Pharmacogenetics of OATP (SLC21/SLCO), OAT and OCT (SLC22) and PEPT (SLC15) transporters in the intestine, liver and kidney

The role of carrier-mediated transport in determining the pharmacokinetics of drugs has become increasingly evident with the discovery of genetic variants that affect expression and/or function of a given drug transporter. Drug transporters are expressed at numerous epithelial barriers, such as intestinal epithelial cells, hepatocytes, renal tubular cells and at the blood-brain barrier. Several recent studies have associated alterations in substrate uptake with the presence of SNPs. Here, we summarize the current knowledge on the functional and phenotypic consequences of genetic variation in intestinally, hepatically and renally expressed members of the organic anion-transporting polypeptide family (OATPs; SLC21/SLCO family), the organic anion and organic cation transporters (OATs/OCTs; SLC22 family) and the peptide transporter-1 (PEPT1; SLC15 family).     

Unraveling ‘The Cancer Genome Atlas’ information on the role of SLC transporters in anticancer drug uptake

Introduction: Anticancer chemotherapy often faces the problem of intrinsic or acquired drug refractoriness due in part to efficient mechanisms of defense present or developed, respectively, in cancer cells. Owing to their polarity and/or high molecular weight, many cytostatic agents cannot freely cross the plasma membrane by simple diffusion and hence depend on SLC proteins to enter cancer cells. The downregulation of these transporters and the appearance of either inactivating mutations or aberrant splicing, hamper the possibility of anticancer drugs to interact with their intracellular targets.

Areas covered: In addition to specific literature, we have revised Gene database of the NCBI PubMed resources and information publicly available at NIH ‘The Cancer Genome Atlas’ (TCGA) (update November 2018) to evaluate the relationship between the profile of expression of SLC transporters playing a major role in the transportome and accounting for drug uptake, in healthy and tumor tissue, and their ability to recognize as substrate several antitumor drugs frequently used in the treatment of different types of cancer, which could affect the overall response to chemotherapy based on regimens including these drugs.

Expert commentary: Changes in the transportome may affect the overall response to chemotherapy based on drugs taken up by SLC transporters.     

Potential for food-drug interactions by dietary phenolic acids on human organic anion transporters 1 (SLC22A6), 3 (SLC22A8), and 4 (SLC22A11)

Phenolic acids exert beneficial health effects such as anti-oxidant, anti-carcinogenic, and anti-inflammatory activities and show systemic exposure after consumption of common fruits, vegetables, and beverages. However, knowledge regarding which components convey therapeutic benefits and the mechanism(s) by which they cross cell membranes is extremely limited. Therefore, we determined the inhibitory effects of nine food-derived phenolic acids, p-coumaric acid, ferulic acid, gallic acid, gentisic acid, 4-hydroxybenzoic acid, protocatechuic acid, sinapinic acid, syringic acid, and vanillic acid, on human organic anion transporter 1 (hOAT1), hOAT3, and hOAT4. In the present study, inhibition of OAT-mediated transport of prototypical substrates (1 μM) by phenolic acids (100 μM) was examined in stably expressing cell lines. All compounds significantly inhibited hOAT3 transport, while just ferulic, gallic, protocatechuic, sinapinic, and vanillic acid significantly blocked hOAT1 activity. Only sinapinic acid inhibited hOAT4 (～35%). For compounds exhibiting inhibition > ～60%, known clinical plasma concentration levels and plasma protein binding in humans were examined to select compounds to evaluate further with dose–response curves (IC₅₀ values) and drug–drug interaction (DDI) index determinations. IC₅₀ values ranged from 1.24 to 18.08 μM for hOAT1 and from 7.35 to 87.36 μM for hOAT3. Maximum DDI indices for gallic and gentisic acid (?0.1) indicated a very strong potential for DDIs on hOAT1 and/or hOAT3. This study indicates that gallic acid from foods or supplements, or gentisic acid from salicylate-based drug metabolism, may significantly alter the pharmacokinetics (efficacy and toxicity) of concomitant therapeutics that are hOAT1 and/or hOAT3 substrates.     

Interactions of organophosphorus pesticides with solute carrier (SLC) drug transporters

1.?Organophosphorus pesticides (OPs) are known to interact with human ATP-binding cassette drug efflux pumps. The present study was designed to determine whether they can also target activities of human solute carrier (SLC) drug transporters.

