Synthesis of a 1,4-benzodiazepine containing palladacycle with in vitro anticancer and cathepsin B activity

Article abstract of DOI:10.1039/b819061e

The reaction of the five-membered C,N-palladacycle [(L)PdCl]₂, where LH = 1-methyl-5-phenyl-1H-1,4-benzodiazepin-2(3H)-one, with 1,2-ethanebis(diphenylphosphine), dppe, leads to the formation of the bridged palladacycle. [Pd₂L_2<

Full text of DOI:10.1039/b819061e

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis of a 1,4-benzodiazepine containing palladacycle with in vitro
anticancer and cathepsin B activity†
John Spencer,*a Rajendra P. Rathnam,a Mahesh Motukuri,a Arun K. Kotha,a Simon C. W. Richardson,a
Ali Hazrati,b John A. Hartley,b Louise Malec and Michael B. Hursthousec
Received 28th October 2008, Accepted 24th December 2008
First published as an Advance Article on the web 29th January 2009
DOI: 10.1039/b819061e
The reaction of the five-membered C,N-palladacycle [(L)PdCl]2, where LH = 1-methyl-5-phenyl-
1H-1,4-benzodiazepin-2(3H)-one, with 1,2-ethanebis(diphenylphosphine), dppe, leads to the formation
of the bridged palladacycle. [Pd2L2(m-dppe)Cl2] 3, which was characterised in solution by 1H and 31
NMR spectroscopy and in the solid state by X-ray crystallography. Complex 3 was tested in vitro
against a number of cell lines. For example, it inhibited K562 leukaemia cells with an IC50 value of
4.3 mM (1 h exposure) and displayed cathepsin B inhibitory action with an IC50 value of 3 mM.
P
Introduction
Historically transition metals have played an important role in
medicinal chemistry.1 Many have anticancer activity due to their
cytotoxic action and a number are now marketed as drugs or in
advanced clinical trials.2 New generation complexes are currently
being developed whereby the metal can act as an inert scaffold and
many enable binding modes and geometries that are not accessible
to traditional organic, carbon-based, chemistry.3
We have a long-standing interest in palladium chemistry,
Fig. 1 Non-cytotoxic palladacycles.
especially palladacycles, which are molecules containing a C–Pd
bond stabilised intramolecularly by a dative bond (e.g. N-, S-,
O-, P-, Se- donor).4 A number of palladacycles have been evaluated
for cytotoxic activity but have yet to make sufficient progress
to clinical trials for evaluation as drugs.5 Nevertheless, a recent
breakthrough from Caires’ group has shown that a dimeric dppf
bridged C,N-palladacycle 1 (dppf = 1,1¢-bis(diphenylphosphine)-
ferrocene) has promising non-cytotoxic anticancer activity; in vivo
animal studies with 1 have shown that significant tumour shrink-
age can be achieved and that very little toxicity is observed,
even at high doses. Compounds 1 can be synthesised from either
(R)- or (S)-dmpa, (N,N -dimethyl-1-phenethylamine) and have
notable cathepsin B inhibitory activity, with the potential to treat
metastatic cancer (Fig. 1).6
The promising results derived from 1 have prompted us to inves-
tigate the reaction of the benzodiazepine containing palladacycle 2
with bidentate phosphine ligands in order to generate compounds
for biological evaluation.7 The premise for such a study stems
from the modular nature, i.e. ease of synthesis of analogues for
structure activity studies (SAR), of the benzodiazepine ligand
and the druglikeness of the benzodiazepine scaffold, which
may lead to favourable properties in the resulting palladacycle
complex.8
Results and discussion
The dimeric palladacycle 2 reacted with an equivalent of dppe to
afford an ash coloured solid. Analysis by 31P NMR spectroscopy
showed the appearance of a major signal at 40.4 ppm (s),
corresponding to the dppe bridged dimer 3, as well as a trace
amount of a product displaying doublets centred at 43.9 and
61.1 ppm, which was attributed to 4 (Scheme 1).9 The structure of
3, which could be isolated in pure form by recrystallisation, was
corroborated by 1H NMR spectroscopy and elemental analysis as
well as by single crystal X-ray analysis (vide infra), which confirmed
the presence of chloroform solvates (Fig. 2).
