Drug-Drug Interaction Study of Entinostat and Exemestane in Postmenopausal Women With ER+ Breast Cancer

SNDX-275-0130 is a Phase 1, two-cohort, open-label, drug-drug interaction study of entinostat
and exemestane. All patients will be enrolled into one of two cohorts to receive either
entinostat monotherapy followed by entinostat plus exemestane (Cohort 1), or exemestane
monotherapy followed by exemestane plus entinostat (Cohort 2).

The cohorts will enroll sequentially, meaning that Cohort 1 will enroll prior to Cohort 2.
Cohort 1 will evaluate exemestane's effect on the PK of entinostat. Cohort 2 will evaluate
entinostat's effect on the PK of exemestane. In both Cohorts, each treatment cycle will be 28
days. Patients will participate in only one cohort.

All patients will be assessed at Screening and at specified times during the conduct of the
study using standard clinical and laboratory assessment. Patients will also be assessed for
tumor response per standard of care after the Screening Period. Response to treatment will be
assessed by computerized tomography (CT), magnetic resonance imaging (MRI) and bone scans as
appropriate. Patients will continue receiving their appropriate cycles of study treatment
until tumor progression or adverse events (AEs) occur which necessitate discontinuing therapy
as determined by the Investigator.

Inclusion Criteria:

- Postmenopausal female patients

- Histologically or cytologically confirmed ER+ breast cancer at initial diagnosis and
have locally recurrent or metastatic disease that has progressed to where the patient
is a candidate to receive exemestane as determined by the Investigator

- Patients receiving palliative radiation at the non-target lesions must be clinically
stable prior to receiving the first dose of study treatment

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

- Patient must have acceptable, applicable laboratory requirements

- Patients may have a history of brain metastasis provided certain protocol criteria are
met

- Able to understand and give written informed consent and comply with study procedures

Exclusion Criteria:

- Rapidly progressive or life-threatening metastases

- Inability to swallow oral medications or gastrointestinal (GI) malabsorption syndromes

- History of significant GI surgery as determined by Investigator

- A medical condition that precludes adequate study treatment or increases patient risk

- Currently enrolled in (or completed within 30 days prior to study drug administration)
another investigational drug study