Since the discovery of cyclin-dependent kinases (CDKs), a family of proteins vital in the early stages of cell development, scientists have tried to target them to disrupt the division and proliferation of cancer cells. This is especially true in breast cancer due to its reliance on this process for growth.

The first generation of these agents, known as CDK inhibitors, targeted both CDK4 and CDK6. That is why they’re known as CDK4...