Glucocorticoid receptor (GR) expression in circulating tumor cells (CTCs) to prognosticate overall survival (OS) for metastatic castration-resistant prostate cancer (mCRPC) patients (pts) treated with androgen receptor signaling inhibitors (ARSi).

Abstract

194 Background: Upregulation of GR protein expression in metastatic biopsies from pts with CRPC has previously been shown to correlate with resistance to enzalutamide and has been validated as a therapeutic target in pre-clinical studies. We sought to determine whether upregulated GR protein expression in CTCs from pts with progressing mCRPC predicted clinical outcomes following treatment with enzalutamide (E) or abiraterone (A). Methods: Pre-therapy blood samples from 54 pts with progressing mCRPC were subjected to CTC analysis using the Epic Sciences platform. Samples were examined to identify CK+ (CK+, CD45- cells, with intact nuclei, morph distinct) CTCs for GR protein expression. GR+ CTCs were defined as having expression greater than the 95th percentile of GR expression in the GR negative LNCAP cell line. Kaplan-Meier analysis was used to test the impact of GR+ CTCs on OS following treatment with A or E. A Cox proportional hazards model with CTC number and GR positivity was used in a multivariate analysis. Results: 37 out of 54 pts (69%) had detectable and viable CK+ CTCs. 28 out of 37 pts (76%) had CTCs with upregulated GR staining with a median of 6 GR+/CK+ cells/ml per patient (range 0.7 – 244 cells/ml). The OS of patients with GR+ CTCs treated with ARSi was significantly worse than that of patients without detectable GR+ CTCs (11.4 mo. vs NA, p < 0.01), an effect independent and additive to the presence of viable CTCs, a previously described prognostic biomarker (see Table). Conclusions: GR protein upregulation in CTCs can be detected in a significant percentage of pts with progressing mCRPC and the presence of GR+ CTCs predicts worse OS in response to ARSi. The data supports previously reported pre-clinical data proposing a pathogenic role for GR in mediating resistance to ARSi therapy. Detection of GR in patient CTCs may be a useful predictive biomarker to guide GR-directed therapies. [Table: see text]