Identification of novel, potent and selective inhibitors of Polo-like kinase 1

PLK1 inhibition dampens NLRP3 inflammasome-elicited response in inflammatory disease models

Unabated activation of the NLR family pyrin domain-containing 3 (NLRP3) inflammasome is linked with the pathogenesis of various inflammatory disorders. Polo-like kinase 1 (PLK1) has been widely studied for its role in mitosis. Here, using both pharmacological and genetic approaches, we demonstrate that PLK1 promoted NLRP3 inflammasome activation at cell interphase. Using an unbiased proximity-dependent biotin identification (Bio-ID) screen for the PLK1 interactome in macrophages, we show an enhanced proximal association of NLRP3 with PLK1 upon NLRP3 inflammasome activation. We further confirmed the interaction between PLK1 and NLRP3 and identified the interacting domains. Mechanistically, we show that PLK1 orchestrated the microtubule-organizing center (MTOC) structure and NLRP3 subcellular positioning upon inflammasome activation. Treatment with a selective PLK1 kinase inhibitor suppressed IL-1β production in in vivo inflammatory models, including LPS-induced endotoxemia and monosodium urate-induced peritonitis in mice. Our results uncover a role of PLK1 in regulating NLRP3 inflammasome activation during interphase and identify pharmacological inhibition of PLK1 as a potential therapeutic strategy for inflammatory diseases with excessive NLRP3 inflammasome activation.

Development of Cell Permeable NanoBRET Probes for the Measurement of PLK1 Target Engagement in Live Cells

PLK1 is a protein kinase that regulates mitosis and is both an important oncology drug target and a potential antitarget of drugs for the DNA damage response pathway or anti-infective host kinases. To expand the range of live cell NanoBRET target engagement assays to include PLK1, we developed an energy transfer probe based on the anilino-tetrahydropteridine chemotype found in several selective PLK inhibitors. Probe 11 was used to configure NanoBRET target engagement assays for PLK1, PLK2, and PLK3 and measure the potency of several known PLK inhibitors. In-cell target engagement for PLK1 was in good agreement with the reported cellular potency for the inhibition of cell proliferation. Probe 11 enabled the investigation of the promiscuity of adavosertib, which had been described as a dual PLK1/WEE1 inhibitor in biochemical assays. Live cell target engagement analysis of adavosertib via NanoBRET demonstrated PLK activity at micromolar concentrations but only selective engagement of WEE1 at clinically relevant doses.

Development of Cell Permeable NanoBRET Probes for the Measurement of PLK1 Target Engagement in Live Cells

PLK1 is a protein kinase that regulates mitosis and is both an important oncology drug target and a potential anti target of drugs for the DNA damage response pathway or anti-infective host kinases. To expand the range of live cell NanoBRET target engagement assays to include PLK1 we developed an energy transfer probe based on the anilino-tetrahydropteridine chemotype found in several selective PLK inhibitors. Probe 11 was used to configure NanoBRET target engagement assays for PLK1, PLK2, and PLK3 and measure the potency of several known PLK inhibitors. In cell target engagement for PLK1 was in good agreement with the reported cellular potency for inhibition of cell proliferation. Probe 11 enabled investigation of the promiscuity of adavosertib, which had been described as a dual PLK1/WEE1 inhibitor in biochemical assays. Live cell target engagement analysis of adavosertib by NanoBRET demonstrated PLK activity at micromolar concentrations but only selective engagement of WEE1 at clinically relevant doses.

Unusually High Affinity of the PLK Inhibitors RO3280 and GSK461364 to HSA and Its Possible Pharmacokinetic Implications

The binding processes of two Polo-like kinase inhibitors, RO3280 and GSK461364, to the human serum albumin (HSA) protein as well as the protonation equilibria of both compounds have been studied combining absorbance and fluorescence spectroscopy experiments together with density functional theory calculations. We found that the charge states of RO3280 and GSK461364 are +2 and +1, respectively, at the physiological pH. Nevertheless, RO3280 binds to HSA in the charge state +1 prior to a deprotonation pre-equilibrium. Binding constants to site I of HSA of 2.23 × 10⁶ and 8.80 × 10⁴ M^-1 were determined for RO3280 and GSK461364, respectively, at 310 K. The binding processes of RO3280 and GSK461364 to HSA are entropy- and enthalpy-driven, respectively. The positive enthalpy found for the RO3280-HSA complex formation could be related to a proton pre-equilibrium of RO3280.

