The path to drug discovery travels from cells to mice, and from non-human primates to humans — at least, researchers wish that the path was always this straight. If a drug works in a mouse, that’s usually a good sign that it will work in a human, but if that were always the case, scientists would have cured diseases from obesity to cancer by now.
Drug carriers such as lipid nanoparticles are no different. Bioengineers create thousands of particles, varying everything from charge to size and lipid composition to find a nanoparticle that will deliver its gene therapy or RNA cargo successfully into human cells.
“We're just hoping that mouse predicts rat, and rat predicts primate,” said James Dahlman, a bioengineer at the Georgia Institute of Technology. “That's where the inspiration for a system that could do a lot of different particles at once — across species — came from.”
In a new Nature Nanotechnology study, Dahlman and his colleagues described how they developed a high-throughput screening system to identify how different species take up nanoparticles. That system? Mice with humanized or primatized livers.
By using this system, the team found that mice and human cells engulf lipid nanoparticles differently, demonstrating that a hybrid mouse approach may better predict nanoparticles that will work in humans, accelerating the development of gene and RNA therapies through the drug discovery pipeline.
To investigate the differences between mouse, non-human primate, and human cell uptake of nanoparticles, Dahlman and his team compared normal mice with mice that had livers populated with cells from different animal species. One mouse model had a liver with both human and mouse cells (humanized mice), one had primate and mouse cells (primatized mice), and the last model had its liver repopulated with mouse cells to serve as a direct comparison for the humanized and primatized mice.
Each nanoparticle contained an mRNA encoding a camel antibody that would be expressed on the cell surface when successfully delivered into the cell. The different nanoparticles also carried a unique DNA barcode so that the researchers could identify which one entered a particular cell.
Dahlman and his team injected the different mouse strains with their library of 89 nanoparticles. They were not surprised to see that many of the nanoparticles that delivered mRNA payloads to human liver cells also successfully delivered mRNA to the primate cells. There were comparatively fewer nanoparticles that entered both mice and human liver cells.
The researchers also noticed that some nanoparticles delivered mRNA very efficiently to mouse cells, but not to human or primate cells. Other nanoparticles delivered well to human cells but not to mouse cells. These results indicate that nanoparticles that look promising in mice may not translate their usefulness to humans, and that scientists may prematurely eliminate many good nanoparticle candidates for humans if they don’t work well in mice.
Using their hybrid mouse screening system, Dahlman and his team compared the transcriptomes of the human and mouse cells in the livers of humanized mice. To their surprise, they found that genes involved in how cells engulf nanoparticles were expressed in opposing directions in human and mouse cells.
“Cells take up stuff in really complicated ways, and I did find it interesting that the complicated way the mouse cells took up particles was different from the complicated way that human cells took up particles,” said Dahlman. He wondered if he could make nanoparticle testing even more efficient by engineering a mouse to express human endocytosis pathway genes and using those mice to screen new nanoparticles.
“It’s again, years and years and years out,” said Dahlman, but “the idea of that mouse would have never crossed my mind until I saw these results.”
For Michael Mitchell, a nanoparticle bioengineer at the University of Pennsylvania who was not involved in the study, the strength of this work is the high-throughput screening of the nanoparticles in the humanized and primatized mouse models.
“High-throughput in vivo screening is very important for not only discovering new nanoparticles, but also for understanding the translational gaps between animal models as you progress through the process of developing a product for humans. So, I think that this is exciting and important work,” he said. “We can potentially gain a lot of knowledge at an early stage before we move into those larger animal studies. We can get a better idea of what potentially might work.”
Mitchell is interested in seeing how the nanoparticle delivery in the primatized mice compares to nanoparticle delivery in a non-human primate. That experiment would create a useful data set to help predict which nanoparticles may deliver more efficiently to human cells.
Dahlman and his team are currently investigating some new chemistries associated with their nanoparticles that they hope work well in humanized and primatized mouse models to justify future experiments in larger non-human primates. They are planning to manufacture these nanoparticles at the gram scale so that they can one day be used in human clinical trials.
They hope that other nanoparticle researchers will take advantage of humanized or primatized mice to screen their nanoparticles to accelerate the development of new gene therapies and RNA therapeutics.
“Even if you slightly increase the efficiency of one of those nodes on that pipeline, you're going to make it better for everybody, no matter what nanoparticle they're testing,” said Dahlman. “With any pipeline, it's never going to be perfect. It's just, how do you make it as good as it can be?”
- Hatit, M.Z.C., Lokugamage, M.P., Dobrowolski, C.N. et al. Species-dependent in vivo mRNA delivery and cellular responses to nanoparticles. Nat. Nanotechnol (2022).