Inappropriate gene activity is being linked to a growing number of diseases. The ability to modulate or silence such activity via RNA interference (RNAi) technologies could revolutionize the way we treat disease. RNAi, as a new class of drugs, has already been convincingly demonstrated in a number of preclinical models of diabetes, hepatitis, oncology, influenza, macular degeneration, Parkinson and Huntington diseases, and others.
RNAi therapeutics have enormous potential. Because they inhibit gene expression at the level of translation, they offer a unique opportunity to rapidly identify and potently inhibit disease targets. The total market for RNAi therapeutics could reach $580 million by 2012, according to Market Research. CHI’s recent “From Tools to Therapies” meeting highlighted challenges facing this young field, exciting new applications, delivery, and cutting-edge platforms.
The best RNAi product in the world is useless if it cannot be delivered effectively to the target. “Delivery of RNAi therapeutics is one of the biggest challenges the industry faces,” reported Ian MacLachlan, Ph.D., CSO of Tekmira Pharmaceuticals. “Nucleic acids are comparatively large molecules that also are unstable in the blood. Additionally, they cannot readily cross membranes into target cells. To reach the maximum therapeutic potential of RNAi-based drugs, effective delivery is crucial.”
While the mechanism is called RNAi, the therapeutic agents are the so-called short interfering RNAs (siRNA) that consist of ~21 nucleotides of double-stranded RNA. Dr. MacLachlan said that nanoparticles coated with lipids encasing the payload siRNA represent an emerging new approach for delivery. “In the early days, researchers were using naked siRNA or chemically modified versions. Although initially there were some good in vivo results, further studies were less encouraging. We found, however, specially coated nanoparticles can potentiate delivery by 1,000-fold.”
Tekmira’s siRNA delivery technology platform is called SNALP for stable nucleic acid-lipid particles. SNALP are specialized lipid nanoparticles that encapsulate and systemically deliver a variety of nucleic acid molecules such as siRNA.
“Our preclinical studies have shown them to be effective in delivering the drug to target organs and into cells where the nucleic acid-based drug can carry out its desired effect while minimizing systemic toxicity,” Dr. MacLachlan reported. “Tekmira and its partners will have five product candidates in clinical trials by the end of 2010.”
One application targets hypercholesterolemia. The technology, called ApoB SNALP, targets ApoB, a protein that is essential to the assembly and secretion of very low density lipoprotein, a precursor to low density lipoprotein (LDL), both of which are required for the transport and metabolism of cholesterol.
In animal studies, the ApoB SNALP are delivered with high efficiency into liver hepatocytes, the cells that produce ApoB. The payload siRNA acts to knock down the precursor mRNA coding for ApoB protein resulting in significant reductions in LDL.
Although the nanoparticle technology is currently used for systemic application, according to Dr. MacLachlan the field is moving toward a targeted modality. “A variety of research is under way to see how RNAi can be delivered to specific target cells. That’s the race for the future.”
Cancer Cell Vulnerability
RNAi can be directed against various types of cancers. But one of the key challenges for cancer therapeutics is the ability to destroy only the tumor cell and not normal ones. A novel way to specifically attack cancer cells is to take advantage of their “oncogenic addictions and associated dependencies,” reported Roderick L. Beijersbergen, Ph.D., group leader, division of molecular carcinogenesis at The Netherlands Cancer Institute.
“It turns out that cancer cells can be highly dependent on the mutated proteins associated with their cancerous behavior. For example, breast cancer cells are dependent on the continuous activation of Her2. Inhibition of this receptor blocks the growth of those tumor cells when normal cells are not affected.
“Many cancerous proteins cannot be inhibited with drugs, but it is likely that these cells are dependent on other proteins for their survival. A major challenge is to identify those proteins we should attack to eliminate only those cancer cells that carry a specific genetic alteration.”
According to Dr. Beijersbergen, RNAi screening can help. “It is still impossible to predict those specific dependencies in tumor cells. We perform large-scale functional screens using genome-wide RNAi collections to determine for each gene, one by one, whether inactivation results in the specific killing of cells carrying a tumor-associated mutation. To achieve this we have built panels of cell lines that are either normal or contain a mutated gene frequently present in cancer cells.
“By comparing the reduction in cell viability between the normal and mutant cells, we are able to identify, in an unbiased, genome-wide scale, those proteins that can be explored as drug targets for cancer therapy. These screens involve thousands of individual experiments and can only be done in a highly automated system using robotics.”
Dr. Beijersbergen believes that RNAi methodologies are well suited for seeking such novel therapeutic targets, although the field still has many challenges to overcome. “We are in the early days. What is most needed is to obtain better insights into why proteins show this functional phenotypic partnership. If we can better understand these relationships, maybe we can identify signaling networks and other proteins that may be even more efficient targets. This would help determine what is most druggable and then to develop improved strategies for attack.”