Browsing by Subject "OATP1B1"

The effect of genes on drug response is studied in the field of pharmacogenetics. Genetic polymorphism occurs in several genes that code drug metabolizing enzymes or drug transporters. A protein coded by a variant gene may be dysfunctional, which can affect the efficiency and safety of the substrate drug individually. The common polymorphisms of the gene ABCG2 coding the efflux transporter BCRP and the gene SLCO1B1 coding the influx transporter OATP1B1 are associated with the interindividual variation in the effectiveness and tolerability of the cholesterol-lowering statins. In this study, the effects of the polymorphisms ABCG2 c.421C>A and SLCO1B1 c.521T>C on rosuvastatin concentration in plasma and the liver were studied with two different pharmacokinetic models. The developed liver model illustrating the enterohepatic circulation of drugs was compared to a commercial Simcyp model. According to the simulations with both models, the effect of the polymorphisms of OATP1B1 and BCRP on the plasma concentration of rosuvastatin is additive. The plasma concentration increases up to fourfold if the same individual has homozygous polymorphic forms of both the OATP1B1 and the BCRP. Based on the modellings, the change of the rosuvastatin concentration in the liver owing to polymorphism does not follow the same pattern as in plasma. In consequence of the polymorphism of the BCRP, the rosuvastatin concentration rises two to three times larger in the liver, which is the site of action of the statins. The polymorphism of the OATP1B1 instead causes the liver concentration to decrease little compared to the wild type. In conclusion, the efflux transporter BCRP seems to have a greater significance on regulating the concentration of rosuvastatin in the liver than the influx transporter OATP1B1. Computer modelling is worth exploiting as a supportive method of other study methods in the pharmacogenetic research, for example when the relative significance of separate transporter proteins is evaluated.

The liver is the major site of drug metabolism and excretion. Within the liver endogenous and exogenous compounds are eliminated through many metabolizing enzymes. Drug removal is not only dependent on metabolic enzymes, but also on transporters. Before cellular metabolism can occur, a drug must first enter the hepatocyte. Very lipophilic drugs enter the cell membrane through passive diffusion, but polar or ionized organic compounds can enter the cell membrane only by transporters. Transporters in the basolateral membrane of the hepatocyte facilitate drug entry and access to drug metabolizing enzymes. Transporters in the canalicular domain (apical) of the hepatocyte faclitate removal of drugs or metabolites from the cell interior. Recent studies have shown that transporters can mediate drug-drug interactions, and transporter genes are subject to genetic polymorphism which may affect pharmacokinetic parameters of a drug, such as absorption, distribution, and excretion. This Master's thesis consists of two parts, a literature review and an experimental section. In the literature review two transporters, OATP1B1 and MRP2, are discussed in detail. OATP1B1 is expressed on the basolateral and MRP2 on the apical membrane of the hepatocyte. These transporters are responsible for the vectorial transcellular hepatobiliary transport of various organic anions in humans. The experimental section aims at modelling vectorial hepatobiliary transport of three compounds in a double-transfected (OATP1B1/MRP2) MDCKII cell line. All three compounds studied, rosuvastatin, estrone sulphate, and estradiol glucuronide, are substrates of both transporters. Wild type (WT) MDCKII cells were used as a control. Tight junctions form a barrier between cells. This barrier regulates the paracellular passage of, for example, water, ions, large molecules, and drugs. In the experimental section the tight junctions were reversibely opened to distinguish between trans- and paracelluar routs of transport of the three compounds studied. Permeation of rosuvastatin and estradiol glucuronide in the basolateral to apical direction was faster in the double-transfected cell line compared to the MDCKII-WT cell line. Permeation of estrone sulphate, however, behaved unexpectedly in the double-transfected cell line. The permeation of this compound was almost equal in the apical to basolateral and basolateral to apical direction. The reason for this unexpected finding remains unclear. By opening the tight junctions the permeation of all compounds in both cell lines was increased, indicating that the compounds studied preferred the paracellular route and the importance of transporters were reduced. The double-transfected MDCKII cell line is a useful in vitro model of hepatic vectorial transport of organic anions in humans.