Expression of EGFR, PD‐L1, and the mismatch repair proteins before and following therapy in malignant serous effusions with metastatic high‐grade serous tubo‐ovarian carcinoma

Abstract Aim To compare the immunochemical expression of EGFR, PD‐L1, and the mismatch repair (MMR) proteins MLH1, PMS2, MSH2, and MSH6 between matched malignant effusions obtained before and following the administration of chemotherapy in patients with high‐grade serous tubo‐ovarian carcinoma (HGSC). Methods In the enrolled HGSCs, matched formalin‐fixed and paraffin‐embedded cell blocks (CBs) from effusions sampled before (treatment‐naïve patients) and during recurrence (following chemotherapy administration), in addition to their matched HGSC tissues obtained from the ovaries at initial diagnosis (treatment‐naïve patients), were subjected to EGFR, PD‐L1, and MMR immunochemical analysis. Results EGFR was more often overexpressed in effusions obtained after chemotherapy administration compared to both effusions (100% vs. 57.1%) and their matched tubo‐ovarian tumors (100% vs. 7.1%) from treatment‐naïve patients, respectively. EGFR immunochemistry was concordant in just 9.1% of the effusions sampled during recurrence and their paired ovarian samples before recurrence. Whereas all HGSC treatment‐naïve samples (ovarian lesions and effusions) were PD‐L1 negative, 3/11 (27.3%) malignant effusions obtained during recurrence showed PD‐L1 overexpression. Lastly, none of the tested HGSC samples exhibited MMR deficiency. Conclusion Measuring biomarkers using CBs from malignant effusions may provide clinicians with significant information related to HGSC prognosis and therapy selection, especially in patients with resistance to chemotherapy.