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Stealth Biotech Looks to Enter microRNA Rx Field with Novel Gene-Specific Approach

As the science of microRNA grows, so too does the interest in targeting the small non-coding RNA therapeutically. But while most companies working in the field are focused on inhibiting a particular miRNA, and therefore all its gene targets, Dutch startup Stealth Biotech aims at blocking miRNA activity on specific genes.
According to Stealth Founder and CEO Thorleif Moller, the company’s technology involves using single-stranded antisense oligos, termed blockmirs, to prevent single miRNA/mRNA interactions.
Blockmirs “are not capable of activating RNase H or recruiting RISC … [but] are inactive oligonucleotides that act as steric blockers,” he told RNAi News last week. “So when they bind to a microRNA binding site on a messenger RNA, they prevent the microRNA from inhibiting that particular messenger RNA” while allowing it to regulate its other target mRNA.
In doing so, a blockmir-based drug may be able to treat a disorder caused by an miRNA’s effect on a single gene without triggering potential side effects caused by disrupting the miRNA’s control of genes not involved in the condition.
Moller said that he came up with the idea for blockmirs after attending a conference in 2006 during which miRNA and their therapeutic potential was discussed.
“One of the main problems in the microRNA field is to figure out which mRNAs are regulated by which microRNAs,” he said. “Then it occurred to me that instead of inhibiting microRNAs directly, it might be [beneficial] to inhibit microRNAs gene-specifically.
“I thought that was pretty obvious … [but] it turned out that nobody had done it at that time,” he added. “Since I was pretty sure it would work, I filed patents” on the approach.
Moller said that he has already filed patent applications in both the US and Europe on blockmirs and their use against specific targets.
The patent applications have yet to be published, but according to the abstract of the first application, which Moller provided to RNAi News, the intellectual property claims “oligonucleotides that bind to microRNA target sites in target RNAs, such as mRNAs, [and can] mediate RNase H degradation of the target RNA, mediate RNAi of the target RNA, or prevent microRNA regulation of the target RNA.
“The oligonucleotides of the invention are useful, [for example,] as research tools for studying microRNA/mRNA interactions and for therapeutic development,” the abstract states. The application also describes “methods of identifying microRNA target sites, methods of validating microRNA target sites, methods of identifying oligonucleotides of the invention, and methods of modulating the activity of a target RNA.”
Moller conceded that this first patent application is “broad” and “covers a lot of different sequences and a lot of different chemistries.
“Chances are that it will not be possible to keep all of those in one patent application,” he added. “But … there is a second [application] covering specific uses of these blockmirs — and that is being updated continuously.”
Stealth Mode
Although the blockmir concept is new, there are some data in the literature supporting it. In August, researchers from the Montreal Heart Institute reported in the Journal of Cellular Physiology that antisense oligos designed against miRNA target sites in the 3’ UTRs of two pacemaker channel genes could enhance those genes’ expression and function.
Also in August, investigators from Harvard University published a report in the online early edition of Science that morpholinos could be used to disrupt the interaction of specific miRNA/mRNA pairs in zebrafish.
Stealth, which was founded less than six months ago, has one employee and a handful of patent applications as its primary asset. However, Moller said he expects this to change over 2008 as he works to close a Series A financing round.
He said he has “made contact” with potential investors,” and plans to “start up a real company,” with additional staffers once funding is secured.
He declined to provide an expected figure for the Series A.
At the same time, Moller said he has made contact with various academic groups and companies within the miRNA and antisense arenas to discuss possible collaborations.
Since it is not yet clear what technology or technologies Stealth will use to suppress specific miRNA/mRNA interactions, Moller hopes to be able to arrange deals under which the blockmir approach can be evaluated using, for example, locked nucleic acids.

“One of the main problems in the microRNA field is to figure out which mRNAs are regulated by which microRNAs. Then it occurred to me that instead of inhibiting microRNAs directly, it might be [beneficial] to inhibit microRNAs gene-specifically.”

“We may want to use 2’-O-methyl [antisense oligos] or LNAs or morpholinos,” he explained. As such, “one of the main priorities at the start would be to test different chemistries and then get back to these companies to sort out licenses” to the gene-silencing technologies.
Further, an alliance could allow Stealth Biotech to share some of the costs associated with testing different blockmir approaches, while tapping into a partner’s chemistry and R&D expertise, Moller said.
A collaboration could also include cross-licensing chemistry with certain targets Stealth may not wish to pursue on its own, he added.
“I’m looking at all options,” Moller said. “We can’t pursue all opportunities … therefore I want to enter into as many collaborations as possible.”
If the price is right, he said he would also consider selling off the IP estate he is building. However, “I think more value can be created by keeping [the company] as a separate entity,” he noted.
Although the timing of any kind of partnership is uncertain, Moller said that he hopes to have inked some kind of deal by the summer. Once this happens, Stealth can better evaluate its drug-development options and start building a pipeline.
Thus far, Stealth has not committed to any specific drug programs, but Moller noted that a hepatitis C program targeting miR-122 would be “an obvious choice.”
This miRNA has been associated with hepatitis C replication and linked to fatty acid metabolism. It is also the focus of at least two companies’ efforts to develop an anti-HCV drug: Regulus Therapeutics and Santaris Pharma (see RNAi News, 9/13/2007 and 12/20/2007).


But while most efforts to targeting miR-122 to treat HCV focus on inhibiting the miRNA directly, Moller thinks Stealth’s blockmirs could potentially offer a better approach.
“MicroRNA-122 is pretty abundant in liver cells,” he said. “If you’re going to inhibit it completely, you might expect some side effects. With the blockmir approach, you could block just the activation of hepatitis C replication.”
Specifically, a blockmir drug would target the 5’ end of the hepatitis C transcript, the site to which miR-122 binds in order to facilitate HCV replication, Moller said.
“There may be other binding sites, but as of now the primary binding site would be in the 5’ region of the hepatitis C transcript,” he added.

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