Purpose: STING agonists require frequent intratumoral injections over months to achieve efficacy, leading to poor patient adherence, repeated disruption of the tumor microenvironment, increased metastasis risk, and limited applicability to hard-to-reach tumors. A single-injection delivery system that mimics multiple dosing would improve adherence, reduce metastasis risk, and expand clinical utility.
Hypothesis: If STING agonist is encapsulated in microfabricated PLGA microparticles engineered to release cargo in programmable pulses at predetermined time points, then a single intratumoral injection will trigger potent local and systemic antitumor immunity, inhibit tumor growth, prolong survival as effectively as multiple soluble doses, reduce metastasis, and improve outcomes in combination with immune checkpoint blockade and after surgical resection.
Aims: Primary aim: Develop microfabricated PLGA microparticles with a fully enclosed cavity for drug loading that release encapsulated STING agonist in discrete pulses at predetermined times with no premature leakage.
- Secondary aim 1: Characterize in vitro and in vivo release kinetics, cargo stability, and bioactivity of released cGAMP.
- Secondary aim 2: Evaluate antitumor efficacy of a single injection of cGAMP-loaded microparticles versus multiple soluble cGAMP injections in multiple mouse tumor models (B16F10 melanoma, 4T1 breast cancer, contralateral B16F10, orthotopic pancreatic KPC, and surgical resection models).
- Secondary aim 3: Assess STING pathway activation, local and systemic immune responses, immune memory, metastasis inhibition, toxicity, biodegradability, and platform versatility with other agents (pemetrexed, CpG DNA).
Delivery system:
Component: Polymer; Description: PLGA (polylactic-co-glycolic acid), FDA-approved and biodegradable
Component: Particle Type; Description: Microfabricated cubic microparticles (PLGA-MPs) produced by soft lithography / Stamp Assembly of Polymer Layers
Component: Dimensions; Description: Base: 200 × 200 × 100 µm; sealed particle: 400 × 400 × 300 µm; wall thickness 100 µm; volumetric loading capacity 8.4%
Component: Alternative Larger Design; Description: 300 × 300 × 200 µm cavity with 50 µm wall; loads ~10 µg cGAMP per particle
Component: Payload; Description: 3′3′-cGAMP (STING agonist), 2 µg per particle in standard design; also AF647-dextran (model), pemetrexed, CpG DNA
Component: Targeting Ligand; Description: None; local intratumoral or surgical-bed administration
Component: Release Programming; Description: Different PLGA properties yield pulses at predetermined times: PLGA-1 (day 4), PLGA-2 (day 8), PLGA-3 (day 11)
Component: Dosing Strategy; Description: Single injection combines cGAMP-S + cGAMP-loaded PLGA-1 + PLGA-2 to mimic multiple doses (e.g., 30 µg total cGAMP: 10 µg soluble + 10 µg PLGA-1 + 10 µg PLGA-2)
Component: Key Feature; Description: Pulsatile release with essentially no leakage before the intended release window
Approach: In vitro: Release kinetics in PBS pH 6.84 at 37°C; LC-MS for cGAMP structural integrity; RAW-Lucia ISG reporter cells for bioactivity. - In vivo mouse models: SKH1-E hairless mice for subcutaneous release imaging; C57BL/6 for B16F10 melanoma, contralateral B16F10, surgical resection B16F10, and orthotopic KPC pancreatic cancer; BALB/c for orthotopic 4T1 breast cancer. - Treatment groups: Untreated, empty PLGA-1/2 (EP), soluble cGAMP multiple injections (3× or 4× cGAMP-S), single injection cGAMP-MPs, cGAMP-MPs + anti-PD-1 (3× ICB), and in surgical models 1× cGAMP-S + EP vs 3× cGAMP-S vs 1× cGAMP-MPs. - Doses: cGAMP-MPs: 30 µg cGAMP total (10 µg cGAMP-S, 10 µg PLGA-1, 10 µg PLGA-2); soluble cGAMP: 10 µg per injection. Anti-PD-1: 100 µg intraperitoneally at days 7, 11, 15. - Sample sizes: Typically n = 4–10 per group; no blinding; power analysis not used.
