OBP-801 (spiruchostatin A) is an inhibitor of histone deacetylase (HDAC) enzymes, with potential antineoplastic activity. OBP-801 was originally identified as an enhancer of PAI-1 gene expression and was established as a new HDAC inhibitor by a p21 promoter reporter screen. Upon administration, OBP-801 inhibits the activity of HDACs; this results in an accumulation of highly acetylated chromatin histones, the induction of chromatin remodeling and an altered pattern of gene expression. This leads to selective transcription of tumor suppressor genes, tumor suppressor protein-mediated inhibition of tumor cell division and induction of tumor cell apoptosis. This may inhibit proliferation of susceptible tumor cells. HDAC, which is upregulated in many tumor cell types, deacetylates chromatin histone proteins. OBP‑801 induces M‑phase arrest and apoptosis in rhabdomyosarcoma cells. OBP‑801 is expected to show anticancer effect by promoting an expression of tumor suppressor genes in cancer cell and inducing apoptotic and autophagic cell death. The results of pre-clinical studies on OBP-801 indicated the most potent HDAC inhibitory activity as compared to other HDAC inhibitors including and Zolinza® and Istodax®, and its efficacy on a wide range of cancers is expected. Furthermore, Oncolys has been exploring the potential ophthalmic use of OBP-801 in collaboration with Kyoto Prefectural University of Medicine. OBP-801 has been used in trials studying the treatment of solid tumor.