High Throughput Screening of a Prescription Drug Library for Inhibitors of Organic Cation Transporter 3, OCT3

Several orphan solute carriers functionally identified as organic cation transporters: substrates specificity compared with known cation transporters

Organic cations comprise a significant part of medically relevant drugs and endogenous substances. Such substances need organic cation transporters (OCT) for efficient transfer via cell membranes. However, the membrane transporters of most natural or synthetic organic cations the membrane transporters are still unknown. To identify these transporters, genes of 10 known OCTs and 18 orphan solute carriers (SLC) were overexpressed in HEK293 cells and characterized concerning their transport activities with a broad spectrum of low molecular weight substances emphasizing organic cations. Several SLC35 transporters and SLC38A10 significantly enhanced the transport of numerous relatively hydrophobic organic cations. Significant organic cation transport activities have been found in gene families classified as transporters of other substance classes. For instance, SLC35G3 and SLC38A10 significantly accelerated the uptake of several cations, such as clonidine, 3,4-methylenedioxymethamphetamine, and nicotine, which are known as substrates of a thus far genetically unidentified proton/organic cation antiporter. The transporters SLC35G4 and SLC35F5 stood out by their significantly increased choline uptake, and several other SLC transported choline together with a broader spectrum of organic cations. Overall, there are many more polyspecific organic cation transporters than previously estimated. Several transporters had one predominant substrate but accepted some other cationic substrates, and others showed no particular preference for one substrate but transported several organic cations. The role of these transporters in biology and drug therapy remains to be elucidated.

Proteomic Profiling Reveals Age-Related Changes in Transporter Proteins in the Human Blood-Brain Barrier

The Blood-Brain Barrier (BBB) is a crucial, selective barrier that regulates the entry of molecules including nutrients, environmental toxins, and therapeutic medications into the brain. This function relies heavily on brain endothelial cell proteins, particularly transporters and tight junction proteins. The BBB continues to develop postnatally, adapting its selective barrier function across different developmental phases, and alters with aging and disease. Here we present a global proteomics analysis focused on the ontogeny and aging of proteins in human brain microvessels (BMVs), predominantly composed of brain endothelial cells. Our proteomic profiling quantified 6,223 proteins and revealed possible age-related alteration in BBB permeability due to basement membrane component changes through the early developmental stage and age-dependent changes in transporter expression. Notable changes in expression levels were observed with development and age in nutrient transporters and transporters that play critical roles in drug disposition. This research 1) provides important information on the mechanisms that drive changes in the metabolic content of the brain with age and 2) enables the creation of physiologically based pharmacokinetic models for CNS drug distribution across different life stages.

Cardiac Uptake of the Adrenergic Imaging Agent meta-Iodobenzylguanidine (mIBG) Is Mediated by Organic Cation Transporter 3 (Oct3)

