Cyclopalladated benzophenone imines: Synthesis, antitumor activity, cell accumulation, dna interaction, and cathepsin b inhibition

Article abstract of DOI:10.1021/om501060f

The synthesis of the endo five-membered cyclo-ortho-palladated benzophenone imines [Pd{C₆H₄(Ph)C=NR}]₂(μ-X)₂ [1 (X = OAc), 2 (X = Cl), a (R = phenyl), b (R = 1-naphthyl), c (R = benzyl), d (R = α-methylbenzyl)], and trans-N,P-[Pd{C₆H₄(Ph)C=NR}X(PPh₃)] [3 (X = OAc), 4 (X = Cl), a (R = phenyl), b (R = 1-naphthyl), c (R = benzyl), d (R = α-methylbenzyl)] and the X-ray molecular structure of 1a, 1c, 1d, 4a, 4b, and 4c are reported. The antitumor activity, DNA interaction, and cathepsin B inhibition of palladium compounds a-d were studied and compared with those previously reported for palladium compounds e with R = H and compound 4f analogous to 4e but with a platinum(II) center. The IC₅₀ values against a panel of human cancer cell lines allowed the establishment of a qualitative relationship between their structure and antitumor activity. Compounds 3e, 4e, and 4f were the most active ones in relation to their in vitro anticancer activity. Compounds 3e and 4e were about 4 times more active than cisplatin against the MDA-MB-231 and MCF-7 breast human cancer lines, and compound 4f was about 4 times more active than cisplatin against the cisplatin-resistant HCT-116 colon human cancer cell line. In addition, compound 3e was 3 times less cytotoxic than cisplatin toward the quiescent HUVEC cells. Accumulation of palladium compounds e and b in the MDA-MB-231 cell line was considerably greater than that of cisplatin in the same cell line, but palladium compounds b were noncytotoxic. Some of these complexes altered the DNA tertiary structure in a similar way to cisplatin but at higher concentration, and most cytotoxic ones did not present a high efficiency as cathepsin B inhibitors.

Full text of DOI:10.1021/om501060f

Article
pubs.acs.org/Organometallics
Cyclopalladated Benzophenone Imines: Synthesis, Antitumor
Activity, Cell Accumulation, DNA Interaction, and Cathepsin B
Inhibition
,
†,⊥
†,⊥
∥,⊥
†
‡,⊥
§
Joan Albert,* Jaume Granell, Romana Qadir, Josefina Quirante, Carme Calvis,
§
∥,⊥
Merce
̀
Font-Bardia, and Teresa Calvet#
#,¶
Ramon Messeguer, Josefa Badía, Laura Baldoma,
̀
†
‡
§
∥
Departament de Química Inorgan
Laboratori de Química Organica, Facultat de Farmac
Biomed Division, LEITAT Technological Center, Parc Científic, Edifici Hel
Departament de Bioquímica i Biologia Molecular, Facultat de Farmacia, Universitat de Barcelona, Av. Joan XXIII s/n, 08028
Barcelona, Spain
̀
ica, Facultat de Química, Universitat de Barcelona, Martí i Franques
ia, Universitat de Barcelona, Av. Joan XXIII s/n, 08028 Barcelona, Spain
ix, Baldiri Reixach 15-21, 08028 Barcelona, Spain
̀
1-11, 08028 Barcelona, Spain
̀
̀
̀
̀
⊥
Institut de Biomedicina, Universitat de Barcelona, 08028 Barcelona, Spain
#
Departament de Crystallografia, Mineralogia i Dipos
Spain
̀
̀
its Minerals, Universitat de Barcelona, Martí i Franques s/n, 08028 Barcelona,
¶
Unitat de Difraccio
Spain
́
de Raigs-X, Centre Científic i Tecnolog
̀
ic de la Universitat de Barcelona, Sole i Sabarís 1-3, 08028 Barcelona,
́
*
S Supporting Information
ABSTRACT: The synthesis of the endo five-membered cyclo-
ortho-palladated benzophenone imines [Pd{C H (Ph)C
6
4
NR}] (μ-X) [1 (X = OAc), 2 (X = Cl), a (R = phenyl), b
2
2
(
R = 1-naphthyl), c (R = benzyl), d (R = α-methylbenzyl)],
and trans-N,P-[Pd{C H (Ph)CNR}X(PPh )] [3 (X =
6
4
3
OAc), 4 (X = Cl), a (R = phenyl), b (R = 1-naphthyl), c
R = benzyl), d (R = α-methylbenzyl)] and the X-ray
(
molecular structure of 1a, 1c, 1d, 4a, 4b, and 4c are reported.
The antitumor activity, DNA interaction, and cathepsin B
inhibition of palladium compounds a−d were studied and
compared with those previously reported for palladium
compounds e with R = H and compound 4f analogous to 4e but with a platinum(II) center. The IC50 values against a panel
of human cancer cell lines allowed the establishment of a qualitative relationship between their structure and antitumor activity.
Compounds 3e, 4e, and 4f were the most active ones in relation to their in vitro anticancer activity. Compounds 3e and 4e were
about 4 times more active than cisplatin against the MDA-MB-231 and MCF-7 breast human cancer lines, and compound 4f was
about 4 times more active than cisplatin against the cisplatin-resistant HCT-116 colon human cancer cell line. In addition,
compound 3e was 3 times less cytotoxic than cisplatin toward the quiescent HUVEC cells. Accumulation of palladium
compounds e and b in the MDA-MB-231 cell line was considerably greater than that of cisplatin in the same cell line, but
palladium compounds b were noncytotoxic. Some of these complexes altered the DNA tertiary structure in a similar way to
cisplatin but at higher concentration, and most cytotoxic ones did not present a high efficiency as cathepsin B inhibitors.
INTRODUCTION
has provided palladium(II) compounds that are less reactive
■
and with promising cytotoxicity against different cancer cell
In recent years, coordination and organometallic palladium(II)
compounds have been explored as an alternative to platinum-
II) compounds as anticancer drugs, in part due to the
2
lines.
