The structure of PEG-modified poly(ethylene imines) influences biodistribution and pharmacokinetics of their complexes with NF-κB decoy in mice

Purpose. To study the relationship between structure of poly(ethylene imine-co-ethylene glycol), PEI-PEG, copolymers and physicochemical properties as well as in vivo behavior of their complexes with NF-kappaB decoy. Methods. A variety of copolymers of PEG grafted onto PEI as well as PEI grafted onto PEG were synthesized and their complexes with a double stranded 20mer oligonucleotide were examined regarding size, surface charge, biodistribution and pharmacokinetics. Results. Polyplexes of copolymers were smaller compared to polyplexes formed by non-PEGylated PEI 25 kDa (58-334 nm vs. 437 nm for a nitrogen/phosphate ratio of 3.5 and 85-308 nm vs. 408 nm for N/P 6.0) and showed reduced zeta potential (-2.5-6.4 mV vs. 14.5 mV for N/P 6.0). IV injection into mice revealed liver (35-76% of injected dose), kidney (3-22%) and spleen (2-16%) to be the main target organs for all injected complexes. Complexes formed by copolymers with few PEG blocks of higher molecular weight (5 kDa and 20 kDa) grafted onto PEI 25 kDa did not show different blood levels from PEI 25 kDa. In contrast, a copolymer with more short PEG blocks (550 Da) grafted onto PEI showed elevated blood levels with an increase in AUC of 62%. Conclusions. A sufficiently high density of PEG molecules is necessary to effectively prevent opsonization and thereby rapid clearance from blood stream.