ESR1 mutations have been reported that will contribute to the resistance of the aromatase inhibitor (AI) in HR+ breast cancer patients in several past studies. However, the role of ESR1 mutation remains unknown when the AI is combined with the CDK4/6 inhibitors. In phase 3 PADA-1 study, 33 of the 1017 enrolled patients (3.2%) harbored an ESR1 mutation before receiving the 1st-line AI plus palbociclib.
