The "De-Risking" of RNAi

Last week, Alnylam Pharmaceuticals announced that its researchers, in collaboration with Protiva Biotherapeutics, had demonstrated the therapeutic use of systemically delivered RNAi in non-human primates. In particular, they have shown in a Nature published paper that they could block the expression of a gene encoding apoB, which is involved in cholesterol transport. The researchers achieved a >90% reduction in apoB mRNA concentrations, a 70% reduction in protein levels, and better than a 60% reduction in cholesterol levels, with a single injection of an RNAi drug. This is better cholesterol control than one would expect with Lipitor, which is the best selling drug in the world.

Possibly even more significant is what Alnylam’s President and CEO John Manganore called the “de-risking” of RNAi. If RNAi works in primates, it will likely work in humans, making potential major pharmaceutical partners and venture capitalists a whole lot more comfortable. The prospects of RNAi type drugs as a general new class of therapeutics just took an enormous leap.

What is RNAi?

The earliest macromolecule of life (at least the earliest one that is still around), seems to have been RNA. When I was a graduate student, RNA had not yet become fully appreciated. It was known to carry the information from the genome to the ribosome where proteins were made. Other specialized RNAs performed functions as part of the ribosomes. But the proteins were presumed to be the main actors in carrying out living functions.

Now we know that RNA can do a lot of things all by itself. Given the right conditions, it can self-replicate. Some RNAs have enzymatic activity and can degrade other RNAs. RNAs also can retain spatial information and react in the manner of antibodies to antigens. RNA can bind proteins, DNA, other RNAs and even small molecules.

RNAi wasn’t discovered until a few years ago. It is an ancient, evolutionarily conserved system in which one RNA transcript regulates the expression of other transcripts, especially mRNAs. What makes the system so attractive to pharmaceutical companies is the potentially broad range in the activity of RNAi. In principle, any gene, including those of viruses, could be controlled through the use of RNA. The therapeutic potential is enormous. RNAi is also valuable as a tool for unraveling molecular biology in the development of other types of drugs.

Basically, small interfering RNAs (siRNAs) bind to the target RNA strand through strand complementarity. This triggers destruction of the now double-stranded RNA pair by an enzyme called Slicer [originally posted as "Dicer" which is a separate enzyme involved in siRNA maturation. Thanks to David Frendewey for the correction]. Thus, the therapeutic RNA takes advantage of a pre-existing system for a new purpose.

Though RNAs are a natural product, siRNAs can be chemically synthesized, which makes them cheaper and easier to purify than the protein drugs of a similar size. Also, so far RNAi does not seem to trigger immune reactions in the way that foreign proteins would. They are also easily metabolized and seem to have no significant side effects. RNAi can also be introduced into the cell on a permanent basis through the use of genetic engineering, which would allow the cell to produces its own siRNA transcripts.

Problems with RNA therapeutics

Though RNA is not inherently unstable in a chemical sense, in the real world enzymes that degrade RNA are literally everywhere: in soil, in bodily secretions, even fingerprint, and especially in serum. Stability can be increased by chemical alterations of the RNA, but any such alteration must not interfere with the recognition function of the RNA or with its degradation by Dicer, the enzyme involved ultimately in the therapeutic action.

Alnylam’s candidate siRNA therapeutic was chemically altered modestly from normal RNA. Delivery through the serum was accomplished through the use of liposomes, little lipid particles that were, in this case, targeted to the liver where apoB is formed. Another RNAi company, Calandro Pharmaceuticals, uses cyclodextrin nanoparticles to transport RNAi. Engineered virus particles could likewise be used as transport vessels.

An industry in the wings

Alnylam [ALNY: NASDAQ] is not the only company attempting to develop RNAi therapeutics. Others include Sirna Therapeutics [RNAI: NASDAQ], CytRX [CytR—NASDAQ], Nastech Pharmaceuticals [NSTK: NASDAQ] and Calandro Pharmaceuticals, which is a portfolio company or Arrowhead Research [ARWR: NASDAQ], a component of the Edwards’ Real Nanotech Index. A number of others are not publicly traded yet.