Engineers have come up with a straightforward and cost-effective method to guide lipid nanoparticles (LNPs) — the groundbreaking molecules responsible for delivering COVID-19 vaccines — towards specific tissues, marking the beginning of a new era in personalized medicine and gene therapy. The central idea involves making minor adjustments to the LNPs’ chemical structure, such as adding siloxane, a chemical group containing silicon. This adjustment enhances membrane flexibility and facilitates improved mRNA absorption by target cells.
Penn Engineers have developed an innovative technique for directing lipid nanoparticles (LNPs), the groundbreaking molecules that delivered the COVID-19 vaccines, allowing them to target specific tissues, marking a significant step forward in personalized medicine and gene therapy.
Previous research, including studies conducted at Penn Engineering, has utilized “libraries” of LNPs to discover specific versions that target organs like the lungs, an approach that resembles trial and error. “We never fully understood how the structure of a key component of LNPs, the ionizable lipid, influences the final destination of LNPs beyond the liver,” explains Michael J. Mitchell, Associate Professor of Bioengineering.
In a recent publication in Nature Nanotechnology, Mitchell’s research group details how minor changes to the ionizable lipid’s chemical structure enable targeted delivery to specific tissues, particularly the liver, lungs, and spleen.
The crucial insight was the incorporation of siloxane compounds—silicon and oxygen-based substances already used in the medical field, cosmetics, and drug delivery—into the ionizable lipids integral to LNPs.
Similar to silicon kitchenware known for its durability and cleanliness, siloxane compounds have shown, in previous studies, to possess high stability and low toxicity. “We aimed to investigate whether these characteristics could be harnessed to design highly stable and minimally toxic LNPs for mRNA delivery,” the researchers discuss in their paper.
By meticulously testing hundreds of variations of the newly named siloxane-containing lipid nanoparticles (SiLNPs), the researchers identified specific chemical properties that influenced mRNA delivery. “Determining their in vivo delivery presented a significant challenge,” notes Lulu Xue, a postdoctoral fellow in the Mitchell Lab and co-first author of the paper.
The team initially utilized the SiLNP variants to deliver mRNA encoding firefly luciferase—the gene responsible for the glow of fireflies—to cancerous liver cells in animal models, serving as a test for potential liver cancer treatments. The appearance of glowing cells indicated successful transfer of the mRNA from SiLNPs to the target cells.
When glowing cells were also detected in the animals’ lungs, the researchers realized that certain SiLNP variants had effectively navigated beyond the liver—an impressive achievement in LNP research, since LNPs generally accumulate in the liver due to its complex blood vessel structure.
The modifications made by the group included simple changes, such as swapping one chemical group for another—substituting an amide for an ester—that resulted in a remarkable 90% success rate for mRNA delivery to lung tissue in the animal models.
“We merely altered the lipid structure,” Xue says, “but this slight variation in lipid chemistry significantly enhanced delivery beyond the liver.”
The research team further discovered that several chemical features influenced the overall efficiency of the SiLNPs, including the number of silicon groups present, the lipid tails’ length, and the lipids’ structure.
Moreover, the SiLNPs exhibited a strong attraction to endothelial cells; since blood vessels are composed of these cells, SiLNPs may have potential clinical applications in regenerative medicine targeting damaged blood vessels, particularly in the lungs. Indeed, the team found that SiLNPs delivering agents that stimulate the growth of new blood vessels significantly improved blood oxygen levels and lung function in animal models affected by viral infections that had harmed their lung blood vessels.
The researchers suggested that one explanation for the SiLNPs’ effectiveness could be the larger size of silicon atoms compared to carbon atoms. With atoms being less densely packed, when SiLNPs merge with target cell membranes, they potentially increase the latter’s fluidity. This added flexibility assists the mRNA within SiLNPs to enter target cells, facilitating protein production. As the SiLNPs circulate through the bloodstream, proteins adhering to their surfaces aid in guiding them to the appropriate tissues.
Ultimately, the SiLNPs demonstrated up to a sixfold enhancement in delivering mRNA compared to existing leading LNP types, indicating that the distinctive attributes of siloxane compounds significantly boost the molecules’ clinical potential. “These SiLNPs hold promise for protein replacement therapies, regenerative medicine, and CRISPR-Cas-based gene editing,” states Xue.
“We aspire for this research to pave the way for new clinical applications for lipid nanoparticles by illustrating how straightforward modifications to their chemical structure can lead to highly targeted mRNA delivery to desired organs,” Mitchell adds.
This study was carried out at the University of Pennsylvania’s School of Engineering and Applied Science (Penn Engineering), School of Veterinary Medicine (PennVet), Perelman School of Medicine (Penn Medicine); in collaboration with the University of Electronic Science and Technology of China; the University of Delaware; and Temple University, with support from a U.S. National Institutes of Health (NIH) Director’s New Innovator Award (DP2 TR002776), a Burroughs Wellcome Fund Career Award at the Scientific Interface (CASI), a U.S. National Science Foundation CAREER Award (CBET-2145491), an American Cancer Society Research Scholar Grant (RSG-22-122-01-ET), and the National Institutes of Health (NICHD R01 HD115877).
Additional co-authors include Ningqiang Gong (co-first author), Xuexiang Han, Sarah J. Shepherd, Rohan Palanki, Junchao Xu, Kelsey L. Swingle, Rakan El-Mayta, Il-Chul Yoon, and Jingchen Xu from Penn Engineering; Gan Zhao (co-first author), Zebin Xiao, and Andrew E. Vaughan from PennVet; Vivek Chowdhary, Mohamad-Gabriel Alameh, Claude Warzecha, Lili Wang, James M. Wilson, and Drew Weissman from Penn Medicine; Xinhong Xong and Jiaxi Cui from the University of Electronic Science and Technology of China; Darrin J. Pochan from the University of Delaware; and Karin Wang from Temple University.