Heart failure (HF) is a chronic disease affecting 1%-2% of the global population.123I-labeled meta-iodobenzylguanidine (mIBG) is US Food and Drug Administration-approved for cardiac imaging and prognosis risk assessment in patients with HF. As a norepinephrine analog, mIBG is believed to be transported into adrenergic nerve terminals by the neuronal norepinephrine transporter (NET) and hence image sympathetic innervation of the myocardium. We previously showed that mIBG is an excellent substrate of organic cation transporter 3 (OCT3), an extraneuronal transporter expressed in cardiomyocytes. Here, we evaluated the in vivo impact of Oct3 on mIBG disposition and tissue distribution using Oct3 knockout mice. Oct3 +/+ and Oct3 -/- mice were administered with mIBG intravenously, and mIBG plasma pharmacokinetics and tissue exposures were determined. In Oct3 +/+ mice, mIBG exhibited extensive accumulation in multiple tissues (heart, salivary gland, liver, and adrenal gland). No difference was observed in overall plasma exposure between Oct3 +/+ and Oct3 -/- mice. Strikingly, cardiac mIBG was depleted in Oct3 -/- mice, resulting in 83% reduction in overall cardiac exposure (AUC0-24 h: 12.7 vs. 2.1 μg × h/g). mIBG tissue exposure (AUC0-24 h) was also reduced by 66%, 36%, and 31% in skeletal muscle, salivary gland, and lung, respectively, in Oct3 -/- mice. Our data demonstrated that Oct3 is the primary transporter responsible for cardiac mIBG uptake in vivo and suggested that cardiac mIBG imaging mainly measures OCT3 activity in cardiomyocytes but not NET-mediated uptake in adrenergic nerve endings. Our findings challenge the current paradigm in interpreting cardiac mIBG imaging results and suggest OCT3 as a potential genetic risk marker for HF prognosis. SIGNIFICANCE STATEMENT: 123I-labeled meta-iodobenzylguanidine is used for cardiac imaging and risk assessment in heart failure patients. Contrary to the current belief that meta-iodobenzylguanidine (mIBG) tracks cardiac sympathetic innervation due to its uptake by the neuronal norepinephrine transporter, the authors demonstrated that cardiac mIBG uptake is mediated by the extraneuronal transporter Oct3. Their findings warrant a re-evaluation of the scientific rationale behind cardiac mIBG scan and further suggest organic cation transporter 3 as a risk factor for disease progression in heart failure patients.

OCTN1 (SLC22A4) displays two different transport pathways for organic cations or zwitterions

BACKGROUND

OCTN1 belongs to the SLC22 family, which includes transporters for cationic, zwitterionic, and anionic substrates. OCTN1 function and role in cells are still poorly understood. Not only cations, such as TEA, but also zwitterions, such as carnitine and ergothioneine, figure among transported molecules.

METHODS

In this work, we carried out transport assays measuring [14C]-TEA and [3H]-Carnitine in proteoliposomes reconstituted with the recombinant human OCTN1 in the presence of Na+ or other cations. The homology model of OCTN1 was built using the structure of OCT3 as a template for docking analysis.

RESULTS

TEA and carnitine did not inhibit each other. Moreover, carnitine uptake was not affected by the presence of Na+ and TEBA, whereas TEA was strongly inhibited by both compounds. Computational data revealed that TEA, Na+, and carnitine can interact with E381 in the OCTN1 substrate site. Differently from TEA, in the presence of Na+, carnitine is still able to interact with the binding site via R469.

CONCLUSIONS

The lack of mutual inhibition of the two prototype substrates, the different effect of Na+ and TEBA on their transport reaction, together with the computational analysis supports the existence of two transport pathways for cations and zwitterions.

GENERAL SIGNIFICANCE

The results shed new light on the transport mechanisms of OCTN1, helping to get further insights into the structure/function relationships. The described results correlate well with previous and very recent findings on the polyspecificity of the OCT group of transporters belonging to the same family.

Organic cation transporters in psychiatric and substance use disorders

Psychiatric and substance use disorders inflict major public health burdens worldwide. Their widespread burden is compounded by a dearth of effective treatments, underscoring a dire need to uncover novel therapeutic targets. In this review, we summarize the literature implicating organic cation transporters (OCTs), including three subtypes of OCTs (OCT1, OCT2, and OCT3) and the plasma membrane monoamine transporter (PMAT), in the neurobiology of psychiatric and substance use disorders with an emphasis on mood and anxiety disorders, alcohol use disorder, and psychostimulant use disorder. OCTs transport monoamines with a low affinity but high capacity, situating them to play a central role in regulating monoamine homeostasis. Preclinical evidence discussed here suggests that OCTs may serve as promising targets for treatment of psychiatric and substance use disorders and encourage future research into their therapeutic potential.

