Overcoming delivery barriers in immunotherapy for glioblastoma

Delivery Strategy: Convection-Enhanced Delivery (CED); Description: Pressure-driven bulk flow via intracranial catheter; drug distributes through brain interstitial space; Key Advantages: Bypasses BBB; compatible with nanoparticles; can be combined with implantable pumps; Key Challenges: Invasive; requires surgery; inhomogeneous distribution; oedema risk

Delivery Strategy: MRI-Guided Focused Ultrasound (MRgFUS); Description: Focused ultrasound with microbubble cavitation to temporarily disrupt BBB; Key Advantages: Non-invasive; high spatiotemporal control; reversible BBB opening; Key Challenges: Complex parameters; slow clinical adoption; repeatability concerns

Delivery Strategy: Intrathecal Administration; Description: Injection into cerebrospinal fluid via spinal canal; Key Advantages: 100% CSF bioavailability; lower doses needed; avoids systemic toxicity; Key Challenges: Primarily spinal; requires position optimisation for brain delivery

Delivery Strategy: Cell-Based Targeting; Description: MSCs, CAR T cells, or engineered macrophages home to brain tumours; Key Advantages: Tumour-homing; can cross BBB; potential for sustained local therapy; Key Challenges: Complex manufacturing; safety concerns; limited clinical data

Delivery Strategy: Ligand-Mediated BBB Crossing; Description: Conjugation of targeting ligands (TfR, angiopep-2, glucose) to nanocarriers; Key Advantages: Active transport across BBB; dual tumour targeting potential; Key Challenges: Ligand affinity optimisation; protein corona masking; functional display challenges

Parameter: Scope; Details: Review of physiological, immunological, and drug delivery barriers for GBM immunotherapy; synthesis of preclinical and clinical studies

Parameter: Physical Barriers Discussed; Details: BBB (tight junctions, limited vesicular transport); Blood-tumour barrier (interstitial fluid pressure); Brain ECM (porosity, diffusion limitations)

Parameter: Immunological Barriers Discussed; Details: CNS immune privilege; "Cold" tumour (low TILs); HLA class I downregulation; GBM heterogeneity; cancer stem cells

Parameter: Delivery Technologies Reviewed; Details: CED, MRgFUS, intrathecal administration, cell-based therapies, ligand-mediated targeting

Parameter: Immunotherapy Strategies Reviewed; Details: Cancer vaccines (neoantigen, in situ reprogramming); Immunostimulatory scaffolds; Immune checkpoint blockade; Viral gene therapy (HSV-tk/Flt3L); VEGF-C-mediated lymphangiogenesis

Parameter: Animal Models Discussed; Details: Orthotopic murine GBM models (GL261, U87, B16F1-ova); Importance of immunocompetent models and BBB preservation

Parameter: Clinical Translation; Details: CED in phase I/II trials (paclitaxel, HSV-tk); MRgFUS first-in-human trials; Neoantigen vaccine phase I/Ib trial in GBM

Analysis Category: BBB Permeability Assessment; Methods Described: MRI contrast agents; Evans blue dye; fluorescent tracers; in vitro BBB models

Analysis Category: Drug Distribution; Methods Described: Fluorescence imaging (whole brain, sections); Bioluminescence (luciferase reporter); Autoradiography

Analysis Category: Nanoparticle Characterisation; Methods Described: Size, zeta potential, ligand density, stability in physiological conditions

Analysis Category: Immune Response Evaluation; Methods Described: Flow cytometry (TILs, macrophages, MDSCs); Immunohistochemistry (CD3, CD8, CD206); Cytokine profiling (Luminex); ELISPOT

Analysis Category: Tumour Burden Assessment; Methods Described: Caliper measurements; IVIS bioluminescence imaging; MRI; Survival analysis (Kaplan-Meier)

Analysis Category: Lymphatic Imaging; Methods Described: LYVE1 staining; Dural confluence quantification; Deep cervical lymph node ligation

Analysis Category: Gene Expression; Methods Described: qPCR; RNAseq; Western blot

Barrier: BBB; Key Finding: Passive transport limited to <400 Da or lipid-soluble molecules; all GBM patients have tumour regions with intact BBB

Barrier: EPR Effect; Key Finding: Up to 97% of nanoparticles enter tumours through active transcytosis, not passive extravasation

Barrier: Brain ECM; Key Finding: Historical belief required <64 nm for diffusion; dense PEG coating enabled 114 nm NP penetration

Barrier: Interstitial Fluid Pressure; Key Finding: Hinders convective transport from blood to tumour (blood-tumour barrier)

Application: Polymeric NPs + anti-tumour miRNAs (Lopez-Bertoni et al.); Outcome: Significant survival extension in orthotopic human brain cancer models

Application: PBAE NPs + HSV-tk suicide gene (Choi et al.); Outcome: Significant survival extension in paediatric CNS malignancy models

