Low-Intensity Focused Ultrasound-Responsive Phase-Transitional Liposomes Loaded with STING Agonist Enhances Immune Activation for Breast Cancer Immunotherapy

Background: Pharmacologically targeting the STING pathway offers a novel approach to cancer immunotherapy. However, small-molecule STING agonists face challenges such as poor tumor accumulation, rapid clearance, and short-lived effects within the tumor microenvironment, thus limiting their therapeutic potential. To address the challenges of poor specificity and inadequate targeting of STING in breast cancer treatment, herein, we report the design and development of a targeted liposomal delivery system modified with the tumor-targeting peptide iRGD (iRGD-STING-PFP@liposomes). With LIFU irradiation, the liposomal system exploits acoustic cavitation, where gas nuclei form and collapse within the hydrophobic region of the liposome lipid bilayer (transient pore formation), which leads to significantly enhanced drug release. Methods: Transmission electron microscopy (TEM) was used to investigate the physicochemical properties of the targeted liposomes. Encapsulation efficiency and in vitro release were assessed using the dialysis bag method, while the effects of iRGD on liposome targeting were evaluated through laser confocal microscopy. The CCK-8 assay was used to investigate the toxicity and cell growth effects of this system on 4T1 breast cancer cells and HUVEC vascular endothelial cells. A subcutaneous breast cancer tumor model was established to evaluate the tumor-killing effects and therapeutic mechanism of the newly developed liposomes. Results: The liposome carrier exhibited a regular morphology, with a particle size of 232.16 ± 19.82 nm, as indicated by dynamic light scattering (DLS), and demonstrated low toxicity to both HUVEC and 4T1 cells. With an encapsulation efficiency of 41.82 ± 5.67%, the carrier exhibited a slow release pattern in vitro after STING loading. Targeting results indicated that iRGD modification enhanced the system's ability to target 4T1 cells. The iRGD-STING-PFP@liposomes group demonstrated significant tumor growth inhibition in the subcutaneous breast cancer mouse model with effective activation of the immune system, resulting in the highest populations of matured dendritic cells (71.2 ± 5.4%), increased presentation of tumor-related antigens, promoted CD8+ T cell infiltration at the tumor site, and enhanced NK cell activity. Conclusions: The iRGD-STING-PFP@liposomes targeted drug delivery system effectively targets breast cancer cells, providing a new strategy for breast cancer immunotherapy. These findings indicate that iRGD-STING-PFP@liposomes could successfully deliver STING agonists to tumor tissue, trigger the innate immune response, and may serve as a potential platform for targeted immunotherapy.