Sustained microglial depletion with CSF1R inhibitor impairs parenchymal plaque development in an Alzheimer's disease model.

Many risk genes for the development of Alzheimer's disease (AD) are exclusively or highly expressed in myeloid cells. Microglia are dependent on colony-stimulating factor 1 receptor (CSF1R) signaling for their survival. We designed and synthesized a highly selective brain-penetrant CSF1R inhibitor (PLX5622) allowing for extended and specific microglial elimination, preceding and during pathology development. We find that in the 5xFAD mouse model of AD, plaques fail to form in the parenchymal space following microglial depletion, except in areas containing surviving microglia. Instead, Aß deposits in cortical blood vessels reminiscent of cerebral amyloid angiopathy. Altered gene expression in the 5xFAD hippocampus is also reversed by the absence of microglia. Transcriptional analyses of the residual plaque-forming microglia show they exhibit a disease-associated microglia profile. Collectively, we describe the structure, formulation, and efficacy of PLX5622, which allows for sustained microglial depletion and identify roles of microglia in initiating plaque pathogenesis.

Amyloid-ß 11C-PiB-PET imaging results from 2 randomized bapineuzumab phase 3 AD trials.

OBJECTIVE: To evaluate the effects of bapineuzumab on brain ß-amyloid (Aß) burden using (11)C-Pittsburgh compound B ((11)C-PiB)-PET. METHODS: Two phase 3 clinical trials, 1 each in apolipoprotein APOE ε4 carriers and noncarriers, were conducted in patients with mild to moderate Alzheimer disease dementia. Bapineuzumab, an anti-Aß monoclonal antibody, or placebo, was administered by IV infusion every 13 weeks for 78 weeks. PET substudies assessed change in brain fibrillar Aß over 71 weeks using an (11)C-PiB-PET standardized uptake value ratio (SUVr) global cortical average (GCA) comprising the average SUVr from 5 cortical regions of interest with cerebellar gray matter as the reference region. RESULTS: A total of 115 carriers and 39 noncarriers were analyzed. The difference (δ) in mean baseline to 71 week change in (11)C-PiB-PET GCA between bapineuzumab and placebo was significant in carriers (0.5 mg/kg vs placebo δ = -0.101; p = 0.004) and in pooled analyses of both carriers and noncarriers (0.5 mg/kg vs placebo δ = -0.068; p = 0.027; 1.0 mg/kg vs placebo δ = -0.133; p = 0.028) but not in the noncarrier trial separately. Analyses by individual region of interest and in mild disease yielded findings similar to the main trial results. CONCLUSIONS: The (11)C-PiB-PET imaging results demonstrated reduction of fibrillar Aß accumulation in patients with Alzheimer disease treated with bapineuzumab; however, as no clinical benefit was observed, the findings are consistent with the hypotheses that bapineuzumab may not have been initiated early enough in the disease course, the doses were insufficient, or the most critical Aß species were inadequately targeted.