The use of covalent irreversible binding inhibitors is an established concept for drug development. Usually the discovery of new irreversible kinase inhibitors occurs serendipitously showing that efficient rational approaches for the rapid discovery of new drugs are needed.Herein, we report a virtual screening strategy that led to the discovery of irreversible inhibitors of the FMS-like tyrosine kinase 3 (FLT3) involved in the pathogenesis of acute myeloid leukemia (AML). A virtual screening library was designed to target the highly conserved Cys828 residue preceding the DFG motif by modification of reported reversible inhibitors with chemically reactive groups. Prospective covalent docking allowed the identification of two lead series, resulting in a massive increase in inhibition of kinase activity and cell viability by irreversible inhibitors compared to the corresponding reversible scaffolds. Lead compound 4b (BSc5371) displays superior cytotoxicity in FLT3-dependent cell lines to compounds in recent clinical trials and overcomes drug-resistant mutations.

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The ubiquitin‐proteasome system (UPS) is an established therapeutic target for approved drugs to treat selected hematologic malignancies. While drug discovery targeting the UPS focuses on irreversibly binding epoxyketones and slowly‐reversibly binding boronates, optimization of novel covalent‐reversibly binding warheads remains largely unattended. We previously reported α‐ketoamides to be a promising reversible lead motif, yet the cytotoxic activity required further optimization. This work focuses on the lead optimization of phenoxy‐substituted α‐ketoamides combining the structure‐activity relationships from the primed and the non‐primed site of the proteasome β5 subunit. Our optimization strategy is accompanied by molecular modeling, suggesting occupation of P1′ by a 3‐phenoxy group to increase β5 inhibition and cytotoxic activity in leukemia cell lines. Key compounds were further profiled for time‐dependent inhibition of cellular substrate conversion. Furthermore, the α‐ketoamide lead structure 27 does not affect escape response behavior in Danio rerio embryos, in contrast to bortezomib, which suggests increased target specificity.

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