A marked synergistic effect in antitumor activity of salan titanium(IV) complexes bearing two differently substituted aromatic rings

Article abstract of DOI:10.1021/ja208219f

Salan titanium(IV) complexes of differently substituted aromatic rings, where one ring is para-nitrated and another is ortho,para-halogenated, demonstrate exceptionally high anticancer activity, with IC₅₀ values of <1 μM, exceeding that of cisplatin by ～30-fold. Whereas an additive effect in hydrolytic stability was detected for these highly stable complexes, an unexpected synergistic effect in anticancer activity makes these hybrid complexes substantially more active than both their symmetrical analogues alone and their equimolar mixture.

Full text of DOI:10.1021/ja208219f

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A Marked Synergistic Effect in Antitumor Activity of Salan Titanium(IV)
Complexes Bearing Two Differently Substituted Aromatic Rings
Hagai Glasner and Edit Y. Tshuva*
Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, 91904 Israel
S Supporting Information
b
Scheme 1
ABSTRACT: Salan titanium(IV) complexes of differently
substituted aromatic rings, where one ring is para-nitrated
and another is ortho,para-halogenated, demonstrate excep-
tionally high anticancer activity, with IC50 values of <1 μM,
exceeding that of cisplatin by ∼30-fold. Whereas an additive
effect in hydrolytic stability was detected for these highly
stable complexes, an unexpected synergistic effect in anti-
cancer activity makes these hybrid complexes substantially
Scheme 2
more active than both their symmetrical analogues alone
and their equimolar mixture.
he discovery oftheanticancertherapeuticpropertiesofcisplatin
initiated wide research on complexes of other transition
T
metals.1À7 In particular, the titanium(IV) complexes budotitane
((bzac)2Ti(OEt2)), titanocene dichloride (Cp2TiCl2) and their
derivatives demonstrated high anticancer activity toward a range of
cell lines with relatively minor toxicity, where the main disadvantage
is in their relatively rapid hydrolysis.8À19 We have recently intro-
duced the cytotoxic C2-symmetrical TiIV complexes that include
diaminobis(phenolato) “salan” ligands (Scheme 1).20À24 Leading
compounds demonstrate high hydrolytic stability in mixed organic/
water solutions with markedly higher anticancer activity than those of
(bzac)2Ti(OiPr)2, Cp2TiCl2, and cisplatin toward several cell lines.
In addition, structural parameters of the ligands, including different
substitutions on the aromatic rings, substantially influenced the
complex features. For instance, ortho-halogenation increased both
the cytotoxicity and hydrolytic stability,20 whereas nitro substituents
enhanced solubility in a biological environment. Herein we present
C1-symmetrical salan TiIV “hybrid” complexes bearing different
substituents on the two aromatic rings of the ligand, demonstrating
exceptionally high antitumor activity with a pronounced inherent
synergistic effect.
the nitrated benzylchloride.25,26 1H NMR verified that the
desired Cs-symmetrical ligands were obtained based, for instance,
on five different signals for the aromatic region. L1,2Ti(OiPr)2
and L1,3Ti(OiPr)2 (Scheme 2) were synthesized quantitatively
1
by reacting H2L1,2 and H2L1,3 with Ti(OiPr)4. The H NMR
features are consistent with the formation of C1-symmetrical
complexes, with two different septet signals of the isopropoxo
groups and AB doublets of the methylene protons. All C2-
symmetrical analogues (Scheme 2) were prepared as previously
described.20,30À32
Single crystals of L1,2Ti(OiPr)2 suitable for X-ray crystal-
lography were obtained from dichloromethane. The X-ray
structure (Figure 1) exhibits similar features both to those of
related C1-symmetrical ZrIV complexes with differently substi-
tuted aromatic rings25,26 and to those of C2-symmetrical TiIV
complexes previously reported.20À22,30,33À37 The octahedral
metal center is bound to the asymmetrical L1,2 ligand with a
trans configuration of the phenolato groups, leaving the two
additional isopropoxo ligands in a cis geometry. This indicates
substantial similarity in the structure of the hybrid complex to
that of its symmetrical analogues.
