Activity of Combined Androgen Receptor Antagonism and Cell Cycle Inhibition in Androgen Receptor Positive Triple Negative Breast Cancer

Triple-negative breast cancer (TNBC) is an aggressive subtype, with a peak recurrence rate within the first few years after diagnosis. Few targeted therapies are available to treat this breast cancer subtype, defined by the lack of estrogen receptor (ER) and progesterone receptor and without amplification of human epidermal growth factor receptor 2 (HER2). Although cell cycle cyclin-dependent kinase (CDK) 4/6 inhibitors are approved for treatment of ER-positive (ER⁺) breast cancer, they have not proven effective as monotherapy in patients with TNBC. The androgen receptor (AR) has emerged as a therapeutic target in a subset of TNBCs and with significant clinical benefit observed in multiple trials. The purpose of this study was to investigate the preclinical activity of the CDK4/6 inhibitor, abemaciclib, in combination with an agent that targets both androgen biosynthesis and AR activity, seviteronel, using TNBC cell lines expressing high AR, cell line xenografts, and an AR-positive (AR⁺), androgen-responsive TNBC patient-derived xenograft (PDX). Single-cell RNA sequencing demonstrated heterogeneity in AR levels, even in a highly AR⁺ cell line, and identified cell cycle pathway activation in AR^High- versus AR^Low-expressing cells. Combination treatment with the cell cycle CDK4/6 inhibitor, abemaciclib, and seviteronel showed synergy in an AR⁺ TNBC model compared with each drug alone. Although cell cycle inhibitors are FDA approved for use in ER⁺ breast cancer, our studies suggest that they may also be effective in AR⁺ TNBC, perhaps combined with AR-targeted agents.

Role of androgen and microRNA in triple-negative breast cancer

Breast cancer (BC) is the most frequent type of malignancy affecting females worldwide. Molecular-based studies resulted in an identification of at least four subtypes of breast carcinoma, including luminal A and luminal B, Human growth factor receptor (HER-2)-enriched and triple-negative tumors (basal-like and normal breast-like). A proportion of BC cases are of the triple-negative breast cancer (TNBC) type. TNBC lacks the expression of estrogen receptor (ER), progesterone receptor (PR), and HER-2, and is known to express androgen receptor (AR) at considerable levels. AR has been shown to promote the progression of TNBC. However, the exact mechanisms have yet to be unraveled. One of these mechanisms could be through regulating the expression of microRNA (miRNA) molecules, which play an important regulatory role in BC through post-transcriptional gene silencing. Activation of AR controls the expression of miRNA molecules, which target selective mRNAs, consequently, affecting protein expression. In this review we attempt to elucidate the relations between AR and miRNA in TNBC.

Efficacy and Safety of Targeting Androgen Receptor in Advanced Breast Cancer: A Systematic Review

Background::

Androgen receptor (AR) upstreams complex signaling pathways that regulate cell proliferation and contribute to breast tumorignensis. Several clinical trials were initiated to investigate the clinical relevance of targeting AR especially in hormone-receptor-negative breast cancer.

Methods::

The search was performed in PubMed and the meeting libraries of ASCO, ESMO, SABCS, ImpakT congresses from January 2005 to July 2017. The following key words were used: Breast cancer, Androgen receptor, androgen agonist/antagonist, Flutamide, Abiraterone, Bicalutamide, Enzalutamide, Enobosarm, selective androgen receptor modulator.

Results::

Screening of title/abstracts yielded a total of 20 relevant results. Of those, twelve studies were found eligible: eleven clinical trials along with one case report. Response rates ranged from 0 to 12% while clinical benefit rates reached up to 35% in 2 studies (with enzalutamide and enobosarm). Progression-free survival ranged from 2.8 to 4.5 months. The most widely used cutoff for AR expression was 10%. High expression of AR was associated with more clinical benefit. Regarding safety, anti-androgens were generally well tolerated with hot flushes, elevated transaminases and fatigue being the most commonly reported across all agents.

Conclusion::

Androgen receptor pathway targeting in advanced breast cancer remains a valid option with reasonable clinical benefit in non-selected patients. Future studies are needed to define an AR addicted cohort with better responses and outcome.