RA-9 is a potent and selective proteasome-associated deubiquitinating enzymes (DUBs) inhibitor with favorable toxicity profile and anticancer activity. RA-9 blocks ubiquitin-dependent protein degradation without impacting 20S proteasome proteolytic activity. RA-9 selectively induces onset of apoptosis in ovarian cancer cell lines and primary cultures derived from donors. RA-9 induces endoplasmic reticulum (ER)-stress responses in ovarian cancer cells.