Breast cancer remains one of the most common malignancies in women worldwide, and although overall survival has improved in recent decades, metastatic breast cancer—especially bone metastasis—continues to be a major cause of poor prognosis and mortality.

Current treatments are still largely focused on controlling primary tumours, with limited efficacy against metastatic lesions, which has driven growing interest in therapeutic strategies that can both eradicate tumours and reprogram the immunosuppressive tumour microenvironment.

“Sonodynamic therapy (SDT), with its deep tissue penetration and strong spatiotemporal controllability, has emerged as a promising approach for solid tumour treatment; however, currently available multifunctional sonosensitizers still face key limitations, including poor biodegradability in inorganic systems and limited multifunctionalization in organic small molecules.” said the author Ming Wu, a researcher at Gongli Hospital of Pudong New Area, “Against this backdrop, covalent organic frameworks (COFs), owing to their tunable structures, favourable biocompatibility, and functionalization potential, are increasingly regarded as promising platforms for next-generation multifunctional sonodynamic therapy.”

In this study, the authors developed ferrocene-modified nanoscale covalent organic frameworks (mCOFs) through a top-down strategy by reacting microscale COFs with aminoferrocene, thereby achieving both nanosizing and Fenton-like catalytic functionality.

The research is published in the journal Cyborg and Bionic Systems.

Under ultrasound irradiation, the platform was designed to generate singlet oxygen, while the Fe²⁺ centres in ferrocene catalysed the conversion of endogenous H₂O₂ in the tumour microenvironment into hydroxyl radicals, enabling amplified oxidative damage through a dual ROS mechanism.

The authors first performed systematic characterisation of particle size, dispersibility, crystalline structure, and ROS-generation capability, and then evaluated cellular uptake, intracellular ROS production, apoptosis, ferroptosis, and immunogenic cell death-related effects in 4T1 breast cancer cells.

Finally, using orthotopic breast tumour and bone metastasis mouse models together with intravenous administration and ultrasound activation, they assessed the therapeutic performance of this nanoplatform in suppressing primary tumours, remodelling the tumour immune microenvironment, and inhibiting bone metastasis.

The results showed that the engineered mCOFs could markedly amplify oxidative damage under ultrasound activation and simultaneously induce both apoptosis and ferroptosis in breast cancer cells.

Compared with the controls, mCOF + US exhibited substantially enhanced singlet oxygen and hydroxyl radical generation, leading to a pronounced increase in intracellular ROS.

In vitro, treatment with mCOF + US reduced 4T1 cell viability to 24.3% and increased the apoptosis rate to 84.51%, accompanied by strong lipid peroxidation, indicating a synergistic tumoricidal effect mediated by combined apoptosis and ferroptosis.

In addition, this treatment triggered robust immunogenic cell death, as reflected by an increase in ATP release from 0.37 nM to 1.75 nM, an elevation in dendritic cell maturation from 5.27% to 23.6%, and 3.81-fold and 3.14-fold increases in IL-6 and TNF-α levels, respectively.

In vivo, intravenously injected mCOFs showed high tumour accumulation at 12 h post-injection, and ultrasound activation led to marked inhibition of orthotopic tumour growth while alleviating osteolytic destruction associated with bone metastasis, as evidenced by improved bone volume fraction and bone mineral density.

Further immune analyses revealed increased infiltration of mature DCs, CD4+ and CD8+ T cells, NK cells, and IFN-γ-positive CD8+ T cells, suggesting that this strategy not only directly kills tumour cells but also remodels the immunosuppressive tumour microenvironment to synergistically inhibit breast cancer progression and bone metastasis.

Overall, this study developed a ferrocene-modified nanoscale COF-based sonodynamic platform that integrates ultrasound-triggered ROS generation with Fenton-like catalysis, ferroptosis induction, and immune microenvironment remodelling, thereby achieving synergistic inhibition of both primary breast tumours and bone metastasis.

The significance of this work lies in the fact that it not only provides a biocompatible and multifunctional organic nanotherapeutic system with potent antitumor activity, but also demonstrates that combining sonodynamic therapy with immunomodulation may help overcome the traditional limitation of therapies that mainly focus on primary tumour control while offering a broader strategy for metastatic intervention.

“By introducing therapeutically active ferrocene moieties during the nanosizing of COFs, we provide a new design strategy for multifunctional sonosensitizers and further demonstrate the promising potential of nanomedicine-based combination therapies for suppressing breast cancer progression and bone metastasis.” said Ming Wu.

Authors of the paper include Ming Wu, Yiqing Zeng, JianGang Chen, Zhen Yang, Siyuan Song, Rongkai Yan, Taofik Al Hassan, and Yan Zhang.

Source: Beijing Institute of Technology Press Co., Ltd