The cytotoxicity of a significant number of cyclopalladated
compounds has been explored against a variety of cancer cell
lines with interesting outcomes and with some chances of
(
1,2
similarity of their substitution reactions.
A significant
difference between the two metal centers is the higher kinetic
lability of palladium(II) compared to that of platinum(II). This
renders palladium(II) less attractive for anticancer studies since
hydrolysis of Pd−Cl bonds is quite fast and gives way to very
reactive species that are unable to reach the target biomolecules
4
−20
progressing for evaluation as drug candidates.
For instance,
compounds A and B in Figure 1 presented IC50 values against
the B16-next2 murine melanoma cell line less than 1.25 μM.
3
into the cancer cells. In spite of this, the use of bulky
Received: October 21, 2014
monodentate ligands or chelate bidentate or terdentate ligands
©
XXXX American Chemical Society
A
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Organometallics
Article
accumulation of the palladium compounds b and e, (iii) the
interaction of compounds a−d with the pBluescript SK +
plasmid DNA, (iv) the interaction of compounds 2b and 2d
with the pBluescript SK + plasmid DNA in the presence of
topoisomerase I, and (v) the cathepsin B inhibition test for the
most cytotoxic palladium compounds a−d. The cytotoxicity of
compound 3e, one of the most antiproliferative complexes, was
also studied toward the human umbilical vein endothelial cells
Figure 1. Structural formula of cyclopalladated compounds with
chances of progressing for evaluation as drug candidates.
(
HUVEC) in quiescent and normal culture conditions. It
Compound B was the most potent in vivo, delaying tumor
growth and prolonging animal survival in mice subcutaneously
inoculated with the B16F10-Nex2 melanoma cell line. On the
other hand, cyclopalladated compound C with a bridging 1,1′-
bis(diphenylphosphane)ferrocene showed a notable cathepsin
B inhibitory activity, with the potential to treat metastatic
should be noted that these cells in quiescent conditions are
used as a model for nonproliferative normal cells. These
analyses are compared with those previously reported by our
group for palladium compounds e and for the platinum
1
9,20
compound 4f.
The aim of these studies is to establish structure−activity
relationships upon the structure of the proposed compounds
and their anticancer activity and the identification of plausible
primary target biomolecules for them.
1
0−18
cancers.
It should be noted that cyclopalladated
compounds A−C shown in Figure 1 were patented as
1
8
antitumor drugs.
Recently, we have reported the synthesis of the cyclo-
palladated benzophenone imines 1e−4e and the cycloplati-
nated benzophenone imine 4f (see Scheme 1 for their
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Compounds.
The structural formula and the numbering of the compounds
under study are given in Scheme 1.
a
Scheme 1
Benzophenone imines a−d were prepared by a condensation
reaction between benzophenone and the corresponding amine
using TiCl as Lewis acid promoter and drying agent in a molar
4
ratio of 1:1:5 TiCl /benzophenone/amine by an adaptation of
4
21
a previously reported method. Imines a−d were obtained in
moderate to high yield and were characterized by EM, IR, and
1
13
1
H and C{ H} NMR. Imine e (Ph CNH) was available
2
commercially and was used as received.
Compounds 1e−4e and 4f were prepared according to the
19,20
procedures previously described in our research group.
Thus, treatment of imine e (Ph CNH) with Pd(OAc) in a
2
2
1
:1 molar ratio in acetic acid at 60 °C for 24 h generated the
dinuclear cyclopalladated compound 1e with acetato bridging
ligands in high yield. A subsequent metathesis reaction of
compound 1e with LiCl in acetone at room temperature
produced the dinuclear cyclopalladated compound 2e with
chlorido bridging ligands. The mononuclear compounds 3e and
a
Reactions and conditions: (i) TiCl (Lewis acid promoter and drying
4
agent) + benzophenone + amine, toluene, 24 h, reflux, molar ratio
TiCl /benzophenone/amine = 1:1:5; (ii) benzophenone imine +
Pd(OAc) , HOAc, 60 °C, 24 h, molar ratio Pd(OAc) /benzophenone
imine = 1:1; (iii) compound 1 + LiCl excess, acetone, room
4
2
2
4e were obtained by bridge-splitting reactions of the dinuclear
temperature, 2 h; (iv) compounds 1 or 2 + PPh , acetone, room
3
temperature, 2 h, molar ratio 1 or 2/PPh3 = 1:2; (v) cis-
compounds 1e and 2e with PPh3 in acetone at room
temperature. Mixture A, the precursor of compound 4f, was
prepared by reaction of cis-[PtCl (DMSO) ] and NaOAc in a
[
PtCl (DMSO) ], NaOAc (stoichiometric), benzophenone imine
2 2
(
excess), methanol, reflux, 24 h, under N ; (vi) mixture A + PPh ,
2
3
2
2
acetone, room temperature, 2 h, molar ratio mixture A/PPh = 1:1.
3
1:1 molar ratio with an excess of benzophenone imine in dry
methanol under nitrogen and at reflux for 24 h. A subsequent
structural formula), and we have studied their (i) antitumor
activity toward MDA-MB-231 and MCF-7 breast and cisplatin-
resistant HCT-116 colon human cancer cell lines, (ii)
interaction with pBluescript SK + plasmid DNA, (iii) cathepsin
B inhibition test, and (iv) antibacterial and antioxidant
reaction of mixture A with PPh yielded compound 4f. The
3
1
purity of compounds 1e−4e and 4f was checked by IR and H
NMR, and purity of compounds 3e, 4e, and 4f was checked in
addition by 31P{ H} NMR.
1
Compounds 1−4 from series a−d were obtained by an
adaptation of the procedures given above for compounds 1e−
4e. It should be noted that the synthesis of compounds 2a, 2c,
1
9,20
activity.
