MODL-03. ADAPTING PALBOCICLIB FOR MEDULLOBLASTOMA THERAPY BY IMPROVING DRUG DELIVERY AND ADDRESSING RESISTANCE

CDK4/6 inhibition may be a promising therapy for medulloblastoma. All medulloblastoma subgroups show D-cyclin/CDK4/RB pathway activity, suggesting broad potential for efficacy. To address drug delivery and systemic toxicity limitations, we developed a nanoparticle formulation of CDK 4/6 inhibitor, palbociclib, in poly (2-oxazoline) micelles (POx-palbo). POx-palbo showed reduced systemic toxicity in transgenic mice engineered to develop medulloblastoma, allowing for higher dosing. Pharmacodynamic studies showed POx-palbo suppressed RB phosphorylation acutely and after 24hrs, the effect diminished. This inhibition produced a longer lasting suppression of SHH pathway activity, demonstrated by Gli-luc reporter tumor mice. Importantly, POx-palbo therapy, administered daily, reduced tumor growth and improved the survival of mice with medulloblastoma. While POx-palbo was clearly effective as a single agent, all mice treated with POx-palbo eventually developed progressive disease, as resistant populations of tumors cells emerged. To understand the mechanisms of resistance, we compared tumors early and late in the course of therapy. We found that after 5 days of treatment, palbociclib altered cell cycle progression to produce an extended period of S-phase and that the fractions of cell expressing the stem cell marker Olig2 were markedly increased. Based on these data, we propose that tumors respond to the initial suppressive effect of palbociclib by increasing the pool of Olig2+ stem cells, that these cells show discernably different cell cycle kinetics and are resistant to CDK4/6 inhibition. Combining POx-palbo with additional therapies that target Olig2+ stem cells, by disrupting their prolonged S-phase, or by disrupting Olig2 function, may lead to newly effective medulloblastoma treatment.