RNA interference (RNAi) represents a novel therapeutic strategy allowing the knockdown of any pathologically relevant target gene. Since it relies on the action of small interfering RNAs (siRNAs), the in vivo delivery of siRNAs is instrumental. Polyethylenimines (PEIs) and PEGylated PEIs have been shown previously to complex siRNAs, thus mediating siRNA protection against nucleolytic degradation, cellular uptake and intracellular release.

The present study determines in vivo pharmacokinetics, tissue distribution/efficacy of siRNA delivery and adverse effects of a broad panel of PEI(–PEG)-based siRNA complexes. The aim is to systematically evaluate the effects of different degrees and patterns of PEGylation in PEI–PEG copolymers on the in vivo behavior of PEI(–PEG)/siRNA complexes in mice.

Upon i.v. injection of radioactively labeled, PEI(–PEG) complexed siRNAs, marked differences in the pharmacokinetics and biodistribution of the complexes are observed, with the fate of the PEI(–PEG)/siRNA complexes being mainly dependent on the degree of uptake in liver, spleen, lung and kidney. Thus, the role of these tissues is investigated in greater detail using representative PEI(–PEG)/siRNA complexes. The induction of erythrocyte aggregation and hemorrhage is dependent on the degree and pattern of PEGylation as well as on the PEI/siRNA (N/P) ratio, and represents one important effect in the lung. Furthermore, siRNA uptake in liver and spleen, but not in lung or kidney, is mediated by macrophage and is dependent on macrophage activity. In the kidney PEI(–PEG)/siRNA uptake is mostly passive and reflects the total stability of the complexes.

Liver, lung, spleen and kidney are the major players determining the in vivo biodistribution of PEI(–PEG)/siRNA complexes. Beyond their physicochemical and in vitro bioactivity characteristics, PEI(–PEG)/siRNA complexes show marked differences in vivo which can be explained by distinct effects in different tissues. Based on these data, our study also identifies which PEGylated PEIs are promising tools for in vivo siRNA delivery in future therapeutic studies and which major determinants require further investigation.