Functional expression and characterization of a sodium-dependent nucleoside transporter hCNT2 cloned from human duodenum

Separate Evaluation of Fraction Absorbed and Intestinal Availability after Oral Administration of Drugs Based on the Measurement of Portal and Systemic Plasma Concentrations and Luminal Concentration

There are several experimental methods to estimate the product of the fraction absorbed (Fa) and intestinal availability (Fg) in vivo after oral administration of drugs. Metabolic enzyme inhibitors are typically used to separate Fg from Fa·Fg. Since Fa·Fg can be regarded as Fa under metabolism-inhibited conditions, Fg can be isolated by dividing Fa·Fg by Fa. However, if the inhibition of intestinal metabolism is insufficient, Fa is overestimated, which results in an underestimation of Fg compared to the actual value. In this study, to avoid this problem, an experimental method for the separate estimation of Fa and Fg in rats without utilizing metabolic enzyme inhibitors was established. Buspirone, a CYP3A substrate, and ribavirin, a substrate of purine nucleoside phosphorylase and adenosine kinase, were selected as models. Following oral administration of the drugs with fluorescein isothiocyanate dextran 4000 (FD-4, an unabsorbable marker), Fa·Fg was pharmacokinetically calculated from portal and systemic plasma concentration-time profiles of model drugs and Fa was calculated from the difference in the ileal concentration profiles of the drugs and FD-4. Fg was evaluated by dividing Fa·Fg by Fa. Following oral administration, buspirone was not detected in any segment of the small intestine, indicating that the administered buspirone was completely absorbed. In addition, buspirone was extensively metabolized in enterocytes (Fg = 20.1). Ribavirin was primarily absorbed in the upper segment of the small intestine, and 64.4% of the ribavirin was absorbed before it reached the ileum. In addition, it was revealed that ribavirin was metabolized more extensively in the intestine than in the liver. Our method may be effective in quantitatively assessing Fa and Fg in vivo, which can help in the formulation design and prediction of drug-drug interactions.

Identification of 8-aminoadenosine derivatives as a new class of human concentrative nucleoside transporter 2 inhibitors

Purine-rich foods have long been suspected as a major cause of hyperuricemia. We hypothesized that inhibition of human concentrative nucleoside transporter 2 (hCNT2) would suppress increases in serum urate levels derived from dietary purines. To test this hypothesis, the development of potent hCNT2 inhibitors was required. By modifying adenosine, an hCNT2 substrate, we successfully identified 8-aminoadenosine derivatives as a new class of hCNT2 inhibitors. Compound 12 moderately inhibited hCNT2 (IC50 = 52 ± 3.8 μM), and subsequent structure-activity relationship studies led to the discovery of compound 48 (IC50 = 0.64 ± 0.19 μM). Here we describe significant findings about structural requirements of 8-aminoadenosine derivatives for exhibiting potent hCNT2 inhibitory activity.

Impact of genetic SLC28 transporter and ITPA variants on ribavirin serum level, hemoglobin drop and therapeutic response in patients with HCV infection

BACKGROUND & AIMS

In the last decade, pegylated interferon-α (PegIFN-α) plus ribavirin (RBV) was the standard treatment of chronic hepatitis C for genotype 1, and it remains the standard for genotypes 2 and 3. Recent studies reported associations between RBV-induced anemia and genetic polymorphisms of concentrative nucleoside transporters such as CNT3 (encoded by SLC28A3) and inosine triphosphatase (encoded by ITPA). We aimed at studying genetic determinants of RBV kinetics, efficacy and treatment-associated anemia.

METHODS

We included 216 patients from two Swiss study cohorts (61% HCV genotype 1, 39% genotypes 2 or 3). Patients were analyzed for SLC28A2 single nucleotide polymorphism (SNP) rs11854484, SLC28A3 rs56350726, and SLC28A3 rs10868138 as well as ITPA SNPs rs1127354 and rs7270101, and followed for treatment-associated hemoglobin changes and sustained virological response (SVR). In 67 patients, RBV serum levels were additionally measured during treatment.

RESULTS

Patients with SLC28A2 rs11854484 genotype TT had higher dosage- and body weight-adjusted RBV levels than those with genotypes TC or CC (p=0.02 and p=0.06 at weeks 4 and 8, respectively). ITPA SNP rs1127354 was associated with hemoglobin drop ≥3 g/dl during treatment, in genotype (relative risk (RR)=2.1, 95% CI 1.3-3.5) as well as allelic analyses (RR=2.0, 95%CI 1.2-3.4). SLC28A3 rs56350726 was associated with SVR in genotype (RR=2.2; 95% CI 1.1-4.3) as well as allelic analyses (RR=2.0, 95% CI 1.1-3.4).

