Enhancing abiraterone acetate efficacy in androgen receptor-positive triple negative breast cancer: Chk1 as a potential target

Purpose: Our aim was to identify predictive factors of abiraterone acetate (AA) efficacy and putative new druggable targets in androgen receptor (AR)-positive triple-negative breast cancer (TNBC) treated in the UCBG 2012-1 trial. Experimental Design:We defined AA response as either complete or partial response, or stable disease at 6 months. We sequenced 91 general and breast cancer-associated genes from the tumor DNA samples. We analyzed transcriptomes from the extracted RNA samples on a Nanostring platform and performed immunohistochemistry (IHC) using tissue microarrays. We assessed AA and Chk1 inhibitors (GDC-0575 and AZD7762) efficacies, either alone or in combination, on cell lines grown in vitro and in vivo. Results: Classical IHC apocrine markers including AR, FOXA1, GGT1 and GCDFP15, from patients' tumors allowed identifying AA responders and non-responders. All responders had clear apocrine features. Transcriptome analysis revealed that 31 genes were differentially expressed in the two subgroups, 9 of them being linked to proliferation and DNA damage repair. One of the most significant differences was the overexpression, in non-responders, of CHEK1, a gene encoding Chk1, a protein kinase that can be blocked by specific inhibitors. Based on cell line experiments, AA and Chk1 inhibitor combination showed at least additive effect on cell viability, cell cycle, apoptosis and accumulation of DNA damages. In vivo, orthotopic xenograft experiments confirmed the efficacy of this combination therapy. Conclusions: This study suggests that apocrine features can be helpful in the identification of AA-responders. We identified Chk1 as a putative drug target in AR-positive TNBCs.

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