Abstract 1241: Converting high affinity peptides to non-peptidic inhibitors via REPLACE: Novel and new strategies in targeting PLK1 Polo Box Domain (PBD) for anti-cancer therapy
Authors: Chapagai Danda , Ramamoorthy Guru , Baxter Merissa , Craig Sandra , Nurmemmedov Elmar , Campbell McInnes , Wyatt Michael 
Published: July 2021

Abstract
Polo Like Kinase 1 (PLK1) is only expressed in dividing cells and plays a critical role in several stages of mitosis. PLK1 is highly expressed in tumors of various origins while its expression is largely absent in normal tissues. PLK1 inhibition selectively kills cancer cells because they are dependent on the mitotic functions of PLK1 overexpression. PLK1 consists of a highly conserved N-terminal catalytic kinase domain and a unique, functionally essential C-terminal Polo Box Domain (PBD). The PBD of each PLK is a phospho-peptide binding motif that determines substrate recognition and sub-cellular localization. PLK1 catalytic inhibitors have advanced to clinical trials but they inhibit other family members PLK2 and PLK3. An alternative approach to developing potent and selective PLK1 inhibitors is to allosterically target the PBD. An iterative strategy called REPLACE involving computational and synthetic approaches was utilized to generate fragment-ligated inhibitory peptides (FLIPs) by rationally replacing amino acid residues in two PLK1 substrate peptides with drug-like small molecule fragments to generate FLIPs. Further application of REPLACE resulted in non-peptidic compounds named abbapolins. Confirmation of engagement of cellular PLK1 by abbapolins was assessed using a cellular thermal shift assay (CETSA), which determines drug binding to target proteins by measuring the shift in a protein's solubility and hence its thermal stability. Abbapolins specifically bound to full-length cellular PLK1 over PLK3 as determined by CETSA in a pattern consistent with in vitro binding affinities to the purified PBD of PLK1 and PLK3. We made an additional unique observation that mitotic PLK1 had a lower thermal stability than non-mitotic PLK1, suggesting that mitotic PLK1 might adopt a different conformation that is less soluble in the CETSA assay. Moreover, we observed that ATP-based inhibitors decreased the thermal stability of mitotic PLK1, whereas PBD inhibitors abbapolins increased the thermal stability of mitotic PLK1 as measured by CETSA. Additional evidence for engagement of cellular PLK1 was obtained through the unique mechanistic observation that abbapolins induced a striking dose dependent reduction in PLK1 protein levels as measured by western blotting and immunofluorescence. The loss of PLK1 was partially due to proteasome mediated degradation because the proteasome inhibitor MG132 partially prevented PLK1 degradation. Moreover, abbapolins caused preferential loss of nuclear PLK1 as determined by cellular fractionation. The therapeutic potential of these compounds was indicated through their antiproliferative activity on a cell line expressing a PLK1 point mutant that is dramatically resistant to ATP competitive PLK1 inhibitors. Future studies will examine in vivo activity of selected abbapolins.
Citation Format: Danda Chapagai, Guru Ramamoorthy, Merissa Baxter, Sandra Craig, Elmar Nurmemmedov, McInnes Campbell, Michael Wyatt. Converting high affinity peptides to non-peptidic inhibitors via REPLACE: Novel and new strategies in targeting PLK1 Polo Box Domain (PBD) for anti-cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1241.