Complex 3 has been characterised in the solid phase by a
single crystal X-ray diffraction study. The centre of mass of the
palladium dimer lies on an inversion centre (see Fig. 2), such
that only half of the molecule is crystallographically unique. The
i
˚
Pd(1) ◊ ◊ ◊ Pd(1) distance is 7.55 (1) A, showing that the two metal
aSchool of Science, University of Greenwich at Medway, Chatham Maritime,
UK ME4 4TB. E-mail: j.spencer@gre.ac.uk; Fax: +44 (0)208 331 9805
atoms in the dimer are not bonded. The structure contains four
molecules of chloroform per palladium dimer. The geometry about
the palladium atom is distorted square planar, with the maximum
deviation from the least squares plane through atoms Pd(1), Cl(1),
bCancer Research UK Drug-DNA Interactions Research Group, UCL Cancer
Institute, Paul O’Gorman Building, 72 Huntley Street, University College
London, UK WC1E 6BT
cEPSRC National Crystallography Service, School of Chemistry, University
of Southampton, Highfield, Southampton, UK SO17 1BJ
˚
P(1), N(2) and C(15) being 0.08 A for atom N(2). The bond angle
at Pd(1) involving the bidentate ligand, N(2)–Pd(1)–C(15) = 80.5
† CCDC reference number 706386 for 3. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b819061e
(3)◦, is smaller than the other three bond angles at the palladium
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 4299–4303 | 4299
©

Scheme 1 Synthesis of a dppe bridged palladacycle.
Fig. 2 Ortep diagram for complex 3 with ellipsoids drawn at the 50% probability level. Hydrogen atoms and chloroform molecules have been omitted
for clarity. The centre of mass of the dimeric molecule li◦es on an inversion centre. The symmetry transformation used to generate equivalent atoms
˚
is: (i) -x, -y, -z. Selected bond lengths (A), and angles ( ): Pd(1)–Cl(1) 2.381 (2), Pd(1)–P(1) 2.259 (2), Pd(1)–N(2) 2.099 (8), Pd(1)–C(15) 2.016 (9),
P(1)–C(29) 1.833 (9), C(29)–C(29)i 1.543 (17), N(2)–Pd(1)–C(15) 80.5 (3), C(15)–Pd(1)–P(1) 93.7 (3), P(1)–Pd(1)–Cl(1) 93.43 (8), Cl(1)–Pd(1)–N(2)
92.5 (2), P(1)–Pd(1)–N(2) 172.4 (2), Cl(1)–Pd(1)–C(15) 172.7 (3), Pd(1)–P(1)–C(29) 118.8 (3), P(1)–C(29)–C(29)i 114.2 (8), Pd(1)–N(2)–C(9) 114.7 (6),
Pd(1)–N(2)–C(16) 126.3 (6), Pd(1)–N(2)–C(9)–C(10) 4.3 (10), Pd(1)–C(15)–C(10)–C(9) 0.1 (10), P(1)–Pd(1)–C(15)–C(14) 9.6 (8).
centre, the average of which is 93.2 (1–3)◦ (See Fig. 2). The
five-membered ring formed by the bonding of the bidentate
ligand to the metal centre, Pd(1), N(2), C(9), C(10) and C(15), is
planar with the maximum deviation from the least squares plane
˚
through the atoms being 0.03 A for atom N(2). The larger group
C(8)–C(16), Pd(1), Cl(1), N(2) and P(1) is also close to being
planar with the maximum deviation from the least squares plane
˚
through all the atoms being 0.20 A for atom P(1). There is a twist in
this group, with the torsion angle P(1)–Pd(1)–C(15)–C(14) being
10 (1)◦.