Application: Lipopolymeric NPs + multiplexed siRNAs (Yu et al.); Outcome: Survival benefit with implantable osmotic pump (no repeat invasive procedures)

Application: Paclitaxel via CED (phase I/II); Outcome: Significant tumour volume reduction in recurrent GBM

Application: HSV-tk liposomes via CED (phase I/II); Outcome: Tumour volume reduction; some complications (meningitis, oedema)

Parameter: Interstitial flow velocity (Curley et al.); Result: Doubled with MRgFUS

Parameter: NP dispersion (Curley et al.); Result: >100% increase

Parameter: Transfection of brain tumours (Curley et al.); Result: Fourfold increase vs. NP administration alone

Parameter: ALS first-in-human trial; Result: Successful BBB opening; normalised within 24 h; no serious adverse events

Ligand: Transferrin Receptor (TfR); Key Finding: Low affinity antibodies increased brain accumulation 5-fold vs. high affinity

Ligand: Engineered Fc fragments (Kariolis et al.); Key Finding: ~40-fold higher brain uptake vs. native antibodies

Ligand: Acid-labile Tf linker (Clark and Davis); Key Finding: Enabled parenchymal dispersion; non-cleavable linker trapped NPs in blood vessels

Ligand: Glucose (Anraku et al.); Key Finding: 56-fold increase in brain accumulation with fasting + glycemic control

Ligand: Glucose (Min et al.); Key Finding: ~7% whole brain accumulation; 30% gene knockdown; ~50% knockdown in cerebral cortex

Ligand: Angiopep-2 (Qiao et al.); Key Finding: 80% long-term survivors in GL261 model with TMZ + TGF-β siRNA

Ligand: Angiopep-2 (Zheng et al.); Key Finding: Significant tumour burden decrease in U87 model

Strategy: Neoantigen vaccine (Keskin et al., phase I/Ib GBM); Key Finding: Circulating neoantigen-specific CD4⁺/CD8⁺ T cell responses in patients not on dexamethasone; increased TILs

Strategy: In situ tumour reprogramming (Tzeng et al.); Key Finding: Long-term survivors with immunological memory in B16-F10 and MC38 models when combined with anti-PD-1

Strategy: IL-36γ + IL-23 + OX40L mRNA LNPs (Hewitt et al.); Key Finding: >90% long-term survival (MC38); >40% (B16-F10) + checkpoint blockade

Strategy: VEGF-C mRNA NPs (Song et al.); Key Finding: ~100% increase in dural lymphatic confluence; >80% long-term survival + checkpoint blockade

Strategy: HSV-tk + Flt3L gene therapy (Ali et al.); Key Finding: >80% survival extension in large tumours (macrophage- and CD4⁺ T cell-dependent)

Strategy: HSV-tk + Flt3L (King et al.); Key Finding: 70% long-term survival (single tumour); 50% (primary + distal secondary tumour)

---

1. Review article: This is a review, not original research; all conclusions are based on synthesis of published literature rather than new experimental data.

2. EPR effect controversy: The review discusses the challenge to the EPR effect paradigm (up to 97% of NPs enter tumours via active transcytosis), but this finding is based on a single recent study and remains debated.

3. Clinical translation gap: While many preclinical strategies show promise, the review acknowledges that CED and MRgFUS have had slow clinical adoption, and many immunotherapy approaches remain in early-stage trials.

4. Immunocompetent model limitations: The review notes that "the selection of animal models for studying drug delivery to brain tumours has also been shown to be critical," and emphasises that immunocompetent models are needed to assess true antitumor immune responses.

5. GBM heterogeneity: The review notes that "GBM cancer stem cells have been identified as the main drivers behind chemotherapeutic resistance and tumour recurrence," and effective therapeutics must target these populations.

6. Dexamethasone confounding: In the neoantigen vaccine trial, the authors note that patients receiving dexamethasone during vaccine priming showed reduced T cell responses, highlighting the immunosuppressive effect of standard care steroids.

7. No specific therapeutic cargo: The review covers delivery platforms broadly without focusing on a single therapeutic payload; clinical translation will require specific drug selection.

8. Cranial oedema intolerance: The review notes that "the cranial space is inherently oedema-intolerant," which "poses further limitations for flow rates and pressures associated with bulk transport of therapeutics as well as potential immunotoxicities."

9. Lack of head-to-head comparisons: The review does not directly compare the efficacy of different delivery strategies, as this is a synthesis of the literature.

10. Potential conflicts of interest: The corresponding author (J.J.G.) has potential conflicts (co-founder of Dome Therapeutics, SAB member of Tidal Therapeutics), which are disclosed.

Report prepared based on the published Drug Delivery and Translational Research review article. For full experimental details, supplementary materials, and complete references, please refer to the original publication.