sildenafil-citrate and estrone-sulfate

The choroid plexuses (CP) are responsible for transport of micronutrients into brain and clearance of toxic compounds, in addition to its barrier function and production of CSF. Multidrug resistance-associated protein (Mrp) 4 is one transport protein highly expressed in CP tissue and is characterized as a versatile pump for toxicants and signalling molecules. Aim of the study was to determine transport characteristics of a fluorescent cAMP analog in rat CP and to define whether fluo-cAMP can be used for analyses of function, substrate/inhibitor specificity and regulation of Mrp4. Confocal imaging was used to analyze transport mechanisms in absence and presence of various modulators of organic anion transport in freshly isolated and functionally intact CP. Fluo-cAMP transport was saturable, selective, concentrative and metabolism-dependent, following an active two-step mechanism composed of apical uptake into epithelial cells and basolateral efflux. Uptake included a Na(+) -dependent and a Na(+) -independent component and was inhibited by estrone sulfate, taurocholate and sildenafil indicating involvement of organic anion transporting polypeptide Oatp1a5. Efflux was composed of an indirect Na(+) -dependent component and a component inhibitable by, for example, the MRP4 substrates/inhibitors, sulindac sulfide and 4-(2-aminoethyl) benzenesulfonyl fluoride. Therefore, fluo-cAMP can be used as fluorescent model compound for studying involvement of Mrp4 in signalling pathways and neuroprotection in CP.

Topics: Animals; Anti-Inflammatory Agents, Non-Steroidal; Biological Transport, Active; Choroid Plexus; Cyclic AMP; Estrone; Fluorescent Dyes; Male; Microscopy, Confocal; Multidrug Resistance-Associated Proteins; Neuroprotective Agents; Organic Anion Transporters, Sodium-Independent; Phosphodiesterase Inhibitors; Piperazines; Potassium; Potassium Cyanide; Purines; Rats; Rats, Sprague-Dawley; Reverse Transcriptase Polymerase Chain Reaction; RNA; Signal Transduction; Sildenafil Citrate; Sodium; Sulfones; Sulindac; Taurocholic Acid

The elimination process of the endothelin receptor antagonist bosentan (Tracleer) in humans is entirely dependent on metabolism mediated by two cytochrome P450 (P450) enzymes, i.e., CYP3A4 and CYP2C9. Most interactions with concomitantly administered drugs can be rationalized in terms of inhibition of these P450 enzymes. The increased bosentan concentrations observed in the presence of cyclosporin A, rifampicin, or sildenafil, however, are incompatible with this paradigm and prompted the search for alternative mechanisms governing these interactions. In the present article, we identify bosentan and its active plasma metabolite, Ro 48-5033 (4-(2-hydroxy-1,1-dimethyl-ethyl)-N-[6-(2-hydroxy-ethoxy)-5-(2-methoxy-phenoxy)-[2,2']bipyrimidinyl-4-yl]-benzenesulfonamide), as substrates of the human organic anion transporting polypeptides (OATP) OATP1B1 and OATP1B3. Bosentan uptake into Chinese hamster ovary cells expressing these OATP transporters was efficiently inhibited by cyclosporin A and rifampicin with IC(50) values significantly below their effective plasma concentrations in humans. The phosphodiesterase-5 inhibitor sildenafil was also shown to interfere with OATP-mediated transport, however, at concentrations above those achieved in therapeutic use. Therefore, inhibition of bosentan hepatic uptake may represent an alternative/complementary mechanism to rationalize some of the pharmacokinetic interactions seen in therapeutic use. A similar picture has been drawn for drugs like pitavastatin and fexofenadine, drugs that are mainly excreted in unchanged form. Bosentan elimination, in contrast, is entirely dependent on metabolism. Therefore, the described interactions with rifampicin, cyclosporin A, and, to a lesser extent, sildenafil represent evidence that inhibition of hepatic uptake may become the rate-limiting step in the overall elimination process even for drugs whose elimination is entirely dependent on metabolism.

Topics: Animals; Aryl Hydrocarbon Hydroxylases; Biological Transport; Bosentan; CHO Cells; Cricetinae; Cricetulus; Cyclosporine; Cytochrome P-450 CYP2C9; Cytochrome P-450 CYP3A; Cytochrome P-450 Enzyme Inhibitors; Cytochrome P-450 Enzyme System; Dehydroepiandrosterone Sulfate; Drug Interactions; Enzyme Inhibitors; Estradiol; Estrone; Humans; Liver-Specific Organic Anion Transporter 1; Molecular Structure; Organic Anion Transporters; Organic Anion Transporters, Sodium-Independent; Piperazines; Purines; Pyrimidines; Rifampin; Sildenafil Citrate; Solute Carrier Organic Anion Transporter Family Member 1B3; Sulfonamides; Sulfones; Warfarin