Activation of the Complement System by Synthetic DNA Complexes: A Potential Barrier for Intravenous Gene Delivery

Abstract

We have examined the complement-activating properties of synthetic cationic molecules and their complexes with DNA. Commonly used gene delivery vehicles include complexes of DNA with polylysine of various chain lengths, transferrin-polylysine, a fifth-generation poly(amidoamine) (PAMAM) dendrimer, poly(ethyleneimine), and several cationic lipids (DOTAP, DC-Chol/DOPE, DOGS/DOPE, and DOTMA/DOPE). These agents activate the complement system to varying extents. Strong complement activation is seen with long-chain polylysines, the dendrimer, poly(ethyleneimine), and DOGS (half-maximal at about 3 μM amine content in the assay used). Compared to these compounds, the other cationic lipids (in liposome formulations) are weak activators of the complement system (half-maximal ≈ 50–100 μM positive charge in assay). Complement activation by polylysine is strongly dependent on the chain length. Short-chain oligolysines are comparable to cationic lipids in their activation of complement. Incubation of these compounds with DNA to form complexes reduces complement activation in virtually all cases. The degree of complement activation by DNA complexes is strongly dependent on the ratio of polycation and DNA (expressed as the charge ratio) for polylysine, dendrimer, poly(ethyleneimine), and DOGS. To a lesser degree, charge ratio also influences complement activation by monovalent cationic lipid–DNA complexes. For polylysine–DNA complexes, complement activation can be considerably reduced by modifying the surface of preformed DNA complexes with polyethyleneglycol (half-maximal ≈ 20 μM amine content). The data suggests that, by appropriate formulation of DNA complexes, complement activation can be minimized or even avoided. These findings should facilitate the search for DNA complex formulations appropriate for reproducible intravenous gene delivery. Successful gene delivery upon intravenous administration of DNA complexes has been reported in several cases. However, there is considerable variability and poor reproducibility with such systems. The variability makes it difficult to understand which of the multitude of factors that impact this mode of gene delivery limits the reproducibility. Limiting parameters may include the stability of DNA complexes in blood and their interaction with blood components. Such interactions may limit the half-life and targetability of DNA complexes. The components of the complement system are likely candidates to interact with (opsonize) intravenously administered DNA complexes. To find out whether DNA complexes activate the complement system, we have examined a number of synthetic cationic molecules frequently used in gene delivery and their complexes with DNA for their complement-activating properties. The results indicate that complement activation is a potential limiting factor for gene delivery by synthetic DNA complexes. Furthermore, appropriate formulation of DNA complexes can minimize or avoid complement activation and possibly other interactions with blood components.