Human Transporters


CNT1 (concentrative nucleoside transporter 1)

Aliases: HCNT1, cit
Gene name: SLC28A1 (Solute carrier family 28 member 1)


CNT1 (SLC28A1) is a sodium-dependent transporter for pyrimidine nucleosides, belonging to the SLC28 family. Besides naturally occurring nucleosides, CNT1 is capable of transporting nucleoside-based anticancer and antiviral drugs. Current FDA and EMA recommendations do not include testing for CNT1 transporter liabilities.

Localization and expression

CNT1 is expressed primarily on the apical side of epithelial and endothelial cells in various tissues including the kidney, liver, small intestine, bone marrow macrophages, and some regions of the brain [1-3]. CNT1 is expressed in HepaRG cells, a cellular hepatocyte model frequently used in ADMETox research [4]. Expression of CNT1 was quantifiable by LC-MS/MS-based proteomics in microsomal fractions of intestine and kidney [5].
Studies on rats showed that the supply of nucleotides modulates CNT1 expression in a tissue-specific manner [6-8], and that the mRNA levels of rCnt1 were significantly altered in the diabetic heart, liver, and kidney [9]. Studies performed on a hepatoma cell line showed cell cycle-dependent CNT1 expression [10]. Lactation has increased CNT1 mRNA levels 4-fold in lactating mammary epithelial cells indicating a role for CNT1 in nucleoside accumulation in milk [11].

Function, physiology, and clinically significant polymorphisms

CNT1, cloned in 1997 by Ritzel et al. [12], is a 72-kDa protein with a putative structure of 13 transmembrane domains [13]. Both CNT2 and CNT3 accept purine and pyrimidine nucleosides, while CNT1 transports only pyrimidine and its analogues. Data about the transport of the purine nucleoside adenosine are controversial as it was reported to bind to CNT1 with high affinity and considered as substrate by many groups, whereas Larrayoz et al. found no translocation through the membrane, and they reported it as an inhibitor [14]. The stoichiometry of nucleoside:sodium transport appears to be 1:1 [15]. 
In physiological conditions, CNT1-mediated nucleoside uptake is dedicated to supply the synthesis of RNA rather than DNA; hence, high CNT1 activity is characteristic of cells with active transcription, and less so of proliferating cells where nucleoside import is mainly mediated by ENTs. E.g., activation of macrophages and differentiation in hepatocytes was shown to be associated with higher CNT1 levels (e.g. [16, 17]). Importantly, nucleoside-based antiviral and anticancer agents are also transported by CNT1 (reviewed e.g. in [18]). For example, the transporter increases chemosensitivity against nucleoside-based drugs such as gemcitabine and the capecitabine-derived metabolite 5′-DFUR [19, 20].
So far, 58 SNPs within the 153 haplotypes of the SLC28A1 gene were identified [21]. There is no reported disease or altered physiological condition associated with mutations of this gene. However, SNPs can affect the bioavailability of certain drugs. For example, Naito et al. demonstrated significantly lower bioavailability of mizoribine in Japanese patients receiving kidney transplantation with the allele 565-A/A (Val189Ile) than in subjects with 565-G/G [22]. In healthy individuals carrying the mutant allele, increasing salt intake could improve intestinal absorption of the drug because of the elevated sodium concentration in the lumen [23]. However, increased salt intake in this particular patient group might be counter-effective as there are reports proving the protective impact of salt restriction on kidney damage progression [24, 25]. Val189Ile is also associated with decreased affinity to gemcitabine [21]. Surprisingly, gemcitabine-treated NSCLC patients carrying the 565A allele showed slightly better initial response to anticancer therapy, although there was no effect on overall survival [26]. The variant D521N is associated with higher risk of hepatic toxicity upon gemcitabine treatment [27]. 

Clinical significance

Pyrimidine analogue drugs are effective tools against blood cancers and solid tumors. In vitro transfection of pancreatic cancer cells with the transporter increases the sensitivity against gemcitabine by 2.5 fold, and CNT1 expression in pancreatic cancers correlates with response to gemcitabine therapy [28]. CNT1 also contributes to the absorption of trifluridine, an effective anticancer drug used in colorectal cancer [29], and emtricitabine, a first line antiviral drug to treat AIDS in pregnancy [30]. 
As CNT1 is expressed in bone marrow macrophages, which are host cells of HIV-1 infection, the transporter represents one of the possible targets for HIV antiviral therapy. The commonly administered HIV antiviral agents, however, bind to the protein with low efficacy [31]. 

