Identification of a novel Polo-like kinase 1 inhibitor that specifically blocks the functions of Polo-Box domain

Structure-guided Development of Novel Benzothiophene Derivatives as PLK1-PBD Inhibitors

Background:

Polo-like kinase 1 (PLK1), a validated target for tumor therapy, plays a key role in mitosis and is over-expressed in many tumors. In addition to its N-terminal kinase domain, PLk1 also harbors a C-terminal polo-box domain (PBD).

Objective:

A candidate based on PLK1-PBD was developed as a promising compound for future development.

Methods:

Seventeen small molecule PLK1-PBD inhibitors were designed, synthesized and evaluated for PLK1-PBD inhibitory activities by fluorescence polarization (FP) assay. The compounds with better inhibitory activities were further assessed for their anti-proliferative activities using a CCK-8 method.

Results:

The inhibitory rates of compounds 7a, 7d, 14a, 14d, 14e and 14f exceeded 98%. The IC50 values of compounds 7d, 14d, 14e, and 14f were 0.73 μM, 0.67 μM, 0.89 μM and 0.26 μM, proving better than MCC1019. Compound 14f showed the best inhibitory activity (IC50: 0.26 μM) and antiproliferative activities against three cancer cell lines (HeLa, HepG2 and MG63). Especially, compound 14f also exhibited acceptable safety profiles in the human ether-a-go-go related gene (hERG) and normal cell tests. The results of docking and prediction studies indicated that compound 14f had a high binding affinity to the target, with good drug-like absorption, distribution, metabolism, elimination, and toxicity (ADMET) properties.

Conclusion:

Compound 14f can be a promising compound for future development.

Inhibitors of the PLK1 polo-box domain: drug design strategies and therapeutic opportunities in cancer

INTRODUCTION

Polo Like Kinase 1 (PLK1) is a key regulator of mitosis and its overexpression is frequently observed in a wide variety of human cancers, while often being associated with poor survival rates. Therefore, it is considered a potential and attractive target for cancer therapeutic development. The Polo like kinase family is characterized by the presence of a unique C terminal polobox domain (PBD) involved in regulating kinase activity and subcellular localization. Among the two functionally essential, druggable sites with distinct properties that PLK1 offers, targeting the PBD presents an alternative approach for therapeutic development.

AREAS COVERED

Significant progress has been made in progressing from the peptidic PBD inhibitors first identified, to peptidomimetic and recently drug-like small molecules. In this review, the rationale for targeting the PBD over the ATP binding site is discussed, along with recent progress, challenges, and outlook.

EXPERT OPINION

The PBD has emerged as a viable alternative target for the inhibition of PLK1, and progress has been made in using compounds to elucidate mechanistic aspects of activity regulation and in determining roles of the PBD. Studies have resulted in proof of concept of in vivo efficacy suggesting promise for PBD binders in clinical development.

A robust high-throughput fluorescence polarization assay for rapid screening of SARS-CoV-2 papain-like protease inhibitors

The global scourge of COVID-19 is a serious threat to public health, but effective therapies remain very limited for this disease. Therefore, the discovery of novel antiviral agents is urgently needed to fight against COVID-19. In the lifecycle of SARS-CoV-2, the causing pathogen of COVID-19, papain-like protease (PLpro) is responsible for the cleavage of polyprotein into functional units as well as immune evasion of vaccines. Hence, PLpro has been regarded as an attractive target to develop antiviral agents. Herein, we first developed a robust and simple sandwich-like fluorescence polarization (FP) screening assay for the discovery of PLpro inhibitors, and identified anacardic acid as a novel competitive inhibitor against PLpro in vitro with an IC₅₀ value of 24.26 ± 0.4 μM. This reliable FP screening assay could provide a prospective avenue for rapid discovery of antiviral agents targeting PLpro in a large-scale screening.

Development of a Polo-like Kinase-1 Polo-Box Domain Inhibitor as a Tumor Growth Suppressor in Mice Models

Polo-like kinase-1 (Plk1) plays a key role in mitosis and has been identified as an attractive anticancer drug target. Plk1 consists of two drug-targeting sites, namely, N-terminal kinase domain (KD) and C-terminal polo-box domain (PBD). As KD-targeting inhibitors are associated with severe side effects, here we report on the pyrazole-based Plk1 PBD inhibitor, KBJK557, which showed a remarkable in vitro anticancer effect by inducing Plk1 delocalization, mitotic arrest, and apoptosis in HeLa cells. Further, in vivo optical imaging analysis and antitumorigenic activities in mouse xenograft models demonstrate that KBJK557 preferentially accumulates in cancer cells and selectively inhibits cancer cell proliferation. Pharmacokinetic profiles and partition coefficients suggest that KBJK557 was exposed in the blood and circulated through the organs with an intermediate level of clearance (t1/2, 7.73 h). The present investigation offers a strategy for specifically targeting cancer using a newly identified small-molecule inhibitor that targets the Plk1 PBD.

