Abstract 4727: Antitumor activity of the MDM2 antagonist, RG7112

The p53 tumor suppressor is a transcription factor which plays a central role in prevention of tumor development by inducing cell cycle arrest or apoptosis in response to diverse stresses. In unstressed cells, p53 level is tightly controlled by MDM2 which binds p53 and negatively regulates its activity and stability. MDM2 is overproduced in many human cancers as a mechanism to impair p53 function. Antagonists of p53-MDM2 interaction could reactivate p53 and may offer a novel approach to cancer therapy. The first potent and selective small-molecule inhibitors of p53-MDM2 binding, the Nutlins, provided preclinical proof-of-concept for MDM2 antagonists as a new therapeutic option for patients with tumors expressing wild-type p53. RG7112 is a member of the Nutlin family of MDM2 antagonists with improved potency, selectivity and pharmacological properties which is currently in Ph1 clinical evaluation. Here, we describe its biological activity in models of human cancer in vitro and in vivo. RG7112 binds to MDM2 with high affinity in vitro (KD ∼11 nM) and effectively blocks its interaction with p53. Crystal structures of RG7112-MDM2 complexes revealed that this imidazoline molecule interact with the p53-binding pocket of MDM2 by projecting functional groups that mimic critical amino acid residues from the p53 molecule. Treatment of cancer cells expressing wild-type p53 with RG7112 stabilized p53 and activated the p53 pathway as revealed by the induction of multiple p53-regulated genes. This led to cell cycle arrest in G1 and G2 phases followed by apoptosis. It showed potent antitumor activity in vitro against a panel of solid tumor and leukemia cells expressing wild-type p53 with EC50 in the 0.2 - 2.2 µM range. The overall selectivity between the panels of seven mutant and fifteen wild-type p53 lines, expressed as fold difference in the average EC 50 values was 14-fold. As previously shown with nutlin-3, RG7112 demonstrated variable apoptotic activity in a panel of solid tumor cells with the best apoptotic response in cells with mdm2 gene amplification. Interestingly, inhibition of caspase activity did not change the kinetics of p53-induced cell death. Oral administration of RG7112 to nude mice bearing established human tumor xenografts caused dose-dependent tumor inhibition and tumor regression at non-toxic concentration. The in vivo mode of action was confirmed by pharmacodynamic markers, indicating p53 pathway activation and induction of growth arrest and apoptosis in tumor tissues. Our results showed that RG7112 is a potent p53 activator and antitumor agent in vitro and in vivo that may have therapeutic utility in the treatment of solid tumors and leukemia with wild-type p53. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4727. doi:1538-7445.AM2012-4727