Regulatory Requirements

Currently, there is no recommendation by the FDA or EMA for the evaluation of CNT1. However, in vitro evaluations may be required on a case-by-case basis.

Location Endogenous substrates In vitro substrates used experimentally Substrate drugs Inhibitors
intestine, kidney, liver, brain uridine, thymine, cytosine uridine zidovudine, lamivudine, zalcitabine, cytarabine, stavudine, gemcitabine, mizoribine, 5’-DFU, ddC, ara-C, dFdC, trifluridine, emtricitabine no potent inhibitor reported (adenosine?; partial inhibition by phloridzin [15])
vidarabine, fludarabine-des-phosphate, decitabine, gemcitabine [32]



1.    Huang, Q.Q., et al., Functional expression of Na(+)-dependent nucleoside transport systems of rat intestine in isolated oocytes of Xenopus laevis. Demonstration that rat jejunum expresses the purine-selective system N1 (cif) and a second, novel system N3 having broad specificity for purine and pyrimidine nucleosides. J Biol Chem, 1993. 268(27): p. 20613-9.
2.    Anderson, C.M., et al., Demonstration of the existence of mRNAs encoding N1/cif and N2/cit sodium/nucleoside cotransporters in rat brain. Brain Res Mol Brain Res, 1996. 42(2): p. 358-61.
3.    Pennycooke, M., et al., Differential expression of human nucleoside transporters in normal and tumor tissue. Biochem Biophys Res Commun, 2001. 280(3): p. 951-9.
4.    Mayati, A., et al., mRNA Expression and Activity of Nucleoside Transporters in Human Hepatoma HepaRG Cells. Pharmaceutics, 2018. 10(4).
5.    Nakamura, K., et al., Large-scale multiplex absolute protein quantification of drug-metabolizing enzymes and transporters in human intestine, liver, and kidney microsomes by SWATH-MS: Comparison with MRM/SRM and HR-MRM/PRM. Proteomics, 2016. 16(15-16): p. 2106-17.
6.    Mercader, J., et al., Nucleoside uptake in rat liver parenchymal cells. Biochem J, 1996. 317 ( Pt 3): p. 835-42.
7.    Patil, S.D. and J.D. Unadkat, Sodium-dependent nucleoside transport in the human intestinal brush-border membrane. Am J Physiol, 1997. 272(6 Pt 1): p. G1314-20.
8.    Valdes, R., et al., Nutritional regulation of nucleoside transporter expression in rat small intestine. Gastroenterology, 2000. 119(6): p. 1623-30.
9.    Pawelczyk, T., M. Podgorska, and M. Sakowicz, The effect of insulin on expression level of nucleoside transporters in diabetic rats. Mol Pharmacol, 2003. 63(1): p. 81-8.
10.    Valdes, R., F.J. Casado, and M. Pastor-Anglada, Cell-cycle-dependent regulation of CNT1, a concentrative nucleoside transporter involved in the uptake of cell-cycle-dependent nucleoside-derived anticancer drugs. Biochem Biophys Res Commun, 2002. 296(3): p. 575-9.
11.    Alcorn, J., et al., Transporter gene expression in lactating and nonlactating human mammary epithelial cells using real-time reverse transcription-polymerase chain reaction. J Pharmacol Exp Ther, 2002. 303(2): p. 487-96.
12.    Ritzel, M.W., et al., Molecular cloning and functional expression of cDNAs encoding a human Na+-nucleoside cotransporter (hCNT1). Am J Physiol, 1997. 272(2 Pt 1): p. C707-14.
13.    Podgorska, M., K. Kocbuch, and T. Pawelczyk, Recent advances in studies on biochemical and structural properties of equilibrative and concentrative nucleoside transporters. Acta Biochim Pol, 2005. 52(4): p. 749-58.
14.    Larrayoz, I.M., et al., Electrophysiological characterization of the human Na(+)/nucleoside cotransporter 1 (hCNT1) and role of adenosine on hCNT1 function. J Biol Chem, 2004. 279(10): p. 8999-9007.
