UK-101 is a potent and selective immunoproteasome β1i (LMP2) inhibitor with an IC50 value of 104 nM, displays 144- and 10-fold selectivity over β1c (IC50=15 μM) and β5 subunit (IC50=1 μM), respectivey. UK-101 induces cell apoptosis and can be used for the study of prostate cancer. IC50 & Target: IC50: 104 nM (LMP2) IC50: 15 μM (immunoproteasome β1c) IC50: 1 μM (immunoproteasome β5)In Vitro: UK-101 (2-8 μM; 24 hours) induces cell cycle arrest and increases the number of the PC-3 cells arrest in G1 phase of the cell cycle. UK-101 (2-8 μM; 24 hours) induces cell apoptosis, shows a minimal increase in late apoptosis, but has no significant increase in early apoptosis. UK-101 (1-8 μM; 24 hours) induces cells accumulation in the G1 phase of the cell cycle, it increases p27 accumulation and significantly increases PARP cleavage as a dose-dependent manner. In Vivo: UK-101 (intraperitoneal injection; 1-3 mg/kg; twice a week; 3 weeks) decreases tumor volume as a dose-dependent manner, it significantly decreases tumor volume at a dose of 3 mg/kg. Additionally, UK-101-treated mice is suffering less systemic toxicity and the weights of mice treated with UK-101 remains steady over the 3-week treatment period.