Role of Transport Proteins in Drug Clearance

For hepatic clearance, passive diffusion through the lipid core of the hepatocyte membranes (available only to lipophilic drugs) is augmented by sinusoidal carrier systems, particularly for ionized molecules (either anionic or cationic) of molecular weights above 400. The presence of these carrier systems provides access to the interior of the hepatocyte to drugs with a wide range of physicochemical properties, ranging from hydrophilic to lipophilic. A schematic illustrating the role of these transport systems both into and out of the liver is shown in Figure 5.2. The transporters exist on the sinusoid face to remove drugs from the blood and transport them into the interior of the hepatocyte [1].

Likewise a different family of transporters exists on the canalicular face to transport drugs or their metabolites into bile. This complex system was originally termed biliary clearance but it is really two separate processes, hepatic uptake and biliary excretion. With small sized lipophilic drugs that readily traverse membranes hepatic uptake is probably not a major factor, since even if compounds are substrates, rapid redistribution across the membrane can occur. With higher molecular weight compounds (molecular weight greater than about 500) and those containing considerable H- bonding functionality (i.e. those that do not readily cross membranes) hepat-

Fig. 5.2 Schematic showing key sinusoidal and canalicular transport proteins and their substrate characteristics.

5.2 Role of Transport Proteins in Drug Clearance 61

5.2 Role of Transport Proteins in Drug Clearance 61

ic uptake can become the key clearance process, even if metabolism occurs subsequent to this. Figure 5.3 shows the structure of two combined thromboxane synthase inhibitors/thromboxane A2 receptor antagonists (TxSI/TxRAs). Both compounds show high hepatic extraction (E = 0.9) in the isolated perfused rat liver [2].

Compound A appears mainly as unchanged drug in the bile whereas compound B appears partly as metabolites. Administration of ketoconazole, a potent cytochrome P450 inhibitor, to the preparation dramatically decreases the metabolism of B and the compound appears mainly as unchanged material in the bile. Despite the inhibition of metabolism, hepatic extraction remains high (0.9). This indicates that clearance is dependent on hepatic uptake, via a transporter system, for removal of the compounds from the circulation. Metabolism of compound B is a process that occurs subsequent to this rate-determining step and does not influence overall clearance. This model for the various processes involved in the clearance of these compounds is illustrated in Figure 5.4.

The affinity of compounds for the various transporter proteins vary, but charge, molecular weight and additional H-bonding functionality seem to be particularly important.

Fig. 5.4 Model for the hepatic processes involved in the clearance of the combined TxSI/TxRAs (see Figure 5.3). The clearance by hepatic uptake (Clup) is the rate-determining step in the removal of the compound from the perfusate. Compounds accumulate within the liver and are subsequently cleared by biliary (Clbil) or metabolic clearance (ClM) (modified from reference [2]).

Fig. 5.4 Model for the hepatic processes involved in the clearance of the combined TxSI/TxRAs (see Figure 5.3). The clearance by hepatic uptake (Clup) is the rate-determining step in the removal of the compound from the perfusate. Compounds accumulate within the liver and are subsequently cleared by biliary (Clbil) or metabolic clearance (ClM) (modified from reference [2]).

Lipophilic drugs are metabolized by intracellular membrane-bound enzyme systems (e. g. cytochrome P450s and glucuronyl transferases) to more water soluble derivatives. The active sites of the major forms of the cytochrome P450 superfamily rely heavily on hydrophobic interactions with their substrates, although ion-pair and hydrogen bonding interactions also occur. Exit from the hepatocyte may be by simple passive diffusion back into the plasma or as outlined above via canalicular active transport systems which excrete drugs and their metabolites, again with wide ranging physical properties, into the bile.

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