Novel vitamin D receptor ligands bearing a spherical hydrophobic core structure-Comparison of bicyclic hydrocarbon derivatives with boron cluster derivatives

Article abstract of DOI:10.1016/j.bmcl.2011.12.137

Vitamin D receptor (VDR) is a nuclear receptor for 1a,25-dihydroxyvitamin D3 (1a,25(OH)2D3), and is an attractive target for multiple clinical applications. We recently developed novel non-secosteroidal VDR ligands bearing a hydrophobic p-carborane cage,

Full text of DOI:10.1016/j.bmcl.2011.12.137

Bioorganic & Medicinal Chemistry Letters 22 (2012) 1756–1760
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel vitamin D receptor ligands bearing a spherical hydrophobic core
structure—Comparison of bicyclic hydrocarbon derivatives with boron
cluster derivatives
Angsuma Wongmayura a, Shinya Fujii b, Shigeru Ito b, Atsushi Kano b, Yoshiyuki Taoda c, Emiko Kawachi b,
Hiroyuki Kagechika b, Aya Tanatani a,
⇑
a Department of Chemistry, Faculty of Science, Ochanomizu University, 2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan
b Graduate School of Biomedical Science, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10, Kanda-Surugadai,
Chiyoda-ku, Tokyo 101-0062, Japan
c Graduate School of Pharmaceutical Sciences, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Vitamin D receptor (VDR) is a nuclear receptor for 1a,25-dihydroxyvitamin D3 (1a,25(OH)2D3), and is an
Received 25 November 2011
Revised 15 December 2011
Accepted 16 December 2011
Available online 10 January 2012
attractive target for multiple clinical applications. We recently developed novel non-secosteroidal VDR
ligands bearing a hydrophobic p-carborane cage, thereby establishing the utility of this spherical hydro-
phobic core structure for development of VDR ligands. Here, we synthesized two series of novel non-secos-
teroidal VDR ligands with different spherical hydrophobic cores, that is, bicyclo[2.2.2]octane derivatives
and p-carborane derivatives, and compared their biological activities in order to examine the difference
between the interactions of the C–H hydrocarbon surface and the B–H carborane surface with the receptor.
Carborane derivatives exhibited more potent differentiation-inducing activity toward HL-60 cells than did
the corresponding bicyclo[2.2.2]octane derivatives. These results suggest that the hydrophobic carborane
cage may interact more efficiently than the hydrocarbons with the hydrophobic surface of VDR. This find-
ing further supports the view that carborane structure is a promising option for drug development.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Vitamin D
Carborane
Pharmacophore
Cell differentiation
Vitamin D receptor (VDR) is the ligand-inducible nuclear recep-
tor for vitamin D,1 and is involved in many physiological processes,
including calcium and phosphate homeostasis, bone metabolism,
immune regulation, cell proliferation and differentiation.2 VDR is
steroid hormones, as well as non-secosteroidal VDR ligands with
unique biological activities, such as bisphenol derivative 4,4 have
been developed in the last decade.
Previously, we obtained novel non-secosteroidal VDR ligands by
using dicarba-closo-dodecaboranes (carboranes) as the hydropho-
bic pharmacophore.5 Carboranes are carbon-containing boron
clusters, and have noteworthy chemicophysical characteristics,
including spherical geometry and a hydrophobic B–H surface.6,7
Carboranes can be used as the hydrophobic core of biologically ac-
tive molecules, especially nuclear receptor ligands, such as andro-
gen receptor ligands,8 estrogen receptor ligands9 and retinoids.10
We developed a potent non-secosteroidal VDR ligand 5 using
p-carborane (1,12-dicarba-closo-dodecaborane), instead of the CD
ring of the secosteroid structure of 1. X-ray crystallographic analy-
sis of the VDR ligand-binding domain (LBD) complexed with 5
revealed that the carborane cage of 5 is surrounded by several
hydrophobic amino acid residues in the ligand-binding pocket,
and thus, the spherical B–H surface of carborane interacts
effectively with hydrophobic sites at the protein surface.5 There-
fore, in the present work, we planned to synthesize and compare
two series of novel VDR ligands: one in which the spherical hydro-
phobic core is a hydrocarbon derivative and the other in which it is
a carborane derivative, in order to investigate the difference
activated by binding of its endogenous agonist, 1
a,25-dihydroxyvi-
tamin D3 [1 ,25(OH)2D3: 1] (Fig. 1), and regulates expression of
a
specific target genes. VDR and its ligands have significant roles in
the pathogenesis and therapy of diseases such as osteoporosis,
arthritis, psoriasis and cancers. Thus, VDR is an attractive target
for drug discovery, and thousands of VDR ligands have been devel-
oped, leading to the clinical application of eldecalcitol (2) and
maxacalcitol (3).3 Most of the reported synthetic derivatives with
high potency as VDR ligands possess the secosteroidal skeleton,
like 1, consisting of the A-ring bearing two hydroxyl groups, a con-
jugated olefin linker, the CD-ring and a side chain. Although mod-
ification of the secosteroid structure has provided highly active
analogs, their structural complexity, synthetic inconvenience and
chemical instability are disadvantageous for potential clinical
application. Therefore, development of novel non-secosteroidal
VDR ligands is important, and various non-steroidal analogs of
⇑
Corresponding author. Tel./fax: +81 3 5978 2716.
E-mail address: tanatani.aya@ocha.ac.jp (A. Tanatani).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.12.137

