The use of surface-based methods for the delivery of therapeutics has recently generated increasing interest. These platforms have tremendous potential to minimize detrimental side effects associated with systemic delivery by localizing the therapeutic vehicle and thus providing higher local doses for improved efficacy. Cationic lipids are one of the most commonly used synthetic carriers for nanoparticle-based delivery of genetic cargo such as DNA and RNA. The cationic surfactants interact electrostatically with the negatively charged nucleic acids, forming a complex that protects the DNA or RNA from degradation and facilitates cellular uptake. A similar strategy can be implemented for localized delivery from a solid film.
We have performed a detailed structural investigation of a solid film composed of DNA and the cationic lipid dimethyldidodecylammonium bromide (DDAB). These films are prepared first by electrostatic complexation of DNA and lipid in water. The resulting water-insoluble complex is then dried and cast as a thin film using an organic solvent. The properties of similar bulk films have been reported previously for a variety of applications including coatings, delivery of therapeutics, nanoelectronics, photonics, and optoelectronics. A unique feature demonstrated for DNA-DDAB films is that, in the dry state, the DNA is maintained in a single-stranded conformation, rendering it resistant to degradation by nucleases. However, upon hydration, the structure of the film switches, restoring the DNA to its biologically active double-stranded form. This structural transition facilitates the easy storage and use of these films in a clinical setting, as the dry film could be stored indefinitely and then activated by hydration upon implantation at the site of therapeutic interest within the body. Additionally, hydrated DNA within these films denatures at elevated temperatures in the same fashion that it does naturally, allowing for the formation of solid films composed of surfactant bilayers and single stranded DNA.
Despite the prevalence of lipid-based nanoparticle transfection strategies, reports on the use of lipid-based films for gene delivery are scarce. We investigated the use of a lipid-based film for the in vitro delivery of plasmid DNA. Initial results show very low levels of transfection from solid DNA-lipid films compared to nanoparticle delivery. Detailed analysis suggested that this loss of activity is a consequence of both the monovalent interaction between a single surfactant molecule and DNA as well as the hydrophobic driving force for micellization. Based on this structural understanding, we are then able to suggest strategies for circumventing these challenges and restoring the efficacy of these films as gene delivery platforms.
Schematic depiction of the structure of a DNA-DDAB film as a function of water content and temperature. Dry films exhibit a structure of alternating monolayers of the DDAB surfactant and single-stranded DNA. Upon the addition of water, the DNA hybridizes to form biologically active double-stranded DNA and the surfactants convert to bilayers. Subsequent heating drives the denaturation of the double-stranded DNA. Each of these transformations is reversible.