BACKGROUND: MLN4924 is an investigational first-in-class small molecule inhibitor of NEDD8-activating enzyme (NAE). NAE is an essential component of the NEDD8 conjugation pathway, controlling the activity of a subset of ubiquitin-proteasome system (UPS) E3 ligases, multiprotein complexes that transfer ubiquitin molecules to substrate proteins. PROCEDURES: MLN4924 was tested against the PPTP in vitro panel using 96-hour exposure time at concentrations ranging from 1.0 nM to 10 µM. It was tested in vivo at a dose of 100 mg/kg [66 mg/kg for the acute lymphoblastic leukemia (ALL) xenografts] administered orally twice daily × 5 days. Treatment duration was 3 weeks. RESULTS: The median relative IC(50) for MLN4924 against the PPTP cell lines was 143 nM, (range: 15-678 nM) with that for the Ewing panel being significantly lower (31 nM). MLN4924 induced significant differences in EFS distribution compared to control in 20 of 34 (59%) evaluable solid tumor xenografts. MLN4924 induced intermediate activity (EFS T/C values >2) in 9 of the 33 evaluable xenografts (27%), including 4 of 4 glioblastoma xenografts, 2 of 3 Wilm's tumor xenografts, 2 of 5 rhabdomyosarcoma xenografts, and 1 of 4 neuroblastoma xenografts. For the ALL panel, 5 of 8 evaluable xenografts showed intermediate activity for the EFS T/C measure. MLN4924 did not induce objective responses in the PPTP solid tumor or ALL panels. CONCLUSIONS: MLN4924 showed potent activity in vitro and in vivo showed tumor growth inhibitory activity against a subset of the PPTP solid tumor and ALL xenografts.