Abstract B164: ASTX029 is a dual mechanism ERK1/2 inhibitor with activity in models of MAPK-inhibitor resistance
Authors: Zhang Zhiqiang , Hindley Christopher , Biondo Andrea , Wallis Nicola , Lyons John , Munck Joanne 
Published: December 2023

Clinical efficacy of RAF, MEK1/2 (MEK) and KRASG12C inhibitors has been demonstrated in MAPK-activated cancers. However, response is often short-lived due to resistance mechanisms, which commonly confer reactivation of ERK1/2 (ERK). As the primary downstream effector of the MAPK pathway, ERK is an attractive therapeutic target for overcoming resistance to upstream inhibition. ASTX029 is a potent and selective dual-mechanism ERK inhibitor. Due to its distinctive ERK-binding mode, ASTX029 inhibits both ERK catalytic activity and the phosphorylation of ERK by MEK. It is currently in Phase 1/2 trial in patients with advanced solid tumors (NCT03520075). Here we demonstrate the potent activity of ASTX029 in models of acquired MAPK inhibitor resistance. Two MAPK inhibitor-resistant models were generated by continuous culture of A375 (BRAFV600E -mutant melanoma) cells in the presence of the BRAF inhibitor vemurafenib (A375R) or by introduction of an NRASQ61K mutation (A375-NRASQ61K). The A375-NRASQ61K and A375R cell lines were resistant to vemurafenib in cell proliferation assays (IC50 &gt;3 µM and &gt;10 µM respectively, compared to IC50 110 nM in parental A375 cells). They were also less sensitive to the MEK inhibitor selumetinib (IC50 270 nM and 1000 nM, respectively, compared to IC50 43 nM in parental A375). Both models were highly sensitive to ASTX029, with proliferation assay IC50 values of 12 nM (A375-NRASQ61K) and 7.2 nM (A375R), values which were not significantly different to the ASTX029 IC50 of parental A375. ASTX029 inhibited MAPK signalling in both models, whereas vemurafenib and selumetinib had no effect. Treatment of A375R tumour-bearing mice with 50 mg/kg bid vemurafenib (which causes significant tumor growth inhibition in parental A375 xenografts), did not result in significant tumor growth inhibition. In contrast, 75 mg/kg qd ASTX029 conferred significant anti-tumor activity (P&lt; 0.001). The acquisition of MEK mutations is a known resistance mechanism to RAF, MEK and KRASG12C inhibitors. We performed a CRISPR-tiling screen on MEK1 and identified mutations that conferred resistance to MEK inhibitors. The identified MEK1 mutations either caused MEK activation (indicated by elevated pERK levels) or were predicted to prevent MEK inhibitor binding (supported by MEK1 structural analysis). All mutations conferred resistance to selumetinib (7- to 200-fold increase in cell proliferation IC50 values relative to parental cells).  Mutations predicted to prevent MEK inhibitor binding were sensitive to vemurafenib and the pan-RAF inhibitor LY3009120, whereas those that caused MEK1 activation were resistant (5- to 10-fold increase in cell proliferation IC50 relative to parental cells). However, they were all sensitive to ASTX029 (IC50 values of 4 to 17 nM). We conclude that the MAPK inhibitor resistance mechanisms described did not confer resistance to ASTX029. These data highlight the therapeutic potential of ASTX029 for the treatment of cancers, which have acquired resistance to inhibitors of upstream components of the MAPK pathway.

Citation Format: Zhiqiang Zhang, Christopher Hindley, Andrea Biondo, Nicola Wallis, John Lyons, Joanne Munck. ASTX029 is a dual mechanism ERK1/2 inhibitor with activity in models of MAPK-inhibitor resistance [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B164.