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

Abstract

The organic cation transporter 1 (OCT1, SLC22A1) transports a large number of structurally diverse endogenous and exogenous substrates. There are numerous known competitive and non-competitive inhibitors of OCT1, but there are no studies systematically analyzing the relationship between transport, stimulation, and inhibition. Here, we tested in vitro OCT1 inhibition by OCT1 substrates and transport of OCT1 inhibitors under uniform analytical conditions. Beyond inhibition testing with two model substrates, we tested nine additional OCT1 substrates for their mutual inhibition. Inhibition of ASP+ uptake by most OCT1 substrates was weak. The model substrate sumatriptan, with its moderately stronger inhibitability, was used to confirm this. Interestingly, OCT1 substrates exhibiting stronger OCT1 inhibition were mainly biaromatic β-agonistic drugs, such as dobutamine, fenoterol, ractopamine and ritodrine. Biaromatic organic cations were both, strong inhibitors and good substrates, but many OCT1 substrates showed little pairwise inhibition. Surprisingly, sumatriptan did significantly enhance dobutamine uptake. This effect was concentration dependent and additional experiments indicated that efflux inhibition may be one of the underlying mechanisms. Our data suggests, that OCT1 substrates are mainly weak OCT1 inhibitors and among those inhibiting well, noncompetitive inhibition could be responsible. Weak competitive inhibition confirms that OCT1 inhibition screenings poorly predict OCT1 substrates. Additionally, we showed that the OCT1 substrate sumatriptan can enhance uptake of some other OCT1 substrates. OCT1 transport stimulation was already observed earlier but is still poorly understood. Low OCT1 uptake inhibition and strong OCT1 efflux inhibition could be mechanisms exploitable for enhancing transport.

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

“…To extend our analyses on the selectivity of OCTs toward OCT1, we included averaged published data on OCT1 transport, and we also calculated averaged OCT2 and OCT3 data where additional published data were available. ,,,, In general, substrates of OCT1 had a higher molecular weight (MW) and a larger topological polar surface area (TPSA) (Table for a general overview comparing chemical descriptors between substrates of the OCTs, Figure S2 for histograms showing the distribution of TPSA and molecular weight for substrates of OCT1, OCT2, and OCT3).…”

“…But large-scale comparisons with different model substrates exist for OCT2 only. , Moreover, transporter substrates and inhibitors have been mainly characterized separately and rarely compared in the same study. Recently, we have shown that OCT1 substrates are mostly weak OCT1 inhibitors, but the relationship between substrate and inhibitor profiles has not been studied for the other OCTs so far. Lastly and most importantly, to our knowledge, there are no larger-scale substrate screenings for OCT3, limiting the comparison of substrate spectra across all three OCTs to heterogeneous data from many sources.…”

Substrates of the Human Brain Proton-Organic Cation Antiporter and Comparison with Organic Cation Transporter 1 Activities

“…Intracellular concentrations of probe substrates and substrates-in-question were performed using HPLC-MS/MS analyses as essentially as described earlier [ 1 , 2 , 16 ]. Briefly, the cell lysate was centrifuged at 13,000 rpm in a tabletop centrifuge for 15 min to pellet potential cell debris.…”

“…More lipophilic substances may not only pass membranes by nonionic diffusion but also with these substances, influx and efflux transport may be significantly accelerated by membrane transporters. For several organic cations, organic cation transporters such as OCT1, OCT2, OCT3, OCTN1, OCTN2, THTR1, THTR2, MATE-1, MATE-2K and several others have been identified as relevant transporters [ 1 , 2 , 3 ]. However, these known transporters do not contribute in a relevant manner to the membrane transport of many cationic drugs.…”

“…In addition, the H + OC antiporter is characterized by typical inhibitors such as imipramine, diphenhydramine, clonidine or memantine [ 17 ], but to our best knowledge, there is no inhibitor specific for the H + OC antiporter. Many of these inhibitors may also inhibit, for instance, OCT1 [ 2 ] and inhibitors such as verapamil or quinidine are also very pleiotropic and, therefore, less useful for transporter specification. A third line of evidence comes from disturbing the pH gradient by protonophores such as carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP) [ 18 ].…”