Polo-like Kinase 1 Inhibitors in Human Cancer Therapy: Development and Therapeutic Potential

Polo-like kinase 1 (PLK1) plays an important role in a variety of cellular functions, including the regulation of mitosis, DNA replication, autophagy, and the epithelial-mesenchymal transition (EMT). PLK1 overexpression is often associated with cell proliferation and poor prognosis in cancer patients, making it a promising antitumor target. To date, at least 10 PLK1 inhibitors (PLK1i) have been entered into clinical trials, among which the typical kinase domain (KD) inhibitor BI 6727 (volasertib) was granted "breakthrough therapy designation" by the FDA in 2013. Unfortunately, many other KD inhibitors showed poor specificity, resulting in dose-limiting toxicity, which has greatly impeded their development. Researchers recently discovered many PLK1i with higher selectivity, stronger potency, and better absorption, distribution, metabolism, and elimination (ADME) characteristics. In this review, we emphasize the structure-activity relationships (SARs) of PLK1i, providing insights into new drugs targeting PLK1 for antitumor clinical practice.

Shedding light on the binding mechanism of kinase inhibitors BI-2536, Volasetib and Ro-3280 with their pharmacological target PLK1

In the present work, the interactions of the novel kinase inhibitors BI-2536, Volasetib (BI-6727) and Ro-3280 with the pharmacological target PLK1 have been studied by fluorescence spectroscopy and molecular dynamics calculations. High Stern-Volmer constants were found in fluorescence experiments suggesting the formation of stable protein-ligand complexes. In addition, it was observed that the binding constant between BI-2536 and PLK1 increases about 100-fold in presence of the phosphopeptide Cdc25C-p that docks to the polo box domain of the protein and releases the kinase domain. All the determined binding constants are higher for the kinase inhibitors than for their competitor for the active center (ATP) being BI-2536 and Volasertib the inhibitors that showed more affinity for PLK1. Calculated binding free energies confirmed the higher affinity of PLK1 for BI-2536 and Volasertib than for ATP. The higher affinity of the inhibitors to PLK1 compared to ATP was mainly attributed to stronger van der Waals interactions. Results may help with the challenge of designing and developing new kinase inhibitors more effective in clinical cancer therapy.

PELO facilitates PLK1-induced the ubiquitination and degradation of Smad4 and promotes the progression of prostate cancer

PLK1 and Smad4 are two important factors in prostate cancer initiation and progression. They have been reported to play the opposite role in Pten-deleted mice, one is an oncogene, the other is a tumor suppressor. Moreover, they could reversely regulate the PI3K/AKT/mTOR pathway and the activation of MYC. However, the connections between PLK1 and Smad4 have never been studied. Here, we showed that PLK1 could interact with Smad4 and promote the ubiquitination and degradation of Smad4 in PCa cells. PLK1 and PELO could bind to different domains of Smad4 and formed a protein complex. PELO facilitated the degradation of Smad4 through cooperating with PLK1, thereby resulting in proliferation and metastasis of prostate cancer cell. Changes in protein levels of Smad4 led to the alteration of biological function that caused by PLK1 in prostate cancer cells. Further studies showed that PELO upregulation was positively associated with high grade PCa and knockdown of PELO expression significantly decreased PCa cell proliferation and metastasis in vitro and vivo. PELO knockdown in PCa cells could enhance the tumor suppressive role of PLK1 inhibitor. In addition, blocking the interaction between PELO and Smad4 by using specific peptide could effectively inhibit PCa cell metastasis ability in vitro and vivo. Overall, these findings identified a novel regulatory relationship among PLK1, Smad4 and PELO, and provided a potential therapeutic strategy for advanced PCa therapy by co-targeting PLK1 and PELO.