To broaden the range of structural influence on the properties
of the TiIV center in the salan complexes, we explored the
cytotoxicity of asymmetric complexes, prepared similarly to
known ZrIV catalysts,25,26 relative to their two symmetrical
analogues alone and to the combination of these analogues,
especially as synergistic effects in combination anticancer therapy
are not uncommon.27À29 In an attempt to combine high activity
and stability with improved solubility, complexes halogenated on
one ring and nitrated on the other were prepared (Scheme 2).
H2L1,2 and H2L1,3 (Scheme 2) were synthesized based on
known procedures by reacting the halogenated salicylaldehyde
1
Hydrolytic stability was assessed by H NMR, adding 10%
D2O to a THF-d8 solution of the complex, as reported for
Received: August 31, 2011
Published: October 03, 2011
ꢀ
with N,N-dimethylethylenediamine, followed by reaction with
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja208219f J. Am. Chem. Soc. 2011, 133, 16812–16814
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Figure 1. ORTEP drawing of L1,2Ti(OiPr)2 in 50% probability ellip-
soids; H-atoms omitted for clarity. Selected bond lengths (Å) and angles
(deg): Ti(1)ÀN(1) 2.362(2), Ti(1)ÀO(1) 1.920(2), Ti(1)ÀO(5)
1.772(2). O(1)ÀTi(1)ÀO(2) 161.93(8), N(1)ÀTi(1)ÀN(2) 76.18(8),
O(5)ÀTi(1)ÀO(6) 106.4(1).
Figure 3. Dependence of HT-29 cell viability on administered con-
centration of L1,3Ti(OiPr)2 and comparison to its C2-symmetrical
analogues and their combination (1:1 mixture; concentration deter-
mined per TiIV center), following 72 h of incubation based on 3 Â 3
repetitions.
Table 1. Relative IC50 (μM) Valuesa for L1,2Ti(OiPr)2 and
L
1,3Ti(OiPr)2 on HT-29 and OVCAR-1 Cells and Compar-
ison to Analogous C2-Symmetrical Complexes, Their Com-
binations, and Cisplatin
Complex
HT-29 (μM)
OVCAR-1 (μM)
L1,2Ti(OiPr)2
0.7 ( 0.4
3.3 ( 0.3
2.8 ( 0.7
2.4 ( 0.9
0.9 ( 0.3
inactive
0.7 ( 0.4
2.5 ( 0.9
3.0 ( 0.7
1.8 ( 0.7
0.9 ( 0.4
inactive
L
L
L
L
L
L
1,1Ti(OiPr)2
2,2Ti(OiPr)2
1,1/L2,2Ti(OiPr)2
1,3Ti(OiPr)2
3,3Ti(OiPr)2
1,1/L3,3Ti(OiPr)2
b
b
Figure 2. Dependence of HT-29 cell viability on administered con-
centration of L1,2Ti(OiPr)2 and comparison to its C2-symmetrical
analogues and their combination (1:1 mixture; concentration deter-
mined per TiIV center), following 72 h of incubation based on 3 Â 3
repetitions.
3.1 ( 0.6
20 ( 2
2.4 ( 0.6
13 ( 1
Cisplatin39
a Error values are based on standard deviations. b 1:1 mixture of the two;
concentration determined per TiIV center.
symmetrical derivatives.20,21 The rates of isopropoxo hydrolysis
obtained for the hybrid complexes were, as expected, between
those of the corresponding C2-symmetrical complexes, with a
t1/2 value of 10 h for L1,2Ti(OiPr)2 (relative to 1.3 and 110 h for
L1,1Ti(OiPr)2 and L2,2Ti(OiPr)2, respectively)20 and 17 h for
L1,3Ti(OiPr)2 (relative to 1.3 and 110 h). Presuming a salan-
bound polynuclear complex forming under these conditions as
observed for the symmetrical analogues,20,21,36 this behavior is
apparently a consequence of combined steric and electronic
influences on the metal center.
concentration, where participation of both compounds in activity
is apparent, with a markedly less pronounced synergistic effect.
