Abstract 4574: Polo box-targeted PLK1 inhibitors: structure activity relationships and activity in cells resistant to ATP-based inhibitors

Polo-like Kinase 1 (PLK1) performs critical roles in the coordination of mitosis. PLK1 is also an oncogene over-expressed in many cancer types and is being targeted therapeutically. ATP-binding site inhibitors of PLK1 have progressed to clinical trials, and one was recently given FDA breakthrough status to treat acute myeloid leukemia. However, recent data suggests that there are major disadvantages of the conventional approach to blocking the kinase activity of PLK1. First, both general kinome and PLK family specificity is an issue with ATP competitive compounds because they commonly inhibit all paralogs in the PLK family, including PLK3, a known tumor suppressor. Second, a single point mutant in the active site of PLK1 (Cys67Val) results in dramatic resistance to structurally distinct ATP competitive inhibitors in clinical trials (Burkard, Santamaria, and Jallepalli, ACS Chem. Biol. 2012, 7, 978−981), suggesting that the emergence of resistance in the clinic against these agents is a near certainty. We have validated the innovative REPLACE strategy by discovering non-ATP competitive and PLK1 selective inhibitors as a potential therapeutic strategy. The Polo-box domain (PBD) of each PLK is a phospho-peptide binding motif that determines substrate recognition and subcellular localization. Our approach is to target the PBD of PLK1 to achieve desired PLK1 selectivity and improve efficacy. Our first generation of fragment-ligated inhibitory peptides (FLIPs) (Mol. Cancer Ther., 2012;11:1683-1692) included a compound that contained a butyl-benzamide group and possessed a binding affinity to the PLK1 PBD of 8.6 μM, and insignificant levels of PLK3 binding (IC50 > 600 μM). Subsequent modeling and design produced a FLIP with an octyl-benzamide group, which possessed IC50 of 0.3 μM for the PLK1 PBD, and an IC50 of 93.5 μM for PLK3, a >300-fold selectivity (unpublished results). We also report the investigation of our PBD inhibitors in cells expressing the mutant C67V PLK1 that is resistant to ATP-based inhibitors. Indeed, we find that our compounds are equally active in cells expressing wild-type or C67V PLK1, whereas cells expressing C67V PLK1 are dramatically resistant to a model ATP-based inhibitor, BI 2536. We also further explore the structure-activity relationship and structural basis of PLK1 versus PLK3 selectivity with selected modifications to PBD binding peptides and FLIPs. In summary, these exciting developments demonstrate the validity of our approach to produce drug-like lead PBD inhibitors that are PLK1 selective and will be active against tumors resistant to ATP inhibitors. Citation Format: Merissa Baxer, Sandra Craig, Campbell McInnes, Michael Wyatt. Polo box-targeted PLK1 inhibitors: structure activity relationships and activity in cells resistant to ATP-based inhibitors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4574. doi:10.1158/1538-7445.AM2014-4574