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antidepressants, and antacids. In addition, and distinct from its paralogs, OCT3 transports a number of hormones, including progesterone and testosterone. Model substrates for OCT3 include the cation MPP⁺, the fluorescent substrate ASP⁺, and the anti‐diabetic drug metformin (Table 2.2). Corticosterone serves as a model inhibitor for OCT3. Because the transporter is expressed at lower levels in comparison to its paralogs in organs of drug absorption (e.g., the intestine) and elimination (liver and kidney), OCT3 is not generally considered a target for DDIs.

      2.5.2 Regulation

      OCT3 is regulated by various transcription factors including specificity protein 1 (Sp1), myeloid zinc finger 1 (MZ1), p300, and Ap4. These transcription factors bind to DNA response elements in the basal promoter of OCT3 [2]. In addition, methylation of the basal promoter of OCT3, which is tissue specific, suggests that regulation of the transporter is epigenetic in some tissues. In addition, Sp1 and USF1 bind to response elements in the upstream promoter, modulating the expression of OCT3 [2].

      2.5.3 Animal Models

      Considerable information about the physiologic role of OCT3 has been obtained in various elegant studies in Oct3 knockout mice. As with Oct1 and Oct2 knockout mice, Oct3 ^−/− mouse are viable, fertile, have a normal lifespan, and show no obvious physiological defect. In mice, deletion of Oct3 results in substantially reduced levels of MPP⁺ in the heart, embryo, and salivary glands. However, no significant differences in the endogenous levels of two OCT3 substrates, norepinephrine and dopamine, were observed in the embryo or placenta of wild‐type versus Oct3 ^−/− mice [46].

      Though OCT3 is expressed in the liver, no obvious liver phenotype has been observed between Oct3 ^−/− and wild‐type mice on normal chow. However, after induction by diethylnitrosamine and phenobarbital [47], Oct3 ^−/− mice exhibit increased tumorigenesis in the liver, suggesting the transporter may play a role in hepatocellular carcinoma.

      As an excellent transporter for histamine, OCT3 appears to play a role in determining histamine levels and associated immunologic phenotypes. For example, a significant increase in histamine content in the spleen has been observed in Oct3 ^−/− mice compared with wild‐type mice. Consistent with these observations, in an endotoxemia model involving LPS injection (20 mg/kg) into the peritoneal cavity, Oct3 ^−/− mice exhibit a significant decrease in survival rate compared with wild‐type mice. These studies suggest that OCT3 plays a role in immunological response in endotoxemia through its role as a histamine transporter.

      Because of its expression in the central nervous system, OCT3 has been investigated for effects on behavior, particularly regarding locomotion and anxiety. Oct3 ^−/− mice have a decreased dopamine concentration in several brain regions (i.e., substantia nigra, tegmental area) [48]. Several behavioral tests reveal higher scores for anxiety compared with wild‐type mice [49]. Further high doses of amphetamine or cocaine increase locomotor activity in Oct3 ^−/− mice, but not in wild‐type mice. The mechanism for its effects on behavior may be that the transporter takes up biologically active amines and without it, the amines are present at higher levels, causing effects on locomotion and other CNS phenotypes.

      With respect to the exogenous drug metformin, Oct3 ^−/− mice exhibit substantial decreases in oral bioavailability compared with wild‐type mice. Tissue‐to‐plasma ratios are significantly lower in the liver, kidney, adipose tissue, and skeletal muscle in Oct3 ^−/− mice, suggesting OCT3 plays an important role in tissue distribution of metformin [2]. However, blood glucose levels in Oct3 ^−/− mice do not significantly change after metformin administration, indicating the altered pharmacokinetic profiles may not translate into an effect on pharmacodynamic response.

      2.5.4 Human Genetic Studies

      Consistent with its expression in the prostate and intestine, single nucleotide variants in hOCT3 have been linked in genome‐wide association studies (GWAS) to prostate cancer and colorectal cancer [50, 51]. In an Asian population, a single nucleotide variant (SNV) within intron 5 of SLC22A3 is associated with prevalence of distal colon cancer [2, 51].

