Abstract P5-09-23: Programmed death-ligand 1 expression in a Mexican cohort of triple-negative breast cancer

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Abstract Programmed death-ligand 1 (PD-L1) expression predicts response to immunotherapy in a range of malignancies, including triple-negative breast cancer (TNBC). However, observational studies show a significant variability in the rates of PD-L1 positivity in TNBC. Currently, PD-L1 expression rates remains unknown in the Mexican population. A retrospective multicenter study of women with newly diagnosed TNBC between 2006 and 2021 was carried out. PD-L1 was assessed at a central laboratory using the IHC 22C3 assay. Expression was characterized according to the combined positive score (CPS), with positivity defined as >1%. Descriptive statistics were employed to calculate means, percentages, and standard deviations. Among 298 cases with available tissue for analysis, 285 (96%) samples had sufficient cellularity for adequate CPS evaluation. In the entire cohort, the PD-L1 positivity rate was 29.1%, while 13.3% had a CPS >10%. The prevalence of PD-L1 positivity differed significantly according to histological grade, percentage of tumor-infiltrating lymphocytes, and the tissue type analyzed. The majority of CPS evaluations were performed on tissue from biopsies of the primary tumor, which can underestimate PD-L1 positivity by 20-30% when compared to matched surgical specimens. Furthermore, biopsies from metastatic sites were not analyzed for PD-L1 expression, which can impact overall CPS positivity rates. Our data underscore the importance of considering the source of the tissue sample when interpreting PD-L1, as different types and sites of biopsy may yield different expressions. Citation Format: Alejandro Aranda-Gutierrez, Cynthia Villarreal-Garza, César Octavio Lara-Torres, J. Edgardo Hernandez, Daniela Vázquez Juárez, Gabriela Sofía Gómez-Macías, Alejandro Aranda-Gutierrez, Paula Anel Cabrera-Galeana, Fany Iris Porras-Reyes, Víctor Manuel Pérez-Sánchez, Antonio Nateras-Pérez, Alejandro Mohar. Programmed death-ligand 1 expression in a Mexican cohort of triple-negative breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P5-09-23.

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