Effect of Polymer Molecular Weight to Tumor Cells 1579 Fig. 9. Luciferase expression, 24 h after i.v. administration of pDNA/ ppz290 complexes (a) and pDNA/ppz130 complexes (b) into mice bearing a Neuro 2 A s.c. tumor (30 mg DNA/mouse). White bars: N/P ratio 12, hatched bars: N/P ratio 20. Luciferase expression is plotted as relative light units per organ (RLU/organ) (mean + SD, n = 4). Average tumor weight was 600 mg. molecular weight ppz950 and ppz570 could not be determined at these N/P ratios. At a lower N/P ratio of 6, polyplexes of the latter polymers did not show significant gene expression in any of the organs. Possibly, the absence of gene expression is due to the limited amount of polymer present, leading to complex neutralization or complex dissociation after opsonization by serum proteins (1,13). The administration of polyplexes of the lower weight polymers (ppz290 and ppz130) resulted in considerable tumor gene expression, when prepared at an N/P ratio of 20 (Fig. 9a and b). However, gene expression was not only confined to the tumor tissue, but was also observed in liver and lung. At an N/P ratio of 12, intravenous administra- tion of polyplexes based on the lowest molecular weight polymer (ppz130) resulted in gene expression which was confined to the tumor tissue. This tumor selective gene expression is in good agreement with earlier findings on p(DMAEA)ppz based polyplexes. The extent of luciferase expression is similar to that observed for polyplexes based on PEI22 (N/P 6) (5). CONCLUSION p(DMAEA)-ppz_s with different molecular weights ranging from 130 to 950 kDa and with narrow molecular weight distributions were obtained by size exclusion chroma- tography. 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