15.    Smith, K.M., et al., Electrophysiological characterization of a recombinant human Na+-coupled nucleoside transporter (hCNT1) produced in Xenopus oocytes. J Physiol, 2004. 558(Pt 3): p. 807-23.
16.    Soler, C., et al., Macrophages require different nucleoside transport systems for proliferation and activation. FASEB J, 2001. 15(11): p. 1979-88.
17.    del Santo, B., et al., Developmental regulation of the concentrative nucleoside transporters CNT1 and CNT2 in rat liver. J Hepatol, 2001. 34(6): p. 873-80.
18.    Errasti-Murugarren, E. and M. Pastor-Anglada, Drug transporter pharmacogenetics in nucleoside-based therapies. Pharmacogenomics, 2010. 11(6): p. 809-41.
19.    Mackey, J.R., et al., Functional nucleoside transporters are required for gemcitabine influx and manifestation of toxicity in cancer cell lines. Cancer Res, 1998. 58(19): p. 4349-57.
20.    Mata, J.F., et al., Role of the human concentrative nucleoside transporter (hCNT1) in the cytotoxic action of 5[Prime]-deoxy-5-fluorouridine, an active intermediate metabolite of capecitabine, a novel oral anticancer drug. Mol Pharmacol, 2001. 59(6): p. 1542-8.
21.    Gray, J.H., et al., Functional and genetic diversity in the concentrative nucleoside transporter, CNT1, in human populations. Mol Pharmacol, 2004. 65(3): p. 512-9.
22.    Naito, T., et al., Impact of concentrative nucleoside transporter 1 gene polymorphism on oral bioavailability of mizoribine in stable kidney transplant recipients. Basic Clin Pharmacol Toxicol, 2010. 106(4): p. 310-6.
23.    Ishida, K., et al., Effect of salt intake on bioavailability of mizoribine in healthy Japanese males. Drug Metab Pharmacokinet, 2013. 28(1): p. 75-80.
24.    Cianciaruso, B., et al., Salt intake and renal outcome in patients with progressive renal disease. Miner Electrolyte Metab, 1998. 24(4): p. 296-301.
25.    He, F.J., et al., Effect of modest salt reduction on blood pressure, urinary albumin, and pulse wave velocity in white, black, and Asian mild hypertensives. Hypertension, 2009. 54(3): p. 482-8.
26.    Muller, P.J., et al., Polymorphisms in ABCG2, ABCC3 and CNT1 genes and their possible impact on chemotherapy outcome of lung cancer patients. Int J Cancer, 2009. 124(7): p. 1669-74.
27.    Soo, R.A., et al., Distribution of gemcitabine pathway genotypes in ethnic Asians and their association with outcome in non-small cell lung cancer patients. Lung Cancer, 2009. 63(1): p. 121-7.
28.    Garcia-Manteiga, J., et al., Nucleoside transporter profiles in human pancreatic cancer cells: role of hCNT1 in 2',2'-difluorodeoxycytidine- induced cytotoxicity. Clin Cancer Res, 2003. 9(13): p. 5000-8.
29.    Takahashi, K., et al., Involvement of Concentrative Nucleoside Transporter 1 in Intestinal Absorption of Trifluridine Using Human Small Intestinal Epithelial Cells. J Pharm Sci, 2015. 104(9): p. 3146-53.
30.    Zeng, Q., et al., Multiple Drug Transporters Contribute to the Placental Transfer of Emtricitabine. Antimicrob Agents Chemother, 2019. 63(8).
31.    Cano-Soldado, P., et al., Interaction of nucleoside inhibitors of HIV-1 reverse transcriptase with the concentrative nucleoside transporter-1 (SLC28A1). Antivir Ther, 2004. 9(6): p. 993-1002.
32.    Vasko, B., et al., Inhibitor selectivity of CNTs and ENTs. Xenobiotica, 2019. 49(7): p. 840-851.


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