Identification of a New Heterocyclic Scaffold for Inhibitors of the Polo-Box Domain of Polo-like Kinase 1

As a mitotic-specific target widely deregulated in various human cancers, polo-like kinase 1 (Plk1) has been extensively explored for anticancer activity and drug discovery. Although multiple catalytic domain inhibitors were tested in preclinical and clinical studies, their efficacies are limited by dose-limiting cytotoxicity, mainly from off-target cross reactivity. The C-terminal noncatalytic polo-box domain (PBD) of Plk1 has emerged as an attractive target for generating new protein-protein interaction inhibitors. Here, we identified a 1-thioxo-2,4-dihydro-[1,2,4]triazolo[4,3-a]quinazolin-5(1H)-one scaffold that efficiently inhibits Plk1 PBD but not its related Plk2 and Plk3 PBDs. Structure-activity relationship studies led to multiple inhibitors having ≥10-fold higher inhibitory activity than the previously characterized Plk1 PBD-specific phosphopeptide, PLHSpT (Kd ∼ 450 nM). In addition, S-methyl prodrugs effectively inhibited mitotic progression and cell proliferation and their metabolic stability was determined. These data describe a novel class of small-molecule inhibitors that offer a promising avenue for future drug discovery against Plk1-addicted cancers.

Optimizations of a novel fluorescence polarization-based high-throughput screening assay for β-catenin/LEF1 interaction inhibitors

Aberrant activation of the Wnt/β-catenin signaling pathway is prominent in the development and metastasis of non-small cell lung cancer (NSCLC). Highly effective inhibition of this pathway highlights a therapeutic avenue against NSCLC. Moreover, β-catenin/LEF1 interaction regulates β-catenin nuclear transport as well as the transcriptions of the key oncogenes in Wnt/β-catenin signaling pathway. Therefore, interruption of this interaction would be a promising therapeutic strategy for NSCLC metastasis. To date, no economical and rapid high-throughput screening (HTS) assay has been reported for the discovery of β-catenin/LEF1 interaction inhibitors. In this study, we developed a novel fluorescence polarization (FP)-based HTS assay to identify β-catenin/LEF1 interaction inhibitors. The FITC-LEF1 sequence, incubation time, temperature, and DMSO resistance were optimized, and then a high Z' factor of 0.77 was achieved. A pilot screening of a natural product library via this established FP screening assay identified sanguinarine analogues as potential β-catenin/LEF1 interaction inhibitors. GST pull-down and surface plasmon resonance (SPR) assay demonstrated that β-catenin/LEF1 interaction is a potential anticancer target of sanguinarine in vitro. This newly developed FP screening assay will be vital for the rapid discovery of novel Wnt inhibitors targeting β-catenin/LEF1 interaction.

Discovery of Targeted Covalent Natural Products against PLK1 by Herb-Based Screening

Natural products (NPs) are a rich source of drug discovery, and some of them act by covalently binding to the targets. Recently, targeted covalent natural product (TCNP) design has gained considerable attention since this approach offers significant benefits in improving biological efficacy and decreasing the off-target side effects. However, most of the known TCNPs were discovered by chance. Rational approaches for a systematic screen of TCNPs are much needed. Here, we developed a combined computational and experimental approach to carry out herb-based screening to identify TCNPs against proper cysteine residues in the target proteins. The herb-based TCNP screening approach (HB-TCNP) starts from a druggable pocket and cysteine residue prediction, followed by virtual screening of a covalent NP database and herb-based mapping to identify candidate herbs for experimental validation. Herbs with time-dependent activity are selected, and their NPs are experimentally tested to further screen covalent NPs. We have successfully applied HB-TCNP to screen anti-PLK1 herbs and NPs with high efficacy. Cys67 and Cys133 in the ATP binding pocket of PLK1 were used in the search. Five herbs were tested and exhibited PLK1 inhibition activity to some extent, among which Scutellaria baicalensis showed the most potent activity with time dependency. Further experimental studies showed that the main active compounds in Scutellaria baicalensis, baicalein and baicalin, covalently bind PLK1 through Cys133. Our study provided an efficient way to rationally design TCNPs and to make better use of herb medicines. The Cys133 residue in PLK1 serves as a novel covalent site for further drug discovery against PLK1.

