CDK4 and neoplasm: Among CDK4/6 inhibitors, abemaciclib has the highest lipid solubility and lowest SA (3.8 vs. class average of 2.6, 75 vs. class average of 89 Å2, respectively), which could support its ability to attain a high non-tumor brain concentration (3.6 μmoL/L) and brain-to-plasma ratio in vivo as well as explain its clinical efficacy in phase 2 trials (23, 24).