HGG-08. COMBINATORIAL ACTIVITY OF CDK4/6-INHIBITORS AND TRAMETINIB IN PEDIATRIC HIGH-GRADE GLIOMA WITH BRAFV600E MUTATION AND HOMOZYGOUS CDKN2A DELETION

BRAFV600E mutations occur in 10-15% and homozygous CDKN2A deletions in approximately 40% of adolescent and pediatric high-grad glioma (HGG) and is associated with significantly decreased overall survival. Moreover, a subset of pHGG is defined by combined BRAFV600E mutation and concomitant CDKN2A homozygous deletion. Standard therapy has remained unchanged and consists of maximal safe resection and focal radiotherapy still resulting in poor overall survival. Therefore, new therapeutic approaches to increase patient survival are of utmost importance. Targeted therapy of BRAFV600E mutant HGG has emerged as effective treatment but remains challenging due to therapy resistance. CDK4/6 inhibitors represent a promising target in CDKN2A altered HGG. However, palbociclib demonstrated only low efficacy in HGG patients. Therefore, combination approaches may be more effective, and we investigated CDK4/6-inhibitors alone and in combination with trametinib in HGG cell models with BRAFV600E mutation and homozygous CDKN2A deletion. Efficacy of CDK4/6-inhibitor monotherapy and combinatorial approach with trametinib was assessed by short- and long-term viability assays in four patient-derived HGG cell models with homozygous CDKN2A deletion and BRAFV600E mutation. Furthermore, effects of CDK4/6 inhibitor therapy alone or in combination with trametinib on downstream signaling pathways (MAPK, PI3K) were analyzed with Western blots. Abemaciclib showed the highest activity in all cell models with IC50-values ranging from 0,5 – 2µM. Combined treatment approaches with trametinib showed synergistic effects across all cell models. Long-term viability assays revealed distinct sensitivity in the nanomolar range for CDK4/6-inhibitors and combined treatment with trametinib in all cell models. Exposure to abemaciclib and trametinib significantly decreased pRB, pERK, pS6, and pAKT protein expression levels when compared to monotherapy alone. Summarizing, combined treatment with CDK4/6-inhibitors and trametinib showed promising therapeutic effects on HGG models with homozygous CDKN2A deletion and BRAFV600E mutation. Currently, effects on cell cycle distribution and downstream molecular mechanisms are investigated to identify potential predictive biomarkers.