Abstract B154: Characterization of a novel ERK1/2 inhibitor, which modulates the phosphorylation and catalytic activity of ERK1/2

The MAPK pathway is commonly hyper-activated in human cancers due to the occurrence of oncogenic mutations in RAF, RAS and the upregulation of RTKs. The therapeutic potential of MAPK pathway inhibition has been demonstrated by the clinical efficacy of RAF and MEK1/2 (MEK) inhibitors in the treatment of BRAF-mutant melanoma. However, response to such agents is short-lived due to the onset of resistance mechanisms, which in the majority of cases result in the reactivation of ERK1/2 (ERK) signalling. Therefore, the direct targeting of ERK is an attractive therapeutic approach to overcoming the limitations of RAF or MEK inhibitors. Here, we describe a novel, potent, and selective ERK inhibitor, which inhibits both ERK catalytic activity and also the phosphorylation of ERK by MEK. Using fragment-based drug discovery we have developed a selective ERK inhibitor, which inhibits in vitro ERK catalytic activity with a low nM IC 50 value. This lead compound has strong antiproliferative effects in a wide range of MAPK-activated cell lines, including the BRAF-mutant cell lines A375 (melanoma) and Colo205 (colorectal), the KRAS-mutant cell lines HCT116 (colorectal), Calu6 (lung) and Panc05.04 (pancreatic), and the NRAS-mutant cell line Ma-mel-27 (melanoma). The lead compound potently inhibits ERK cell signalling. The potent (nM) inhibition of RSK phosphorylation (a direct ERK substrate) was confirmed in A375 (BRAF-mutant melanoma) cells, using MSD analysis. In addition to inhibiting downstream ERK signalling, we demonstrated by ELISA and Western blotting that the lead compound confers a decrease in phospho-ERK levels in both BRAF-mutant and KRAS-mutant cell lines. We investigated the biochemical mechanism of the modulation of ERK phosphorylation in vitro and demonstrated that the compound prevents the phosphorylation of ERK by MEK (at key ERK activation loop residues, T202/Y204), without directly inhibiting MEK activity. The compound was profiled in a range of subcutaneous xenograft models including A375 (BRAF-mutant melanoma) and Calu-6 (KRAS-mutant lung). Once-daily oral dosing of the lead compound conferred significant antitumor activity in a range of in vivo efficacy studies. The compound potently inhibited the phosphorylation of downstream ERK substrates (including RSK) in tumor xenograft tissue. There was a clear relationship between in vivo compound concentrations and the modulation of ERK substrate phosphorylation. Furthermore, as was demonstrated in vitro, we confirmed that in addition to inhibiting ERK catalytic activity the compound potently inhibited the phosphorylation of ERK itself, in both KRAS and BRAF-mutant tumor xenografts. Here, we characterize a novel, highly potent, selective ERK inhibitor, which inhibits both ERK catalytic activity and also the upstream phosphorylation of ERK by MEK. These data support the further optimization of this series of compounds for clinical development. Citation Format: Joanne M. Munck, Valerio Berdini, Luke D. Bevan, Hannah Braithwaite, Ildiko M. Buck, Megan Cassidy, Juan Castro, Aurelie Courtin, James E. Day, Charlotte East, Lynsey Fazal, Brent Graham, Charlotte M. Griffiths-Jones, Tom D. Heightman, Chris J. Hindley, Birikiti Kidane, Justyna Kucia-Tran, John F. Lyons, Vanessa Martins, Sandra Muench, Chris W. Murray, David Norton, Marc O9Reilly, Nick Palmer, Puja Pathuri, Mike Reader, David C. Rees, Sharna J. Rich, Caroline J. Richardson, Harpreet K. Saini, Alpesh Shah, Lukas Stanczuk, Neil T. Thompson, Hugh Walton, Nicola E. Wilsher, Alison J. Woolford, Nicola G. Wallis. Characterization of a novel ERK1/2 inhibitor, which modulates the phosphorylation and catalytic activity of ERK1/2 [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B154.