Potentiation of Carboplatin-Mediated DNA Damage by the Mdm2 Modulator Nutlin-3a in a Humanized Orthotopic Breast-to-Lung Metastatic Model

Triple-negative breast cancers (TNBC) are typically resistant to treatment, and strategies that build upon frontline therapy are needed. Targeting the murine double minute 2 (Mdm2) protein is an attractive approach, as Mdm2 levels are elevated in many therapy-refractive breast cancers. The Mdm2 protein-protein interaction inhibitor Nutlin-3a blocks the binding of Mdm2 to key signaling molecules such as p53 and p73α and can result in activation of cell death signaling pathways. In the present study, the therapeutic potential of carboplatin and Nutlin-3a to treat TNBC was investigated, as carboplatin is under evaluation in clinical trials for TNBC. In mutant p53 TMD231 TNBC cells, carboplatin and Nutlin-3a led to increased Mdm2 and was strongly synergistic in promoting cell death in vitro. Furthermore, sensitivity of TNBC cells to combination treatment was dependent on p73α. Following combination treatment, γH2AX increased and Mdm2 localized to a larger degree to chromatin compared with single-agent treatment, consistent with previous observations that Mdm2 binds to the Mre11/Rad50/Nbs1 complex associated with DNA and inhibits the DNA damage response. In vivo efficacy studies were conducted in the TMD231 orthotopic mammary fat pad model in NOD.Cg-Prkdc(scid)Il2rg(tm1Wjl)/SzJ (NSG) mice. Using an intermittent dosing schedule of combined carboplatin and Nutlin-3a, there was a significant reduction in primary tumor growth and lung metastases compared with vehicle and single-agent treatments. In addition, there was minimal toxicity to the bone marrow and normal tissues. These studies demonstrate that Mdm2 holds promise as a therapeutic target in combination with conventional therapy and may lead to new clinical therapies for TNBC.

Abstract 1680: Modulation of MDM2 in context of DNA damage enhances cell death in a metastatic breast-to-lung xenograft model

Metastatic breast cancer is highly refractive to current treatment strategies, and new multi-targeted treatments need to be elucidated. In metastatic disease, inhibiting key protein-protein interactions with the murine double minute 2 (MDM2) could be beneficial for developing new treatment modalities since this signaling pathway is a critical regulatory point in cancer progression. Inhibition of protein binding to the hydrophobic pocket of MDM2 by Nutlin-3a can activate pro-apoptotic proteins such p73 and E2F1 as well as decrease pro-angiogenic Hif-1α. Since the DNA damaging agent carboplatin is currently being studied in clinical trials of triple-negative breast cancers (TNBCs), our objective was to evaluate the effects of carboplatin and Nutlin-3a in combination in TNBC in a mutant p53 background. Using a TNBC cell line TMD231 derived from the MDA-MB-231 human breast cancer cell line, we performed combination studies using different ratios of carboplatin to Nutlin-3a in vitro to evaluate the range of carboplatin-mediated DNA damage required to obtain synergism with inhibition of MDM2 function. A fixed ratio of 1:1 carboplatin:Nutlin-3a was strongly synergistic with a combination index of <0.5. In cell proliferation assays there was increased sensitivity to the drugs when given in combination (p<0.05). TMD231 cells implanted into the mammary fat pad of NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice showed enhanced tumor growth, and metastasis was evident in the lungs. Dose-finding studies were performed to determine an optimal carboplatin dosing schema. NSG mice were randomized based on fluorescent imaging of E2-crimson expressing TMD231 cells allowing for a sensitive measurement of early tumor burden. Following Nutlin-3a and carboplatin combination treatment in vivo, there was a statistically significant reduction in tumor volume and lung metastases compared to vehicle and single drug treated mice (p<0.001). Following Kaplan-Meier analysis, the combination treated mice had a significant increase in survival, (54.3±1.5 days) compared to the vehicle (39.3±0.6 days) and each single drug (Nutlin-3a: 39±1 and carboplatin: 47.5±1.8 days) (p<0.001). While there was a decrease in bone-marrow cellularity, this did not lead to bone-marrow aplasia, and body weights recovered to normal levels within 7 days post-treatment. Pharmacodynamic studies are ongoing to further understand at the molecular level how the DNA damage response and repair is modulated by MDM2 resulting in a robust synergistic response. These studies will lead to a better understanding of how to potentiate DNA damage and may lead to new clinical therapies in the future for metastatic breast cancer.

Citation Format: Eva Tonsing-Carter, Harlan E. Shannon, Barbara J. Bailey, Anthony L. Sinn, Kacie M. Peterman, Lindsey D. Mayo, Karen E. Pollok. Modulation of MDM2 in context of DNA damage enhances cell death in a metastatic breast-to-lung xenograft model. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1680. doi:10.1158/1538-7445.AM2014-1680