Hepatic efflux process

Prediction of Hepatic Efflux Process from In Vitro Data... 

One of the popular experimental systems to investigate the hepatic efflux process is canalicular membrane vesicle (CMV). It is difficult to evaluate the transport activity of efflux transporters in cell systems because substrates cannot easily access the intracellular compartment, so CMV system is often used to rapidly determine the ATP-dependent efflux transport of substrates across bile canalicular membrane. 

OATP/SLCO superfamily is classified into six families in mammalians (64). This chapter described the characteristics of three families (OATP1, OATP2, and OATP4), which have been suggested to be involved in the drug disposition, i.e., hepatic uptake process, basolateral uptake and reabsorption in the kidney, intestinal uptake, and efflux transport in the barriers of central nervous system. 

FIGURE 6.1 The barriers that a lipophilic drug has to transverse along the intestinal absorption process (1) dissolution and solubilization in the intestinal milieu, (2) narrow absorption window, (3) unstirred water layer, (4) efflux pumps, (5) intra-enterocyte metabolism, and (6) first pass hepatic metabolism. 

The use of in vitro systems to evaluate hepatic drug uptake and efflux is an essential part of the drug development process. Primary hepatocyte culture is one technique to address this issue. 

Fig. 4 This cartoon depicts the various processes leading to an oral bioavailability of 27%, following an oral dose of the Sandimmune formulation of cyclosporine. The values at the bottom of the figure indicate the average fraction of the dose lost in each of the processes i.e., 14% of the dose is either unabsorbed, counter transported (effluxed) by P-gp, or degraded in the gut lumen, 51% of the drug is metabolized in the enterocytes of the gut wall, and only 8% is lost due to hepatic first-pass metabolism.

Third, the equations employed in a PBPK model should be consistent with the state of knowledge or biologically plausible hypotheses of the mechanisms of ADME for the particular chemical. In this regard, the uptake of chemicals in systemic circulation is described as either a diffusion-limited or perfusion-limited process (Gerlowski and Jain 1983), and metabolic clearance in individual tissues or tissue groups is described using a maximal velocity and Michaelis constant, intrinsic clearance, or hepatic extraction ratio (Krishnan and Andersen 2007). The mass balance differential equations accounting for uptake clearance, efflux clearance, and metabolic clearance are formulated as a function of identifiable input parameters (Table 21.1). 

Under normal conditions, the administration of L-tryptophan affects the transcriptional and translational controls involving hepatic protein synthesis.220 Earlier studies have reported enhanced outflow of mRNA from hepatic nuclei in rats treated with chemical carcinogens,326 similar to that described with the administration of L-tryptophan to normal controls.298 Altered or uncontrolled release of nuclear RNAhas been considered as a potential basis for the phenotypic expression of chemically induced neoplasms.327 Possibly the stimulation of nuclear mRNA efflux induced by L-tryptophan in conjunction with that induced by the chemical carcinogen may in some cases potentiate the overall process involved in carcinogenesis. 

As far as HDL levels and metabolism are concerned, one result of the LCAT- and transfer protein-catalyzed reactions is the production of a dynamic spectrum of particles with a wide range of sizes and lipid compositions (Chapter 19). Nascent HDL particles contain mostly apo A1 and phospholipids, and undergo modulation and maturation in the circulation. For instance, the unesterified cholesterol incorporated into plasma HDL is converted to cholesteryl esters by LCAT, creating a concentration gradient of cholesterol between HDL and cell membranes, which is required for efficient cholesterol efflux from cells to HDL. In addition, cholesteryl ester transfer protein transfers a significant amount of HDL cholesteryl ester to VLDL, IDL, and LDL for further transport, primarily to the liver. Thus, a substantial fraction of cell-derived cholesterol is delivered as part of HDL indirectly to the liver via hepatic endocytic receptors for IDL and LDL this process is termed reverse cholesterol transport . However, receptor-mediated delivery of HDL cholesterol to cells is fundamentally different from the classic LDL receptor-mediated endocytic pathway, as described in Section 7.3.2. 

Bi-directional flux of free cholesterol between cells and lipoproteins occurs, and rate constants characteristic of influx and efflux can be measured [17]. The direction of any net transfer of free cholesterol is determined by the relative free cholesterol/phospholipid molar ratios of the donor and acceptor particles. Cholesterol diffuses down its gradient of chemical potential generally partitioning to the phospholipid-rich particle. Such a surface transfer process can lead to delivery of cholesterol to cells. This mechanism operates independently of any lipoprotein internalization by the receptor-mediated endocytosis. The influence of enzymes such as lecithin-cholesterol acyltransferase and hepatic lipase on the direction of net transfer of free cholesterol between lipoproteins and cells can be understood in terms of their effects of the pool sizes and the rate constants for influx and efflux. 

Steady state the hepatic intrinsic clearance of pravastatin, a substrate for OATP2 and MRP2 (Tokui et al., 1999 Yamazaki et al., 1997), was regulated by the uptake process, followed by rapid metabolism and/or biliary excretion with minimal efflux to the systemic circulation in rats after infusion. The total hepatic elimination rate at steady state exhibited Michaelis-Menton saturation with the drug concentration and the and V ax obtained in rats with different mathematical models (i.e., well stirred, parallel tube, and dispersion models) were comparable with the initial uptake velocity measured from in vitro hepatocytes (Tokui et al., 1999). 

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