Study to Evaluate Exemestane With and Without Entinostat (SNDX-275) in Treatment of Postmenopausal Women With Advanced Breast Cancer

Inclusion Criteria:

Postmenopausal female patients
Histologically or cytologically confirmed estrogen receptor positive (ER+) breast cancer
Relapsed or progressed on prior treatment with aromatase inhibitor (AI)
Metastatic disease must be measurable
Patients receiving palliative radiation at the non-target lesions must have a 2 week wash out period following completion of the treatment prior to enrollment
Patient may have had one prior chemotherapy as part of first line therapy as long as it was received before initiation of prior AI
Eastern Cooperative Oncology Group (ECOG) performance status: 0 to 1
Laboratory parameters: a)Hemoglobin ≥ 9.0 g/dL; platelets ≥ 100.0 x 10^9/L; Absolute Neutrophil Count (ANC ≥) 1.5 x 10^9/L without the use of hematopoietic growth factors b)Creatinine less than 2.5 times the upper limit of normal for the institution c)Aspartate transaminase (AST) and alanine transaminase (ALT) less than 2.5 times the upper limit of normal for the institution
Able to understand and give written informed consent and comply with study procedures

Exclusion Criteria:

Relapse on treatment with non-steroidal AI after less than 12 months for patients in the adjuvant setting
Progressive disease after less than 3 months treatment with most recent AI for patients with metastatic disease
Rapidly progressive, life-threatening metastases
Any palliative radiotherapy to the measurable lesion
Previous treatment with SNDX-275 or any other histone deacetylase (HDAC) inhibitor including valproic acid
Allergy to benzamides or inactive components of the study drug
A history of allergies to any active or inactive ingredients of exemestane
Any concomitant medical condition that precludes adequate study treatment compliance
Patient is currently enrolled in (or completed within 30 days before study drug administration) another investigational drug study
Patient is currently receiving treatment with valproic acid, Zolinza (vorinostat) or any other HDAC inhibitor or deoxyribonucleic acid (DNA) methyltransferase inhibitor or any systemic anticancer treatment (with the exception of Lupron)