Effect of neoadjuvant stereotactic body radiation therapy (SBRT) on the immune microenvironment of high-risk localized prostate cancer in paired transcriptomic analysis of pretreatment biopsy and irradiated prostatectomy specimens.

371 Background: SBRT is a treatment option for men with prostate cancer. PREPARE-SBRT (NCT03663218) is a clinical trial testing the safety of neoadjuvant MRI-guided SBRT for men with high-risk localized prostate cancer. We leveraged paired samples from pre-treatment biopsy (Bx) and irradiated prostatectomy (RP) specimens to evaluate immune-related transcriptomic changes in irradiated tumors at acute timepoints following neoadjuvant SBRT. Methods: Transcriptomic profiles were generated using Decipher GRID (Veracyte, Inc). Differences in transcriptional signatures were assessed between Bx samples taken before treatment and RP samples post-treatment using a paired T-Test. A control cohort of transcriptomic profiles of 803 untreated Bx samples and matching RP samples from the same patients were used to account for differential effects in tissue preservation between Bx and RP. Linear regression model with the interaction effect of cohort (SBRT vs control) and by treatment status (Bx vs RP) to select signatures significantly impacted by neoadjuvant SBRT. Signatures with interaction p-value <0.01 and paired T-test value <0.05 were considered statistically significant. Results: 10 patients with paired pre-treatment Bx and post-SBRT RP specimens (n=20 samples) were analyzed with a median interval from completion of SBRT to RP of 5 days. Neoadjuvant SBRT was associated with upregulation of the T-cell inflamed signature (p=0.028) and immune190 signature (p=0.0031) supported by enrichment of transcriptomic features associated with IFN-gamma response (p=0.022), TCR signaling (p=0.0052) and antigen presentation (p=0.0048). Increased gene expression associated with effector memory/activated CD4 T-cells (p=0.0005 and p=0.0013, respectively) and effector memory/activated CD8 T-cells (p=0.0015 and p=0.013, respectively) were paralleled by a significant reduction in immunosuppressive regulatory T-cell (Treg; p=0.0015) and myeloid derived suppressor cell (MDSC; p=0.005) signatures. Additionally, macrophage-specific signatures were significantly enriched among SBRT-irradiated samples including CD68 and CSF1 gene clusters (both p=0.0041) as well as a decreased expression of the M2-to-M1 ratio signature (p=0.0052). Interestingly, WNT/β-catenin signaling, a putative oncogenic mediator of immune exclusion, was significantly downregulated in irradiated RP samples relative to pre-treatment Bx (p=2.6x10-5). Conclusions: Neoadjuvant SBRT was associated with significant immune remodeling of the irradiated prostate microenvironment. Collectively, immune-related transcriptional signatures skewed towards immune activation, increased effector T-cell and macrophage signatures and a reduction in immunosuppressive transcriptomic features.