Kinetic PEG Shedding Tunes mRNA-LNP Delivery

Research Question

PEG can protect lipid nanoparticles (LNPs) and support transport through biological barriers, but its surface coating can also impede cell interactions. Sun and colleagues asked whether PEG removal could be programmed to occur on a useful timescale: slow enough to retain shielding during transport, then fast enough to improve cellular delivery. They designed PEG-lipids with oxanorbornadiene (OND) linkers that cleave through retro-Diels–Alder fragmentation at physiological temperature, and investigated whether different shedding rates suit inhaled and intravenous mRNA delivery.

Relevance to ALC-0315 research: this is a study of PEG-lipid design and delivery behavior, not a comparison of ionizable lipids. The reported LNP preparation uses SM-102, DSPC, cholesterol, and PEG-lipid; ALC-0315 is not evaluated. Its findings therefore inform questions about PEG shielding and shedding but do not establish how an ALC-0315 formulation would perform.

Study Design and Methods

The authors synthesized OND-linked PEG-lipids and varied linker structure to tune cleavage. They first characterized degradation by nuclear magnetic resonance and used computational calculations to examine how linker structure related to the cleavage barrier. They then formulated mRNA LNPs and monitored PEG release using a fluorescence resonance energy transfer (FRET) assay based on labeled PEG-lipid and DiI in the particles. Formulations were compared for particle properties and mRNA encapsulation, and the study included noncleavable PEG-lipid controls.

Cell experiments separated a labeled-RNA uptake readout from functional reporter output. Cy5-labeled mEGFP mRNA fluorescence was used to assess cell-associated uptake by flow cytometry; this signal is not, by itself, evidence of translation. Reporter expression was assessed separately, including firefly luciferase mRNA output. The authors also used lysosomal colocalization as an indicator relevant to endosomal escape.

In animals, the researchers compared PEG-shedding formulations after inhaled or intravenous administration. They used fluorescent lipid labels such as DiD to assess particle-associated distribution, and reporter mRNA expression to assess functional delivery. Therapeutic studies used interleukin-12 mRNA in mouse models of pulmonary metastasis or subcutaneous tumors. These therapeutic experiments provide context for delivery consequences; the central design question concerns PEG shedding from mRNA LNPs.

Main Findings

Paper findings: OND-linker structure altered PEG shedding rates at 37 °C. In serum-containing conditions, the long-residing DSPE anchor reduced nonspecific PEG loss compared with the DMG anchor, helping preserve the differences programmed by linker design. The DSPE formulations had particle size and mRNA encapsulation comparable to conventional DSPE-PEG LNPs at formulation, although prolonged incubation at 37 °C was associated with particle growth and declining encapsulation, particularly for faster-shedding formulations.

In cell studies, thermoresponsive DSPE-PEG LNPs showed greater Cy5-labeled mRNA uptake and reporter expression than the noncleavable PEG control, with the reported trend increasing from slow to fast shedding. In a PEG-preset experiment, reducing the residual PEG level before cell exposure was associated with increased uptake and expression. The authors also report lower lysosomal colocalization for faster-shedding formulations, consistent with improved escape from lysosomal compartments; this imaging-based measure is not a direct quantitative measurement of cytosolic mRNA.

Route influenced which shedding rate performed best. For inhaled delivery, dense PEG coverage improved early lung retention of DiD-labeled particles, while the noncleavable dense-PEG formulation produced little reporter expression. Fast-shedding 5% DSPE-PEG-J LNPs gave the highest pulmonary reporter expression among the compared formulations. The distinction matters: DiD fluorescence reports particle-associated signal and does not establish that mRNA reached cells or was translated.

After intravenous delivery, fast shedding was associated with lower tissue accumulation than slower or intermediate shedding in the comparisons reported. Yet intermediate-shedding DSPE-PEG-G LNPs produced greater tumor firefly-luciferase expression than the noncleavable DSPE-PEG control, despite lower tumor-associated DiD signal. Thus, particle-associated distribution and functional mRNA expression did not track identically. In the reported mouse tumor models, mIL-12 LNPs with fast shedding improved outcomes in the pulmonary-metastasis study, while the intermediate-shedding formulation improved tumor control in the subcutaneous model relative to the noncleavable control.

Interpretation: the results support a route-dependent design principle rather than a universally optimal PEG level or shedding rate. Fast shedding appears suited to the short sequence of mucus traversal followed by pulmonary cell interaction; intravenous delivery requires a longer shielding phase, making intermediate shedding a better balance in the tested system. These are conclusions about the formulations and animal models studied, not proof that the same timing will generalize to other LNP compositions or delivery settings.

Limitations

The study uses preclinical cell and animal models, so it does not establish human performance or clinical benefit. The authors acknowledge that changing linker chemistry may also alter LNP membrane properties and cellular interactions, making it difficult to isolate shedding rate as the sole cause of delivery differences. Lysosomal colocalization supports an endosomal-escape interpretation but is an indirect readout. The authors also identify time-dependent protein-corona remodeling as a possible contributor to circulation and tissue handling that requires further study.

For formulation comparisons, the ionizable lipid was SM-102 rather than ALC-0315, and the work does not isolate the effect of changing ionizable-lipid identity. DiD distribution, Cy5-labeled RNA uptake, and translated reporter output are distinct measurements and should not be treated as interchangeable. The findings therefore support investigation of dynamic PEG-lipid behavior, but not an ALC-0315-specific performance claim.

Source

Sun, S. et al. “Kinetically Programmed Thermoresponsive Polyethylene Glycol Shedding from Lipid Nanoparticles Improves mRNA Delivery.” Journal of the American Chemical Society. https://doi.org/10.1021/jacs.6c09907.