Concurrent MEK targeted therapy prevents MAPK pathway reactivation during BRAF V600E targeted inhibition in a novel syngeneic murine glioma model

// Stefan Grossauer 1, *, # , Katharina Koeck 1, *, # , Nicole E. Murphy 1 , Ian D. Meyers 1 , Mathieu Daynac 1 , Nathalene Truffaux 1 , Albert Y. Truong 1, 2 , Theodore P. Nicolaides 1, 2 , Martin McMahon 3 , Mitchel S. Berger 1 , Joanna J. Phillips 1 , C. David James 4 , Claudia K. Petritsch 1, 5, 6 1 Department of Neurological Surgery, Brain Tumor Research Center, San Francisco, CA 94158, USA 2 Department of Pediatrics, University of California San Francisco, San Francisco, CA 94158, USA 3 Huntsman Cancer Institute, University of Utah, Salt Lake City, UT 84112, USA 4 Department of Neurological Surgery, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA 5 Helen Diller Comprehensive Cancer Research Center, University of California San Francisco, San Francisco, CA 94158, USA 6 Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, University of California San Francisco, San Francisco, CA 94158, USA * These authors have contributed equally to this work # Co-first authorship Correspondence to: Claudia K. Petritsch, email: Claudia.petritsch@ucsf.edu Keywords: MAPK pathway reactivation, dabrafenib, primary adaptive therapy resistance, syngeneic high-grade astrocytoma model Received: May 30, 2016     Accepted: September 15, 2016     Published: October 03, 2016 ABSTRACT Inhibitors of BRAF V600E kinase are currently under investigations in preclinical and clinical studies involving BRAF V600E glioma. Studies demonstrated clinical response to such individualized therapy in the majority of patients whereas in some patients tumors continue to grow despite treatment. To study resistance mechanisms, which include feedback activation of mitogen-activated protein kinase (MAPK) signaling in melanoma, we developed a luciferase-modified cell line (2341 luc ) from a Braf V600E mutant and Cdkn2a- deficient murine high-grade glioma, and analyzed its molecular responses to BRAF V600E - and MAPK kinase (MEK)-targeted inhibition. Immunocompetent, syngeneic FVB/N mice with intracranial grafts of 2341 luc were tested for effects of BRAF V600E and MEK inhibitor treatments, with bioluminescence imaging up to 14-days after start of treatment and survival analysis as primary indicators of inhibitor activity. Intracranial injected tumor cells consistently generated high-grade glioma-like tumors in syngeneic mice. Intraperitoneal daily delivery of BRAF V600E inhibitor dabrafenib only transiently suppressed MAPK signaling, and rather increased Akt signaling and failed to extend survival for mice with intracranial 2341 luc tumor. MEK inhibitor trametinib delivered by oral gavage daily suppressed MAPK pathway more effectively and had a more durable anti-growth effect than dabrafenib as well as a significant survival benefit. Compared with either agent alone, combined BRAF V600E and MEK inhibitor treatment was more effective in reducing tumor growth and extending animal subject survival, as corresponding to sustained MAPK pathway inhibition. Results derived from the 2341 luc engraftment model application have clinical implications for the management of BRAF V600E glioma.