Metaplastic breast cancer (MBC) is a rare form of breast cancer characterized by histological presence of two or more cellular types, typically epithelial and mesenchymal components. MBC represents 0.2 to 5% of all breast cancers. Most MBC typically displays a triple-negative breast cancer phenotype in which they lack expression of ER, PR, and HER2 and they also exhibit basal-like or claudin-like molecular subtypes. The current treatment guidelines for MBC have yet to be established due to limited knowledge of its pathogenesis. Prior studies have shown that MBC harbor a subtype mutation in RPL39, which is correlated with higher levels of iNOS, and aberrations in the PI3K/Akt signaling pathway. Considering that aberrations in both of these pathways may play a role in disease pathogenesis, our goal is to further elucidate whether there is a synergistic relationship between the iNOS and PI3K/Akt pathways in MBC. Our hypothesis is that isoform-specific PIK3CA inhibitor Alpelisib, will interact synergistically with pan-NOS inhibitor L-NMMA, to act as an improved therapeutic combination against MBC. We used MBC cell lines Hs578t and BT549 and MBC PDX models RPL39-positive BCM 4664 and RPL39-negative BCM 3807 in our preliminary studies. Using flow cytometry to detect Annexin V+/DAPI+ cells, we found increased levels of cell death in MBC cell lines treated with LNMMA+Alpelisib combination treatment in comparison to single agent treatment. Western blot analysis of samples from single (LNMMA or Alpelisib) or combination treated cell lines and PDXs showed increased PARP degradation and cleaved caspase 3/9 with combination treatment. Pathway-focused RT-PCR analysis was performed using RNA collected from Hs578t cells treated for 48 hrs. with LNMMA, Alpelisib, or combination treatment. RT-PCR data were analyzed using Ingenuity Pathway Analysis (IPA; Qiagen) software which showed that insulin signaling and PI3K/AKT pathways were the most enriched pathways with single and combination treatment. In vivo data using PDX BCM 4664 showed that combination treatment of LNMMA+Alpelisib was most effective at reducing tumor volume in comparison to single treatment. Our preliminary results suggest that LNMMA and Alpelisib combination treatment have therapeutic potential in the treatment of MBC. Our results indicated that simultaneous inhibition of the NOS and PI3K/AKT pathways may represent a clinically relevant therapeutic concept against hard-to-treat MBC, and warrants further studies.

Citation Format: Tejaswini Parlapalle Reddy, Roberto Rosato, Liliana Guzman, Wei Qian, Jianying Zhou, Anthony J Kozielski, Jenny C Chang. Elucidating the synergistic relationship between the iNOS and PI3K/Akt pathways for treatment of metaplastic breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-03-05.