Abstract 955: Combined inhibition of MEK and mTOR pathways is effective in NRAS Q61K mutant small cell lung cancer
Authors: Ogino Atsuko , Lin Mika , Gokhale Prafulla C. , Wilkens Margaret K. , Choi Jihyun , Calles Antonio , Xu Man , Capelletti Marzia , Oxnard Geoffrey , Gray Nathanael S. , Kirschmeier Paul , Jänne Pasi A. 
Published: August 2018

Abstract
Background: Small cell lung cancer (SCLC) is a heterogeneous disease, consisting of a phenotypically diverse population of cells, often described as neuroendocrine (NE) and non-NE. Though previous studies suggest that aberrant RAS signaling is involved in the transition from NE to non-NE profile, the characteristics of SCLC with RAS mutations have not been fully examined due to the rarity of the occurrence and the lack of resources available to study its biology.
Methods: We created a cell line (DFCI168) from the pleural effusion of the SCLC patient with a rare NRASQ61K mutation. A patient-derived xenograft (PDX) model from this patient was successfully established by subcutaneous implantation of cancer cells in NSG mice. The efficacy of MEK and mTOR inhibitors was evaluated in DFCI168 and the commercially available lung cancer cell lines with NRAS Q61 mutations (SW1271, H1299, H2087, H2347). The Chou-Talalay combination index was used to evaluate drug synergy. Retroviral NRASQ61K transduction was performed on a classical SCLC cell line Glc16 to examine whether NRAS activation leads to acquisition of non-NE features.
Results: DFCI168 showed non-NE/mesenchymal features both morphologically and biologically, which is exemplified by an elongated spindle shape, mesenchymal marker (VIM) expression and silenced NE markers (SYP, ASCL1) expression. NRASQ61K transduction on Glc16 achieved the transition from NE to non-NE/mesenchymal phenotype. Although DFCI168 showed marked sensitivity to MEK inhibitors both in vitro (trametinib IC50=2nmol/L, selumetinib IC50=50nmol/L) and in vivo, the tumors regrew after withdrawal of treatment with trametinib in xenograft mice, and caused sustained activation of the mTOR pathway during MEK inhibitor treatment in vitro. The combination of MEK and mTOR inhibitors (trametinib and TORIN2) prevented mTOR activation at 100nmol/L and yielded synergistic effects (CI=0.61). The commercially available lung cancer cell lines (SW1271, H1299, H2087, H2347) also responded synergistically to the trametinib/TORIN2 combination except H1299, a cell line with a known PTEN promoter methylation.
Conclusion: We demonstrate that the oncogenic NRAS mutation defines a distinct molecular subset of SCLC, and is sufficient to cause the phenotypic switch from NE to non-NE/mesenchymal profile. Such tumors are likely to benefit from the dual targeted therapy with MEK and mTOR inhibitors. Our findings highlight the future potential of targeted therapies for SCLC patients with NRAS activating mutations.
Citation Format: Atsuko Ogino, Mika Lin, Prafulla C. Gokhale, Margaret K. Wilkens, Jihyun Choi, Antonio Calles, Man Xu, Marzia Capelletti, Geoffrey Oxnard, Nathanael S. Gray, Paul Kirschmeier, Pasi A. Jänne. Combined inhibition of MEK and mTOR pathways is effective in NRAS Q61K mutant small cell lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 955.