NUCLEOSIDE TRANSPORTERS: CNTs AND ENTs - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 182

gemcitabine, fludarabine, cytarabine, clofarabine, and ribavirin have been demon-
strated to be transported by hENT1. 28 , 43 , 76 , 77 However, hENT1 does not transport
AZT and only weakly transports ddC and ddI. 78 Coronary vasodilators such as dipyri-
damole, dilazep, and draflazine are potent hENT1 inhibitors ( K i values in nanomolar
concentrations). 79 These compounds bind to the ENT1 protein but are not transported
into the cells. There are significant species differences among ENT1 in interacting
with coronary drugs. Rat ENT1 is much less sensitive to dipyridamole and dilazep than
human ENT1, and mouse ENT1 is only intermediately sensitive to dipyridamole. 69 , 79
In addition to NBMPR and coronary vasodilators, a number of protein kinase in-
hibitors, including the Bcr-Abl tyrosine kinase inhibitor STI-571 (Gleevec) used for
treatment of chronic myelogenous leukemia, were shown to be effective in inhibiting
ENT1 and/or ENT2, with IC 50 values ranging from 60 to 1000 nM. 80 Interestingly,
Carrier et al. recently showed that plant-derived cannabinoids are potent inhibitors of
ENT1 ( K i <

250 nM). 81

ENT2 (SLC29A2) The human ENT2 transporter was cloned from HeLa cells and
human placenta by two independent groups. 82 , 83 hENT2 (456 a.a.) is 46% identical
in amino acid sequence to hENT1. The most homologous regions between hENT1
and hENT2 are the transmembrane segments, while the least are the hydrophilic ter-
mini and loops. The human hENT2 gene is localized to chromosome 11q13. Two
mRNA-spliced variants, encoding nonfunctional truncated transporter proteins, were
reported. 72 , 83 The rodent orthologs rENT2 and mENT2 were also cloned. 68 , 69 The
456-residue mENT2 and rENT2 are 88% identical to hENT2. hENT2 mRNA is par-
ticularly abundant in skeletal muscle, but is also expressed in a wide range of tissues,
including brain, heart, placenta, thymus, pancreas, prostate, and kidney. 12 , 83 In MDCK
cells, GFP-tagged hENT2 is localized exclusively to the basolateral membrane. A C-
terminal dileucine repeat is implicated in the surface expression of hENT2. 72
Recombinant human and rat ENT2 proteins transport a broad range of purine
and pyrimidine nucleosides, and they confer NBMPR-insensitive Na + -independent
nucleoside transport typical to the ei system. Although both hENT1 and hENT2
transporters are broadly selective for purine and pyrimidine nucleosides, hENT2 ex-
hibits 7.7- and 19.3-fold lower affinities for guanosine and cytidine, respectively, but
a four fold higher affinity for inosine. 75 More important, ENT2 transports a wide
range of purine and pyrimidine nucleobases, whereas ENT1 does not. 84 Specifically,
hENT2 and rENT2 transport hypoxanthine, adenine, guanine, uracil, and thymine
with low affinities ( K m ranging from 0.7 to 2.6 mM). 84 hENT2, but not rENT2,
also transports cytosine. h/rENT2 transports AZT, ddC, and ddI with much higher
efficiency than h/rENT1. Gemcitabine and clofarabine have been shown to be trans-
ported by hENT2. 28 , 43 , 79 A number of uridine and cytidine analogs also interact with
hENT2. 85

ENT3 (SLC29A3) The third member of the ENT family, ENT3, was first identified
and cloned from human and mouse due to sequence similarity to ENT1 and ENT2. 86
The hENT3 gene is localized to chromosome 10q22.1. The 475-residue hENT3 and
mENT3 proteins are 73% identical in amino acid sequence and exhibit 30 to 33%

identities to ENT1 and ENT2. Structurally, ENT3 differs from ENT1 and ENT2

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