3-(1,2,3-Triazol-1-yl)-1-thio-galactosides as small, efficient, and hydrolytically stable inhibitors of galectin-3

Article abstract of DOI:10.1016/j.bmcl.2005.05.084

Copper(I)-catalyzed addition of alkynes to methyl 3-azido-3-deoxy-1-thio- β-d-galactopyranoside afforded stable and structurally simple 3-deoxy-3-(1H-1,2,3-triazol-1-yl)-1-thio-galactosides carrying a panel of substituents at the triazole C4 in high yield

Full text of DOI:10.1016/j.bmcl.2005.05.084

Bioorganic & Medicinal Chemistry Letters 15 (2005) 3344–3346
3-(1,2,3-Triazol-1-yl)-1-thio-galactosides as small, efficient,
and hydrolytically stable inhibitors of galectin-3
Bader A. Salameh,a Hakon Lefflerb and Ulf J. Nilssona,*
aOrganic Chemistry, Lund University, PO Box 124, SE-221 00 Lund, Sweden
bSection MIG, Department of Laboratory Medicine, Lund University, So¨lvegatan 23, SE-223 62 Lund, Sweden
Received 14 February 2005; revised 11 May 2005; accepted 11 May 2005
Abstract—Copper(I)-catalyzed addition of alkynes to methyl 3-azido-3-deoxy-1-thio-b-D-galactopyranoside afforded stable and
structurally simple 3-deoxy-3-(1H-1,2,3-triazol-1-yl)-1-thio-galactosides carrying a panel of substituents at the triazole C4 in high
yields. The 3-(1H-[1,2,3]-triazol-1-yl)-1-thio-galactoside collection synthesized contained inhibitors of the tumor- and inflam-
mation-related galectin-3 with Kd values as low as 107 lM, which is as potent as the natural disaccharide inhibitors lactose and
N-acetyllactosamine.
Ó 2005 Elsevier Ltd. All rights reserved.
Galectins are a family of carbohydrate-binding proteins
(i.e., lectins), which are defined by having at least one
carbohydrate recognition domain (CRD) with specifici-
ty for b-D-galactosides.1,2 Fourteen different galectins
have been characterized so far; they are numbered
according to the chronology of their discovery (galec-
tin-1 to galectin-14) and are widely distributed from
lower to higher vertebrates. Although the exact func-
tions of galectins are still unknown, it is clear that they
play a role in cell–cell communication, cell-matrix adhe-
sion, cell growth regulation, and intracellular processes,
such as pre-mRNA splicing. They show a combination
of the properties expected for both intra- and extracellu-
lar proteins. Their activity as extracellular proteins,
where they usually bind glycoconjugates, has attracted
most attention.3–11 Strong evidence from several reports
suggests that galectins play an important role in inflam-
mation and immunity,5,12–14 as well as cancer.6,7,10
Recently, it was demonstrated that a C-terminal frag-
ment of galectin-3 containing the CRD decreased tumor
growth in a human breast cancer mouse model by acting
as a dominant negative inhibitor.15 Moreover, glycocon-
jugates, which decrease metastasis in mice, have been
suggested to act by the inhibition of galectins.16,17 Thus,
efficient inhibition of galectins may lead to anti-inflam-
matory and anti-cancer properties.
Natural small ligands of the galectins, such as N-acetyl
lactosamine (LacNAc) and lactose, show low inhibition
potency18 and development of new inhibitors with a
higher affinity for galectins has therefore emerged as
an important task. The crystal structure of a LacNAc:
galectin-3 complex19 shows a possible extended binding
groove close to HO-3(Gal). This binding groove has
been recently exploited in the design of high-affinity
inhibitors through synthesis of 30-benzamido derivatives
of LacNAc from a 3-azido-3-deoxy galactose precur-
sor.20,21 Such a 3-azido-3-deoxy-galactose derivative is
particularly versatile, as the azide functionality poten-
tially allows for various different chemical transforma-
tions. Cycloaddition of azides with acetylenes appears
to be attractive in this context, as the resulting [1,2,3]-
triazoles are non-natural heterocycles well known for
their stability towards oxidation, reduction, hydrolysis,
etc.22 They have been traditionally prepared through
thermal Huisgen 1,3-dipolar cycloaddition23 between
azides and monosubstituted alkynes, yielding both 1,4-
and 1,5-disubstituted triazoles. In recent years, an
important development in triazole chemistry has been
reported,24–27 in that the presence of a copper catalyst
leads to a regioselective reaction toward 1,4-disubsti-
tuted triazoles. Moreover, mild conditions and high
yield of the reaction attract attention, as neither com-
plex reagents nor anhydrous conditions are needed.
In this communication, we report on the discovery of
novel galectin-3 inhibitors based on Cu(I)-catalyzed
cycloaddition of methyl 3-azido-3-deoxy-1-thio-b-D-
Keywords: Galectin-3; 1,2,3-Triazole; Inhibitor.
*
Corresponding author. Tel.: +46 46 2228218; fax: +46 46 2228209;
e-mail: ulf.nilsson@organic.lu.se
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.05.084

