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| Charge-switching ionizable lipids |
Charge-switching ionizable lipids may separate efficient RNA delivery from one of the biggest liabilities of conventional LNPs
Lipid nanoparticles changed the trajectory of RNA medicine.
They solved one of the fundamental problems facing nucleic-acid therapeutics: how do you move a fragile, highly charged molecule such as mRNA through the body, into a cell, and finally into the cytoplasm where it can function?
But the same chemistry that makes lipid nanoparticles effective can also make them inflammatory.
That trade-off has become increasingly important as researchers try to move LNP technology beyond vaccines toward repeated treatment of cancer, inflammatory disease, genetic disorders and other chronic conditions.
A new study in Nature Nanotechnology proposes an intriguing way around the problem.
Instead of asking how to make conventional ionizable lipids slightly less inflammatory, the researchers redesigned their electrostatic behavior.
Their solution is a class of charge-switching ionizable lipids that are negatively charged under physiological conditions but change their charge as they encounter the acidic environment used during formulation and intracellular trafficking.
The resulting particles—called switchable nanoparticles, or SNPs—were able to deliver mRNA and DNA while avoiding several inflammatory pathways normally activated by conventional LNPs.
If the approach translates beyond the current preclinical models, it could address one of the central engineering constraints in RNA delivery.
The paradox at the heart of lipid nanoparticles
Modern LNPs rely heavily on ionizable lipids.
These molecules are extraordinarily useful because their charge changes with pH.
During formulation under acidic conditions, the lipids become positively charged. That helps them associate with negatively charged RNA and package it efficiently.
After administration, their reduced charge at physiological pH improves tolerability compared with permanently cationic lipids.
Then, after a nanoparticle is taken into a cell, acidification within endosomes helps ionizable lipids interact with endosomal membranes and promote release of the RNA cargo.
It is an elegant system.
But it contains a fundamental contradiction.
The properties needed for efficient nucleic-acid encapsulation and endosomal escape can also stimulate inflammation.
Conventional LNPs can activate pathways including:
complement signalling,
Toll-like receptor 4,
galectin-associated responses following endosomal damage,
and platelet-activating factor signalling.
These effects become particularly problematic in tissues that are already inflamed.
And many of the patients who could benefit most from RNA therapeutics—people with inflammatory, pulmonary, metabolic or degenerative diseases—may already have substantial baseline inflammation.
So the challenge is not merely:
Can an LNP deliver RNA?
It is increasingly:
Can it deliver enough RNA repeatedly without making the underlying disease worse?
A lipid that changes its electrostatic personality
The new study approaches this problem through lipid chemistry.
The researchers designed what they call S-lipids.
Their head group contains both:
a carboxylic acid, and
a tertiary amine.
That combination allows the lipid to switch its charge state across different pH conditions.
At physiological pH, around 7.4, the particles are negatively charged.
Under more acidic conditions, the charge state changes, supporting nucleic-acid encapsulation and intracellular delivery.
This is important because conventional ionizable LNPs are often discussed mainly in terms of their transition between neutral and positively charged states.
Here, the design deliberately introduces a negative surface state at physiological pH.
That apparently changes how the nanoparticle interacts with immune and inflammatory pathways.
The authors therefore attempt to decouple two properties that have traditionally been difficult to separate:
efficient delivery and inflammatory cationic chemistry.
Screening 144 new ionizable lipids
This was not based on a single fortunate molecule.
The team synthesized a library of 144 S-lipids and systematically evaluated their ability to deliver mRNA.
Several structural rules emerged.
The spacing between the amine and carboxyl group mattered.
So did the architecture of the hydrophobic tails.
Lipids with branched tails performed dramatically better than some corresponding linear-tail structures—in certain comparisons, by thousands-fold.
The best-performing particles had charge-transition properties suited to acidic intracellular compartments while remaining negatively charged at physiological pH.
Importantly, many candidates retained strong transfection activity.
Of 18 top-performing S-lipids investigated further, 11 produced nanoparticles capable of transfecting primary fibroblasts approximately as efficiently as LNPs containing highly effective established ionizable lipids.
So reducing inflammatory signalling did not necessarily require sacrificing RNA delivery.
That is the central engineering advance.
The particles still escaped the endosome
Avoiding positive charge alone would not be particularly useful if the RNA remained trapped inside endosomes.
