TR-10: AUTOPHAGY INHIBITION REVERSES RESISTANCE TO TARGETED BRAF THERAPY IN PEDIATRIC CNS TUMORS

Autophagy inhibition is a potential therapeutic strategy in cancer. We have previously shown that autophagy-dependent tumors, such as those with BRAF^V600E, there may be broad potential application of autophagy inhibition. Targeted inhibition against the BRAF^V600E mutation (BRAFi) has been identified as important in pediatric CNS tumors and is being more broadly provided to these patients. Patients treated with BRAFi will develop resistance mechanisms and new approaches to reversing this resistance are needed. Our findings suggest that pediatric CNS tumors with BRAF^V600E are autophagy-dependent in parental tumors and those with inherent or acquired BRAFi resistance. Autophagy was evaluated by flow cytometry and Western blot. Autophagy was inhibited by clinically available chloroquine and BRAFi was provided by vemurafenib. Cell survival was evaluated using a variety of techniques. Ex vivo organotypic culture of resistant tumor was used to verify in vitro findings. We found that mutant (but not WT) cells display high rates of induced autophagy, and these levels remain similar in BRAFi resistant cells. Both parental and resistant cells are sensitive to autophagy inhibition and display synergy when chloroquine was combined with BRAFi. We also demonstrate chloroquine can improve vemurafenib sensitivity in a resistant ex vivo primary culture. Autophagy inhibition decreases the ratio of pERK:ERK levels when used in combination with BRAFi in resistant tumor cells, suggesting a potential mechanism for improved response in combination treatment. Importantly, chloroquine improved vemurafenib sensitivity in a patient with acquired BRAFi resistance and improved clinical outcomes. These data highlight an exciting possibility for identifying genetic markers, such as BRAF^V600E, which sensitize tumors to autophagy inhibition combination therapy. Synergy was identified in both parental and resistant cells suggesting that autophagy-dependence is driven by its underlying mutations. Therefore, in autophagy-dependent BRAF^V600E tumors there may be broad potential application of autophagy inhibition in both BRAFi naive and resistant patients.