Biodistribution of Long-Circulating PEG-Grafted Nanocapsules in Mice: Effects of PEG Chain Length and Density

Purpose: To study the pharmacokinetics and biodistribution of novel polyethyleneglycol (PEG) surface-modified poly(rac-lactide) (PLA) nanocapsules (NCs) and to investigate the influence of PEG chain length and content. Methods: The biodistribution and plasma clearance in mice of different NC formulations were studied with ³H]-PLA. PLA-PEG copolymers were used in NC preparations at different chain lengths (5 kDa and 20 kDa) and PEG contents (10% and 30% w/w of total polymer). In vitro and in vivo stability were also checked. Results: Limited ³H]-PLA degradation was observed after incubation in mouse plasma for 1 h, probably because of to the large surface area and thin polymer wall. After injection into mice, NCs prepared with PLA-PEG copolymers showed an altered distribution compared to poloxamer-coated PLA NCs. An increased concentration in plasma was also observed for PLA-PEG NCs, even after 24 h. A dramatic difference in the pharmacokinetic parameters of PLA-PEG 45-20 30% NCs compared to poloxamer-coated NCs indicates that covalent attachment, longer PEG chain lengths, and higher densities are necessary to produce an increased half-life of NCs in vivo. Conclusions: Covalently attached PEG on the surface of NCs substantially can reduce their clearance from the blood compartment and alter their biodistribution.