Understanding the transport mechanisms of BSEP to improve the prediction of DILI compounds
Creators: name: De Kruijf, Robbin
Publisher: name: Apollo - University of Cambridge Repository
Year: 2020
Subjects: bile salt export pump
BSEP
Drug induced liveer injury
DILI
Transporter
Bile acids
primary human hepatocyte
toxicity
liver

description: At present, Drug-Induced Liver Injury (DILI) is the most frequent cause of acute liver injury, with an incidence ranging from 0.023-1.5‰ in developed countries. DILI can be separated into two different types. Whereas intrinsic DILI concerns predictable dose-dependent hepatoxicity, idiosyncratic DILI refers to unpredictable adverse drug reactions for which there is no existing pharmacological explanation. Many prescription and over-the-counter drugs causing DILI do so in an undiscovered and idiosyncratic pattern. It is the latter that causes DILI to be the most frequent cause of safety-related drug withdrawals. This causes delays in the availability of treatments and is responsible for 20% to 40% of liver transplants and hepatic failures. While several mechanisms might be responsible for idiosyncratic DILI, a broad industry-wide consensus was reached in 2016 on the importance of the liver bile salt export pump (BSEP) and the need to test for BSEP inhibition at an early stage of drug development. An assay for BSEP-mediated bile acid transport was established to study the biochemical mechanisms of inhibition of BSEP activity by organic compounds. The four selected compounds showed competitive inhibition of taurocholate transport in Lineweaver-Burk plots but also other types of inhibition in Eadie-Hofstee plots. These results indicate that these inhibitors might compete with taurocholate for binding in the substrate-binding pocket and possibly, that the inhibitors are transported by BSEP. Interestingly, this work shows a strong inhibitory effect of excess Mg2+ on the ability of BSEP to transport glycocholic acid; the transport of taurocholic acid was not inhibited by Mg2+. To get more insight in structure-function relationships in BSEP, a condense and accurate database of all known BSEP point mutations from published literature was compiled. This database shows a total of 264 known mutations associated with several pathogenic and non-pathogenic phenotypes. To date, there is still little known about the effects of many of these mutations on BSEP activity. Only 82 of the 264 mutations have currently been investigated for their impact on function and expression level of BSEP. 2D and 3D primary cultured hepatocytes were used to establish the effect of DILI-inducing drugs on the synthesis of bile acids and their distribution across the cell and the media. Multivariate analyses of the data demonstrate that drugs can change the bile acid composition in hepatocytes and their environment. This might occur by regulating bile acid synthesis pathways and/or by modulating the selectivity of BSEP itself. Interestingly, some of the detected changes in bile acid concentrations might be early indicators of cell stress and cell death. The experiments also suggest that drugs can cause a change in BSEP expression in the plasma membrane, through changes in the trafficking of protein to and from the plasma membrane. These changes could explain the existence of non-pathogenic BSEP inhibitors. These findings increase our understanding of the influence of compounds on bile acid production and BSEP distribution and their potential roles in idiosyncratic DILI.
description: MRC ITTP and AstraZeneca