NOVEL APPROACAHES FOR IMPROVING PEDIATRIC BRAFV600E GLIOMA PATIENT OUTCOMES

University Of California, San Francisco, San Francisco CA

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Abstract

Gliomas are aggressive brain tumors that affect children. Current treatment of high-grade glioma, most often involving surgery, radiation, and cytotoxic chemotherapy, only extend median survival of affected children to14 months, and results in significant morbidity. Low-grade glioma, that cannot be completely resected aretreated with radiation, typically after failure of chemotherapy approaches, with significant resulting neuro-cognitive and developmental risks. This project addresses a specific oncogene alteration, BRAFV600E occurring in as many as 14% of pediatric high-grade glioma. For certain subtypes of pediatric glioma thefrequency of BRAFV600E is much higher (66% in pleomorphic xanthoastrocytoma). Inhibitors that specifically target BRAFV600E have been developed and have shown remarkable efficacy against melanomas that harborthis mutation. One such inhibitor, vemurafenib, has now been FDA-approved for melanoma, and is underinvestigation for treating a number of other BRAFV600E malignancies. We have shown that BRAFV600E inhibitor treatment significanfiy extends survival of intracranial BRAFV600E glioma-bearing animals. We have also noted the activation of tumor feedback mechanisms may limit drug efficacy, suggesting that combination therapies targeting these mechanisms may further improve the response of BRAFV600E glioma to BRAF inhibitor therapy. Our preliminary studies have highlighted three specific and targetable activities that might be exploitable for this purpose. First, we have observed that CDKN2A inactivation and resultant increased CDK4/6 signaling occurs in the majority of BRAFV600E high grade glioma, and that concurrent treatment with CDK4/6 and BRAFV600E inhibitors provides additional survival benefit to animals bearing BRAFV600E and CDKN2A deficient tumors. Second, we observed that the reactivation of ERK signaling following BRAF inhibitor monotherapy is prevented by addition of a MEK inhibitor, which also increases anti-proliferative effect and survival for animais with intracranial BRAFV600E glioma. Finally, we have observed that BRAF-targeted treatment of BRAFV600E glioma results in EGFR feedback activation, suggesting that BRAF + EGFR combination therapy may augment the anti-tumor effect of BRAF monotherapy. Although these combination strategies appear to enhance BRAF inhibitor efficacy we do not fully understand the mechanistic basis for their actions, which of these combination approaches may be the most effective alone or combined with radiotherapy, or if BRAF inhibitors alone or in combination are safe in the pediatric population. These issues will be addressed in association with the proposed research that follows.