Dalpiciclib Plus Fulvestrant With Pyrotinib in Hormone Receptor-positive, HER2-low Advanced Breast Cancer That Progressed on Previous CDK4/6i Plus AI Therapy

The purpose of this study is to determine if the triplet combination of dalpiciclib, fulvestrant, and pyrotinib is safe and effective in the treatment of hormone receptor-positive, HER2-low locally advanced/metastatic breast cancer following treatment with an aromatase inhibitor plus a CDK4/6 inhibitor (palbociclib abemaciclib or ribociclib). The study will employ a Bayesian Optimal Phase II (BOP2) design which explicitly controls the type I error rate, thereby bridging the gap between Bayesian designs and frequentist designs, and has favorable operating characteristics with higher power and lower risk of incorrectly terminating the trial than some existing Bayesian phase II designs.