Implementing a CTC-based clinical trial assay for selecting AR-V7+ patients for enrollment in a pivotal phase 3 study (ARMOR3-SV).

Abstract

348 Background: ARMOR3-SV is a randomized phase 3, multi-center study comparing galeterone to enzalutamide in men with metastatic (M1) castrate resistant prostate cancer (CRPC) expressing androgen receptor splice variant-7 mRNA (AR-V7) who have not yet received chemotherapy, or other second generation androgen signaling inhibitors. AR-V7 is a truncated, constitutively active splice variant of the androgen receptor (AR) that lacks the ligand binding domain (LBD) and has been implicated in prostate cancer progression. Recently, the presence of AR-V7 in circulating tumor cells (CTCs) has been associated with a lack of responsiveness to both abiraterone and enzalutamide, whereas galeterone, a selective small molecule that potently degrades the androgen receptor, has demonstrated activity in preclinical model systems expressing AR-V7 and clinically in patients with C-terminal loss, of which AR-V7 is the most common form. Methods: Utilizing the lab test developed at JHU as a model system, we sought to develop a clinical trial assay utilizing immunomagnetic bead isolation of CTCs followed by mRNA isolation and cDNA generation. We then developed a duplexed PCR assay using Taqman chemistry to detect the full-length AR (AR-FL) and AR-V7. Negative patient samples were used to establish the limit of blank and assay cut-off and synthetic oligonucleotides to establish the limit of detection. Results: The AR-FL and AR-V7 assay shows linear response to template dilution to the assay cut-off Ct. Blood spiked with LNCaP and LNCaP95 cells were used to demonstrate assay suitability and sensitivity. We have successfully detected AR-FL and AR-V7 in select CRPC patient samples. We have also demonstrated assay reproducibility and have shown that the assay remains consistent in the presence of select interfering substances. Conclusions: For the ARMOR3-SV trial, this clinical trial assay is being utilized globally at central laboratories to screen patient blood samples for AR-V7 expression in evaluable CTCs as the first criterion for inclusion in this study. Clinical trial information: NCT02438007.