Inclusion Criteria:

- Have a diagnosis of HR+, HER2- breast cancer.

- Relapsed or progressed following endocrine therapy.

- Have received prior treatment with at least 2 chemotherapy regimens, of which at
least 1 but no more than 2 have been administered in the metastatic setting.

- Have the presence of measureable disease as defined by the Response Evaluation
Criteria in Solid Tumors (RECIST 1.1).

- Have a performance status 1 on the Eastern Cooperative Oncology Group (ECOG) scale.

- Have discontinued previous therapies for cancer (including specifically, aromatase
inhibitors, anti-estrogens, chemotherapy, radiotherapy, and immunotherapy) for at
least 21 days for myelosuppressive agents or 14 days for nonmyelosuppressive agents
prior to receiving study drug, and recovered from the acute effects of therapy (until
the toxicity resolves to either baseline or at least Grade 1) except for residual
alopecia or peripheral neuropathy.

- Have adequate organ function.

- Have negative serum pregnancy test within 7 days prior to randomization and agree to
use medically approved precautions to prevent pregnancy during the study and for 12
weeks following last dose of study treatment.

- Are able to swallow oral medication.

Exclusion Criteria:

- Have clinical evidence or history of central nervous system metastasis.

- Have received treatment with a drug that has not received regulatory approval for any
indication within 14 or 21 days prior to randomization of study drug for a
nonmyelosuppressive or myelosuppressive agent, respectively.

- Have had major surgery within 14 days prior to randomization of study drug to allow
for post-operative healing of the surgical wound and site(s).

- Have a personal history of any of the following conditions: presyncope or syncope of
either unexplained or cardiovascular etiology, ventricular tachycardia, ventricular
fibrillation, or sudden cardiac arrest.

- Have active bacterial or fungal infection, or detectable viral infection.

- Have received treatment with a prior cyclin-dependent kinase (CDK4) and CDK 6
inhibitor.

- Have a preexisting chronic condition resulting in persistent diarrhea.

- Have a history of any other cancer (except nonmelanoma skin cancer or carcinoma
in-situ of the cervix or breast), unless in complete remission with no therapy for a
minimum of 3 years.

Minimum Eligible Age: 18 Years

Maximum Eligible Age: N/A

Eligible Gender: Female