According to the latest global cancer statistics, the number of patients with solid tumors far exceeds that of patients with non-solid tumors [1]. Immune checkpoint blockers (ICBs) have been widely used in the clinical treatment of solid tumors. However, a few patients with a favorable tumor immune microenvironment (e.g., abundant immune infiltration and active immune response) demonstrate a favorable therapeutic response to ICBs. Therefore, there is an urgent need to explore novel therapeutic strategies that can format the immunologically favorable solid tumor microenvironment, thereby enhancing the efficacy of ICBs in patients with solid tumors.
Recently, we explored that inhibition of ribosome biogenesis in solid tumors can remodel the tumor immune microenvironment, thereby enhancing the therapeutic efficacy of ICB targeting LAG3 (The ribosomal state plasticity is essential to forming the tumor-permissive microenvironment) [2]. Additionally, histone deacetylase inhibitors (HDACis), such as Vorinostat (SAHA) and Panobinostat, have been reported to enhance the infiltration and cytotoxicity of CD8⁺ T cells, as well suppress their exhaustion in solid tumors. The two agents also reprogram M2-like macrophages to an M1-like phenotype, consequently improving the efficacy of PD-1 immune checkpoint blockade [3, 4].
FK228 (Romidepsin) is another HDACi that has been approved by the FDA for the treatment of non-solid tumors such as cutaneous and peripheral T-cell lymphoma. Due to it is relatively expensive, our team has focused on improving the production yield of FK228 to make it more accessible to patients. Based on our previously established Burkholderia-specific recombineering system, we successfully obtained FK228 at a yield three times higher than the previously reported maximum [5]. However, what is the efficacy of FK228 in the treatment of solid tumors? As early as the 1990s, studies using ex vivo cell-based assays and human cancer xenograft models in nude mice demonstrated that FK228 could inhibit the growth of solid tumors [6]. At that time, the concept of the tumor immune microenvironment had not yet been widely recognized, which limited the comprehensive evaluation of FK228’s therapeutic effects in solid tumors. Subsequent numerous clinical trials on solid tumors also indicated that monotherapy with FK228 only resulted in moderate therapeutic efficacy. Consequently, FK228 has not been approved for the treatment of solid tumors to date. Surprisingly, in a Phase II clinical trial involving patients with metastatic castration-resistant prostate cancer, two subjects who had undergone standard hormone therapy exhibited significant antitumor responses after treatment with FK228 [7], suggesting that FK228 may possess considerable clinical activity in combination regimens for solid tumors.
Although two previous studies have demonstrated that FK228 significantly enhanced the response to PD-1 blockade immunotherapy in multiple lung or colon tumor models[8, 9], the underlying mechanism by which FK228 enhances sensitivity to this treatment has not yet been elucidated. In this study, we found that FK228 can serve as a novel necroptosis inducer in cancer cells by triggering endoplasmic reticulum stress. Necroptotic cancer cells treated with FK228 functioned immunogenic cell death. We further characterized the tumor immune microenvironment landscapes pre- and post- FK228-treatment using single-cell RNA sequencing (scRNA-seq) to further analyze whether FK228-induced tumor cell death could alter the tumor immune microenvironment. FK228-treatment significantly enhanced the infiltration of tumor-killing immunocytes, including CD8+ T and natural killer cells, particularly activating tumor-infiltrated CD8+ T cells, as well as shifting macrophages toward the pro-inflammatory phenotype. Therefore, we observed that the treatment with FK228 created a favorable immune microenvironment in solid tumors.
Furthermore, following FK228 treatment, the expression of the immune checkpoint PD-L1 on both cancer cells and macrophages, as well as PD-1 on T cells, was significantly upregulated. This phenomenon reflects that, following single-agent FK228 treatment, although the infiltration and activity of antitumor immune cells increase, their exhaustion was also accelerated. This may be an intrinsic reason why FK228 monotherapy showed the moderate efficacy and has not been approved for clinical treatment of solid tumors. However, the FK228-therapeutic tumor immune microenvironment is more suitable for treatment with PD-L1 immune checkpoint blockers. Therefore, we treated CT26 and 4T1 tumor-bearing mouse models with a combination of FK228 and a PD-L1 inhibitor, which significantly delayed tumor growth and extended the survival of the tumor-bearing mice. Surprisingly, approximate 36% mice (total eleven mice) in the combination therapy group had their tumors completely regressed in CT26-bearing mouse model.