Endosomal escape is one of the major bottlenecks in nucleic-acid delivery.
Conventional LNPs help destabilize endosomal membranes. But membrane disruption itself can also activate cellular stress and inflammatory pathways.
The switchable nanoparticles behaved differently.
According to the study, they were capable of releasing nucleic acids efficiently without producing the same degree of endosomal membrane damage.
This was reflected in reduced activation of galectin-8, a cellular marker associated with damaged endosomal membranes.
The particles also avoided substantial activation of platelet-activating factor signalling.
That suggests a potentially important distinction:
successful endosomal release does not necessarily require extensive destructive membrane disruption.
If confirmed across different cell types and formulations, that concept could influence the next generation of delivery-vector design.
Less complement and TLR4 activation
The negative physiological charge of the switchable nanoparticles also affected extracellular inflammatory interactions.
The particles showed reduced activation of complement and TLR4-related signalling compared with conventional LNP systems examined in the study.
That matters because complement activation is a long-standing concern for nanoparticle therapeutics.
Nanoparticle surfaces immediately encounter proteins when they enter biological fluids, forming a protein corona that affects distribution, clearance and immune recognition.
Changing surface electrostatics can fundamentally alter those interactions.
The S-lipid design therefore does more than modify intracellular trafficking.
It changes the nanoparticle's biological identity before it even reaches the cell.
Human immune cells showed the same trend
A particularly relevant part of the study involved human peripheral blood mononuclear cells.
The researchers compared conventional LNPs and switchable nanoparticles carrying nucleic acids.
Both systems could deliver their cargo.
But traditional LNPs produced substantially greater cytokine responses, whereas the switchable formulations produced much less inflammatory stimulation.
That does not establish clinical safety.
PBMC experiments cannot reproduce the complexity of an entire human immune system.
But they provide an important bridge between standard cell-line experiments and animal studies.
The question now becomes whether this lower immunostimulatory profile persists across different human donors, doses, routes of administration and therapeutic cargos.
The difference became striking in already-inflamed animals
One of the most interesting aspects of the study was its focus on pre-existing inflammation.
This is biologically important.
A nanoparticle that looks reasonably tolerable in a healthy animal may behave very differently in an organism whose immune system is already activated.
The researchers therefore treated mice with lipopolysaccharide, or LPS, to induce inflammation before administering the nanoparticles.
Conventional LNPs substantially increased inflammatory cytokine responses.
The switchable nanoparticles caused much less additional inflammatory stimulation.
That finding gets to the practical significance of the technology.
Future RNA medicines will often not be administered to healthy individuals.
They will be administered to sick patients.
Nanoparticle safety therefore needs to be tested in disease-relevant physiological states—not only under idealized healthy conditions.
A dramatic DNA-delivery experiment
The researchers also examined plasmid DNA delivery.
This is notable because LNP development is usually dominated by mRNA and siRNA, but the underlying platform could potentially carry several classes of nucleic acids.
Switchable nanoparticles successfully delivered nanoplasmid DNA to mice.
The comparison with conventional particles was striking: traditional LNP formulations used for the DNA-delivery experiment produced severe acute toxicity, whereas the switchable formulation was substantially better tolerated.
This result reinforces a broader point.
Nanoparticle toxicity does not depend only on the lipid.
It emerges from the interaction among:
lipid chemistry + nucleic-acid cargo + dose + route + tissue environment.
A delivery system that works well for one cargo cannot automatically be assumed safe for another.
Treating acute lung injury with IL-22 mRNA
The researchers then moved from reporter delivery to a therapeutic experiment.
They used an LPS-induced mouse model of acute lung injury and delivered mRNA encoding interleukin-22 (IL-22).
IL-22 can promote tissue-protective and regenerative responses in epithelial barriers.
The switchable nanoparticles carrying IL-22 mRNA performed better than several conventional LNP formulations.
The proposed reason is especially interesting.
It was not simply that the switchable nanoparticles delivered more therapeutic mRNA.
Rather, conventional LNPs added inflammatory stress to an organ that was already inflamed.
The therapeutic payload was therefore competing against inflammatory effects generated by its own delivery vehicle.
The low-inflammatory nanoparticle avoided much of that penalty.
This illustrates a principle that deserves more attention in drug-delivery research:
The biological effect of a nucleic-acid medicine is the sum of the cargo and the carrier.
