N-acetylcysteine overcomes NF1 loss-driven resistance to PI3K alpha inhibition in breast cancer

A genome-wide PiggyBac transposon-mediated screen and a resistance screen in a PIK3CAH1047R-mutated murine tumor model reveal NF1 loss in mammary tumors resistant to the phosphatidylinositol 3-kinase a (PI3Ka)-selective inhibitor alpelisib. Depletion of NF1 in PIK3CAH1047R breast cancer cell lines and a pa-tient-derived organoid model shows that NF1 loss reduces sensitivity to PI3Ka inhibition and correlates with enhanced glycolysis and lower levels of reactive oxygen species (ROS). Unexpectedly, the antioxidant N-acetylcysteine (NAC) sensitizes NF1 knockout cells to PI3Ka inhibition and reverts their glycolytic pheno-type. Global phospho-proteomics indicates that combination with NAC enhances the inhibitory effect of al-pelisib on mTOR signaling. In public datasets of human breast cancer, we find that NF1 is frequently mutated and that such mutations are enriched in metastases, an indication for which use of PI3Ka inhibitors has been approved. Our results raise the attractive possibility of combining PI3Ka inhibition with NAC supplementa-tion, especially in patients with drug-resistant metastases associated with NF1 loss.