LAUSR

Abstract Background A subgroup of MEK1 mutations in human tumors were recently described as RAF- and phosphorylation independent and as insensitive to currently clinically used allosteric MEK inhibitors. We established a novel cell line that carries a MEK1 mutation of this class (pGlu102_Lys104delinsGln) and tested a novel ATP competitive MEK inhibitor (MAP855) in this model. According to our knowledge this is the first patient derived cell line with this type of MEK mutation and the first spitzoid melanoma cell model. Methods PF130 cell line was established from the pleural effusion sample of a 37-year-old male patient with spitzoid melanoma. NGS was used for mutational analysis. Cells were treated either with allosteric MEK inhibitor selumetinib or with newly developed ATP competitive MEK inhibitor MAP855. We analyzed changes in cell number, cell cycle distribution and ERK activation after treatment in vitro. Furthermore, the in vivo tumorigenicity was tested by injecting the cells subcutaneously, intrapleurally or intravenously into female SCID mice. Results We found that ATP competitive MEK inhibitor MAP855 strongly decreased the viability of PF130 cells, while allosteric inhibitor selumetinib did not. Cell cycle analysis showed that MAP588 treatment could induce cell death in PF130 cells in a concentration dependent manner and slightly reduced the amount of cells in the S and the G2M phases. Furthermore, MAP855 was able to reduce ERK protein activation in PF130 cells effectively while selumetinib treatment had no effect. In vivo, intrapleurally injected cells established macroscopic tumors in the chest cavity after two months while intravenously and subcutaneously delivered cells showed limited growth. Conclusions We established a new cellular model of tumors with RAF- and phosphorylation independent MEK mutation. We found that recently developed ATP-competitive MEK inhibitor MAP855 is a potent inhibitor of these cells in vitro. The in vivo efficacy is currently under investigation. Furthermore, we describe here to the best of our knowledge the first patient derived spitzoid melanoma cell model to test both in vitro and in vivo novel therapeutic modalities. Legal entity responsible for the study Balazs Hegedus. Funding Has not received any funding. Disclosure All authors have declared no conflicts of interest.