A phase 1 study conducted on pracinostat (SB939) 4 (Fig. 3) in 2011 reported that it is tolerable in patients with advanced solid tumors and shows side effects consistent with those of other HDAC inhibitors.53 Another phase I study in 2013 showed that it is well tolerated in children with refractory solid tumors.54 A phase II study of pracinostat conducted in 2015 on patients with castration resistant prostate cancer (CRPC) revealed no sufficient activity to warrant further study, however, it was well tolerated and showed a decline in circulating tumor cells (CTC).55 Here, HDAC9 is linked to neoplasm.