• Part B [MTD Expansion Cohort(s)]: When the MTD(s) of palbociclib plus nab-P has been estimated with confidence, enrollment will proceed into 1 or 2 MTD expansion cohort(s) of up to 20 patients each at the MTD(s). The objective of the MTD expansion cohort(s) will be to provide additional information on safety, tolerability, biomarkers, PD activity, and PK/PD relationship for the combination regimen in order to determine the RP2D. The MTD expansion cohort(s) will only enroll patients who have not received previous treatment for their metastatic disease in order to evaluate preliminary activity of the combination in the target patient population.