LG-47TYPE II RAF INHIBITORS INHIBIT BRAF MUTATIONS AND TRUNCATED FUSIONS IN PEDIATRIC LOW-GRADE GLIOMAS

LG-47. TYPE II RAF INHIBITORS INHIBIT BRAF MUTATIONS AND TRUNCATED FUSIONS IN PEDIATRIC LOW-GRADE GLIOMAS Mark W. Kieran1,2, Yu Sun1, Catherine Pilarz1, David Calligaris3, Emily J. Chadwick1, John A. Alberta1, Shakti H. Ramkissoon1,2, Lori A. Ramkissoon1, Veronica Matia Garcia1, Kim Wilkinson1, Michael Kane1, Liliana Goumnerova1,2, Susan N. Chi1,2, Peter Manley1,2, Karen D. Wright1,2, Nathalie Y. Agar1,3, Keith L. Ligon1,3, Rameen Beroukhim1,3, Pratiti Bandopadhayay1,2, Geoffrey Kannan1,2, Rosalind A. Segal1, Levi A. Garraway1, Nathanael S. Gray1, Michael Eck1, Charles D. Stiles1, and Sara J. Buhrlage1; Dana-Farber/Harvard Cancer Center, Boston, MA, USA; Boston Children’s Hospital/Harvard Medical School, Boston, MA, USA; BrighamandWomen’sHospital/HarvardMedical School,Boston,MA,USA PURPOSE: Abnormal signaling of the ras/raf pathway predominates in pediatric low-grade gliomas (PLGGs). Most (75%) non-NF1 low-grade gliomas have Ras-independent BRAF activation, either due to KIAA1549:BRAF truncated fusion duplication or V600E point mutation. While the latter BRAF oncoprotein functions as a monomer, the truncated fusion form requires dimerization. Type I BRAF V600E inhibitors approved for melanoma effectively abrogate monomeric signaling by targeting the “DFG-in” kinase conformation but paradoxically activate wild type RAF dimers. We therefore evaluated Type II RAF inhibitor activity (targeting the “DFG-out” kinase conformation). METHODS: We developed multiple pathway relevant model systems for different BRAF point mutations and KIAA1549 variants using neural progenitor cells from mouse embryos for in vitro, in vivo and mass spectrometry analysis. Additionally, we developed an organoid ex vivo avatar assay from direct patient samples to identify drugs inhibiting these targets. We subsequently treated a single patient with one of two identified compounds. RESULTS: Two different type II BRAF inhibitors capable of inhibiting downstream activation of pERK and tumor progression in vivo for multiple point mutations and BRAF truncated fusion forms were identified. One compound was active on authentic human PLGA cells ex vivo and showed excellent CNS penetration. CONCLUSIONS: Type II BRAF inhibitors possess a unique mechanism of action targeting the BRAF/MEK complex and inhibiting both BRAF point mutations and common structural abnormalities (KIAA1549) in PLGGs. These results have led to the development of a phase I/0/II trial. Neuro-Oncology 18:iii78–iii96, 2016. doi:10.1093/neuonc/now075.47 #The Author(s) 2016. Published by Oxford University Press on behalf of the Society for Neuro-Oncology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.