Programmed Death-Ligand 1 Expression in Triple-Negative Breast Cancer: Insights from a Mexican Cohort.

Background: Pembrolizumab-containing regimens have become the standard of care across the spectrum of triple-negative breast cancer (TNBC). While their use in the neoadjuvant setting is independent of biomarker status, their application in metastatic disease remains strictly contingent upon PD-L1 expression. Given that PD-L1 prevalence can vary significantly by ethnicity and geography, the lack of specific data for the Mexican population creates a challenge for optimizing treatment in the metastatic setting. This study sought to characterize PD-L1 positivity rates in a Mexican TNBC cohort to better define the local molecular landscape. Methods: We conducted a retrospective study across two cancer centers in Mexico to assess PD-L1 positivity in a cohort of women with TNBC (stages I-IV) diagnosed between 2006 and 2021. PD-L1 expression was assessed and evaluated centrally using the 22C3 pharmDx assay, with a Combined Positive Score (CPS) of ≥1 considered positive. We explored the association between PD-L1 expression and clinicopathological features. Results: Of the 298 TNBC patients identified, 285 (96%) had sufficient tissue for CPS evaluation and thus were included in the analysis. PD-L1 positivity was observed in 29.1% of the cohort, and 13.3% of patients had a CPS ≥ 10. PD-L1 positivity was associated with higher histological grades (91.3% vs. 78.5%, p = 0.035) and TILs ≥ 30% (22.2% vs. 10.0%, p = 0.007). Additionally, pre-treatment surgical specimens were more frequently PD-L1 positive than tumor biopsies (56.6% vs. 30.7%, p < 0.001). Conclusions: This study characterizes the PD-L1 landscape in Mexican women with TNBC, reporting a 29.1% prevalence of CPS ≥ 1. The strong association between PD-L1 positivity and high TILs/histological grade highlights the role of the immune microenvironment in these aggressive phenotypes. Given the significant variability observed between specimens (biopsy vs. surgical), clinicians should consider the dynamic nature of PD-L1 expression when choosing treatment strategies.