A Phase II Trial Assessing the Tolerability of Palbociclib in Combination with Letrozole or Fulvestrant in Patients Aged 70 and Older with Estrogen Receptor-Positive, HER2-Negative Metastatic Breast Cancer

This phase II trial studies the side effects and how well palbociclib and letrozole or
fulvestrant works in treating patients aged 70 years and older with estrogen receptor
positive, HER2 negative breast cancer that has spread to other places in the body.
Palbociclib may stop the growth of tumor cells by blocking some of the enzymes needed for
cell growth. Drugs used in chemotherapy, such as letrozole or fulvestrant, work in different
ways to stop the growth of tumor cells, either by killing the cells, by stopping them from
dividing, or by stopping them from spreading. Giving palbociclib and letrozole or fulvestrant
may work better in treating patients with breast cancer. The trial will explore factors other
than chronologic age that can affect toxicity rates as identified using a cancer-specific
geriatric assessment.

Inclusion Criteria:

- Documentation of disease: estrogen receptor positive and/or progesterone receptor (PR)
positive, HER2 negative metastatic breast cancer; histologic confirmation is required

- Measurable disease or non-measurable disease

- Planning to begin palbociclib for metastatic disease; one prior line of endocrine
therapy and/or chemotherapy for metastatic disease is allowed; patients may begin or
have already begun endocrine therapy before they start palbociclib treatment, but no
more than two weeks prior to registration

- No prior therapy with a CDK inhibitor

- Resolution of all acute toxic effects of prior therapy or surgical procedures to CTCAE
grade =< 1 (except alopecia) or to baseline toxicities prior to previous therapy or
surgical procedures, prior to registration

- No untreated brain metastases; patients with treated brain metastases must have
completed treatment with steroids to be eligible

- No known interstitial lung disease

- No second malignancies other than non-melanoma skin cancers or cervical carcinoma in
situ; however, patients are not considered to have a "currently active" malignancy if
they have completed therapy and are free of disease for >= 3 years

- No active infection requiring treatment with antibiotics

- Patients must be able to swallow and retain oral medication

- Eastern Cooperative Oncology Group (ECOG) performance status 0, 1 or 2

- Patients must be able to read and comprehend English or Spanish

- Absolute neutrophil count (ANC) >= 1500/mm^3 (1.5 x 10^9/L)

- Platelet count >= 100,000/mm^3 (100 x 10^9/L)

- Creatinine clearance >= 30 ml/min calculated using the Cockcroft-Gault formula

- Total serum bilirubin =< 1.5 upper limit of normal (ULN) (< 3 ULN if Gilbert's
disease)

- Aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) =< 3 x ULN (=<
5.0 x ULN if liver metastases present)

- Alkaline phosphatase =< 2.5 x ULN (=< 5 x ULN if bone or liver metastases present)