Molecular characterization of longitudinally tracked prostate cancer foci in men on active surveillance for low-risk disease.

Abstract

56 Background: The biological trajectory of Gleason score 6 (GS6) prostate cancer (PCa) in men on active surveillance (AS) is unknown. We herein evaluate the potential for high grade PCa to arise clonally from GS 6 disease and determine the capacity of tissue-based biomarkers to predict grade progression. Methods: Men on AS with GS6 PCa who underwent magnetic resonance imaging/ultrasound (MRI/US) fusion biopsy on two occasions from 2012 through 2017 were enrolled. Tumor foci were tracked and re-sampled using the MRI/US fusion biopsy platform. ERG immunohistochemistry (IHC) and DNA/RNA next generation sequencing (NGS) were performed on formalin-fixed paraffin-embedded (FFPE) initial and repeat biopsy specimens to assess tumor clonality and evaluate candidate molecular markers of PCa grade progression. Results: Sixty-seven men of median age 64 years (IQR 59-69) and PSA 4.9 ng/ml (IQR 3.3-6.4) underwent repeat sampling of a single tracked tumor focus using MRI/US fusion biopsy. The median interval to repeat biopsy was 11 months (IQR 6-13). On repeat biopsy, 31 (46%) men progressed to high-grade (GS≥7) disease (n = 24, GS 3+4 = 7; n = 7, GS ≥4+3 = 7). Among the 67 subjects, ERG IHC status was concordant between initial and repeat biopsy in 64 (96%). Of 134 total specimens obtained (67 initial + 67 repeat biopsies), ERG status determined by NGS was concordant with ERG status by IHC in 132 (99%). Comparing the initial biopsy specimens in men who did versus did not undergo grade progression on follow up biopsy, derived cell cycle progression (CCP) scores (median 57.3 vs. 44.0, p = 0.11) and genomic prostate scores (GPS; median 73.8 vs. 64.4, p = 0.15) were not significantly different. Similarly, expression of FOLH1, PCAT4, SCHLAP1, and SPINK1 on initial biopsy did not significantly differ among men who did and did not undergo grade progression. Conclusions: Use of MRI/US fusion biopsy facilitated resampling of the same clonal focus of cancer over time, with high concordance of ERG status determined by both IHC and NGS. Derived genomic classifiers and candidate individual gene expression markers measured on initial biopsy tissue were not significantly different between patients who progressed and those who did not.