Abstract B234: LY2835219, a potent oral inhibitor of the cyclin-dependent kinases 4 and 6 (CDK4/6) that crosses the blood-brain barrier and demonstrates in vivo activity against intracranial human brain tumor xenografts.

Effective treatments for primary brain tumors and brain metastases represent a major unmet medical need. The blood-brain barrier (BBB) arises from both a structural barrier and drug efflux transporters that prevent most anti-cancer drugs from efficiently reaching brain tumors or metastases. The CDK4/6 pathway (CDK4/6-cyclin D1-Rb-CDKN2) plays a key role in regulating cellular proliferation. The importance of this pathway is highlighted by its inactivation in a majority of human tumors including glioblastoma multiforme. We have identified and characterized a potent and selective dual cdk4/6 inhibitor, LY2835219. Preclinical characterization was performed with the monomesylate salt (LY2835219-MsOH), which inhibits these kinases with IC 50 values of 2 and 10 nM for CDK4 and CDK6, respectively. LY2835219-MsOH is a potent inhibitor of Rb phosphorylation in vitro and in vivo that induces G1 specific arrest and inhibition of tumor growth. To determine the potential of LY2835219-MsOH for the treatment of brain tumors and metastases, we assessed the ability of LY2835219-MsOH to cross the BBB and its interaction with the P-gp and BCRP efflux pumps that are expressed at the BBB. Using MDCK cells over-expressing either human ABCB1 (P-gp) or mouse abcg2 (Bcrp), LY2835219-MsOH and a second CDK4/6 inhibitor (PD0332991) are substrates for these two pumps, but each cross the murine BBB in vivo to a different degree. Unlike PD0332991, LY2835219-MsOH saturates BBB efflux with an unbound plasma IC50 of ∼95 nM (1.8 uM total plasma). The percent of dose in brain for LY2835219-MsOH is 0.5–3.9% and is comparable to that for temozolomide (1.9% plasma). In both a subcutaneous and intracranial human glioblastoma model (U87MG), LY2835219-MsOH suppressed tumor growth in a dose-dependent manner both as a single agent, and in combination with temozolomide. In summary, LY2835219-MsOH is a potent and selective oral CDK4/6 inhibitor that crosses the BBB and inhibits the growth of intracranial tumors alone or in combination with other agents. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B234.