B. A. Salameh et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3344–3346
3345
galactoside 120 with acetylene derivatives. The expected
1,4-disubstituted triazoles 2–5 were obtained in high
yield and as single regioisomers (Scheme 1). The reac-
tion took place at room temperature in the case of meth-
yl propiolate (!2), whereas for aliphatic or aromatic
substituted acetylenes (!3–5) long reaction times and
heating at 45° were necessary. This was probably due
to a lower dipolarophilicity of these acetylenes com-
bined with steric hindrance of the azide 1. The mono-
substituted triazole 6 was obtained by heating 1 and
propiolic acid without Cu(I) at 65° in toluene; the initial-
ly formed 4-carboxy-triazole was decarboxylated in
situ.28 Deprotection of 3–6 with methylamine in water
gave 9–12, whereas the ester 8 was obtained by the treat-
ment of 2 with methanolic sodium methoxide. The reac-
tion of methyl ester 2 with different amines gave a panel
of 4-carbamoyltriazoles (13–17). Amide formation was
fast in the case of aliphatic primary amines, while heat-
ing at 40° was necessary in the case of benzylamines
(!15).
Table 1. Kd values for galectin-3 with inhibitors 8–19 and the methyl
b-glycosides of D-galactose 20, lactose 21, and LacNAc 22
Compound
Kd (lM)
Affinity relative to 20
8
9
1408
4600
147
3.2
1.0
29
10
11
12
13
14
15
16
17
18
19
20
21
22
1750
>20000
230
2.5
<0.2
19
124
107
36
42
386
571
12
7.5
1.7
16
2692
272
440029
22229
6721
1
20
64
The triazoles 8–17 and the references 18–22 were evaluat-
ed as inhibitors of galectin-3 in a fluorescence polariza-
tion assay that had been recently developed by us30,31
(Table 1). As reference compounds in the evaluation of
triazoles 8–17 against galectins were included methyl
3-benzamido-3-deoxy-1-thio-b-D-galactoside 18 and
methyl 3-(2-naphthamido)-3-deoxy-1-thio-b-D-galacto-
side 19 (Scheme 2), together with methyl b-D-galactoside
20, methyl b-lactose 21, and methyl b-LacNAc 22. The
references 18 and 19 are monosaccharide analogues of
earlier reported high-affinity 30-benzamido LacNAc-de-
rived galectin-3 inhibitors.20,21 A first and rather discour-
aging observation was that the triazole ring itself
appeared to be detrimental to the interaction with galec-
tin-3, as the monosubstituted 3-(triazol-1-yl)-galactoside
12 was virtually non-inhibitory. Fortunately, substitu-
ents at C4 of the triazole regain, as for the propyl substi-
Scheme 2. Reagents and conditions: (a) H2/C-Pd, MeOH; (b) Acyl
chloride, pyridine; (c) MeNH2 40% in H2O.
tuted one (9), and even improve the inhibitory power (8,
and 10, 11, and 13–17) compared to that of the reference
galactoside (20). In particular, a phenyl substituent (10)
or amides (13–17) at C4 of the triazole enhance the affin-
ity for galectin-3. The triazole ring of 8–17 can thus be
regarded as an Ôambivalent spacer,Õ because it decreases
inhibitory potency, but places its C4-substituents in posi-
tions allowing for favorable interactions with galectin-3.
The affinity-enhancing effects of triazole C4-substituents
clearly surpass the affinity-decreasing effect of the tria-
zole ring itself.
The three best inhibitors, the phenyl-substituted 10, and
the butyl and benzyl amides 14 and 15, are about 40
times better than the simple methyl b-D-galactoside 20,
which indicates that the interactions between galectin-3
and the triazole C4-substituent are indeed strong. Fur-
thermore, the Kd values of 10 (147 lM), 14 (124 lM),
and 15 (107 lM) are significantly lower than those of
the methyl lactoside 21 (222 lM) and almost as low as
the LacNAc methyl glycoside 22 (67 lM). LacNAc is
the most potent natural disaccharide inhibitor of
galectin-3.18 However, it should be emphasized that
modified monosaccharide derivatives, such as the
triazoles 10, 14, and 15, have several advantages over
oligosaccharides as inhibitors of galectin-3, as they
may possess a longer half-life in vivo due to lack of
hydrolytically labile glycosidic bonds, they are often eas-
ier to prepare, and they are typically less polar, which
could improve cell membrane permeability. Finally, sev-
eral triazole-derivatives (10 and 13–15) are better than
the C3-naphthamido reference compound 19. The C3-
naphthamido group has been reported to be the most
efficient affinity-enhancing C3-substituent on LacNAc
derivatives,21 much better than natural saccharides,
Scheme 1. Reagents and conditions: (a) Cu(I), DIPEA, toluene, 40 °C;
(b) Propiolic acid, toluene, 80 °C; (c) NaOMe/MeOH; (d) MeNH2 40%
in H2O; (e) RNH2, MeOH.

