Effect of oxyethylene numbers on the pharmacokinetics of menatetrenone incorporated in oil-in-water lipid emulsions prepared with polyoxyethylene-polyoxypropylene block copolymers and soybean oil in rats.

We have prepared lipid emulsions of approximately 200 nm in diameter with soybean oil (SO) and a series of Pluronics with various numbers of oxyethylene units and about 60 oxypropylene units (SO/Pluronics), and studied the pharmacokinetics of menatetrenone incorporated into SO/Pluronics in rats. Emulsions of approximately 200 nm in diameter were obtained when SO contents were 2.5% and 20% (w/w) for 2.4% (w/w) PL101 and Pluronics that more than 30% was made up by oxyethylene units, respectively. The half-life of menatetrenone in plasma when oxyethylene units made up less than 30% of the Pluronic (SO/PL101 and SO/PP103) was similar to that for SO/egg yolk phosphatides (SO/EYP), but longer than that when oxyethylene units composed more than 40% of the Pluronic (SO/PP104 and SO/PF108, by 3- and 10-fold, respectively). Pretreatment with dextran sulfate 500000, an inhibitor of emulsion uptake by the reticuloendothelial system (RES), resulted in a higher plasma concentration and a lower liver uptake of menatetrenone as SO/PL101 at 10 min and SO/PP103 at 60 min, indicating that both SO/PL101 and SO/PP103 were taken up by the RES, although SO/PP103 required some time to be recognized by the RES. These findings suggested that larger numbers of oxyethylene units of Pluronics with 60 oxypropylene units were required for the longer plasma circulation of SO/Pluronics due to evasion of the RES.