AB025. Evaluation of potential therapeutic immunohistochemical targets with experimental or FDA-approved therapies in thymic epithelial tumor microarrays

Abstract

Background: Thymus epithelial tumors (TETs) are rare malignancies of the anterior mediastinum. The current standard of care for metastatic TETs is a combination of platinum-based chemotherapy. Here, we have evaluated the experimental and FDA-approved makers in a large TET tissue array with the hope of identifying a new therapeutic option. Methods: A tissue microarray (TMA) containing ninety malignant thymic tumors (A, AB, B1 and B2, n=62, B2/B3 and B3, n=16, and thymic carcinoma, n=12) and seven normal adult thymus were assembled. The protein expressions of GLUT1, TROP2, PSMA, ROS1, ALK, HER2, and PDL-1 were tested with immunohistochemical assays. Expression was quantified using a “staining score (SS)”, which is a 0–3 numerical score that results from the product of the intensity of expression: 0= negative, 1= weak, 2= moderate, 3= strong, and the area of expression in fractions of a percent (0= no expression, 1= 100% area). Expression of HER2 and PDL-1 was quantified according to existing guidelines [HER2 score and combined positive score (CPS)]. Results: Trop-2 had the highest expression in thymic carcinoma (TC) (100%, SS 2.6±0.6) followed by thymoma B2/B3 (78%, SS 1.3±1.3), types A/AB/B1 (54%, SS 1.1±1.1) (P=0.01). In TC, all patients with squamous histology had immunohistochemistry (IHC) SS of 3. Patients with thymoma who had Trop-2 expression experienced significantly worse survival [hazard ratio (HR): 3.3, P=0.008]. GLUT1 was highly expressed in TC (81.8%, SS: 2.1±1, TC vs. normal thymus, P=0.0003). PDL-1 was expressed in all TET tissues (mean, 2.5–52 CPS). No significant expression of ALK, ROS1, or HER2 observed in normal thymus or TETs. Conclusions: Trop-2 expression is a prognostic marker in TETs. High expression of Trop-2 protein in thymoma and TC appears a promising therapeutic target for Trop-2 antibody-drug conjugates.