Mutation-specific dual potentiators maximize rescue of CFTR gating mutants.

BACKGROUND: The potentiator ivacaftor (VX-770) has been approved for therapy of 38 cystic fibrosis (CF) mutations (∼10% of the patient population) associated with a gating defect of the CF transmembrane conductance regulator (CFTR). Despite the success of VX-770 treatment of patients carrying at least one allele of the most common gating mutation G551D-CFTR, some lung function decline and P. aeruginosa colonization persist. This study aims at identifying potentiator combinations that can considerably enhance the limited channel activity of a panel of CFTR gating mutants over monotherapy. METHODS: The functional response of 13 CFTR mutants to single potentiators or systematic potentiator combinations was determined in the human bronchial epithelial cell line CFBE41o- and a subset of them was confirmed in primary human nasal epithelia (HNE). RESULTS: In six out of thirteen CFTR missense mutants the fractional plasma membrane (PM) activity, a surrogate measure of CFTR channel gating, reached only ∼10-50% of WT channel activity upon VX-770 treatment, indicating incomplete gating correction. Combinatorial potentiator profiling and cluster analysis of mutant responses to 24 diverse investigational potentiators identified several compound pairs that improved the gating activity of R352Q-, S549R-, S549N-, G551D-, and G1244E-CFTR to ∼70-120% of the WT. Similarly, the potentiator combinations were able to confer WT-like function to G551D-CFTR in patient-derived human nasal epithelia. CONCLUSION: This study suggests that half of CF patients with missense mutations approved for VX-770 administration, could benefit from the development of dual potentiator therapy.

Cell-Based Optimization of Covalent Reversible Ketoamide Inhibitors Bridging the Unprimed to the Primed Site of the Proteasome ß5 Subunit.

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 ß5 subunit. Our optimization strategy is accompanied by molecular modeling, suggesting occupation of P1' by a 3-phenoxy group to increase ß5 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.

Virtual Screening Identifies Irreversible FMS-like Tyrosine Kinase 3 Inhibitors with Activity toward Resistance-Conferring Mutations.

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 FMS-like tyrosine kinase 3 (FLT3) involved in the pathogenesis of acute myeloid leukemia. 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.

Elastase-like Activity Is Dominant to Chymotrypsin-like Activity in 20S Proteasome's ß5 Catalytic Subunit.

The ubiquitin/proteasome system is the major protein degradation pathway in eukaryotes with several key catalytic cores. Targeting the ß5 subunit with small-molecule inhibitors is an established therapeutic strategy for hematologic cancers. Herein, we report a mouse-trap-like conformational change that influences molecular recognition depending on the substitution pattern of a bound ligand. Variation of the size of P1 residues from the highly ß5-selective proteasome inhibitor BSc2118 allows for discrimination between inhibitory strength and substrate conversion. We found that increasing molecular size strengthens inhibition, whereas decreasing P1 size accelerates substrate conversion. Evaluation of substrate hydrolysis after silencing of ß5 activity reveals significant residual activity for large residues exclusively. Thus, classification of the ß5 subunit as chymotrypsin-like and the use of the standard tyrosine-containing substrate should be reconsidered.

Computer-guided design, synthesis, and biological evaluation of quinoxalinebisarylureas as FLT3 inhibitors.

Activating mutations of FMS-like tyrosine kinase 3 (FLT3) are present in ∼30 % of patients with acute myeloid leukemia (AML) and are associated with poor prognosis. Point mutations in the tyrosine kinase domain (TKD) are observed as primary mutations or are acquired as secondary mutations in FLT3 with internal tandem duplications (ITDs) after treatment with tyrosine kinase inhibitors (TKIs). Although dozens of potent inhibitors against FLT3 ITD have been reported, activating TKD point mutations, especially at residues F691 and D835, remain the leading cause for therapy resistance, highlighting the consistent need for new potent inhibitors. Herein we report the identification and characterization of novel quinoxaline-based FLT3 inhibitors. We used the pharmacophore features of diverse known inhibitors as a starting point for a new optimization algorithm for type II TKIs, starting from an in silico library pharmacophore search and induced-fit docking in the known FLT3 structure. This led to the design of a set of diverse quinoxalinebisarylureas, which were profiled in an FLT3 kinase activity assay. The most promising compounds were further evaluated in a zebrafish embryo phenotype assay.