Complex Fusion Protein Stabilizes and Targets Therapeutic siRNAs

A recent paper described the development of novel method for delivering targeted doses of therapeutic small interfering RNAs (siRNAs) that was successfully tested in a mouse breast-cancer model.

Investigators at Harvard Medical School (Boston, MA, USA) constructed a fusion protein-based delivery system for siRNAs. The complex macromolecule consisted of single-chain fragmented antibodies (ScFvs) specific for the breast cancer-marker protein HER2. These antibodies were attached to the desired siRNA via a bridge composed of the positively charged peptide protamine. For the current study, the investigators used a siRNA that blocked the gene responsible for synthesis of the enzyme Polo-like kinase 1 (PLK1). Plk1 is considered a proto-oncogene involved in normal cell division whose overexpression is often observed in tumor cells.

Results published in the April 18, 2012, online edition of the journal Science Translational Medicine revealed that in a mouse model intravenously injected ScFvs-Protamine/PLK1-siRNA complexes concentrated in Her2+ breast cancer xenografts and persisted for at least 72 hours, leading to suppressed PLK1 gene expression and tumor cell apoptosis. The intravenously injected siRNA complexes retarded Her2+ breast tumor growth, reduced metastasis, and prolonged survival without evident toxicity.

“The challenge is to deliver the siRNAs to the site of the tumor without creating toxicity or inflammation in other portions of the body, said senior author Dr. Judy Lieberman, professor of pediatrics at Harvard Medical School. “ Naked siRNAs are cleared by the kidneys very quickly, and cells do not take them up readily. Other delivery systems, like liposomes, tend to make siRNAs collect in the liver.

“The protamine stabilizes the siRNAs and protects them from being degraded by enzymes in the blood stream,” said Dr. Lieberman. “And the ScFv antibody fragment makes sure that they only get delivered to and act in HER2-positive cells. PLK1 is a ubiquitously expressed gene, but because we targeted siRNA delivery only to HER2-expressing tumor cells, we could silence it with great specificity and no toxicity to other tissues. The platform could be used to target siRNAs therapeutically to any cell for which we have a cell surface marker to target. We have also used it to target lymphocytes, suggesting it could be useful against lymphomas and leukemias and as a way of countering organ rejection.”

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