2.?The interactions of 13 OPs with SLC transporters involved in drug disposition, such as organic cation transporters (OCTs), multidrug and toxin extrusion proteins (MATEs), organic anion transporters (OATs) and organic anion transporting polypeptides (OATPs), were mainly investigated using transporter-overexpressing cell clones and fluorescent or radiolabeled reference substrates.

3.?With a cut-off value of at least 50% modulation of transporter activity by 100?µM OPs, OAT1 and MATE2-K were not impacted, whereas OATP1B1 and MATE1 were inhibited by two and three OPs, respectively. OAT3 activity was similarly blocked by three OPs, and was additionally stimulated by one OP. Five OPs cis-stimulated OATP2B1 activity. Both OCT1 and OCT2 were inhibited by the same eight OPs, including fenamiphos and phosmet, with IC₅₀ values however in the 3–30?µM range, likely not relevant to environmental exposure.

4.?These data demonstrated that various OPs inhibit SLC drug transporter activities, especially those of OCT1 and OCT2, but only when used at high concentrations not expected to occur in environmentally-exposed humans.     

Pharmacotherapy in pregnancy; effect of ABC and SLC transporters on drug transport across the placenta and fetal drug exposure

Pharmacotherapy during pregnancy is often inevitable for medical treatment of the mother, the fetus or both. The knowledge of drug transport across placenta is, therefore, an important topic to bear in mind when deciding treatment in pregnant women. Several drug transporters of the ABC and SLC families have been discovered in the placenta, such as P-glycoprotein, breast cancer resistance protein, or organic anion/cation transporters. It is thus evident that the passage of drugs across the placenta can no longer be predicted simply on the basis of their physical-chemical properties. Functional expression of placental drug transporters in the trophoblast and the possibility of drug–drug interactions must be considered to optimize pharmacotherapy during pregnancy. In this review we summarize current knowledge on the expression and function of ABC and SLC transporters in the trophoblast. Furthermore, we put this data into context with medical conditions that require maternal and/or fetal treatment during pregnancy, such as gestational diabetes, HIV infection, fetal arrhythmias and epilepsy. Proper understanding of the role of placental transporters should be of great interest not only to clinicians but also to pharmaceutical industry for future drug design and development to control the degree of fetal exposure.     

Targeting uptake transporters for cancer imaging and treatment

Cancer cells reprogram their gene expression to promote growth, survival, proliferation, and invasiveness. The unique expression of certain uptake transporters in cancers and their innate function to concentrate small molecular substrates in cells make them ideal targets for selective delivering imaging and therapeutic agents into cancer cells. In this review, we focus on several solute carrier (SLC) transporters known to be involved in transporting clinically used radiopharmaceutical agents into cancer cells, including the sodium/iodine symporter (NIS), norepinephrine transporter (NET), glucose transporter 1 (GLUT1), and monocarboxylate transporters (MCTs). The molecular and functional characteristics of these transporters are reviewed with special emphasis on their specific expressions in cancers and interaction with imaging or theranostic agents [e.g., I-123, I-131, ¹²³I-iobenguane (mIBG), ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) and ¹³C pyruvate]. Current clinical applications and research areas of these transporters in cancer diagnosis and treatment are discussed. Finally, we offer our views on emerging opportunities and challenges in targeting transporters for cancer imaging and treatment. By analyzing the few clinically successful examples, we hope much interest can be garnered in cancer research towards uptake transporters and their potential applications in cancer diagnosis and treatment.     

Pathogenetics of the human SLC26 transporters

Over the past decade, 11 human genes belonging to the solute linked carrier (SLC) 26 family of transporters, have been identified. The SLC26 proteins, which include SAT-1, DTDST, DRA/CLD, pendrin, prestin, PAT-1/CFEX and Tat-1, are structurally related and have been shown to transport one or more of the following substrates: sulfate, chloride, bicarbonate, iodide, oxalate, formate, hydroxyl or fructose. Special interest has focused on four members of the SLC26 family that are associated with distinct recessive diseases: (i) Mutations in SLC26A2 lead to four different chondrodysplasias (diastrophic dysplasia, atelosteogenesis type II, achondrogenesis type IB and multiple epiphyseal dysplasia); (ii) SLC26A3 is associated with congenital chloride diarrhea; (iii) SLC26A4 is associated with Pendred syndrome and non-syndromic deafness, DFNB4; and (iv) SLC26A5 is defective in non-syndromic hearing impairment. This review article summarizes current information on the pathophysiological consequences of mutations in the human SLC26A2 to A5 genes.     

