Stereoselectivity in Drug Membrane Transport by Solute Carriers (SLCs)Cumulative thesis
Date of Examination:2023-06-30
Date of issue:2023-07-10
Advisor:Prof. Dr. Jürgen Brockmöller
Referee:Prof. Dr. Jürgen Brockmöller
Referee:Prof. Dr. Christian Griesinger
Sponsor:Deutsche Forschungsgemeinschaft (DFG - project number 437446827)
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EnglishAbout 50% of all small molecule drugs are racemic mixtures of usually two stereoisomers. In the living organism, these drugs interact with a mostly chiral environment of unichiral amino acids, sugars, hormones, and other biologically relevant molecules. Chiral drugs might interact highly stereospecific with other chiral (macro-) molecules. This was shown to be relevant for many ligand-receptors interactions and numerous drug metabolism reactions. For drug membrane transporters, the impact of stereochemistry on substrate translocation has received little attention. This thesis aimed to characterize stereoselectivity in two families of cation transporters. In general, transporters are membrane proteins which facilitate the movement of molecules across cellular membranes which cannot cross those easily by simple diffusion. Organic cation transporters (OCTs) are polyspecific membrane transporters which are likely to be involved in handling numerous positively charged drugs and endogenous substances. In comparison, monoamine transporters (MATs) have a well assigned physiological function by mediating the reuptake of released monoamine neurotransmitters. As a result of my studies, stereoselectivity in membrane transporters was relatively low in most of the studied substrate-transporter pairs. Nevertheless, several OCT substrates were transported highly stereoselectively. For instance, the enantiomers of clidinium, mepenzolate, terbutaline, and zolmitriptan were transported with high stereoselectivity by OCT2 and OCT3. In contrast, OCT1 rarely mediated stereoselective transport. Interestingly, the high-affinity MATs showed only minor stereoselectivity as well. This lack of stereoselectivity was unexpected considering the high substrate selectivity of MATs compared to the polyspecific OCTs. Beyond stereoselectivity, transporter specificity of OCT substrates depended more on minor substitutions than on the general scaffold of the substrate. In transporter inhibition, the extent of stereoselectivity was also only moderate with most inhibitors. The pattern of stereoselectivity in inhibition was identical to transport. OCT1 showed negligible selectivity, whereas OCT2 was inhibited with at least moderate stereoselectivity by 30% of chiral inhibitors. OCT3 showed remarkably high stereoselectivity in its inhibition by tolterodine and zolmitriptan. However, these were exceptions and the enantiomers of most chiral inhibitors had highly similar inhibitory potencies. The parallelism in substrate and inhibitor specificity might indicate that most inhibitors were competitive inhibitors binding to similar protein domains as the substrates. Stereoselectivity in pharmacokinetics might become even more pronounced if both, drug transporters and metabolizing enzymes, act selectively. Analyzing combined action of OCT1 and CYP2D6 revealed only a small overlap of the substrate specificity of both proteins. Nevertheless, the cellular disposition of shared substrates was largely depending on the action of OCT1 and CYP2D6. Moreover, a combined stereoselectivity of OCT1 and CYP2D6 was shown exemplarily for shared chiral substrates. Altogether, I could show that stereoselectivity, if existent at all, is a highly substrate and transporter-specific property. Several examples in my thesis projects showed that minor structural alterations of the MAT and OCT substrates resulted in a large change or even opposite stereoselectivity. Conversely, selectivity also varied substantially between the closely related (highly homologous) transporters. Since stereochemistry had only moderate effects on OCTs, for most drugs, clinical consequences are unlikely. Biochemically, in combination with improved structural data on OCTs, stereoselective transport analyses might assist in understanding the mechanisms underlying substrate binding and translocation.
Keywords: Chirality; Drug membrane transport; Monoamine transporters; Organic cation transporters; Pharmacology; Stereoselectivity