Differential BET Bromodomain Inhibition by Dihydropteridinone and Pyrimidodiazepinone Kinase Inhibitors

BRD4 and other members of the bromodomain and extraterminal (BET) family of proteins are promising epigenetic targets for the development of novel therapeutics. Among the reported BRD4 inhibitors are dihydropteridinones and benzopyrimidodiazepinones originally designed to target the kinases PLK1, ERK5, and LRRK2. While these kinase inhibitors were identified as BRD4 inhibitors, little is known about their binding potential and structural details of interaction with the other BET bromodomains. We comprehensively characterized a series of known and newly identified dual BRD4-kinase inhibitors against all eight individual BET bromodomains. A detailed analysis of 23 novel cocrystal structures of BET-kinase inhibitor complexes in combination with direct binding assays and cell signaling studies revealed significant differences in molecular shape complementarity and inhibitory potential. Collectively, the data offer new insights into the action of kinase inhibitors across BET bromodomains, which may aid the development of drugs to inhibit certain BET proteins and kinases differentially.

Polo-like Kinase 1 as an emerging drug target: structure, function and therapeutic implications

Polo-like kinase 1 (PLK1) is a conserved mitotic serine-threonine protein kinase, functions as a regulatory protein, and is involved in the progression of the mitotic cycle. It plays important roles in the regulation of cell division, maintenance of genome stability, in spindle assembly, mitosis, and DNA-damage response. PLK1 is consist of a N-terminal serine-threonine kinase domain, and a C-terminal Polo-box domain (regulatory site). The expression of PLK1 is controlled by transcription repressor in the G1 stage and transcription activators in the G2 stage of the cell cycle. Overexpression of PLK1 results in undermining of checkpoints causes excessive cellular division resulting in abnormal cell growth, leading to the development of cancer. Blocking the expression of PLK1 by an antibody, RNA interference, or kinase inhibitors, causes a subsequent reduction in the proliferation of tumour cells and induction of apoptosis in tumour cells without affecting the healthy cells, suggesting an attractive target for drug development. In this review, we discuss detailed information on expression, gene and protein structures, role in different diseases, and progress in the design and development of PLK1 inhibitors. We have performed an in-depth analysis of the PLK1 inhibitors and their therapeutic implications with special focus to the cancer therapeutics.

Discovery of methyl 3-((2-((1-(dimethylglycyl)-5-methoxyindolin-6-yl)amino)-5-(trifluoro-methyl) pyrimidin-4-yl)amino)thiophene-2-carboxylate as a potent and selective polo-like kinase 1 (PLK1) inhibitor for combating hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the second leading cause of cancer-related death worldwide and targeted therapeutics exhibit limited success. Polo-like kinase 1 (PLK1), a Ser/Thr kinase, plays a pivotal role in cell-cycle regulation and is considered a promising target in HCC. Here, via structural optimization using both biochemical kinase assays and cellular antiproliferation assays, we discovered a potent and selective PLK1 kinase inhibitor, compound 31. Compound 31 exhibited biochemical activity with IC50 of < 0.508 nM against PLK1 and a KINOMEscan selectivity score (S(1)) of 0.02 at a concentration of 1 μM. Furthermore, 31 showed broad antiproliferative activity against a variety of cancer cell lines, with the lowest antiproliferative IC50 (11.1 nM) in the HCC cell line HepG2. A detailed mechanistic study of 31 revealed that inhibition of PLK1 by 31 induces mitotic arrest at the G2/M phase checkpoint, thus leading to cancer cell apoptosis. Moreover, 31 exhibited profound antitumor efficacy in a xenograft mouse model. Collectively, these results establish compound 31 as a good starting point for the development of PLK1 targeted therapeutics for HCC.

RO3280: A Novel PLK1 Inhibitor, Suppressed the Proliferation of MCF-7 Breast Cancer Cells Through the Induction of Cell Cycle Arrest at G2/M Point

BACKGROUND

As a member of serine/threonine-protein kinase, Polo.like kinase 1 (PLK1) plays crucial roles during mitosis and also contributes to DNA damage response and repair. PLK1 is aberrantly expressed in many types of tumor cells and increased levels of PLK1 are closely related to tumorigenesis and poor clinical outcomes. Therefore, PLK1 is accepted as one of the potential targets for the discovery of novel anticancer agents. The objective of this study was to assess the cytotoxic effects of a novel PLK1 inhibitor, RO3280, against MCF-7, human breast cancer cells; HepG2, human hepatocellular carcinoma cells; and PC3, human prostate cancer cells, as well as non-cancerous L929 fibroblast cells.

