PD is a highly selective, orally administered inhibitor of both CDK4 and CDK6 kinase activity and is currently being evaluated in clinical trials for a variety of adult cancers. PD is selective for CDK4 and CDK6, with IC50 values for CDK4/cyclinD1, CDK4/cyclinD3, and CDK6/cyclinD2 of 11, 9, and 15 nM, respectively, and has low activity against a panel of 36 additional protein kinases, including CDK2/cyclin E2, CDK2/cyclin A, and CDK1/cyclin B. In addition, recent publications suggest that PD has a good safety EMD 66684 profile and demonstrates efficacy in cancers such as mantle cell lymphoma that harbor aberrant expression of cyclin D1 due to a translocation. Since radiation therapy, the current standard of care for children with DIPG provides only temporary relief, we were interested to evaluate whether treatment with PD immediately following RT would enhance the survival benefit of RT. Although there have been numerous clinical trials for DIPG combining chemotherapy or targeted agents with RT, none have successfully provided a survival advantage over radiation alone. Here we evaluated the efficacy of PD alone and PD with RT in our immunocompetent, genetically relevant BSG mouse model. We were first interested to determine if CDK4/6 and D-type cyclins are overexpressed at the protein level in two PDGF-B driven BSG mouse models: Ink4a-ARF deficient and p53 deficient. Interestingly, western blot analysis demonstrated that CDK4/6 and all three D-type cyclins were overexpressed in both BSG models relative to the normal brainstem. These results provided an initial rationale that a CDK4/6 inhibitor could be a potential therapeutic agent in the two BSG mouse models. Between successful early phase clinical trials with PD in adults, relevant murine models, and potential for therapeutic targeting in BSGs, we were interested to determine how PD would affect our BSG cell lines in vitro. To examine the efficacy of PD we performed a variety of in vitro assays on two independent PDGF-B driven Ink4a-ARF-/- BSG cell lines. Using MTT assays, we observed that treatment with PD for 48 hours was minimally cytotoxic to the Ink4a-ARF deficient BSG cells at a dose of 5��M but not at lower doses. We also noted that BRL 15572 hydrochloride proliferation of these cells was indeed inhibited by PD, with an IC50 of 1.8��M as assessed with BrdU assays. Since proliferation was inhibited, we were also interested to determine if PD induced apoptosis. Surprisingly, PD induced a very small but significant increase in caspase 3/7 activity levels at doses. To assess if the mechanism of action of PD was in fact through inhibition of CDK.