Noteworthy, compounds 3e and 4e were about 4
times more active than cisplatin against the MDA-MB-231 and
MCF-7 human breast cancer lines, and compound 4f was also
about 4 times more active than cisplatin against the cisplatin-
resistant HCT-116 human colon cancer cell line. In addition,
compounds 1e−4e and 4f presented a moderate antibacterial
2
2,23
and 4c was previously claimed,
but their detailed
preparative method and their characterization data were not
2
3
reported, except for the C, H, and N elemental analyses of 2a.
24
According to the literature, we propose an endo five-
membered ortho-cyclopalladated structure for the palladium
compounds a−d. The endo descriptor denotes that the CN
20
and antioxidant activity.
Following these studies, we report herein the preparation of
cyclopalladated benzophenone imines a−d (Scheme 1) and the
study of (i) the antitumor activity against MDA-MB-231 and
MCF-7 human breast and cisplatin-resistant HCT-116 human
colon cancer cell lines for compounds a−d, (ii) the cellular
25
bond is included in the metalacycle. This assumption was
established unambiguously by the XRD molecular crystal
structure determination of 1a·2MeOH, 1c, 1d, 4a, 4b·MeOH,
and 4c·MeOH (see below). Compounds 1−4 from series a−d
B
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Article
1
afforded satisfactory EM, IR, and H NMR. In addition, the
3
1
1
P{ H} NMR spectra of compounds 3 and 4 from series a−d
26
were also consistent with their proposed structure.
Details of the preparative method and isolation in pure form
of the benzophenone imines a−d and their cyclopalladated
derivatives 1−4 and selected characterization data for these
compounds are given in the Supporting Information.
XRD Molecular Structures of 1a, 1c, 1d, 4a, 4b, and 4c.
Figures 2−7 show ball-and-stick models of the molecular
Figure 4. XRD molecular structure of compound 1d. Selected bond
distances (Å) and angles (deg): Pd1−C1 1.957(4), Pd1−N1 2.038(3),
Pd1−O1 2.141(3), Pd1−O2 2.050(3), Pd(1)−Pd(1) 3.2037(6), O1−
Pd1−C1 171.07(13), N1−Pd1−O2 172.97(12), C1−Pd1−N1
81.19(14), C1−Pd1−O2 92.50(15), O2−Pd1−O1 87.34(11), N1−
Pd1−O1 98.40(12).
Figure 2. XRD molecular structure of compound 1a. Selected bond
distances (Å) and angles (deg): Pd1−C9 1.950(2), Pd1−N1
2
2
2
.0191(19), Pd1−O4 2.0576(17), Pd1−O1 2.1247(16), Pd1−Pd2
.9069(3), Pd2−C21 1.943(2), Pd2−N2 2.0177(19), Pd2−O2
.0374(16), Pd2−O3 2.1573(16), C9−Pd1−N1 80.77(9), C9−
Pd1−O4 93.80(8), N1−Pd1−O4 173.36(7), C9−Pd1−O1
1
76.17(8), N1−Pd1−O1 95.47(7), O4−Pd1−O1 89.89(7), C21−
Pd2−N2 81.27(9), C21−Pd2−O2 92.79(8), N2−Pd2−O2 173.53(7),
C21−Pd2−O3 176.31(8), N2−Pd2−O3 95.15(7), O2−Pd2−O3
9
0.83(7).
Figure 5. XRD molecular structure of compound 4a. Selected bond
distances (Å) and angles (deg): Pd1−C31 2.015(2), Pd1−Cl1
2.3508(6), Pd1−N1 2.1028(19), Pd1−P1 2.2695(6), N1−Pd1−P1
176.59(5), C(31)−Pd(1)−Cl(1) 170.94(7), C31−Pd1−N1 80.70(8),
N1−Pd1−Cl1 90.86(6), P1−Pd1−Cl1 92.42(2), C31−Pd1−P1
96.09(7).
Figure 3. XRD molecular structure of compound 1c. Selected bond
distances (Å) and angles (deg): Pd1−C44 1.968(3), Pd2−C3
1
.947(3), Pd1−O2 2.155(2), Pd2−O3 2.147(3), Pd1−N2 2.010(3),
Pd2−N1 2.022(3), Pd1−O4 2.055(2), Pd2−O1 2.042(2), Pd1−Pd2
.8698(4), O2−Pd1−C44 176.26(12), O3−Pd2−C3 176.91(12),
O4−Pd1−N2 173.65(11), O1−Pd2−N1 174.78(11), C44−Pd1−N2
1.66(14), C3−Pd2−N1 81.26(13), C44−Pd1−O4 93.70(12), C3−
Pd2−O1 93.56(12), O4−Pd1−O2 89.13(10), O1−Pd2−O3
9.42(10), N2−Pd1−O2 95.33(12), N1−Pd2−O3 95.78(10).
2
1
c, 1d, 4a, 4b·MeOH, and 4c·MeOH were grown by slow
8
evaporation of the solvents of a solution of the corresponding
compound 1 in CH Cl , MeOH, and acetone in 1:1:1 volume
8
2
2
ratio or by the slow evaporation of the solvents of a solution of
the corresponding compound 4 in CH Cl and MeOH in a
volume ratio of 1:1. The crystal structure reports and CIF files
for these X-ray molecular crystal structure determinations are
2
2
structure of 1a, 1c, 1d, 4a, 4b, and 4c determined by single-
crystal X-ray diffraction analysis. Single crystals of 1a·2MeOH,
C
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These molecular structures confirm the proposed structural
formula for the compounds under study. Thus, all these
molecules can be classified as endo five-membered ortho-
cyclopalladated benzophenone imines. In addition, molecules
1a, 1c, and 1d are dinuclear with the acetato ligands bridging
two cyclopalladated units and present a trans-folded structure,
and molecules 4a, 4b, and 4c are mononuclear and present a
trans-N,P configuration.
In the crystal of compound 1d, both halves of the molecules
were equivalent because these molecules were situated over
crystallographic binary proper rotation axes, but in the crystals
of 1a·2MeOH and 1c, the molecules of compounds 1a and 1c
presented two nonequivalent halves, which differed between
them in small differences in distances, angles, and torsion
angles.