METHODS

Antiproliferative activity of RO3280 was examined using the XTT assay. Flow cytometry assay was performed to evaluate cell cycle distribution, apoptosis, multicaspase activity, mitochondrial membrane potential, and DNA damage response. Apoptosis with fluorescence imaging studies was also examined.

RESULTS

According to the results of XTT assay, although RO3280 displayed potent cytotoxicity in all treated cancer cells, the most sensitive cell line was identified as MCF-7 cells that were selected for further studies. The compound induced a cell cycle arrest in MCF-7 cells at G2/M phase and significantly induced apoptosis, multicaspase activity, DNA damage response, and decreased mitochondrial membrane potential of MCF-7 cells.

CONCLUSION

Overall, RO3280 induces anticancer effects promoted mainly by DNA damage, cell cycle arrest, and apoptosis in breast cancer cells. Further studies are needed to assess its usability as an anticancer agent with specific cancer types.

Inhibition of Prostate Smooth Muscle Contraction by Inhibitors of Polo-Like Kinases

Background: Prostate smooth muscle contraction plays an important role for pathophysiology and treatment of male lower urinary tract symptoms (LUTS) but is incompletely understood. Because the efficacy of available medication is limited, novel options and improved understanding of prostate smooth muscle contraction are of high demand. Recently, a possible role of polo-like kinase 1 (PLK1) has been suggested for smooth muscle contraction outside the lower urinary tract. Here, we examined effects of PLK inhibitors on contraction of human prostate tissue.

Methods: Prostate tissues were obtained from radical prostatectomy. RT-PCR, Western blot and immunofluorescence were performed to detect PLK expression and phosphorylated PLK. Smooth muscle contractions were induced by electric field stimulation (EFS), α₁-agonists, endothelin-1, or the thromboxane A₂ analog U46619 in organ bath.

Results: RT-PCR, Western blot, and immunofluorescence suggested expression of PLK1 in the human prostate, which may be located and active in smooth muscle cells. EFS-induced contractions of prostate strips were reduced by SBE 13 (1 μM), cyclapolin 9 (3 μM), TAK 960 (100 nM), and Ro 3280 (100 nM). SBE 13 and cyclapolin 9 inhibited contractions by the α₁-agonists methoxamine, phenylephrine, and noradrenaline. In contrast, no effects of SBE 13 or cyclapolin 9 on endothelin-1- or U46619-induced contractions were observed.

Conclusion: Alpha1-adrenergic smooth muscle contraction in the human prostate can be inhibited by PLK inhibitors. PLK-dependent signaling may be a new pathway, which promotes α₁-adrenergic contraction of prostate smooth muscle cells. As contractions by endothelin and U46619 are not susceptible to PLK inhibition, this reflects divergent regulation of adrenergic and non-adrenergic prostate smooth muscle contraction.

Current progress and future perspectives in the development of anti-polo-like kinase 1 therapeutic agents

Although significant levels of side effects are often associated with their use, microtubule-directed agents that primarily target fast-growing mitotic cells have been considered to be some of the most effective anti-cancer therapeutics. With the hope of developing new-generation anti-mitotic agents with reduced side effects and enhanced tumor specificity, researchers have targeted various proteins whose functions are critically required for mitotic progression. As one of the highly attractive mitotic targets, polo-like kinase 1 (Plk1) has been the subject of an extensive effort for anti-cancer drug discovery. To date, a variety of anti-Plk1 agents have been developed, and several of them are presently in clinical trials. Here, we will discuss the current status of generating anti-Plk1 agents as well as future strategies for designing and developing more efficacious anti-Plk1 therapeutics.

Assessing the performance of docking scoring function, FEP, MM-GBSA, and QM/MM-GBSA approaches on a series of PLK1 inhibitors

Over-expressed polo-like kinases 1, a key regulator of cell mitosis, is associated with carcinogenesis and poor prognosis. It is very necessary to develop a reliable computational affinity prediction protocol targeting PLK1. In this study, the performance of different docking scoring function, free energy perturbation, MM-GBSA and QM/MM-GBSA were evaluated. The ranking capability of FEP is the best with r_s = 0.854. However, the r_s obtained from MM-GBSA can reach 0.767, which requires only about one-eighth of the simulation time of FEP. As for the sampling method, single long molecular dynamics (SLMD) surpass the multiple short molecular dynamics (MSMD) in ranking of the 20 congeneric compounds by about 0.1 in r_s. In addition, ligands treated by QM can significantly improve the ranking performance. As for the docking scoring functions, a force field-based scoring function is more suitable for ranking congeneric compounds.

