Abstract P5-01-15: Anti-ER monoclonal antibody SP1 seems more sensitive in the identification of ER alpha positive breast cancer cases than anti-ER monoclonal antibody 1D5

Background: Accurate pathological analysis of biopsies is crucial for conclusive and precise disease diagnosis, prognosis and therapeutic guidelines. Evaluation of estrogen receptor (ER) expression in breast cancer (BC) is imperative in determining the disease prognosis, and patients' eligibility for hormonotherapy. Herein, ER alpha status was evaluated in 61 BC cases using two different anti-ER alpha monoclonal antibodies, SP1 (rabbit) and 1D5 (mouse). Methods: Primary BC cases registered in two reference hospitals, with correspondent written informed consent, were used to generate in house tissue microarray platforms. ER alpha expression was accessed by immunohistochemistry using the monoclonal antibodies anti-ER alpha, SP1 and 1D5. Staining scoring followed the Allred method (PS, proportion score or percentage of stained cells; IS, intensity score). Concordance and correlation parameters were calculated using Kappa factor; Pearson, Spearman's, or intraclass correlation coefficient (CF). Staining patterns were compared by paired T-Test and Wilcoxon test. Findings: SP1 and 1D5 provided equivalent results (Concordance rate, 96.7%; Kappa factor, 0.921), whereas SP1 was more prone for positive results than 1D5 (2 samples diverged). Total concordance of PS was obtained (Pearson and intraclass CF, 0.7351 and 0.6193, respectively); however, concordance between the antibodies seems more accurate in higher PS values. Excellent SI correlation between antibodies was observed throughout the population (Spearman's CF, ρ=0.9150). Following the Allred score, 17 out of 42 positive BC samples diverged; always pointing 1D5 to weaker staining than SP1. When calculating Spearman's CF of Total Score (TS) within the population, excellent correlation between both the antibodies (ρ=0.9238) was noted; nonetheless, results were less concordant result among the BC positive cases (ρ=0.7743). Indeed, 20 samples were differentially classified using the antibodies (only 3 had higher TS with 1D5). Considering the mean TS of all samples, or of invasive ductal carcinoma, SP1 provided higher scores than 1D5 (p < 0.05). Interpretation: Despite the equivalence between the anti-ER alpha monoclonal antibodies, SP1 and 1D5, the former has proven superior to the latter with respect to the IS and TS. Therefore, we suggest the clinical use of SP1 while diagnosing BC, due to its higher accuracy in examining ER status; thus, enabling more accurate therapeutic decisions to treat the disease. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P5-01-15.