Distances and angles around of the palladium(II) centers for
26−29
all these molecules were between the normal intervals.
The chelate bite angles were those from the coordination
sphere of the palladium(II) centers with the largest deviations
from the ideal angles of 90 and 180° and were 80.77(9) and
8
1.27(9)° for 1a, 81.66(14) and 81.26(13)° for 1c, 81.19(14)°
for 1d, 80.70(8) Å for 4a, 81.11(7)° for 4b, and 80.69(7) Å for
4c. The five-membered metalacycles in all these molecules were
planar, and maximum deviated atoms from the five-membered
planar metalacycles were C1 0.064 Å and C21 −0.78 Å for 1a,
C39 −0.011(3) Å and N1 0.068(3) for 1c, C6 0.008(4) Å and
C1 −0.008(4) for 1d, N1 −0.038(2) for 4a, N1 0.021(2) for
Figure 6. XRD molecular structure of compound 4b. Selected bond
distances (Å) and angles (deg): Pd1−C23 2.0226(19), Pd1−Cl1
2
.3647(10), Pd1−N1 2.1162(17), Pd1−P1 2.2651(9), Cl1−Pd1−C23
72.91(6), P1−Pd1−N1 175.62(5), N1−Pd1−C23 81.11(7), Cl1−
1
Pd1−N1 91.83(5), Cl1−Pd1−P1 92.54(3), P1−Pd1−C23 94.52(6).
4b, and N1 0.049 Å for 4c.
In compounds 1, Pd−O bonds trans to the metalated carbon
atoms were longer than Pd−O bonds trans to the iminic
nitrogen atom [Pd1−O1 2.1247(16) Å and Pd2−O3
2
2
2
1
.1573(16) Å but Pd1−O4 2.0576(17) Å and Pd2−O2
.0374(16) Å for 1a, Pd1−O2 2.155(2) Å and Pd2−O3
.147(3) Å but Pd1−O4 2.055(2) and Pd2−O1 2.042(2) Å for
c, and Pd1−O1 2.141(3) but Pd1−O2 2.050(3) Å for 1d].
These results were in agreement with the greater trans influence
of the palladated carbon atom in relation to the iminic nitrogen
30
atom. Also, the greater trans influence of the phosphorus
atom of PPh in relation to the oxygen atom of the acetato
3
ligand explained the longer Pd−N bonds in compounds 4 in
relation to compounds 1 [Pd1−N1 2.1028(19) Å for 4a, Pd1−
N1 2.1162(17) Å for 4b, Pd1−N1 2.0964(14) Å for 4c but
Pd1−N1 2.0191(19) Å and Pd2−N2 2.0177(19) Å for 1a,
Pd1−N2 2.010(3) Å and Pd2−N1 2.022(3) Å for 1c, and
30
Pd1−N1 2.038(3) Å for 1d].
The angle between the palladacycles in compounds 1a, 1c,
and 1d were 25.11, 27.66(14), and 54.16°, respectively, and the
distances between palladium atoms were 2.90693(3),
2
.8698(4), and 3.204 Å, respectively. These distances were
too long to be considered a palladium−palladium single
31
bond.
Figure 7. XRD molecular structure of compound 4c. Selected bond
distances (Å) and angles (deg): Pd(1)−C(1) 2.0135(19), Pd(1)−
N(1) 2.0964(16), Pd(1)−P(1) 2.2639(6), Pd(1)−Cl(1) 2.3837(5),
C(1)−Pd(1)−N(1) 80.69(7), C(1)−Pd(1)−P(1) 93.70(6), N(1)−
Pd(1)−P(1) 174.15(5), C(1)−Pd(1)−Cl(1) 173.10(6), N(1)−
Pd(1)−Cl(1) 92.62(5), P(1)−Pd(1)−Cl(1) 92.942(19).
The angles between the palladacycles and the phenyls
bonded to the methinic carbon atoms in these molecules were
59.38 and 58.78° for 1a, 78.4(2) and 62.42(18)° for 1c,
86.1(2)° for 1d, 85.11(11)° for 4a, 66.69(12)° for 4b, and
7
6.75° for 4c. In compounds 1a and 4a, the angles between the
phenyls bonded to the iminic nitrogen atoms and the
palladacycles were 61.57 and 61.24° for 1a and 79.75(11)°
for 4a, and the angle between the 1-naphthyl group bonded to
the iminic nitrogen atom and the palladacycle was 75.23(10)°
in compound 4b.
given in the Supporting Information. These structures have
been also deposited in the Cambridge Crystallographic Data
Centre with the following deposition numbers: 1025622 (4b·
MeOH), 1025623 (1c), 1025624 (1d), 1025625 (1a·2MeOH),
Finally, in all these molecules, the metalated phenyl and the
metalacycle were almost coplanar, with the angles between
1025626 (4a), and 1025627 (4c·MeOH).
D
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Organometallics
Article
them being 7.05 and 5.82° for 1a, 1.73(16) and 7.23(15)° for
From data given in Table 1, the following trend for the in
vitro antitumor activity of compounds 1−4 from series a−e and
compound 4f can be derived: 3e ≈ 4e (more cytotoxic than
cisplatin) > 4f (more cytotoxic than cisplatin) > 2e ≈ 1e (less
cytotoxic than cisplatin) > 2d ≈ 2c ≈ 2a (less cytotoxic than
cisplatin) > 3a (less cytotoxic than cisplatin) > the other
compounds (inactive or poorly active).
1
c, 4.15(18)° for 1d, 5.50(11)° for 4a, 1.00(10)° for 4b, and
4
.52° for 4c.
Biological Studies. Antiproliferative Activity. The cyto-
toxicity of palladium compounds a−d against MDA-MB-231
and MCF-7 breast and cisplatin-resistant HCT-116 colon
adenocarcinoma human cell lines was studied, and their IC50
values against the precedent cell lines are collected in Table 1.
