Nanoparticles for mRNA delivery benefit from chemically evolved lipids

To create lipid A1 nanoparticles to deliver mRNA to target cells, scientists from the University of Pennsylvania used compound A3. A3 refers to an amine-aldehyde-alkyne coupling reaction that scientists at the University of Pennsylvania used to iteratively accelerate the structural optimization of propargylamine-based ionizable lipids. Naturally, these lipids are called A3 lipids. And when A3 lipids that have been properly optimized self-assemble into lipid nanoparticles, the result is an mRNA delivery vehicle that can safely travel throughout the body to target cells, efficiently release its contents, and degrade through biodegradation.

So how did the lipid nanoparticles containing lipid A3 developed by Penn’s team qualify as A1? They showed superior results in preclinical models for two high-priority applications: delivering genes that can be used to treat hereditary amyloidosis and delivering an mRNA COVID-19 vaccine. In both cases, the engineered lipids performed better than lipids meeting current industry standards.

Details appeared recently in Natural Biomedical Engineeringin a paper entitled “Optimizing the activity and biodegradability of ionizable lipids for mRNA delivery through directed chemical evolution.” The paper describes how Penn’s team used the A3 coupling reaction as a tool for iterative chemical derivatization and combinatorial chemistry. With it, Penn’s team achieved step-by-step optimization of the structure of ionizable lipids, a feat that had not previously been achieved despite the use of rational design and combinatorial synthesis to develop potent and biodegradable ionizable lipids.

“Over five cycles of this directed chemical evolution, we identified dozens of biodegradable and asymmetric A3 lipids with delivery activities comparable to or superior to the reference ionizable lipid,” the paper’s authors write. “We then derived structure-activity relationships for the head group, ester linkage and tail.

“Compared with standard ionizable lipids, lead A3 lipid improved liver delivery of an mRNA genome editor and intramuscular delivery of an mRNA vaccine against SARS-CoV-2. Structural criteria for ionizable lipids discovered through directed chemical evolution may accelerate the development of LNPs for mRNA delivery.”

This new method for developing ionizable lipids is expected to have broad applications for mRNA-based vaccines and therapeutics that are intended to treat a range of conditions, from genetic disorders to infectious diseases. In addition, the new approach could potentially speed up the development of mRNA therapies in general. While developing an effective lipid using traditional methods can take years, the team’s process of directed evolution can reduce that time to months or even weeks.

“We hope this method will accelerate the development of mRNA therapeutics and vaccines, bringing new treatments to patients faster than ever before,” said Michael J. Mitchell, assistant professor of bioengineering at the University of Pennsylvania.

Penn’s team’s nanoparticles are based on ionizable lipids, which are lipids that can switch between charged and neutral states depending on their environment. This switch is important for the travel of nanoparticles: in the bloodstream, ionizable lipids remain neutral, preventing toxicity. But once inside the target cell, they become positively charged, causing the release of their mRNA payload.

To develop safer and more effective ionizable lipids, Penn State engineers took a unique approach that combines two prevailing techniques: medicinal chemistry, which involves slowly and painstakingly designing molecules step by step, and combinatorial chemistry, which involves quickly creating many different molecules by simply reactions. The first has high accuracy but low speed, and the second has low accuracy and high speed.

“We thought it was possible to achieve the best of both worlds—high speed and high precision,” said Xuexiang Han, first author of the paper and until recently a postdoctoral fellow in the Mitchell lab. “But we had to think outside the traditional boundaries of the field.”

“We found that the A3 reaction was not only efficient, but also flexible enough to provide precise control over the molecular structure of lipids,” Mitchell said. This flexibility has been key to fine-tuning the properties of ionizable lipids for safe and efficient delivery of mRNA.