RXi Next Generation Therapeutic Platform: sd-rxRNA®
A successful RNAi therapeutic platform includes stable, specific and potent RNAi compounds and the ability to deliver these compounds to the tissue(s) of choice. One conventional solution to the delivery problem involves encapsulation into a lipid-based particle, such as a liposome, to improve circulation time and cellular uptake. Scientists at RXi have used an alternative approach to delivery in which drug-like properties were built into the RNAi compound itself. These novel compounds are termed ‘self-delivering’ RNAi compounds or sd-rxRNA.
The proprietary combination of chemical modifications that results in spontaneous cellular uptake of sd-rxRNA without the need for a delivery vehicle was discovered through systematic medicinal chemistry screening. sd-rxRNAs are hybrid oligonucleotide compounds that RXi believes combine the beneficial properties of both conventional RNAi and antisense technologies.
Traditional, single-stranded antisense compounds have favorable tissue distribution and cellular uptake properties. However, they do not have the intracellular potency that is a hallmark of double-stranded RNAi compounds. Conversely, the duplex structure and hydrophilic character of traditional RNAi compounds results in poor tissue distribution and cellular uptake. In an attempt to combine the best properties of both technologies, sd-rxRNA has a single-stranded phosphorothioate region, a short duplex region, and contains a variety of nuclease-stabilizing and lipophilic chemical modifications. The combination of these features allows sd-rxRNA to achieve efficient spontaneous cellular uptake and potent, long-lasting intracellular activity. RXi’s sd-rxRNA compounds are designed for therapeutic use and have drug-like properties, such as high potency, target specificity, serum stability, reduced immune response activation, and efficient cellular uptake.
RXI-109, a potent anti-fibrotic agent, was developed based on the novel chemistry of the sd-rxRNA platform. RXI-109 is designed to reduce the expression of CTGF, a critical regulator of several biological pathways involved in fibrosis, including scar formation in the skin. RXI-109 entered into RXi’s first clinical trial in June 2012.
sd-rxRNA: Spontaneous uptake in multiple cell types in vitro and in vivo
Treatment of multiple cell types with fluorescently-labeled sd-rxRNA (DY547, red in pictures below) results in efficient and universal cellular uptake. All cell types tested (primary, neuronal and non-adherent) internalize sd-rxRNA compounds uniformly and efficiently. Potent and long lasting silencing of targeted genes has been demonstrated in many cell systems (data not shown).
Efficient cellular uptake is observed both in vitro and in vivo, including tissues like skin, retina, lung, spinal cord and liver. The tissue distribution profile is defined by the route of administration (e.g., intravenous, subcutaneous, etc.) and the compound’s pharmacokinetic properties. Data available to date suggests that efficient uptake of sd-rxRNA compounds might be achieved in any tissue, as long as the route of administration enables local delivery of a relatively high concentration of the compound. RXi believes that sd-rxRNA technology may enable rapid progression toward a pipeline of clinical programs in a wide range of diseases, where local administration is an option.
The built-in drug like properties of sd-rxRNAs, including an extended circulation time and better tissue distribution, may make them amenable for systemic delivery. Further optimization efforts are underway to expand this technology to systemic applications.
Target classes (mRNA and lncRNA)
In order for a gene to guide the production of a protein, it must first be copied into a single-stranded chemical messenger (messenger RNA or mRNA), which is then translated into protein. Abnormal expression of certain genes (too much or too little) can result in disease, as can expression of an abnormal protein from a gene with a mutation.
RNA interference (RNAi) is a naturally occurring process by which a particular mRNA can be destroyed before it is translated into protein. The process of RNAi can be artificially induced by introducing a small double-stranded fragment of RNA that corresponds to a particular mRNA into a cell. A protein complex within the cell called RISC (RNA-Induced Silencing Complex) recognizes this double-stranded RNA fragment and uses one strand, the guide strand, to bind to and destroy its corresponding cellular mRNA target. If the mRNA is destroyed in this way, the encoded protein cannot be made. Thus, RNAi provides a way to potentially block the expression of specific proteins. Since the overexpression of certain proteins plays a role in many diseases, the ability to inhibit gene expression with RNAi provides a potentially powerful tool to treat human disease.
The sequence of the entire human genome is now known, and the mRNA coding sequence for many proteins is already available. Supported by numerous gene-silencing reports and our own research, we believe that this sequence information can be used to design RNAi compounds to interfere with the expression of almost any specific gene. We also believe that our RNAi platform may allow us to develop therapeutics with significant potential advantages over traditional drugs. These advantages include:
- High specificity for targeted genes;
- High potency (low doses);
- Ability to interfere with the expression of potentially any gene;
- Accelerated generation of lead compounds; and
- Low toxicity, natural mechanism of action.