Complex 3 was tested for in vitro anticancer activity against
a K562 human leukaemic cell line via a medium throughput
screen. For comparison, a number of palladacycles 1 and 5 were
synthesised and tested (Table 1). Both (R)- and (S)-1 have very
weak cytotoxic action (Entries 1–2), confirming Caires’ findings,6
and most of the benzodiazepine palladacycle analogues 5 display
poor activity, as does the commercially available coordination
complex dppfPdCl2 (Entries 4–11). However, complex 3 displays
good in vitro activity, with an IC50 of 4.3 mM (Entry 3).
Having established 3 as a “hit” from this primary screen,
we have evaluated it in other immortal cell lines namely B16
(Murine Melanoma) and Vero (African Green Monkey Kidney
Epithelia).10 Preliminary data show 3 to have submicromolar
activity (Fig. 3 and 4).
Fig. 3 Toxicity of palladacycle 3 and cisplatin against Vero cell lines.
Furthermore, compound 3 has been tested for cathepsin B
inhibitory activity11 and registered an IC50 value of 2.98 mM, which
is in the same range as that of 1 (Fig. 5).
4300 | Dalton Trans., 2009, 4299–4303
This journal is
The Royal Society of Chemistry 2009
©

Table 1 In vitro activity of palladacycles against a K562 cell line
Fig. 5 Inhibitory action of 3 towards (human liver) cathepsin B.
proteases.12 All these exciting aspects of palladacycle chemistry
will be divulged in due course.
Entry
Compound
R1
R3
L
X
IC50/mMa
1
2
3
4
5
6
7
8
9
(R)-1b
(S)-1b
3
—
—
—
—
Me
Me
Me
Me
Me
Bn
Bn
—
—
—
—
H
H
H
i-Pr
i-Pr
H
—
—
—
—
PPh3
pyr
PPh3
PPh3
pyr
pyr
PPh3
—
—
—
—
Cl
Cl
OAc
OAc
OAc
Cl
400
300
4.3
>1000
550
>1000
>1000
>1000
>1000
1000
Experimental
dppfPdCl2
5ac
General
5bc
5cc
Starting materials and commercial grade solvents were purchased
from Sigma-Aldrich or Alfa Aesar and used without further pu-
rification. Reactions were carried out under argon. NMR spectra
were measured on a Jeol EX270 spectrometer at 270 MHz (1H),
109.4 MHz (31P) and 68 MHz (13C) in CDCl3. Elemental analysis
was performed on a CE Instruments Eager 300 apparatus. Unless
otherwise stated all cell culture reagents were from Invitrogen. Vero
cells were from the American Tissue Culture Collection (ATCC-
CCL-81). The B16 cells were kindly donated by Professor I. Hart
(St Thomas’ Hospital). Compounds 1 were synthesised according
to ref. 6, compounds 2, 5a–g were synthesised according to ref. 7.
5dc
5ec
10
11
5fc
5gc
H
Cl
200
a MTT assay from DMSO stock solution on human leukaemic K562 cells
(1 h exposure). b From ref. 6. c From ref. 7.
Palladacycle 3. Complex 2 (0.668 g, 0.85 mmol of dimer) was
stirred with dppe (0.338 g, 0.85 mmol) in dichloromethane (10 cm3)
for 2 h. The reaction mixture was filtered over Celite and the filter
cake was washed with further dichloromethane (20 ml). Thereafter
the resulting combined filtrates were evaporated to nearly dryness
and hexane was added to afford an ash colour precipitate. The
latter was collected by filtration and dried in vacuo. Yellow crystals
were obtained from CHCl3–hexane (0.945 g, 94%). NMR (CDCl3)
d
H 8.16–7.90 (8H, m, aryl), 7.64 (4H, m, aryl), 7.38–7.24 (16H, m,
aryl), 6.98 (2H, d, J = 7.7 Hz, aryl), 6.77 (2H, t, J = 6.8 Hz, aryl),
6.53 (4H, m, aryl), 6.01 (2H, d, J = 11.0 Hz, CH2), 3.72 (2H, d,
J = 11.0 Hz, CH2), 3.31 (6H, s, CH3), 2.92 (4H, brs, CH2): dP 40.40
(s): dC 175.6, 164.1, 153.6, 153.5, 141.5, 137.9, 129.4 (overlapping
carbons), 126.4, 125.0 (overlapping carbons), 122.6 (overlapping
carbons), 118.3, 117.5, 115.7, 47.8, 28.9, 21.2 (m). Found C, 58.1;
H, 4.3; N, 4.2. C58H50Cl2N4O2P2Pd2 0.25 CHCl3 requires C, 57.8;
H, 4.2; N, 4.6.
