Abstract 378: Predictive and pharmacodynamic biomarkers of the androgen receptor in circulating tumor cells

Abstract

Abstract Purpose: The androgen receptor (AR) signaling inhibitors (e.g. Enzalutamide) and lyase inhibitors (e.g Abiraterone) modulate the AR signaling pathway and improve outcomes for patients with castrate resistant prostate cancer (CRPC). Molecular analysis of the AR signaling pathway can serve as predictive and pharmacodynamic markers of therapeutic response and resistance. Proposed resistance mechanisms to these agents include AR overexpression, expression of AR splice variants and up-regulation of the Glucocorticoid Receptor (GR). Circulating tumor cells (CTCs) have been utilized as one potential source of tumor cells for these analyses. We employed an integrated CTC capture and analysis technology known as VERSA to perform CTC enumeration, subcellular localization of AR and GR, and gene expression analysis of AR splice variants and AR dependent genes in patients with CRPC. Methods: Using the VERSA platform, we captured and enumerated EpCAM+ CTCs from 25 patients with CRPC who have been treated with AR signaling inhibitors or lyase inhibitors. Protein expression and nuclear localization of the AR and GR were quantified. mRNA was extracted from EpCAM+ CTCs for analysis of AR splice variants and AR targeted genes. CTC results were correlated with clinical outcomes of radiographic progression, PSA progression and stable disease. Results: Analysis of AR/GR protein expression revealed significant intra-patient heterogeneity in patients with radiographic progression, compared to patients with stable disease, treated with AR therapies. Patients with radiographic progression exhibit a dynamic range of AR localized in the nucleus (17 to 100%) in EpCAM+/AR+ CTCs. We then identified expression of V1 and V7 variants of the AR in 5 of 7 men showing radiographic progressionon their current therapy, only 1 out of 11 patients with rising PSA but no radiographic progression, and none of the patients with stable disease. 4 of 6 patients showing expression of AR V7 still maintained significant activity in the AR signaling pathway while 2 patients had no detectable expression of PSA. Conclusions: Multiplexed assays for protein quantification and gene expression of CTCs are suitable for pharmacodynamic analysis of AR signaling inhibitors. Results from patients with CRPC identify heterogeneous activity in the AR signaling pathway and expression of multiple AR splice variants in men with radiographic progression. Protein analysis reveals a subset of CTCs that retain phenotypic alterations consistent with persistent inhibition of the AR pathway. These results suggest a subset of men may benefit from continued AR targeting therapies despite the presence of AR splice variants. Rational combinatorial strategies could be employed based on these pharmacodynamic biomarkers to inhibit the AR pathway in sensitive tumor clones while utilizing alternative treatments, such as chemotherapies, for resistant tumor populations. Citation Format: Jamie M. Sperger, Lindsay Strotman, Benjamin P. Casavant, Chorom Pak, Sacha Horn, Erika Heninger, Scott M. Berry, David J. Beebe, Joshua M. Lang. Predictive and pharmacodynamic biomarkers of the androgen receptor in circulating tumor cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 378. doi:10.1158/1538-7445.AM2015-378