Abexinostat (PCI-24781) is a novel, second-generation phenyl hydroxamic acid–based, orally bioavailable HDAC inhibitor that has previously been shown to have activity in vitro and in vivo against a broad array of cancers, including hematopoietic malignancies and bone and soft-tissue sarcomas. Abexinostat is a pan-HDAC inhibitor mostly targeting HDAC1 with Ki of 7 nM, modest potent to HDACs 2, 3, 6, and 10 and greater than 40-fold selectivity against HDAC8. Abexinostat exhibits potent antitumor activity against a variety of tumor cell lines with GI50 ranging from 0.15 uM to 3.09 uM. PCI-24781 also has an antiproliferative effect on HUVEC endothelial cells with GI50 of 0.43 uM. Abexinostat treatment causes dose-dependent accumulation of both acetylated histones and acetylated tubulin in HCT116 or DLD-1 cells, induces expression of p21, and leads to PARP cleavage and accumulation of the γH2AX. It has also shown good tolerability and activity in Phase I and II clinical trials against lymphoma, as well as against solid tumors in Phase-I trials. Additionally, it acts as a potent radiosensitizing agent and is synergistic with cytotoxic chemotherapy, such as doxorubicin in preclinical models.