ABSTRACT
The field of nonviral gene delivery was created as a result of breakthrough work involving the utilization of cationic lipids for gene transfer.1 In general, the ability to vary and control process parameters in liposome dispersions is relatively limited. In addition, the lipoplex (DNA-lipid complex) systems are poorly water soluble and their macroscopic characteristics are unstable over time, limiting their pharmaceutical applications. The lipoplex instability is due to the fact that such complexes may dissociate because of the charge screening effect of the electrolytes (and polyelectrolytes) in biological fluids. In contrast, self-assembly of polyplexes (DNA-polymer complexes) offer greater flexibility by varying the composition, the polymer molecular mass, and the polycation architecture. However, all such systems are inherently less efficient as compared to viral gene systems. The estimated gap between the efficiency of viral and nonviral systems can be as much as 10,000-fold.2 Lately, it has been reported that adenoviral transduction of cells that are normally refractory to infection can be enhanced by complexing virus particles with cationic lipids or cationic polymers. The mechanism of cellular uptake of adenovirus has recently been clarified and shown to consist of two distinct steps. Initial attachment to the cell occurs via an interaction between the fiber knob and several cellular receptors that are still being identified but include the Coxsackie’s virus and adenovirus receptor (CAR)3,4 and the α2 domain of major histocompatibility complex (MHC) class I molecules.5 This initial binding allows close proximity with the cell and subsequent binding of the viral penton base protein to cellular αv β3 and αv β5 integrins, promoting receptormediated endocytosis of the viral particle.6 The abundance of the CAR receptor, MHC class I
receptors, and αv β3/αv β5 integrins on the surface of different cell types greatly influences the level of infection by adenoviral vectors.7,8 Thus, the cells lacking such receptors are refractory to AdV gene transfer. The cationic lipid component ensures efficient cellular uptake, but endosomal escape and nuclear uptake are significant barriers to non-virus-mediated gene transfer.9 Adenovirus possesses an inherent capacity to destabilize the endosome and a nuclear targeting function,10 and a combination of polycations and adenovirus should permit efficient delivery to the nucleus. Precomplexing adenovirus with polycations enables a smaller dose of adenovirus to be administered in comparison with adenovirus alone, to achieve efficient transduction. In addition, a partial protection of adenovirus particles against the neutralizing effect of adenovirus antiserum when complexed with polycations has been observed.11
