A. Gupte, J. K. Buolamwini / Bioorg. Med. Chem. Lett. 19 (2009) 917–921
921
with phloretin (37, Ki = 32.28 lM) highlights this observation.
Supplementary data
Additionally, the presence of hydroxyl groups on the aromatic
aglycone subunit of the phloridzin scaffold is important for high
binding affinities at the hCNT3. Removal of the hydroxyl groups
present on the A and B rings leads to a substantial loss in binding
affinity. This is exemplified by the binding affinity of compound 19
Supplementary data associated with this article can be found, in
References and notes
(Ki > 100 lM), which has a structure similar to phloridzin but lacks
the 40-, 60- and 4-position hydroxyl groups on the aromatic A and B
rings. Also, compounds possessing both the A and B rings in their
aglycone subunits bind better than their corresponding analogs
possessing just the B ring. A comparison between phloridzin (3,
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Ki = 16.00
lM) with compound 26 (Ki = 30.77
lM) and compound
16 (Ki = 2.88
l
M) with compound 7 (Ki = 12.40 M) reveals this
l
trend.
In addition to the [3H]-uridine uptake experiments, a flow cyto-
metric competitive binding assay was also undertaken to gain
knowledge regarding the selectivity of these analogs.27 Although
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NBMPR (1) inhibits hCNT3 with a Ki of 88.05 lM it is known to
bind the hENT1 nucleoside transporter in subnanomolar concen-
trations (Ki = 0.7 nM). The most active hCNT3 inhibitor in this ser-
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ies, compound 16 (Ki = 2.88
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19. Ritzel, M. W.; Ng, A. M.; Yao, S. Y.; Graham, K.; Loewen, S. K.; Smith, K. M.;
Ritzel, R. G.; Mowles, D. A.; Carpenter, P.; Chen, X. Z.; Karpinski, E.; Hyde, R. J.;
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(Ki = 16.00 M), exhibited Ki values of >1 mM at the hENT1 thus
l
indicating that these compounds are highly selective for hCNT3
relative to hENT1.
Contrary to hENTs, the knowledge regarding inhibitors of hCNTs
has been strikingly limited. In this SAR study, we have introduced
specific modifications in the phloridzin scaffold while demonstrat-
ing the importance of a sugar subunit at the 20-hydroxyl position
for exhibiting binding affinity at the hCNT3. Replacing the gluco-
pyranose of phloridzin with a ribofuranose, results in a binding
improvement at the hCNT3. Presence of hydroxyl groups on the
aromatic A and B rings is important for binding at the hCNT3. Addi-
tionally, phloridzin and its most potent analog, compound 16 do
not exhibit significant binding affinity at the hENT1 (Ki > 1 mM),
highlighting their selectivity for hCNT3 when compared with
hENT1. The knowledge gained from this study will be significant
in our efforts in developing selective high affinity ligands of the
hCNT3 transporter. This study identifies compound 16 as a poten-
tial lead that may be modified to yield more selective and potent
hCNT3 inhibitors.
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