Abstract C128: Small-molecule antagonists of MDM2 and MDMX.

The p53 tumor suppressor is controlled by MDM2 and MDMX that negatively modulate its activity. Both proteins possess p53-binding sites within their N-terminal domains and can inhibit the transcriptional activity of p53. However, only MDM2 can serve as E3 ubiquitin ligase for p53 and is thus responsible for its stability. MDMX does not have intrinsic ligase activity but can modulate the ligase activity of MDM2. Many human tumors overproduce MDM2 or MDMX to impair p53 function. Small-molecule MDM2 antagonists, the nutlins, interact specifically with the p53-binding pocket of MDM2 and can release p53 from negative control. Treatment of cancer cells expressing wild-type p53 with nutlins stabilizes p53 and activates the p53 pathway, leading to cell cycle arrest and apoptosis in vitro and in vivo. However, nutlins and other published MDM2 antagonists do not inhibit the p53-MDMX interaction and their effectiveness can be compromised in tumors overexpressing MDMX. We identify the first small molecules that potently block p53 interaction with both MDM2 and MDMX (in vitro IC50 of 17 nM and 24 nM, respectively) by inhibitor-driven homo- and/or hetero-dimerization of MDM2 and MDMX proteins. Structural studies revealed that these idole-hydantoin compounds bind into and occlude p53 pockets of MDM2 and/or MDMX by inducing the formation of dimeric protein complexes kept together by a dimeric small-molecule core. This novel mode of inhibiting protein-protein interactions effectively stabilized p53 and activated p53 signaling in cancer cells, leading to cell cycle arrest and apoptosis. MDM2/MDMX antagonist, RO-5963, restored p53 apoptotic activity in cultured osteosarcoma cells in the presence of high levels of MDMX. RO-5963 showed a significantly better apoptotic activity against MCF7 and other solid tumor cell lines with higher MDMX levels than the MDM2-specific inhibitor, nutlin-3a, suggesting that dual antagonists may offer a more effective therapeutic modality for MDMX-overexpressing cancers. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C128.