“RNAi as a tool for genome-wide screening is in its early days,” said Hakim Djaballah, Ph.D., director of the high-throughput screening core facility at Memorial Sloan Kettering Cancer Center (MSKCC), and the challenges it presents largely reflect the overall poor quality of the early findings and misleading conclusions published in the scientific literature.
“Off-target effects were too often ignored,” and the belief still persists that siRNA sequences are specific and will only hit the intended target, he added. Chemical modifications to the passenger strand of the siRNA hairpin are helping to minimize these off-target effects and remedy this problem.
The data generated from genome-wide siRNA screens and, in particular, the high hit rates, make it difficult to prioritize hits for further analysis. “We have struggled with different ways to analyze the data,” Dr. Djaballah said. “Most of the fancy methods described don’t really make sense,” he asserted, adding, “I gave up on the up-front algorithms. If something comes out of a screen we want to make sure it is not an artifact before going after the biology.” Recently, his group began using back-end knowledge-based strategies for RNAi hit prioritization, including relying on database searching to rule out unlikely candidates. “We have also built some database tools around 3´-UTR regions.”
Researchers at MSKCC are using chemically modified siRNA duplexes to perform genome-wide RNAi screens to identify genes involved in cell-cycle regulation and cell viability and proliferation. At “RNAi Asia,” Dr. Djaballah presented data from RNAi screens intended to probe the biogenesis of microRNAs (miRNAs) and to help define the role of miRNAs in the transformation of cancer cells. The miRNA signatures described to date have shown that in cancer cells some miRNAs are greatly overexpressed and others may be deleted.
In addition to studying the rewiring of cancer cells, the group is employing drug modifier screens to understand why some drug candidates fail in late-stage clinical testing. MSKCC is using high-content assays to screen drug-sensitive and drug-resistant cells against high and low concentrations of a drug to identify genes that affect drug sensitivity in a defined patient population. This information could then be used to predict patient response and drive patient recruitment and selection for clinical trials.
Dr. Lu spoke about the work under way at Sirnaomics to develop a cancer therapeutic targeting breast carcinoma and non-small-cell lung carcinoma using a multitargeted siRNA cocktail designed to silence multiple disease-related genes. At the core of this program is Sirnaomics’ algorithm for siRNA design and its Tri-Blocker™ platform for producing the active pharmaceutical ingredient (API).
Using a polymer-based carrier for systemic delivery of the API, the company has validated its siRNA cocktails in mouse xenograft and orthotopic models. Sirnaomics is exploring other delivery vehicles—including ligand-directed cell targeting, infrared-activated nanoparticles, and nanomicrospheres for oral delivery—as well as combination therapy with monoclonal antibodies and other drugs to enhance the antitumor effects of the siRNA cocktails.
Design of the cocktail begins with a predictive algorithm that uses pathway analysis to select sequences most likely to inhibit disease gene-related targets. The algorithm aims to optimize the thermodynamic properties and RNA-induced silencing complex (RISC) binding activity of the siRNAs, avoid immune-stimulating motifs, minimize off-target effects, protect against potential intellectual property conflicts, and ensure homology between human and mouse to facilitate testing and development. The selection process identifies up to three genes that can be targeted by one API that contains a mixture of siRNA duplexes.
In his presentation, Dr. Lu demonstrated the synergistic effects evident both in vitro and in vivo of the multitarget platform, which can take advantage of the cross-talk between disease pathways.
In addition to the STP-502 therapeutic against solid tumors, Sirnaomics has four other siRNA products in development: STP-601 for ocular neovascularization diseases, STP-702 for pandemic flu (H5N1 and H1N1), and STP-705 for scarless wound healing.
Jinkang Wang, Ph.D., vp and CSO at Biomics Biotechnologies, introduced his company’s two main technology platforms and its therapeutic siRNA development programs. The first platform is a multiplex siRNA technique in which three siRNA sequences—each of which can target a different gene—are combined on a single strand. The strand is digested inside a cell to release the three distinct siRNAs.
“The technology can increase efficacy, reduce the development cost, and bypass the 21-mer patent protection,” said Dr. Wang. Biomics has also demonstrated a greatly reduced interferon response with the multiplex siRNA compared to a control siRNA.
The company’s other technology platform presents a new method for gene-specific Entire siRNA Target (EsT) library construction that uses the EcoP 151 enzyme to generate siRNAs that cover all possible 19–23 base-pair target binding sites.
The RNAi therapeutics initiatives under way at Biomics are targeting age-related macular degeneration (AMD), liver cancer, and hepatitis B virus (HBV). Dr. Wang described the company’s nonvirus liver delivery system, which, he says, has demonstrated its ability to deliver an siRNA to the liver and knock-down the ApoB gene, resulting in reduced blood cholesterol levels.
AMD, a degenerative disease affecting the eye, lends itself to localized siRNA delivery via direct injection into the eye. The goal of treatment is to inhibit the abnormal angiogenesis characteristic of AMD that can lead to blindness. Biomics has designed a small ligand interfering RNA (sliRNA) that does not silence a target gene via a RISC mechanism, but instead targets a toll-like receptor (TLR-3) on the cell surface, activating signaling pathways that block angiogenesis.
In animal studies, the sliRNA “performed with comparable or even better efficacy compared to Avastin (bevacizumab, a monoclonal antibody-based angiogenesis inhibitor), and much better than VEGF-targeted siRNA, stated Dr. Wang.
Biomics is collaborating with Benitec and using the Australian company’s viral delivery system together with its EsT library technology as the basis for siRNA drug development to treat HBV.