HSP90 Inhibition Overcomes Resistance to Molecular Targeted Therapy in BRAF(V600E)-mutant High-grade Glioma

Purpose: Molecular targeted therapy using BRAF and/or MEK inhibitors has been applied to BRAFV600E-mutant high-grade gli-omas (HGG); however, the therapeutic effect is limited by the emergence of drug resistance. Experimental Design: We established multiple paired BRAFV600E-mutant HGG patient-derived xenograft models based on tissues collected prior to and at relapse after molecular targeted therapy. Using these models, we dissected treatment-resistant mechanisms for molecular targeted therapy and explored thera-peutic targets to overcome resistance in BRAFV600E HGG models in vitro and in vivo. Results: We found that, despite causing no major genetic and epigenetic changes, BRAF and/or MEK inhibitor treatment deregu-lated multiple negative feedback mechanisms, which led to the reactivation of the MAPK pathway through c-Raf and AKT signaling. This altered oncogenic signaling primarily mediated resistance to molecular targeted therapy in BRAFV600E-mutant HGG. To overcome this resistance mechanism, we performed a high-throughput drug screening to identify therapeutic agents that potently induce additive cytotoxicity with BRAF and MEK inhibi-tors. We discovered that HSP90 inhibition combined with BRAF/ MEK inhibition coordinately deactivated the MAPK and AKT/ mTOR pathways, and subsequently induced apoptosis via dephos-phorylation of GSK3b (Ser9) and inhibition of Bcl-2 family pro-teins. This mediated potent cytotoxicity in vitro and in vivo in refractory models with acquired resistance to molecular targeted therapy. Conclusions: The combination of an HSP90 inhibitor with BRAF or MEK inhibitors can overcome the limitations of the current therapeutic strategies for BRAFV600E-mutant HGG.