Dual targeting of PI3K and NOS pathways in Metaplastic BreastCancer (MBC) - ABSTRACT Metaplastic breast cancer (MpBC) is a rare subset accounting for <5% of all breast cancers. MpBC is a significant health challenge as it exhibits the most dismal prognosis of all breast cancer subtypes, worse than non-MpBC triple-negative breast cancer (TNBC), with median survival rate of 8 months or less in patients with metastatic disease. Due to a lack of druggable targets, the main therapeutic option for metastatic MpBC remains systemic chemotherapy, despite known resistance to most cytotoxic drugs. One common molecular alteration in MpBC is hyperactivation of the phosphoinositide 3-kinase and protein kinase B (PI3K/AKT) pathway. Additionally, we published that MpBC also displays a gain-of-function oncogenic mutation in ribosomal protein L39 (RPL39), which is responsible for treatment resistance, stem cell self-renewal, and lung metastasis. The mechanistic function of RPL39 is mediated through inducible nitric oxide synthase (iNOS)-mediated nitric oxide production. In a recently published clinical trial targeting this nitric oxide synthase (NOS) pathway with a pan-NOS inhibitor NG-methyl-L-arginine acetate (L-NMMA), high efficacy in chemorefractory TNBC patients was demonstrated. Furthermore, in vivo studies performed showed a significant reduction in tumor growth, associated with a significant increase in apoptosis after the alpelisib/L-NMMA combinatorial regimen. Therefore, we hypothesize that the NOS and PI3K signaling pathways may exert their oncogenic responses synergistically to promote aggressive tumor growth. To test this hypothesis, Specific Aim 1 seeks to demonstrate the therapeutic efficacy of simultaneous inhibition of NOS and PI3K pathways with chemotherapy in MpBC pre- clinical models on primary tumor growth and metastasis. Specific Aim 2 will investigate the global and RPL39- specific ribosome translation landscape in response to NOS/PI3K inhibition in MpBC. In Specific Aim 3, the cell- cell interactions among tumor cells, myeloid cells, lymphoid cells, and stromal cells within the tumor microenvironment and their role in supporting cancer niche populations will be evaluated at the single-cell level using spatial transcriptomics, immunofluorescence, CyTOF imaging systems, and a multi-modal data analysis model. This study thus proposes a mechanistic investigation of a combinatorial targeted approach against the two key pathways in MpBC, identifies cell–cell interactions, and develops unique crosstalk models that will effectively predict outcome and treatment response and complement our recently funded U01 clinical trial on MpBC patients.
