Scientists have discovered that replacing amines with phosphonium-based cations can significantly enhance the effectiveness of mRNA-loaded polymeric micelles. This novel approach has led to improved stability and mRNA delivery efficiency of micelle nanocarriers within tumor tissue in living organisms. Their findings emphasize the need to investigate alternative cations for mRNA drug delivery, potentially leading to better treatments for difficult diseases.
Among the various strategies for addressing diseases, influencing gene expression in cells stands out as one of the most effective methods. In recent decades, researchers have developed numerous strategies that utilize messenger RNA (mRNA) to prompt cells to produce specific proteins. These mRNA-based therapies have recently become more prominent, particularly as vaccines for infectious diseases like COVID-19. Beyond that, they offer significant promise for treating cancers and genetic disorders.
Since mRNA is inherently unstable and easily degraded by enzymes within the body, mRNA-based therapies depend on effective drug delivery techniques. The main concept involves encapsulating mRNA molecules in nanostructures that can safely transport them into target cells. Currently, the most commonly studied mRNA nanocarriers consist of amine-containing cationic lipids or polymers, which create small protective spheres to efficiently deliver their contents into cells. However, these existing designs often struggle with stability issues, resulting in increased costs and higher doses needed to achieve the desired effects.
In light of these challenges, a research team from Japan has investigated alternatives to amine-based materials for mRNA nanocarriers. Their recent study, published in Materials Horizons on July 10, 2024, examined the effectiveness of triphenyl phosphonium (TPP) as a substitute for amine groups used to form mRNA-loaded micelles.
“Phosphonium-based cations exhibit distinct ionic characteristics that enhance interactions with anions like mRNA, influenced by their unique charge distribution and binding strength, which differ from those of nitrogen due to the variations in electronegativity between phosphorus and nitrogen,” explained Associate Professor Yasutaka Anraku from the Tokyo Institute of Technology, who led the research. “Additionally, the three phenyl groups help facilitate hydrophobic interactions, ensuring stable mRNA complexation. Therefore, using TPP instead of amines can boost the efficiency of mRNA delivery,” he added.
To test their theory, the researchers created polymeric micelles made from polyethylene glycol (PEG), TPP, and mRNA. They successfully designed a highly effective method to replace the amine groups in PEG-poly(L-lysine) copolymers with TPP. The resulting polymers naturally form a core-shell structure in environments rich in anions due to their hydrophobic nature and charge distribution. Since mRNA is composed of multiple negatively charged phosphates, the positive TPP groups attract these phosphates, promoting high and stable mRNA incorporation within the micelles.
The research team thoroughly evaluated their strategy through extensive analysis, including thermodynamic, physicochemical, and computational methods. They also assessed the ability of the proposed system to deliver mRNA to tumor cells in vivo using a mouse model. “Following intravenous injection, TPP-containing micelles exhibited a remarkable increase in mRNA bioavailability, leading to efficient protein production in solid tumors,” noted Anraku. Remarkably, the experiments demonstrated that the levels of intact mRNA in the bloodstream were significantly higher after 30 minutes when using the TPP-based micelles compared to amine-based ones. Additionally, protein expression in tumor tissues was more than tenfold greater with the TPP micelles.
In conclusion, this groundbreaking approach shows promise for mRNA therapeutics and targeted drug delivery. “As polymeric micelles can be directed to specific tissues by attaching ligands, TPP-bearing polymeric micelles could become a powerful system for mRNA delivery across various tissues,” stated Anraku. With continued development, this technology may lead to effective treatments for some of humanity’s most pressing health challenges.
