. The interaction of cyclosporin A (CyA) with p‐glycoprotein during intestinal uptake was investigated by a combination of in vitro experiments with human Caco‐2 cells and an intubation study in healthy volunteers. . CyA uptake into the cells was not saturable and exhibited only a low temperature sensitivity, suggesting passive diffusion. When the permeation of CyA across Caco‐2 monolayers from the apical to the basolateral side was determined, overall transport had an apparently saturable component up to a concentration of 1 μm. At higher concentrations permeation increased over‐proportionally. Calculation of the kinetic parameters of apical to basolateral permeation suggested a diffusional process with a KD of 0.5 μl min−1 per filter, which was overlayed by an active system in basolateral to apical direction with a KM of 3.8 μm and a Jmax of 6.5 picomol min−1 per filter. . CyA permeation was significantly higher when the drug was given from the basolateral side as compared to the permeation from the apical side. Apical to basolateral transport of CyA was increased in the presence of vinblastine, daunomcyin and a non‐immunosuppressive CyA‐derivative. All compounds inhibit p‐glycoprotein‐mediated transport processes. Basolateral to apical permeation of CyA showed a dose‐dependent decrease in the presence of vinblastine. Permeation of daunomycin across Caco‐2 cell monolayers was also higher from the basolateral to the apical side than vice versa. Basolateral to apical permeation was decreased in the presence of SDZ PSC 833 and cyclosporin A. . Western blot analysis of Caco‐2 cells with the monoclonal antibody C219 confirmed the presence of p‐glycoprotein in the used cell system. . When the absorption of CyA in the gastrointestinal (G***I)‐tract of healthy volunteers was determined, a remarkable decrease of the plasma AUC could be observed dependent on the location of absorption in the rank order stomach > jejunum/ileum > colon. The decrease in absorption exhibited a marked correlation (r = 0.994) to the expression of mRNA for p‐glycoprotein over the G**I‐tract (stomach < jejunum < colon). . All data provide evidence that CyA is a substrate of p‐glycoprotein in the G***I‐tract, which might explain the local differences and the high variability in cyclosporin absorption found in vivo.

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1 Morphine-6-glucuronide is one of the major metabolites of morphine. The potent analgesic action of this compound together with its potential lower apparent toxicity in man, when compared with morphine, indicated its clinical importance. 2 Primary cultures of porcine brain capillary endothelial cells were used to study brain penetration of morphine-6-glucuronide. Biochemical characterization of the cell cultures revealed a marked enrichment in enzymatic activity of alkaline phosphatase (56 fold) and angiotensin converting enzyme (230 fold) as compared to whole brain tissue. By immunostaining the presence of vimentin, factor VIII, the tight junction associated protein ZO-1, and P-glycoprotein was shown. Functional characterization revealed that the carrier system responsible for transport of neutral amino acids was intact. 3 Uptake and transport of morphine-6-glucuronide was marginal and in the range of the extracellular marker sucrose. However, uptake of morphine-6-glucuronide was enhanced significantly (P<0.0001) in presence of the inhibitors of P-glycoprotein, verapamil or vincristine. The finding that morphine-6-glucuronide may serve as a substrate for P-glycoprotein was confirmed in multidrug-resistant P388 tumour cells.4 We conclude that penetration of the blood-brain barrier by morphine-6-glucuronide may depend on the expression of the product of the multidrug-resistance (MDR) gene in brain capillary endothelial cells.

Immunoliposomes conjugated with the OX26 monoclonal antibody to the rat transferrin receptor can be used for brain delivery of small molecules. In the present study the uptake of OX26-immunoliposomes by target cells as well as their transcytosis across the blood-brain barrier was investigated. Microscopy of RG2 rat glioma cells incubated with fluorescence labeled OX26-immunoliposomes revealed intracellular co-localization of liposomal cargo, the liposomal membrane bilayer and the OX26 monoclonal antibody. The distinct particulate staining pattern was indicative for accumulation of OX26-immunoliposomes within endosomal or lysosomal compartments. Prolonged incubations demonstrated endosomal release of the liposomal cargo propidium iodide to the cytoplasm. A maximum of 50% of propidium iodide was released from the endosomal compartment after 24 hours of incubation. Transcytosis was studied using an in vitro model of the blood-brain barrier consisting of immortalized RBE4 rat brain endothelial cells. OX26-immunoliposomes did permeate across the RBE4 cell monolayer and showed a permeability coefficient of P(app) = 1.6 x 10(-5) ml/s. Transport was inhibited at low temperature, by competition with free OX26 or by exchanging the OX26 monoclonal antibody for an unspecific isotype antibody. Transcytosis of OX26-immunolipsomes was confirmed in vivo by the brain perfusion and capillary depletion technique. OX26-immunoliposomes were detected within the post-vascular compartment of brain parenchyma (PS product = 2.4 microl/g/min.) and were not associated with the brain microvasculature.