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MIT, Alnylam Team Reports on New Lipopeptide Delivery Strategy

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Taking cues from the structures of lipoproteins, scientists from the Massachusetts Institute of Technology have developed a new class of lipopeptide nanoparticles that can selectively deliver siRNAs to liver cells better than other non-viral approaches, according to a new study.

The delivery system also proved highly efficient, with siRNAs contained in the nanoparticles cutting levels of their target protein by around 95 percent at 0.3 mg/kg in non-human primates. As such, the researchers believe it has "significant potential for use as a therapeutic delivery system," they wrote in a paper appearing in the Proceedings of the National Academy of Sciences.

Although nanomaterials inspired by natural molecules are under development as drug carriers, particularly for liver-based diseases, due to their delivery efficiency and specificity, none have shown to be as potent as traditional lipids or lipidoids, according to the MIT team, which included collaborators from Alnylam Pharmaceuticals.

"One approach to improve efficacy, tissue, and cell-type selectivity and tolerability of synthetic nanoparticles is to explore the broad chemical space of biomaterials," such as apolipoproteins, phospholipids, cholesterol, cholesterol esters, and triglycerides, they wrote.

To that end, the team synthesized a collection of synthetic lipopeptides designed to mimic apolipoproteins by conjugating lipid tails to amino acids, peptides, and polypeptide head groups. Notably, these could be produced on a large scale without using cells or animals, it stated in PNAS.

Using an iterative screening process and examining the compounds structure-activity relationships, the researchers pinpointed four criteria for their next-generation siRNA delivery agents: lysine-derived lipopeptides; lysine-based ring structure as a core; epoxide and aldehyde-derived lipid tails; and carbon tail length between 12 and 14. Following these rules, they discovered a lead material dubbed cKK-E12.

To test the agent's silencing activity in different organs, cKK-E12-based nanoparticles were loaded with siRNAs against phosphatase and tensin homolog (Pten), which is a protein ubiquitously expressed in various cell types. After intravenous administration to mice, the nanoparticles triggered significant silencing in the liver compared with the lung, spleen, kidney, heart, or brain. Pten expression was also found to be downregulated by over 80 percent in hepatocytes, but unaffected in endothelial cells or leukocytes.

Noting that apolipoprotein E (apoE) has been shown to enhance cellular uptake and gene silencing for ionizable lipid nanoparticles, the scientists set out to study the effects various apolipoproteins would have on cKK-E12.

In vitro experimentation revealed that the addition of an apoE isoform called apoE3 boosted cell uptake of the nanoparticle over nine-fold, while improving the escape of siRNAs from endosomes into cytosol. In apoE knockout mice, meanwhile, the silencing activity of cKK-E12 was "dramatically reduced."

To further extend their findings, the team tested the effects of cKK-E12 nanoparticles carrying siRNAs against transthyretin, which is mutated in the genetic disease TTR-mediated amyloidosis, in non-human primates. (Alnylam is developing two drugs to treat this condition, including one that uses a lipid nanoparticle technology developed by Tekmira Pharmaceuticals.)

With a single 0.3 mg/kg dose, TTR levels were suppressed in the monkeys by more than 95 percent, suggesting that cKK-E12 has potential for use with RNAi therapeutics, according to the researchers. Their "bioinspired approach" to biomaterial design may also prove a useful strategy in other areas of biomedical research, they concluded.

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