The hallmark pathologies of the Alzheimer's disease (AD) brain are amyloid beta (Aβ)-containing senile plaques and neurofibrillary tangles formed from the microtubule (MT)-binding tau protein. Tau becomes hyperphosphorylated and disengages from MTs in AD, with evidence of resulting MT structure/function defects. Brain-penetrant MT-stabilizing compounds can normalize MTs and axonal transport in mouse models with tau pathology, thereby reducing neuron loss and decreasing tau pathology. MT dysfunction is also observed in dystrophic axons adjacent to Aβ plaques, resulting in accumulation of amyloid precursor protein (APP) and BACE1 with the potential for enhanced localized Aβ generation. We have examined whether the brain-penetrant MT-stabilizing compound CNDR-51657 might decrease plaque-associated axonal dystrophy and Aβ release in 5XFAD mice that develop an abundance of Aβ plaques. Administration of CNDR-51657 to 1.5-month-old male and female 5XFAD mice for 4 or 7 weeks led to decreased soluble brain Aβ that coincided with reduced APP and BACE1 levels, resulting in decreased formation of insoluble Aβ deposits. These data suggest a vicious cycle whereby initial Aβ plaque formation causes MT disruption in nearby axons, resulting in the local accumulation of APP and BACE1 that facilitates additional Aβ generation and plaque deposition. The ability of a MT-stabilizing compound to attenuate this cycle, and also reduce deficits resulting from reduced tau binding to MTs, suggests that molecules of this type hold promise as potential AD therapeutics.

Studies in tau and Aβ plaque transgenic mouse models demonstrated that brain-penetrant microtubule (MT)-stabilizing compounds, including the 1,2,4-triazolo[1,5-a]pyrimidines, hold promise as candidate treatments for Alzheimer's disease and related neurodegenerative tauopathies. Triazolopyrimidines have already been investigated as anticancer agents; however, the antimitotic activity of these compounds does not always correlate with stabilization of MTs in cells. Indeed, previous studies from our laboratories identified a critical role for the fragment linked at C6 in determining whether triazolopyrimidines promote MT stabilization or, conversely, disrupt MT integrity in cells. To further elucidate the structure-activity relationship (SAR) and to identify potentially improved MT-stabilizing candidates for neurodegenerative disease, a comprehensive set of 68 triazolopyrimidine congeners bearing structural modifications at C6 and/or C7 was designed, synthesized, and evaluated. These studies expand upon prior understanding of triazolopyrimidine SAR and enabled the identification of novel analogues that, relative to the existing lead, exhibit improved physicochemical properties, MT-stabilizing activity, and pharmacokinetics.

Microtubule (MT)-stabilizing 1,2,4-triazolo[1,5-a]pyrimidines (TPDs) hold promise as candidate therapeutics for Alzheimer's disease (AD) and other neurodegenerative conditions. However, depending on the choice of substituents around the TPD core, these compounds can elicit markedly different cellular phenotypes that likely arise from the interaction of TPD congeners with either one or two spatially distinct binding sites within tubulin heterodimers (i.e., the seventh site and the vinca site). In the present study, we report the design, synthesis, and evaluation of a series of new TPD congeners, as well as matched molecular pair analyses and computational studies, that further elucidate the structure-activity relationships of MT-active TPDs. These studies led to the identification of novel MT-normalizing TPD candidates that exhibit favorable ADME-PK, including brain penetration and oral bioavailability, as well as brain pharmacodynamic activity.