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

Palbociclib at a dose of 125 mg should be taken by mouth with food on a 5 days on/2 days off schedule

Patients who are receiving letrozole will take it daily by mouth, every day of each 28-day cycle, at a dose of 2.5 mg.

Patients who are receiving fulvestrant will receive it at a dose of 500 mg as two 5 mL intramuscular injections (one into each buttock) on Days 1 and 15 of Cycle 1 and then on Day 1 of each cycle thereafter.

Patients may consent to paired tumor biopsies at baseline and time of progression.

Goserelin is given as a subcutaneous injection every 28 days. It is preferred to be given on Day 1 of each cycle, but it may be administered on any day of the treatment cycle to accommodate its specific Q28-day cycle. It will be given to pre- and peri-menopausal women only.

"-Blood will be drawn at the following time points for serum, plasma, cfDNA, and germline DNA (only at baseline):~* Baseline~* C1D15~* C2D1~* Every 2-3 months thereafter (to coincide with imaging studies)~* Time of progression"

-Baseline, cycle 2 day 1, post 2 or 3 months of therapy (to coincide with first tumor imaging), and progression

-Baseline and progression