Targeted inhibition of Polo-like kinase 1 by a novel small-molecule inhibitor induces mitotic catastrophe and apoptosis in human bladder cancer cells

Bladder cancer is a common cancer with particularly high recurrence after transurethral resection. Despite improvements in neoadjuvant chemotherapy, the outcome of patients with advanced bladder cancer has changed very little. In this study, the anti-tumour activities of a novel Polo-like kinase 1 (PLK1) inhibitor (RO3280) was evaluated in vitro and in vivo in the bladder carcinoma cell lines 5637 and T24. MTT assays, colony-formation assays, flow cytometry, cell morphological analysis and trypan blue exclusion assays were used to examine the proliferation, cell cycle distribution and apoptosis of bladder carcinoma cells with or without RO3280 treatment. Moreover, real-time RT-PCR and Western blotting were used to detect the expressions of genes that are related to these cellular processes. Our results showed that RO3280 inhibited cell growth and cell cycle progression, increased Wee1 expression and cell division cycle protein 2 phosphorylation. In addition, RO3280 induced mitotic catastrophe and apoptosis, increased cleaved PARP (poly ADP-ribose polymerase) and caspase-3, and decreased BubR1 expression. The in vivo assay revealed that RO3280 retarded bladder cancer xenograft growth in a nude mouse model. Although further laboratory and pre-clinical investigations are needed to corroborate these data, our demonstration of bladder cancer growth inhibition and dissemination using a pharmacological inhibitor of PLK1 provides new opportunities for future therapeutic intervention.

Inhibiting PLK1 induces autophagy of acute myeloid leukemia cells via mammalian target of rapamycin pathway dephosphorylation

Decreased autophagy is accompanied by the development of a myeloproliferative state or acute myeloid leukemia (AML). AML cells are often sensitive to autophagy‑inducing stimuli, prompting the idea that targeting autophagy can be useful in AML cytotoxic therapy. AML NB4 cells overexpressing microtubule-associated protein 1 light chain 3-green fluorescent protein were screened with 69 inhibitors to analyze autophagy activity. AML cells were treated with the polo-like kinase 1 (PLK1) inhibitors RO3280 and BI2536 before autophagy analysis. Cleaved LC3 (LC3-II) and the phosphorylation of mammalian target of rapamycin (mTOR), adenosine monophosphate-activated protein kinase, and Unc-51-like kinase 1 during autophagy was detected with western blotting. Autophagosomes were detected using transmission electron microscopy. Several inhibitors had promising autophagy inducer effects: BI2536, MLN0905, SK1-I, SBE13 HCL and RO3280. Moreover, these inhibitors all targeted PLK1. Autophagy activity was increased in the NB4 cells treated with RO3280 and BI2536. Inhibition of PLK1 expression in NB4, K562 and HL-60 leukemia cells with RNA interference increased LC3-II and autophagy activity. The phosphorylation of mTOR was reduced significantly in NB4 cells treated with RO3280 and BI2536, and was also reduced significantly when PLK1 expression was downregulated in the NB4, K562 and HL-60 cells. We demonstrate that PLK1 inhibition induces AML cell autophagy and that it results in mTOR dephosphorylation. These results may provide new insights into the molecular mechanism of PLK1 in regulating autophagy.

PLK1, A Potential Target for Cancer Therapy

Polo-like kinase 1 (PLK1) plays an important role in the initiation, maintenance, and completion of mitosis. Dysfunction of PLK1 may promote cancerous transformation and drive its progression. PLK1 overexpression has been found in a variety of human cancers and was associated with poor prognoses in cancers. Many studies have showed that inhibition of PLK1 could lead to death of cancer cells by interfering with multiple stages of mitosis. Thus, PLK1 is expected to be a potential target for cancer therapy. In this article, we examined PLK1’s structural characteristics, its regulatory roles in cell mitosis, PLK1 expression, and its association with survival prognoses of cancer patients in a wide variety of cancer types, PLK1 interaction networks, and PLK1 inhibitors under investigation. Finally, we discussed the key issues in the development of PLK1-targeted cancer therapy.

