Multicomponent syntheses enable the discovery of novel quisinostat-derived chemotypes as histone deacetylase inhibitors

In this study, we synthesized and evaluated novel histone deacetylase (HDAC) inhibitors derived from the clinical candidate quisinostat. A library of 16 compounds categorized in three novel chemotypes was rapidly generated using multicomponent reactions (MCRs), enabling efficient structure-activity relationship studies. First, the compounds were evaluated for their activity against the Plasmodium falciparum strains 3D7 and Dd2, the main malaria-causing parasite, identifying compound 18b of the type C series as the most potent. It demonstrated low nanomolar IC₅₀ values (IC₅₀ (3D7) = 0.023 μM; IC₅₀ (Dd2) = 0.047 μM) and high parasite selectivity (SI^{MRC−5/Pf3D7} > 2174). HDAC inhibition assays confirmed substantial inhibition of the P. falciparum enzyme PfHDAC1 (IC₅₀ = 0.037 μM) as well as of human HDAC1 (IC₅₀ = 0.021 μM) and HDAC6 (IC₅₀ = 0.25 μM). Docking studies suggested distinct binding modes of 18b in P. falciparum and human HDAC1. Additionally, the in vitro anticancer activity was evaluated in Cal27 (head-neck carcinoma), HepG2 (hepatocellular carcinoma), A2780 (ovarian carcinoma), and U87 (glioblastoma) cell lines. Compounds 9b, 9d, and 13f showed potent antiproliferative activity and caspase 3/7 activation, in contrast to 18b. Furthermore, these compounds caused hyperacetylation of histone H3 and α-tubulin, indicating robust cellular target engagement. Overall, in this work we have identified the HDAC inhibitor 18b with selective antiplasmodial and 9b, 9d, and 13f with selective anticancer activities, providing valuable hits for further drug development efforts aimed at creating derivatives with reduced cytotoxicity against non-cancer cells compared to quisinostat.