VR23 is a potent proteasome inhibitor. Data shows IC50 =1 nM for trypsin-like proteasomes, IC50=50-100 nM for chymotrypsin-like proteasomes, and IC50=3 μM for caspase-like proteasomes.

Chymotrypsin-like proteasome:50-100 nM, Caspase-like proteasomes:3 μM, Trypsin like proteasomes:1 nM

In HeLa cells, VR23 induces ubiquitinated proteins accumulation. In RPMI 8226 and KAS 6 cells, VR23 inhibits cell growth with IC50 of 2.94 and 1.46 μM, respectively. VR23 is also equally effective on both bortezomib (BTZ)-sensitive and -resistant RPMI 8226 and ANBL6 cells cells. When used in combination of bortezomib in the cells above, VR23 shows synergistic effects on cell growth inhibition. In addition, VR23 selectively induces cancer cell apoptosis by causing the accumulation of ubiquitinated cyclin E. [1]

In ATH490 athymic mice engrafted with MDA-MB-231 metastatic breast cancer cells, VR23 (30 mg/kg, i.p.) shows effective antitumor and antiangiogenic activities. VR23 also reduces adverse effects caused by paclitaxel in mice. [1]

Proteasome assay: Exponentially growing cells on a 96-well clustered plate are treated with different concentrations of drugs or left untreated (control) for 6 hours. Proteasomes extracted with 0.5% NP40 buffer are mixed with equal amounts of samples in 100 μL total volume, and then incubated with 25 μmol/L of fluorogenic substrates (LRR- specific for trypsin-like activity, LLE-specific for caspase-like activity, and SUVY-specific for chymotrypsin-like activity) in black-bottom 96-well plates at 37°C. Fluorescence is monitored every 5 minutes at the wavelength of 360 nm (excitation) and 480 nm (emission).