The Phase I study will illustrate the safety profile of the combination and identify the appropriate dosing regimen to be used in the Phase II portion.
Translational studies will evaluate the mechanism of action for the combination in tumor cells while identifying biomarkers for patient selection in subsequent studies.
Syndax president and chief executive officer Joanna Horobin said: "With our recently reported positive data in ENCORE 301 targeting ER+ breast cancer and the NCI sponsored study NCT01234532 combining entinostat with Arimidex in ER-, PR-, HER2- triple negative breast cancer we are pleased to be in a position to provide proof-of-concept clinical data across all segments of breast cancer."