Abstract 1403: Early tumor regression after endocrine and kinase inhibitor therapy in an experimental model of breast cancer.
Authors: Polo Maria Laura , Riggio Marina , Lanari Claudia , Novaro Virginia 
Published: August 2014

Abstract
More than 70% of breast cancers express estrogen and progesterone receptors (ER and PR), and are thus susceptible to adjuvant endocrine therapy. Treatments for these patients are mainly focused on blocking ER function at multiple levels. In the last few years, evidence has linked progestin to breast cancer pathogenesis. According to that, PR would be a valid target for breast cancer therapy. In addition, up to 40% of hormone-receptor positive breast tumors have PI3K activating mutations, and then PI3K/AKT inhibition is becoming a new approach for breast cancer treatment.
The aim of this work was to study early stages of tumor regression in an experimental model of breast cancer. We tested the effect of an antiprogestin and a PI3K inhibitor for 48 hs, alone or in combination. We used the medroxyprogesterone acetate (MPA)-induced breast cancer model. Tumors of this model express high levels of ER and PR. Initially, they are MPA-dependent, but after a few passages MPA-independent variants appear spontaneously. These variants may respond to an endocrine therapy or may progress to a hormonal resistant phenotype.
We have previously demonstrated that the MPA-independent variant C4-HI regresses after 48 hs administration of the antiprogestin Mifepristone (MFP, 12 mg/kg/day) or the PI3K inhibitor Wortmannin (WN, 1 mg/kg/day), displaying an increase in ductal differentiation. We now explored the mechanisms involved in these inhibitory effects. Proliferation of tumor cells, determined as mitotic and Ki67 indexes, is reduced (p&lt;0.01) similarly by MFP and WN respect to vehicle. Consistently, we detected a downregulation in the expression of cell cycle proteins c-MYC and Cyclin D1 determined by western blot and immunohistochemistry. However, after 48hs only WN increased necrosis, apoptosis (p&lt;0.01) and the BAX/BCL-XL ratio. As expected, only WN decreased pAKT and pS6 levels. Furthermore, when given in combination, MFP and WN improved all the above mentioned parameters.
In an antiprogestin resistant tumor variant, PI3K inhibition still suppressed tumor growth. We detected that after 7 days of treatment WN-treated C4-2-HI tumors were smaller than vehicle-treated tumors (p&lt;0.001). However, no evident signs of regression were histologically observed. Ongoing experiments are being conducted to evaluate histologically tissue response at earlier time points (48hs) after WN therapy.
Taken together, these results suggest that inhibiting PI3K pathway could represent a useful tool for the treatment of hormone receptor-positive tumors, even of those that have progressed to a hormonal resistant phenotype. Understanding the early mechanisms involved in tumor regression may improve the design and monitoring of cancer therapies.
Citation Format: Maria Laura Polo, Marina Riggio, Claudia Lanari, Virginia Novaro. Early tumor regression after endocrine and kinase inhibitor therapy in an experimental model of breast cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1403. doi:10.1158/1538-7445.AM2013-1403