This trend seems to indicate that an accurate balance
between hydrophilicity and lipophilicity could be regulating the
in vitro antitumor activity of these cyclometalated benzophe-
none imines. For instance, for palladium compounds e, which
should be the most hydrophilic ones due to the presence of the
N−H function in their structural formula, when passing from
Table 1. IC50 (μM) Values against the Indicated Cancer Cell
a
Lines and clog P for the Compounds under Study
3e and 4e to 1e and 2e, a substantial drop in activity is
observed. This should be ascribed to the decrease in
lipophilicity on going from 3e and 4e to 1e and 2e (see clog
P values in Table 1). On the other hand, for palladium
compounds a, c, and d, which are more lipophilic than
palladium compounds e, since in this series the group bonded
to the iminic nitrogen is phenyl, benzyl, or α-methylbenzyl,
respectively, the maximum activity corresponds to compounds
2a, 2c, and 2d, which are the ones with the lowest lipophilicity
of these series (see clog P values in Table 1). Finally,
compound 2b is not active at all since this compound is more
lipophilic (R = 1-naphthyl) than compounds 2a (R = phenyl),
2c (R = benzyl), and 2d (R = α-methylbenzyl). At present, we
cannot find a reliable explanation for the IC50 value of
compound 3a (12 μM) toward the MDA-MB-231 cell line. In
addition, the great cytotoxicity observed for palladium
compounds e could also be related to their better solubility
and/or to the ability of their NH moiety to establish hydrogen
33
bonds with a target biomolecule.
It should be noted that water solubility for the organo-
metallic compounds under study is in fact very low, but we
expect that, once in solution in the biological media, palladium
compounds a−d should be converted into the ionic aqua
a
Data are shown as the mean values of two experiments performed in
triplicate with the corresponding standard deviation. cis-
b
[
PtCl (NH ) ] is taken as a reference compound. clog P is the
2
3 2
complexes [Pd(C,N)(H O) ]X (X = OAc or Cl) (compounds
calculated logarithmic value of the n-octanol/water partition coefficient
and was estimated using the program ChemBioDraw Ultra 12.0
2
2
I) and trans-N,P-[Pd(C,N)(PPh )(H O)]X (X = OAc or Cl)
3
2
c
19,20
(
ChemBioOffice 2010). Previously reported.
Cells color code:
(compounds II) by substitution of their chlorido ligands by
water molecules. Therefore, we propose that the responsible
species for the biological activities studied here are the ionic
aqua complexes I and II commented above. Similar aqua
complexes were proposed to be the responsible species for the
biological activities observed for palladium compound e and
green = more cytotoxic than cisplatin, yellow = less cytotoxic than
cisplatin, red = inactive, white = poorly active or inactive, blue =
cisplatin.
For comparison, the IC50 values against the above cited cell
lines for cisplatin, free imines, palladium compounds e, and
platinum compound 4f are also included in Table 1. It should
be noted that the IC50 values for palladium compounds e and
compound 4f against the cancer cell lines cited above have been
1
9,20
compound 4f.
The cytotoxicity of compound 3e, one of the most potent
complexes in the cancer cell lines evaluated, and cisplatin was
also tested against HUVEC cells in starving culture conditions,
in which cells are almost quiescent or in low proliferative
conditions, and in normal cell culture conditions, in which cells
are in highly proliferative conditions (see Experimental Section
for the details). Table 2 gives the IC50 values for these studies.
Interestingly, compound 3e was 2 times and 1.5 times less
cytotoxic than cisplatin toward the HUVEC cells in starving
and in normal cell culture conditions, respectively.
1
9,20
previously reported by our research group.
Table 1 gives
also the clog P for the compounds under study. This is the
calculated logarithmic value of the n-octanol/water partition
coefficient of the compound under consideration and can be
32
related to its lipophilicity.
For a better understanding of Table 1, we have included a
color code for the cells. Thus, green ones mean that the
compounds under consideration are more cytotoxic than
cisplatin against the specified tumor cell lines; yellow ones
are compounds that are cytotoxic but less cytotoxic than
cisplatin; red ones are compounds that are not cytotoxic with
IC values greater than 100 μM, and cells not filled with color
indicate compounds that are poorly cytotoxic (IC50 > 50 μM)
or noncytotoxic. Cells corresponding to cisplatin are filled with
blue color.
Cell Accumulation. The cellular accumulation of platinum
drugs in vivo is well-documented, and it is believed to be
dependent on both passive diffusion and active transport.
Active transport can involve the entry into the cell with the help
of copper transporters and organic cation transporters, as well
5
0
34
as active efflux of the drug by copper efflux transporters. The
relationship between lipophilicity, cellular accumulation, and
cytotoxicity of oxaliplatin derivatives with different substituents
E
dx.doi.org/10.1021/om501060f | Organometallics XXXX, XXX, XXX−XXX

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Table 2. IC50 (μM) Values for Compound 3e toward
HUVEC Cells in Starving and Normal Culture Conditions
MDA-MB-231 cell line in quite high amounts. This result
suggests that these compounds could be noncytotoxic because
of their bulky 1-naphthyl group, which should render their
palladium(II) centers less reactive, or because of their
accumulation in cellular organelles such as lysosomes due to
a
b
c
compound
HUVEC starving
HUVEC normal
3
e
49 ± nd
16 ± 3
29 ± 7
18 ± 2
a
35
cisplatin
their elevated lipophilicity.
a
Data are shown as the mean values of two experiments performed in
triplicate with the corresponding standard deviation. cis-
DNA Interaction. The binding of palladium compounds a−d
to DNA was studied by their ability to modify the
electrophoretic mobility of the supercoiled closed circular
b
[
PtCl (NH ) ] is taken as a reference compound. Low proliferative
2 3 2
c
culture conditions. Highly proliferative culture conditions.
(
ccc) and the open circular (oc) forms of pBluescript SK +
plasmid DNA.
at the 4 position of the cyclohexane ring has been investigated.
