A Phase 2, Randomized, Double-Blind, Multicenter Study of Exemestane With and Without SNDX-275 in Postmenopausal Women with Locally Recurrent or Metastatic Estrogen Receptor-Positive Breast Cancer, Progressing on Treatment with a Non-Steroidal Aromatase Inhibitor

INTERVENTION: Product Name: entinostat Product Code: SNDX‐275 Pharmaceutical Form: Coated tablet INN or Proposed INN: entinostat Current Sponsor code: SNDX‐275 Other descriptive name: MS‐275 Concentration unit: mg milligram(s) Concentration type: equal Concentration number: 1‐ INN or Proposed INN: entinostat Current Sponsor code: SNDX‐275 Other descriptive name: MS‐275 Concentration unit: mg milligram(s) Concentration type: equal Concentration number: 5‐ Pharmaceutical form of the placebo: Coated tablet Route of administration of the placebo: Oral use CONDITION: ER+ Breast Cancer ; MedDRA version: 9.1 Level: LLT Classification code 10006187 Term: Breast cancer PRIMARY OUTCOME: Main Objective: To compare the efficacy of exemestane alone with exemestane plus entinostat, as determined by the duration of progression free survival (PFS) measured from the date of randomization Primary end point(s): Progression‐free survival (PFS) Secondary Objective: • Efficacy: To compare objective response rate (ORR) and clinical benefit response (CBR); ; • Safety: To evaluate the safety and tolerability of entinostat in combination with exemestane as measured by adverse events and laboratory safety parameters ; INCLUSION CRITERIA: 1. Postmenopausal female patients 2. Histologically or cytologically confirmed ER+ breast cancer at initial diagnosis, with current disease relapse or progression that occurred while receiving a non‐steroidal aromatase inhibitor (anastrozole “Arimidex®” or letrozole “Femara®”), meeting either of the following criteria a. Relapse on non‐steroidal AI given as adjuvant AI after at least 12 months treatment with the AI OR b. Progressive disease on prior non‐steroidal AI after at least 3 months of treatment in the advanced disease setting 3. Evidence of metastatic disease based on imaging studies (bone scan, CT, MRI ): a. Patients with bone only metastases are eligible provided there is a positive bone scan confirmed with MRI or PET‐CT, according to local standard of care, within 4 weeks prior to study entry. OR b At least 1 measurable lesion = 20 mm by conventional techniques or = 10 mm by spiral CT scan wi