IN-VITRO DISPLACEMENT BY RAT SERUM OF ADSORBED RADIOLABELED POLOXAMERAND POLOXAMINE COPOLYMERS FROM MODEL AND BIODEGRADABLE NANOSPHERES

Poloxamer 407 and poloxamine 908 have been used by many research group s to modify the surface of both model latex and biodegradable nanosphe res, thereby producing nanospheres that have shown reduced protein ads orption in vitro and extended circulation times in vivo. A potential l imitation of such systems is the desorption of the copolymer coating l ayer. We describe a two-stage process to radiolabel poloxamer 407 and poloxamine 908 that has facilitated an investigation into this potenti al desorption, in vitro. The first stage of the labeling procedure inv olved the substitution of the terminal hydroxyl groups in each poly(et hylene oxide) (PEO) chain of poloxamer 407 and poloxamine 908 with an amino group. The aminated copolymers were then radioabeled with (125)I odine Bolton-Hunter reagent. The efficiency of labeling was calculated to be similar to 20% for the tetramine poloxamine 908 and similar to 33% for the diamine poloxamer 407, Remaining free amino groups were th en either acetylated, using acetic anhydride, or left in the free amin o form. Covalent linkage of the radiolabel to the copolymer was confir med by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy . The stability of the link between radiolabel and copolymer to hydrol ysis was also confirmed, <4% loss of radiolabel occurred from poloxami ne 908 after incubation in phosphate-buffered saline (PBS) at 37 degre es C for 8 days. The radiolabeled copolymers (with the free amino grou ps acetylated) were then used in experiments that have given the first direct evidence that adsorbed copolymers can be displaced by serum pr oteins in significant amounts from the surface of model and biodegrada ble nanospheres. The displacement was highly dependent on copolymer-na nosphere compatibility, with up to 78% of I-125 tetramine poloxamine 9 08 being displaced from poly(lactide-co-glycolide) (PLGA) nanospheres in 24 h, compared with 20% displacement of I-125 tetramine poloxamine 908 in 24 h from polystyrene nanospheres. These results have direct im plication for the future design of drug delivery systems based on coat ed nanospheres.