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The Endogenous and Exogenous Substrate Spectrum of the Human Organic Cation Transporter OCT1 – a Comprehensive Characterization

dc.contributor.advisorBrockmöller, Jürgen Prof. Dr.
dc.contributor.authorJensen, Ole
dc.titleThe Endogenous and Exogenous Substrate Spectrum of the Human Organic Cation Transporter OCT1 – a Comprehensive Characterizationde
dc.contributor.refereeBrockmöller, Jürgen Prof. Dr.
dc.description.abstractengThe organic cation transporter 1 (OCT1) is most extensively expressed in the human liver. OCT1 is involved in the hepatic uptake of several drugs and endogenous compounds, which then undergo bioactivation, recycling, metabolism, or elimination. The aim of this work was to extend the knowledge about OCT1 substrates and non-substrates of endogenous and exogenous origin. Establishing an in vitro model for studying uptake and subsequent metabolism was an additional goal, as well as finding an endogenous biomarker for OCT1 activity. These studies should contribute to our basic understanding of the biological role of this transporter and to the understanding of the role of OCT1 in pharmacology and toxicology. Influx transport of 18 psychostimulant or hallucinogenic compounds, which all meet the conventional physicochemical criteria of OCT1 substrates, was investigated for OCT1 and related transporters. Mescaline was newly identified as a substrate of OCT1. To more systematically and comprehensively search for additional substrates, a machine learning-based model was used. This approach exploited existing knowledge about OCT1 substrates. Machine learning-aided prediction of new substrates was highly reliable as subsequent in vitro validation showed. The in silico prediction, which was based on two-dimensional structures, did not include three-dimensional information, which is important for enantiomers. Therefore, stereoselectivity of OCT1 transport was investigated in vitro. It revealed a surprisingly stereoselective cell uptake for some substrates, e.g. fenoterol, but not all, e.g. salbutamol. For the purpose of in vitro-to-in vivo translation in more complex biological systems, a cell model was developed, which allowed the chromosomal integration of two genes of interest in a targeted manner, to investigate uptake and metabolism in a more holistic fashion. Both genes can be transfected simultaneously and are expressed with equal strength. As a first proof of the value, the sequential uptake of proguanil and subsequent CYP-dependent activation to cycloguanil resembled indeed the uptake and metabolism in primary hepatocytes. Several European and American investigators have already expressed their interest to adopt the system for their research. Thiamine (Vitamin B1) had been proposed as a biomarker for in vivo OCT1 activity. Although thiamine was a substrate of OCT1 in vitro, thiamine was rejected as a suitable biomarker for OCT1 activity through a human clinical trial. After the intake of a large thiamine dose, neither thiamine trough concentrations nor maximum concentrations showed any correlation with individual OCT1 activity according to genotype. Results suggested that other transporters play the central role in thiamine uptake into the liver and other organs. In total, I could add more than 20 previously unknown substrates to the list of well characterized OCT1 substrates. I could show that OCT1 transport can be quite different depending on subtle structural differences between enantiomers. Moreover, the developed cell model may serve as an interesting tool to mimic the complex interplay between hepatic uptake and metabolism, and human pharmacokinetics of thiamine is not dependent on OCT1 - at least not in a quantitatively relevant fashion. Together, these findings may contribute to a gradually improving understanding of OCT1 functionality and biomedical
dc.contributor.coRefereeRizzoli, Silvio Prof. Dr.
dc.subject.engOrganic Cation Transporter 1de
dc.affiliation.instituteMedizinische Fakultät
dc.subject.gokfullMedizin (PPN619874732)de
dc.subject.gokfullMolekularbiologie {Medizin} (PPN619875186)de
dc.subject.gokfullPharmakologie / Toxikologie / Pharmakotherapie - Allgemein- und Gesamtdarstellungen (PPN61987550X)de

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