Concurrent MEK targeted therapy prevents MAPK pathway reactivation during BRAFV600E targeted inhibition in a novel syngeneic murine glioma model.

Stefan Grossauer,Joanna J Phillips,Mitchel S Berger,Claudia K Petritsch,Nicole E Murphy,Theodore P Nicolaides,Albert Y Truong,Nathalene Truffaux,Mathieu Daynac,C David James,Ian D Meyers,Martin Mcmahon,Katharina Koeck

doi:10.18632/oncotarget.12419

Abstract

Inhibitors of BRAFV600E kinase are currently under investigations in preclinical and clinical studies involving BRAFV600E 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 (2341luc) from a BrafV600E mutant and Cdkn2a- deficient murine high-grade glioma, and analyzed its molecular responses to BRAFV600E- and MAPK kinase (MEK)-targeted inhibition. Immunocompetent, syngeneic FVB/N mice with intracranial grafts of 2341luc were tested for effects of BRAFV600E 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 BRAFV600E inhibitor dabrafenib only transiently suppressed MAPK signaling, and rather increased Akt signaling and failed to extend survival for mice with intracranial 2341luc 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 BRAFV600E 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 2341luc engraftment model application have clinical implications for the management of BRAFV600E glioma.

Highlights

Elevated mitogen-activated protein kinase (MAPK) signaling is common in adult and pediatric gliomas
There are early indications that this personalized approach benefits some patients with BRAFV600E positive ganglioglioma [16,17], recurrent pleomorphic xanthoastrocytoma (PXA) [18] and recurrent glioblastoma [19]
The observation of progressive tumor growth during treatment is consistent with our more recent preclinical studies that showed no significant impact on survival rates from PLX4720 monotherapy when treating mice with distinct BRAFV600E mutant and CDKN2A deficient tumors models

Summary

Introduction

Elevated mitogen-activated protein kinase (MAPK) signaling is common in adult and pediatric gliomas. We previously reported that five out of seven (71%) of BRAFV600E mutant pediatric grade II-IV astrocytoma have homozygous deletion of CDKN2A, which encodes the p16 tumor suppressor [6]. A subsequent study reported that the BRAFV600E mutation and CDKN2A deletion are more frequent in pediatric low-grade glioma that transform to high-grade malignancy than in nontransforming tumors [8]. BRAFV600E is less common in adult glioma, in which MAPK signaling is most often stimulated by gene amplification and mutation of upstream receptor tyrosine kinases [9, 10]. The results of mutation screening studies indicate that BRAFV600E is important to multiple pediatric glioma types, and suggest that this oncogenic alteration cooperates with CDKN2A deletion to promote neoplastic transformation and tumor malignant progression

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Conclusion