Abstract B5: Preclinical characterization of a novel orally-available MDM2 inhibitor DS-3032b: Anti-tumor profile and predictive biomarkers for sensitivity
Authors: Nakamaru Kenji , Seki Takahiko , Tazaki Koichi , Tse Archie 
Published: May 2016

Abstract
The p53 protein exerts its tumor suppressor function by inducing cell cycle arrest, apoptosis, or senescence in cells under stress conditions. Inactivation of p53 by somatic mutations is a frequent tumorigenic event. In human tumors that retain wildtype p53 protein, its activity is often inhibited by interaction with the negative regulator MDM2. DS-3032b, a novel, specific, small-molecule inhibitor of MDM2, disrupts the MDM2-p53 interaction in tumor cells and is being developed as a cancer therapeutic based on a p53 reactivation mechanism.
Cell-free study using recombinant MDM2 and p53 proteins demonstrated inhibition of MDM2-p53 binding by DS-3032b with an IC50 value of 5.57nM. Treatment with graded concentrations of DS-3032b resulted in a dose-dependent increase of p21 and PUMA mRNA levels in an osteosarcoma cell line (SJSA-1) harboring MDM2 gene-amplification. DS-3032b inhibited cell growth in p53 wildtype (GI50s: 0.043 - 0.276μM), but not p53 mutant or null cell lines (GI50s: &gt;10μM). Studies in athymic nude mice inoculated with SJSA-1 xenografts showed tumor regression following treatment with DS-3032b given once daily for 10 days at a dose of 50 mg/kg.
To identify biomarkers predictive of sensitivity to MDM2 inhibitors, we treated a panel of 240 molecularly-annotated cancer cell lines (OncoPanel, Eurofins Panlabs) with the prototypic MDM2 inhibitor DS-5272 structurally unrelated to DS-3032b. Cells were dichotomized as sensitive or resistant to DS-5272, based on IC50 cut-off value of &lt;1.5μM and &gt;4 μM, respectively. As expected, TP53 mutation was identified as a strong predictor for resistance. In addition, a 175-gene signature was identified as an indicator for sensitivity. This gene signature predicts for response to 3 different MDM2 inhibitors (DS-5272, nutlin3a, and DS-3032a (free form of DS-3032b)). A combination of TP53 genotype and gene signature showed further enrichment of sensitive cell lines compared to using either of the biomarker alone. In vivo studies using patients derived tumor xenografts have further validated the predictive value of the gene signature. Bioinformatic reduction of the complexity of the gene signature to a clinically applicable level is on-going.
Mapping of this signature to genomic data from 15,000+ clinical samples in the OncoMine database has revealed that only a subset of TP53 wildtype tumors contain high expression of the signature genes. The top-ranked genes in the signature were well-known p53-inducible genes, suggesting that the gene signature might be a surrogate for functional p53. Consistent with this postulate, we showed low signature gene expression and resistance to MDM2 inhibitors in cervical cancers in which p53 function is inactivated by the E6 viral oncoprotein. Cancer subtypes enriched for a low incidence of TP53 mutation and high signature gene expression are melanoma, lymphoma, myeloma, renal cell cancer, and leukemia.
In summary, DS-3032b is a specific and potent MDM2 inhibitor showing promising anti-tumor activity in vitro and in vivo. The sensitivity gene signature might serve as a pharmacologically defined surrogate for p53 functionality that is critical for predicting clinical benefits from DS-3032b treatment.
Citation Format: Kenji Nakamaru, Takahiko Seki, Koichi Tazaki, Archie Tse. Preclinical characterization of a novel orally-available MDM2 inhibitor DS-3032b: Anti-tumor profile and predictive biomarkers for sensitivity. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B5.