Background: Women with “triple-negative breast cancer” (TNBC), are currently treated with chemotherapy, and have a very poor prognosis. TNBC tumors also frequently have p53 and BRCA1 gene mutations. Dysregulation of PI3K-mTOR pathway has been commonly associated with ER-negative breast cancers with poor prognosis. The mTOR inhibitor everolimus is used to treat ER-positive tumors that have become resistant to anti-estrogen therapy. We hypothesized that targeting mTOR may prevent development of ER-negative and BRCA1-mutant breast cancers and asked whether the mTOR inhibitor everolimus exhibited tumor preventive efficacy in several mouse models of breast cancer.

Methods: p53-null mammary gland donor mice were transplanted into cleared fat pads of p53 wild-type mice. BRCA/p53-deficient mice: We produced BRCA1 mice by breeding males and females. All mice were separated into two groups 1) Control and 2) everolimus. MMTV-erbB2 and p53 null mammary gland mice were treated with everolimus (5mg/kg, by oral gavage). BRACA1 mice were given 2mg/kg and 5mg/kg 2X a week of everolimus. All of these mice spontaneously developed mammary tumors within 12 months. Mice were observed daily, toxicity and the percentage of tumor free mice were recorded. Tumor incidence and time to tumor formation was visualized using Kaplan- Meier curves and analyzed using the generalized Wilcoxon test.

Results: In MMTV-erbB2 mice everolimus reduced tumor incidence and was associated with an increase in median survival time from 240 days to 410 days. At 365 days, when all mice in control group died, only half of the mice treated with everolimus had developed tumors (p=0.0001). Everolimus also reduced tumor incidence in p53 null mammary gland mice. At 420 days, 50% of the control mice developed mammary tumors compare to only 7 % of the everolimus treated mice (p=0.04). Everolimus also reduced tumor incidence in BRCA1 mice. At 262 days, when 13 (out of 18) (72%) of the control mice developed mammary tumors compare to only 7 (out of 18) (38%) in everolimus treated group developed tumors (p=0.02). Long term treatment (>150 days) of everolimus was associated with mild toxicity that includes slight weight loss (<10%) and skin changes (matted hair, skin erythema) in MMTV-erbB2 and p53-null mammary gland models. We have not observed any visible toxicity in BRCA1 mice yet. Results of mammary tissue biomarkers will be presented.

Conclusions: The mTOR inhibitor everolimus prevented mammary tumorigenesis in all three mouse models. Our results suggest that everolimus can be an effective cancer preventive drug and that further studies with reduced everolimus dose alone or in combination with other targeted therapies are warranted. In the future, clinical trials of the everolimus should be considered for the prevention of breast cancer in high-risk patients. Supported by NCI PREVENT Cancer Preclinical Drug Development Program (HHSN-2612015000-18I (PB)).

Citation Format: Abhijit Mazumdar, Jamal Hill, Yun Zhang, Lakshmi Reddy Bollu, Alejandro Contreras, Michelle Savage, Shizuko Sei, Altaf Mohammed, Powel Brown. Targeting the mTOR/TORC Pathway for the prevention of er-negative and triple-negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5072.