Key methods:
Technique: SEM and microCT; Purpose: Particle morphology, array fidelity, and intratumoral localization
Technique: IVIS fluorescence imaging; Purpose: In vivo release kinetics of AF647-dextran-loaded microparticles
Technique: LC-MS and HPLC; Purpose: Quantify cGAMP, pemetrexed, and CpG release; confirm cGAMP structural integrity
Technique: NanoDrop; Purpose: Quantify cGAMP release
Technique: RAW-Lucia ISG reporter cells; Purpose: Measure bioactivity of released cGAMP
Technique: qPCR; Purpose: Cxcl10 and Irf7 mRNA expression in tumors
Technique: Western blot; Purpose: p-TBK1 and p-IRF3 activation
Technique: Flow cytometry; Purpose: Immune cell subsets in tumor microenvironment and circulation
Technique: Immunofluorescence and TUNEL; Purpose: CD8+ T cell infiltration and apoptosis in tumor sections
Technique: H&E staining; Purpose: Organ toxicity and lung metastasis quantification
Technique: Tumor volume and survival analysis; Purpose: Therapeutic efficacy
Technique: Rechallenge; Purpose: Protective immune memory
Key results: Pulsatile release: Microparticles released cargo in pulses at approximately 1, 4, 8, 11, 15, 18, and 97 days in vitro with no detectable leakage before release. In vivo release times for PLGA-1, PLGA-2, and PLGA-3 were 3.9 ± 1.1, 8.1 ± 1.5, and 11.5 ± 1.4 days, respectively. - Cargo integrity and bioactivity: Released cGAMP showed identical LC-MS elution and molecular mass to standard; >95% bioactivity was maintained in IRF reporter cells. - Antitumor efficacy: A single intratumoral injection of cGAMP-MPs inhibited tumor growth and prolonged survival as effectively as four intratumoral injections of soluble cGAMP in B16F10 melanoma and 4T1 breast tumor models. - Immune activation: cGAMP-MPs increased Cxcl10 and Irf7 mRNA, p-TBK1, and p-IRF3 in tumors; increased CD8+CD3+ and CD4+CD3+ T cells, NK cells, dendritic cells, and M1/M2 macrophage ratio in the tumor microenvironment. - Systemic immunity and memory: cGAMP-MPs increased IFN-γ+ CD8+ T cells in serum at day 21 by 5.1- and 4.9-fold vs untreated; increased memory CD62L−CD44+ CD4+ T cells ~6.2-fold and CD8+ T cells ~5.4-fold in the tumor microenvironment. Inhibited distant tumors in a contralateral B16F10 model; combination with anti-PD-1 further inhibited primary and distant tumor growth. - Metastasis reduction: In orthotopic 4T1 breast cancer, cGAMP-MPs significantly decreased metastatic foci on lung surfaces (P < 0.0001) and reduced relative lung tumor area (P < 0.01) compared with untreated; greater reduction than 3× cGAMP-S (P < 0.05). - Post-surgical recurrence: In a surgical resection B16F10 model, cGAMP-MPs reduced tumor recurrence rate from 100% (untreated) to 25%; 3× cGAMP-S also 25%; 1× cGAMP-S + EP was 87.5%. Tumor-free mice rechallenged with B16F10 showed slower tumor growth and prolonged survival. - Hard-to-reach tumors: In orthotopic pancreatic KPC tumors, cGAMP-MPs significantly inhibited primary tumor growth and lung metastasis vs untreated (P < 0.001); a single high dose of soluble cGAMP + empty particles did not show benefit. - Toxicity and biodegradation: No weight loss or behavioral changes; H&E of major organs showed no obvious morphology changes; empty PLGA-MPs caused no detectable inflammation; particles completely degraded and cleared by day 30 after subcutaneous injection.
Interpretation: The authors conclude that microfabricated PLGA microparticles capable of pulsatile STING agonist release can replace frequent intratumoral injections with a single administration, improving adherence, reducing metastasis risk, expanding applicability to hard-to-reach tumors, and serving as an adjuvant after surgery. The platform is modular for hydrophilic and hydrophobic drugs and holds promise for rapid clinical translation because it uses only FDA-approved PLGA.
Limitations: All efficacy and safety data are from mouse models; no large-animal or human validation. - Local administration still requires an accessible tumor or surgical bed; microparticles are too large for systemic delivery. - No active targeting ligand; delivery relies on local retention. - Release times are predetermined by PLGA formulation properties; on-demand or adjustable pulsatile timing is not demonstrated in vivo. - Researchers were not blinded; power analysis was not used to predetermine sample size. - Long-term toxicity and immune safety beyond the study period were not fully assessed. - The surgical resection model removed ~99% of tumor, which may not reflect minimal residual disease in all clinical settings. - No direct comparison with clinically approved sustained-release formulations or hydrogels in vivo.
