NSC139021 (ERGi-USU) is a highly selective inhibitor for the growth of ERG-positive cancer cells with IC₅₀s ranging from 30 to 400 nM. IC50 & Target: IC50: 30 to 400 nM (ERG-positive cancer cells)^[1] In Vitro: NSC139021 selectively inhibits growth of ERGpositive cancer cell lines with minimal effect on normal prostate or endothelial cells or ERG-negative tumor cell lines. The IC₅₀ of NSC139021 for cell growth inhibition of responsive cell lines range between 30 nM to 400 nM. Combination of NSC139021 with enzalutamide shows additive effects in inhibiting growth of VCaP cells. A screen of kinases reveal that NSC139021 directly bound the ribosomal biogenesis regulator atypical kinase RIOK2 and induces ribosomal stress signature^[1]. In Vivo: NSC139021 treatment inhibits growth of ERG-positive VCaP tumor xenografts with no apparent toxicity. Significant (P<0.05, P<0.005) inhibition of tumor growth is noted at day 26 in treatment groups indicating 44% (100 mg/kg) and 65% (150 mg/kg) reduction of tumor burden. At 100 mg/kg and 150 mg/kg, no apparent toxicity including weight loss, lethargy, diarrhea, loss of appetite, respiratory distress, or overall drug related toxicity is observed. ^[1].