Human Polo-like Kinase Research Articles

Human Polo-like Kinase Inhibitors as Antiplasmodials.

Protein kinases have proven to be a very productive class of therapeutic targets, and over 90 inhibitors are currently in clinical use primarily for the treatment of cancer. Repurposing these inhibitors as antimalarials could provide an accelerated path to drug development. In this study, we identified BI-2536, a known potent human polo-like kinase 1 inhibitor, with low nanomolar antiplasmodial activity. Screening of additional PLK1 inhibitors revealed further antiplasmodial candidates despite the lack of an obvious orthologue of PLKs in Plasmodium. A subset of these inhibitors was profiled for their in vitro killing profile, and commonalities between the killing rate and inhibition of nuclear replication were noted. A kinase panel screen identified PfNEK3 as a shared target of these PLK1 inhibitors; however, phosphoproteome analysis confirmed distinct signaling pathways were disrupted by two structurally distinct inhibitors, suggesting PfNEK3 may not be the sole target. Genomic analysis of BI-2536-resistant parasites revealed mutations in genes associated with the starvation-induced stress response, suggesting BI-2536 may also inhibit an aminoacyl-tRNA synthetase.

An In-silico Approach: Design, Homology Modeling, Molecular Docking, MM/GBSA Simulations, and ADMET Screening of Novel 1,3,4-oxadiazoles as PLK1inhibitors.

Breast cancer is the most commonly diagnosed and major cause of cancer-related deaths in women worldwide. Disruption of the normal regulation of cell cycle progression and proliferation are the major events leading to cancer. Human Polo-like Kinase 1 (PLK1) plays an important role in the regulation of cellular division. High PLK1 expression is observed in various types of cancer including breast cancer. 1,3,4-oxadiazoles are the fivemembered heterocycles, that serve as versatile lead molecules for designing novel anticancer agents and they mainly act by inhibiting various enzymes and kinases. A novel series of 1,3,4-oxadiazole derivatives (A1-A26) were designed and subjected to an in-silico analysis against PLK1 enzyme (PDB ID:1q4k), targeting breast cancer. The chemical structure of each compound (A1-26) was drawn using ChemDraw software. The 3D structure model of protein target (PDB ID:1q4k) was built using the SWISSMODEL server. Molecular docking simulation was performed to determine the designed compound's probable binding mode and affinity towards the protein target (PDB ID:1q4k). The designed compounds were subjected to ADME screening, as well as Prime MM/GBSA simulations using Schrodinger suite 2020-4. Furthermore, the safety profile of compounds was examined through the OSIRIS property explorer program and the results were compared with the standard drugs, 5-fluorouracil and cyclophosphamide. Based on the binding affinity scores, the compounds were found selective to target protein 1q4k through hydrogen bonding and hydrophobic interactions. The compounds A11, A12, and A13 were found to have higher G scores and binding free energy values. The ADME screening results were also found to be within the acceptable range. Moreover, the in-silico toxicity prediction assessments suggest that all designed compounds have a low risk of toxicity, and have higher efficiency for the target receptor. The study showed that the substitution of electron-donating groups at the various position of the aromatic ring, which is bonded at the second position of the substituted 1,3,4- oxadiazole nucleus resulted in compounds with good binding energy and G score compared to the standard drugs, and hence, they can be further developed as potent PLK1 enzyme inhibitors.

Molecular Docking Studies of 1, 3- Thiazolidin-4-Ones as Potential Human Polo Like Kinase 1 (Plk 1) Inhibitors

