(227n) Gene Silencing Ability of Multivalent siRNA-PAMAM Dendrimer Conjugates

RNA interference (RNAi) promotes silencing of gene expression in a post-transcriptional manner.  RNAi is a potential therapeutic in the treatment of several medically relevant pulmonary diseases, such as lung cancer, cystic fibrosis, asthma, chronic obstructive pulmonary disease, respiratory syncytial virus, and severe acute respiratory syndrome viral infection, and there are several ongoing clinical trials related to lung and siRNA.  However, progress in this area has been hindered largely by the lack of efficient carriers capable of overcoming the lung architecture, extra and intracellular barriers present in the lung tissue to the efficient delivery of siRNA to the cell cytosol, and also due to formulation challenges.

In this work we demonstrate the gene silencing ability of multivalent siRNA-poly(amidoamine) (PAMAM) dendrimer conjugates.  siRNA was conjugated onto generation four, amine-terminated PAMAM dendrimers (G4NH2) through a reducible disulfide bond for facilitated cytosolic release.  The conjugates had a hydrodynamic diameter of 10 nm, and high siRNA loading efficiency.  Gene silencing in airway epithelial cells showed a marked dose response as a function of the conjugate concentration.  An efficiency of 55% was achieved at a G4NH2 molar concentrations several thousand-fold lower than its IC50, thus suggesting the potential of the strategy.  We also show that the gene silencing ability of siRNA is not affected by exposing the biomolecule to hydrofluoroalkanes propellants used in pressurized metered-dose inhalers, thus paving the way to the translational aspects of this work related to the oral inhalation delivery of siRNA.  This work is relevant as it represents a novel platform for the delivery of therapeutic siRNA to the lungs, which has potential in the treatment of a large number of medically relevant diseases.

Key words:  pulmonary siRNA delivery, alveolar epithelium, siRNA-dendrimer conjugates, poly(amidoamine) (PAMAM) dendrimers, pressurized-metered dose inhalers (pMDIs); gene silencing