Identification of inhibitors of the polo-box domain of polo-like kinase 1 from natural and semisynthetic compounds

PLK1 has an important role in the regulation of cell cycle and represents an important target for cancer treatment. This enzyme belongs to the Polo-like kinases family, which is characterized by a regulatory domain named Polo-box domain (PBD). Rather than regular kinase inhibitors, this domain provides high selectivity to PLK1. Here, we report on four novel PLK1 PBD inhibitors identified by cytotoxicity screening and fluorescence polarization assay of a chemical library of natural and semisynthetic compounds. These compounds revealed two- to three-fold higher selectivity to the PDB of PLK1 than to those of the related family members, PLK2 and PLK3. These four substances inhibited tumor cell growth of sensitive CCRF-CEM and multidrug-resistant CEM/ADR5000 leukemia cells. The tested compounds increased the apoptotic cell fraction, which indicates apoptosis as a major mechanism of cell death. Cell cycle analysis showed compound (5) arrested the cell cycle of CCRF-CEM cells in the G2/M phase, while the other three molecules ((compound (3), compound (4), and compound (6)) exerted pronounced cytotoxicity with an increase of cells in the sub-G1 population. Molecular docking was performed for the understanding of ligand-protein interaction, the tested candidates showed strong binding affinity to PLK1 PBD. In conclusion, we identified four new chemical scaffolds that may serve as lead compounds for the development of selective PLK1 inhibitors in the future.

Advances in covalent kinase inhibitors

This comprehensive review details recent advances, challenges and innovations in covalent kinase inhibition within a 10 year period (2007–2018).

Poloxin-2HT+: changing the hydrophobic tag of Poloxin-2HT increases Plk1 degradation and apoptosis induction in tumor cells

We report the hydrophobically-tagged Plk1 PBD inhibitor Poloxin-2HT+, which selectively degrades the tumor target Plk1 and induces apoptosis in human tumor cells with higher potency than the hydrophobically-tagged inhibitor Poloxin-2HT. Our data provide further evidence that hydrophobically tagged inhibitors of protein-protein interactions can target and destroy disease-relevant proteins.

High expression of CDK1 and BUB1 predicts poor prognosis of pancreatic ductal adenocarcinoma

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is one of the most common causes of cancer-related death. Increasing evidence suggests that cell cycle dysregulation is one of the hallmarks of cancer. In this study, by using the GEO database, we predicted the cell cycle-related protein CDK1 and BUB1 to be significantly overexpressed in PDAC tissues. Thus, this study aimed to investigate the clinical pathological significance of CDK1 and BUB1 in PDAC.

METHODS

To explore the role of CDK1 and BUB1 in PDAC progression and evaluate their prognostic value, we investigated the expression patterns of CDK1 and BUB1 by using immunohistochemical staining in 99 PDAC and 71 normal pancreatic tissues with complete pathological parameters and survival data.

RESULTS

CDK1 and BUB1 were significantly overexpressed in PDAC tissues. The expression of CDK1 was correlated with tumor size and histological grade, and the expression of BUB1 was correlated with the tumor size of PDAC. With regard to survival, a high expression of either CDK1 or BUB1 was correlated with a short survival of PDAC patients. Additionally, PDAC patients with a concurrent high expression of CDK1 and BUB1 showed the shortest survival.

CONCLUSIONS

Our study demonstrated that CDK1 and BUB1 may play a role in PDAC progression and could be prognostic biomarkers for PDAC patients.

Selective Degradation of Polo-like Kinase 1 by a Hydrophobically Tagged Inhibitor of the Polo-Box Domain

Hydrophobic tagging (HT) of bioactive compounds can induce target degradation via the proteasomal pathway. The first application of hydrophobic tagging to an existing inhibitor of protein-protein interactions is now presented. We developed Poloxin-2HT by fusing an adamantyl tag to Poloxin-2, an inhibitor of the polo-box domain of the protein kinase Plk1, which is a target for tumor therapy. Poloxin-2HT selectively reduced the protein levels of Plk1 in HeLa cells and had a significantly stronger effect on cell viability and the induction of apoptosis than the untagged PBD inhibitor Poloxin-2. The change in cellular phenotype associated with the addition of the hydrophobic tag to Poloxin-2 demonstrated that Poloxin-2HT targets Plk1 in living cells. Our data validate hydrophobic tagging of selective inhibitors of protein-protein interactions as a novel strategy to target and destroy disease-relevant proteins.

IMB-T130 targets 3-dehydroquinate synthase and inhibits Mycobacterium tuberculosis

The anti-tuberculosis (TB) agent IMB-T130 was speculated to be a multi-target compound. In this research, we found that IMB-T130 inhibits the catalytic activity of Mycobacterium tuberculosis 3-dehydroquinate synthase (MtDHQS), the enzyme in the second step of the shikimate pathway. IMB-T130 was identified as a selective inhibitor of MtDHQS with an IC₅₀ value of 0.87 μg/mL. The interaction between the compound and protein was analysed by surface plasmon resonance and circular dichroism. Based on the in silico molecular docking results, the essential amino acids in the binding pocket were then confirmed by site-directed mutagenesis. Overexpression of DHQS reduced the antibacterial activity of IMB-T130 in cells, verifying that DHQS is the target of IMB-T130. IMB-T130 inhibited standard and drug-resistant M. tuberculosis strains by targeting DHQS. Our findings improve our understanding of MtDHQS and make it to be a potential target for new anti-TB drug discovery.

