Globally, cancer remains a significant public health threat [1]. Despite great progression for cancer therapies in past decades, the most of cancers remain incurable. Therefore, there is a pressing need to address this increasingly serious challenge, and new strategies are essential to complement existing treatments, such as surgery, chemotherapy, radiotherapy, and recently developed immunotherapy [[2], [3], [4]].

Exploiting existing knowledge regarding the redox homeostasis of cancer cells to develop effective therapeutics, distinct strategies have been designed and applied for cancer therapy based on the use of reactive oxygen species (ROS) [5,6]. The key to these new approaches is to stimulate ROS production in a controllable manner. Photodynamic therapy (PDT) is a type of cancer treatment that controls ROS production through light irradiation to kill tumor cells in a highly specific manner [5]. However, photosensitizers are severely limited because of the insufficient penetration depth of light [7]. Sonodynamic therapy (SDT), which inhibits cancer cells by ultrasound stimulation toward sonosensitizers to produce ROS, is an potent therapeutic modality for precision oncology treatment due to its unique characteristics, including operational convenience, non-invasiveness, and more importantly, high tissue penetration depth (up to 10 cm) of ultrasound [8]. However, the high concentrations of the antioxidant glutathione (GSH) in cancer cells could compromise the effect of ROS on tumor suppression, which greatly restricts the therapeutic efficacy of SDT [[9], [10], [11]]. Therefore, developing a novel strategy to tip the balance of the cellular redox is a top priority to improve SDT.

To address this challenge, a multifunctional nanoplatform endowed simultaneously with the properties of GSH depletion and ROS production should be developed. For example, to tackle the redox homeostasis inside tumor cells, sulfhydryl-reactive agents, such as Fe3+ or Cu2+, are frequently introduced into nanoplatforms to scavenge the antioxidants such as GSH [12,13]. In addition, to boost ROS production within tumor sites, chemodynamic therapy (CDT) has been developed. CDT is a common modality which uses Fe2+, Mn2+, Cu+, and Co2+-based inorganic catalysts to convert the mild oxidant H2O2 into strongly oxidizing and toxic hydroxyl radicals (·OH) through Fenton/Fenton-like reactions [14,15]. Notably, CDT that specifically produces ROS at tumor sites, relies on the relatively high amount of H2O2 in cancer cells rather than O2, which does not impair the therapeutic effect of oxygen-based strategies against the hypoxic nature of tumors [16,17].

Although nanomaterial-based nanomedicine currently offers many powerful tools to treat cancer, most focus on the “immunosilent” apoptosis process. In contrast, ferroptosis is a non-apoptotic form of cell death with iron-dependent regulation that has been shown to enhance or alter the immune system [[18], [19], [20]]. The abundant accumulation of ROS and lipid peroxidation (LPO) products are hallmarks of ferroptosis [[21], [22], [23]]. Recently, it has reported that immunogenic cell death (ICD) occurs simultaneously with ferroptosis [24,25]. ICD is a mode of cell death that can induce immunogenicity through the release of damage-associated molecular patterns (DAMPs) from injured cancer cells. Many DAMPs have been identified, including calreticulin (CRT), endoplasmic reticulum protein 57 (ERp57), high mobility group box 1 (HMGB1), and adenosine triphosphate (ATP) [26,27]. Crucially, the secretion of DAMPs can enhance the immunogenicity of cancer cells and the ability of dendritic cells (DCs) to process and present antigens, which facilitate T-cell-dependent anti-tumor immunity [28]. Furthermore, endoplasmic reticulum (ER) stress plays an essential role in intracellular pathways that influence ICD and regulate the induction of DAMPs, which is a critical factor in determining the immunogenicity of cancer cells [25,29].

In this study, we designed a novel nanoplatform with HMME@Fe-TA nanoparticles (HFT NPs) that simultaneously integrate the properties of SDT and CDT to affect tumors by activating intracellular apoptosis, ferroptosis, ER stress, and ICD (Scheme.1). First, HMME in the HFT NPs can generate singlet oxygen (1O2) upon ultrasound irradiation. Second, Fe3+ is used to consume the high levels of GSH in cancer cells and further also catalyze the conversion of H2O2 into ·OH through the Fenton reaction. Taken together, the GSH depletion and LPO accumulation result in the induction of ferroptosis; the production of abundant ROS promotes cancer cell apoptosis, ER stress and finally initiates ICD. Furthermore, HFT NPs plus US can trigger DC maturation and activate T-cell-related antitumor immunity in vivo, possibly mediated by ferroptosis and ICD. More importantly, the addition of PD-1 blockade in the regimen of HFT NPs plus ultrasound can remarkably enhance the anti-tumor efficiency of immunotherapy. Conclusively, this study not only provides a new nanoplatform demonstrating effective antitumor activity but also offers an alternative approach to synergistically improve the efficiency of SDT in tumor treatment and ultimately augment the anti-tumor performance of immunotherapy using simple materials.