Abstract 5031: Effects of BRAF and MEK inhibitors, dabrafenib and trametinib, on the immune system and in combination with immunomodulatory antibodies targeting PD1, PD-L1 and CTLA-4
Authors: Liu Li , Mayes Patrick , Eastman Stephen , Shi Hong , Yadavilli Sapna , Pan Xiaoyu , Yang Jingsong , Seestaller-Wehr Laura , Zhang Shu-Yun , Hopson Chris , Tsvetkov Lyuben , Jing Junping , Smothers James , Pardoll Drew M. , Hoos Axel 
Published: May 2015

Abstract
The immunological effects of dabrafenib and trametinib and whether they potentiate or antagonize the activity of immunomodulatory antibodies are not well understood. We assessed the immunological effects of dabrafenib and trametinib at clinically relevant exposure concentrations on both immune and tumor cells in vitro and in vivo, and tested their anti-tumor efficacy in combination with immunomodulatory antibodies in immune-competent syngeneic mouse models. Human CD4+ and CD8+ T cells isolated from healthy volunteers were treated with trametinib and dabrafenib either alone or in combination, and with or without anti-CD3/anti-CD28 bead activation (concurrently or sequentially). Dabrafenib alone enhanced pERK expression levels with no changes of pAKT and pS6 proteins, and had no suppressive impact on human CD4+ or CD8+ T cell proliferation, apoptosis and cytokine production in response to T cell activation. Trametinib alone reduced the pERK levels with no changes in pAKT and apoptosis. However trametinib resulted in partial inhibitory effects on T cell proliferation, pS6 proteins and cytokine expression. These inhibitory effects were transient and only observed if cells were treated with trametinib prior to or simultaneously with T cell activation, while trametinib had little or no suppressive effects on activated T cells. Adding dabrafenib partially offset the transient inhibitory effects caused by trametinib alone. Similarly, gene expression profiling showed that trametinib partially decreased the expression levels of a subset of cytokines and chemokines (e.g. IL1, IL2, IL8, IL10, TNFa, CCL2) and activation/regulation markers (e.g. CD69, CD25, PD1, CTLA4) when trametinib was added prior to or simultaneously with T cell activators. Multi-color flow cytometry confirmed cell surface changes in the expression of CD69, CD25, PD1, OX40 and CTLA4. However, the expression levels of CD69 and OX40 were still well above non-activated T cells. On tumor cells, dabrafenib and trametinib up-regulated HLA molecules and melanoma antigen MART1 expression, and down regulated immune-suppressive factors such as PD-L1, VEGF and IL8 etc in BRAFV600E melanoma cells. Combinations of trametinib with immunomodulators targeting PD1, PD-L1 or CTLA4 in murine syngeneic tumor models are underway and will be presented at the meeting. These findings to date support clinical exploration of dabrafenib and/or trametinib in combination with specific immunomodulatory antibodies.
Citation Format: Li Liu, Patrick Mayes, Stephen Eastman, Hong Shi, Sapna Yadavilli, Xiaoyu Pan, Jingsong Yang, Laura Seestaller-Wehr, Shu-Yun Zhang, Chris Hopson, Lyuben Tsvetkov, Junping Jing, James Smothers, Drew M. Pardoll, Axel Hoos. Effects of BRAF and MEK inhibitors, dabrafenib and trametinib, on the immune system and in combination with immunomodulatory antibodies targeting PD1, PD-L1 and CTLA-4. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5031. doi:10.1158/1538-7445.AM2014-5031