Aliases: HCNT2, HsT17153, SPNT1
Gene name: Solute carrier family 28 member 2 (SLC28A2)
Concentrative Nucleotide Transporter 2 (SLC28A1, CNT2), also referred as SPNT (sodium-dependent purine nucleoside transporter), is a sodium-dependent transporter mainly for purine nucleosides and uridine. Besides naturally occurring nucleosides, CNT2 is capable of transporting nucleoside-based anticancer and antiviral drugs. Current FDA and EMA recommendations do not include testing for CNT2 transporter liabilities.
Localization and expression
CNT2, cloned in 1998 by Ritzel et al. , is expressed in various tissues including the kidney, liver, heart, placenta, pancreas, spleen, skeletal muscle, colon, rectum and small intestine [2, 3]. Expression of CNT2 was quantifiable by LC-MS/MS-based proteomics in microsomal fractions of intestinal tissue .
CNT2 was also found ubiquitously in the immune system [3, 5]. CNT2 is widespread in the brain, being most abundant in the amygdala, hippocampus, cerebellum and certain neocortical regions . CNT2 is the only concentrative nucleoside transporter that is present in myocytes . The expression levels of CNT2 in rat lymphocytes were shown to be modulated by insulin , while experiments showed that the mRNA levels of rCnt2 were significantly altered in the diabetic heart, liver, and kidney . Studies conducted on hepatocytes have indicated that hepatocarcinogenesis leads to a selective loss of CNT2 . Histone hypoacetylation in colorectal cancer due to increased expression of histone deacetylase (HDAC) 7 results in chromatin condensation and reduced expression of CNT2, leading to resistance to CNT2 substrate nucleoside analogs; consistently, HDAC inhibitors were shown to counter resistance .
Function, physiology, and clinically significant polymorphisms
CNT2 is a 72-kDa protein with a putative structure of 13 transmembrane domains. CNT2 preferably transports purine nucleosides, but uridine is also a substrate of the transporter . The stoichiometry of nucleoside:sodium transport appears to be 1:1 .
Extracellular level of adenosine, a substrate of several nucleoside transporters with CNT2 among them  is responsible for the regulation of anti-inflammatory signaling  and energy pathways across the body [16, 17]. Extracellular adenosine also offers a cardioprotective effect during ischemic stress or high myocardial workload , and counteracts neuronal damage caused by metabolic or physiological stress . Adenosine uptake through the BBB is also modulated by CNT2 . Important drug substrates of the transporter include anticancer and antiviral nucleoside analogues, and have been neatly summarized in a couple of reviews (e.g. [12, 21]).
CNT2 expression, similar to that of CNT1, is coupled with cell differentiation, as differentiation inducing glucocorticoids increase CNT2 protein and mRNA levels in rats . In contrast to CNT1, CNT2 has a lower overall genetic variation at the amino acid level. Only six non-synonymous variants and 17 haplotypes were described by Owen et al . No change in ribavirin transport by these variants was observed, although uridine transport by variant Phe355Ser was decreased. The study of Li et al. investigating CNT2 mutations from Asian populations found variant Glu385Lys to be less sensitive to uridine, inosine and ribavirin .
Transport of naturally occurring purines contributes to the development of hyperuricemia. Inhibiting CNT2-mediated absorption of these nucleosides in the intestine can lower the urinary excretion of uric acid, the end product of purine degradation . CNT2 is also considered to play an important role in the absorption of purine nucleoside analogs .
Purine analog nucleoside drugs are highly effective against blood cancers. For example, drug sensitivity of resistant leukemia cells against fluoropyrimidine nucleosides can be increased by CNT2 transgenic expression . CNT2 transports antiviral drugs, such as didanosine (ddI) and ribavirin, which are used in HIV and Hepatitis C therapy. It also transports entecavir that has high efficacy against Hepatitis B Virus (HBV) and used as a first line treatment in chronic hepatitis B infections .
Currently, there is no recommendation by the FDA or EMA for the evaluation of CNT2. However, in vitro evaluations may be required on a case-by-case basis.
|Location||Endogenous substrates||In vitro substrates used experimentally||Substrates drugs||Inhibitors|
|intestine, kidney, liver, brain, immune system, skeletal muscle, spleen, placenta, pancreas, heart||adenosine, uridine, inosine, guanosine
||adenosine, uridine, inosine||ribavirin, zidovudine,
mizoribine, clofarabine, cladribine, fluoropyrimidine, formycin B, didanosine (ddI), maribavir, floxuridine, entecavir
|KGO-2142 and KGO-2173 
nelarabine, vidarabine, fludarabine-des-phosphate, decitabine, gemcitabine 
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