Antitumoral Activity of the MEK Inhibitor Trametinib (TMT212) Alone and in Combination with the CDK4/6 Inhibitor Ribociclib (LEE011) in Neuroendocrine Tumor Cells In Vitro

SIMPLE SUMMARY: Systemic treatment options for advanced neuroendocrine tumors have significantly been improved in the last decade. However efficacy of systemic therapy is limited by tumor resistance and therefore there is a need for further treatment options. Inhibition of the Ras-Raf-Mek-Erk signaling cascade might be a promising new treatment strategy in neuroendocrine neoplasms. In this study we investigated the effects of the MEK inhibitor trametinib, the ERK inhibitor SCH772984 and the CDK4/6 inhibitor ribociclib in human neuroendocrine tumor cell lines BON1, QGP1 and NCI-H727 in vitro. Trametinib alone and in synergism with ribociclib demonstrated antiproliferative effects. Combination therapy of MEK inhibitors and CDK4/6 inhibitors might be a potential strategy to overcome CDK4/6 inhibitor resistance in neuroendocrine tumors. ABSTRACT: Objectives: This study assessed the antitumoral activity of the MEK inhibitor trametinib (TMT212) and the ERK1/2 inhibitor SCH772984, alone and in combination with the CDK4/6 inhibitor ribociclib (LEE011) in human neuroendocrine tumor (NET) cell lines in vitro. Methods: Human NET cell lines BON1, QGP-1, and NCI-H727 were treated with trametinib or SCH772984, alone and in combination with ribociclib, to assess cell proliferation, cell cycle distribution, and protein signaling using cell proliferation, flow cytometry, and Western blot assays, respectively. Results: Trametinib and SCH772984, alone and in combination with ribociclib, significantly reduced NET cell viability and arrested NET cells at the G1 phase of the cell cycle in all three cell lines tested. In addition, trametinib also caused subG1 events and apoptotic PARP cleavage in QGP1 and NCI-H727 cells. A western blot analysis demonstrated the use of trametinib alone and trametinib in combination with ribociclib to decrease the expression of pERK, cMyc, Chk1, pChk2, pCDK1, CyclinD1, and c-myc in a time-dependent manner in NCI-H727 and QGP-1 cells. Conclusions: MEK and ERK inhibition causes antiproliferative effects in human NET cell lines in vitro. The combination of the MEK inhibitor trametinib (TMT212) with the CDK4/6 inhibitor ribociclib (LEE011) causes additive antiproliferative effects. Future preclinical and clinical studies of MEK inhibition in NETs should be performed.