Design, synthesis, and structure–activity relationship studies of 6 <i>H</i> -benzo[ <i>b</i> ]indeno[1,2- <i>d</i> ]thiophen-6-one derivatives as DYRK1A/CLK1/CLK4/haspin inhibitors

A series of sulfur-containing tetracycles was designed and evaluated for their ability to inhibit protein kinase DYRK1A, a target known to have several potential therapeutic applications including cancers, Down syndrome or Alzheimer's disease. Our medicinal chemistry strategy relied on the design of new compounds using ring contraction/isosteric replacement and constrained analogy of known DYRK1A inhibitors, thus resulting in their DYRK1A inhibitory activity enhancement. Whereas a good inhibitory effect of targeted DYRK1A protein was observed for 5-hydroxy compounds 4i-k (IC₅₀ = 35-116 nM) and the 5-methoxy derivative 4e (IC₅₀ = 52 nM), a fairly good selectivity towards its known DYRK1B off-target was observed for 4k. In addition, the most active compound 4k, having an ATP-competitive mechanism of action, proved to be also a potent inhibitor of CLK1/CLK4 (IC₅₀ = 20 and 26 nM) and, to a lesser extent, of haspin (IC₅₀ = 76 nM) kinases. <i>In silico</i> docking studies within the DYRK1A, CLK1/CLK4 and haspin ATP binding sites were carried out to understand the interactions of our tetracyclic derivatives 4 with these targets. Antiproliferative activities on U87/U373 glioblastoma cell lines of the most potent compound 4k showed a moderate effect (IC₅₀ values between 33 and 46 μM). Microsomal stabilities of the designed compounds 4a-m were also investigated, showing great disparities, depending on benzo[<i>b</i>]thiophene ring 5-substitution.