In this study, the more lipophilic compounds revealed a lower
cytotoxicity, although the early influx rate (passive diffusion)
was increased in the case of more lipophilic platinum
complexes. Alterations in the reactivity in the intracellular
environment of the platinum compounds with bulky
substituents or increased sequestration of the most lipophilic
platinum complexes in cellular organelles such as lysosomes
could be responsible for their reduced cytotoxicity in the
investigated cell lines. This explanation could account for the
lower cytotoxicity observed for palladium compounds a−d with
regard to that of palladium compounds e and the absence of
antiproliferative activity of palladium compounds b, which
present the bulkier and more lipophilic substituent in series a−
d.
To get more insight on the antitumor activity of the
benzophenone cyclopalladated imines, the cellular accumu-
lation of palladium compounds b and e was studied and
compared with that of cisplatin. In these studies, palladium was
used as a measure of the cellular accumulation of the palladium
compounds, and platinum was used as a measure of the cellular
accumulation of cisplatin. Mole of palladium or platinum per
cell was determined by ICP-MS. Table 3 gives the cellular
accumulation for the tested compounds and for cisplatin after 4
h of incubation on the breast cancer cell line MDA-MB-231.
Figure 8 shows the electrophoretic mobility of pBluescript
SK + plasmid DNA incubated with the free ligands a−d and
palladium compounds a−d at increasing concentrations. To
provide a basis for comparison, incubation of DNA with
cisplatin and ethidium bromide (EB) was also performed using
the same concentrations and conditions. The electrophoretic
mobility of pBluescript SK + plasmid DNA incubated with
palladium compounds e and compound 4f was previously
reported by our research group and has also been included in
35
19,20
Figure 8 for a comparative purpose.
As expected, cisplatin greatly altered the electrophoretic
mobility of pBluescript SK + plasmid DNA at 2.5 μM, but for
EB, only a very slight decrease in the electrophoretic mobility of
DNA was detected between 25 and 100 μM. For the free
ligands a−d, no shift in the rate of migration of the supercoiled
band was observed.
Palladium compounds a−e were less efficient than cisplatin
in removing the supercoils from DNA, in spite of the fact that
some were more cytotoxic than cisplatin. Only compound 3b
produced at 10 μM concentration (lane 4), a significant effect
on the mobility of pBluescript DNA. At 25 μM concentration
19
(
lane 5), complexes 1a−1c induced significant changes in
plasmid DNA mobility. Complexes 1a−1e, 2e, and 3a−3d
showed the same effect of coalescence and positive supercoiling
observed for cisplatin but at a higher concentration. Compound
4
f was also less efficient than cisplatin for removing the
Table 3. Palladium and Platinum Accumulation in the MDA-
MB-231 Cell Line
supercoils from pBluescript SK + plasmid DNA, suggesting that
the unwinding of the DNA is not the key factor responsible for
its cytotoxicity.
20
a
compound
cell accumulation
Compounds with the greatest effect on DNA mobility did
not necessarily correlate to those with the lowest IC50 values.
For instance, modifications induced on DNA mobility for 1a
were greater than those produced for 2a, but 2a was more
potent than 1a in the three cancer cell lines assayed. On the
other hand, noncytotoxic cyclopalladated compounds (i.e., 1b
and 3b, IC50 values >100 μM) induced significant changes on
DNA mobility. It is hypothesized that, in the conditions of the
gel mobility assay, compounds such as 1b, 3b, or 4f interacted
with DNA in a similar way as that of cisplatin but at a higher
concentration.
1
2
3
4
1
2
3
4
b
b
b
b
e
e
e
e
0.40 ± 0.15
0.56 ± 0.09
0.7 ± 0.4
0.02 ± 0.01
0.2 ± 0.1
0.072 ± 0.006
0.9 ± 0.2
1.6 ± 0.6
cisplatin
0.015 ± 0.002
a
Cell accumulation for compounds b and e and for cisplatin is given as
(
mol of Pd or of Pt per cell ± SD) × 10−15 and was measured by ICP-
To evaluate the ability of the investigated complexes to
intercalate into DNA, a topoisomerase-based gel assay was
performed on 2b and 2d. Supercoiled pBluescript plasmid
DNA was incubated in the presence of topoisomerase I at
increasing concentrations of compounds 2a and 2b. The results
are given in Figure 9, and they show that 2b and 2d do not
prevent unwinding of DNA by the action of topoisomerase I,
indicating that these compounds are neither intercalators nor
MS after 4 h of treatment at 50 μM with the indicated compounds.
Experiments were performed in duplicate. SD = standard deviation.
The accumulation of the tested compounds in the cell line
studied was considerably greater than that of cisplatin, and
compounds 3e and 4e, which are the compounds with the
highest antitumor activity against the MDA-MB-231 cell line,
were those with the highest amounts of palladium accumulated
per cell. The accumulation of these latter compounds was about
8
inhibitors of topoisomerase I.
The precedent results suggest that DNA is not a primary
target biomolecule for the cyclopalladated and cycloplatinated
benzophenone imines under study.
5
0 and 100 times higher than that of cisplatin. Interestingly, the
noncytotoxic compounds 1b−4b were accumulated on the
F
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Article
Figure 9. Analysis of 2b and 2d as putative DNA intercalators or
topoisomerase I inhibitors. Conversion of supercoiled pBluescript
plasmid DNA (0.8 μg) to relaxed DNA by the action of topoisomerase
I (3 units) in the absence or in the presence of increasing amounts of
compounds 2b and 2d was analyzed by agarose gel stained with
ethidium bromide. EB was used as a control of intercalating agent and
etoposide (E) as a control of nonintercalating agent. Lane 1: DNA
only. Lane 2: 0 μM drug. Lane 3: 10 μM drug. Lane 4: 25 μM drug.
Lane 5: 50 μM drug. Lane 6: 100 μM drug. Except for lane 1, all lanes
included topoisomerase I; ccc = supercoiled closed circular DNA; oc =
open circular DNA.
20
reported. In spite of these results, we have recently reported
that compounds 1e−4e and 4f are not efficient inhibitors of
cathepsin B although they are quite cytotoxic in vitro.