The SLC36 family of proton-coupled amino acid transporters and their potential role in drug transport

Members of the solute carrier (SLC) 36 family are involved in transmembrane movement of amino acids and derivatives. SLC36 consists of four members. SLC36A1 and SLC36A2 both function as H(+) -coupled amino acid symporters. SLC36A1 is expressed at the luminal surface of the small intestine but is also commonly found in lysosomes in many cell types (including neurones), suggesting that it is a multipurpose carrier with distinct roles in different cells including absorption in the small intestine and as an efflux pathway following intralysosomal protein breakdown. SLC36A1 has a relatively low affinity (K(m) 1-10 mM) for its substrates, which include zwitterionic amino and imino acids, heterocyclic amino acids and amino acid-based drugs and derivatives used experimentally and/or clinically to treat epilepsy, schizophrenia, bacterial infections, hyperglycaemia and cancer. SLC36A2 is expressed at the apical surface of the human renal proximal tubule where it functions in the reabsorption of glycine, proline and hydroxyproline. SLC36A2 also transports amino acid derivatives but has a narrower substrate selectivity and higher affinity (K(m) 0.1-0.7 mM) than SLC36A1. Mutations in SLC36A2 lead to hyperglycinuria and iminoglycinuria. SLC36A3 is expressed only in testes and is an orphan transporter with no known function. SLC36A4 is widely distributed at the mRNA level and is a high-affinity (K(m) 2-3 μM) transporter for proline and tryptophan. We have much to learn about this family of transporters, but from current knowledge, it seems likely that their function will influence the pharmacokinetic profiles of amino acid-based drugs by mediating transport in both the small intestine and kidney.     

The emerging physiological roles of the SLC14A family of urea transporters

In mammals, urea is the main nitrogenous breakdown product of protein catabolism and is produced in the liver. In certain tissues, the movement of urea across cell membranes is specifically mediated by a group of proteins known as the SLC14A family of facilitative urea transporters. These proteins are derived from two distinct genes, UT-A (SLC14A2) and UT-B (SLC14A1). Facilitative urea transporters play an important role in two major physiological processes - urinary concentration and urea nitrogen salvaging. Although UT-A and UT-B transporters both have a similar basic structure and mediate the transport of urea in a facilitative manner, there are a number of significant differences between them. UT-A transporters are mainly found in the kidney, are highly specific for urea, have relatively lower transport rates and are highly regulated at both gene expression and cellular localization levels. In contrast, UT-B transporters are more widespread in their tissue location, transport both urea and water, have a relatively high transport rate, are inhibited by mercurial compounds and currently appear to be less acutely regulated. This review details the fundamental research that has so far been performed to investigate the function and physiological significance of these two types of urea transporters.     

Improving cancer chemotherapy with modulators of ABC drug transporters

ATP-binding cassette (ABC) transporters, P-glycoprotein (P-gp, ABCB1) and ABCG2, are membrane proteins that couple the energy derived from ATP hydrolysis to efflux many chemically diverse compounds across the plasma membrane, thereby playing a critical and important physiological role in protecting cells from xenobiotics. These transporters are also implicated in the development of multidrug resistance (MDR) in cancer cells that have been treated with chemotherapeutics. One approach to blocking the efflux capability of an ABC transporter in a cell or tissue is inhibiting the activity of the transporters with a modulator. Since ABC transporter modulators can be used in combination with chemotherapeutics to increase the effective intracellular concentration of anticancer drugs, the possible impact of modulators of ABC drug transporters is of great clinical interest. Another possible clinical use of modulators that has recently attracted attention is their ability to increase oral bioavailability or increase tissue penetration of drugs transported by the transporters. Several preclinical and clinical studies have been performed to evaluate the feasibility and the safety of this approach. The primary focus of this review is to discuss progress made in recent years in the identification and applicability of compounds that may serve as ABC transporter modulators and the possible role of these compounds in altering the pharmacokinetics and pharmacodynamics of therapeutic drugs used in the clinic.     

