Microbubbles are 1–10 μm diameter gas-filled acoustically-active particles, typically stabilized by a phospholipid monolayer shell. In cancer immunotherapy, the microbubble shell can be engineered through the bioconjugation of molecular ligands to facilitate the delivery and uptake of drugs, genes, or cells. This approach allows for precise control of immune stimulation and improves the delivery and pharmacokinetics of immunomodulatory agents at the target site. It has emerged as an attractive strategy for treating various cancer immunotherapy modalities, including monoclonal antibodies, immune checkpoint inhibitors, adoptive cell transfer, cytokine therapy, and vaccines.
1. Ultrasound Targeted Microbubbles
Ultrasound Targeted Microbubble (tMB) formulations have been engineered as ultrasound-responsive carriers to promote and enhance the local delivery and uptake of a wide variety of drugs, genes, and cells for immunotherapy applications. tMBs play a crucial role in facilitating the targeted delivery of immune cells, cytokines, antigens, and antibodies, thereby promoting the activation and infiltration of immune cells at various levels, including single cells, tissues, organs, and physiological systems [1][2][3][4]. In the context of cancer treatment, for instance, the aim is to induce modulation and modification of the tumor microenvironment, leading to enhanced adaptive immune-cell activation and subsequent destruction of the primary tumor and its metastases.
2. Monoclonal Antibody Immunotherapy
Monoclonal antibody immunotherapy aims to induce cell death by targeting specific antigens, sequences, or epitopes expressed at the disease target site
[5]. Therapeutic monoclonal antibodies (mAbs) can be administered unconjugated or conjugated with chemotherapeutic drugs and radioisotopes to specifically target tumors and minimize the toxicity effects of conventional chemotherapy
[6]. Although mAb immunotherapy has potential therapeutic benefits, poor penetration and heterogenous distribution can impact the therapeutic effectiveness
[5][6]. High concentrations of mAbs are often required, leading to adverse side effects due to the rapid metabolism and clearance rate through the kidneys
[7]. The use of mAb-tMBs combined with ultrasound (US) (mAb-tMBs+US) has been proposed as an image-guided delivery method to enhance targeting, improve local penetration, and potentiate the therapeutic effect of mAbs in different cancer therapies (
Table 1). Liao et al.
[8] demonstrated improved glioma treatment by administrating EGFR-tMB + US, resulting in increased tumor vessel permeability and enhanced tumor-suppressing effects. Kang et al.
[9] found that combining anti-DLL4-tMBs + US with DAPT for gastric tumor therapy was more effective than DAPT alone, showing synergistic antitumor proapoptotic effects. These effects were attributed to the regulation of apoptosis-related proteins Bcl-2 and BAX, as well as the tumor suppressor protein P53. Recently, Sun et al.
[10] developed a tMB construct that delivers pyropheophorbide sensitizer and therapeutic trastuzumab mAbs for targeted combination of sonodynamic and antibody therapies in gastric cancer. This therapy enhanced antibody accumulation at the tumor site, increased tumor cell apoptosis and tumor growth inhibition by suppressing AKT phosphorylation.
Table 1. Targeted microbubbles used for cancer immunotherapy.