In conclusion, our study firstly provides a novel mechanism and the single-cell landscapes of FK228-induced tumor immune microenvironment remodeling in solid tumors. This histone deacetylase inhibitor turns the unfavorable TIME into a favorable TIME, contributing to enhancing the efficacy of immunotherapy in solid tumors. In line with this notion, our findings highlight FK228 as a novel ICB sensitizer that could be considered to synergize with ICB in solid tumors in the future. Currently, many clinical trials have not been completed. Our investigation of FK228 in solid tumor therapy provides a reference for improving current clinical studies. Noted that FK228 may exhibit certain toxicities in clinical treatment, including cardiotoxicity [10]. Therefore, our previously developed tumor-targeting nanocarrier platform to deliver FK228 are necessary to achieve precise tumor treatment [11].
1 Bray F, Laversanne M, Sung H, Ferlay J, Siegel RL, Soerjomataram I et al. Global cancer statistics 2022: GLOBOCAN estimates of incidence and mortality worldwide for 36 cancers in 185 countries. CA Cancer J Clin. 2024; 74: 229-263.
2 Cui K, Liu B, Gong L, Wan Q, Tang H, Gong Z et al. Targeting ribosomes reprograms the tumour microenvironment and augments cancer immunotherapy. Br J Cancer. 2025.
3 He Y, Fang Y, Zhang M, Zhao Y, Tu B, Shi M et al. Remodeling "cold" tumor immune microenvironment via epigenetic-based therapy using targeted liposomes with in situ formed albumin corona. Acta Pharm Sin B. 2022; 12: 2057-2073.
4 Peng H, Chen B, Huang W, Tang Y, Jiang Y, Zhang W et al. Reprogramming Tumor-Associated Macrophages To Reverse EGFR(T790M) Resistance by Dual-Targeting Codelivery of Gefitinib/Vorinostat. Nano Lett. 2017; 17: 7684-7690.
5 Gong K, Wang M, Duan Q, Li G, Yong D, Ren C et al. High-yield production of FK228 and new derivatives in a Burkholderia chassis. Metab Eng. 2023; 75: 131-142.
6 Ueda H, Manda T, Matsumoto S, Mukumoto S, Nishigaki F, Kawamura I et al. FR901228, a novel antitumor bicyclic depsipeptide produced by Chromobacterium violaceum No. 968. III. Antitumor activities on experimental tumors in mice. J Antibiot (Tokyo). 1994; 47: 315-323.
7 Molife LR, Attard G, Fong PC, Karavasilis V, Reid AH, Patterson S et al. Phase II, two-stage, single-arm trial of the histone deacetylase inhibitor (HDACi) romidepsin in metastatic castration-resistant prostate cancer (CRPC). Ann Oncol. 2010; 21: 109-113.
8 Shi Y, Fu Y, Zhang X, Zhao G, Yao Y, Guo Y et al. Romidepsin (FK228) regulates the expression of the immune checkpoint ligand PD-L1 and suppresses cellular immune functions in colon cancer. Cancer Immunol Immunother. 2021; 70: 61-73.
9 Zheng H, Zhao W, Yan C, Watson CC, Massengill M, Xie M et al. HDAC Inhibitors Enhance T-Cell Chemokine Expression and Augment Response to PD-1 Immunotherapy in Lung Adenocarcinoma. Clin Cancer Res. 2016; 22: 4119-4132.
10 Stadler WM, Margolin K, Ferber S, McCulloch W, Thompson JA. A phase II study of depsipeptide in refractory metastatic renal cell cancer. Clin Genitourin Cancer. 2006; 5: 57-60.
11 Gong L, Tian L, Cui K, Chen Y, Liu B, Li D et al. An off-the-shelf small extracellular vesicle nanomedicine for tumor targeting therapy. J Control Release. 2023; 364: 672-686.