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Purpose Tumor gene expression after the intravenous (i.v.) administration of current polymer-based gene delivery systems is generally low and short-lived. Immune stimulatory CpG dinucleotides, present within the plasmid DNA of the polyplexes are likely to contribute to this. The effect of CpG replacement on the levels of transgene expression was studied, after the i.v. administration of polyethylenimine (PEI) polyplexes. Methods Tumor transfection and immune stimulation of PEI polyplexes containing plasmid DNA encoding for luciferase and rich in CpG motifs was monitored and compared to polyplexes containing the same gene but devoid of CpG motifs. Lipoplexes based on 1,2-dioleyl-3-trimethylammonium-propane/dioleoylphosphatidylethanolamine liposomes were included as a control. Results The replacement of CpGrich DNA by CpGfree DNA did neither affect the physical properties of the DNA complexes nor did it affect their in vitro transfection activity or cytotoxicity. The immune stimulation (interleukin-12) after i.v. administration of the PEI DNA complexes was low and unaffected by the presence of CpG motifs. The absence of CpG motifs within the different DNA complexes improved the degree and the duration of organ and tumor gene expression. Conclusion The depletion of CpG dinucleotides within the plasmid DNA of polyplexes enhances the degree and duration of in vivo transgene expression.

Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases collectively capable of degrading essentially all extracellular matrix components. At present, MMP gene family consists of 16 members, which, according to their substrate specificity and structure, are divided into subgroups of collagenases, gelatinases, stromelysins, and membrane type MMPs (MT-MMPs) (1). The expression of MMPs is upregulated in normal remodeling processes including fetal development, tissue repair, and angiogenesis, as well as in pathological conditions, e.g. rheumatoid arthritis, periodontitis, autoimmune blistering disorders of skin, tumor cell invasion, and metastasis (1). The original members of the collagenase subgroup, i.e. collagenase-1 (MMP-1) and neutrophil collagenase (MMP-8) have long been the only known secreted neutral proteinases capable of degrading native fibrillar collagens of type I, II, and III, and they are apparently responsible for controlled degradation of collagenous extracellular matrix in various situations (1).