Macrophages confer resistance to PI3K inhibitor GDC-0941 in breast cancer through the activation of NF-κB signaling

The PI3K pathway is one of the most dysregulated signaling pathways in epithelial cancers and has become an attractive therapeutic target under active preclinical and clinical development. However, recent clinical trial studies revealed that blockade of PI3K activity in advanced cancer often leads to the development of resistance and relapse of the diseases. Intense efforts have been made to elucidate resistance mechanisms and identify rational drug combinations with PI3K inhibitors in solid tumors. In the current study, we found that PI3K inhibition by GDC-0941 increased macrophage infiltration and induced the expression of macrophage-associated cytokines and chemokines in the mouse 4T1 breast tumor model. Using the in vitro co-culture system, we showed that the presence of macrophages led to the activation of NF-κB signaling in 4T1 tumor cells, rendering tumor cells resistant to PI3K inhibition by GDC-0941. Furthermore, we found that Aspirin could block the activation of NF-κB signaling induced by PI3K inhibition, and combined use of GDC-0941 and Aspirin resulted in attenuated cell growth and enhanced apoptosis of 4T1 cells in the in vitro co-culture system with the presence of macrophages. Consistently, the combination treatment also effectively reduced tumor burden, macrophage infiltration and pulmonary metastasis in in vivo 4T1 breast tumor model. Together, our results suggested macrophages in microenvironment may contribute to the resistance of breast cancer cells to PI3K inhibition and reveal a new combination paradigm to improve the efficacy of PI3K-targeted therapy.