Combining DNA methylation inhibition and STING agonist in the treatment of metastatic triple-negative breast cancer

Triple negative breast cancer (TNBC) is an aggressive cancer associated with early metastatic events leading to a poor prognosis. According to the American Cancer Society, the 5-year relative survival rate is 91% in patients with localized TNBC but only 12% for those with distant metastatic TNBC. Thus, there is an urgent need to understand the mechanisms that drive TNBC metastasis to uncover more effective therapeutic approaches. In this regard, we profiled RNA Seq and DNA methylation in 8 metastatic TNBC cell lines with lung, liver, and bone organotropism compared to 3 parental TNBC lines. RNA-Seq revealed downregulation of IFN-I pathways associated with metastatic organotropism in the lung, liver and bone. Surprisingly, DNA methylation is enriched within the promoters of STING/IFN-related genes in these metastatic TNBC cells. In line with these findings, the analysis of TCGA human TNBC tumors showed a decreased expression of STING, an IFN-I pathway activator, in metastases versus primary tumors. In addition, a low STING expression associates with poorer TNBC patient survival. Consistent with transcriptomic data, the treatment with decitabine, a DNA methylation inhibitor, restored IFNbeta and STING expression in metastatic TNBC cells, supporting epigenetic silencing with decitabine. Furthermore, combining decitabine with a STING agonist synergistically reduced viability in metastatic TNBC lines, indicating that targeting DNA methylation and activating the IFN-I pathway may represent therapeutic vulnerabilities in metastatic TNBC. More importantly, the decitabine/STINGa therapy showed a potent effect in targeting TNBC metastatic lesions in vivo with lung and liver organotropisms and increased the median survival of mice by almost 50% compared to the control single drug-treatment groups. Mechanistically, the inhibition of IFNbeta with specific blocking anti-IFNbeta mAb and the knockdown of TBK1, a mediator of IFN-I pathway, both abrogate the synergy between decitabine and STINGa in targeting metastatic tumor xenografts suggesting that the decitabine/STINGa targets metastatic TNBC via TBK1/IFNbeta pathway. Furthermore, the spectral flow cytometry profiling of metastatic TNBC xenografts revealed that decitabine/STINGa therapy significantly enhanced NK cell infiltration and activation within metastatic tumors. Finally, the in vivo NK cell depletion showed that decitabine STINGa therapy is dependent on NK cells in treating metastatic TNBC. Altogether, this work suggests that the DNA methylation inhibition with decitabine and the stimulation of the IFNbeta pathway with STINGa represent a new approach to efficiently target the metastatic TNBC that currently lacks effective targeted therapies and immunotherapies. This study also brought new evidences that decitabine/STINGa combination therapy is dependent on the TBK1/IFNbeta pathway and NK cells in targeting metastatic TNBC in vivo.