OTHR-13. Effectivity and toxicity of off-label treatment with Trametinib monotherapy or in combination with Dabrafenib in children with relapsed or refractory brain tumor

INTRODUCTION: Most pediatric low grade gliomas, and a substantial part of PXAs, have a BRAF alteration/activated MAPK pathway. This group often requires several lines of therapy, which coincides with significant morbidity. There is growing evidence that molecular targeted treatment may be beneficial for this population. Here we report the toxicity and efficacy of patients treated off-trial with either trametinib monotherapy or in combination with dabrafenib. METHODS: We performed a single-center retrospective chart review of all neuro-oncological patients who received trametinib monotherapy or in combination with dabrafenib (compassionate-use) in the Princes Maxima Center between April 1st, 2018 and December 31st, 2021. RESULTS: 22 patients (ages 1-21 years) were included of whom 14 received trametinib monotherapy (BRAF-KIAA fusion, n=10) and eight received trametinib/dabrafenib combination therapy (BRAFV600 mutation, n=8). All patients on trametinib monotherapy (n=14) developed skin problems such as rash (100%), dry skin (86%), paronychia (71%) and eczema (64%). Eight patients (57%) had at least one adverse event (AE) ≥grade 3. Patients on trametinib and dabrafenib showed similar toxicity, although with lower prevalence and no paronychia. One patient (13%) had an AE ≥grade 3. Median treatment time of trametinib was 514 days (IQR 455) and for trametinib and dabrafenib 360 days (IQR 512). Five patients (36%) on trametinib are still on treatment and nine patients (64%) stopped treatment due to e.g. tumor progression or toxicity. All patients on trametinib and dabrafenib are still on treatment. Best overall response of trametinib monotherapy (n=14 evaluated) was observed as partial response (50%), stable disease (33%) and progressive disease (17%). Combination therapy (n=7 evaluated) brought 100% partial response. CONCLUSION: Dermatological toxicities are mostly seen in trametinib monotherapy or in combination with dabrafenib. Despite moderate toxicity patients seem to benefit from treatment. Results suggest that combination therapy has a more favorable toxicity profile than monotherapy.