“…However, the earlier class of CDK inhibitors had limited specificity, inadequate clinical activity, poor pharmacokinetic properties, and unacceptable toxicity profiles (10,11,14,15). These disappointing initial efforts now have been followed by the development of the specific CDK4/6 inhibitors palbociclib (PD0332991), ribociclib (LEE011), and abemaciclib (LY2835219), which have demonstrated manageable toxicities, improved pharmacokinetic properties, and impressive antitumor activity, especially in certain forms of breast cancer (14,16). Successful early clinical trials with these three CDK4/6 inhibitors have generated cautious enthusiasm that these drugs may emerge as a new class of anticancer agents (14,17).…”