Polo-like kinase 1 (PLK1) has a plethora of roles within the cell of which mitotic regulation is the best characterized. This kinase is overexpressed and upregulated in cancer cells, making it an attractive target for therapeutics. Drug discovery efforts have mainly focused on the ATP binding site however have met with little clinical success to date. Targeting substrate recognition and subcellular localization through the Polo-Box domain represents an attractive alternative to generating PLK1 specific compounds and to avoid the tumor suppressor functions of PLK3. REplacement with Partial Ligand Alternatives through Computational Enrichment, REPLACE, is validated strategy used to convert peptide inhibitors of protein-protein interactions into more drug like compounds. Through the use of this methodology, we have discovered promising non-ATP competitive inhibitors based on a 2-(4-AlkylBenzamido) Benzoic Acid (ABBA) pharmacophore which bind to the polo-box domain of PLK1. These compounds, designated as abbapolins, engage PLK1 as evidenced by their ability to block phosphorylation of TCTP, a key substrate of PLK1, induce PLK1 degradation and are potently anti-proliferative in prostate cancer cell lines. Based on these observations, lead compounds were investigated further for their pharmacokinetic properties and in vivo efficacy and key compounds showed development potential based upon preliminary results. To further investigate their potential in other tumors, selected abbapolins were tested against the NCI-60 tumor cell panel and other cancers with poor prognosis. Results indicate that neuroblastoma, lung, colorectal and certain leukemias are the most sensitive cell lines and thus provide impetus to investigate the abbapolins for difficult to treat and resistant tumors. We present the further optimization of the abbapolins to determine their structure-activity relationship and identify compounds with novel structures and improved drug-like properties. This includes their ability to engage PLK1 and anti-proliferative activities against selected cell lines that represent therapeutically challenging cancers. Furthermore, these compounds serve as chemical biology probes to elucidate roles of the PBD in conformationally regulating PLK1 activity, through in vitro experiments and specifically in the abbapolin sensitive cell lines. Citation Format: Jessy Stafford, Danda Chapagai, George Merhej, Justin Pressley, Chintada Nageswara-Rao, Michael Wyatt, Campbell Mcinnes. Development of abbapolin inhibitors of the PLK1 PBD as potential therapeutics for prostate and other challenging cancers. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4993.
Polo Like Kinase 1 (PLK1) is only expressed in dividing cells and plays a critical role in several stages of mitosis. PLK1 is overexpressed in many tumor types. PLK1 inhibition selectively kills cancer cells because they are dependent on the mitotic functions of PLK1. PLK1 consists of a highly conserved N-terminal catalytic kinase domain and a unique, functionally essential C-terminal Polo Box Domain (PBD). The PBD is a phospho-peptide binding motif that determines substrate recognition and sub-cellular localization. PLK1 catalytic inhibitors have advanced to clinical trials but not demonstrated convincing efficacy. Targeting the PBD offers an attractive alternative to pursue PLK1 inhibition. An iterative strategy called REPLACE, involving computational and synthetic approaches, was utilized to generate fragment-ligated inhibitory peptides and further application of REPLACE resulted in non-peptidic compounds named abbapolins. In recently published studies, abbapolins were found to specifically bind to the PBD of PLK1 in biochemical and cellular assays. The phosphorylation of TCTP, a specific PLK1 substrate, was measured in abbapolin treated cancer cells. Abbapolins produced a dose dependent reduction in p-TCTP. We also made a novel observation that abbapolins upon binding to PLK1 induced its intracellular loss in a mechanism at least partially dependent on the proteasome. Spurred on by this unique mechanism of action, we initiated studies on the interactions of catalytic inhibitors and PBD-binding abbapolins with PLK1 in vitro and in cellular contexts. Biochemical assays were performed with full length PLK1 in vitro and intracellular PLK1 binding was measured by a Cellular Thermal Shift Assay (CETSA). We report novel findings during mitosis inferred from our collective data, namely that catalytic binding by BI2536 or volasertib unexpectedly decreased soluble PLK1 as determined by CETSA, inferring induction of a conformational change in intracellular PLK1. In contrast, abbapolins produced the expected right shift in the melting curve of PLK1. Intriguingly, these differential effects on PLK1 thermal stability have opposing impacts on the fate of intracellular PLK1. Binding by catalytic inhibitors cause accumulation of PLK1, whereas PBD binding by abbapolins ultimately lead to its loss. Results from quantifying intracellular PLK1 were also supported in vitro by evidence of cooperative binding between catalytic inhibitors and abbapolins in the context of the FL protein and suggest that conformational changes induced by binding to the catalytic site increase affinity of abbapolins for the PDB. Collectively, the results shed further insight into the unique mechanism of action for abbapolins potentially due to their engagement of a cryptic hydrophobic pocket of the PBD. Abbapolins are thus a compelling alternative to catalytic-based inhibitors for the development of novel therapeutics targeting PLK1. Citation Format: Danda Chapagai, Merissa Baxter, Sandra Craig, Guru Ramamoorthy, Jessy Stafford, Sikirzhytski Vitali, Elmar Nurmemmedov, Campbell McInnes, Michael D. Wyatt. A novel strategy for PLK1 kinase inhibition by allosterically targeting the Polo Box Domain [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2572.
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