Palbociclib / Letrozole or Fulvestrant in African American Women With HR+ HER2- Breast Cancer

The study is designed to assess the rate of completion of planned oncology therapy in the
absence of a hematological event defined as episodes of febrile neutropenia and treatment
discontinuation due to neutropenia. A completion rate of 80% is considered of clinical
relevance as to benefit breast cancer patients who are at a higher risk of having ethnic
neutropenia where as a completion rate of 60% is considered poor and to justify additional
safety studies. A two stage design with a total of 35 patients is used to test if the
completion rate is at least 80% versus if it is below 60% with 80% power at a significance
level of 5%.

An exact confidence interval of the completion rate will be calculated. Investigators
estimate there will be no more than a 10% rate of febrile neutropenia. Due to the small
sample size, the analysis of secondary endpoints will be descriptive and will not include
specific hypothesis testing.

Inclusion Criteria:

1. Self-identified Black, African or African American women of ≥ 18 years of age with
proven diagnosis of advanced adenocarcinoma of the breast (locoregionally recurrent or
metastatic disease)

2. ER-positive and/or PgR-positive tumor based on local laboratory results

3. HER2-negative breast cancer based on local laboratory results (test to be used as per
local practice)

4. Patients must be appropriate candidates for letrozole or fulvestrant therapy

5. Eastern Cooperative Oncology Group (ECOG) performance status 0-2

6. Adequate bone marrow function:

- Absolute Neutrophil Count (ANC) ≥ 1,000/mm3 (1.0 x 109/L);

- Platelets ≥100,000/mm3 (100 x 109/L);

- Hemoglobin ≥9 g/dL (90 g/L).

Exclusion Criteria:

1. Current use of food or drugs known to be potent inhibitors or inducers of CYP3A4

2. Active uncontrolled or symptomatic brain metastases. Previously treated and clinically
stable, as per Investigator's judgment, brain metastases are permitted.

3. Previous CDK4/6 inhibitor