RES uptake depends strongly on drug carrier surface prop-
The adsorption of poly(ethylene glycol) (PEG)-esterified fatty erties including hydrophobicity, charge, and chemical strucacids at methylated silica, phosphatidic acid, and phosphatidylture. In particular, it has been found that coatings of poly choline surfaces was investigated with in situ ellipsometry. For a (ethylene glycol) (PEG) strongly reduce the RES uptake, series of PEG-fatty acid esters of ethoxy groups and acyl tails of which has resulted in a large interest in PEG-coated drug type C i: j -EO 151 (16 ยฃ i ยฃ 18, 0 ยฃ j ยฃ 2) adsorption at methylated carriers (often referred to as stealth carriers) (1-12). PEG silica was independent of bulk micellization, and a plateau was coating of colloidal drug carriers is typically accomplished reached below the critical micellization concentration (CMC). The by incorporating a PEG-derivatized phospholipid into the plateau adsorbed amount for the investigated fatty acid esters was carrier's phospholipid layer or adsorbing copolymers of PEG only weakly dependent on the nature of the hydrophobic moiety. Instead, saturation adsorption was largely determined by the inter-and a more hydrophobic polymer such as poly(propylene actions between PEG chains. Adsorption isotherms were therefore glycol) (PPG) onto the carrier surface. essentially identical on all three of the quite different surfaces. At
The mechanisms behind surface-localized PEG-chains resaturation adsorption, the adsorbed layer thickness was 10-15 ducing carrier uptake are expected to be related to the ennm, while the average adsorbed layer concentration was 0.07 g/ hanced biocompatibility of PEG-coated implant materials cm 3 . Formation of the PEG-surfactant coatings thus appeared to and extracorporeal devices, as well as PEG derivatized proinvolve significant molecular alterations of PEG from a random teins, and to decreasing problems associated with passive coil. The ability of the PEG-ester coatings to inhibit protein adadsorption in solid-phase diagnostics (13). In the case of sorption was also investigated. At the adsorption plateau, all coatcolloidal drug carriers, uptake is thought to be initiated by ings investigated displayed quite good ability to inhibit adsorption the adsorption of certain serum proteins, so-called opsonines, by a number of serum proteins. For the surfaces studied this ability decreased below 0.2 CMC. These findings are discussed in relation including immunoglobulins (e.g., IgG), complement comto the ability of PEG-derivatized lipids to control the in vivo fate ponents (e.g., C3 and C1q), and adhesins (e.g., fibrinogen of colloidal drug carriers.