WI231696: Bosutinib, Palbocicilib and Fulvestrant for HR+HER2- Advanced Breast Cancer Refractory to a CDK4/6 Inhibitor

This is an open-label, single-arm, phase I trial. It is designed with a conservative dose escalation plan to ensure patient's safety and with a strong translational component to inform if target inhibition is achieved. With concerns regarding safety, based on extensive available pharmacokinetic data and clinical efficacy experience, bosutinib will be given 5-days in a row followed by 2 days rest in a weekly basis, instead of daily. The protocol will enroll patients per 3+3 escalation design. The Dose Limiting Toxicity (DLT) observation period is 28 days. At the end of DLT observation period of each cohort of 3 patients, decision will be made regarding further escalation or de-escalation according to this plan. Once the MTD of the combination is reached, the safety data will be analyzed. There will be no dose reductions during DLT observation period. Dose reduction within patients (individually) is allowed after the 4-week DLT observation period. Treatment in this phase I trial will be administered until there is disease progression or unacceptable toxicity.