With delivery still the primary problem facing those trying to develop RNAi-based therapeutics, the number of researchers trying to come up with a solution keeps growing.
The latest to join the ranks is Tatiana Segura, a postdoc at the Swiss Federal Institute of Technology, who has begun work on using the block copolymers polyethylene glycol (PEG) and polypropylene sulfide (PPS) to deliver siRNA systemically.
“Most delivery systems that I’m aware of, for both plasmid DNA and small interfering RNAs, are based on cationic polymers and lipids,” Segura told RNAi News. “In this case, we’re making something that self-assembles by itself … through hydrophilic-hydrophilic and hydrophobic-hydrophobic interactions instead of using charge.”
Earlier this year, Nicola Tirelli and colleagues at the Swiss Federal Institute of Technology reported in Nature Materials their work on the use of the hydrophile PEG and hydrophobe PPS to develop block copolymeric amphilphiles that self-assemble into unilamellar vesicles. The authors noted that the PEG is resistant to protein absorption and has low toxicity (it is also currently being used with small molecule drugs to reduce their immunogenicity and boost their circulation time). Meanwhile, the authors added, PPS is extremely hydrophobic, has a low glass-transition temperature, and is capable of oxidative conversion from a hydrophobe to a hydrophile.
Segura said that while her work is similar to that of Tirelli and colleagues, she is developing a PEG-PPS-siRNA compound rather than a PEG-PPS-PEG one. The siRNA will be bound to the copolymer through a reduction-sensitive bond in order to allow the oligo to be released from its carrier in the endosome, and will be directed to specific cell types by attaching various ligands to the PEG molecule.
As for getting the siRNA out of the endosome, Segura wrote in an e-mail to RNAi News this week that “we will either see if the released PEG-PPS acts as a surfactant and breaks the endosomal membrane, or we will add fusiogenic peptides to the structure. Surfactant molecules have similar structures … to PEG-PPS diblock and thus this structure by itself may provide [for the] release of the siRNA from the endosome,” she added.
Segura has already begun testing siRNAs, which were purchased from Dharmacon, using them to knock down Lamin A/C, and expects to conduct some in vitro work in order to determine stability of the copolymers. However, she said that the bulk of her experiments will be done in vivo, as she tries to down-regulate genes that play a role in fibrin production.
“For the siRNAs, you can down-regulate any gene that you want,” she said. “What I wanted to down-regulate was a gene that was important for something that was physically made by cells.”
The initial focus of Segura’s effort will be in preventing abdominal adhesion. “You get a lot of blood in your abdomen during surgery, and this blood turns into fibrin, which then turns into collagen,” she said. “Then you have fibrin formation between organs, and this creates a lot of problems. The idea is to target the siRNA against the proteins that promote the fibrin plaque formation.”
Specifically, she said she will be targeting the plasminogen activator inhibitor-1 (PAI-1) gene and the transcription factor hypoxia-inducible factor-1 (HIF-1) alpha.
Ultimately, Segura said she hopes to use RNAi to cause stem cells to differentiate into specific cell types, but that project would most likely use a more traditional delivery system.