Combination of PRMT5 and Mcl-1 inhibitors overcomes immune checkpoint inhibitor resistance in pleural mesothelioma

Introduction: Pleural mesothelioma (PM) is an aggressive cancer with a poor prognosis. Primary resistance to chemotherapy and immune checkpoint inhibitors (ICI) is common, demanding new therapeutic strategies. Our previous work showed the anti-apoptotic protein, Myeloid Cell Leukemia 1(MCL-1) drives chemoresistance. What is unknown is whether targeting MCL-1 will sensitize PM to ICIs. Our multi-omic data linked Mcl-1 to ICI resistance through the methionine metabolic pathway, in which PRMT5 is a critical enzyme. This study evaluates whether targeting the PRMT5/MCL-1 axis will overcome ICI resistance in PM to increase response rates. Methods: Inhibition of PRMT5 and Mcl-1 was assessed in PM cell lines (H28 and H2452) and Patient-Derived Xenograft (PDX) models. Transcriptomics, metabolomics and reverse-phase protein array were performed on PDX model comparing treatment arms of PRMT5 inhibition, MCL-1 inhibition, and the combination (Cmb) versus vehicle control (Cnt). Additionally, ICI treatment response was evaluated by in vitro T-cell co-culture assays. The synergistic effects of PRMT5 and Mcl-1 inhibition were investigated in PM lines by Western Blot with both shRNA-mediated gene knockdown or inhibitor treatment. Annexin V/propidium Iodide staining and flow cytometry analysis were performed. Results: In vitro, the combination of PRMT5 and Mcl-1 inhibition significantly increased apoptosis compared to either single agent alone and control (Cnt vs Cmb, H28: 8.1% vs 34.1%; H2452: 7.2% vs 25.1%). These cells also displayed synergistic lack of proliferation (Cnt vs Cmb: H28: ~1.6-fold; H2452: ~2-fold). In the PDX model, PRMT5 and Mcl-1 inhibition showed a significant decrease of tumor growth (Cnt vs Cmb: ~1.8-fold). Multi-omic analyses implicated the PI3K/Akt signaling pathway in tumor growth suppression with the combined treatment. Western blot analysis confirmed this mechanism based on phosphorylation of proteins in PI3K/Akt and mTOR signaling pathway, including GSK-3a/b, PRAS40, mTOR, TSC2, P70S6K and AKT1. The combination of PRMT5 and Mcl-1 inhibitors also showed a significant improvement of ICI (anti-PD-L1) response in vitro (Cnt vs Cmb: ~1.4-fold). Conclusion: Co-targeting PRMT5 and Mcl-1 synergistically enhance apoptosis and proliferation via PI3K/Akt/mTOR pathway in PM. Inhibition of PRMT5/Mcl-1 axis may improve treatment response and overcome primary resistance. Our findings provide a novel strategy to translate into clinical trials.