Heterogeneity and Scoring Reproducibility of Folate Receptor 1 Immunohistochemistry in High-grade Serous Carcinoma

Mirvetuximab soravtansine (MIRV) is an antibody-drug conjugate approved for the treatment of adult patients with folate receptor 1 (FRα; FOLR1) positive, platinum-resistant epithelial ovarian, fallopian tube, or primary peritoneal cancer, who have received one to three prior systemic treatment regimens. Per the FDA approval, FOLR1 positivity is defined as ≥75% of viable tumor cells showing moderate (2+) or strong (3+) membranous immunostaining ("PS2+"). Given this disease's high recurrence rate and relatively limited therapeutic options, there is utility in exploring consistency in FOLR1 reporting. Tubo-ovarian high-grade serous carcinoma (HGSC) samples from our institution's archives were included in tissue microarrays (n=806), whole tissue sections (n=51), or cell blocks (n=30) and evaluated using the Ventana FOLR1 (FOLR1-2.1) RxDx Assay. FOLR1 staining was heterogeneous across different anatomic sites (average FOLR1 PS2+ was 50.2 from adnexal sites compared with 47.4 from omental sites, P =0.015). Similarly, heterogeneity was noted in pre- versus post- neoadjuvant chemotherapy specimens (on average, FOLR1 PS2+ score increased by 17.7 from pre- to post- therapy, P =0.0089). Lastly, specimen type may also influence FOLR1 staining (average abdominal fluid FOLR1 PS2+ score was 25.5 and average surgical FOLR1 PS2+ score was 56.9, P =0.000034). Agreement among 9 readers was initially substantial, with a Fleiss kappa of 0.661 (95% CI: 0.636-0.685). For the subset of cases with the worst agreement initially, a training session with reference cases improved interobserver agreement. Our study highlights several factors contributing to heterogeneity in FOLR1 reporting. Future studies are needed to better understand the impact of FOLR1 heterogeneity on patient response to therapy.