A. Wongmayura et al. / Bioorg. Med. Chem. Lett. 22 (2012) 1756–1760
1757
Figure 1. Structures of VDR ligands.
between the interactions of the C–H surface of hydrocarbons and
the B–H surface of carborane with the receptor.
dienes 21 and 22, respectively. Hydrogenation, followed by
Grignard reaction, gave diethylcarbinol 25 and 26, and removal
of the benzylidene group under acidic conditions gave the target
compounds 6 and 7, respectively. The 1,2-diol derivative 8 was
synthesized by a similar method to that used for synthesis of 6
and 7. Ether formation of 13 using tosylate 27 corresponding to
the 1,2-diol moiety afforded 28, and oxidation of alcohol gave alde-
hyde 29. Then, the Horner–Wadsworth–Emmons type reaction
afforded diene 30, followed by hydrogenation, Grignard reaction
and removal of the acetonide group to give the target compound
8, although the yield was low due to the formation of by-product
caused by ether cleavage reaction (Scheme 1).
The synthesis of the carborane derivatives is summarized in
Scheme 2. Reaction of the C-lithiated form of carborane13 and para-
formaldehyde gave carboranylmethanol 34, and ether formation
using tosylates 14 or 15 afforded 35 and 36, respectively. The
side-chain moiety was introduced at the other carbon atom of car-
borane using bromide 37 to afford triol precursors 38 and 39,
respectively. Finally, removal of protective groups under acidic
conditions gave the target compounds 9 and 10. The 1,2-diol deriv-
ative 11 was similarly synthesized. Ether formation of 34 using tos-
ylate 27 afforded 40, and introduction of the side-chain part gave
41. Then, removal of protective groups under acidic conditions
gave the target compound 11 (Scheme 2).
Four structural elements are required for effective binding to
VDR; one is a hydrophobic core with appropriate bulkiness, and
the others are three appropriately positioned hydroxyl groups, as
in 1. As a spherical hydrocarbon core structure of novel VDR ligand
candidates, we chose the bicyclo[2.2.2]octane skeleton, based on
its spatial volume and its symmetric structure, resembling that of
p-carborane. As a side chain structure, we chose diethylcarbinol,
as in 5, in place of dimethylcarbinol, as in 1, based on reports that
the diethylcarbinol moiety enhances vitamin D potency.11 Our pre-
vious study indicated that the 1,3-diol moiety of 5 corresponds to
the two hydroxyl groups of the A-ring of 1, and the 1,2-diol struc-
ture can also serve this purpose. Therefore, we designed bicy-
clo[2.2.2]octane derivatives with a 1,2- or 1,3-dihydroxylalkoxyl
side chain 6–8, as well as the corresponding p-carborane deriva-
tives 9–11 (Fig. 2). The designed derivatives have the ether oxygen
atom at a different position from the spherical core structure, com-
pared to compound 5. X-ray analysis of the complex of the VDR–
LBD with 5 suggested that the ether oxygen atom of 5 does not
form a hydrogen bond to amino acid residues of the receptor,
and so the position of the oxygen atom may not affect the vitamin
D potency. We anticipated that examination of the structure–activ-
ity relationships of compounds 6–11 would clarify this point.
Scheme 1 summarizes the synthetic route to the designed
bicyclo[2.2.2]octane derivatives using 1,4-bisethoxycarbonylbicy-
clo[2.2.2]octane 1212 as the starting material. Reduction of 12 gave
diol 13, and ether formation using tosylates 14 or 15 corresponding
to the 1,3-diol moiety afforded 16 and 17, respectively. Oxidation
of alcohol gave aldehydes 18 and 19, and then Horner–Wads-
worth–Emmons type reaction using phosphonate 20 afforded
Vitamin D activity of the synthesized molecules was evaluated
in terms of cell differentiation-inducing activity toward human
acute promyelocytic leukemia cell line HL-60.14 The bicy-
clo[2.2.2]octane derivatives exhibited HL-60 cell differentiation-
inducing activity at the concentration of 10À5 M (Fig. 3A), and
the most potent compound 7, bearing 1,3-diol structure, exhibited
moderate activity at the concentration of 10À6 M. The activity of
compound
7 was weaker than that of 1 by approximately
two orders of magnitude. Compound 6 bearing a shorter 1,3-
dihydroxylalkoxyl structure than that of 7 and compound 8 with
1,2-diol structure exhibited lower potency than did 7. On the other
hand, the corresponding carborane derivatives exhibited more po-
tent activity in HL-60 cell assay (Fig. 3B). Compound 10 with 1,3-
diol structure exhibited the highest HL-60 cell differentiation-
inducing potency among the three compounds. The potency of
10 was approximately one-tenth of that of 1, and 10 is one of the
most potent non-secosteroidal VDR ligands so far discovered. Com-
pound 11 with 1,2-diol structure also exhibited potent activity,
while compound 9, bearing a shorter 1,3-dihydroxylalkoxyl struc-
ture than that of 10, showed lower potency.
Figure 2. Structures of novel VDR ligand candidates bearing a spherical hydropho-
bic core.

