Blood-brain-barrier Transport of Lipid Microspheres Containing Clinprost, a Prostaglandin I2 Analogue

Because the permeability of the blood‐brain barrier to lipid microspheres (LMs) has not hitherto been demonstrated, blood‐brain‐barrier permeability to LM containing the prostaglandin I2 analogue clinprost has been evaluated for an in‐vitro system of primary cultured monolayers of bovine brain capillary endothelial cells (BCECs), by a capillary depletion study in rats and by an in‐situ brain perfusion study in normal and 4‐vessel‐occluded fore brain ischaemic rats. Although energy‐dependency was not observed in [3H]clinprost uptake by BCECs, in accordance with results for simple diffusional transport, uptake of [3H]clinprost contained in lipid microspheres (denoted [3H]clinprost(LM)) was significantly inhibited by the endocytosis inhibitor, dansylcadaverine. The transport of LM into BCECs by endocytosis was also confirmed by fluorescence microscopy and flow‐cytometric analysis using LM labelled with a fluorescent probe, 1,1′‐dioctadecyl‐3,3,3′,3′‐tetramethylindocarbocyanine perchlorate (DiI). The absolute uptake of DiI(LM) by BCECs, measured by HPLC, was, however, almost 1/10 that of [3H]clinprost(LM), results which suggest the superiority of simple diffusion of clinprost over endocytosis of its LM form in the uptake of clinprost(LM) by BCECs. In the capillary‐depletion study with rat‐brain‐perfused [3H]clinprost(LM) from the internal carotid artery, the parenchyma apparent distribution volume was about 45 times larger than that of the capillary, showing that [3H]clinprost(LM) was transported through the blood‐brain barrier into the brain. The permeability coefficients of [3H]clinprost and [3H]clinprost(LM) determined by in‐situ brain perfusion in normal rats were considerably higher than those of the active metabolite [3H]isocarbacyclin and its LM form. In addition, the Blood‐brain‐barrier permeabilities to [3H]clinprost, [3H]isocarbacyclin and their LM forms in ischaemic rats were almost identical to those in normal rats. It was concluded that clinprost(LM) was transported through the blood‐brain barrier by endocytosis of LM, simple diffusion of clinprost released from LM, and transport of isocarbacyclin generated by hydrolysis of clinprost. The blood‐brain‐barrier permeability of clinprost(LM) is not reduced in ischaemic conditions, because the simple diffusion of clinprost released from LM contributed mainly to clinprost(LM) transport.