Polo-like kinase 1 (Plk1) belongs to a subfamily of Ser/ Thr protein kinases, collectively termed polo-like kinases (Plks). A growing body of evidence suggests that Plk1 plays important roles during multiple stages of M-phase progression and, as a result, in cell proliferation. In a series of preclinical studies, PLK1 has been validated as a cancer target. This has prompted many research scientists to develop selective PLK1 inhibitors for the treatment of cancer. The recent strategy employed in PLK1 inhibition involves the designing of small molecule inhibitors that targets the classical ATP-binding site of PLK1. Purpurogallin (PPG) and poloxin have been reported as inhibitors of the polo-box domain (PBD) of human polo-like kinase 1. Recently, new generation of PLK1 inhibitors targeting the second druggable domain of PLK1, the polo box domain, is currently being tested preclinically. We propose to design and develop 1, 3-thiazolidin-4-ones which could inhibit PLK1 for potent anticancer and anti-inflammatory activities. The 1, 3-thiazolidin – 4-ones could show anticancer and anti-inflammatory potential through inhibition of polo-like kinase 1 (PLK1) and cyclooxygenase-2 (COX 2) via NF-Kb (nuclear factor kappa-light- chain-enhancer of activated B cells) protein. Molecular docking of a set of ligands was performed using Vlife MDS 4.4 software. The 3D ligand protein complex structure of Polo Like Kinase 1 (2rku) was obtained from the protein database bank (RCSB-PDB) and processed for protein preparation and optimization. The computational studies suggest that aromatic group with non-polar electronegative substituents at second position and nitro-heterocycle either directly or through amide linkage at third position of the 1, 3-thiazolidin – 4-ones could result in better anticancer compound through inhibition of enzyme polo-like kinase 1.

A cryptic hydrophobic pocket in the polo-box domain of the polo-like kinase PLK1 regulates substrate recognition and mitotic chromosome segregation

The human polo-like kinase PLK1 coordinates mitotic chromosome segregation by phosphorylating multiple chromatin- and kinetochore-binding proteins. How PLK1 activity is directed to specific substrates via phosphopeptide recognition by its carboxyl-terminal polo-box domain (PBD) is poorly understood. Here, we combine molecular, structural and chemical biology to identify a determinant for PLK1 substrate recognition that is essential for proper chromosome segregation. We show that mutations ablating an evolutionarily conserved, Tyr-lined pocket in human PLK1 PBD trigger cellular anomalies in mitotic progression and timing. Tyr pocket mutations selectively impair PLK1 binding to the kinetochore phosphoprotein substrate PBIP1, but not to the centrosomal substrate NEDD1. Through a structure-guided approach, we develop a small-molecule inhibitor, Polotyrin, which occupies the Tyr pocket. Polotyrin recapitulates the mitotic defects caused by mutations in the Tyr pocket, further evidencing its essential function, and exemplifying a new approach for selective PLK1 inhibition. Thus, our findings support a model wherein substrate discrimination via the Tyr pocket in the human PLK1 PBD regulates mitotic chromosome segregation to preserve genome integrity.

In silico study on interaction between human polo-like kinase 1 and cyanobacterial sheath pigment scytonemin by molecular docking approach

Cancer is one of the major causes of death throughout the globe. It is expected that the number of new reports with cancer cases will reach twenty two million in the coming two decades. Africa, Central and South America and Asia will have more than 60 percent of the world’s new cancer reports and 70 percent of the world’s deaths from cancer will be contributed by these continents. Hence, drug discovery for treatment of cancer is the most worked area of this century. Polo-like kinase 1 (PLK-1) is highly expressed in human tumors and is important target of anti-cancerous drugs owing to its role in cell cycle events. It is crucial in maintenance of stability of genome and during different stages of mitosis. Scytonemin is a lipid-soluble and yellow-brown pigment exclusively synthesized by several cyanobacterial species in response to ultraviolet-A radiation. It functions as a photoprotective compound and can act as non-competitive and competitive inhibitor of PLK-1. Other kinases such as Myt1, cyclin-dependent kinase 1/cyclin B, checkpoint kinase 1 and protein kinase C are also inhibited by scytonemin. In the present study, molecular docking approach has been employed for positioning the inhibitor (dimethoxyscytonemin) into the active site of PLK-1 for determining the most probable binding mode. Based on the docking studies, the models which are developed could be utilized for understanding the structure-activity relationships of the scytonemins and for the prescreening and designing of novel inhibitors of PLK-1.

An oncoinformatics study to predict the inhibitory potential of recent FDA-approved anti-cancer drugs against human Polo-like kinase 1 enzyme: a step towards dual-target cancer medication.

