Abstract P5-08-04: LMP7-specific inhibitor M3258 modulates the tumor microenvironment of aggressive breast cancer
Authors: Xie Xuemei , Lee Jangsoon , Manyam Ganiraju C , Pearson Troy , Walter-Bausch Gina , Friese-Hamim Manja , Goodstal Samantha M , Tripathy Debu , Jing Wang , Sanderson Michael , Ueno Naoto T 
Published: March 2022

Background: LMP7 (beta5i/PSMB8) is a proteolytic subunit of the immunoproteasome that is predominantly expressed in leukocytes and inflamed tissues. The immunoproteasome degrades ubiquitinated proteins for maintenance of cell homeostasis and generation of peptides for MHC class I presentation. LMP7 has been implicated in multiple myeloma, autoimmune and inflammatory diseases, and inflammation-related cancers. Here, we evaluated the effect of M3258, an orally bioavailable, highly-selective, and reversible inhibitor of LMP7, on the tumor microenvironment (TME) using in vitro and in vivo models of triple-negative breast cancer (TNBC) and inflammatory breast cancer (IBC). Methods: The proteolytic activity of M3258 was determined using the fluorogenic LMP7-specific peptidic substrate (Ac-ANW)2R110. Its anti-proliferation activity was determined using the CellTiter-Blue cell viability and sulforhodamine B staining assays. Expression of immunoproteasome subunits under different conditions was examined by Western blotting. All in vitro assessments were done with or without IFNγ, which is known to increase immunoproteasome expression. The in vivo anti-tumor efficacy of M3258 was evaluated using a SUM-149 PT TN-IBC xenograft mouse model implanted in humanized mice. Single-cell RNA sequencing (scRNA-seq) was used in this same study to examine the impact of M3258 on the TME. Results: M3258 inhibited LMP7 activity (IC50 = 6.5-212.8 nM) and had no effects on LMP7 protein expression in a panel of TNBC cells (DU4475, HCC1187), IBC cells (SUM190, IBC3, KPL4), and TN-IBC cells (SUM-149 PT, BCX-010, FC-IBC02). M3258 inhibited proliferation of TNBC and IBC cell lines in vitro (IC50 = 1-20 µM). In keeping with previous reports, IFNγ treatment strongly induced LMP7 expression and enhanced its proteolytic activity in these cell lines in vitro and increased M3258 sensitivity (2- to 5-fold). In the humanized SUM-149 PT xenograft mouse model, M3258 orally administered at 10 mg/kg significantly inhibited tumor growth versus vehicle (31.4%, P &lt; 0.01) and was tolerated. ScRNA-seq analysis of the terminal tumor samples from this study revealed that M3258 reduced the abundance of CD4+ and CD8+ T cells, Tregs, M1 and M2 macrophages, and dendritic cells. Gene expression profiling indicated a particularly pronounced inhibitory effect of M3258 on M1 and M2 macrophages. Furthermore, M3258 treatment activated CD8+ T cells and suppressed expression of genes involved in inflammatory response, IFNα signaling, IFNγ signaling, and TNFα signaling in immune cells. In light of the scRNA-seq findings, we assessed the impact of M1 and M2 macrophages on the invasiveness of TN-IBC cells in vitro. Co-cultures with M1 or M2 macrophages enhanced invasiveness of SUM-149 PT cells (4.6-fold by M1, P &lt; 0.001; 6.3-fold by M2, P &lt; 0.01) and BCX-010 cells (2.1-fold by M1, P &lt; 0.05; 4.0-fold by M2, P &lt; 0.01). Since M2 macrophages showed a superior effect than M1 macrophages on TN-IBC cell invasiveness, we further assessed M3258’s ability to inhibit such activity. M3258 at 500 nM significantly inhibited M2 macrophage-induced invasion of SUM-149 PT cells (29.6%, P &lt; 0.05) and BCX-010 (49.5%, P &lt; 0.01). Conclusion: Using the exquisitely selective LMP7 inhibitor M3258, we were able to demonstrate for the first time that LMP7 plays an important role in the inflammatory microenvironment, proliferation and invasiveness of TNBC and IBC cells, in particular by modulating the pathogenic role of M2 macrophages. These data warrant future in vivo studies into the antitumor efficacy of M3258 when used with immunotherapy agents.

Citation Format: Xuemei Xie, Jangsoon Lee, Ganiraju C Manyam, Troy Pearson, Gina Walter-Bausch, Manja Friese-Hamim, Samantha M Goodstal, Debu Tripathy, Wang Jing, Michael Sanderson, Naoto T Ueno. LMP7-specific inhibitor M3258 modulates the tumor microenvironment of aggressive breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-08-04.