Presentation In vivo evaluation of the therapeutic potential of novel Translocator protein (TSPO) ligands for the treatment of Alzheimer’s disease

Asih, Prita  ,  Ji, Bin  ,  Katsifis, Andrew  ,  Mattner, Filomena  ,  Caruso, Donatella  ,  Melcangi, Roberto  ,  Higuchi, Makoto  ,  N Martins, Ralph  ,  Barron, Anna

BackgroundLigands of the translocator protein (TSPO) have been identified as promising candidate therapeutic agents for several neurodegenerative disorders due to their ability to enhance neurosteroidogenesis. We have previously demonstrated that the classic TSPO ligands, Ro5-4864 and PK11195, increase brain steroid levels, reduce Aβ accumulation, and improve cognition in a mouse model of AD. Here we evaluated the steroidogenic efficacy and acute behavioral effects of three new generation TSPO imidazopyridine ligands, CLINDE, PBR175, and PBR162, which we have previously shown to be non-toxic in vitro. CLINDE, which exhibited the most favorable behavioral and pharmacokinetic profile, was investigated further for its specificity and binding affinity in human brain.MethodsThe acute behavioural effects of the three novel TSPO ligands, CLINDE, PBR-175 and PBR-162, were compared with the well characterized TSPO ligand, Ro5-4864, in 3 month old castrated C57BL/6J mice. Two hours after ligand injection (3 mg/kg i.p.), exploratory-, anxiety-, depression-, and memory-related behaviors were assessed in the elevated-plus maze (EPM), open field maze, tail-suspension test, and object recognition tests respectively. Brain steroids levels were measured by LC-MS/MS. To determine if the behavioral effects of TSPO ligands were mediated via increased neurosteroidogenesis, mice were pretreated with a steroidogenesis inhibitor, aminogluthetimide (10μl/g ). The pharmacokinetic properties of the novel ligands, including biodistribution and receptor occupancy, were assessed by competitive PET imaging using [11C]PK-11195. The specificity of action of the most promising novel TSPO ligand, CLINDE, was confirmed in TSPO knockout mice, and the affinity for TSPO in human brain homogenate was investigated in vitro by competitive binding assay using [11C]PK-11195.ResultsCLINDE and PBR175 improved learning and memory performance in the object recognition task equally as well as the classic TSPO ligand, Ro5-4864. These behavioral improvements were completely ablated by pretreatment with the steroidogenesis inhibitor, AG, confirming that the beneficial effects were mediated through an increase in neurosteroid levels. Exploratory and locomotor activity, anxiety- and depression-related behaviors were unaffected by the TSPO ligands. Competitive PET studies with the highly specific TSPO ligand [11C]PK-11195, indicated that while all three novel ligands rapidly penetrated the brain, CLINDE was the most stable. The specificity of CLINDE was confirmed in TSPO knockout mice, with no effect on learning and memory observed in these mice. Since affinity for TSPO of many second generation TSPO ligands is greatly reduced by a common polymorphism, we evaluated the affinity of CLINDE in brain homogenates from high-affinity binder (HAB) and low-affinity binder (LAB) subjects, with Ki values of 5.5 and 29.7 in HAB and LABs respectively. ConclusionThese findings indicate that the new generation TSPO imidazopyridine ligands, CLINDE, PBR-175 and PBR162 rapidly promote neurosteroidogenesis, thereby enhancing learning and memory function in vivo. These second generation TSPO ligands are promising therapeutic candidates with improved pharmacokinetic properties compared classic TSPO ligands.
Alzheimer`s Association International Conference

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