The higher efficiency of the hybrid TiIV salan complexes as
anticancer agents may be attributed to several possible para-
meters: (a) the particular symmetry is of importance to the
interaction with the biological target, where the hybrid complex
exhibits an overall different structure with different electronic and
steric features; (b) formation of a different polynuclear salan-
bound complex is expected upon hydrolysis, which as we
previously reported might be involved in the cytotoxicity
mechanism;20,21,36 this may also explain the observation with
the combinations of symmetrical analogues where heteroligated
clusters may form; (c) indirect parameters affecting delivery to
the target, such as cell penetration and solubility, might be
of influence, although, at the low concentrations applied to
establish the high cytotoxicity, no solubility problems were
detected. In fact, an additional related advantage of the hybrid
complexes is their enhanced solubility in DMSO, which is of
particular interest to therapeutic applications, adding to their
promise as new anticancer agents. Altogether, these observa-
tions further emphasize the importance of fine-tuning structur-
al parameters of the ligand to achieve high anticancer activity,
and the ligand involvement in the valuable cellular interactions,
all of which should encourage continuous development of
improved systems.
The cytotoxicity of the complexes was studied on colon HT-29
(Figures 2, 3) and ovarianOVCAR-138 cancer cell lines, employing
the methylthiazolyldiphenyltetrazolium bromide (MTT) assay.20
The IC50 values are summarized in Table 1.
It is evident that the cytotoxic activity of both C1-symmetrical
compounds on the cells analyzed is especially high, with IC50
values corresponding to activity exceeding that of cisplatin by up
to 30-fold. Most importantly, the cytotoxicity is significantly
higher than that of each of the C2-symmetrical analogues
(Figures 2, 3), marking a synergistic rather than additive effect
of the two parts of the molecule. It is particularly interesting that,
for L1,3Ti(OiPr)2, a marked enhancement of activity relative to
the nitrated symmetric analogue L1,1Ti(OiPr)2 is observed,
despite the complete inactivity of the brominated analogue
L3,3Ti(OiPr)2 on the cells tested (Figure 3), which is probably
a result of increased steric effects.20,21 Of further interest
is the comparison to the performance of the 1:1 mixtures of
C2-symmetrical analogues to give an identical final TiIV
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’ ASSOCIATED CONTENT
(30) Chmura, A. J.; Davidson, M. G.; Jones, M. D.; Lunn, M. D.;
Mahon, M. F.; Johnson, A. F.; Khunkamchoo, P.; Roberts, S. L.; Wong,
S. S. F. Macromolecules 2006, 39, 7250.
(31) Gendler, S.; Segal, S.; Goldberg, I.; Goldschmidt, Z.; Kol, M.
Inorg. Chem. 2006, 45, 4783.
S
Supporting Information. Crystallographic data for
b
L1,2Ti(OiPr)2 (CCDC 838323), experimental details, and cyto-
toxicity plots for OVCAR-1 cells. This material is available free of
charge via the Internet at http://pubs.acs.org.
(32) Immel, T. A.; Debiak, M.; Groth, U.; Burkle, A.; Huhn, T.
ChemMedChem 2009, 4, 738.
(33) Balsells, J.; Carroll, P. J.; Walsh, P. J. Inorg. Chem. 2001, 40,
5568.
’ AUTHOR INFORMATION
(34) Groysman, S.; Sergeeva, E.; Goldberg, I.; Kol, M. Eur. J. Inorg.
Chem. 2005, 2005, 2480.
(35) Manna, C. M.; Tshuva, E. Y. Dalton Trans. 2010, 39, 1182.
(36) Meker, S.; Manna, C. M.; Peri, D.; Tshuva, E. Y. Dalton Trans.
2011, 40, 9802.
Corresponding Author
tshuva@chem.ch.huji.ac.il
’ ACKNOWLEDGMENT
(37) Yeori, A.; Groysman, S.; Goldberg, I.; Kol, M. Inorg. Chem.
We thank Dr. Shmuel Cohen for Crystallography. This research
received funding from the European Research Council under the
European Community’s Seventh Framework Programme (FP7/
2007-2013)/ERC Grant Agreement No. 239603 and partially
from the Israel Science Foundation (Grant No. 124/09).
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(38) See Supporting Information.
(39) Tzubery, A.; Tshuva, E. Y. Inorg. Chem. 2011, 50, 7946.
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