Trans titanium(IV) complexes of salen ligands exhibit high antitumor activity

Article abstract of DOI:10.1021/ic201296h

Salen-titanium(IV) complexes are introduced as a new family of highly efficient antitumor complexes, being the first cytotoxic titanium(IV) complexes of trans labile ligands, as characterized crystallographically. Four complexes with different aromatic substitutions were analyzed, reveling a meaningful effect of the ligand structure on the complex performance. All complexes exhibit high hydrolytic stability, where the labile OAr ligands hydrolyze in a 10% D₂O solution with t_1/2 ranging from 2 to 11 h. The IC ₅₀ values obtained for three of the salen complexes studied on HT-29 colon and OVCAR-1 ovarian cells demonstrate activity that exceeds those of the known tianium(IV) complexes Cp₂TiCl₂ and (bzac) ₂Ti(OiPr)₂ and that of cisplatin, where the most active para-chlorinated complex exhibits activity enhancement relative to cisplatin by 10-fold.

Full text of DOI:10.1021/ic201296h

COMMUNICATION
pubs.acs.org/IC
Trans Titanium(IV) Complexes of Salen Ligands Exhibit High
Antitumor Activity
Avia Tzubery and Edit Y. Tshuva*
The Hebrew University of Jerusalem, Jerusalem 91904, Israel
S Supporting Information
b
Scheme 1
ABSTRACT: SalenÀtitanium(IV) complexes are intro-
duced as a new family of highly efficient antitumor com-
plexes, being the first cytotoxic titanium(IV) complexes of
trans labile ligands, as characterized crystallographically.
Four complexes with different aromatic substitutions were
analyzed, reveling a meaningful effect of the ligand structure
on the complex performance. All complexes exhibit high
hydrolytic stability, where the labile OAr ligands hydrolyze
in a 10% D2O solution with t1/2 ranging from 2 to 11 h. The
IC50 values obtained for three of the salen complexes
studied on HT-29 colon and OVCAR-1 ovarian cells
demonstrate activity that exceeds those of the known
tianium(IV) complexes Cp2TiCl2 and (bzac)2Ti(OiPr)2
and that of cisplatin, where the most active para-chlorinated
complex exhibits activity enhancement relative to cisplatin
by 10-fold.
Scheme 2
Scheme 3
ince the discovery of the cytotoxic activity of cisplatin, there is
S
increasing interest in finding additional inorganic compounds
of different metals with cytotoxic activity.1 Two titanium(IV)
complexes that reached clinical trials are titanocene dichloride
(Cp2TiCl2) and budotitane [(bzac)2Ti(OEt)2] (Scheme 1),
demonstrating cytotoxic activity toward cisplatin-resistant
and -sensitive cells with reduced toxicity relative to platinum
compounds, where modified complexes with various ligand
substitutions exhibited improved activity.2À11 The main general
disadvantage of the titanium(IV) complexes is their low stabi-
lity in aqueous environments and their tendency to undergo
rapid hydrolysis, within seconds to minutes for labile ligand
dissociation,5,7,12,13 which also impeded the medicinal utility of
Cp2TiCl2 and (bzac)2Ti(OEt)2.
showed cytotoxic activity by different binding mechanisms.20
Nevertheless, the mechanism of activity of the titanium com-
pounds is still unknown, as is the importance of this particular
structural factor.
Herein we report on the investigation of titanium(IV) com-
plexes with two labile ligands in a trans configuration, with the
aim of gaining additional information on the mechanism and
possible diversity of activity relating to diaminobis(phenolato)
complexes. We employed ligands from the salen family that are
similar in coordination features to the ligands of the salan family
but are known to afford equatorial binding due to the planar
imine moiety, leading to trans labile ligands.21 We employed
We have recently introduced a new family of cytotoxic titanium-
(IV) complexes based on diaminobis(phenolato) “salan” ligands
(Scheme 2).14À18 These complexes demonstrate exceptional
antitumor activity and hydrolytic stability, and structureÀactivity
relationship studies have revealed that the ligand significantly influ-
ences both parameters.
All titanium complexes that were investigated so far as
antitumor agents include two labile ligands in a cis configura-
tion, a feature deigned to enable chelate binding to a proper
target as occurs for cisplatin.19 Transplatin, the geometric isomer
of cisplatin, is clinically ineffective because of its inability to
perform the necessary intrastrand cross-linking to two adja-
cent bases in the DNA, although other trans-platinum complexes
Received: June 16, 2011
Published: July 28, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic201296h Inorg. Chem. 2011, 50, 7946–7948
|