Following these studies, we have determined the cathepsin B
inhibition activity for compounds 2a, 2c, 2d, and 3a, which are
the more active ones from series a−d in relation to their in vitro
antitumor activity. The results are given in Table 4. This table
also includes our previously reported results for the cathepsin B
inhibition test for palladium compounds e and compound 4f
20
for a comparative purpose.
Table 4. IC50 (μM) Values and Percent of Residual Activity
of Cathepsin B at 100 μM for Compounds 2a, 2c, 2d, and
a
3a
compound IC50 (μM) vs cathepsin B % of residual activity at 100 μM
2
2
2
3
1
2
a
c
d
a
>100
>50
58.0 ± 0.8
47.0 ± 1.1
45.0 ± 1.9
47.0 ± 0.9
62.0 ± 2.1
60.0 ± 1.7
70.0 ± 1.5
50.0 ± 0.9
72.0 ± 1.0
>50
>50
b
e
e
>100
>100
>100
>50
b
b
b
Figure 8. Interaction of pBluescript SK + plasmid DNA (0.8 μg) with
increasing concentrations of compounds under study, cisplatin and
ethidium bromide. Lane 1: DNA only. Lane 2: 2.5 μM. Lane 3: 5 μM.
Lane 4: 10 μM. Lane 5: 25 μM. Lane 6: 50 μM. Lane 7: 100 μM. Lane
3e
4e
b
4f
>100
8
: 150 μM. Lane 9: 200 μM; ccc = supercoiled closed circular DNA;
a
The enzyme was preincubated for 2 h with 50 or 100 μM of each
oc = open circular DNA.
compound. The activity is given as a percentage of the enzyme activity
determined in the absence of the test compound. Data are shown as
the mean values of the experiment performed in triplicate with the
Cathepsin B Inhibition. Cathepsin B is a cysteine metal-
loprotease highly upregulated in a wide variety of cancers by
mechanisms ranging from gene amplification to post-transcrip-
tional modification. The exact role of cathepsin B in solid
tumors has yet to be defined, but it has been proposed to
participate in metastasis, angiogenesis, and tumor progression.
Recently, compounds based on palladium, platinum, ruthe-
nium, rhenium, gold, and tellurium were shown to be effective
b
20
corresponding standard deviation. Previously reported.
Compounds 2a, 2c, 2d, and 3a inhibited cathepsin B in a
dose-dependent manner, but they were not efficient as
cathepsin B inhibitors since 50% of residual activity of the
enzyme is not observed until quite high concentrations, greater
than 50 or 100 μM depending on the compound. These results
confirm that cathepsin B is not a primary target biomolecule for
the cyclopalladated and cycloplatinated benzophenone imines
studied here.
36
inhibitors of cathepsin B. In addition, an excellent correlation
between cathepsin B inhibition and cytotoxicity for some
3
7
dinuclear biphosphane palladacycles and mononuclear
38
platinacycles containing a fluorinated phosphane has been
G
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Organometallics
Article
seeded in 6 mm tissue culture dishes for 16 h in DMEM/high glucose
plus 10% FCS. Then, compounds were added at 50 μM for 4 h at 37
CONCLUSIONS
■
1
9,20
The present study and previous ones
confirm that
°
C, 5% CO . Cisplatin was used as a positive control at the same
2
compounds 3e, 4e, and 4f are the most active in vitro
antitumor agents against the evaluated cell lines between a
quite large number of cyclometalated benzophenone imines
studied. Compounds 3e and 4e were about 4 times more active
than cisplatin against the MDA-MB-231 and MCF-7 breast
human cancer lines, and compound 4f is about 4 times more
active than cisplatin against the cisplatin-resistant HCT-116
colon human cancer cell line. The in vitro cytotoxicity of the
cyclopalladated benzophenone imines a−e against the studied
cancer cell lines seems to be regulated by an adequate balance
between their hydrophilicity and lipophilicity. In addition,
accumulation of palladium compounds e and b in the MDA-
MB-231 cell line was considerably greater than that of cisplatin,
but palladium compounds b were noncytotoxic. Furthermore,
compound 3e, one of the most potent complexes in the cell
lines evaluated, was 2 times and 1.5 times less cytotoxic than
cisplatin toward HUVEC cells in starving (quiescent) and
normal cell culture conditions, respectively. Concerning the
determination of plausible primary target biomolecules for
these cyclopalladated and cycloplatinated benzophenone
imines, DNA migration studies and cathepsin B inhibition
tests suggest that these biomolecules are not their primary
target biomolecules.
concentration and DMSO as a vehicle control. After treatment, cells
were washed twice with PBS, tripsinized, and harvested in PBS. Cell
suspension was centrifuged, and pellets were digested with 12 M HCl
and diluted to 1.2 M HCl. Each treatment was done in duplicate. The
samples were analyzed by ICP-MS.
DNA Migration Studies. A stock solution (10 mM) of each
compound was prepared in high-purity DMSO. Then, serial dilutions
were made in Milli-Q water (1:1). Plasmid pBluescript SK+ was
obtained using a kit as described by the manufacturer. Interaction of
drugs with pBluescript SK+ plasmid DNA was analyzed by agarose gel
electrophoresis following a modification of the method previously
4
0
−1
described. Plasmid DNA aliquots (40 μg mL ) were incubated in
TE buffer (10 mM Tris-HCl, 1 mM EDTA, pH 7.5) with different
concentrations of test compounds ranging from 0 to 200 μM at 37 °C
for 24 h. Final DMSO concentration in the reactions was always lower
than 1%. For comparison, cisplatin and ethidium bromide were used as
reference controls. Aliquots of 20 μL of the incubated solutions of
compounds containing 0.8 μg of DNA were subjected to 1% agarose
gel electrophoresis in TAE buffer (40 mM Tris-acetate, 2 mM EDTA,
pH 8.0). The gel was stained in TAE buffer containing EB (0.5 mg
−1
mL ) and visualized and photographed under UV light.
