Abstract 2740: Mechanistic basis of Palbociclib combinatorial activity in ER+ breast cancer and non-breast indications

Phosphorylation of the retinoblastoma protein (Rb) by cyclin-dependent kinases 4 and 6 (CDK4/6) is a critical checkpoint for G 1 /S cell cycle progression and commitment to cellular proliferation. Human malignancies often subvert these control mechanisms through a range of genetic and biochemical adaptations. Accordingly, tumors that depend on CDK4/6 activity for proliferation and survival are particularly sensitive to inhibition of this pathway by palbociclib (Ibrance TM ), a highly selective inhibitor of CDK4/6 kinase activities. Treatment regimen of palbociclib with letrozole significantly improved progression-free survival in a randomized phase 2 study of women with advanced estrogen receptor-positive (ER+), HER2-negative breast cancer. Likewise, in ER+ breast cancer models palbociclib and estrogen antagonists combine for greater anti-proliferative activity, increased hallmarks of cellular senescence and prolonged durability of response following drug removal. Dual inhibition of CDK4/6 and ER signaling demonstrated robust anti-tumor activity in xenograft studies. The addition of Palbociclib to other targeted therapeutics elicits improved activity in pre-clinical models of several non-breast indications and these effects also manifest through modulation of cellular proliferation, senescence and growth arrest. Data will be presented on the molecular basis of combination benefit with Palbociclib in ER+ breast and other oncology indications. Citation Format: Stephen Dann, Jing Yuan, John Chionis, Chaoting Liu, Tao Xie, Nathan V. Lee, Enhong Chen, Ping Wei, Paul A. Rejto, David J. Shields, Todd VanArsdale. Mechanistic basis of Palbociclib combinatorial activity in ER+ breast cancer and non-breast indications. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2740.