Should Ki-67 be adopted to select breast cancer patients for treatment with adjuvant abemaciclib?

On 12 October 2021, the US Food and Drug Administration (FDA) approved the cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor abemaciclib for the adjuvant treatment of hormone receptor-positive (HR+)/HER2-negative (hereafter defined HR+) breast cancer,1 making abemaciclib the first CDK4/6 inhibitor approved for the adjuvant treatment of HR+ breast cancer.2 This approval was granted based on the results of the monarchE trial, in which patients who were randomized to receive adjuvant abemaciclib in addition to endocrine treatment achieved a statistically significant and clinically meaningful improvement in invasive disease-free survival (iDFS).