Cancer prevalence has increased at an alarming rate worldwide. Complexity, resistance mechanism and multiple compensatory survival pathways of cancer cells have abated the response of currently available cancer medications. Therefore, multi-target agents rather than single target might provide a better solution to these cancer therapy issues. In the present study, anti-PLK1 (Polo-like kinase 1) potential of the eight FDA-approved (2017) anti-cancer drugs have been explored using molecular docking approach. Out of all the tested drugs, brigatinib, niraparib and ribociclib showed better binding affinity towards the 'kinase domain' of PLK1. The Gibbs free binding energy (ΔG) and inhibition constant (K i) values for brigatinib, niraparib and ribociclib interaction with the kinase domain of PLK1 were '- 8.05kcal/mol and 1.26µM', '- 8.35kcal/mol and 0.729µM' and '- 7.29kcal/mol and 4.52µM', respectively. Interestingly, the docking results of these three drugs were better than the known PLK1 inhibitors (BI-2536 and rigosertib). The ΔG and K i values for BI-2536 and rigosertib interaction with the kinase domain of PLK1 were '- 6.8kcal/mol and 10.38µM' and '- 6.6kcal/mol and 14.51µM', respectively. Brigatinib, niraparib and ribociclib have been approved by FDA for the treatment of non-small cell lung cancer, ovarian/fallopian tube cancer and breast cancer, respectively. PLK1 is regarded as a potential cancer target, and it is specifically over-expressed in different types of cancer cells, including aforementioned cancers. Actually, the target enzymes for anti-cancer action of brigatinib, niraparib and ribociclib are tyrosine kinase, poly(ADP-ribose) polymerase and cyclin-dependent kinase 4/6, respectively. However, based on our outcomes, we could safely state that PLK1 might plausibly emerge as an add-on target for each of these three anti-cancer drugs. We strongly believe that this study would assist in the development of better dual-targeting cancer therapeutic agent in the near future.

Polo-like kinase 1 expression is suppressed by CCAAT/enhancer-binding protein α to mediate colon carcinoma cell differentiation and apoptosis

Human polo-like kinase 1 (PLK1), a highly conserved serine/threonine kinase is a key player in several essential cell-cycle events. PLK1 is considered an oncogene and its overexpression often correlates with poor prognosis of cancers, including colorectal cancer (CRC). However, regulation of PLK1 expression in colorectal cells was never studied earlier and it is currently unknown if PLK1 regulates differentiation and apoptosis of CRC. PLK1 expression was analyzed by real-time PCR and western blotting. Transcriptional regulation was studied by reporter assay, gene knock-down, EMSA and ChIP. PLK1 expression was down-regulated during butyrate-induced differentiation of HT-29 and other CRC cells. Also, PLK1 down-regulation mediated the role of butyrate in CRC differentiation and apoptosis. We report here a novel transcriptional regulation of PLK1 by butyrate. Transcription factors CCAAT/enhancer-binding protein α (C/EBPα) and Oct-1 share an overlapping binding site over the PLK1 promoter. Elevated levels of C/EBPα by butyrate treatment of CRC cells competed out the activator protein Oct-1 from binding to the PLK1 promoter and sequestered it. Binding of C/EBPα was associated with increased deacetylation near the transcription start site (TSS) of the PLK1 promoter, which abrogated transcription through reduced recruitment of RNA polymerase II. We also found a synergistic role between the synthetic PLK1-inhibitor SBE13 and butyrate on the apoptosis of CRC cells. This study offered a novel p53-independent regulation of PLK1 during CRC differentiation and apoptosis. Down-regulation of PLK1 is one of the mechanisms underlying the anti-cancer role of dietary fibre-derived butyrate in CRC.

Interactions of Prototype Foamy Virus Capsids with Host Cell Polo-Like Kinases Are Important for Efficient Viral DNA Integration.

Unlike for other retroviruses, only a few host cell factors that aid the replication of foamy viruses (FVs) via interaction with viral structural components are known. Using a yeast-two-hybrid (Y2H) screen with prototype FV (PFV) Gag protein as bait we identified human polo-like kinase 2 (hPLK2), a member of cell cycle regulatory kinases, as a new interactor of PFV capsids. Further Y2H studies confirmed interaction of PFV Gag with several PLKs of both human and rat origin. A consensus Ser-Thr/Ser-Pro (S-T/S-P) motif in Gag, which is conserved among primate FVs and phosphorylated in PFV virions, was essential for recognition by PLKs. In the case of rat PLK2, functional kinase and polo-box domains were required for interaction with PFV Gag. Fluorescently-tagged PFV Gag, through its chromatin tethering function, selectively relocalized ectopically expressed eGFP-tagged PLK proteins to mitotic chromosomes in a Gag STP motif-dependent manner, confirming a specific and dominant nature of the Gag-PLK interaction in mammalian cells. The functional relevance of the Gag-PLK interaction was examined in the context of replication-competent FVs and single-round PFV vectors. Although STP motif mutated viruses displayed wild type (wt) particle release, RNA packaging and intra-particle reverse transcription, their replication capacity was decreased 3-fold in single-cycle infections, and up to 20-fold in spreading infections over an extended time period. Strikingly similar defects were observed when cells infected with single-round wt Gag PFV vectors were treated with a pan PLK inhibitor. Analysis of entry kinetics of the mutant viruses indicated a post-fusion defect resulting in delayed and reduced integration, which was accompanied with an enhanced preference to integrate into heterochromatin. We conclude that interaction between PFV Gag and cellular PLK proteins is important for early replication steps of PFV within host cells.

