Conjugates of polyhedral boron compounds with carbohydrates 1. New approach to the design of selective agents for boron neutron capture therapy of cancer

Full text of DOI:10.1023/B:RUCB.0000019902.98142.33

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Russian Chemical Bulletin, International Edition, Vol. 52, No. 12, pp. 2766—2768, December, 2003
Conjugates of polyhedral boron compounds with carbohydrates
1. New approach to the design of selective agents
for boron neutron capture therapy of cancer
b
b
A. V. Orlova,a A. I. Zinin,a N. N. Malysheva,a L. O. Kononov,a I. B. Sivaev, and V. I. Bregadze
ꢀ
аN. D. Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5328. Eꢀmail: kononov@ioc.ac.ru
bA. N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5085. Eꢀmail: bre@ineos.ac.ru
Boron neutron capture therapy (BNCT) of cancer is a
glycosides containing a triple bond in the aglycone
and the subsequent addition of decaborane(14) to give
ωꢀ(orthoꢀcarboranyl)alkylglycosides (derivatives of a numꢀ
ber of monosaccharides and simple disaccharides includꢀ
ing lactose have been described). The main drawback of
this approach is the lack of flexibility and versatility for
the preparation of a large set of neoglycoconjugates
because of the neccesity of performing the nonꢀ
trivial glycosylation and the addition of highly toxic deꢀ
caborane(14) for each alkynol.
Here we report a new approach to the preparation of
conjugates of orthoꢀcarboranes with the carbohydrate
ligands of lectins. It is based on the use of prespacer
strategy,4 which is highly flexible and efficient and allows
reliable preparation of large sets (libraries) of neoꢀ
glycoconjugates from only one (oligo)saccharide glycoꢀ
side with a functional group in the terminal position of a
rather simple (short) prespacer aglycone (Scheme 1). Then
spacers of an appropriate structure are introduced into the
aglycone (the introduction of a sufficiently long spacer
between the carbohydrate part and the relatively bulky
carborane cage is needed to ensure the accessibility of the
carbohydrate fragment of the neoglycoconjugate for the
interaction with lectin), and the resulting set of spacered
binary (chemoꢀradiotherapeutic) method for the treatꢀ
ment of cancer based on the introduction of the stable
10B isotope into a tumor. Subsequent irradiation of the
tumor by a flux of thermal neutrons gives rise to highꢀ
energy fission products with a path length comparable
with cell dimensions, which allows selective destruction
of the tumor cells without affecting the surrounding
healthy tissue.1 The secondꢀgeneration BNCT agents used
currently in clinical practice do not exhibit the required
high selectivity of accumulation in the tumor. A way of
increasing the selectivity of BNCT agents may be the use
of targeted delivery of boron compounds to the tumor
cells, which may be based, for example, on carboꢀ
hydrate—protein interactions. Endogenous lectins (recepꢀ
tors of the protein nature) located on the surface of many
normal and tumor cells function as specific receptors and
are mediators in the carbohydrateꢀspecific endocytosis of
(neo)glycoconjugates.2 Conjugates of polyhedral boron
compounds with carbohydrates representing ligands of
the lectins that are expressed on the surface of cancer cells
can serve as promising agents for BNCT.3а A known apꢀ
proach3 to the conjugation of carbohydrates with polyheꢀ
dral boron compounds is based on the preparation of
Scheme 1
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2617—2620, December, 2003.
1066ꢀ5285/03/5212ꢀ2766 $25.00 © 2003 Plenum Publishing Corporation

Sugar—carborane conjugates
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 12, December, 2003 2767
Scheme 2
Reagents, conditions, and yields: a. HOCH2CH2Cl, Hg(CN)2, refluxing, Ar, yield 59%; b. HOCH2CH2Cl, SnCl4, MeCN, Ar,
yield 50%; c. NaN3, 18ꢀcrownꢀ6, DMF, ∆, yield 95%; d. NaBH4, H3BO3, NiCl2•6H2O, EtOH, yield 71%; e. BocNH(CH2)5COOH,
DCC, Et3N, yield 55%; f. (1) TFA, (2) Et3N, yield ∼100%.
(oligo)saccharides is used for linking with the fragments
to be conjugated (in this particular case, carboranes).
