Abstract BACKGROUND BRAF/MEK-inhibitors have been transformative for pediatric gliomas with RAS/MAPK pathway aberrations. The RAS/MAPK pathway has a complex role in immune regulation. Though well-known in adults, the systemic immune-related adverse effects (irAE) of BRAF/MEKi in children are not widely reported. METHODS We performed a retrospective chart review for irAE following BRAF/MEKi in children treated at SickKids between 2016-2023. Additional patients were included through a pan-Canadian survey. RESULTS 136 patients on BRAF/MEKi were included (MEKi: 74%, BRAFi: 13%, BRAF+MEKi: 13%); irAE was reported in 6/136 (4.5%). One additional patient was included through the national survey. Median age was 14 years (range: 7-16). Male:female was 3:4. Three patients were on BRAFi, and two each on MEKi, and BRAF+MEKi. Early events developing within weeks of drug initiation included hyper-immune systemic inflammatory responses in three children on BRAFi and one child on MEKi. There was no evidence of any concomitant infection. Two children needed intensive care and showed evidence of multi-organ dysfunction. Delayed irAE developing after months to years on therapy included multi-systemic sarcoid-like reaction, parasympathetic dysautonomia (both on BRAF+MEKi) and brachial neuritis/Parsonage-Turner syndrome (MEKi). Management included drug holidays, dose reductions, and immunosuppressive treatment with steroids. Long-term steroids were needed in two of these seven patients. CONCLUSIONS Pediatric oncologists need to be aware of rare yet possibly under-reported systemic immune toxicity in children treated with BRAF/MEKi. They can present acutely, or while on chronic therapy. Management may be challenging in view of issues with rebound tumor growth on stopping some of these drugs, with select patients needing concomitant, long-term immunosuppressive therapy. Genetic susceptibility and biomarkers may need to be systematically studied in future clinical trials of novel RAS/MAPK inhibitors. Continued surveillance and reporting are needed for the marketed compounds, especially for children on long-term use of these inhibitors.

LGG-51. SYSTEMIC IMMUNE TOXICITY OF BRAF AND MEK-INHIBITORS IN CHILDREN: A PAN-CANADIAN SERIES