CDK 4/6 inhibitors for adjuvant therapy in early breast cancer—Do we have a clear winner?

CDK4/6 inhibitors have become the mainstay of treatment for patients with advanced hormone receptor positive and Human Epidermal Receptor -2 [ HER-2 ] negative breast cancer. Three CDK 4/6 inhibitor drugs are currently approved and available, including Palbociclib, Ribociclib and Abemaciclib. All three of these drugs have similar mechanism of action and other pharmacokinetic and pharmaco-dynamic properties and hold equivalent positions in cancer care guidelines. Surprisingly, however, in the adjuvant setting of early breast cancer, two trials of palbociclib have failed to show any benefit while abemaciclib has shown some early benefits in disease-free survival and has received approval for its use in adjuvant setting. In this article, we explore several reasons for this discrepancy in the results of CDK4/6 inhibitors in the adjuvant setting. We also question if we should already adopt adjuvant abemaciclib in our clinical practice given the uncertainty in data.