Abstract 3447: NOS inhibition reverses epithelial-to-mesenchymal transition and synergizes with alpelisib in metaplastic breast cancer

Metaplastic breast cancer (MpBC) is a rare and highly chemoresistant breast cancer subtype, with a median survival of 8 months after metastatic disease, and no standard therapeutic options. MpBCs are enriched for epithelial-to-mesenchymal transition (EMT)/cancer stem cell (CSC) markers, produce high levels of nitric oxide (NO), and have a hyperactive phosphoinositide 3-kinase (PI3K) signaling pathway. NO activates multiple oncogenic pathways, such as PI3K and transforming growth factor beta (TGFβ), a regulator of EMT. Therefore, we hypothesized that pan-NOS inhibitor NG-monomethyl-l-arginine (L-NMMA) could augment the efficacy of α-specific PI3K inhibitor alpelisib in MpBC in vitro and in vivo models. Immunostaining analysis revealed that MpBC PDX tumors had elevated co-expression of iNOS and pAkt (60% vs 23%, p=0.04) relative to triple-negative breast cancer (TNBC) PDX tumors. MpBC PDX tumors had higher RPL39 A14V (66% vs 4.7%, p< 0.00) and PIK3CA hotspot mutation rates (50% vs 19.1%, p=0.31) than TNBC PDX tumors. L-NMMA was synergistic with alpelisib in MpBC cell lines with PIK3CA/PIK3R1 mutations and antagonistic in PIK3CA-wild type and PTEN-deleted models. In vivo evaluation using MpBC PDX tumors found that L-NMMA augmented the efficacy of alpelisib in reducing tumor volume in PIK3CA-mutated MpBC PDX models. Transcriptomic analysis found gene sets associated with EMT reversal, such as the formation of cornified envelope (NES = 2.04 Nom p