Immunotherapy Modality |
Therapeutic Molecule |
Conjugation Chemistry |
MicrobubbleComposition |
Ultrasound Parameters |
Animal/Disease Model |
Reference |
Monoclonal antibody |
EGFR mAb |
Avidin-biotin |
Targestar™-SA Shell: Phospholipid Gas: C4F10 Diameter: 2.5 μm Dose: 3.6 × 109 MBs/kg or 29.7 µL/kg MVD |
f = 400 kHz PRF = 1 Hz Power = 5 W Time = 3–4 min |
Mouse/Glioma tumor |
[8] |
|
DLL4 mAb |
Avidin-biotin |
Targestar™-SA Shell: Phospholipids Gas: C4F10 Diameter: 2 μm Dose: 5.3 × 108 MBs/kg or 2.2 µL/kg MVD |
f = 1 MHz DC= 50% I = 2 W/cm2 Time = 1.5 min |
Mouse/Gastric cancer |
[9] |
|
Trastuzumab mAb |
NHS |
Shell: DPSC, DSPE-PEG-2000-NHS, Cholesterol and pyropheophorbide Gas: SF6 Diameter: 1.654 ± 1.07 μm Dose: unknown |
f = 1 MHz DC= 50% I = 2 W/cm2 Time = 5 min |
Mouse/Gastric cancer |
[10] |
Immune checkpoint inhibitors |
PDL-1 mAb |
NHS |
Shell: DSPC, DSPE-PEG-2000-NHS. Gas: SF6 Diameter: 1.06 ± 0.31 μm Dose: 6.9 × 1010 MBs/kg or 43.3 µL/kg MVD |
f = 1.1 MHz DC = 5% PRF = 100 Hz Time = 0.5 min |
Mouse/Colon cancer |
[11] |
|
PDL-1 mAb and Cisplatin |
Avidin-biotin Unbounded |
Shell: DPSC, DSPE-PEG-2000, -Biotin Gas: C3F8 Diameter: 1.01 ± 0.14 μm Dose: 1 × 108 MBs/kg or 0.06 µL/kg MVD |
f = 1 MHz DC = 50% I = 1W/cm2 PRF = 1 kHz Time = 1.5 min |
Mouse/Cervical cancer |
[12] |
|
PDL-1 mAb and miR-34a |
Avidin-biotin Electrostatic |
Shell: DSPC, DSPE-PEG-2000, DSPE-PEG-2000-Biotin, PEI-600 Gas: C3F8 Diameter: 0.940 ± 0.080 μm Dose: 4 × 109 MBs/kg or 1.7 µL/kg MVD |
f = 18 MHz DC = 50% I = 1 W/cm2 Time = 1.5 min |
Mouse/Cervical cancer |
[13] |
Vaccine |
CD11b mAb and CGAMP |
Maleimide |
Shell: DSPC, DSPE-PEG-2000, DSPE-PEG-5000-Maleimide Gas: C4F10 Diameter: 2.6 μm Dose: 1.4 × 109 MBs/kg or 13.1 µL/kg MVD |
f = 1 MHz DC = 50% I = 4 W/cm2 Time = 2 min |
Mouse/Breast cancer |
[14] |
|
HSP70-MAGEA1 |
Electrostatic |
Shell: Span 60 and Tween 80 Gas: SF6 Diameter: 6 μm Dose: 1.3 × 109 MBs/kg or 144.7 µL/kg MVD |
MI = 0.75 |
Mouse/Melanoma tumor |
[15] |
|
Dendritic cell plasma membrane fragments |
Hydrophobic |
Shell: DPPC, DPPA, DSPE-PEG5000 Gas: C3F8 Diameter: 1.21 ± 1.0 μm Dose: 5 × 108 MBs/kg or 0.5 µL/kg MVD |
f = 18 MHz MI = 0.75 Time = 5 min |
Mouse/Breast cancer |
[16] |
3. Immune Checkpoint Inhibitor Therapy
Immune checkpoint inhibitors (ICI) are variants of mAbs that activate T cells by blocking immune checkpoint receptors
[5]. Approved ICIs for therapy include programmed cell death protein-1 (PD-1), programmed death ligand-1 (PD-L1), and cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4)
[5][17]. However, systemic ICI therapy is limited by severe side effects associated with dosage, low treatment response, and overactivation of the immune response
[18]. To address these limitations, a controlled delivery strategy called PD-L1 mAb–tMBs + US has been proposed (
Table 1). Kim et al.
[11] reported that PD-L1 mAb–tMBs can improve therapeutic efficacy, increase the therapeutic index, reduce toxicity, and prevent immune responses and fatalities associated with systemic administration of PD-L1 mAb in colon cancer treatment. Ma et al.
