Protein adsorption to polyethylene glycol modified liposomes from fibrinogen solution and from plasma

Unmodified and polyethylene glycol (PEG) modified neutral and negatively ch arged liposomes were prepared by freeze-thaw and extrusion followed by chro matographic purification. The effects of PEG molecular weight (PEG 550, 200 0, 5000), PEG loading (0-15 mol%), and liposome surface charge on fibrinoge n adsorption were quantified using radiolabeling techniques. All adsorption isotherms increased monotonically over the concentration range 0-3 mg/ml a nd adsorption levels were low. Negatively charged liposomes adsorbed signif icantly more fibrinogen than neutral liposomes. PEG modification had no eff ect on fibrinogen adsorption to neutral liposomes. An inverse relationship was found between PEG loading of negatively charged liposomes and fibrinoge n adsorption. PEGs of all three molecular weights at a loading of 5 mol% re duced fibrinogen adsorption to negatively charged liposomes. Protein adsorp tion from diluted plasma (10% normal strength) to four different liposome t ypes (neutral, PEG-neutral, negatively charged, and PEG-negatively charged) was investigated using gel electrophoresis and immunoblotting. The profile s of adsorbed proteins were similar on all four liposome types, but distinc tly different from the profile of plasma itself, indicating a partitioning effect of the lipid surfaces. alpha2-macroglobulin and fibronectin were sig nificantly enriched on the liposomes whereas albumin, transferrin, and fibr inogen were depleted compared to plasma. Apolipoprotein Al was a major comp onent of the adsorbed protein layers. The blot of complement protein C3 ads orbed on the liposomes suggested that the complement system was activated. (C) 2001 Published by Elsevier Science B.V.