Disposition in rats of a polyoxypropylene-polyoxyethylene copolymer used in plasma fractionation

A polyoxypropylene-polyoxyethylene block copolymer of about 4750 daltons (Poloxamer 108, Pluronic F-38) used in a new protein fractionation procedure may be infused into patients receiving therapeutic plasma fractions. We studied the disposition and pharmacokinetics of Poloxamer 108 in rats as an initial step towards understanding its behavior in man. After iv administration in rats, about 94% of 7 or 100 mg/kg doses of ethylene-14C-labeled polymer was excreted in the urine in 3 days. About 6% of the label appeared in feces. Erythrocyte membranes were not permeable to the polymer, and only the parent compound was demonstrable in urine. Twenty hours after dosing, small residues were detectable only in the kidney, liver, small intestine, and carcass. The third phase of the plasma disappearance pattern was evident only at the larger dose, but plasma disappearance kinetics were independent of the dose in the range used here. Thus, most of poloxamer 108 was eliminated rapidly in rats by renal excretion, and a smaller portion probably was removed by biliary excretion. These results will be applied to continuing studies of Poloxamer 108 disposition in man.