Abstract A25: Autophagy inhibition reverses resistance to targeted BRAF therapy in CNS tumors

Autophagy inhibition is a potential therapeutic strategy in cancer. We have previously shown that in autophagy-dependent tumors, such as those with BRAFV600E, there may be broad potential application of autophagy inhibition. Targeted inhibition against the BRAFV600E mutation (BRAFi) has been identified as important in 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 CNS tumors with BRAFV600E are autophagy-dependent in both parental tumors and those with inherent or acquired BRAFi resistance. Autophagy was evaluated by flow cytometry and Western blot. It was inhibited by clinically available chloroquine (CQ) and BRAFi was provided by vemurafenib. Cell survival was evaluated using a variety of techniques. Ex vivo organotypic slice 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, with equal CQ response despite BRAFi resistance. Furthermore, both display synergy when CQ was combined with BRAFi in long-term growth assays. The addition of CQ increased apoptosis compared to BRAFi only cells. We also demonstrate CQ can improve vemurafenib sensitivity in a resistant ex vivo primary culture by decreasing Edu incorporation and increasing cytotoxicity as measured by LDH release. The mechanism underlying these effects is still under investigation. We have shown the mechanism is different than those reported in melanoma studies where BRAFi resistant cells show increased autophagic responses and ER stress responses as a mechanism for reversal of resistance. In CNS BRAFi resistant cells, we show no compensatory increase in autophagy levels and no up-regulation of ER stress response. This highlights the importance of specific studies within CNS tumors. More importantly, we have shown how CQ 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 BRAFV600E, 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 BRAFV600E tumors there may be broad potential application of autophagy inhibition in both BRAFi naive and resistant patients. Citation Format: Shadi Zahedi, Andrea M. Griesinger, Todd C. Hankinson, Michael H. Handler, Nicholas K. Foreman, Andrew Thorburn, Jean M. Mulcahy Levy. Autophagy inhibition reverses resistance to targeted BRAF therapy in CNS tumors. [abstract]. In: Proceedings of the AACR Special Conference: Advances in Brain Cancer Research; May 27-30, 2015; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2015;75(23 Suppl):Abstract nr A25.