Drug Disposition in Liver Disease: Liver histopathology and hepatic drug transporter activity as determinants of drug disposition in liver diseases

Hepatic drug uptake and intrinsic clearance are key determinants of hepatic drug disposition in normal and disease states. Drug metabolism enzymes and transporters in the liver play important roles in hepatic uptake, metabolism and biliary clearance and they are also well known critical factors in hepatic drug-drug interactions. In particular, studies about the mechanism of hepatic drug disposition in the progression of disease states are limited. In this thesis, hepatic drug disposition was studied in normal rats and in three different rat models of human diseases, including cirrhosis, fatty liver disease (FLD), and right heart failure (RHF), which have different types of liver damage. Hepatic pharmacokinetics in normal and diseased conditions was studied with the established in situ perfused rat liver (IPRL) using multiple indicator dilution (MID). The outflow data were fitted with a two-phase physiologically based pharmacokinetic model in which the vascular dispersion was described by a two-inverse Gaussian distribution. To evaluate the liver damage during diseases, different quantitative histology indexes, including fibrosis index (FI), steatosis index (SI) and necrosis index (NI), were acquired from the computer-assisted image analysis of histology slides. Drug metabolism enzymes and drug transporter expression were semi-quantitatively determined with both real-time reverse transcription polymerase chain reaction (RT-PCR) and western blot analysis. The following studies were undertaken. Firstly, the hepatic pharmacokinetics of five cationic drugs in normal, FLD and RHF rat livers were studied to evaluate the relationship between liver damages and physicochemical properties of a drug and changes in hepatic pharmacokinetics. Central lobule necrosis was observed in the RHF rat livers and extensively quantified as increased NI. Both FLD and RHF rat livers showed collagen deposition in the liver sinusoids with a significantly increased FI. The hepatic extraction ratio (E), permeability-surface area product (PS) and hepatic intrinsic elimination clearance (CLint) for all drugs decreased with the increasing of FI. Stepwise regression analysis showed that E, PS and CLint are related to the physicochemical properties of cationic drugs (logarithm of octanol/water partition coefficient (logPapp)) and liver histopathology (FI, NI and cytochrome P450 (CYP) concentration) during the progression of FLD and RHF (r2>0.7). Stepwise regression analysis showed that the apparent distribution ratio (Kv) of cationic drugs was related to their pKa and SI in the liver (r2>0.7). Real-time RT-PCR analysis result showed increased expression of Cyp3a2, Cyp2d2 and Cyp2e1 in FLD rat livers, which explains the increased CLint of some cationic drugs. In RHF rat livers, real-time RT-PCR analysis showed decreased expression of Cyp3a2 and Cyp2d2, which is consistent with the decreased CLint. The predicted pharmacokinetic parameters calculated from physicochemical properties and liver histopathology changes during the disease states are well correlated to the observed pharmacokinetic parameters. The second part of the study was focused on the role of drug transporters in hepatic drug disposition. Hepatic pharmacokinetics of specific substrates for drug transporters was undertaken in normal, cirrhosis, FLD and RHF rat livers. In cirrhotic livers, the expression of all hepatic uptake transporters studied was decreased. In the FLD rat livers, Oatp1 and Oct1 showed a decreased expression, whereas Oatp2 and Oat2 showed an increased expression. Enhanced Oatp2 and Oat2 expression were found in RHF rat livers. A good linear relationship was found between values for the changes in logarithm expression levels of hepatic transporters (ΔlogProtein and ΔlogmRNA) and values for the changes in logarithm of PS (ΔlogPS). These relationships suggest that altered transporter expression is a key determinant of changes in transporter function in different liver diseases. The good correlation between mRNA expression and transporter protein also indicates that the mRNA expression could also be used as an alternate predictor for transporter protein expression, and in turn transporter functions. In summary, hepatic pharmacokinetics studies of different cationic drugs and transporter substrates were performed in different types of diseased rat livers. A relationship between liver histopathological damage and drug hepatic disposition was shown by stepwise regression analysis. The changes in drug transporter function were also shown in the various diseased livers. These changes could be related to alterations in transporter proteins and mRNAs expression in various diseased livers