As a representative example, we chose the conjugates
of 1,2ꢀdicarbaꢀclosoꢀdodecaborane(12) (orthoꢀcarborane)
with the disaccharide lactose, which is the ligand of lectins
that are expressed on the surface of melanoma cells.5
Lactose derivatives with a free amino group in the
aglycone were synthesized from acetobromolactose (1a)6
or from lactose octaacetate (1b);7 the key steps of the
synthesis included glycosylation8a of 2ꢀchloroethanol and
subsequent introduction8 of the azido group into the aglyꢀ
cone of the lactoside 2a (Scheme 2). The reduction of the
azido function in lactoside 2b by nickel boride generꢀ
ated in situ furnished the desired selectively protected
2ꢀaminoethyl lactoside 2с. Lactoside 3a with an elonꢀ
gated spacer was prepared by condensation of amine 2с
with NꢀBocꢀ6ꢀaminohexanoic acid in the presence of
DCC (yield 55%). Removal of the Boc group in 3a by
treatment with TFA smoothly gave the target amine 3b.
The condensation of amines 2c and 3b with an actiꢀ
vated ester of carboranylacetic acid prepared in situ from
acid 4 9 (Nꢀhydroxysuccinimide (NHS), DCC, THF) reꢀ
sulted in acetylated conjugates 5 and 6 in 19 and 32%
yields, respectively (Scheme 3). The Oꢀacetyl protective
groups present in conjugates 5 and 6 can, apparently, be
easily removed by treatment with MeONa in MeOH,
as has been carried out previously3 for other carboꢀ
rane—carbohydrate conjugates.
The synthesized compounds were isolated by chromatograꢀ
phy on silica gel and characterized by 1H, 13C, and 11B NMR
spectroscopy (Bruker ACꢀ200, CDCl3), IR spectroscopy
(Specord Mꢀ80, thin film from a solution in CDCl3), and mass
spectrometry (Finnigan MAT LCQ, electrospray (ESI)).
2ꢀChloroethyl 2,3,6ꢀtriꢀOꢀacetylꢀ4ꢀOꢀ(2,3,4,6ꢀtetraꢀOꢀ
acetylꢀβꢀDꢀgalactopyranosyl)ꢀβꢀDꢀglycopyranoside (2a), 2ꢀazidoꢀ
ethyl 2,3,6ꢀtriꢀOꢀacetylꢀ4ꢀOꢀ(2,3,4,6ꢀtetraꢀOꢀacetylꢀβꢀDꢀ
galactopyranosyl)ꢀβꢀDꢀglucopyranoside (2b), 2ꢀaminoethyl
2,3,6ꢀtriꢀOꢀacetylꢀ4ꢀOꢀ(2,3,4,6ꢀtetraꢀOꢀacetylꢀβꢀDꢀgalactoꢀ
pyranosyl)ꢀβꢀDꢀglucopyranoside (2c). Characteristic signals in
the 13C NMR spectra, δ: 42.3 (2a, X = Cl), 50.1 (2b, X = N3),
41.6 (2c, X = NH2) (OCH2CH2X); 100.9 and 100.7 (2a), 100.9
and 100.3 (2b), 101.0 and 100.7 (2c) (C(1), Glc and Gal,
respectively). Mass spectrum of 2a, m/z 721.0 [M + Na].
C28H39ClNaO18. Calculated: m/z 721.2 [M + Na]. IR spectrum
of 2b (thin film from a solution in CDCl3), ν/cm–1: 1752 (CO),
2108 (N3). Mass spectrum of 2b, m/z 728.2 [M + Na].
C28H39N3NaO18. Calculated: m/z 728.2 [M+Na].
2ꢀ[6ꢀ(tertꢀButoxycarbonylamino)hexanoylamino]ethyl 2,3,6ꢀ
triꢀOꢀacetylꢀ4ꢀOꢀ(2,3,4,6ꢀtetraꢀOꢀacetylꢀβꢀDꢀgalactopyranosyl)ꢀ
βꢀDꢀglucopyranoside (3a). 13C NMR, δ: 20.4, 20.5, 20.7, 20.8
(CH3CO); 25.1 (CH2CH2CH2CH2CH2); 26.3 (CH2CH2CO),
28.3 (CMe3); 29.7 (CH2CH2NH); 36.2 (CH2CH2CH2CO); 39.0
(CH2NH); 40.3 ((CH2)4CH2NH); 60.6 (C(6), Gal); 61.7 (C(6),
Glc); 66.5 (C(4), Gal); 69.0 (C(2), Gal); 69.3 (OCH2CH2NH);
70.5 (C(5), Gal); 70.8 (C(3), Gal); 71.5 (C(2), Glc); 72.5 (C(5),
Glc); 72.7 (C(3), Glc); 76.0 (C(4), Glc); 100.7 (C(1), Gal);
100.9 (C(1), Glc); 168.9, 169.6, 170.0, 170.3, 172.9 (C=O).
