Characterization of FOLH1 expression in renal cell carcinoma (RCC).

713 Background: The FOLH1 gene encodes prostate-specific membrane antigen (PSMA), a transmembrane glycoprotein that is highly expressed in prostate cancer cells and on endothelial cells in the neovasculature of solid tumors, including RCC. PSMA has been used as a target for diagnostic imaging and therapeutic radioligand therapy. We utilized a database of molecularly profiled RCC tumors to evaluate associations with FOLH1 expression. Methods: NextGen sequencing of DNA (592-gene/whole exome) and RNA (whole transcriptome) was performed for RCC patient specimens (n=1765) through Caris Life Sciences (Phoenix, AZ). FOLH1-High/Low expression were defined as ≥75th/<25th-percentile of RNA transcripts per million (TPM). Angiogenic, T-effector, and Myeloid expression signatures were calculated using previously defined gene sets (McDermott, 2018). Immune cell infiltration in tumor microenvironments (TMEs) was estimated using MCP-Counter (Becht, 2016). Tumor cell PD-L1+ expression (≥2+, ≥%5; SP142) was assessed by IHC. Kaplan-Meier estimates were calculated from time of tissue collection or therapy start. Results: FOLH1 expression was similar between sexes (71% male/29% female, 11.2 vs. 11.3 median TPM, p=0.54) and was not correlated with patient age at time of profiling (median 63 years, range 1-90+, spearman = 0.02, P=0.42). FOLH1 expression was significantly higher in clear cell RCC (ccRCC; 71.1% prevalence) compared to non-ccRCC tumors (19.0 vs 3.3 TPM, P<0.001). FOLH1 expression varied by specimen site (45% kidney/55% metastatic, 13.6 vs. 9.9 TPM, P<0.001), with notably lower expression in lymph nodes (5.3 TPM, P<0.001, 8.2% prevalence). FOLH1 expression was strongly correlated with angiogenic gene expression compared to T-effector and myeloid signatures (spearman = 0.76 vs 0.33 and 0.20, respectively, each P<0.001), with similar correlation strength observed for endothelial cell abundance in TMEs (spearman = 0.76 vs. 0.04-0.50 for immune cell types, P<0.001). PD-L1+ IHC frequency was numerically lower yet not significantly different in FOLH1-High compared to -Low tumor among ccRCC (10 vs. 17%, P=0.07), but was similar among non-ccRCC (31 vs 32%, P=0.95). For patients stratified by median FOLH1 expression, no difference in overall survival from time of tissue sampling was observed for ccRCC (HR 1.2, P=0.57) or non-ccRCC cohorts (HR 0.77, P=0.59), while FOLH1-High was associated with numerically longer cabozantinib time-on-treatment (223 vs. 61 days, HR 0.60, P=0.08). Conclusions: We observed differential patterns of FOLH1 expression by histology and tumor site. FOLH1 expression was strongly correlated with angiogenic gene expression and distinct differences in TME composition, including endothelial cell abundance . FOLH1 gene expression was positively correlated with increased duration of anti-angiogenic treatment. Additional studies are needed to test the efficacy of PSMA-based diagnostics/therapeutics in RCC.