Cardiac contraction and relaxation are regulated by distinct subcellular cAMP pools

Cells interpret a variety of signals through G-protein-coupled receptors (GPCRs) and stimulate the generation of second messengers such as cyclic adenosine monophosphate (cAMP). A long-standing puzzle is deciphering how GPCRs elicit different physiological responses despite generating similar levels of cAMP. We previously showed that some GPCRs generate cAMP from both the plasma membrane and the Golgi apparatus. Here we demonstrate that cardiomyocytes distinguish between subcellular cAMP inputs to elicit different physiological outputs. We show that generating cAMP from the Golgi leads to the regulation of a specific protein kinase A (PKA) target that increases the rate of cardiomyocyte relaxation. In contrast, cAMP generation from the plasma membrane activates a different PKA target that increases contractile force. We further validated the physiological consequences of these observations in intact zebrafish and mice. Thus, we demonstrate that the same GPCR acting through the same second messenger regulates cardiac contraction and relaxation dependent on its subcellular location.

Stereoselectivity in Cell Uptake by SLC22 Organic Cation Transporters 1, 2, and 3

Stereoselectivity can be most relevant in drug metabolism and receptor binding. Although drug membrane transport might be equally important for small-molecule pharmacokinetics, the extent of stereoselectivity in membrane transport is largely unknown. Here, we characterized the stereoselective transport of 18 substrates of SLC22 organic cation transporters (OCTs) 1, 2, and 3. OCT2 and OCT3 showed highly stereoselective cell uptake with several substrates and, interestingly, often with opposite stereoselectivity. In contrast, transport by OCT1 was less stereoselective, although (R)-tamsulosin was transported by OCT1 with higher apparent affinity than the (S)-enantiomer. Using OCT1 and CYP2D6 co-overexpressing cells, an additive effect of the stereoselectivities was demonstrated. This indicates that pharmacokinetic stereoselectivity may be the result of combined effects in transport and metabolism. This study highlights that the pronounced polyspecificity of OCTs not contradicts stereoselectivity in the transport. Nevertheless, stereoselectivity is highly substrate-specific and for most substrates and OCTs, there was no major selectivity.

Stereoselective Inhibition of High- and Low-Affinity Organic Cation Transporters

Many drugs have chiral centers and are therapeutically applied as racemates. Thus, the stereoselectivity in their interactions with membrane transporters needs to be addressed. Here, we studied stereoselectivity in inhibiting organic cation transporters (OCTs) 1, 2, and 3 and the high-affinity monoamine transporters (MATs) NET and SERT. Selectivity by the inhibition of 35 pairs of enantiomers significantly varied among the three closely related OCTs. OCT1 inhibition was nonselective in almost all cases, whereas OCT2 was stereoselectively inhibited by 45% of the analyzed drugs. However, the stereoselectivity of the OCT2 was only moderate with the highest selectivity observed for pramipexole. The (R)-enantiomer inhibited OCT2 4-fold more than the (S)-enantiomer. OCT3 showed the greatest stereoselectivity in its inhibition. (R)-Tolterodine and (S)-zolmitriptan inhibited OCT3 11-fold and 25-fold more than their respective counterparts. Interestingly, in most cases, the pharmacodynamically active enantiomer was also the stronger OCT inhibitor. In addition, stereoselectivity in the OCT inhibition appeared not to depend on the transported substrate. For high-affinity MATs, our data confirmed the stereoselective inhibition of NET and SERT by several antidepressants. However, the stereoselectivity measured here was generally lower than that reported in the literature. Unexpectedly, the high-affinity MATs were not significantly more stereoselectively inhibited than the polyspecific OCTs. Combining our in vitro OCT inhibition data with available stereoselective pharmacokinetic analyses revealed different risks of drug-drug interactions, especially at OCT2. For the tricyclic antidepressant doxepine, only the (E)-isomer showed an increased risk of drug-drug interactions according to guidelines from regulatory authorities for renal transporters. However, most chiral drugs show only minor stereoselectivity in the inhibition of OCTs in vitro, which is unlikely to translate into clinical consequences.

