Adsorption of amphiphilic hyperbranched polyglycerol derivatives onto human red blood cells

Hydrophobically derivatized hyperbranched polyglycerol (HPG)-polyethylene glycol (PEG) polymers bearing stearoyl chains (HPG-C18-PEG) were originally developed as human serum albumin substitutes and further as a unimolecular drug delivery system. In view of these in vivo applications and the potential for membrane interaction by these materials due to their amphiphilic structure, determining the adsorption of the polymers to human red blood cells (RBCs) is an important issue. This paper reports on the in vitro adsorption to RBCs of tritium-radiolabeled HPG-C18-PEG polymers. The morphological changes of RBCs associated with the adsorption were also examined by light and scanning electron microscopy (SEM). Laser scanning confocal microscopy (LSCM) suggests that the binding site of the polymers on RBCs is the cell membrane. Adsorption experiments show that, in the medium of either saline or plasma, the binding amount of the polymers to RBCs increases with increased polymer concentration in a manner which implies simple Langmurian behavior. The binding amount in saline is of the order of 10(5) molecules/cell at an equilibrium concentration of 1 mg/mL of HPG-C18-PEG polymer. The RBC morphology depends on the adsorbed amount; the cells become crenated in high concentrations (5 and 10 mg/mL) of the polymer solutions in the absence of plasma proteins. Interestingly, a large amount of polymers remain bound to RBCs even after washes with plasma (of the order of 10(4) molecules/cell). Thus, the bound polymers might have an extended circulating time by "hitchhiking" on RBCs in the bloodstream. These results provide significant information and insight for related studies of the interaction of amphiphilic molecules with cell membranes and for in vivo applications of biopolymers as drug delivery systems.