MS, m/z 917.2 [M + Na]. C39H62N2NaO21. Calculated: m/z
917.4 [M + Na].
2ꢀ[(1,2ꢀDicarbaꢀclosoꢀdodecaboran(12)ꢀ1ꢀyl)acetylꢀ
amino]ethyl 2,3,6ꢀtriꢀOꢀacetylꢀ4ꢀOꢀ(2,3,4,6ꢀtetraꢀOꢀacetylꢀβꢀ
Dꢀgalactopyranosyl)ꢀβꢀDꢀglucopyranoside (5). 13C NMR, δ:
20.5, 20.6, 20.8, 21.0 (CH3CO); 39.6 (CH2NH); 42.8

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Russ.Chem.Bull., Int.Ed., Vol. 52, No. 12, December, 2003
Orlova et al.
Scheme 3
Reagents and conditions: i. NHS, DCC, THF.
((C2HB10H10)CH2CO); 58.7 ((CHB10H10C)); 60.7 (C(6), Gal);
61.2 (C(6), Glc); 66.5 (C(4), Gal); 69.1 (OCH2CH2NH); 69.1
(C(2), Gal); 70.7 (C(5), Gal); 70.9 (C(3), Gal); 71.5 (C(2),
Glc); 72.5 (C(3), Glc); 73.2 (C(5), Glc); 75.6 (C(4), Glc);
100.8 (C(1), Gal); 101.0 (C(1), Glc); 166.5, 170.1 (CO).
Based on Them", and by the State Program for the Supꢀ
port of Leading Scientific Schools of the RF (Grant NSh
1557.2003.3).
References
11B{1H} NMR, δ: –2.5 (1 B); –5.6 (1 B); –9.9 (8 B). IR, ν/cm–1
1752 (CО), 2592 (BH), 3380 (NH). MS, m/z 887.0 [M + Na].
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2002, 51, 1362].
C
32H53B10NNaO19. Calculated: m/z 888.4 [M + Na].
2 ꢀ { 6 ꢀ [ ( 1 , 2 ꢀ D i c a r b a ꢀc l o s o ꢀ d o d e c a b o r a n ( 1 2 ) ꢀ 1 ꢀ
yl)acetylamino]hexanoylamino}ethyl 2,3,6ꢀtriꢀOꢀacetylꢀ4ꢀOꢀ
(2,3,4,6ꢀtetraꢀOꢀacetylꢀβ ꢀD ꢀgalactopyranosyl)ꢀβ ꢀD ꢀ
glucopyranoside (6). 13C NMR, δ: 20.5, 20.6, 20.8 (CH3CO);
23.8 (CH2CH2CH2CH2CH2); 25.7 ((CH2)3CH2CH2CO);
27.8 ((CH2)3CH2CH2NH); 35.5 ((CH2)4CH2CO); 38.9
(OCH2CH2NH);
39.3
((CH2)4CH2NH);
43.3
((C2HB10H10)CH2CO); 58.7 ([CHB10H10C]); 60.7 (C(6), Gal);
61.7 (C(6), Glc); 66.5 (C(4), Gal); 69.1 (C(2), Gal); 69.4
(OCH2CH2NH); 70.7 (C(5), Gal); 70.9 (C(3), Gal); 71.6
(C(2), Glc); 72.5 (C(5), Glc); 72.9 (C(3), Glc); 76.0 (C(4),
Glc); 100.8 (C(1), Gal); 101.0 (C(1), Glc); 170.3, 173.1
(CO). 11B{1H} NMR, δ: –2.6 (1 B), –5.5 (1 B), –10.0 (8 B).
IR, ν/cm–1
: 1752 (CО), 2593 (BH), 3444 (NH). MS,
m/z 1001.3 [M + Na]. C38H64B10N2NaO20. Calculated: m/z
1001.5 [M + Na].
The approach we propose to the synthesis of carboꢀ
rane—carbohydrate conjugates can also be used successꢀ
fully to prepare conjugates with polyhedral boron comꢀ
pounds of various structures, for example, closoꢀdodecaꢀ
borates (see a review10 dealing with their use in BNCT),
which is impossible within the framework of the previꢀ
ously described approach3 to the synthesis of carboꢀ
rane—carbohydrate conjugates based on the addition of
decaborane(14) to acetylene glycosides.
The authors are grateful to B. G. Kimel´ for recording
the mass spectra. This work was financially supported by
the Russian Foundation for Basic Research (Project No.
03ꢀ03ꢀ32622), the Program of the Presidium of the RAS,
"Directed Synthesis of Substances with Predetermined
Properties and Development of Functional Materials
Received November 27, 2003