The p53 tumor suppressor is a potent transcription factor that plays a key role in the regulation of cellular responses to stress. It is controlled by its negative regulator MDM2, which binds directly to p53 and inhibits its transcriptional activity. MDM2 also targets p53 for degradation by the proteasome. Many tumors produce high levels of MDM2, thereby impairing p53 function. Restoration of p53 activity by inhibiting the p53-MDM2 interaction may represent a novel approach to cancer treatment. RG7112 (2g) is the first clinical small-molecule MDM2 inhibitor designed to occupy the p53-binding pocket of MDM2. In cancer cells expressing wild-type p53, RG7112 stabilizes p53 and activates the p53 pathway, leading to cell cycle arrest, apoptosis, and inhibition or regression of human tumor xenografts. KEYWORDS: MDM2, p53, RG7112, protein−protein interaction, cancer p53 is a potent tumor suppressor that activates the transcription of a subset of genes controlling cell-cycle progression and apoptosis.1−3 Dysregulation of the p53 pathway, including mutation or deletion of the p53 gene and changes in downstream signaling molecules, is the most frequent alteration in human cancers.4 MDM2 is a negative regulator of p53 that binds the transactivation domain of p53 and inhibits its ability to activate transcription.5−8 MDM2 is also an E3 ubiquitin ligase that targets p53 for proteosomal degradation.9 In a variety of solid tumors and hematologic malignancies, MDM2 overexpression is one of the mechanisms by which the wildtype p53 function is impaired.10 Given the central role of MDM2 in regulating p53 activity and stability, developing small-molecule inhibitors of MDM2 could offer a novel approach to treating cancers. 11,12The crystal structure of a p53-derived peptide bound to the p53 binding domain of MDM2 revealed the existence of a deep hydrophobic clef on the surface of the MDM2 molecule. 13Three amino acid residues from the p53 peptide (Phe19, Trp23, and Leu26) play critical roles in the binding between the two proteins by projecting hydrophobic side-chains deep into the cavity of the MDM2 molecule. These structural features of the p53-MDM2 complex suggested the likelihood of identifying small-molecule inhibitors that can successfully block the interaction between the two proteins. Compounds with the ability to inhibit the binding between p53 and MDM2 have been reported. 14−17 We previously reported the discovery of a series of 4,5-dihydroimidazolines called Nutlins. These compounds, exemplified by compound 1 (Figure 1), were discovered through screening and subsequent medicinal chemistry optimization. 18 Compound 1, also known as Nutlin-3a, has become a tool of choice to study p53 biology and therapeutic applications.19 Although these early lead compounds have shown good cellular activity and provided the mechanistic proof-of-concept for inhibiting p53-MDM2 interaction for cancer therapy, their pharmacological properties were suboptimal for clinical development. Here, we describe

MDM2 negatively regulates p53 stability and many human tumors overproduce MDM2 as a mechanism to restrict p53 function. Thus, inhibitors of p53-MDM2 binding that can reactivate p53 in cancer cells may offer an effective approach for cancer therapy. RG7112 is a potent and selective member of the nutlin family of MDM2 antagonists currently in phase I clinical studies. RG7112 binds MDM2 with high affinity (K D $ 11 nmol/L), blocking its interactions with p53 in vitro. A crystal structure of the RG7112-MDM2 complex revealed that the small molecule binds in the p53 pocket of MDM2, mimicking the interactions of critical p53 amino acid residues. Treatment of cancer cells expressing wild-type p53 with RG7112 activated the p53 pathway, leading to cell-cycle arrest and apoptosis. RG7112 showed potent antitumor activity against a panel of solid tumor cell lines. However, its apoptotic activity varied widely with the best response observed in osteosarcoma 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 human xenograft-bearing mice at nontoxic concentrations caused dosedependent changes in proliferation/apoptosis biomarkers as well as tumor inhibition and regression. Notably, RG7112 was highly synergistic with androgen deprivation in LNCaP xenograft tumors. Our findings offer a preclinical proof-of-concept that RG7112 is effective in treatment of solid tumors expressing wild-type p53.

No abstract

No abstract

A carbon-shift analysis of labdanic diterpenes of the manool, agathic acid, and sciadin types is presented. The C-12 and C-20 configurations of sciadin have been determined and the structure of potamogetonin has been revised. 'Dedicated to Professor Edgar Lederer on the occasion of his seventy-fifth birthday. 2Carbon-13 Nuclear Magnetic Resonance Spectroscopy of Naturally Occurring Substances, LXXXI.