Aurora-PLK1 cascades as key signaling modules in the regulation of mitosis

Mitosis is controlled by reversible protein phosphorylation involving specific kinases and phosphatases. A handful of major mitotic protein kinases, such as the cyclin B-CDK1 complex, the Aurora kinases, and Polo-like kinase 1 (PLK1), cooperatively regulate distinct mitotic processes. Research has identified proteins and mechanisms that integrate these kinases into signaling cascades that guide essential mitotic events. These findings have important implications for our understanding of the mechanisms of mitotic regulation and may advance the development of novel antimitotic drugs. We review collected evidence that in vertebrates, the Aurora kinases serve as catalytic subunits of distinct complexes formed with the four scaffold proteins Bora, CEP192, INCENP, and TPX2, which we deem "core" Aurora cofactors. These complexes and the Aurora-PLK1 cascades organized by Bora, CEP192, and INCENP control crucial aspects of mitosis and all pathways of spindle assembly. We compare the mechanisms of Aurora activation in relation to the different spindle assembly pathways and draw a functional analogy between the CEP192 complex and the chromosomal passenger complex that may reflect the coevolution of centrosomes, kinetochores, and the actomyosin cleavage apparatus. We also analyze the roles and mechanisms of Aurora-PLK1 signaling in the cell and centrosome cycles and in the DNA damage response.

Identification of nitroimidazole-oxime derivatives targeting the polo-box domain of polo-like kinase 1

Recent progress in the development of small molecular skeleton-derived polo-like kinase (PLK1) catalytic domain (KD) inhibitors has led to the synthesis of multiple ligands with high binding affinity. However, few systematic analyses have been conducted to identify key PLK1-PBD domain and characterize their interactions with potent PLK1 inhibitors. Therefore, we designed a series of PLK1-PBD inhibitors with an in silico scaffold modification strategy. A docking simulation combined with a primary screen in vitro were performed to filter for the lead compound, which was then substituted, synthesized and evaluated by a variety of bioassays. The biological profile of 4v suggests that this compound may be developed as a potential anticancer agent.

Low BUB1 expression is an adverse prognostic marker in gastric adenocarcinoma

Gastric adenocarcinomas are associated with a poor prognosis due to the fact that the tumor has often metastasized by the time of diagnosis and prognostic markers are urgently needed to tailor treatment.

We examined the expression of the mitotic spindle checkpoint protein BUB1 (budding uninhibited by benzimidazoles 1) and Ki-67 protein expression by immunohistochemistry in 218 patients with primary gastric adenocarcinomas.

Tumors with low frequency of BUB1 expression were associated with larger tumor size (pT) (p < 0.001), higher incidence of lymph node metastases (pN) (p = 0.027), distant metastases (pM) (p = 0.006) and higher UICC stage (p < 0.001). Furthermore, BUB1 expression was inversely correlated with residual tumor stage (p = 0.038). Abundant BUB1 protein expression correlated with frequent Ki-67 protein expression (p < 0.001) and low BUB1 expression was associated with shorter survival (p < 0.001). Univariate and multivariate analyses confirmed BUB1 to be an independent prognostic marker in gastric cancer (p = 0.021).

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.

Several inhibitors of the Plk1 Polo-Box Domain turn out to be non-specific protein alkylators

For almost a decade, there has been much interest in the development of chemical inhibitors of Polo-like kinase 1 (Plk1) protein interactions. Plk1 is a master regulator of the cell division cycle that controls numerous substrates. It is a promising target for cancer drug development. Inhibitors of the kinase domain of Plk1 had some success in clinical trials. However, they are not perfectly selective. In principle, Plk1 can also be inhibited by interfering with its protein interaction domain, the Polo-Box Domain (PBD). Selective chemical inhibitors of the PBD would constitute tools to probe for PBD-dependent functions of Plk1 and could be advantageous in cancer therapy. The discovery of Poloxin and thymoquinone as PBD inhibitors indicated that small, cell-permeable chemical inhibitors could be identified. Other efforts followed, including ours, reporting additional molecules capable of blocking the PBD. It is now clear that, unfortunately, most of these compounds are non-specific protein alkylators (defined here as groups covalently added via a carbon) that have little or no potential for the development of real Plk1 PBD-specific drugs. This situation should be minded by biologists potentially interested in using these compounds to study Plk1. Further efforts are needed to develop selective, cell-permeable PBD inhibitors.