Structural analysis of human polo-like kinase 1 polo box domain in complex with peptide inhibitors

Structural analysis of human polo-like kinase 1 polo box domain in complex with peptide inhibitors

Decoding Polo-like kinase 1 signaling along the kinetochore-centromere axis.

Protein kinase signaling along the kinetochore-centromere axis is crucial to assure mitotic fidelity, yet its spatial coordination is obscure. Here, we examined how pools of human Polo-like kinase 1 (Plk1) within this axis control signaling events to elicit mitotic functions. To do this, we restricted active Plk1 to discrete subcompartments within the kinetochore-centromere axis using chemical genetics and decoded functional and phosphoproteomic signatures of each. We observe distinct phosphoproteomic and functional roles, suggesting that Plk1 exists and functions in discrete pools along this axis. Deep within the centromere, Plk1 operates to assure proper chromosome alignment and segregation. Thus, Plk1 at the kinetochore is a conglomerate of an observable bulk pool coupled with additional functional pools below the threshold of microscopic detection/resolution. Although complex, this multiplicity of locales provides an opportunity to decouple functional and phosphoproteomic signatures for a comprehensive understanding of Plk1’s kinetochore functions.

Structure-Bioactivity Relationship for Benzimidazole Thiophene Inhibitors of Polo-Like Kinase 1 (PLK1), a Potential Drug Target in Schistosoma mansoni.

BackgroundSchistosoma flatworm parasites cause schistosomiasis, a chronic and debilitating disease of poverty in developing countries. Praziquantel is employed for treatment and disease control. However, its efficacy spectrum is incomplete (less active or inactive against immature stages of the parasite) and there is a concern of drug resistance. Thus, there is a need to identify new drugs and drug targets.Methodology/Principal FindingsWe show that RNA interference (RNAi) of the Schistosoma mansoni ortholog of human polo-like kinase (huPLK)1 elicits a deleterious phenotypic alteration in post-infective larvae (schistosomula or somules). Phenotypic screening and analysis of schistosomula and adult S. mansoni with small molecule inhibitors of huPLK1 identified a number of potent anti-schistosomals. Among these was a GlaxoSmithKline (GSK) benzimidazole thiophene inhibitor that has completed Phase I clinical trials for treatment of solid tumor malignancies. We then obtained GSKs Published Kinase Inhibitor Sets (PKIS) 1 and 2, and phenotypically screened an expanded series of 38 benzimidazole thiophene PLK1 inhibitors. Computational analysis of controls and PLK1 inhibitor-treated populations of somules demonstrated a distinctive phenotype distribution. Using principal component analysis (PCA), the phenotypes exhibited by these populations were mapped, visualized and analyzed through projection to a low-dimensional space. The phenotype distribution was found to have a distinct shape and topology, which could be elicited using cluster analysis. A structure-activity relationship (SAR) was identified for the benzimidazole thiophenes that held for both somules and adult parasites. The most potent inhibitors produced marked phenotypic alterations at 1–2 μM within 1 h. Among these were compounds previously characterized as potent inhibitors of huPLK1 in cell assays.Conclusions/SignificanceThe reverse genetic and chemical SAR data support a continued investigation of SmPLK1 as a possible drug target and/or the prosecution of the benzimidazole thiophene chemotype as a source of novel anti-schistosomals.

Upregulated Polo-Like Kinase 1 Expression Correlates with Inferior Survival Outcomes in Rectal Cancer.