[12] and Liu et al.
[13] both demonstrated that combining PD-L1 mAb–tMBs with chemotherapeutic drugs or loading PD-L1 mAb–tMBs with genes exhibited strong synergistic effects in inhibiting cervical tumor growth, improving survival rates, and reducing side effects compared to using either the drug/gene or PD-L1 mAb–tMBs alone. The combination treatment showed better immunological activity, as indicated by increased infiltration of CD8+ T cells and cytokine expression, ultimately resulting in an effective antitumor immune-killing effect.
4. Adoptive Cell Immunotherapy
Adoptive cell-mediated immunotherapy (ACT) involves the intravenous transfer of resident T cells or genetically modified T cells that specifically target tumor antigens and mediate anti-tumor functions. There are three types of ACT: tumor-infiltrating lymphocytes (TIL), T cell receptor (TCR) gene therapy, and chimeric antigen receptor-modified T cells (CAR-T)
[5][19]. However, ACT is limited by the lack of in vivo persistence of transferred cells, toxicities
associated with lymphodepletion, immune response, and cytokine release
[19]. Preliminary studies utilizing MBs targeted with specific antibodies and retroviruses (CD3, CD8, CD45RA, CD62L, CD3/CD28, and CD-19CAR) have demonstrated that tMBs have the potential to stimulate
[20][21], activate, transduce, and precisely sort specific phenotypes of CAR-T cells
[20]. These simulations result
in increased in vivo persistence, reduced toxicity, and improved antitumor response of adoptively transferred CAR-T cells compared to CAR-T cells obtained through conventional methods
[21].
5. Cytokine Immunotherapy
Cytokine-mediated immunotherapy involves the systemic administration of cytokines to enhance the immune response
[5]. Commonly used cytokines for immunotherapy in clinical and research settings include Interferon-alpha (IFN-α), Interleukins (ILs), and granulocyte-macrophage colony-stimulating factor (GM-CSF)
[5][22][23]. However, this therapy has several limitations, including low efficacy, high levels of toxicity, and immune response activation
[24]. IL-27-tMBs have shown promising results by enhancing cytokine bioactivity, inhibiting prostate tumor growth, and efficiently improving the recruitment of natural killer cells (NKT) and CD8+ cells to the tumor site compared to untargeted delivery
[25]. Additionally, IL-16–tMBs
[26] have been introduced for evaluating myocardial ischemia–reperfusion, detecting atherosclerosis, and detecting ovarian tumors, respectively.
6. Vaccine Immunotherapy
Vaccine-mediated immunotherapy involves administering specific antigens or protein fragments to stimulate an immune response
[5]. Different types of vaccines are used, including peptide-based vaccines
[27], DNA-based vaccines
[28], and cell-based vaccines such as NK cells, dendritic cells (DC), and CAR-T cells
[29]. However, the effectiveness of vaccine therapy is limited by tissue-specific antigens, low humoral responses, and heterogenous immune responses
[5][30]. Gene- and protein-loaded MBs targeted with tumor-specific antigens offer promising image-guided vaccine immunotherapy for breast and melanoma tumors, as demonstrated by Li et al.
[14] and Gao et al.
[15]. This approach can increase delivery efficiency, prolong survival rates, activate systemic antitumor immunity, inhibit and delay tumor growth
[14][15], reduce systemic toxicity, and inhibit cancer metastasis by bridging the innate and adaptive immune responses
[14]. Recently, Jungio et al.
[16] introduced the first cell-free vaccine in the form of tMBs with activated DC plasma membranes to enhance breast cancer tumor targeting, reduce tumor growth, and increase survival rates. Studies have also shown tumor growth inhibition and/or antigen-specific protection through DCs activated by mRNA-loaded MBs
[31] or tMBs conjugated with NK cells to promote the controlled delivery at the target site
[32].