Overlap and Specificity in the Substrate Spectra of Human Monoamine Transporters and Organic Cation Transporters 1, 2, and 3

Abstract

Human monoamine transporters (MATs) are cation transporters critically involved in neuronal signal transmission. While inhibitors of MATs have been intensively studied, their substrate spectra have received far less attention. Polyspecific organic cation transporters (OCTs), predominantly known for their role in hepatic and renal drug elimination, are also expressed in the central nervous system and might modulate monoaminergic signaling. Using HEK293 cells overexpressing MATs or OCTs, we compared uptake of 48 compounds, mainly phenethylamine and tryptamine derivatives including matched molecular pairs, across noradrenaline, dopamine and serotonin transporters and OCTs (1, 2, and 3). Generally, MATs showed surprisingly high transport activities for numerous analogs of neurotransmitters, but their substrate spectra were limited by molar mass. Human OCT2 showed the broadest substrate spectrum, and also the highest overlap with MATs substrates. Comparative kinetic analyses revealed that the radiotracer meta-iodobenzylguanidine had the most balanced uptake across all six transporters. Matched molecular pair analyses comparing MAT and OCT uptake using the same methodology could provide a better understanding of structural determinants for high cell uptake by MATs or OCTs. The data may result in a better understanding of pharmacokinetics and toxicokinetics of small molecular organic cations and, possibly, in the development of more specific radiotracers for MATs.

Relationships between Inhibition, Transport and Enhanced Transport via the Organic Cation Transporter 1

“…As they will, for sure interact with OCT1, known OCT1 substrates are interesting screening candidates to find more complex drug-transporter interactions. As OCT1 is an efficient serotonin transporter and OCT2 is an efficient serotonin and dopamine transporter [23,29], potential complex interactions of other OCT substrates with serotonin and dopamine could prove valuable for pharmacological interventions modulating peripheral blood serotonin concentrations.…”

“…This resulted in a total of 326 compounds (including published data), for which uptake ratios were determined in a uniform manner (Figure S2). When taking a conservative ratio of 3 as a cutoff for the differentiation between substrate and non-substrate as applied in previous publications [16,23], the overall database now included 84 substrates (most already known as substrates of OCT1). When using the less conservative criterion of a twofold increase by transporter-mediated uptake, which is suggested by the FDA, that number was 107.…”

“…For known OCT1 substrates, for which no uptake ratios were available to us, we performed in vitro transport assays at a concentration of 2.5 µM, as established and applied previously [10,23]. This concentration was considered a trade-off between linearity in sub-K m concentrations and detectability in mass spectrometric analysis, and it had been used in larger comparable screenings before [10].…”

Substrates and Inhibitors of the Organic Cation Transporter 3 and Comparison with OCT1 and OCT2

“…Especially with the substrates, we could also demonstrate that this overlap has limits. Interestingly, while OCT3 shares some endogenous substrates with OCT2 that are comparably well transported, such as histamine, many other monoamine neurotransmitters or trace amines, such as dopamine, serotonin, and deoxyepinephrine, show a strong preference for OCT2 . Given its discussed role in the blood–brain barrier, e.g., strong OCT3 transport of dopamine would run afoul of dopamine’s limited capability of crossing the blood–brain barrier anyway.…”

Physiology, Biochemistry and Pharmacology of Transporters for Organic Cations 2.0

“…Using HEK cells overexpressing human monoamine transporters (MATs) or OCTs, in the paper “Overlap and Specificity in the Substrate Spectra of Human Monoamine Transporters and Organic Cation Transporters 1, 2, and 3”, Gebauer et al [ 12 ] compared the uptake of 48 compounds, mainly phenethylamine and tryptamine derivatives, including matched molecular pairs, across noradrenaline, dopamine, and serotonin transporters and OCTs (1–3). They found that MATs showed high transport activities for numerous analogues of neurotransmitters, but their substrate spectra were limited by molar mass.…”