BackgroundHuman polo-like kinase 1 (PLK1) expression has been associated with inferior outcomes in colorectal cancer. Our aims were to analyse PLK1 in rectal cancer, and its association with clinicopathological variables, overall survival as well as tumour regression to neoadjuvant treatment.MethodsPLK1 expression was quantified with immunohistochemistry in the centre and periphery (invasive front) of rectal cancers, as well as in the involved regional lymph nodes from 286 patients. Scores were based on staining intensity and percentage of positive cells, multiplied to give weighted scores from 1–12, dichotomised into low (0–5) or high (6–12).ResultsPLK1 scores in the tumour periphery were significantly different to adjacent normal mucosa. Survival analysis revealed that low PLK1 score in the tumour periphery had a hazard ratio of death of 0.59 in multivariate analysis. Other predictors of survival included age, tumour depth, metastatic status, vascular and perineural invasion and adjuvant chemotherapy. There was no statistically significant correlation between PLK1 score and histological tumour regression in the neoadjuvant cohort.ConclusionLow PLK1 score was an independent predictor of superior overall survival, adjusting for multiple clinicopathological variables including treatment.

Vanillin-derived antiproliferative compounds influence Plk1 activity

We synthesized a series of vanillin-derived compounds and analyzed them in HeLa cells for their effects on the proliferation of cancer cells. The molecules are derivatives of the lead compound SBE13, a potent inhibitor of the inactive conformation of human polo-like kinase 1 (Plk1). Some of the new designs were able to inhibit cancer cell proliferation to a similar extent as the lead structure. Two of the compounds ((({4-[(6-chloropyridin-3-yl)methoxy]-3-methoxyphenyl}methyl)(pyridin-4-ylmethyl)amine) and (({4-[(4-chlorophenyl)methoxy]-3-methoxyphenyl}methyl)(pyridin-4-ylmethyl)amine)) were much stronger in their capacity to reduce HeLa cell proliferation and turned out to potently induce apoptosis and reduce Plk1 kinase activity in vitro.

Polo-like kinase 1 as a biomarker in rectal cancers.

e14542 Background: Human polo-like kinase 1 (PLK1) is important in mitotic progression. It is overexpressed in many cancers including colorectal cancer. Decreased PLK1 expression has been associate...

Phosphopeptides with improved cellular uptake properties as ligands for the polo-box domain of polo-like kinase 1

Human polo-like kinase 1 (Plk1) is involved in cell proliferation and overexpressed in a broad variety of different cancer types. Due to its crucial role in cancerogenesis Plk1 is a potential target for diagnostic and therapeutic applications. Peptidic ligands can specifically interact with the polo-box domain (PBD) of Plk1, a C-terminal located phosphoepitope binding motif. Recently, phosphopeptide MQSpTPL has been identified as ligand with high binding affinity. However, a radiolabeled version of this peptide showed only insufficient cellular uptake. The present study investigated peptide dimers consisting of PBD-targeting phosphopeptide MQSpTPL and a cell-penetrating peptide (CPP) moiety. The new constructs demonstrate superior uptake in different cancer cell-lines compared to the phosphopeptide alone. Furthermore, we could demonstrate binding of phosphopeptide-CPP dimers to PBD of Plk1 making the compounds interesting leads for the development of molecular probes for imaging Plk1 in cancer.

Fate of primary cells at the G1/S boundary after polo-like kinase 1 inhibition by SBE13

Human polo-like kinase 1 (Plk1), a key regulator of mitosis, is over-expressed in various human tumors. It is a negative prognostic factor for cancer patients and a measure for the aggressiveness of a tumor. Thus, targeting Plk1 might be a promising approach for cancer therapy. Plk1 inhibitors represent attractive tools for cancer research and for the mechanistic investigation of checkpoint control. Here, we show the impact of Plk1 inhibition on cell cycle regulation in primary cells. After treatment with SBE13 the G1/S checkpoint was intact, indicated by reduced pRb, resulting in slower cell cycle progression but overall cell proliferation was not significantly impaired. Thus, we provide strong evidence that SBE13 leaves checkpoint control in primary cells unaffected making it a remarkable future anti-cancer therapeutic.