Fig. 4 Toxicity of palladacycle 3 and cisplatin against B16 cell lines.
Cell culture. B16 and Vero cells were maintained in an
atmosphere of 5% (v/v) CO2 at 37 ◦C. IC50 values were obtained
using published methodologies. Briefly, 5 ¥ 103 cells/well were used
to seed 96 well cell culture treated plates (Sigma). Compounds
were dissolved at 5 mg ml-1 in sterile DMSO and further diluted
with the appropriate complete cell culture medium. After 72 h cell
viability was assessed using MTT (Sigma) also following published
protocols.10
Conclusion
Palladacycle 3 is cytotoxic and inhibits cathepsin B with IC50 values
in the low mM range. Current studies are aiming to address some
remaining questions including (i) whether 3 displays selectivity
towards cancer over normal cells, (ii) if the cathepsin B inhibitory
action of 3 is exhibited by other related benzodiazepine containing
palladacycles, which preferably display lower toxicity than 3 to
cancer cell lines, (iii) if complexes 3 can be used as biological
probes for proteins and biomolecules e.g. cysteine, selenocysteine
Growth inhibitory activity against the human K562 cell line.
K562 human chronic myeloid leukemia cells were maintained in
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 4299–4303 | 4301
©

RPM1 1640 medium supplemented with 10% fetal calf serum and
2 mM glutamine at 37 ◦C in a humidified atmosphere containing
5% CO2 and were incubated with a specified dose of test agent
˚
at 0.89 A from Cl(13). Figures were produced using ORTEP3 for
Windows18 while structural analysis was carried out in PARST.19
◦
for 1 h at 37 C in the dark. The incubation was terminated by
Acknowledgements
centrifugation (5 min, 300 g) and the cells were washed once with
drug-free medium. Following the appropriate drug treatment, the
cells were transferred to 96-well microtitre plates. Plates were then
kept in the dark at 37 ◦C in a humidified atmosphere containing
5% CO2. The assay is based in the ability of viable cells to reduce
a yellow soluble tetrazolium salt, 3-(4,5-dimethylthiazol-2-yl)-2,5-
diphenyl-2H-tetrazolium bromide (MTT, Sigma Chemical Co.) to
an insoluble purple formazan precipitate. The optical density was
then read at a wavelength of 550 nm on a plate reader, and a dose-
response curve was constructed. For each curve, an IC50 value was
read as the dose required to reduce the final optical density to 50%
of the control value.
JS is grateful for a Royal Society of Chemistry Research Fund
Grant (2007–8) for exploiting palladacycle chemistry. Mrs D.
Amin is thanked for CHN analysis and GRE and the University
of Greenwich are thanked for generous support, notably for the
provision of CHN analysis equipment and cell culture facilities.
Johnson Matthey is thanked for a generous gift of palladium salts.
Dr Jon Roffey, Cancer Research Technology and Robin Leach,
MDS Pharma Services, are thanked for helpful discussions. This
paper is dedicated to Professor Michael Lappert FRS for his
contributions to organometallic chemistry.
References
X-Ray crystallography†
1 S. P. Fricker, Dalton Trans., 2007, 4903–4917 and references
cited.