Arginine-rich molecular transporters for drug delivery: role of backbone spacing in cellular uptake

Short oligomers of arginine, either alone or when conjugated to therapeutic agents or large biopolymers, have been shown to cross readily a variety of biological barriers (e.g., lipid bilayers and epithelial tissue). Molecular modeling suggests that only a subset of the side chain guanidinium groups of these transporters might be required for transport involving contact with a common surface such as a plasma membrane or cell surface receptor. To evaluate this hypothesis, a series of decamers were prepared that incorporated seven arginines and three nonarginine residues. Several of these mixed decamers were comparable to the all arginine decamer in their ability to enter cells. More significantly, these decamers containing seven arginines performed almost without exception better than heptaarginine itself, suggesting that spacing between residues is also important for transport. The influence of spacing was more fully evaluated with a library of oligomers incorporating seven arginines separated by one or more nonconsecutive, non-alpha-amino acids. This study led to the identification of a new series of highly efficient molecular transporters.     

Genetic polymorphisms of drug-metabolising enzymes and drug transporters in the chemotherapeutic treatment of cancer

There is wide variability in the response of individuals to standard doses of drug therapy. This is an important problem in clinical practice, where it can lead to therapeutic failures or adverse drug reactions. Polymorphisms in genes coding for metabolising enzymes and drug transporters can affect drug efficacy and toxicity. Pharmacogenetics aims to identify individuals predisposed to a high risk of toxicity and low response from standard doses of anti-cancer drugs. This review focuses on the clinical significance of polymorphisms in drug-metabolising enzymes (cytochrome P450 [CYP] 2C8, CYP2C9, CYP2C19, CYP2D6, CYP3A4, CYP3A5, dihydropyrimidine dehydrogenase, uridine diphosphate glucuronosyltransferase [UGT] 1A1, glutathione S-transferase, sulfotransferase [SULT] 1A1, N-acetyltransferase [NAT], thiopurine methyltransferase [TPMT]) and drug transporters (P-glycoprotein [multidrug resistance 1], multidrug resistance protein 2 [MRP2], breast cancer resistance protein [BCRP]) in influencing efficacy and toxicity of chemotherapy.The most important example to demonstrate the influence of pharmacogenetics on anti-cancer therapy is TPMT. A decreased activity of TPMT, caused by genetic polymorphisms in the TPMT gene, causes severe toxicity with mercaptopurine. Dosage reduction is necessary for patients with heterozygous or homozygous mutation in this gene.Other polymorphisms showing the influence of pharmacogenetics in the chemotherapeutic treatment of cancer are discussed, such as UGT1A1*28. This polymorphism is associated with an increase in toxicity with irinotecan. Also, polymorphisms in the DPYD gene show a relation with fluorouracil-related toxicity; however, in most cases no clear association has been found for polymorphisms in drug-metabolising enzymes and drug transporters, and pharmacokinetics or pharmacodynamics of anti-cancer drugs. The studies discussed evaluate different regimens and tumour types and show that polymorphisms can have different, sometimes even contradictory, pharmacokinetic and pharmacodynamic effects in different tumours in response to different drugs.The clinical application of pharmacogenetics in cancer treatment will therefore require more detailed information of the different polymorphisms in drug-metabolising enzymes and drug transporters. Larger studies, in different ethnic populations, and extended with haplotype and linkage disequilibrium analysis, will be necessary for each anti-cancer drug separately.     

Solid lipid nanoparticles as intracellular drug transporters: an investigation of the uptake mechanism and pathway

The aim of this work was to develop a systematic analysis of the cellular internalisation mechanism and pathway of solid lipid nanoparticles (SLN) internalisation. To evaluate if SLN show cell uptake and to understand the mechanism of internalisation, four human glioma cell lines (A172, U251, U373 and U87) and a human macrophage cell line (THP1) were used. For this purpose rhodamine 123 (R123) was loaded into SLN coated with polysorbate 60 and 80. Fluorescence microscopy and flow cell cytometry techniques were assessed to study internalisation of these systems within the cells. MTT studies were performed to evaluate the cytotoxicity of the R123-loaded SLN. To assess the SLN internalisation mechanism and intracellular pathway, excluding endocytosis mechanisms were applied. Our results revealed that R123-loaded SLN with mean size below 200 nm and slight negative surface charge (around -20 mV) have the ability to be internalised by gliomas in a higher amount than by macrophages. The mechanism of internalisation was found to be mainly through a clathrin-dependent endocytic pathway. In addition, the cytotoxicity of SLN was higher for gliomas than for macrophages. These results suggest that SLN can be a promising alternative in brain tumours treatment.