Abstract 1882: MERTK drives residual tumor growth in EGFR-mutated non-small cell lung cancer cells treated with osimertinib

Abstract

Osimertinib (OSI) was recently FDA-approved as a front-line agent for newly diagnosed EGFRMT non-small cell lung cancer (NSCLC). However, unmet clinical needs have arisen in conjunction with OSI use, including understanding mechanisms of OSI resistance and developing novel approaches to prevent or reverse resistance and/or enhance OSI efficacy in responsive patients. To address these issues, osimertinib-resistant (osiR) derivatives of five EGFRMT NSCLC cell lines were generated and roles for MERTK, a receptor tyrosine kinase that has been implicated as a potential therapeutic target in NSCLC, were characterized. PI3K-AKT and MAPK-ERK signaling pathways were activated in osiR cells, even when EGFR was not active. Treatment with the MERTK ligands GAS6 or PROS1 stimulated AKT, ERK, and ribosomal S6 phosphorylation in parental cells treated with OSI and in osiR cells, implicating MERTK as a mediator of resistance to OSI. Downstream signaling was responsive to both EGF and GAS6 stimulation in parental cells but was only activated by GAS6 in osiR cells. OSI blocked EGF-dependent signaling through AKT, ERK and S6 in parental cells in the absence of GAS6, but combined treatment with OSI and MRX-2843, a novel MERTK inhibitor currently in Phase I clinical trials, was required to block signaling in the presence of GAS6. However, treatment with MRX-2843 alone had little impact on downstream signaling in the presence of activated EGFR. Thus, MERTK is not the dominant driver of downstream signaling in parental cells. In contrast, treatment with MRX-2843 alone was sufficient to inhibit downstream signaling in osiR cells and osiR cells were also more sensitive to treatment with MRX-2843 in clonogenic assays. Thus, osiR cells have increased dependence on MERTK kinase activity relative to parental cells. Interestingly, EGFR and MERTK co-precipitated from parental cell lysates and GAS6 stimulation enhanced this interaction. In contrast, MERTK and EGFR interaction was not detected in osiR cells, suggesting a more complex interplay between these two receptors. MERTK and the ligand PROS1 were dramatically upregulated in EGFRMT tumors treated with OSI in vivo, consistent with a role for autocrine MERTK activation in osiR tumor growth. Indeed, treatment with OSI alone or in combination with MRX-2843 was sufficient to block tumor growth in vivo, but when treatment was stopped, tumors treated with OSI alone started to grow, while treatment with the combination resulted in durable suppression of tumor growth. Together these data implicate MERTK as a mediator of resistance to OSI and suggest that combining MRX-2843 and OSI therapy will control tumor growth. Citation Format: Dan Yan, Justus Huelse, Rebecca Parker, Zikang Tan, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Deborah DeRyckere, Douglas K. Graham. MERTK drives residual tumor growth in EGFR-mutated non-small cell lung cancer cells treated with osimertinib [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1882.

Targeting MERTK and AXL in EGFR Mutant Non-Small Cell Lung Cancer

“…High levels of MERTK and/or AXL have also been implicated in drug resistance and radioresistance [ 54 , 55 , 60 , 69 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 ]. Increased MERTK or AXL expression in NSCLC correlated with chemotherapy resistance [ 51 , 66 , 70 , 87 , 88 , 89 ].…”

“…AXL dimerizes with and phosphorylates EGFR to promote activation of the PLCγ-PKC-mTOR signaling cascade and tumor cell survival [ 76 ]. Similarly, there is crosstalk between MERTK and EGFR and they are frequently co-expressed on both mt EGFR - and wt EGFR -expressing NSCLC cell lines [ 69 , 80 ]. In fact, MERTK stabilized the EGFR protein on the cell surface, probably by preventing EGFR internalization and degradation, as EGF-dependent EGFR turnover was reversed by inhibition of lysosomal hydrolase activity [ 140 ].…”

“…Other mechanisms that drive secondary resistance to EGFR TKIs are summarized in Table 1 . MERTK and AXL are co-expressed with EGFR in NSCLC [ 58 , 69 , 80 ], and both receptors, but especially AXL, have been studied as causative drivers of secondary EGFR TKI resistance in mt EGFR -expressing NSCLCs [ 53 , 70 , 112 , 196 ]. Forced expression of an active form of AXL, but not a kinase-impaired AXL, in erlotinib-sensitive tumor cells was sufficient to induce erlotinib resistance [ 53 ].…”

“…A more detailed mechanistic understanding is needed to facilitate development of effective approaches to overcome resistance [ 241 ]. Upregulation of MERTK and AXL has been observed in osimertinib-resistant mt EGFR -expressing NSCLC cell lines, implicating these kinases as mediators of resistance [ 54 , 80 ]. In addition, both MERTK and PROS1 were upregulated in osimertinib-resistant mt EGFR NSCLC cell line derivatives and in mt EGFR NSCLC xenograft tumors treated with osimertinib.…”

“…In addition, both MERTK and PROS1 were upregulated in osimertinib-resistant mt EGFR NSCLC cell line derivatives and in mt EGFR NSCLC xenograft tumors treated with osimertinib. Moreover, treatment with MERTK kinase inhibitor MRX-2843 [ 67 , 242 ] re-sensitized resistant cells to osimertinib in cell-based assays and provided durable tumor regression in a mt EGFR NSCLC xenograft model, even after treatment ended [ 80 ]. NSCLCs expressing mt EGFR and high levels of AXL were more tolerant to osimertinib treatment than tumors expressing low levels of AXL [ 54 , 241 ].…”