Inorganic Chemistry
COMMUNICATION
Table 1. IC50 (μM) Values of Lig1À4Ti(OArMe2)2 and
Known Reference Compounds toward HT-29 and OVCAR-1
Cancer Cell Lines and Additional Reference Values
aromatic
complex
substitution
HT-29 (μM)
OVCAR-1 (μM)
Lig1Ti(OArMe2)2
Lig2Ti(OArMe2)2
Lig3Ti(OArMe2)2
Lig4Ti(OArMe2)2
p-Me
none
p-Cl
o-Cl
10 ( 2
9 ( 1
3.5 ( 0.6
1.2 ( 0.3
inactive
3.3 ( 0.5
1.0 ( 0.3
inactive
Figure 1. ORTEP drawings of Lig3Ti(OArMe2)2 (a) and Lig4Ti-
(OArMe2)2 (b). Selected bond lengths (Å) and angles (deg) for (a)
Ti(1)ÀN(1) 2.145(2), Ti(1)ÀO(1) 1.909(2), Ti(1)ÀO(3) 1.857(2),
O(1)ÀTi(1)ÀO(2) 117.18(7), N(2)ÀTi(1)ÀN(1) 73.71(7), and O-
(3)ÀTi(1)ÀO(4) 178.18(7) and for (b) Ti(1)ÀN(1) 2.164(2), Ti-
(1)ÀO(1) 1.912(2), Ti(1)ÀO(3) 1.843(2), O(1)ÀTi(1)ÀO(2)
118.32(8), N(2)ÀTi(1)ÀN(1) 73.77(9), and O(3)ÀTi(1)ÀO(4)
177.00(9).
reference
HT-29 (μM)
OVCAR-1 (μM)
cisplatin
20 ( 2
13 ( 1
Cp2TiCl2
520 ( 90
11.6 ( 0.8
550 ( 120
11.5 ( 0.2
(bzac)2Ti(OiPr)2
reference25
LLC-PK (μM)
titanocene Y
Cp2TiCl2
cisplatin
21 ( 1
2000 ( 1000
3.3 ( 0.5
the titanium(IV) complexes in high yields (Scheme 3). 1H and
13C NMR analyses of the products are consistent with the
formation of an isomer with high symmetry, with a single type
of aromatic system and a single set of labile ligand signals. Single
red crystals suitable for X-ray crystallography of Lig3,4Ti-
(OArMe2)2 were obtained from diethyl ether at room tempera-
ture. ORTEP drawings of the structures along with selected bond
lengths and angles are provided in Figure 1.
The structures of Lig3,4Ti(OArMe2)2 exhibit similar features.
Octahedral C2v-symmetrical complexes are obtained, with trans
binding of the labile dimethylphenoxo ligands, as anticipated,
characterized by an OÀTiÀO angle to labile ligands of
177À178° and an OÀTiÀO angle to the salen ligands of
117À118°. The bond lengths and angles around the metal center
are otherwise similar to those obtained for related salan complexes,
with somewhat shorter TiÀN bonds to the sp2-type donor.14,18
The cytotoxicity was studied on ovarian OVCAR-1 and colon
HT-29 cell lines as previously described,18 employing the MTT
assay for establishing cell viability. The results are depicted in
Figure 2. A summary of the IC50 values is provided in Table 1.
Lig1À3Ti(OArMe2)2 demonstrate high antitumor activity
toward HT-29 and OVCAR-1 cells, higher than that of cisplatin.
When compared to the activity of titanocene Y on other cells,25
similar and more significant improvements relative to Cp2TiCl2
and cisplatin, respectively, are obtained for the salen complexes.
Additionally, the para-chlorinated complex exhibits the highest
activity, which exceeds that of cisplatin by an order of magnitude.
It is also obvious that the structural parameters of the ligand play
an important role in the activity because the ortho-chlorinated
complex is completely inactive. Because the nonsubstituted
complex is more active than the para-methylated one and the
para-chlorinated complex is more active than all in this series, we
may generally suggest that steric effects that are significantly
more pronounced at the ortho positions reduce cytotoxicity and
the effect of the p-chloro substituent is apparently electronic.
Compared to the family of cis-salan complexes,14,18 it is apparent
that the IC50 values obtained for the salens are comparable to
those obtained for the salans, despite the different geometries,
different diamino bridges, and different labile ligands. It is
Figure 2. Dependence of OVCAR-1 (top) and HT-29 (bottom) cell
viability based on the MTT assay following a 3 day incubation period
upon added concentration of Lig1À4Ti(OArMe2)2 presented on a
logarithmic scale.
“salophen” ligands that include a planar phenylenediamine
bridge (Scheme 3),22 which were more efficient than ethylene-
diamine in producing pure trans products. High cytotoxicity is
obtained, identifying salenÀtitanium(IV) complexes as a new
family of antitumor agents and as the first trans-titanium(IV)
complexes with cytotoxic activity.
