(334f)  Ligand-Targeted Conjugate Systems for Delivery of siRNA to Tumors

Small interfering ribonucleic acids (siRNA) are a class of nucleic acid-based drugs that have the potential to knock down any gene with a high degree of specificity. They harness a natural gene regulation and antiviral mechanism for efficient destruction of any mRNA. This is useful for both the understanding of and treatment of diseases ranging from genetic disorders and viral infections to cancer. However, to be effective, the siRNA must travel from the injection/infusion site to the tissue of interest and then reach the cytoplasm of the desired cell type. Nucleic acids are highly charged and cannot cross the cell membrane unaided. Exposure to serum endonucleases also accelerates the destruction of the siRNA payload. Methods to cross these barriers include targeted delivery using vehicles ranging from lipid nanoparticles to polyplexes. Unfortunately, the delivery materials tend to be toxic, limiting the efficacy in patients, and also tend to localize to immune cells, the endothelium, or the liver. Currently, there are very few biomaterials that can successfully deliver siRNA to tissues outside of this group, and none are in Phase II clinical trials.

Recently, advances in siRNA stabilization technology have made unencapsulated siRNA conjugates viable, as chemical modifications to the RNA backbone have been used to reduce susceptibility of the siRNA to endonuclease degradation. These conjugates also have desirable treatment properties, including an in-human safety profile of up to 300 mg/kg and a clear regulatory path due to a well-defined chemical structure. Furthermore, due to their small size and thus ability to diffuse relatively quickly through extracellular matrix, the conjugates can be injected subcutaneously, a home procedure familiar to many diabetics on insulin, as opposed to nanoparticles, which must be infused intravenously at a clinic or hospital.

We have developed an azide-alkyne cycloaddition based system that allows for the delivery of ligand-targeted siRNA conjugates to models of glioblastoma (a form of brain tumor) in vitro and in vivo. The ligand used is based on chlorotoxin. In vitro studies using U87 (glioblastoma) cells show a similar degree of knockdown to commercially available transfection materials relative to a scrambled siRNA control. Confocal microscopy studies using derivatives of these ligands with fluorescently-labeled siRNA demonstrate a high degree of uptake relative to unconjugated siRNA in both cases. These conjugates are also able to traverse the bloodstream, deliver a nucleic acid payload into the tumor cells, and efficiently knock down expression of a housekeeping gene. Using fluorescent derivatives of our siRNA, we are able to observe localization of the siRNA to subcutaneous or intracranial tumor xenografts. Intravital imaging experiments demonstrate the ability of these conjugates to pass the blood-brain barrier and permeate the tumor. In vivo knockdown studies show a moderate degree of tumor-specific knockdown of a housekeeping gene in an intracranial model of glioblastoma using U87 cells.

These results demonstrate that siRNA knockdown of genes in tumors using these modular conjugate systems are feasible and present a way forward for using siRNA in the treatment of cancer. Future work involves using biologically-relevant siRNA to demonstrate anti-tumor effects in this and syngeneic tumor models.