PEG-free LNP: How BioNTech Redefines mRNA Delivery Stability

Introduction

In the world of mRNA therapeutics and lipid nanoparticles (LNPs),

PEG-lipids have long been treated as a default design choice.

They are used to:

• Provide steric shielding to prevent aggregation

• Extend circulation time

• Reduce nonspecific interactions

However, as mRNA technologies move beyond one-time vaccines toward repeat-dose systemic therapies, PEG-associated risks—most notably anti-PEG antibodies and the accelerated blood clearance (ABC) phenomenon—are becoming increasingly relevant at the platform level.

A recent bioRxiv preprint from BioNTech Delivery Technologies asks a deceptively simple but fundamental question:

Is PEG truly indispensable for LNP stability?

Their answer leads to a compelling alternative: PEG-free LNPs stabilized by electrostatic repulsion.

Why PEG-free LNPs Become Unstable at Physiological pH

The scientific strength of this work lies in how clearly the problem is defined before proposing a solution.

The authors systematically examined the zeta potential (ζ-potential) of PEG-free LNPs across different pH conditions and observed:

• pH 5.0 (formulation conditions): positive ζ-potential due to protonated ionizable lipids

• pH 8.5: negative ζ-potential dominated by RNA surface charge

• pH 7.4 (physiological and storage conditions): ζ-potential near 0 mV

This finding is critical.

At pH 7.4—the condition most relevant for in vivo use and long-term storage—PEG-free LNPs lack sufficient electrostatic repulsion, making aggregation thermodynamically favorable.

In other words, the issue is not simply the absence of PEG, but the absence of any stabilizing physical mechanism under physiological conditions.

The Core Mechanism of PEG-free LNPs: Electrostatic Stabilization

Instead of replacing PEG with another polymer, the BioNTech team took a more fundamental approach.

They introduced sodium triphosphate (3P), a multivalent anion, to the surface of PEG-free LNPs.

This design is based on three key properties:

• A −3 charge providing high charge density

• Strong electrostatic interaction with surface-exposed ionizable lipid headgroups

• Formation of a stable, negatively charged surface layer

Experimentally, they showed that:

• At ≥5 mM 3P, PEG-free LNPs maintained a ζ-potential of approximately −4 to −7 mV

• This charge range was sufficient to preserve colloidal stability at pH 7.4

Importantly, not all anions worked:

• Monophosphate and citrate failed to provide long-term stability

This demonstrates that electrostatic stabilization is a tunable and designable mechanism, not a nonspecific ionic effect.

CMC-Relevant Evidence: Long-Term Stability Without PEG

A PEG-free concept only matters if it survives real-world constraints.

To address this, the authors conducted nine-month stability studies at 4°C, using acceptance criteria aligned with CMC and translational considerations:

• Particle size ≤115 nm

• PDI ≤0.3

• RNA integrity ≥65%

Under these conditions, PEG-free 3P-LNPs and conventional PEG-LNPs were nearly indistinguishable:

• Comparable size and size distribution

• Preserved RNA integrity

• Intact morphology confirmed by cryo-TEM

These data establish that PEG-free LNPs can meet product-level stability requirements, not just short-term experimental benchmarks.

In Vivo Performance: PEG-free LNPs Alter Delivery Kinetics

In mouse intravenous dosing studies, PEG-free LNPs displayed a distinct but interpretable behavior:

• At 6 hours post-injection, liver luciferase expression was ~2× higher than PEG-LNPs

• At 24 hours, total expression levels converged

• Biodistribution remained liver-dominant for both formulations

Rather than overclaiming superiority, the authors offered a mechanistically grounded interpretation:

• PEG-lipids delay early uptake by inhibiting ApoE-mediated liver entry

• PEG-free LNPs bypass this kinetic barrier, enabling faster hepatic uptake

This highlights an underappreciated role of PEG:

PEG does not merely stabilize LNPs—it actively shapes delivery kinetics

MALDI-MS Imaging: Biodistribution Is Not Biological Effect

To disentangle physical distribution from functional expression, the study employed MALDI mass spectrometry imaging of liver tissue.

The results revealed:

• Ionizable lipids enriched in periportal regions

• GFP expression localized predominantly in pericentral hepatocytes

This spatial mismatch underscores a critical insight for mRNA delivery:

Where nanoparticles accumulate is not always where transgene expression occurs

Hepatic zonation, cellular metabolism, and translation efficiency all modulate functional outcomes—factors often obscured by bulk biodistribution measurements.

What PEG-free LNPs Change at the Scientific Level

Stripped of hype, this PEG-free LNP study delivers four substantive contributions:

1. It reframes PEG-lipids as one stabilization strategy—not a biological necessity

2. It demonstrates that electrostatic repulsion can replace steric shielding

3. It achieves PEG-free systemic mRNA delivery without sacrificing stability, efficacy, or tolerability

4. It expands the design space of LNP system engineering beyond polymer-centric solutions

Conclusion: PEG-free LNPs as a System Design Signal

As mRNA therapeutics advance toward:

• Repeat dosing regimens

• Chronic indications

• Fine-tuned delivery kinetics

The field must move beyond the assumption that one formulation paradigm fits all use cases.

BioNTech’s PEG-free LNP work does not eliminate PEG from the toolbox—but it decisively proves that PEG is no longer the only viable foundation for stable, systemically delivered LNPs.

And that realization may define the next decade of mRNA delivery engineering.

Looking Ahead: Technical Consulting in LNP & mRNA Delivery Systems

As the field of mRNA therapeutics evolves, challenges are no longer confined to whether delivery works—but how well, how reproducibly, and how sustainably it works across programs, indications, and dosing regimens.

Many teams today are grappling with questions such as:

• How to redesign LNP systems beyond PEG-centric assumptions

• How to balance stability, delivery kinetics, and repeat dosing requirements

• How to translate formulation concepts into CMC-robust, scalable platforms

• How to interpret in vivo data at a system level, rather than as isolated readouts

These are not problems solved by a single formulation tweak—they require system engineering across chemistry, biology, and manufacturing.

Technical Consulting Support

Learn More

I provide technical consulting for biotech startups, platform teams, and R&D organizations working on:

• Lipid nanoparticle (LNP) and nucleic acid delivery system design

• PEG-free and next-generation stabilization strategies

• mRNA delivery optimization for repeat dosing and systemic applications

• Translational thinking across formulation, in vivo biology, and CMC considerations

My focus is not on generic advice, but on helping teams:

• Clarify design trade-offs early

• Avoid common platform-level pitfalls

• Build delivery systems that are scientifically sound and development-ready

If your team is exploring PEG-free LNPs, rethinking delivery kinetics, or scaling an mRNA platform toward the clinic, I’m happy to discuss how a system-level approach can support your goals.

👉 Learn more about my technical consulting work at LuTra Studio or reach out directly to start a focused technical conversation.

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Reference

PEG-free, Triphosphate-Stabilized LNPs Enable Potent RNA Delivery

BioNTech Delivery Technologies（bioRxiv, 2025）