The effect of N/P ratio on the in vitro and in vivo interaction properties of PEGylated poly[2-(dimethylamino)ethyl methacrylate]-based siRNA complexes.

In this study, we employed poly(ethylene glycol)-poly(n-butyl acrylate)-poly(2-(dimethylamino)ethyl methacrylate) (PEG-PnBA-PDMAEMA) triblock copolymer micelles as well as a PEG-PDMAEMA diblock copolymer as model systems for studying the role of N/P ratio on the in vivo behaviors of PEGylated siRNA carriers in mice. Through various in vitro assays, we identified the presence of a free/uncomplexed polymer population coexisting with siRNA complexes, the extent of which was found to be an increasing function of the N/P ratio. Contrary to what one might expect, however, we found that for both the diblock and triblock-based siRNA carrier systems, a change in the N/P ratio exerts insignificant influence on the in vivo biodistribution and ex vivo blood chemistry properties of the respective systems at < ~ 6 hours after systemic injection in mice. On the other hand, histological analysis of major organs at a longer time point (≈ 16 hours) indicates that the presence of uncomplexed polymer elicits toxicity to the organ that is associated with the clearance of the siRNA complexes from the circulation system. This effect can be eliminated by working at N/P ratios near the charge-neutralization point of the complexes.