An investigation of the filtration capacity and the fate of large filtered sterically-stabilized microspheres in rat spleen.

Earlier we demonstrated that coating the surface of large model polystyrene microspheres (220-300 nm in diameter) with the block co-polymer polyoxyethylene/polyoxypropylene poloxamine-908 triggered their accumulation in the rat spleen by a filtration mechanism following intravenous administration [Moghimi, S.M., Porter, C.J.H., Muir, I.S., Illum, L. and Davis, S.S. (1991) Biochem. Biophys. Res. Commun. 177, 861-866]. We have now demonstrated that the macrophages of the red-pulp can effectively phagocytose the filtered poloxamine-coated microspheres 24 h post-administration. This could be the result of either the loss of the surface absorbed poloxamine, and hence the steric barrier, or 'neutralization' of the effect of the anti-phagocytic material poloxamine-908 within the spleen. In order to assess the capacity of the splenic uptake mechanism(s), rats received daily intravenous administration of unlabelled large poloxamine-908 coated microspheres (220 nm in diameter) for 4 days (daily-dosed animals). Control rats received daily saline injection. On the fifth day all animals were injected with either radiolabelled large (220 nm) or small (60 nm in diameter) poloxamine-coated polystyrene microspheres. Predosing dramatically decreased the splenic uptake of the large test microspheres but had no effect on the uptake of small test-microspheres. The failure of the spleen to take up particles was not associated with an increased circulatory level of microspheres. Surprisingly, both small and large coated microspheres were sequestered by the liver and accumulated in Kupffer cells as demonstrated by electron microscopy in daily-dosed animals. In contrast, the liver of control animals did not effectively sequester poloxamine-coated microspheres. Here, microspheres predominantly remained in blood. Sequestration of poloxamine-908 coated microspheres by Kupffer cells of the liver of daily-dosed animals was the result of opsonization by an unknown serum component.