Beyond COVID vaccines: what’s next for lipid nanoparticles?

Lipid nanoparticles (LNPs) transport small molecules into the body. The most well-known LNP cargo is mRNA, the key constituent of some of the early vaccines against COVID-19. But that is just one application: LNPs can carry many different types of payload, and have applications beyond vaccines.

Barbara Mui has been working on LNPs (and their predecessors, liposomes) since she was a PhD student in Pieter Cullis’s group in the 1990s. “In those days, LNPs encapsulated anti-cancer drugs,” says Mui, who is currently a senior scientist at Acuitas, the company that developed the LNPs used in the Pfizer-BioNTech mRNA vaccine against SARS-CoV-2. She says it soon became clear that LNPs worked even better as carriers of polynucleotides. “The first one that worked really well was encapsulating small RNAs,” Mui recalls.

But it was mRNA where LNPs proved most effective, primarily because LNPs are comprised of positively charged lipid nanoparticles that encapsulate negatively charged mRNA. Once in the body, LNPs enter cells via endocytosis into endosomes and are released into the cytoplasm. “Without the specially designed chemistry, the LNP and mRNA would be degraded in the endosome,” says Kathryn Whitehead, professor in the departments of chemical engineering and biomedical engineering at Carnegie Mellon University.

LNPs are an ideal delivery system for mRNA. “COVID accelerated the acceptance of LNPs and people are more interested in them,” says Mui. LNP-mRNA vaccines for other infectious diseases, such as HIV or malaria, or for non-communicable diseases such as cancer, could be next. And the potential doesn’t end with mRNA, there is even more scope to adapt LNPs to carry different types of cargo. But to realize these potential benefits, researchers first need to overcome challenges and decrease toxicity, increase their ability to escape from the endosomes, increase their thermostability, and work out how to effectively target LNPs to organs across the body.

Beyond mRNA vaccines

“The most exciting direction where the field is going right now is gene editing,” says Yulia Eygeris, scientist at EnterX Bio: a company founded in 2021 by Eygeris' postdoc supervisor, Gaurav Sahay, to commercialize LNP research.

LNPs can carry gene editing machinery like Cas9 mRNA or guide RNA into cells. This opens the ability for LNPs to be used as a delivery system for gene therapy. At the moment, there is an LNP-based CRISPR-Cas9 candidate treatment for people with heterozygous familial hypercholesterolemia in clinical trials, which targets the PCSK9 gene in the liver. Other gene therapy possibilities could include manipulating the CFTR gene in people with cystic fibrosis, or for treating rare genetic diseases.

Another potential application for LNPs is immunotherapy. Genetically modifying lymphocytes such as T cells or NK cells with chimeric antibody receptors (CARs) has proven useful in blood cancers. Often this process involves extracting lymphocytes from the blood of the person receiving the treatment, editing the cells in culture to express CARs, and then reintroducing them into the blood. However, LNPs could make it possible to express the desired CAR in vivo, by shuttling CAR mRNA to the target lymphocytes. Mui has been involved in in vivo studies showing this process works in mouse T cells (Rurik, J.G. et al. Science 375, 91-96, 2022). And Vita Golubovskaya, VP of research and development at ProMab Biotechnologies, presented preliminary data (available here) at the CAR-TCR Summit in September 2022, regarding LNPs that direct CAR-mRNA to NK cells, which can then kill target cells. “The RNA-LNP is a very exciting and novel technology that can be used for delivering CAR and bi-specific antibodies against cancer,” she says.

LNPs can also carry small interfering RNA (siRNA), for example in patisiran, the first FDA-approved siRNA drug, which uses LNPs to deliver siRNA against a gene product called transthyretin. This treats a form of amyloidosis by inhibiting the production of the transthyretin protein.

A lot of research still needs to be done for LNPs to act as optimal carriers in all their varied roles. One of the main challenges is that gene therapy and other regular treatments require higher doses or more treatments than vaccines. In these higher doses, LNPs can trigger cytotoxic reactions, so reducing the toxicity of LNPs is high on the agenda.

Reduce toxicity, increase efficacy

There are different ways to make LNP treatments less toxic. One is by studying how lipids affect toxicity.

“There are solutions if the lipids are fully degradable,” says Dan Peer, director of the Laboratory of Nanomedicine at Tel Aviv University. Lipids that linger in the cell after delivering their cargo are more likely to activate an immune response than those that melt away. Peer has been developing a range of new lipids, licensed to his company NeoVac, that show increased biodegradability and less immunogenicity, among other features. “We believe that less immunogenic lipids will be much better for therapy.”

It will also help make LNPs more effective in how they deliver their cargo. One of the obstacles currently hindering efficiency is that LNPs tend to get trapped in endosomes when they’re taken up by the cell and not fully released to their targets. “Improved endosomal escape would be a big deal for future generations of LNPs, given that current LNPs are estimated to escape the endosome less than 5% of the time.” says Whitehead. More escapes would allow for lower doses of LNPs to be used, and in turn reduce any cytotoxic side effects.

Reaching the right organs

Another key challenge to broadening the uses of LNPs is finding ways for them to reach different parts of the body. LNPs naturally move to the liver, but for applications such as targeted gene therapy it’s necessary to direct them to other organs, such as the lung, kidney or brain. “There is this inherent need to bypass the barriers specific to each organ,” says Eygeris. That means preventing liver accumulation, but also directing LNPs to a specific location. For example, they would need to cross the blood-brain barrier to be effective in the brain.

Exactly how LNPs can be better directed to their desired sites of action is not a simple question. “Different people are trying different ways, and no-one has a clear-cut answer,” says Mui. Some groups are examining how the lipids in LNPs affect targeting to different organs, while others are exploring the role of adding targeting ligands to the surface of the LNP to help them bind to specific cells.

Eygeris says that finding new LNP structures is a very active research area. “That's kind of what everybody's working on right now,” she says. “If you have something that is able to bypass the liver and go to any other organ, like lung or spleen, then that significantly increases the potential of your therapy.”

Meanwhile, Peer has also focused on improving nanoparticle thermostability. An obstacle to the widespread delivery of LNP-mRNA COVID-19 vaccines is the need to keep them stored at very low temperatures; thermostable LNPs could potentially be kept at room temperature. Peer’s group is still testing the thermostable lipids they developed, but he hopes they can make mRNA vaccines available to more countries, especially in the Global South. “Thermostable formulations are essential to change the landscape of mRNA vaccines and therapeutics,” says Peer. “It will be more accessible than having freezers.”

Peer is optimistic for the potential of LNP-based treatments beyond the pandemic, although he notes that there is a lot more work to be done. “We learned a lot during COVID,” he says. “Now it's time to move to the next level.”