Editorial Article
Monday March 02, 2009
by Caitlin Smith
News of RNA interference (RNAi) is difficult to avoid lately, even making its way into mainstream media. But there is good reason for the hype, as RNAi holds tremendous promise in therapeutic applications—which may even see significant realizations this year.
As a method to silence gene expression, RNAi offers more specificity and flexibility than traditional drugs in treating diseases. When short pieces of double-stranded RNA (designed to target a particular gene) are introduced into cells, they are separated into single strands, with one binding to the target RNA and causing its demise. Thus the target RNA is no longer expressed. Hypothetically, then, any proteins that cause or contribute to diseases could be removed using RNAi, hopefully with positive therapeutic outcomes. “The challenge is to provide an efficient and safe means by which a circulating siRNA travels to a specific tissue, identifies the appropriate cell, enters that cell, escapes the subcellular compartments and effectively engages the RNAi machinery, thus silencing the gene target,” says Barry Polisky, CSO of MDRNA.
In reality, delivering short inhibitory RNAs (often called siRNAs) into a human body for therapeutic purposes is much more complicated. Yet Phil Haworth, CEO of Intradigm, says that “RNAi therapeutics have two potentially fundamental advantages over ‘traditional’ therapeutics, such as small molecules and antibodies, when it comes to their scope of therapeutic potential.” For example, about one-third of the proteins implicated in diseases by the human genome project are considered inaccessible to small molecules and antibodies. “Until the advent of RNAi, these disease targets were considered ‘undruggable,’” says Haworth. “With RNAi, we have the potential to transform these targets from ‘undruggable’ to ‘druggable’ and, in turn, provide hope and potential treatments to patients whose needs are currently not being met. That is an enormously powerful message when it comes to the potential of RNAi.” Another advantage offered by therapeutic RNAi is that according to Haworth, it “allows for varying degrees of modulation of the expression, which would still allow normal/healthy levels of protein expression,” he says. “In contrast, traditional drugs tend to act as an on/off switch. This potential subtly expands the range of targets that can safely be impacted.”
What is stopping us from reaping the benefits of RNAi therapeutics right now? Though several factors are involved, delivery is the biggest hurdle. Introducing RNA systemically into body fluids can set off siRNA degradation, off-target effects, and immune detection. Therefore many companies are now seeking ways to modify RNAs or attach them to delivery agents that will protect them until they reach their therapeutic destinations.
Delivery as particles or complexes
Tekmira offers a delivery reagent designed to lengthen the time of the RNA therapeutic agent in the body, facilitating its uptake into distal target sites. Tekmira’s technology, known as SNALP (stable nucleic acid-lipid particles), comprises lipid nanoparticles that encapsulate siRNA for delivery to specific disease sites. Tekmira develops agents in partnership with others, such as Alnylam and Roche, that use Tekmira’s delivery technology in developing RNAi therapies. “One of our partners, Alnylam, recently received FDA clearance to begin clinical trials of ALN-VSP, an RNAi drug being developed for the treatment of liver cancer and cancer with liver involvement,” says Mark Murray, Tekmira's president and CEO. “ALN-VSP was developed using Tekmira’s SNALP delivery technology and is scheduled to enter the clinic in the first half of 2009.” Tekmira’s ApoB SNALP for severe high cholesterol is expected to enter a phase 1 human clinical trial in 2009, and their anti-cancer PLK1 SNALP is also under development.
In another approach, Intradigm combines siRNA molecules with their PolyTran™ peptide-based polymers to create nanoparticles for RNAi delivery. Haworth believes that their technology, which can deliver RNA molecules to almost any body tissue, differs from other “liposomal-focused delivery solutions, which, by their nature, can only deliver RNAi therapeutics to a very few select organs in the body.” Intradigm also modifies their nanoparticles according to the delivery challenges faced. For example, “by PEGylating its nanoparticles, Intradigm can increase half-life and tissue accumulation,” says Haworth. They can also target the therapeutic agent to particular tissues by attaching specific targeting moieties to the nanoparticles.
In a new twist on delivery, RXi Pharmaceuticals
and the University of Massachusetts Medical School
(UMMS) are collaborating on the development of orally delivered RNAi therapeutics. RXi’s rxRNA™ molecules are encapsulated with micron-sized particles developed by UMMS for oral delivery, and directed to microphages involved in inflammatory diseases such as asthma, Crohn’s disease, atherosclerosis, diabetes, and rheumatoid arthritis.
Delivery with modifications
Once the RNAi agent has been delivered, the next significant challenge is getting it to the right place. “The challenge past effective in vivo delivery is effective targeted in vivo delivery,” says Chris Cunning, market development manager for Invitrogen molecular biology reagents at Life Technologies. “How completely this is achieved will ultimately decide the fate of RNAi as a therapeutic agent for many disease states.” Invitrogen’s new RNAi delivery reagent, Invivofectamine™, facilitates systemic in vivo delivery and is non-toxic. It is especially effective when used together with their Stealth™ RNAi duplexes, which have been chemically modified so that only one strand participates in RNAi (reducing off-target effects), and the RNA evades the host immune response.
MDRNA takes two approaches to design and delivery with their UsiRNA and meroduplex platforms. “Our UsiRNA platform employs strategically placed non-nucleotide entities termed Unlocked Nucleobase Analogs (UNA) in addition to RNA to form a short double-stranded RNA-based oligonucleotide,” says Polisky. UsiRNAs are protected from degradation and immune detection, and reduce off-target effects. “Meroduplex is based on the concept that placing a nick or gap in the passenger strand will minimize off-target activity related to the passenger strand,” says Polisky. “An optimized meroduplex will effectively silence a gene target while eliminating passenger strand off-target effects, since a nicked or gapped passenger strand does not activate the RNAi pathway. Further, a nicked or gapped passenger strand biases the siRNA to load the guide strand into the RNAi machinery, thus maximizing the likelihood that the guide strand will appropriately silence the gene target.”
While the technology for delivering RNAi therapeutics is progressing, so too is the research that makes it happen. “The most exciting development in the RNAi field today is the breadth of large pharma’s interest in the research and development of siRNA-based therapeutics,” says Polisky. “Small- and mid-sized biotech companies focused in siRNA-based therapeutics are superb at designing novel and proprietary siRNA constructs, and effectively delivering those double-stranded RNAs to specific gene targets in small animal models to demonstrate in vivo efficacy.” But small biotech companies lack the resources and clinical capabilities to develop the agents into human therapeutics. “Through partnerships with large pharma,” says Polisky, “biotech’s breakthroughs in the area of siRNA-based therapeutics have great potential to see clinical success in humans, and ultimately to make these novel therapies available to patients with life-threatening diseases.”