Combination of ribociclib with BET-bromodomain and PI3K/mTOR inhibitors for medulloblastoma treatment in vitro and in vivo.

Despite improvement in the treatment of medulloblastoma (MB) over the last years, numerous patients with MYC- and MYCN-driven tumors still fail current therapies. MBs have an intact retinoblastoma protein RB, suggesting that CDK4/6 inhibition might represent a therapeutic strategy for which drug combination remains understudied. We conducted high throughput drug combination screens in a Group3 (G3) MB line using the CDK4/6 inhibitor (CDK4/6i) ribociclib at IC20, referred to as an anchor, and 87 oncology drugs approved by FDA or in clinical trials. Bromodomain and extra terminal (BET) and PI3K/mTOR inhibitors potentiated ribociclib inhibition of proliferation in an established cell line and freshly dissociated tumor cells from intracranial xenografts of G3 and Sonic hedgehog (SHH) MBs in vitro. A reverse combination screen using the BET inhibitor JQ1 as anchor, revealed CDK4/6i as the most potentiating drugs. In vivo, ribociclib showed single agent activity in MB models while JQ1 failed to show efficacy due to high clearance and insufficient free brain concentration. Despite in vitro synergy, combination of ribociclib with the PI3K/mTOR inhibitor paxalisib did not significantly improve the survival of G3 and SHH MB bearing mice compared to ribociclib alone. Molecular analysis of ribociclib and paxalisib treated tumors revealed that E2F targets and PI3K/AKT/MTORC1 signaling genes were depleted, as expected. Importantly, in one untreated Group3 medulloblastoma model HD-MB03, the PI3K/AKT/MTORC1 gene set was enriched in vitro compared to in vivo suggesting that the pathway displayed increased activity in vitro. Our data illustrate the difficulty in translating in vitro findings in vivo.