A suitable crystal was selected and a dataset for 3 was measured
on a Bruker APEXII CCD diffractometer at the window of a
2 Non exhaustive list of representative references: W. H. Ang and P. J.
Dyson, Eur. J. Inorg. Chem., 2006, 4003–4018; L. Kelland, Nat. Rev.,
2007, 7, 574–584; M. J. Clarke, F. Zhu and D. R. Frasca, Chem. Rev.,
1999, 99, 2511–2533; M. D. Hall, H. R. Mellor, R. Callaghan and
T. W. Hambley, J. Med. Chem., 2007, 50, 3403–3411; M. A. Jakupec,
M. Galanski, V. B. Arion, C. G. Hartinger and B. K. Keppler, Dalton
Trans., 2008, 183–194; G. Sava, F. Frausin, M. Cocchietto, F. Vita,
E. Podda, P. Spessotto, A. Furlani, V. Scarcia and G. Zabucchi, Eur. J.
Cancer, 2004, 40, 1383–1396; Y. K. Yan, M. Melchart, A. Habtemariam
and P. J. Sadler, Chem. Commun., 2005, 38, 4764–4776; D. Wang
and S. J. Lippard, Nat. Rev. Drug Discovery, 2005, 4, 307–320; D.
Kovala-Demertzi, A. Boccarelli, M. A. Demertzis and M. Coluccia,
Chemotherapy, 2007, 53, 148–152.
3 H. Bregman and E. Meggers, Org. Lett., 2006, 8, 5465–5468; U.
Schatzschneider and N. Metzler-Nolte, Angew. Chem. Int. Ed., 2006,
45, 1504–1507; A. Nguyen, A. Vessie`res, E. A. Hillard, S. Top, P. Pigeon
and G. Jaouen, Chimia, 2007, 61, 716–724.
4 D. R. Adams, M. A. J. Duncton, J. R. A. Roffey and J. Spencer,
Tetrahedron Lett., 2002, 43, 7581–7583; J. Dupont, C. S. Consorti and
J. Spencer, Chem. Rev., 2005, 105, 2527–2571; J. Dupont, M. Pfeffer
and J. Spencer, Eur. J. Inorg. Chem., 2001, 1917–1927.
˚
Bruker FR591 rotating anode (lMo Ka = 0.71073 A) at 120 K
(Table 2). The data collection was driven by COLLECT13 and
processed by DENZO.14 An absorption correction was applied
using SADABS.15 The structure was solved in SIR200416 and
was refined by a full-matrix least-squares procedure on F2
in SHELXL-97.17 All non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen atoms were
added at calculated positions and refined by use of a riding model
with isotropic displacement parameters based on the equivalent
isotropic displacement parameter (Ueq) of the parent atom. The
-3
˚
highest difference electron density peak, 1.870 e A , is located at
-3
˚
˚
0.98 A from Pd(1) and the deepest hole, -0.826 e A , is located
Table 2 X-Ray crystallography experimental data
5 A. Ryabov, in Palladacycles, ed. J. Dupont, and M. Pfeffer, Wiley-VCH,
Weinheim, 2008, ch. 13, pp. 307–339 and references cited therein.
6 C. Bincoletto, I. L. S. Tersariol, C. R. Oliveira, S. Dreher, D. M. Fausto,
M. A. Soufen, F. D. Nascimento and A. C. F. Caires, Bioorg. Med.
Chem., 2005, 13, 3047–3055.
7 J. Spencer, D. P. Sharratt, J. Dupont, A. L. Monteiro, V. I. Reis, M. P.
Stracke, F. Rominger and I. M. McDonald, Organometallics, 2005, 24,
5665–5672; J. Spencer, B. Z. Chowdhry, A. I. Mallet, R. P. Rathnam,
T. Adatia, A. Bashall and F. Rominger, Tetrahedron, 2008, 64, 6082–
6089.
8 T. Wunberg, M. Hendrix, A. Hillisch, M. Lobell, H. Meier, C. Schmeck,
H. Wild and B. Hinzen, Drug Discovery Today, 2006, 11, 175–180;
J. R. Proudfoot, Bioorg. Med. Chem. Lett., 2002, 12, 1647–1650.
9 These are comparable to the chemical shifts for the similar dimer
to 3, [Pd2(S(-)C2,N-dmpa)2(m-dppe)Cl2], (31P: 36.8 ppm (s)) and the
monomeric cationic complex similar to 4 [Pd(S(-)C2,N-dmpa(dppe)]Cl
(31P: 42.0, 61.2 ppm). E. G. Rodrigues, L. S. Silva, D. M. Fausto, M. S.
