Drug Development.

Background: DYRK1A overexpression, common in neurodegenerative diseases like Alzheimer's (AD), contributes to neurofibrillary tangles via Tau protein hyperphosphorylation and amyloid plaque formation, key AD hallmarks. Therefore, DYRK1A has been regarded as a novel target for neurodegenerative diseases. However, developing DYRK1A selective inhibitors has been a difficult challenge due to the highly conserved ATP-binding site of protein kinases, particularly among the CMGC family. Here we describe SM15685, a novel highly selective and potent oral DYRK1A/B inhibitor, which reduces Tau hyperphosphorylation both in vitro and in vivo.

Method: Compound selectivity and potency were evaluated in kinase panels and cellular target engagement assays. Tau phosphorylation (pTau) was measured in cell-based assays. Pharmacokinetics (PK) were evaluated in rodents and pharmacodynamics evaluated in 3-month-old JNPL3 female mice (P301L human Tau overexpression) treated orally with SM15685 or Vehicle for 7 days (7 mice/group). pTau at Thr212 was biochemically quantified in subcortical brain and spinal cord fractions.

Result: SM15685 was developed by utilizing structure-based drug design and via iterative medicinal chemistry optimization cycles to achieve high potency, selectivity against the closely related CLKs and GSK3β, and good oral PK profiles characterized by brain-penetration. SM15685 selectively and potently inhibited DYRK1A kinase activity (IC = 1 nM) with >90% inhibition at 1μM and only inhibited <2% of the full kinome. Selectivity and potency for DYRK1A and DYRK1B were confirmed in cellular target engagement assays (IC = 18 and 55 nM, respectively). SM15685 reduced pTau at the Threonine 212 site (IC = 14 nM). In pharmacokinetic studies, SM15685 demonstrated high oral bioavailability (%F >70) and good brain-penetration in rodents (Kpuu >0.6). Compared to vehicle, JNPL3 mice treated with SM15685 at 45mg/kg showed significant reduction of pTau in both subcortical and spinal cord fractions in a pharmacodynamic model (p<0.05, T-test).

Conclusion: SM15685, a highly selective and potent, oral, brain-penetrant, DYRK1A/B inhibitor with the ability to significantly reduce Tau phosphorylation, has potential as a treatment for chronic tauopathies such as AD. These studies support further evaluation of SM15685 as a therapeutic option for AD and other diseases characterized by Tau hyperphosphorylation.