In vitro, RGT-419B showed more robust activity against palbociclib-resistant ER+ breast cancer cells than abemaciclib. In ER+ T47D breast cancer cells with overexpression of Cyclin E1, RGT-419B exhibited better antiproliferation activity than either abemaciclib or palbociclib. RGT-419B also demonstrated more durable in vivo tumor growth inhibition when compared with abemaciclib in an ER+ breast cancer xenograft model.