In vitro activity: Abemaciclib (formerly known as LY2835219) is a potent and selective, orally available dual inhibitor of CDK4 (cyclin-dependent kinase) and CDK6 with IC50 of 2 nM and 10 nM in cell-free assays, respectively. LY2835219 specifically inhibits CDK4 and 6, thereby inhibiting retinoblastoma (Rb) protein phosphorylation in early G1. Inhibition of Rb phosphorylation prevents CDK-mediated G1-S phase transition, thereby arresting the cell cycle in the G1 phase, suppressing DNA synthesis and inhibiting cancer cell growth. Overexpression of the serine/threonine kinases CDK4/6 can cause cell cycle deregulation as seen in certain types of cancer.

Kinase Assay: Cells (5 × 103) are plated in 96 well plates. Cells are treated the next day for 24 to 48 hours and then assessed for caspase-3 activity by Caspase-Glo-3/7 Assay, as per manufacturer's instructions and a luminescence plate reader.

Cell Assay: Cells are seeded in a 96-well plate, allowed to adhere overnight, and treated with DMSO control (0.1% v/v) or the indicated compounds for 72 h. Cell viability and proliferation are determined using a Cell Counting Kit according to the manufacturer's instructions. The interaction between LY2835219 and mTOR inhibitor is determined using CompuSyn. Combination index (CI) values of 1 indicates and additive drug interaction, whereas a CI of < 1 is synergistic and a CI of> 1 is antagonistic.