MODL-14 Inhibition of cell cycle mechanisms to combat high-risk medulloblastoma

Abstract Medulloblastoma (MB) is the most common embryonal brain tumor in children, characterized by a high level of heterogeneity within this entity. Four major molecular groups have been defined - WNT, Sonic hedgehog (SHH), Group 3 (G3), and Group 4 (G4) - differing widely in genetic alterations and patient outcomes. Targeted treatment approaches are limited for high-risk MB including MYC-amplified G3, G4, or MYCN/GLI2-amplified SHH. Based on earlier studies describing the inhibition of cell cycle regulation as a therapeutic vulnerability in high-risk MB, we aimed to explore the efficacy and mode of action of approved CDK4/6 inhibitors in aggressive MB. First the in vitro sensitivity of pediatric G3 (n=5) MB cell models against palbociclib, ribociclib and abemaciclib was evaluated and compared to adult SHH (n=2) cells serving as MYC-negative controls. Abemaciclib most effectively reduced cell viability, exhibiting a mean IC50 value of 0.9 M in G3 compared to 2.1 M in SHH cells (p=0.007). Upon long-term exposure (7-10 days), again abemaciclib most potently decreased clonogenic survival already at 250nM in G3 MB cells. We observed a G1/S phase cell cycle arrest upon abemaciclib treatment and confirmed this by Western blot analyses, showing decreased Rb phosphorylation accompanied by reduced MAPK signaling. Interestingly, abemaciclib treatment distinctly reduced MYC protein and gene expression levels in MYC-amplified G3 cell models. In addition, exposure to abemaciclib decreased GLI2 and TERT mRNA levels, which was accompanied by induction of cellular senescence. Within the ITCC-P4 project, single mouse trials are currently being performed including abemaciclib treatment in orthotopic MB models. The detailed investigation of the precise mode of action in vitro and in vivo is ongoing. Summarizing, the CDK4/6 clinically approved inhibitor abemaciclib shows promising efficacy against high-risk MB in vitro.