      Missense variants in OCT3 are rare. For example, nine nonsynonymous SNPs have been identified and are rare in most investigated populations, with the exception of p.T44M, which has a minor allele frequency (MAF) of 0.017 in African Americans [2]. Variants p.M370I, p.T400I, and p.V423F have functional consequences for OCT3 activity. Specifically, p.M370I reduces norepinephrine uptake by 40% and p.T400I and p.V423F reduce OCT3 V _max for metformin uptake [2, 52]. The impact of these nonsynonymous SNPs on the pharmacokinetics and/or pharmacodynamics of metformin have not been evaluated.

In addition to the missense variants, a number of variants have been identified in the promoter region of OCT3. In particular, eight SNPs with MAFs between 0.01 and 0.29 have been detected in the noncoding promoter region of hOCT3 [2]. Notably, one SNP in the proximal promoter, −2G>A has functional consequences, resulting in decreased transcription of hOCT3 compared with the −2A>G ancestral allele. One SNP (rs2076828, C>G) has been associated in candidate gene studies with reduced metformin response in healthy individuals [2].

2.6 OTHER IMPORTANT CATION TRANSPORTERS: SLC19A2, SLC19A3 (THIAMINE TRANSPORTERS), SLC29A4 (MONOAMINE TRANSPORTER)

      2.6.1 Introduction

      SLC19A2, SLC19A3 and SLC29A4 encode for the transporters THTR‐1, THTR‐2, and PMAT, respectively, and serve as additional important cation transporters while not belonging to the SLC22 family. The main substrate of THTR‐1 and THTR‐2 is thiamine, also known as water‐soluble vitamin B1. Thiamine, a cation at physiological pH, is an essential vitamin, which catalyzes many important reactions involved in cellular metabolism. THTR‐1 and THTR‐2 are high‐affinity, facilitated (energy‐independent) transporters that distribute thiamine and other cationic substrates into the systemic circulation and other tissues in accordance with the substrate’s concentration gradient [3]. SLC19A2 was first cloned in 1999 and encodes for the 497 amino acid THTR‐1 protein with a predicted molecular weight of 55.4 kDa. SLC19A3, cloned a year later, encodes for a 496 amino acid THTR‐2 protein with a predicted molecular weight of 55.6 kDa [53]. The two transporters share 48% sequence identity. Like transporters in the SLC22 family, THTR‐1 and THTR‐2 consist of 12 transmembrane domains and extracellular and intracellular loops [53]. Though crystal structures of a prokaryotic thiamine transporter that binds and transports thiamine are available, neither THTR‐1 nor THTR‐2 have been crystallized [54].

      Plasma membrane monoamine transporter (PMAT) is part of the SLC29 family, which is considered to be a major transporter family for nucleosides and nucleoside analogs. Interestingly, PMAT (SLC29A4) was discovered to transport a variety of organic cations and key neurotransmitters in a low‐affinity, high‐capacity manner. Consisting of 530 amino acids, PMAT has a predicted molecular weight of 58 kDa. PMAT does not share significant homology with known neurotransmitter transporters but exhibits minimal homology (~20%) to other members of the SLC29 (ENT) family [4]. There are no x‐ray crystallography or cryo‐electron microscopy structures of human PMAT, but structures exist for human equilibrative nucleoside transporter 1 (hENT1).

      2.6.2 Tissue Distribution in Human

      Though highly expressed in various tissues such as adipose and breast tissue, THTR‐2 is the major absorptive transporter for thiamine in the intestine [3]. It is located on the apical membrane of intestinal epithelia and renal proximal tubule cells, where it plays a role in renal reabsorption of thiamine. In contrast, while exhibiting low ubiquitous expression across a variety of tissues, THTR‐1 is perhaps most physiologically relevant in the bone marrow, ear, and pancreas/liver, as suggested by the manifestation of megaloblastic anemia, diabetes, and sensorineural

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