Possibly resulting in an improved selectivity profile. The NAE inhibitor MLN4924 was recently reported to be effective against both solidand hematologicalhuman cancer cells. We have previously employed high-throughput virtual screening to identify 6,699-biapigenin as only the second inhibitor of NEDD8-activating enzyme from a natural product and natural product-like database. While transition metal complexes have been widely utilized for the treatment of cancer, their SCH772984 activity against NEDD8-activating enzyme has not been explored. Inspired by the above findings as well as pioneering works from the Meggers’s group on the design of structurally rigid octahedral rutheniumand iridiumcomplexes as shape-complementary inhibitors of protein kinases, we sought to investigate the biological effects of a series of cyclometallated rhodium complexes on the NEDD8 pathway. Cyclometallated rhodium complexes containing the dipyridophenazine dipyridophenazinescaffold were chosen because of the following reasons: 1) the rhodium complex adopts an octahedral geometry rather than a square planar or tetrahedral symmetry, thus allowing much larger structural complexity for potential use in drug design; 2) the octahedral geometry of the rhodium complex provides a globular and rigid scaffold with limited conformational freedoms of the coligands that may interact with the previously inaccessible regions of chemical space in NAE; 3) the synthetic route for 1 is modular and convenient, thus allowing structural modification without the need for lengthy synthetic protocols; and 4) the extended aromatic dppz ligand structurally resembles the planar nature of NAE inhibitor 6,699-biapigenin, potentially functioning as the recognition arm for NAE. We report herein the synthesis and characterization of the racemic mixture of rhodium complex+and its analogues. Complex 1 was found to inhibit NAE activity in vitro and in cellulo. We then investigated the structureactivity relationship of the Rh complexes against NAE activity in vitro. Furthermore, complex 1 inhibited downstream CRLregulated substrate degradation and NF-kB signaling in cellulo, and was also found to exhibit prominent anti-proliferative activity against a human cancer cell line. Molecular modeling analysis revealed that 1 occupied the same binding pocket as MLN4924, the most potent NAE inhibitor to date. NAE promotes ubiquitination and the subsequent degradation of a BKM120 subset of proteins regulated by CRLs such as IkBaand p27. The IkBa protein plays a central role in repressing the activity of the transcription factor NF-kB, which is involved in important cellular processes including the immune response, programmed cell death, as well as cancer initiation and progression. The inhibition of NAE is therefore a potential approach to block the degradation of IkBa thus preventing NF-kB activation. On the other hand, the p27 is a cell cycle regulator which controls cell cycle progression during the G1 phase. The loss of function in p27 can lead to uncontrolled cell proliferation and the development of cancer. Inhibition of NAE activity in Caco-2 cells by 1 should be expected to result in the down-regulation of the CRL activity thus leading to the accumulation of CRL substrates. To examine this, we first investigated the ability of 1 to inhibit NAE-regulated IkBa degradation. Caco-2 cells were stimulated with TNF-a to induce IkBa protein degradation, which was monitored using Western blot analysis. Encouragingly, we observed that the induction of IkBa protein degradation by TNF-a was blocked by 1 in a dose-dependent manner, with potency comparable to the control compound MLN4924 at 2.5 mM.