Alternative Dosing Schedule of Palbociclib in Metastatic Hormone Receptor Positive Breast Cancer

Study Summary

The investigators propose to conduct a study to test an alternative dosing schedule of palbociclib. With the current three-week on and one week off schedule, a significant number of patients develop grade 3 or higher degree of neutropenia and require dose reduction and sometimes discontinuation. This potentially compromises the efficacy of the drug. In addition, as the half-life of palbociclib is 27 hours, 1 week break with the standard 3 weeks on and 1 week off dosing schedule could potentially lead to recovery of Rb phosphorylation during the off week. Hence, the investigators propose a 5 days on and 2 days off schedule each week without any weeks off drug. Although the cumulative doses each 28-day cycle is roughly the same with this schedule compared to conventional dosing, the bone marrow is not exposed to the drug continuously for 21 days and rather gets frequent breaks from therapy. The investigators hypothesize that the 5 days on and 2 days off schedule is more tolerable with less frequent high grade neutropenia and dose interruption/reduction. In addition, this schedule also provides for a more continuous drug delivery to the patient since there is not a week's break in therapy, which could ultimately prove to be more efficacious.

Primary Outcome

* The descriptions and grading scales found in the revised NCI Common Terminology Criteria for Adverse Events (CTCAE) version 4.0 will be utilized for all toxicity reporting. * Grade 3 neutropenia: \<1000-500/mm\^3; \<1.0-0.5 x 10e9/L * Grade 4 neutropenia: \<500/mm\^3; \<0.5 x 10e9/L

Interventions

• DRUG Letrozole
• DRUG Fulvestrant
• DRUG Palbociclib
• DRUG Goserelin
• PROCEDURE Optional research biopsy