LG-64: COMBINATORIAL TARGETING OF ONCOGENIC FGFR1 GENE REARRANGEMENTS IN PEDIATRIC GLIOMAS

Fibroblast growth factor receptor (FGFR) rearrangements that generate the oncogenic FGFR1-TACC1 and FGFR1-tyrosine kinase duplication (FGFR1-TKD) fusion proteins have recently been described in a subset of pediatric low-grade gliomas (pLGGs). To identify kinase inhibitors for targeting FGFR1-driven cells, we performed an unbiased inhibitor screen on transformed Ba/F3 FGFR1-TACC1 and FGFR1-TKD compared to Ba/F3 parental cells with a compound library consisting of 8-graded doses of 109 kinase inhibitors which target ∼2/3rds of the kinome. This functional assay revealed potent activity of previously known FGFR inhibitors, including, BGJ398, AZD4547, ponatinib and axitinib, and an intriguing functional inhibitor hit with the MEK inhibitor trametinib. Enhanced sensitivity to trametinib in Ba/F3 FGFR1-fusion driven cells suggests downstream activation of and addiction to the MAPK pathway. Using a previously described heterologous NIH/3T3 cell based model system expressing FGFR1-TACC1 FGFR1-TKD, we tested the hypothesis that downstream inhibition of the MAPK pathway with the MEK inhibitor trametinib would effectively block oncogenic FGFR signaling. Inhibition of FGFR1-fusion driven pathways in FGFR1-TACC1 and FGFR1-TKD NIH /3T3 cell lines was assessed via soft agar, immuno-blotting and in vivo flank xenografts. Combinatorial drug treatment using everolimus (mTOR inhibitor) was conducted to interrogate possible targeted MEK inhibition escape mechanisms via the PI3K pathway. Treatment of FGFR-TACC1 and FGFR1-TKD with trametinib inhibited downstream MAPK pathway and oncogenic growth in vitro and in vivo. Co-targeting the MAPK pathway and PI3K pathway using trametinib and everolimus demonstrated sustained inhibition of tumor growth in vivo. Our data support the development of combinatorial therapeutic strategies for FGFR-altered PLGGs.