Abstract C34: Senescence as a mechanism of resistance to the Aurora kinase and angiokinase inhibitor, ENMD-2076, in p53 mutated triple-negative breast cancer (TNBC) models
Authors: Ionkina Anastasia A. , Eckhardt S. Gail , Pitts Todd M. , Sartorius Carol , Kabos Peter , Kim Jiyhe , Tan Aik Choon , Tentler John J. , Diamond Jennifer R. 
Published: May 2016

Abstract
Background: Despite advances in targeted therapies for cancer, TNBC remains an aggressive breast cancer subtype with limited treatment options. Mutations in p53 are common in TNBC, however, the exact contribution of individual p53 mutations to response to therapy and mechanisms of acquired resistance are unknown. The purpose of this study was to characterize the activity of ENMD-2076, a multi-target Aurora kinase A and angiokinase inhibitor, against p53 mutated TNBC patient-derived tumor xenografts (PDX) and to identify differences in molecular pathways determining cellular fate in sensitive and resistant models.
Methods: TNBC PDX models harboring different p53 mutations were used for ENMD-2076 treatment studies. Athymic nude mice were injected with tumor tissue and tumor volumes were measured twice a week. When the mean tumor volumes reached 150 mm3, mice were randomized and treated with vehicle control or ENMD-2076 200 mg/kg by oral gavage daily. A subset of animals were sacrificed at Day 4, 30, and at the time of acquired resistance for correlative tissue testing which included: immunofluorescence (IF) for p53, p73, BCL2, BAX, p16, phospho Aurora A (pAA) and phospho histone H3 (pHH3); immunohistochemistry (IHC) for cleaved caspase 3 (CC3) and Ki67; H&E and staining for senescence associated beta-galactosidase (SA -βgal) activity. Tumor growth inhibition (TGI) was calculated at Day 30 and sensitive models were treated until resistance when additional correlative tissue samples were obtained.
Results: ENMD-2076 had significant anti-tumor activity against the CU_002 and CU_005 TNBC PDX models (TGI 71.3%, p value &lt;0.0001; TGI 66.1% p value &lt; 0.0002, respectfully). The CU_004 TNBC PDX model was intrinsically resistant to ENMD-2076 treatment (TGI 37%, p value 0.07). In the two sensitive PDX models, we observed an increase in p53, p73, BAX and the apoptotic marker CC3. This was accompanied by a decrease in the anti-apoptotic protein BCL2 and the proliferation marker Ki67 following treatment at Day 30. Consistent with Aurora kinase A inhibition, we detected an increase in pAA and a decrease in pHH3 expression in both sensitive and resistant PDTX models following treatment at Day 4 and Day 30. At the time of acquired resistance, defined by at least doubling of tumor volumes from the maximal response, we observed loss of p73, p53, and BAX expression and an increase in p16 staining and SA β-gal activity consistent with senescence. These findings were also observed following treatment with ENMD-2076 in the intrinsically resistant CU_004 model.
Conclusions: ENMD-2076 has pro-apoptotic anti-cancer activity in a subset of p53 mutated TNBC PDX models. Sensitivity was associated with the induction of p73, which may mediate the response in the absence of functional p53. Intrinsic and acquired resistance to ENMD-2076 in TNBC PTX models was associated with loss of p73 expression and an increase in markers associated with senescence, including p16 expression and SA β-gal activity. These data support the role of senescence as a potential mechanism of resistance to Aurora kinase inhibitors in p53 mutated TNBC and support the continued development of combination therapies including with inhibitors of pathways that mediate senescence.
Citation Format: Anastasia A. Ionkina, S. Gail Eckhardt, Todd M. Pitts, Carol Sartorius, Peter Kabos, Jiyhe Kim, Aik Choon Tan, John J. Tentler, Jennifer R. Diamond. Senescence as a mechanism of resistance to the Aurora kinase and angiokinase inhibitor, ENMD-2076, in p53 mutated triple-negative breast cancer (TNBC) models. [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 C34.