CONCLUSIONS

The newly identified association between RBV serum levels and SLC28A2 rs11854484 genotype, as well as the replicated association of ITPA and SLC28A3 genetic polymorphisms with RBV-induced anemia and treatment response, may support individualized treatment of chronic hepatitis C and warrant further investigation in larger studies.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Protein kinase inhibitors emodin and dichloro-ribofuranosylbenzimidazole modulate the cellular accumulation and cytotoxicity of cisplatin in a schedule-dependent manner

PURPOSE

Protein kinase inhibitors (PKI) have become prominent agents in cancer therapeutics. However, the specificity for target kinase inhibition can be poor and unwanted effects can emerge in combination regimens. The PKI emodin, for instance, can produce mixed results when combined with cisplatin, and we have sought a biochemical pharmacologic explanation for the negative cytotoxic effects.

METHODS

Human ovarian A2780 tumor cells were exposed to the PKI emodin or dichloro-ribofuranosylbenzimidazole (DRB) with cisplatin using several schedules, and cytotoxicity determined by a growth inhibition assay. Intracellular platinum levels and DNA adducts were estimated by flameless atomic absorption spectrophotometry.

RESULTS

When A2780 cells were exposed first to emodin or DRB and then to cisplatin alone, the cytotoxic effects of cisplatin were significantly enhanced, whereas simultaneous exposure did not enhance the cytotoxicity, but instead inhibited it in the case of DRB. The increase in activity of cisplatin in the sequenced schedule was not due to increases in intracellular levels of cisplatin or DNA adducts, whereas the cytotoxic inhibition was related to a significant fall in both intracellular platinum levels and DNA adducts, which were ascribed to inhibition in cisplatin uptake. Knockdown of hCtr1 (the human copper transporter 1) by siRNA abrogated this inhibition in cisplatin uptake.

CONCLUSION

The results demonstrate that co-exposure of tumor cells to emodin or DRB with cisplatin inhibits platinum drug uptake by impacting the hCtr1 transporter and, thereby, reduce the cytotoxicity of cisplatin. Based on our findings, scheduling of the PKI and the cytotoxic agent should be a major consideration in the clinical design of combination regimens.

Identification of functional promoter haplotypes of human concentrative nucleoside transporter 2, hCNT2 (SLC28A2)

The human concentrative nucleoside transporter 2 (hCNT2) plays a major role in the intestinal absorption of naturally occurring nucleosides as well as some nucleoside analog drugs. To determine if single nucleotide polymorphisms (SNPs) in the promoter region of hCNT2 affect gene expression, we examined approximately 1 kb upstream the hCNT2 transcription start site. Ninety Chinese samples were screened and seven SNPs were identified: -115T>G, -146T>A, -264A>G, -564G>A, -861A>C, -880T>C and -906C>T. Based on these seven variants and their relative positions, eight haplotypes were identified using PHASE v2.1.1. Three naturally occurring haplotypes were cloned into the pGL3-Basic vector and transfected into HEK293 cells. Dual luciferase assay revealed that haplotype 4 (GTAGACC) and 7 (GAGAACT) exhibited significantly lower expression levels compared to the published haplotype 1 (TTAGATC). Results from our in-vitro study showed that the hCNT2 promoter region haplotype may modulate gene expression and cause different drug responses.

Effect of dietary purines on the pharmacokinetics of orally administered ribavirin

Ribavirin is found to be absorbed in the intestine through the human concentrative nucleoside transporter 2 (hCNT2). Cellular uptake of ribavirin was strongly inhibited by purine nucleoside in an in vitro study. This study aims to examine the effects of dietary purine on the pharmacokinetics of orally administered ribavirin in vivo. Twenty healthy participants were enrolled in a randomized, 2-period crossover study. Participants were administered a single 600-mg oral dose of ribavirin after either a high-purine meal or a low-purine meal. Serial blood samples were collected predose and over 144 hours after dosing. Ribavirin concentrations were measured by liquid chromatography/tandem mass spectrometry. In comparison with corresponding plasma values of ribavirin following a high-purine meal, C(max), AUC(0-144) and AUC(0-infinity) of ribavirin following a low-purine meal were 136% (90% confidence internal [CI]: 120%-155%), 134% (90% CI: 118%-153%), and 139% (90% CI: 120%-159%), respectively. This study indicates that dietary purines have an effect on ribavirin absorption. Dosage regimens of ribavirin might need to be adjusted according to the purine content of the meal.

SLC28 genes and concentrative nucleoside transporter (CNT) proteins

The human concentrative nucleoside transporter (hCNT) protein family has three members, hCNT1, 2, and 3, encoded by SLC28A1, A2, and A3 genes, respectively. hCNT1 and hCNT2 translocate pyrimidine- and purine-nucleosides, respectively, by a sodium-dependent mechanism, whereas hCNT3 shows broad substrate selectivity and the unique ability of translocating nucleosides both in a sodium- and a proton-coupled manner. hCNT proteins are also responsible for the uptake of most nucleoside-derived antiviral and anticancer drugs. Thus, hCNTs are key pharmacological targets. This review focuses on several crucial aspects of hCNT biology and pharmacology: protein structure-function, structural determinants for transportability, pharmacogenetics of hCNT-encoding genes, role of hCNT proteins in nucleoside-based therapeutics, and finally hCNT physiology.