PLK-1 Expression is Associated with Histopathological Response to Neoadjuvant Therapy of Hepatic Metastasis of Colorectal Carcinoma

Polo-like kinase 1 (PLK1) is a serine/threonine-protein kinase expressed during mitosis and overexpressed in multiple human cancers, including leukemia and also many solid tumors. PLK1 knockdown has been shown to block proliferation of leukemic cell lines and the clonogenic potential of tumor cells grown from patients with cancer. PLK1 inhibition is a promising strategy for the treatment of some tumors. We aim to analyze expression of PLK1 in metastatic colorectal carcinoma. Retrospective analysis of colorectal carcinomas with hepatic metastasis during follow-up receiving neoadjuvant chemotherapy (NAC), based on oxaliplatin. Immunohistochemistry for PLK-1 in paraffin-embedded tissue from the primary and also from the metastasis. 50 patients. 32% showed good histopathological response. 43% of the primaries were positive for PLK1, as opposed to 23.5% of the metastasis. Expression of PLK1 was significantly reduced in metastasis compared with the primaries (p = 0.05), what could be due to therapy or to a phenotypic change of the metastatic nodule. Analysis of the prognostic influence of PLK1 expression showed significant association between PLK1 expression in metastasis and lower overall survival (p = 0.000). We have also found a significant association between PLK1 expression and histopathological response (p = 0.02). All the tumors with high expression of PLK1 showed minor response (11/11). This study shows the association between survival and poor histopathological response to therapy and high expression of PLK1 in metastasis. Our results could open a new therapeutic approach through the inhibition of PLK1.

Recent Advances and New Strategies in Targeting Plk1 for Anticancer Therapy

Polo-like kinase 1 (Plk1) plays key roles in regulating mitotic processes that are crucial for cellular proliferation. Overexpression of Plk1 is tightly associated with the development of particular cancers in humans, and a large body of evidence suggests that Plk1 is an attractive target for anticancer therapeutic development. Drugs targeting Plk1 can potentially be directed at two distinct sites: the N-terminal catalytic kinase domain (KD), which phosphorylates substrates, and the C-terminal polo-box domain (PBD) which is essential for protein-protein interactions. In this review we summarize recent advances and new challenges in the development of Plk1 inhibitors targeting these two domains. We also discuss novel strategies for designing and developing next-generation inhibitors to effectively treat Plk1-associated human disorders.

Mitotic entry: Non-genetic heterogeneity exposes the requirement for Plk1

The quest to develop novel antimitotic chemotherapy agents has led to the generation of several small molecule inhibitors targeting Plk1, a protein kinase required for multiple aspects of cell division. Previous studies have shown that upon exposure to Plk1 inhibitors, cells enter mitosis, delay briefly in prophase and then arrest in mitosis due to an inability to undergo centrosome separation. Here, we show that four different classes of Plk1 inhibitor block mitotic entry in several cancer cell lines and non-transformed RPE-1 cells. The proportion of cells that arrest in G2 is cell line and concentration dependent, and is subject to non-genetic heterogeneity. Following inhibitor washout, the G2 block is alleviated and cells enter mitosis but then fail to complete cell division indicating that most Plk1 inhibitors are not fully reversible. An exception is CYC140844; in contrast to five other inhibitors examined here, this novel Plk1 inhibitor is fully reversible. We discuss the implications for developing Plk1 inhibitors as chemotherapy agents and research tools.

PLK-1 Targeted Inhibitors and Their Potential against Tumorigenesis

Mitotic kinases are the key components of the cell cycle machinery and play vital roles in cell cycle progression. PLK-1 (Polo-like kinase-1) is a crucial mitotic protein kinase that plays an essential role in both the onset of G2/M transition and cytokinesis. The overexpression of PLK-1 is strongly correlated with a wide spectrum of human cancers and poor prognosis. The (si)RNA-mediated depletion of PLK-1 arrests tumor growth and triggers apoptosis in cancer cells without affecting normal cells. Therefore, PLK-1 has been selected as an attractive anticancer therapeutic drug target. Some small molecules have been discovered to target the catalytic and noncatalytic domains of PLK-1. These domains regulate the catalytic activation and subcellular localization of PLK-1. However, while PLK-1 inhibitors block tumor growth, they have been shown to cause severe adverse complications, such as toxicity, neutropenia, and bone marrow suppression during clinical trials, due to a lack of selectivity and specificity within the human kinome. To minimize these toxicities, inhibitors should be tested against all protein kinases in vivo and in vitro to enhance selectivity and specificity against targets. Here, we discuss the potency and selectivity of PLK-1-targeted inhibitors and their molecular interactions with PLK-1 domains.