Ligands H2Lig1À4 were prepared based on a published
procedure from differently substituted salicylaldehyde com-
pounds and phenylenediamine.23 Because of the difficulty in
isolating clean isopropoxo complexes of these ligands starting
from Ti(OiPr)4 and previous observations suggesting that labile
ligands are of lesser influence on cytotoxicity,7 Ti(OArMe2)4 was
employed instead.24 Thus, Lig1À4Ti(OArMe2)2 were synthe-
sized by reacting H2Lig1À4 with 1 equiv of Ti(OArMe2)4 to give
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Inorganic Chemistry
COMMUNICATION
interesting, however, that the ortho-chlorinated substitution that
yielded an especially active salan complex gave no activity for the
corresponding salen compound, rendering the ortho steric effect
as more meaningful for the salen family of complexes.
’ ACKNOWLEDGMENT
We thank Dr. Shmuel Cohen for crystallography. This re-
search received funding from the European Research Council
under the European Community’s Seventh Framework Pro-
gramme (FP7/2007-2013)/ERC Grant 239603.
Because the main limitation of previously reported titanium-
(IV) complexes as cytotoxic agents is their rapid dissociation
in water solutions, the hydrolytic stability of the salen complexes
was measured under conditions similar to those previously applied
for the analogous salan complexes, as previously described.14,18
The active complexes Lig1À3Ti(OArMe2)2 all gave similar t1/2
values for hydrolysis of the labile groups of 2À4 h, which are
comparable to the values obtained for the corresponding salan
complexes despite the reduced availability of the lone-pair
electrons of nitrogen donors that are occupied by resonance,
and represent substantially higher stability than those of titano-
cene dichloride and budotitane.14,18 Similar to observations with
the salan complexes, no substantial effect of the para substituents on
the hydrolytic stability is observed. In contrast, the ortho-chlorinated
complex Lig4Ti(OArMe2)2 demonstrated some enhancement of
the hydrolytic stability with a t1/2 value of 11 h, which is an effect
similar to that observed for the salan analogue, although substantially
less pronounced.18 Nevertheless, because Lig4Ti(OArMe2)2 is
inactive biologically, we cannot attribute cytotoxicity variations
solely to the hydrolytic stability, and parameters relating to the
size, solubility, and general hydrophobicity and membrane
penetration ability are surely also of significance.
The results explicitly show for the first time that titanium(IV)
complexes with a trans configuration of the labile groups may
exhibit high cytotoxic activity, which is similar to that of the
corresponding cis complexes. We still do not know, however,
whether the two labile positions in the salen complexes operate
separately or whether reorganization on the metal center occurs
upon the first interaction to form chelate binding, due to higher
flexibility of the octahedral titanium(IV) center relative to that of
the planar platinum compounds. This adds to the diversity of the
possible titanium(IV) complexes to be synthesized and analyzed
as antitumor compounds. Additionally, the presence of the
phenylenediamine bridge in the salen complexes, which does
not diminish the hydrolytic stability, adds possibilities for the use
of various substitutions to fine-tune the complex performance
and may be advantageous if planarity is of importance, for
instance, for DNA intercalation. Another advantage of the salen
complexes over the salans is in their enhanced solubility in
biologically relevant solutions, and in their achiral nature, remov-
ing the requirement of chiral separation that involves the
medicinal application of the C2-symmetrical salans.26
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Supporting Information. Crystallographic data for Lig3,4Ti-
S
b
(OArMe2)2 and experimental details. This material is available free of
charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: tshuva@chem.ch.huji.ac.il. Fax: +972-2-6584282.
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