Laterally Stabilized Complexes of DNA with Linear Reducible Polycations:  Strategy for Triggered Intracellular Activation of DNA Delivery Vectors

Target-specific DNA delivery requires vectors that combine stability in the biological milieu, receptor-mediated uptake into target cells, and intracellular activation to mediate transgene expression. This is achieved here using polymer-coated vectors based on plasmid DNA complexed with a reductively degradable polycation (RPC), designed for intercellular degradation. The RPC were prepared by oxidation of the terminal cysteinyl thiol groups of Cys(Lys)10Cys. The complexes were coated and surface-cross-linked using multivalent reactive copolymers of N-(2-hydroxypropyl)methacrylamide (PHPMA), providing a unique combination of steric and reversible lateral stabilization, known to promote extended circulation in the bloodstream. Coated complexes containing RPC exhibited lateral stabilization that was reversible by treatment with 2.5 mM dithiothreitol, releasing free DNA after incubation with a polyanion. In contrast, coated complexes containing nonreducible poly(l-lysine) (PLL) were not destabilized by reduction. The biological usefulness of this trigger mechanism was examined by measuring transfection activity in human retinoblast 911 cells of coated complexes, based on PLL or RPC, targeted to cell surface receptors by covalent linkage of basic fibroblast growth factor. The levels of transgene expression observed for RPC-based targeted vectors indicated efficient intracellular activation, authenticating the concept that lateral stabilization introduced by surface coating with PHPMA can be reversed by intracellular reduction.