JS03.7.A COMBINATION OF CDK4/6 AND MEK INHIBITION INCREASES ANTITUMOR EFFECTS IN PEDIATRIC HIGH-GRADE GLIOMA HARBORING BRAF MUTATION AND CDKN2A/B LOSS

Abstract BACKGROUND Pediatric high-grade gliomas (pHGG) comprise a heterogenous group of tumors characterized by rapid growth and dismal overall survival. Besides maximum safe resection, irradiation and chemotherapy, molecularly-defined therapeutic interventions are still missing. BRAFV600E mutations are detected in 5-10% of pHGG and this subgroup frequently harbors a concurrent homozygous CDKN2A/B loss. Taking advantage of those molecular alterations is of utmost importance to improve overall survival in these patients. Thus, we hypothesize that a combination of MEK and CDK4/6 inhibition lead to prolonged anti-tumor effects in this patient subgroup when compared to monotherapy. MATERIAL AND METHODS Four HGG cell models with BRAFV600E mutation and homozygous CDKN2A/B loss were tested for the efficacy of CDK4/6 inhibitor monotherapy (palbociclib, ribociclib, abemaciclib) and/or combined with the MEK inhibitor trametinib. Drug response, induction of apoptosis, cell cycle alterations and senescence were evaluated upon short- and long-term exposure using cell-based assays. Effects on intracellular signaling cascades were determined by western blot and immunostainings in cell and patient-derived xenograft (subcutaneous and orthotopic) models. Clinical efficacy of combined treatment was further investigated in one patient suffering from progressive metastatic BRAF-altered anaplastic pleomorphic xanthoastrocytoma (aPXA). RESULTS Besides trametinib, the investigated cell models showed the highest sensitivity to abemaciclib, thus we assumed that combination of these two inhibitors would be most suitable in vitro. The simultaneous application of both drugs induced apoptosis and the effect on tumor cell senescence was even superior with the combined treatment. Regarding signal transduction, solely the combination of MEK and CDK4/6 inhibitors reduced PI3K activation at the level of S6. To confirm our findings, in vivo experiments in PDX models were conducted. Significantly increased overall survival and antitumor effects were observed with the combination of both inhibitors when compared to single-agent treatment. Corroboratively, one patient suffering from progressive metastatic aPXA with BRAF mutation and CDKN2A/B loss was successfully treated with trametinib and ribociclib showing stable diseases for two years. CONCLUSION Summarizing, our results suggest that combined treatment with MEK and CDK4/6 inhibitors represent a promising treatment strategy in pHGG harborin