Tripterin (Celastrol) is a proteasome inhibitor which potently and preferentially inhibits the chymotrypsin-like activity of a purified 20S proteasome with IC₅₀ of 2.5 μM. IC50 & Target: IC50: 2.5 μM (20S proteasome)^[1] In Vitro: Tripterin (Celastrol) significantly inhibits the proteasomal chymotrypsin activity in PC-3 cells in a concentration-dependent manner; at 2.5 μM it reaches ~55% inhibition, comparable to its potency to a purified 20S proteasome (IC₅₀=2.5 μM). Furthermore, increased levels of IκB-α, Bax, and p27 are observed, three well known target proteins of the proteasome in PC-3 cells treated with Celastrol^[1]. In Vivo: Treatment of PC-3 tumor-bearing nude mice with Tripterin (Celastrol) (1-3 mg/kg/d, i.p., 1-31 days) results in significant inhibition (65-93%) of the tumor growth^[1]. Following treatment with 3 and 6 mg/kg Tripterin (Celastrol), the levels of malondialdehyde (MDA) are significantly decreased by 35.2 and 36.7% (P<0.05), respectively. Treatment with 3 and 6 mg/kg Tripterin (Celastrol) markedly restores the GSH level (P<0.05) to almost normal levels^[2].