Combined Inhibition of PIM and CDK4/6 Suppresses Both mTOR Signaling and Rb Phosphorylation and Potentiates PI3K Inhibition in Cancer Cells

// Lacey M. Litchfield 1 , Karsten Boehnke 2 , Manisha Brahmachary 2 , Cecilia Mur 3 , Chen Bi 1 , Jennifer R. Stephens 1 , J. Michael Sauder 4 , Sonia M. Gutierrez 3 , Ann M. McNulty 1 , Xiang S. Ye 5 , Wenjuan Wu 1 , Maria Jose Lallena 3 , Xueqian Gong 1 , Farhana F. Merzoug 1 , Valerie M. Jansen 1 and Sean G. Buchanan 1 1 Eli Lilly and Company, Indianapolis, IN, USA 2 Eli Lilly and Company, New York, NY, USA 3 Eli Lilly and Company, Alcobendas, Madrid, Spain 4 Eli Lilly and Company, San Diego, CA, USA 5 Eli Lilly and Company, Shanghai, China Correspondence to: Sean G. Buchanan, email: buchananse@lilly.com Keywords: abemaciclib; CDK4/6; PIM; mTOR; S6 Received: November 04, 2019     Accepted: March 14, 2020     Published: April 28, 2020 ABSTRACT Aberrant activation of mitogenic signaling pathways in cancer promotes growth and proliferation of cells by activating mTOR and S6 phosphorylation, and D-cyclin kinases and Rb phosphorylation, respectively. Correspondingly, inhibition of phosphorylation of both Rb and S6 is required for robust anti-tumor efficacy of drugs that inhibit cell signaling. The best-established mechanism of mTOR activation in cancer is via PI3K/Akt signaling, but mTOR activity can also be stimulated by CDK4 and PIM kinases. In this study, we show that the CDK4/6 inhibitor abemaciclib inhibits PIM kinase and S6 phosphorylation in cancer cells and concurrent inhibition of PIM, CDK4, and CDK6 suppresses both S6 and Rb phosphorylation. TSC2 or PIK3CA mutations obviate the requirement for PIM kinase and circumvent the inhibition of S6 phosphorylation by abemaciclib. Combination with a PI3K inhibitor restored suppression of S6 phosphorylation and synergized to curtail cell growth. By combining abemaciclib with a PI3K inhibitor, three pathways (Akt, PIM, and CDK4) to mTOR activation are neutralized, suggesting a potential combination strategy for the treatment of PIK3CA -mutant ER+ breast cancer.