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A. Wongmayura et al. / Bioorg. Med. Chem. Lett. 22 (2012) 1756–1760
Scheme 1. Synthesis of bicyclo[2.2.2]octane derivatives.
The synthesized vitamin
D
analogs containing bicy-
is in accord with our previous report on the structure–activity
clo[2.2.2]octane or p-carborane as the spherical hydrophobic core
structure were found to exhibit differentiation-inducing activity
toward HL-60 cells. Thus, the bicyclo[2.2.2]octane skeleton can
function as the hydrophobic core structure of VDR ligands, as can
p-carborane. However, the potencies of the compounds in the two
series were quite different. The carborane derivatives exhibited
higher activity than the corresponding bicyclo[2.2.2]octane deriva-
tives by about one order of magnitude. It is noteworthy that com-
pounds bearing similar spherical hydrophobic core structures and
similarly directed substituents exhibit such different potency. This
implies that the hydrophobic B–H surface of the p-carborane cage
functions as a better anchor to the surface of VDR, compared with
the hydrophobic C–H surface of the bicyclo[2.2.2]octane skeleton.
The slight difference of spatial volume between these two spherical
cores might also influence the ligand activity. In both series, the 3,5-
dihydroxypentoxymethyl derivatives, namely 7 and 10, exhibited
the highest activity. The 1,3-diol structure and seven-atom separa-
tion between the terminal hydroxyl group and the core structure
seem to be the most favorable for VDR agonistic activity, and this
relationships of 5.5 Interestingly, compound 10 exhibited potent
activity, similar to or greater than that of 5, though still only about
one-tenth or one-twentieth of that of 1. These results suggest
that the position of the ether oxygen atom is not important for the
ligand potency. These findings are expected to be useful for further
structural development of non-secosteroidal VDR ligands bearing
novel hydrophobic core structures.
In conclusion, we have synthesized two series of novel non-secos-
teroidal VDR ligands bearing different kinds of spherical hydrophobic
core structure, that is, bicyclo[2.2.2]octane and p-carborane, and dem-
onstrated that both structures can function as the hydrophobic anchor
of VDR ligands. Although corresponding compounds in the two series
have similar spatial volume and similarly directed substituents, their
VDR agonistic potency was quite different. Our results suggest that
p-carborane is superior to bicyclo[2.2.2]octane as a hydrophobic core
of VDR ligands. The precise nature of the difference between these
two hydrophobic structures is under investigation. The results re-
ported here are expected to contribute to the further development
of non-secosteroidal VDR ligands.

A. Wongmayura et al. / Bioorg. Med. Chem. Lett. 22 (2012) 1756–1760
1759
Scheme 2. Synthesis of carborane derivatives.
Figure 3. HL-60 cell differentiation-inducing potency of the synthesized bicyclo[2.2.2]octane derivatives (A) and carborane derivatives (B) in the concentration range of 10À9
to 10À5 M. Cell differentiation was determined as the ratio of NBT-positive cells.
2. Vitamin D; Feldman, D., Pike, J. W., Glorieux, F. H., Eds.; Elsevier Academic Press,
Acknowledgments
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3. Kubodera, N. Development of OCT and ED-71. In Vitamin D; Feldman, D., Pike, J.
This study was partially supported by Grants-in-Aid for Scien-
tific Research from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan (Grant Nos. 21790110 to S.F.,
19201044 to H.K., 19689004 to A.T.). A.T. thanks the Takeda Sci-
ence Foundation and the Mitsubishi Foundation for financial
support.
W., Glorieux, F. H., Eds.; Elsevier Academic Press, 2005; p 1525. second ed..
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