Design, synthesis and evaluation of d-amino acid-containing peptidomimetics targeting the polo-box domain of polo-like kinase 1

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.

Novel Macrocyclic Peptidomimetics Targeting the Polo-Box Domain of Polo-Like Kinase 1

The polo-box domain (PBD) of Plk1 is a promising target for cancer therapeutics. We designed and synthesized novel phosphorylated macrocyclic peptidomimetics targeting PBD based on acyclic phosphopeptide PMQSpTPL. The inhibitory activities of 16e on Plk1-PBD is >30-fold higher than those of PMQSpTPL. Both 16a and 16e possess excellent selectivity for Plk1-PBD over Plk2/3-PBD. Analysis of the cocrystal structure of Plk1-PBD in complex with 16a reveals that the 3-(trifluoromethyl)benzoyl group in 16a interacts with Arg516 through a π-stacking interaction. This π-stacking interaction, which has not been reported previously, provides insight into the design of novel and potent Plk1-PBD inhibitors. Furthermore, 16h, a PEGlyated macrocyclic phosphopeptide derivative, induces Plk1 delocalization and mitotic failure in HeLa cells. Also, the number of phospho-H3-positive cells in a zebrafish embryo increases in proportion to the amount of 16a. Collectively, the novel macrocyclic peptidomimetics should serve as valuable templates for the design of potent and novel Plk1-PBD inhibitors.

QSAR-guided pharmacophoric modeling reveals important structural requirements for Polo kinase 1 (Plk1) inhibitors

Targeting Polo-like kinase 1 (Plk1) by molecular inhibitors is being a promising approach for tumor therapy. Nevertheless, insufficient methodical analyses have been done to characterize the interactions inside the Plk1 binding pocket. In this study, an extensive combined ligand and structure-based drug design workflow was conducted to data-mine the structural requirements for Plk1 inhibition. Consequently, the binding modes of 368 previously known Plk1 inhibitors were investigated by pharmacophore generation technique. The resulted pharmacophores were engaged in the context of Genetic function algorithm (GFA) and Multiple linear regression (MLR) analyses to search for a prognostic QSAR model. The most successful QSAR model was with statistical criteria of (r2277 = 0.76, r2adj = 0.76, r2pred = 0.75, Q2 = 0.73). Our QSAR-selected pharmacophores were validated by Receiver Operating Characteristic (ROC) curve analysis. Later on, the best QSAR model and its associated pharmacophoric hypotheses (HypoB-T4-5, HypoI-T2-7, HypoD-T4-3, and HypoC-T3-3) were used to identify new Plk1 inhibitory hits retrieved from the National Cancer Institute (NCI) database. The most potent hits exhibited experimental anti-Plk1 IC50 of 1.49, 3.79. 5.26 and 6.35 μM. Noticeably, our hits, were found to interact with the Plk1 kinase domain through some important amino acid residues namely, Cys67, Lys82, Cys133, Phe183, and Asp194.

Identification of novel peptidomimetics targeting the polo-box domain of polo-like kinase 1

A series of new peptidomimetics targeting the polo-box domain (PBD) of polo-like kinase 1 (Plk1) was identified based on the potent and selective pentapeptide Plk1 PBD inhibitor PLHSpT. Unnatural amino acid residues were introduced to the newly designed compound and the N-terminal substituent of the peptidomimetic was investigated. The optimized compound 9 inhibited the Plk1 PBD with IC50 of 0.267 μM and showed almost no inhibition to Plk2 PBD or Plk3 PBD at 100 μM. Biolayer interferometry studies demonstrated that compound 9 showed potent binding affinity to Plk1 with a Kd value of 0.164 μM, while no Kd were detected against Plk2 and Plk3. Compound 9 showed improved stability in rat plasma compared to PLHSpT. Binding mode analysis was performed and in agreement with the observed experimental results. There are only two natural amino acids remained in the chemical structure of 9. This study may provide new information for further research on Plk1 PBD inhibitors.