Regioselective Control of the SNAr Amination of 5-Substituted-2,4-Dichloropyrimidines Using Tertiary Amine Nucleophiles

The SNAr reaction of 2,4-dichloropyrimidines, further substituted with an electron-withdrawing substituent at C-5, has selectivity for substitution at C-4. Here we report that tertiary amine nucleophiles show excellent C-2 selectivity. In situ N-dealkylation of an intermediate gives the product that formally corresponds to the reaction of a secondary amine nucleophile at C-2. This reaction is practical (fast under simple reaction conditions, with good generality for tertiary amine structure and moderate to excellent yields) and significantly expands access to pyrimidine structures.

Molecular targeting of the oncoprotein PLK1 in pediatric acute myeloid leukemia: RO3280, a novel PLK1 inhibitor, induces apoptosis in leukemia cells

Polo-like kinase 1 (PLK1) is highly expressed in many cancers and therefore a biomarker of transformation and potential target for the development of cancer-specific small molecule drugs. RO3280 was recently identified as a novel PLK1 inhibitor; however its therapeutic effects in leukemia treatment are still unknown. We found that the PLK1 protein was highly expressed in leukemia cell lines as well as 73.3% (11/15) of pediatric acute myeloid leukemia (AML) samples. PLK1 mRNA expression was significantly higher in AML samples compared with control samples (82.95 ± 110.28 vs. 6.36 ± 6.35; p < 0.001). Kaplan-Meier survival analysis revealed that shorter survival time correlated with high tumor PLK1 expression (p = 0.002). The 50% inhibitory concentration (IC50) of RO3280 for acute leukemia cells was between 74 and 797 nM. The IC50 of RO3280 in primary acute lymphocytic leukemia (ALL) and AML cells was between 35.49 and 110.76 nM and 52.80 and 147.50 nM, respectively. RO3280 induced apoptosis and cell cycle disorder in leukemia cells. RO3280 treatment regulated several apoptosis-associated genes. The regulation of DCC, CDKN1A, BTK, and SOCS2 was verified by western blot. These results provide insights into the potential use of RO3280 for AML therapy; however, the underlying mechanisms remain to be determined.

Discovery of TAK-960: an orally available small molecule inhibitor of polo-like kinase 1 (PLK1)

Using structure-based drug design, we identified and optimized a novel series of pyrimidodiazepinone PLK1 inhibitors resulting in the selection of the development candidate TAK-960. TAK-960 is currently undergoing Phase I evaluation in adult patients with advanced solid malignancies.

Discovery of a Highly Selective, Brain-Penetrant Aminopyrazole LRRK2 Inhibitor

The modulation of LRRK2 kinase activity by a selective small molecule inhibitor has been proposed as a potentially viable treatment for Parkinson's disease. By using aminopyrazoles as aniline bioisosteres, we discovered a novel series of LRRK2 inhibitors. Herein, we describe our optimization effort that resulted in the identification of a highly potent, brain-penetrant aminopyrazole LRRK2 inhibitor (18) that addressed the liabilities (e.g., poor solubility and metabolic soft spots) of our previously disclosed anilino-aminopyrimidine inhibitors. In in vivo rodent PKPD studies, 18 demonstrated good brain exposure and engendered significant reduction in brain pLRRK2 levels post-ip administration. The strategies of bioisosteric substitution of aminopyrazoles for anilines and attenuation of CYP1A2 inhibition described herein have potential applications to other drug discovery programs.

1-Eth-oxy-methyl-5-methyl-9-phenyl-6,7,8,9-tetra-hydro-1H-pyrimido[4,5-b][1,4]diazepine-2,4(3H,5H)-dione

The title compound, C(17)H(22)N(4)O(3), comprises a 1,4-diazepine ring in a twist-boat conformation fused to a pyrimidine ring. The dihedral angle between the pyrimidine and phenyl rings is 80.8 (1)°. The crystal packing features N-H⋯O and C-H⋯O hydrogen bonds.