Targeting serum response factor as a novel therapeutic approach for triple-negative breast cancer

e12560 Background: Triple negative breast cancer (TNBC) is an aggressive form of breast cancer (BC) which lacks estrogen receptor, progesterone receptor and HER2 amplification. Due to the fact that up to 50% of TNBC express AR and that AR expression has been associated with poor prognosis, targeting AR in TNBC is attracting increasing interest. However, although targeting AR in prostate cancer results in initial response, resistance eventually develops, resulting in treatment failure. We previously identified Serum Response Factor (SRF) as an important transcription factor in a cellular model of castrate-resistant prostate cancer. We showed a relationship between SRF and AR which potentially plays a role in the resistance to AR-targeted therapy. Therefore, we hypothesise that AR resistance mechanisms could be bypassed by targeting SRF, singly or in combination with AR-inhibitors. Methods: In this study three TNBC cell lines with different SRF/ AR levels were selected to investigate the effect of SRF inhibition on cell viability, cell migration and cell invasion. Immunohistochemistry was used to assess SRF expression in 3 BC TMAs: TMA1 (n = 144) and TMA2 (n = 512) with different subtypes of BC and TMA3 with 138 TNBC patients. Results: MTT assays showed response to CCG1423 in the two cell lines positive for SRF (MDA-MB-231 and HS578t) with IC50 values of 20 uM, while MDA-MB-453 (SRF negative) did not respond (IC50 > 80uM). In addition, a synergistic effect on cell viability was demonstrated when CCG1423 was used in combination with MDV3100. Scratch assays to assess cell migration showed a slower gap-closure post- CCG1423 treatment compared to controls. In addition, CCG1423 treatment significantly reduced both cell lines’ ability to invade by approximately 50% at 48 hours post-treatment (p < 0.05). Analysis of SRF expression in clinical samples showed higher SRF protein expression in TNBC vs. non-TNBC patients. In addition, one TMA (n = 144) showed that higher SRF expression was associated with shorter overall survival (HR 1.99 [CI (1.02, 3.87)], P value 0.039) and shorter disease specific survival (HR 2.93 [CI (1.19, 7.21)], P value 0.014). Conclusions: Our data support the rationale for using SRF as an alternative target to AR, alone or in combination with AR-antagonists.