Clinically approved ALDH inhibitor combined with a chemotherapeutic agent sensitizes triple-negative breast cancer stem cells to PD-L1 blockade

Triple-negative breast cancer (TNBC) is among the most aggressive cancers due to the lack of specific therapeutic targets. Conventional chemotherapy often enriches cancer stem cells, driving therapeutic resistance and tumor relapse. In TNBC, epithelial-like ALDH hi and mesenchymal-like CD44 hi CD24 lo cancer stem cells are associated with cell hyperproliferation, tumor initiation, and metastatic potential. Additionally, PD-L1 expression enhances cancer stem cell survival, further promoting tumor progression and positioning it as a promising immunotherapeutic target. Both the ALDH inhibitor disulfiram and the anti-PD-L1 atezolizumab are FDA-approved drugs investigated in preclinical and clinical studies for metastatic breast cancer. This study explored the therapeutic potential of combining these drugs to target TNBC cancer stem cells enriched by the chemotherapeutic agent doxorubicin. Using human TNBC cell lines and clinically relevant patient-derived xenograft tumor fragments, we assessed post-treatment cell viability, cancer stem cell survival, and the expression of associated genes and proteins. Results suggested that doxorubicin and disulfiram increased PD-L1 expression on both epithelial- and mesenchymal-like cancer stem cells irrespective of baseline PD-L1 expression, and the subsequent addition of the PD-L1 antibody atezolizumab effectively reduced cancer stem cell frequency, expression levels of stemness-associated markers, and bulk TNBC cell viability in an immune-independent context. These results support a clinically translatable therapeutic strategy for targeting chemotherapy-enriched cancer stem cells with varying PD-L1 expression levels, and identify immune-competent in vivo validation as a necessary future direction.