TICBase: Integrated Resource for Data on Drug and Environmental Chemical Interactions with Mammalian Drug Transporters

Environmental health science seeks to predict how environmental toxins, chemical toxicants, and prescription drugs accumulate and interact within the body. Xenobiotic transporters of the ATP-binding cassette (ABC) and solute carrier (SLC) superfamilies are major determinants of the uptake and disposition of xenobiotics across the kingdoms of life. The goal of this study was to integrate drug and environmental chemical interactions of mammalian ABC and SLC proteins in a centralized, integrative database. We built upon an existing publicly accessible platform-the "TransPortal"-which was updated with novel data and searchable features on transporter-interfering chemicals from manually curated literature data. The integrated resource TransPortal-TICBase (https://transportal.compbio.ucsf.edu) now contains information on 46 different mammalian xenobiotic transporters of the ABC- and SLC-type superfamilies, including 13 newly added rodent and 2 additional human drug transporters, 126 clinical drug-drug interactions, and a more than quadrupled expansion of the initial in vitro chemical interaction data from 1,402 to 6,296 total interactions. Based on our updated database, environmental interference with major human and rodent drug transporters occurs across the ABC- and SLC-type superfamilies, with kinetics indicating that some chemicals, such as the ionic liquid 1-hexylpyridinium chloride and the antiseptic chlorhexidine, can act as strong inhibitors with potencies similar or even higher than pharmacological model inhibitors. The new integrated web portal serves as a central repository of current and emerging data for interactions of prescription drugs and environmental chemicals with human drug transporters. This archive has important implications for predicting adverse drug-drug and drug-environmental chemical interactions and can serve as a reference website for the broader scientific community of clinicians and researchers.

The Influence of SLC22A3 Genetic Polymorphisms on Susceptibility to Type 2 Diabetes Mellitus in Chinese Population

Background

Solute carrier family 22 member 3 (SLC22A3) gene had been reported to be associated with the efficacy of metformin in type 2 diabetes mellitus (T2DM). However, few studies reported the relationship between SLC22A3 polymorphism and T2DM. The aim of this study was to investigate the association of SLC22A3 polymorphism and susceptibility to T2DM in Chinese population.

Methods

We identified SLC22A3 rs555754, rs3123636, rs3088442 genotypes of 450 T2DM patients and 220 healthy controls from the Chinese population. The association between SNPs of SLC22A3 and susceptibility of T2DM was evaluated.

Results

The clinical characteristics were significantly different between T2DM patients and healthy controls. The polymorphisms of SLC22A3 rs555754 and rs3123636 were obviously associated with the susceptibility of T2DM which was adjusted for age, sex and BMI, while rs3088442 did not. And there was haplotype association of SLC22A3 rs3088442-rs3123636 with T2DM susceptibility.

Conclusion

SLC22A3 rs555754 and rs3123636 polymorphisms were associated with the susceptibility to T2DM in Chinese Han population. Large sample size studies would be required to verify this association.

Rescue of Misfolded Organic Cation Transporter 3 Variants

Organic cation transporters (OCTs) are membrane proteins that take up monoamines, cationic drugs and xenobiotics. We previously reported novel missense mutations of organic cation transporter 3 (OCT3, SLC22A3), some with drastically impacted transport capabilities compared to wildtype. For some variants, this was due to ER retention and subsequent degradation of the misfolded transporter. For other transporter families, it was previously shown that treatment of misfolded variants with pharmacological and chemical chaperones could restore transport function to a certain degree. To investigate two potentially ER-bound, misfolded variants (D340G and R348W), we employed confocal and biochemical analyses. In addition, radiotracer uptake assays were conducted to assess whether pre-treatment with chaperones could restore transporter function. We show that pre-treatment of cells with the chemical chaperone 4-PBA (4-phenyl butyric acid) leads to increased membrane expression of misfolded variants and is associated with increased transport capacity of D340G (8-fold) and R348W (1.5 times) compared to untreated variants. We herein present proof of principle that folding-deficient SLC22 transporter variants, in particular those of OCT3, are amenable to rescue by chaperones. These findings need to be extended to other SLC22 members with corroborated disease associations.