3346
B. A. Salameh et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3344–3346
9. Hsu, D. K.; Liu, F. T. Glycoconjugate J. 2004, 19, 507.
10. Lahm, H.; Andre, S.; Hoeflich, A.; Kaltner, H.; Siebert,
H. C.; Sordat, B.; von der Lieth, C.-W.; Wolf, E.;
Gabius, H.-J. Glycoconjugate J. 2004, 20, 227.
and the 4-substituted triazole moiety thus appears to be
promising as an even more potent affinity-enhancing
structural motif at galactose C3.
11. Leffler, H. (Ed.), Glycoconjugate J. 2004, 19, 433.
12. Almkvist, J.; Karlsson, A. Glycoconjugate J. 2004, 19, 575.
13. Hirashima, M.; Kashio, Y.; Nishi, N.; Yamauchi, A.;
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Glycoconjugate J. 2004, 19, 593.
14. Sato, S.; Nieminen, J. Glycoconjugate J. 2004, 19, 583.
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Jarvis, G. A. Clin. Cancer Res. 2003, 9, 2374.
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V.; Raz, A. J. Natl. Cancer Inst. 1995, 87, 348.
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13047.
In conclusion, we have developed a new class of
3-[1,2,3]-triazol-1-yl galactosides as highly potent galec-
tin-3 inhibitors. The inhibitors are the best monosaccha-
ride inhibitors of galectin-3 known to date and are
distinguished by their easy synthesis, high stability,
and large potential for further improvement by optimiz-
ing the triazole substituents and by attaching carbohy-
drates or other structures to the galactose residue.
Thus, the triazoles 8–17 are promising lead structures
for the development of high-affinity galectin-3 inhibitors
with potential as tools for studies of galectin-3 functions
in vivo and as galectin-3 blocking drugs.
Acknowledgments
20. So¨rme, P.; Qian, Y.; Nyholm, P.-G.; Leffler, H.; Nilsson,
U. J. ChemBioChem 2002, 3, 183.
This work was supported by the Swedish Research Coun-
cil and by the programs ÔChemistry for the Life SciencesÕ
and ÔGlycoconjugates in Biological SystemsÕ sponsored
by the Swedish Foundation for Strategic Research.
21. So¨rme, P.; Arnoux, P.; Kahl-Knutsson, B.; Leffler, H.;
Nilsson, U. J.;Rini, J. M. J. Am. Chem. Soc. 2005, 127, 1747.
22. Wamhoff, H. In Comprehensive Heterocyclic Chemistry;
Katritzky, A. R., Ress, C. W., Potts, KT, Eds.; Pergamon:
Oxford, 1984; Vol. 5, p 669.
23. Huisgen, R. In 1,3-Dipolar Cycloaddition Chemistry;
Padwa, A., Ed.; Wiley: New York, 1984, p 1.
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