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The Polo-like kinases (Plks) are an evolutionary conserved family of Ser/Thr protein kinases that possess, in addition to the classical kinase domain at the N-terminus, a C-terminal polo-box domain (PBD) that binds to phosphorylated proteins and modulates the kinase activity and its localization. Plk1, which regulates the formation of the mitotic spindle, has emerged as a validated drug target for the treatment of cancer, because it is required for numerous types of cancer cells but not for the cell division in noncancer cells. Here, we employed chemical biology methods to investigate the allosteric communication between the PBD and the catalytic domain of Plk1. We identified small compounds that bind to the catalytic domain and inhibit or enhance the interaction of Plk1 with the phosphorylated peptide PoloBoxtide in vitro. In cells, two new allosteric Plk1 inhibitors affected the proliferation of cancer cells in culture and the cell cycle but had distinct phenotypic effects on spindle formation. Both compounds inhibited Plk1 signaling, indicating that they specifically act on Plk1 in cultured cells.

The receptor tyrosine kinase EPHA2 (Ephrin type-A receptor 2) plays important roles in oncogenesis, metastasis, and treatment resistance, yet therapeutic targeting, drug discovery, or investigation of EPHA2 biology is hampered by the lack of appropriate inhibitors and structural information. Here, we used chemical proteomics to survey 235 clinical kinase inhibitors for their kinase selectivity and identified 24 drugs with submicromolar affinities for EPHA2. NMR-based conformational dynamics together with nine new cocrystal structures delineated drug-EPHA2 interactions in full detail. The combination of selectivity profiling, structure determination, and kinome wide sequence alignment allowed the development of a classification system in which amino acids in the drug binding site of EPHA2 are categorized into key, scaffold, potency, and selectivity residues. This scheme should be generally applicable in kinase drug discovery, and we anticipate that the provided information will greatly facilitate the development of selective EPHA2 inhibitors in particular and the repurposing of clinical kinase inhibitors in general.

Polo-like kinase 1 (Plk1) is a central regulator of mitosis and has been validated as a target for antitumor therapy. The polo-box domain (PBD) of Plk1 regulates its kinase activity and mediates the subcellular localization of Plk1 and its interactions with a subset of its substrates. Functional inhibition of the Plk1 PBD by low-molecular weight inhibitors has been shown to represent a viable strategy by which to inhibit the enzyme, while avoiding selectivity issues caused by the conserved nature of the ATP binding site. Here, we report structure-activity relationships and mechanistic analysis for the first reported Plk1 PBD inhibitor, Poloxin. We present the identification of the optimized analog Poloxin-2, displaying significantly improved potency and selectivity over Poloxin. Poloxin-2 induces mitotic arrest and apoptosis in cultured human tumor cells at low micromolar concentrations, highlighting it as a valuable tool compound for exploring the function of the Plk1 PBD in living cells.