attached to a CatD cleavage sequence followed by a 68Ga labeled DOTA chelator flanked by a fluorescent dye. The CPP allows the agent to cross the blood brain barrier bidirectionally. In the presence of elevated levels of CatD, cleavage of the CatD site removes the CPP resulting in prolonged retention of the imaging moeity. For this study, Tg AD mice (N=8, 5XFAD model) and non-Tg age matched littermates (N=8) at 2 and 4.5 months of age received an intravenous tail vein CA injection of ∼12 MBq of CA under isofluorane anesthesia. Mice were scanned for 3 hours using the Inveon preclinical microPET system (Siemens Medical Solutions, Knoxville TN, USA). Regions of interest were identified in reconstructed images and were used to measure the uptake and washout of the CA in the brain, liver, kidneys and bladder.

      Results

      The Tg mice demonstrated significantly greater uptake (p<0.05) of the CA in the brain in the first two hours following injection at 4.5 months but not 2 months of age compared to controls. No differences in uptake were observed in other organs.

      Conclusions

      This preliminary work further supports the capacity of this CatD targeted CA to help differentiate between AD mice and controls.