Hayashi, S. Dreher, E. L. Santos, J. B. Pesquero, L. R. Travassos and
A. C. F. Caires, Int. J. Cancer, 2003, 107, 498–504.
10 J. Spencer, A. P. Mendham, A. K. Kotha, S. C. W. Richardson, E.
A. Hillard, G. Jaouen, A. Vessie`res, L. Male and M. B. Hursthouse,
Dalton Trans., 2009, 918–921; S. C. W. Richardson, S. C. Winistorfer,
V. Poupon, J. P. Luzio and R. C. Piper, Mol. Biol. Cell., 2004, 15, 1197–
1210; S. C. W. Richardson, P. Ferruti and R. Duncan, J. Drug Targeting,
1999, 6, 391–404.
Empirical formula
Formula weight
Temperature/K
Crystal size/mm
Crystal system
Space group
C58H50Cl2N4O2P2Pd2, 4(CHCl3)
1658.13
120 (2)
0.1 ¥ 0.07 ¥ 0.02
Monoclinic
P21/c
11.3456(5); 10.9170(4);
27.5717(11)
90; 97.760(2); 90
3383.8(2)
2; 0.5
1.627
1.178
0.9768 and 0.8913
1660
˚
a; b; c/A
a; b; g /◦
3
˚
V/A
Z; Z¢
Density (calculated)/Mg m-3
Absorption coefficient Mo Ka/mm-1
Max. and min. transmission
F(000)
q Range for data collection/◦
Index ranges
3.12–25.03
-13 ≤ h ≤ 13, -12 ≤ k ≤ 12,
-32 ≤ l ≤ 32
21908
Reflections collected
Independent reflections
Measured reflections with I ≥ 2s(I)
Completeness to qmax
5894 [Rint = 0.0744]
4287
98.6
5894/0/389
1.091
R1 = 0.0856, wR2 = 0.1567
R1 = 0.1260, wR2 = 0.1796
1.870; -0.826
Data/restraints/parameters
Goodness-of-fit on F2
11 Commercial IC50 determinations were performed by MDS Pharma
Services (http://www.mdsps.com): assay 112250, CTSB (Cathepsin B).
Brief details: substrate 20 mM Boc-Leu-Arg-Arg-AMC, 1% DMSO
vehicle by spectrofluorimetric quantitation of AMC.
Final R indices (observed data)
Final R indices (all data)
Largest diff. peak; hole/e A
-3
˚
4302 | Dalton Trans., 2009, 4299–4303
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The Royal Society of Chemistry 2009
©

12 A. Casini, C. Gabbiani, F. Sorrentino, M. P. Rigobello, A. Bindoli,
T. J. Geldbach, A. Marrone, N. Re, C. G. Hartinger, P. J. Dyson and L.
Messori, J. Med. Chem., 2008, 51, 6773–6781.
13 R. W. W. Hooft, 1998, COLLECT Data Collection Software, Nonius
B. V., Delft.
15 G. M. Sheldrick, 2007, SADABS, Bruker AXS Inc., Madison,
Wisconsin, USA.
16 M. C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G. L. Cascarano,
L. De Caro, C. Giacovazzo, G. Polidori and R. Spagna, J. Appl.
Crystallogr., 2005, 38, 381–388.
14 Z. Otwinowski and W. Minor, in Methods in Enzymology, ed. C. W.
Carter and R. M. Sweet, Academic Press, New York, 1997, vol. 276,
pp. 307–326.
17 G. M. Sheldrick, Acta Crystallogr., 2008, A64, 112–122.
18 L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565–565.
19 M. Nardelli, J. Appl. Crystallogr., 1995, 28, 659–659.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 4299–4303 | 4303
©