Surface hydrophobicity of particles is not necessarily the most important determinant in their in vivo disposition after intravenous administration in rats

The in vivo disposition of polystyrene microsphere (MS) with the particle size of 50 nm (MS-50) and lecithin-coated MS-50 (LMS-50) after intravenous administration to rats was characterized. While a rapid elimination from the systemic circulation was observed for MS-50, much more prolonged circulating property was observed for LMS-50. In addition, this in vivo disposition property of LMS-50 was suggested to be ascribed to its lower affinity to the liver, which is the determining organ of the in vivo disposition of MS-50. The evaluation of surface hydrophobicity of MS-50 and LMS-50 in buffer solution revealed that the surface of MS-50 is more hydrophobic than that of LMS-50. However, LMS-50 was oppositely found to be more hydrophobic than that of MS-50 in rat serum. The profiles of serum proteins associated with MS-50 and LMS-50 were also examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The results showed that the amounts of some adsorbed proteins are greatly different between MS-50 and LMS-50. From these findings, it was suggested that the substantial difference in the in vivo disposition between MS-50 and LMS-50 would not be attributed to the difference in their surface hydrophobicity in the blood, but the difference in the type of serum proteins associated with them.