Stereoselectivity in the Membrane Transport of Phenylethylamine Derivatives by Human Monoamine Transporters and Organic Cation Transporters 1, 2, and 3

Stereoselectivity is well known and very pronounced in drug metabolism and receptor binding. However, much less is known about stereoselectivity in drug membrane transport. Here, we characterized the stereoselective cell uptake of chiral phenylethylamine derivatives by human monoamine transporters (NET, DAT, and SERT) and organic cation transporters (OCT1, OCT2, and OCT3). Stereoselectivity differed extensively between closely related transporters. High-affinity monoamine transporters (MATs) showed up to 2.4-fold stereoselective uptake of norepinephrine and epinephrine as well as of numerous analogs. While NET and DAT preferentially transported (S)-norepinephrine, SERT preferred the (R)-enantiomer. In contrast, NET and DAT showed higher transport for (R)-epinephrine and SERT for (S)-epinephrine. Generally, MAT stereoselectivity was lower than expected from their high affinity to several catecholamines and from the high stereoselectivity of some inhibitors used as antidepressants. Additionally, the OCTs differed strongly in their stereoselectivity. While OCT1 showed almost no stereoselective uptake, OCT2 was characterized by a roughly 2-fold preference for most (R)-enantiomers of the phenylethylamines. In contrast, OCT3 transported norphenylephrine and phenylephrine with 3.9-fold and 3.3-fold preference for their (R)-enantiomers, respectively, while the para-hydroxylated octopamine and synephrine showed no stereoselective OCT3 transport. Altogether, our data demonstrate that stereoselectivity is highly transporter-to-substrate specific and highly diverse even between homologous transporters.

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

Organic cation transporters (OCTs) 1, 2, and 3 facilitate cellular uptake of structurally diverse endogenous and exogenous substances. However, their substrate and inhibitor specificity are not fully understood. We performed a broad in vitro screening for OCT3 substrates and inhibitors, allowing us to compare the substrate spectra and to study the relationship between transport and inhibition of transport. Generally, substrates were smaller and more hydrophilic than OCT3 inhibitors. The best model-based predictor of transport was the positive charge, while the best predictor of inhibition was the aromatic ring count. OCT3 inhibition was well correlated between different model substrates. Substrates of OCT3 were mainly weak inhibitors, and the best inhibitors were not substrates. As tested with 264 substances, OCT3 transport had significantly more overlap with OCT2 than OCT1. Our data further substantiate that specificity of OCT transport varies with minor substitutions rather than with the general scaffolds of substrates.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Increased amisulpride serum concentration in a patient treated with concomitant pregabalin and trazodone: a case report

We report on the case of a 46-year-old woman with generalized anxiety disorder, paranoid personality disorder, and mild reduction in glomerular filtration rate (GFR). She was treated with pregabalin, trazodone, hydroxyzine, and clonazepam before hospital admission. Pharmacotherapy for the patient was changed during her first week in the hospital. Dosing of hydroxyzine and clonazepam was gradually decreased, and then these two drugs were withdrawn. Treatment with amisulpride was started on the fourth day after admission, and amisulpride serum levels were then measured three times as a part of therapeutic drug monitoring (TDM). The serum concentration of amisulpride detected during concurrent use of trazodone and pregabalin was approximately twice the therapeutic range for amisulpride. When the dose of pregabalin was reduced by half, the serum concentration of amisulpride decreased to therapeutic serum levels. We hypothesize that an interaction between amisulpride and pregabalin was responsible for the increased amisulpride concentration since both drugs are almost exclusively excreted from the body by the renal route. Pregabalin-amisulpride interaction might also be influenced by concomitant therapy with trazodone or a mild reduction in GFR. However, we only have clinical evidence for an interaction between amisulpride and pregabalin because after we halved the dose of pregabalin, the amisulpride concentration decreased, and the C/D ratio normalized. This could be helpful information for psychiatrists in order to avoid drug-drug interactions between amisulpride and pregabalin. We recommend TDM of amisulpride in patients treated concomitantly with other drugs eliminated mainly by the kidneys.