Thymoquinone and Poloxin are slow-irreversible inhibitors to human Polo-like kinase 1 Polo-box domain

Objective To provide a kinetic model(s) and reveal the mechanism of thymoquinone and Poloxin blocking an emerging anti-cancer target, human Polo-like kinase 1 (hPlk1) Polo-box domain (PBD). Methods The binding kinetics was determined by using a fluorescence polarization based assay. The putative mechanism was examined with a competition test. Results Thymoquinone follows a one-step binding with an association rate constant ( k 1 ) of 6.635×10 3 L·mol −1·min −1. Poloxin fit a two-step binding with a dissociation constant (Ki) of 118 μmol/L for the intermediate complex and its isomerization rate ( k 4 ) of 0.131 5 min −1 to form an irreversible adduct. No significant dissociation was observed for either ligand up to 13 h. The inhibitors responded insignificantly to the presence of Michael donors as hPlk1-PBD competitors. Conclusion Thymoquinone and Poloxin are slow-tight ligands to the hPlk1-PBD with kinetic models distinct from each other. Michael addition as the mechanism is excluded.

SBE13, a newly identified inhibitor of inactive polo-like kinase 1

Protein kinases are important targets for drug development. The almost identical protein folding of kinases and the common co-substrate ATP leads to the problem of inhibitor selectivity. Type II inhibitors, targeting the inactive conformation of kinases, occupy a hydrophobic pocket with less conserved surrounding amino acids [1]. Human polo-like kinase 1 (Plk1) represents a promising target for approaches to identify new therapeutic agents. Plk1 belongs to a family of highly conserved serine/threonine kinases, and is a key player in mitosis, where it modulates the spindle checkpoint at metaphase/anaphase transition. Plk1 is over-expressed in all today analyzed human tumors of different origin and serves as a negative prognostic marker in cancer patients. The newly identified inhibitor, SBE13, a vanillin derivative, targets Plk1 in its inactive conformation [2]. This leads to selectivity within the Plk family and towards Aurora A. This selectivity can be explained by docking studies of SBE13 into the binding pocket of homology models of Plk1, Plk2 and Plk3 in their inactive conformation. SBE13 showed anti-proliferative effects in cancer cell lines of different origins with EC50 values between 5 μM and 39 μM and induced apoptosis. Increasing concentrations of SBE13 result in increasing amounts of cells in G2/M phase 13 hours after double thymidin block of HeLa cells. The kinase activity of Plk1 was inhibited with an IC50 of 200 pM. Taken together, we could show that carefully designed structure-based virtual screening is well-suited to identify selective type II kinase inhibitors targeting Plk1 as potential anti-cancer therapeutics.

Probing Binding Modes of Small Molecule Inhibitors to the Polo-Box Domain of Human Polo-like Kinase 1

Purpurogallin (PPG) and Poloxin have been reported as inhibitors of the polo-box domain (PBD) of human polo-like kinase 1 (Plk1). However, our experimental results demonstrated that PPG, but not Poloxin, binds to the phospho-binding pocket of the PBD, suggesting that their modes of PBD inhibition are distinct. Induced fit docking (IFD) analyses led us to propose that PPG fills the SpT pocket via pi-pi stacking and hydrogen bonding interactions, thus providing a rationale for designing novel PBD inhibitors. In contrast, Poloxin may fill a different site present near the SpT pocket by forming a covalent bond with a nucleophilic Cys residue.

Biological impact of freezing Plk1 in its inactive conformation in cancer cells

Human polo-like kinase 1 (Plk1), a key regulator of mitosis, is over-expressed in various human tumors. It is a negative prognostic factor for cancer patients and a measure for the aggressiveness of a tumor. Thus, targeting Plk1 might be a promising approach for cancer therapy. Kinase inhibitors are divided in type I inhibitors, targeting the highly conserved active conformation, and the more selective type II inhibitors, targeting the inactive conformation of kinases. We analyzed our previously identified type II Plk1 inhibitor SBE13 which is able to inhibit Plk1 activity. To determine its ability to induce cell death in cancer cells, we applied kinase assays, Western blot analyses, FACScan analyses, Caspase assays and immunofluorescence studies. We detected decreased cell proliferation, delayed progression through the cell cycle in lower SBE13 concentrations, a G2/M arrest using higher SBE13 concentrations followed by apoptosis, and abnormal mitotic figures. Notably, SBE13 did not influence activity of other kinases (Plk2, Plk3, Aurora A), indicating the selectivity of this type II Plk1 inhibitor. This study suggests that Plk1 kinase inhibitors targeting the inactive conformation of Plk1 may be considered for the development of cancer therapeutics.