Folate receptor alpha (FRα; FOLR1) expression and persistence in ovarian cancer in clinical trial samples and real-world patient cohort

FRα protein expression was retrospectively established by immunohistochemistry (IHC) using the VENTANA FOLR1 (FOLR1-2.1) RxDx Assay (FOLR1 CDx) in high-grade serous ovarian cancer tumors from pts in the VELIA trial (NCT02470585) and a real-world (RW) cohort. RW tumors were tested at Caris Life Sciences and linked to health record data from the ConcertAI RWD360 product. FRα-high positivity was defined by a cutoff of ≥75% of viable tumor cells with ≥2+ membrane staining (used for MIRV treatment eligibility with approved FOLR1 CDx). RNA was measured using whole transcriptome RNAseq. IHC and mRNA concordance was determined using Receiver Operating Curve (ROC) analysis.

In the RW cohort (N=611), 40.9% were FRα-high. FRα-high prevalence was 40.3% and 40.5% in tumors with and without BRCA1 mutations, respectively (mutation prevalence, 12.4%). FRα-high prevalence was 47.4% and 39.8% in tumors with and without BRCA2 mutations, respectively (mutation prevalence, 8.8%). Thirty-six pts had 2 biopsies longitudinally collected with IHC results. Consistency of FRα IHC status across biopsies was 86%. FRα mRNA expression associated with IHC status (ROC area under the curve [AUC]=0.88; 95% CI, 0.84-0.91), with sensitivity and specificity of 83% using a cutoff maximizing Youden’s Index. To understand the prognostic nature of pts with tumors with FRα-high expression, samples from the VELIA trial were used. FRα IHC was conducted on a subset of tumors, and 56/186 (30%) were FRα-high. FRα mRNA could robustly predict IHC status (ROC AUC=0.82; 95% CI, 0.75-0.88) with sensitivity of 85% and specificity of 72%. An mRNA cutoff best associated with FRα-high IHC was applied to an extended VELIA cohort in newly diagnosed pts with mRNA data available (N=709), and FRα mRNA was identified as a negative prognostic factor for progression free survival (Hazard Ratio=1.27; 95% CI, 1.05-1.5).

FRα-high expression at both the protein and mRNA level was common and concordant in VELIA trial samples and RW patient samples. The expression status was consistent in longitudinally collected samples in most pts, suggesting that IHC on the diagnostic tumor sample may effectively establish FRα protein expression status over time. High FRα mRNA expression was a negative prognostic factor in VELIA despite positive association with homologous recombination deficiency and BRCA mutations.