Towards Imaging Tau Hyperphosphorylation: Is DYRK1A a Potential Target for Imaging Hyperphosphorylation of Tau? Molecular Modeling Assessment and Synthesis of [125I]Radioiodinated DYRK1A Inhibitor

Dual specificity tyrosine-phosphorylation regulated kinase 1A (DYRK1A), a phosphorylation kinase, is localized within the central nervous system and is linked to hyperphosphorylation of Tau. Imaging of DYRK1A may provide an earlier biomarker for Tauopathies, including Alzheimer's disease (AD). We have used Chimera-Autodock to evaluate potential molecules for binding to the binding site of DYRK1A. Five molecules, 10-bromo-2-iodo-11<i>H</i>-indolo[3,2-c]quinoline-6-carboxylic acid (4E3), 10-iodo-11<i>H</i>-indolo[3,2-c]quinoline-6-carboxylic acid (KuFal184), harmine, 6-(fluoro-3-(1H-pyrrolo[2,3-c]pyridin-1-yl)isoquinolin-5-amine (MK-6240), and 6-iodo-3-(1H-pyrrolo[2,3-c]pyridine-1-yl)isoquinoline (IPPI), were found to have binding energies of -10.4, -10.1, -9.0, -9.1, and -9.4 kcal/mole, respectively. Two molecules, 4E3 and KuFal184, were selective for DYRK1A, while harmine also had a monoamine oxidase A affinity, and MK-6240 and IPPI had affinity for Tau. Tau present in the brain slices of AD subject were labeled with [¹²⁵I]IPPI. KuFal184 had no effect on the binding of [¹²⁵I]IPPI, suggesting the absence of binding overlap of the two molecules. MK-6240, a known Tau agent was, however, able to compete with [¹²⁵I]IPPI. The binding energies of harmine, MK-6240, and IPPI for the DYRK1A site suggest affinities of approximately 80-100 nM, which is insufficient to serve as an imaging agent. The higher affinity of KuFal184 (6 nM for DYRK1A) suggested that [¹²⁵I]KuFal184 may be a potential imaging agent. Electrophilic radioiodination was used to synthesize [¹²⁵I]KuFal184 in modest yields (25%) and high radiochemical purity (>95%). Preliminary binding studies with [¹²⁵I]KuFal184 in AD brain slices showed some selectivity for cortical grey matter regions containing Tau.