Entinostat is a histone deacetylase (HDAC) inhibitor displaying antiproliferative and antineoplastic effects. Entinostat induces cyclin-dependent kinase inhibitor 1A (p21/CIP1/WAF1), which slows cell growth, differentiation, and tumor development in vivo. Entinostat is potentially used as an antitumor drug in combination with other drugs like adriamycin, PARP inhibitors and Hsp90 inhibitors.