An excellent therapeutic RNA delivered by an inflammatory vehicle may produce a weaker therapeutic outcome than expected.
Why this matters beyond one nanoparticle formulation
Most discussion around LNP optimization focuses on parameters such as:
transfection efficiency,
organ targeting,
particle size,
encapsulation efficiency,
pKa,
endosomal escape,
and biodegradability.
This study adds another design dimension:
What charge should the particle have when it is not actively performing a delivery step?
Instead of optimizing one fixed electrostatic state, the researchers designed a lipid whose charge changes according to its biological environment.
That points toward a broader concept of environment-responsive nanomedicine.
Future delivery vehicles may increasingly be engineered to behave differently during:
formulation → circulation → tissue entry → endocytosis → endosomal acidification → cytosolic release.
The best nanoparticle may not have one ideal property.
It may need a sequence of properties activated at precisely the right biological stage.
Implications for RNA therapeutics
For mRNA therapeutics, the implications are substantial.
Lower-inflammatory nanoparticles could potentially make several currently difficult applications more realistic:
Repeated mRNA dosing.
Vaccines may require only a few administrations. Protein-replacement therapies could require dozens or hundreds.
Treatment of inflammatory diseases.
The carrier should ideally not worsen the pathology it is intended to treat.
Higher therapeutic doses.
Delivery toxicity often constrains the amount of nucleic acid that can safely be administered.
Gene editing.
CRISPR components frequently require potent transient delivery, sometimes at relatively high doses.
DNA delivery.
Improved tolerability could expand nanoparticle delivery beyond mRNA and siRNA.
And perhaps most importantly, lower intrinsic carrier toxicity could widen the therapeutic window.
Could the concept matter for plant RNA delivery?
The study concerns mammalian systems, so direct extrapolation to plants would be premature.
Plant cells present completely different delivery barriers, including cell walls, cuticles, extracellular matrices and distinct endosomal trafficking pathways.
Nevertheless, the design principle is relevant.
RNA-delivery materials do not merely transport RNA.
Their charge state determines how they interact with biological surfaces.
In plant biotechnology, positively charged carriers such as polymers, peptides, carbon-based materials and lipid systems are often attractive because they bind negatively charged RNA efficiently.
But excessive charge can also cause aggregation, poor tissue penetration, membrane damage or phytotoxicity.
A carrier capable of switching charge according to its environment could potentially provide strong RNA complexation during formulation while reducing undesirable interactions after delivery.
Whether the specific S-lipid chemistry described here is useful for plants remains an open experimental question.
The broader principle is nevertheless worth exploring.
Important questions remain
The results are impressive, but these are still preclinical experiments.
Several questions will determine whether charge-switching nanoparticles become a broadly useful platform.
How do they behave after repeated administration?
What are their long-term metabolites?
How does their biodistribution compare with clinically established LNPs?
Does their reduced inflammatory profile persist at therapeutically relevant high doses?
Can their tissue tropism be engineered?
Will the advantage remain in non-human primates and eventually humans?
Can different ionizable-lipid structures reproduce the effect?
And perhaps most importantly:
Can low inflammation and high transfection remain linked across many therapeutic applications—or was this balance specific to the formulations tested here?
These questions will require substantial additional work.
The larger lesson
The success of the first generation of lipid nanoparticles came from learning how to package RNA and release it inside cells.
The next generation may require something subtler.
We need nanoparticles that know when to interact strongly with biology—and when not to.
The charge-switching strategy described in this study is therefore interesting not simply because it produces another high-performing lipid.
It represents a change in design philosophy.
Instead of accepting inflammation as an unavoidable cost of efficient nucleic-acid delivery, the researchers asked whether the molecular mechanisms responsible for delivery and toxicity could be separated.
Their results suggest that, at least in these experimental systems, they can.
And that could be an important step toward a future in which the delivery vehicle becomes almost invisible—doing exactly what is required to move RNA into the right cell, and very little else.
Research discussed
Liang D., Qi Y., Murthy N. and colleagues.
Charge-switching ionizable lipids lower the toxicity of lipid nanoparticles.
Nature Nanotechnology (2026).
The study describes ionizable lipids containing both a carboxylic acid and an amine that form charge-switching nanoparticles capable of delivering mRNA and DNA while reducing activation of several inflammatory pathways associated with conventional lipid nanoparticles.
