Inhibition of the MDM2-p53 interaction has been shown to produce an antitumor effect, especially in MDM2 amplified tumors. The isoindolinone scaffold has proved to be versatile for the discovery of MDM2-p53 antagonists. Optimization of previously reported inhibitors, for example, NU8231 (7) and NU8165 (49), was guided by MDM2 NMR titrations, which indicated key areas of the binding interaction to be explored. Variation of the 2-N-benzyl and 3-alkoxy substituents resulted in the identification of 3-(4-chlorophenyl)-3-((1-(hydroxymethyl)cyclopropyl)methoxy)-2-(4-nitrobenzyl)isoindolin-1-one (74) as a potent MDM2-p53 inhibitor (IC(50) = 0.23 ± 0.01 μM). Resolution of the enantiomers of 74 showed that potent MDM2-p53 activity primarily resided with the (+)-R-enantiomer (74a; IC(50) = 0.17 ± 0.02 μM). The cellular activity of key compounds has been examined in cell lines with defined p53 and MDM2 status. Compound 74a activates p53, MDM2, and p21 transcription in MDM2 amplified cells and shows moderate selectivity for wild-type p53 cell lines in growth inhibition assays.

Schizosaccharomyces pombe Rpn10 (SpRpn10) is a proteasomal ubiquitin (Ub) receptor located within the 19 S regulatory particle where it binds to subunits of both the base and lid subparticles. We have solved the structure of full-length SpRpn10 by determining the crystal structure of the von Willebrand factor type A domain and characterizing the full-length protein by NMR. We demonstrate that the single Ub-interacting motif (UIM) of SpRpn10 forms a 1:1 complex with Lys48-linked diUb, which it binds selectively over monoUb and Lys63-linked diUb. We further show that the SpRpn10 UIM binds to SpRpn12, a subunit of the lid subparticle, with an affinity comparable with Lys48-linked diUb. This is the first observation of a UIM binding other than a Ub fold and suggests that SpRpn12 could modulate the activity of SpRpn10 as a proteasomal Ub receptor.

The p53-binding site of MDM2 holds great promise as a target for therapeutic intervention in MDM2-amplified p53 wild-type forms of cancer. Despite the extensive validation of this strategy, there are relatively few crystallographically determined co-complex structures for small-molecular inhibitors of the MDM2-p53 interaction available in the PDB. Here, a surface-entropy reduction mutant of the N-terminal domain of MDM2 that has been designed to enhance crystallogenesis is presented. This mutant has been validated by comparative ligand-binding studies using differential scanning fluorimetry and fluorescence polarization anisotropy and by cocrystallization with a peptide derived from p53. Using this mutant, the cocrystal structure of MDM2 with the benchmark inhibitor Nutlin-3a has been determined, revealing subtle differences from the previously described co-complex of MDM2 with Nutlin-2.

In this study we present a method for defining the binding modes of a set of structurally related isoindolinone inhibitors of the MDM2-p53 interaction. This approach derives the location and orientation of isoindolinone binding, based on an analysis of the patterns of magnitude and direction of chemical shift perturbations for a series of inhibitors of the MDM2-p53 interaction. The MDM2-p53 complex is an attractive target for therapeutic intervention in cancer cells with intact tumor suppressor p53, as it offers the possibility of releasing p53 by blocking the MDM2-p53 binding site with a small molecule antagonist to promote apoptosis. Isoindolinones are a novel class of MDM2-antagonists of moderate affinity, which still require the development of more potent candidates for clinical applications. As the applicability of conventional structural methods to this system is limited by a number of fundamental factors, the exploitation of the information contained in chemical shift perturbations has offered a useful route to obtaining structural information to guide the development of more potent compounds. For a set of 12 structurally related isoindolinones, the data suggests 4 different orientations of binding, caused by subtle changes in the chemical structure of the inhibitors.