Folate Receptor Immunohistochemical Staining and Gynecologic Tumors: Initial Experience With 216 Cases

Folate receptor alpha has been shown to have possible mechanisms of tumorigenesis in malignancies, becoming a potential target for therapy. Mirvetuximab soravtansine is an antifolate receptor alpha monoclonal antibody, with an approved FOLR1-2.1 immunohistochemical biomarker. After IRB approval, a retrospective review of gynecologic pathology cases was performed to identify cases in which FOLR1 immunohistochemistry (IHC) was performed at our institution over a period of 9 months as part of clinical care for therapy eligibility. Clinical data collected included patients' age, tumor histotype, tumor grade, primary tumor site, FIGO stage, dates of recurrence/progression, and use of mirvetuximab therapy. FOLR1 IHC data were recorded, including the date specimen obtained, date IHC was performed, site tested, case type, percentage tumor staining, and intensity. Cases were deemed positive or negative according to current recommendations (75%, 2-3+intensity). Two hundred sixteen cases were identified. Patient ages ranged from 25 to 83 years old (median: 59 yr). Staining intensity was reported as 0 in 15 (6.9%) cases, weak (1+) in 8 (3.7%), moderate (2+) in 27 (12.5%), strong (3+) in 27 (12.5%), weak-to-moderate (1-2+) in 15 (6.9%), and moderate-to-strong (2-3+) in 99 (45.8%); intensity was not provided in 25 (11.6%). Percentage of tumor staining ranged from 0 to 100, with a median of 60. The IHC was overall deemed positive in 98 (45.4%) cases and negative in 118 (54.6%). By histotype, 5 of 17 (29.4%) low-grade serous carcinomas, 88 of 162 (54.3%) high-grade serous carcinomas, 3 of 5 (60%) of carcinosarcomas, and 2 of 6 (33.3%) of mixed carcinomas were positive. No case of clear cell CA, endometrioid CA, Mullerian CA NOS, serous borderline, mucinous CA, or granulosa cell tumor was positive. The primary site of disease was tubo-ovarian in 192 (88.9%) cases, peritoneal in 8 (3.7%) cases, uterine in 3 (1.4%) cases, and unknown in 13 (6%) cases. By site on which immunohistochemical stain was performed: primary site positive in 53 of 96 (55.2%) cases, metastatic site at time of diagnosis/debulking positive in 23 of 41 (52.1%) cases, and metastatic/recurrent cases positive in 22 of 79 (27.8%) cases. There was a statistically significant correlation when comparing the positivity rates between these sites ( P = 0.0004). Survival data were examined with high-grade serous carcinoma, with no statistically significant difference between positive and negative cases in overall survival ( P = 0.622) or progression-free survival ( P = 0.711). Biopsy specimens were positive in 17 (25%) cases, while negative in 51 (75%), whereas resection specimens were positive in 81 (54.7%) and negative in 67 (45.3%), a statistically significant difference ( P < 0.0001). Cases that were <19 months old had 38 (36.2%) positive and 67 (63.8%) negative, compared with cases ≥19 months old that had 60 (54.1%) positive and 51 (45.9%) negative, a statistically significant difference ( P = 0.0084). Significant differences in FOLR1 staining were noted between histotypes, age of the specimen, type of case tested, and site of disease tested. Further testing is needed to help determine the best tissue to be utilized for this new biomarker.