Identification and functional analysis of variants in the human concentrative nucleoside transporter 2, hCNT2 (SLC28A2) in Chinese, Malays and Indians

The human concentrative nucleoside transporter (hCNT2), also known as SLC28A2, plays an important role in the cellular uptake across intestinal membrane of some naturally occurring nucleosides and nucleoside analogs. This study aims to determine the genetic variability of hCNT2 (SLC28A2) in three major Asian ethnic groups residing in Singapore: Chinese, Malay and Indian, and functionally characterize the variants of hCNT2. Healthy participants (n=96) from each group were screened for genetic variations in the exons of hCNT2 (SLC28A2) using denaturing high performance liquid chromatography and sequencing analyses. A total of 23 polymorphisms were identified in the exonic and flanking intronic regions, and ethnic differences in single nucleotide polymorphism frequencies were evident. Five novel nonsynonymous variants (L12R, R142H, E172D, E385K, M612T) were constructed by mutagenesis and functionally characterized in U-251 cells. Expression of these variants in U-251 cells revealed that all except E385K can uptake various substrates of hCNT2: inosine, ribavirin and uridine.

Interaction of intestinal nucleoside transporter hCNT2 with amino acid ester prodrugs of floxuridine and 2-bromo-5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole

Amino acid ester prodrugs of antiviral and anticancer nucleoside drugs were developed to improve oral bioavailability or to reduce systemic toxicity. We studied the interaction of human concentrative nucleoside transporter (hCNT2) cloned from intestine with various amino acid ester prodrugs of floxuridine (FUdR) and 5,6-dichloro-2-bromo-1-beta-D-ribofuranosylbenzimidazole (BDCRB). Na(+)-dependent uptakes of [(3)H]-inosine and [(3)H]-adenosine were measured in U251 cells transiently expressing intestinal hCNT2. FUdR significantly inhibited the uptake of both [(3)H]-inosine and [(3)H]-adenosine (60-70% of control), while its amino acid ester prodrugs including Val, Phe, Pro, Asp, and Lys esters exhibited markedly decreased inhibition potency (10-30% of control). On the other hand, BDCRB and its amino acid prodrugs markedly inhibited the uptake of both [(3)H]-inosine and [(3)H]-adenosine. Val, Phe, and Pro ester prodrugs of BDCRB showed similar inhibition capacities as parent compound BDCRB (80-90% for adenosine and 60-80% for inosine). The amino acid site of attachment (3'- and 5'-monoesters) and stereochemistry (L- and D-amino acid esters), did not significantly affect the uptake of [(3)H]-inosine and [(3)H]-adenosine. These results demonstrate that the hCNT2 favorably interacts with BDCRB and its amino acid prodrugs, compared to those of FUdR, and that neutral amino acid esters of BDCRB have a high affinity toward this transporter. Therefore, the intestinal hCNT2 may be a target transporter as a factor for modulating oral pharmacokinetics of BDCRB prodrugs.

Floxuridine Amino Acid Ester Prodrugs: Enhancing Caco-2 Permeability and Resistance to Glycosidic Bond Metabolism

PURPOSE

The aim of this study was to synthesize amino acid ester prodrugs of 5-fluoro-2'-deoxyuridine (floxuridine) to enhance intestinal absorption and resistance to glycosidic bond metabolism.

METHODS

Amino acid ester prodrugs were synthesized and examined for their hydrolytic stability in human plasma, in Caco-2 cell homogenates, and in the presence of thymidine phosphorylase. Glycyl-L: -sarcosine uptake inhibition and direct uptake studies with HeLa/PEPT1 cells [HeLa cells overexpressing oligopeptide transporter (PEPT1)] were conducted to determine PEPT1-mediated transport and compared with permeability of the prodrugs across Caco-2 monolayers.

RESULTS

Isoleucyl prodrugs exhibited the highest chemical and enzymatic stability. The prodrugs enhanced the stability of the glycosidic bond of floxuridine. Thymidine phosphorylase rapidly cleaved floxuridine to 5-fluorouracil, whereas with the prodrugs no detectable glycosidic bond cleavage was observed. The 5'-L: -isoleucyl and 5'-L: -valyl monoester prodrugs exhibited 8- and 19-fold PEPT1-mediated uptake enhancement in HeLa/PEPT1 cells, respectively. Uptake enhancement in HeLa/PEPT1 cells correlated highly with Caco-2 permeability for all prodrugs tested. Caco-2 permeability of 5'-L: -isoleucyl and 5'-L: -valyl prodrugs was 8- to 11-fold greater compared with floxuridine.

CONCLUSIONS

Amino acid ester prodrugs such as isoleucyl floxuridine that exhibit enhanced Caco-2 transport and slower rate of enzymatic activation to parent, and that are highly resistant to metabolism by thymidine phosphorylase may improve oral delivery and therapeutic index of floxuridine.