N 1 -Benzoylated 5-(4-pyridinyl)indazole-based kinase inhibitors: Attaining haspin and Clk4 selectivity via modulation of the benzoyl substituents.

Haspin and Clk4 are both understudied protein kinases (PKs), offering potential targets for the development of new anticancer agents. Thus, the identification of new inhibitors targeting these PKs is of high interest. However, the inhibitors targeting haspin or Clk4 developed to date show a poor selectivity profile over other closely related PKs, increasing the risk of side effects. Herein, we present two newly developed N 1 -benzyolated 5-(4-pyridinyl)indazole-based inhibitors (18 and 19), derived from a newly identified indazole hit. These inhibitors exhibit an exceptional inhibitory profile toward haspin and/or Clk4. Compound 18 (2-acetyl benzoyl) showed a preference to inhibit Clk4 and haspin over a panel of closely related kinases, with sixfold selectivity for Clk4 (IC 50  = 0.088 and 0.542 μM, respectively). Compound 19 (4-acetyl benzoyl) showed high selectivity against haspin over the common off-target kinases (Dyrks and Clks) with an IC 50 of 0.155 μM for haspin. Molecular docking studies explained the remarkable selectivity of 18 and 19, elucidating how the new scaffold can be modified to toggle between inhibition of haspin or Clk4, despite the high homology of the ATP-binding sites. Their distinguished profile allows these compounds to be marked as interesting chemical probes to assess the selective inhibition of haspin and/or Clk4.