Mirus, which made a name for itself by pioneering the method of hydrodynamic nucleic acid delivery, reported last week that it has developed a technique to deliver genes and siRNA molecules into the muscle tissue of a limb through the bloodstream.
Called Pathway IV, the approach involves using a blood pressure cuff or tourniquet to occlude the upper limb, then rapidly delivering nucleic acids into the bloodstream through a catheter-based injection in the distal portion of the limb. According to Mirus, this elevates the pressure within the occlusion zone, temporarily making the blood vessel wall more permeable and allowing the nucleic acids to enter the adjoining muscle cells.
"All the muscles that are distal to the cuff get genes delivered to them," but delivery to the rest of the body is blocked, Jim Hagstrom, Mirus vice president of scientific operations, told RNAi News. "We put the cuff on, we inject, and we take the cuff off within two minutes — the whole procedure is very short."
Development of Pathway IV is an outgrowth of the company's early work on hydrodynamic delivery, Hagstrom said. While the success of the hydrodynamic method in the research field is encouraging, "the clinical applicability of it is still debatable," he said. "In liver it's still not reduced to practice … and for limb delivery, it hadn't been used clinically either. The big breakthrough we had … is using a methodology like that, but we are able to do it delivering into a vein in the distal limb," he added.
"Earlier methods of gene delivery focused on intramuscular injection of plasmid DNA or the injection of viruses or gold particles that ferry genes of interest into target cells," Jon Wolff, Mirus co-founder and CSO, said in a statement. "These methods suffer from limitations related to the level of gene expression achievable, duration of expression, and/or ability to administer repeat doses. With Pathway IV we have been able to overcome these hurdles and demonstrate high-level, long-term gene expression with either single or multiple doses."
According to Hagstrom, the company, working with colleagues at the University of Wisconsin, has successfully tested Pathway IV in mice, rats, dogs, and non-human primates, revealing one of the important benefits of the technique — efficiency.
"With direct intramuscular injection of plasmid DNA, you get less nucleic acid delivery as you go up the species chain, but with the [Pathway IV] approach, you do not lose [efficiency]," he said. "In fact, it gets easier [in larger animals] because we're dealing with larger veins and vessels."
Preclinical experiments with Pathway IV include delivering dystrophin-expressing DNA plasmids in a muscular dystrophy disease model, erythropoietin-expressing DNA plasmids in rats, and siRNA molecules in rats and rhesus monkeys. The dystrophin work is set to be continued next year by Mirus with partner Transgene in a phase I/II clinical trial in patients with muscular dystrophy.
Data from the dystrophin, erythropoietin, and siRNA work is scheduled for publication in the journal Molecular Therapy. Advanced online release of the data is expected in a week or two.
Because of the pending publication, Hagstrom declined to comment in detail on the siRNA data, but said that "we got knockdowns that are reminiscent of what we'd expect with siRNA. They are effective knockdowns."
Despite the promise Pathway IV holds for siRNA delivery, however, Mirus president Russ Smestad stressed that the company is only currently pursuing the technology in the gene therapy field.
"What we've enabled at this point is a delivery modality to reach limb skeletal muscle," he told RNAi News. "There are a limited number of opportunities [here]. It's a great place for gene therapy opportunities," including muscular dystrophy and arthritis, "but at this point we haven't identified an opportunity to use this for [RNAi]."
Smestad noted that the platform itself is a good one for RNAi, because it is effective. "That's not the problem," he said. "The question is: What do you deliver?"
Hagstrom said that although Mirus doesn't have an RNAi target in hand to which Pathway IV could be applied, the company is thinking about ways in which gene knockdown could be used to treat peripheral ischemia, which is an area of key interest for the company on the gene therapy side.
"Some of the biology is still being worked out on what genes should be knocked down" and which shouldn't, he said. "In most cases, there's more known about what genes should be given back.
"The bottom line is that the delivery method is applicable to both … plasmid DNA and siRNA and siRNA-expressing vectors," he added.
As such, Mirus hasn't totally ruled out the possibility of applying Pathway IV to RNAi.
"If we can identify a good siRNA target where muscle cells are the host site, we would certainly do it either ourselves or through collaborations with others," Smestad said. He noted that although the company has "not been actively promoting" the technology in RNAi, he does anticipate that "discussions exploring whether another party could direct it towards opportunities they have already identified" will occur.
When talks about such a deal might occur is not clear, but Smestad noted that during the Biotechnology Industry Organization's annual meeting in San Francisco he has had "a couple of conversations with folks in the field that would be the start of that." He declined to comment on whom he has met or is scheduled to meet with to discuss potential collaborations.