Abstract 590: Multiplex liquid biopsy for AR pathway activity in metastatic androgen receptor-positive triple negative breast cancer

Abstract

Abstract Introduction: The androgen receptor (AR) is expressed in up to 50% of patients with triple negative breast cancer (TNBC), and a luminal androgen receptor (LAR) subtype has been identified that is dependent on androgen receptor signaling, but AR protein expression by immunohistochemistry (IHC) has been a suboptimal biomarker for response to anti-androgen therapies in AR-TNBC and there remains an unmet need for biomarkers to identify patients likely to benefit from anti-androgens. We have developed a multiplex liquid biopsy encompassing AR protein expression and downstream gene expression analysis in circulating tumor cells (CTCs). We report here the longitudinal evaluation of this liquid biopsy as part of a phase I trial (NCT03090165) of the anti-androgen bicalutamide with selective CDK4/6 inhibitor ribociclib in metastatic AR-TNBC. Methods: Peripheral blood was collected (n=11) prior to treatment, after two cycles of treatment, and at progression and processed using the VERSA (Versatile Exclusion-based Rare Sample Analysis) microfluidic chip that integrates CTC capture and downstream analysis. For protein expression, fixed CTCs were captured immunomagnetically and stained on chip for AR followed by quantitative fluorescent microscopy. For gene expression analysis, live CTCs were captured immunomagnetically followed by RNA isolation on chip, reverse transcription, and RT-qPCR for a panel of 40 luminal AR genes. Results: AR protein and transcript were detected in CTCs from 10/11 patients, and AR target gene expression in 8/11 patients. The pathologic ligand independent AR splice variant AR-V7 was detected in 2/11 patients. Pretreatment CTC number was higher in patients with anti-androgen resistant (range 3-753) than anti-androgen sensitive (range 0-30) disease. Among patients with low AR IHC (<10%) on biopsy, high AR protein and transcript expression in pre-treatment CTCs was associated with anti-androgen sensitivity. Among patients with high AR IHC (>50%) on biopsy, AR-V7 was detected in CTCs of 2/3 of patients with resistant disease and 0/2 of patients with sensitive disease. Conclusions: This study demonstrates the feasibility of a longitudinal multiplex liquid biopsy as a pharmacodynamic biomarker of AR pathway activity in AR-TNBC, and suggests that AR pathway activity in CTCs may provide additional information to solid tumor biopsy AR expression in predicting sensitivity to anti-androgens. This work also demonstrates for the first time the detection of AR-V7 in CTCs in anti-androgen resistant AR-TNBC, suggesting that AR-V7 expression may be a mechanism of anti-androgen resistance in AR-TNBC as it is in advanced prostate cancer. Citation Format: Marina N. Sharifi, Serena K. Wolfe, Jamie M. Sperger, Jennifer L. Schehr, Saswati Bhattacharya, Kari B. Wisinski, Joshua M. Lang, Ruth M. O'Regan. Multiplex liquid biopsy for AR pathway activity in metastatic androgen receptor-positive triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 590.