Copolymers of polylactide and poly(ethylene glycol) (PLA−PEG), which self-disperse in water to form spherical nonionic micelles, have been investigated as a novel biodegradable drug delivery system. These copolymers are defined by the molecular weight ratios of their polylactide to poly(ethylene glycol) components (1.5:2 PLA−PEG and 2:5 PLA−PEG) and gave two peaks when purified by gel permeation chromatography (GPC). The first peak consisted of spherical micelles with a diameter of 15.6 nm for 1.5:2 PLA−PEG, and 18.9 nm for 2:5 PLA−PEG micelles after analysis by dynamic light scattering (DLS) and by transmission electron microscopy (TEM). The second peak was a PLA-depleted species resulting from the synthesis and did not form micelles. Testosterone and sudan black B (SBB), which have different hydrophobicities, were used as “model drugs” to evaluate the drug loading ability of the micelles. Ultracentrifugation sedimentation velocity studies confirmed that solubilization of the model drugs had occurred by micellar incorporation. Higher drug loading was obtained for the 1.5:2 PLA−PEG micelles (63.9% (w/w) of SBB, 0.74% (w/w) of testosterone) than for the 2:5 PLA−PEG micelles (59.0% (w/w) of SBB, 0.34% (w/w) of testosterone). The amount of testosterone solubilized was therefore significantly lower than SBB for both copolymers. Stability testing in the presence of salt suggested that the micelles had sterically stabilized surfaces. In vivo studies in the rat, using a radioactive marker, showed that PLA−PEG micelles demonstrated extended circulation times in the blood during the period of study (3 h). The 1.5:2 PLA−PEG showed increased blood levels and lower uptake of the micelles by the liver compared to the 2:5 PLA−PEG micelles. This is thought to be due to differences in the packing density of the copolymer molecules on the micelle surface.

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The effect of differing densities of poly (ethylene glycol-2000) (PEG2000) at the particle surface of polystyrene-poly (ethylene glycol-2000) (PS-PEG2000) particles was assessed in terms of hydrophobic interaction chromatography (HIC) and the in vitro and in vivo behaviour of the particles. The particles, with different surface densities of PEG, were prepared by varying the copolymerizing reaction of styrene with a PEG macromonomer. There is a clear relationship between the surface density of PEG as determined by X-ray photoelectron spectroscopy and surface hydrophobicity as assessed by hydrophobic interaction chromatography (HIC). Similarly, the interaction of the particles with non-parenchymal liver cells in in vitro studies was shown to decrease as the surface density of PEG increases. The in vivo study investigating the biodistribution of the PS-PEG particles after intravenous injection into rats reveals that a relationship exists between the surface density of PEG and the extent to which the particles remain in the circulation, avoiding recognition by the reticuloendothelial system. Particles with the higher surface densities show increased circulatory times which compared well with data for particles prepared with the surface adsorbed PEO-PPO block copolymer, Poloxamine 908.

The modification of surface properties of biodegradable poly(lactide-co- glycolide) (PLGA) and model polystyrene nanospheres by poly(lactide)-poly(ethylene glycol) (PLA:PEG) copolymers has been assessed using a range of in vitro characterization methods followed by in vivo studies of the nanospheres biodistribution after intravenous injection into rats. Coating polymers with PLA:PEG ratio of 2:5 and 3:4 (PEG chains of 5000 and 2000 Da. respectively) were studied. The results reveal the formation of a PLA:PEG coating layer on the particle surface resulting in an increase in the surface hydrophilicity and decrease in the surface charge of the nanospheres. The effects of addition of electrolyte and changes in pH on stability of the nanosphere dispersions confirm that uncoated particles are electrostatically stabilized, while in the presence of the copolymers, steric repulsions are responsible for the stability. The PLA:PEG coating also prevented albumin adsorption onto the colloid surface. The evidence that this effect was observed for the PLA:PEG 3:4 coated nanospheres may indicate that a poly(ethylene glycol) chain of 2000 Da can provide an effective repulsive barrier to albumin adsorption. The in vivo results reveal that coating of PLGA nanospheres with PLA:PEG copolymers can alter the biodistribution in comparison to uncoated PLGA nanospheres. Coating of the model polystyrene nanospheres with PLA:PEG copolymers resulted in an initial high circulation level, but after 3 hours the organ deposition data showed values similar to uncoated polystyrene spheres. The difference in the biological behaviour of coated PLGA and polystyrene nanospheres may suggest a different stability of the adsorbed layers on these two systems.(ABSTRACT TRUNCATED AT 250 WORDS)

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