Topoisomerase I-based experiments were performed as described
4
1
previously. Supercoiled pBluescript DNA, obtained as described
above, was treated with topoisomerase I in the absence or presence of
increasing concentrations of compounds 2b and 2d. Assay mixtures
contained supercoiled pBluescript DNA (0.8 μg), calf thymus
topoisomerase I (3 units), and complexes 2b and 2d (0−100 μM)
in 20 μL of Tris-HCl buffer (pH 7.5) containing 175 mM KCl, 5 mM
EXPERIMENTAL SECTION
■
MgCl , and 0.1 mM EDTA. Ethidium bromide (10 μM) was used as a
Cell Culture. Colon HCT-116 and breast cancer MCF-7 and
MBA-MD-231 cells were grown as a monolayer culture in minimum
essential medium (DMEM with L-glutamine, without glucose and
without sodium pyruvate) in the presence of 10% heat-inactivated fetal
calf serum (FCS), 10 mM D-glucose, and 0.1% streptomycin/
penicillin, in standard culture conditions (humidified air with 5%
2
control of intercalating agents and etoposide (100 μM) as a control of
the nonintercalating agent. Reactions were incubated for 30 min at 37
°
C and stopped by the addition of 2 μL of agarose gel loading buffer.
Samples were then subjected to electrophoresis, and DNA bands were
stained with ethidium bromide as described above.
Cathepsin B Inhibition Assay. The colorimetric cathepsin B
CO at 37 °C).
2
42
assay was performed as described by Casini et al. with few
Cell Viability Assay for HC-T116, MDA-MB-231, and MCF-7
Cell Lines. A stock solution (50 mM) of each compound was
prepared in high-purity DMSO. Then, serial dilutions were made with
DMSO/DMEM (1:1), and finally a 1:500 dilution on culture medium
was prepared. Final assay concentration of DMSO was the same in all
modifications. Briefly, the reaction mixture contained 100 mM sodium
phosphate (pH 6.0), 1 mM EDTA, and 200 μM sodium N-
carbobenzoxy-L-lysine p-nitrophenyl ester as substrate. To have the
enzyme catalytically active before each experiment, the active site of
the cysteine was reduced by treatment with dithiothreitol (DTT). For
this purpose, 5 mM DTT was added to the cathepsin B sample, before
dilution, and incubated 1 h at 30 °C. To test the inhibitory effect of the
palladium compounds on cathepsin B, activity measurements were
performed in triplicate using fixed concentrations of enzyme (500 nM)
and substrate (200 μM). The palladium compounds were used at
concentrations ranging from 50 to 100 μM. Previous to the addition of
substrate, cathepsin B was incubated with the different compounds at
3
9
experiments and was 0.2%. The assay was performed as described.
HCT-116, MDA-MB-231, and MCF-7 cells were plated at 5000 cells/
well in 100 μL of media in tissue culture 96-well plates. After 24 h,
medium was replaced by 100 μL/well of drug serial dilutions. Control
wells did not contain the compounds under study. Each point
concentration was run in triplicate. Reagent blanks, containing media
and colorimetric reagent without cells, were run on each plate. Blank
values were subtracted from test values and were routinely 5−10% of
the control values. Plates were incubated 72 h. Hexosaminidase activity
was measured according to the following protocol. The medium was
removed, and cells were washed once with PBS. Sixty microliters of
substrate solution of 7.5 mM p-nitrophenol-N-acetyl-β-D-glucosamide,
2
5 °C for 2 h. The cysteine proteinase inhibitor E-64 was used as a
positive control of cathepsin B inhibition. Complete inhibition was
achieved at 10 μM concentration of E-64. Activity was measured over
1
.5 min at 326 nm on a UV spectrophotometer.
0
.1 M sodium citrate at pH 5.0, and 0.25% Triton X-100 was added to
each well and incubated at 37 °C for 1−2 h. After this incubation time,
a bright yellow color appeared. Then, the plates were developed by
adding 90 μL of developer solution (50 mM glycine, pH 10.4; 5 mM
EDTA), and the absorbance was recorded at 410 nm.
Cell Viability Assay for the HUVEC Cells. Cytotoxicity in normal
cells was assayed using primary human umbilical vein endothelial cells,
at passage 6−8. Viability was assayed in two culture conditions: highly
proliferative, where 10% of FCS, supplements, and growth factors were
included in culture media; low proliferative, where cells were almost
quiescent, by restraining FCS at 2% with no supplements or growth
factors. Assays were done as described for tumor cell lines, plating
ASSOCIATED CONTENT
■
*
S
Supporting Information
Preparative method and selected characterization data for
compounds a−d and crystal structure reports and CIF files for
the X-ray molecular crystal structure determinations of 1a·
MeOH, 1c, 1d, 4a, 4b·MeOH, and 4c·MeOH. This material is
available free of charge via the Internet at http://pubs.acs.org.
2
AUTHOR INFORMATION
■
5
000 cell/well for each growing condition.
Cell Accumulation. Cell accumulation of compounds was
Corresponding Author
*Tel.: +34 93 4039131. E-mail: joan.albert@qi.ub.es.
6
measured in the MDA-MB-231 cell line. A total of 1 × 10 cells was
H
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Organometallics
Article
Notes
(22) Clark, P. W.; Dyke, S. F.; Smith, G.; Kennard, C. H. L. J.
Organomet. Chem. 1987, 330, 447−460.
The authors declare no competing financial interest.
(
(
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́
24) Albert, J.; Gomez, M.; Granell, J.; Sales, J.; Solans, X.
ACKNOWLEDGMENTS
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Solans, X. J. Organomet. Chem. 2007, 692, 3070−3080.
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■
(
Financial support from project CTQ2009-11501/BQU from
the Spanish Ministerio de Ciencia e Innovacion
́
and Grant
(
2
009-SGR-1111 from Generalitat de Catalunya is acknowl-
edged. R.Q. acknowledges MAEC-AECID for a master
scholarship.
(
(
́
ez, M.; Granell, J.; Sales, J.
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dx.doi.org/10.1021/om501060f | Organometallics XXXX, XXX, XXX−XXX