Despite considerable challenges, RNA interference (RNAi) as a therapeutic strategy continues to draw excitement and scrutiny. Two recent meetings—Select Biosciences’ “RNAi and miRNA World Congress” and “RNAi 2010”—provided updates on the field. A number of presentations focused on advances in the use of major players in silencing the double-stranded RNA segments termed small (short) interfering RNAs (siRNA). Research highlights included novel ways to stabilize and deliver siRNA, especially the use of nanoparticles for targeted delivery.
“Although the initial buzz about RNAi came from its successes in cell cultures, achieving acceptable efficacy in vivo was another matter,” according to Dmitry Samarsky, vp, technology and development, RXi Pharmaceuticals.
“There are many obstacles facing successful RNAi compound delivery,” Dr. Samarsky explained. “If introduced systemically—that is, into the circulation—compounds are often very labile in biological fluids. They can be quickly inactivated by blood nucleases or cleared from the body. Even when directly injected into tissues of interest, there is no guarantee of delivery inside the target cells.
“It is hard, for example, for an RNAi molecule to cross the cellular membrane, and there are interstitial and cellular nucleases as an added challenge to deal with as well. If successful at penetrating the target cell, the next question is whether they will be able to activate the RNAi induced silencing complex. In addition, RNAi molecules can evoke an undesired immune response through multiple mechanisms.”
To address these multifaceted problems, RXi developed a class of RNA molecules called self-delivering RNAi compounds, or sd-rxRNA™. “Our scientists focused on a strategy that combined particular oligo configurations and chemical modification patterns. These specially designed molecules help resolve many problems associated with therapeutic RNAi delivery,” Dr. Samarsky added.
“They have enhanced nuclease resistance, remain active longer in the circulation, facilitate organ and tissue delivery, and stimulate spontaneous cellular uptake. Because we have chemically modified and optimized the content, they avoid immune responses and increase RNAi specificity.”
Part of the design strategy was to reduce the size of the molecules. “Most conventional siRNA duplexes are between 19 and 25 base pairs. Ours contain duplexes of less than 15 base pairs. We found that reducing oligo content greatly enhances their ability to cross the cell membrane.
“The sd-rxRNAs seem to operate via a universal mechanism and are taken up by various cell types in vitro. It is very fast. Fluorescent molecules can be observed within minutes of administration. The next step for RXi was to assess and verify activity in multiple cell types in vivo. We have already demonstrated the uptake of the molecules by cells in skin, lung, and eye after local administrations, as well as by the liver upon systemic injections. We are continuing to examine applications.”