Synthesis, characterization, lipophilicity and cytotoxic properties of novel bis(carboxylato)oxalatobis(1-propylamine)platinum(IV) complexes

Article abstract of DOI:10.1016/j.ica.2019.03.036

A series of novel bis(carboxylato)oxalatobis(1-propylamine)platinum(IV) complexes as well as an ethylamine analog were synthesized. The compounds are either symmetrical with both axial ligands consisting of monoesters of succinic acid, or unsymmetrical, with one axial ligand being acetate. The compounds were characterized in detail by elemental analysis, mass spectrometry and multinuclear (¹H, ¹³C, ¹⁵N, ¹⁹⁵Pt) NMR spectroscopy. The reduction behavior was followed by NMR spectroscopy, while lipophilicity was determined by analytical reversed-phase HPLC measurements. The capacity of inhibiting proliferation of the human cancer cell lines A549 (non-small cell lung cancer), CH1(PA-1) (ovarian teratocarcinoma) and SW480 (colon carcinoma) was evaluated by the MTT assay. In the most sensitive cell line CH1(PA-1), all compounds exhibited IC₅₀ values in the lower μM range. In general, the IC₅₀ values decreased with increasing lipophilicity within the two compound series. Nevertheless, replacing one of the succinic ester ligands with acetate has a rather marginal impact on antiproliferative activity and is hardly disadvantageous.

Full text of DOI:10.1016/j.ica.2019.03.036

Accepted Manuscript
Synthesis, characterization, lipophilicity and cytotoxic properties of novel
bis(carboxylato)oxalatobis(1-propylamine)platinum(IV) complexes
Selin Hizal, Michaela Hejl, Michael A. Jakupec, Markus Galanski, Bernhard K.
Keppler
PII:
S0020-1693(19)30226-9
https://doi.org/10.1016/j.ica.2019.03.036
ICA 18841
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
16 February 2019
21 March 2019
27 March 2019
Please cite this article as: S. Hizal, M. Hejl, M.A. Jakupec, M. Galanski, B.K. Keppler, Synthesis, characterization,
lipophilicity and cytotoxic properties of novel bis(carboxylato)oxalatobis(1-propylamine)platinum(IV) complexes,
Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.03.036
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Synthesis, characterization, lipophilicity and cytotoxic properties of
novel bis(carboxylato)oxalatobis(1-propylamine)platinum(IV)
complexes
Selin Hizal,a Michaela Hejl,a Michael A. Jakupec,a,b Markus Galanski,a,* Bernhard K.
Kepplera,b,*
a
Institute of Inorganic Chemistry, University of Vienna, Waehringer Strasse 42, A-1090 Vienna,
Austria
b
Research Cluster “Translational Cancer Therapy Research”, University of Vienna, Waehringer
Strasse 42, A-1090 Vienna, Austria
Corresponding authors Tel. +43-1-4277-52600
E-mail address: markus.galanski@univie.ac.at, bernhard.keppler@univie.ac.at
1

Abstract
A series of novel bis(carboxylato)oxalatobis(1-propylamine)platinum(IV) complexes as well
as an ethylamine analog were synthesized. The compounds are either symmetrical with both
axial ligands consisting of monoesters of succinic acid, or unsymmetrical, with one axial
ligand being acetate. The compounds were characterized in detail by elemental analysis,
mass spectrometry and multinuclear (1H, 13C, 15N, 195Pt) NMR spectroscopy. The reduction
behavior was followed by NMR spectroscopy, while lipophilicity was determined by analytical
reversed-phase HPLC measurements. The capacity of inhibiting proliferation of the human
cancer cell lines A549 (non-small cell lung cancer), CH1(PA-1) (ovarian teratocarcinoma)
and SW480 (colon carcinoma) was evaluated by the MTT assay. In the most sensitive cell
line CH1(PA-1), all compounds exhibited IC50 values in the lower µM range. In general, within
the two compound series, the IC50 values decreased with increasing lipophilicity.
Nevertheless, replacing one of the succinic ester ligands with acetate has a rather marginal
impact on antiproliferative activity and is hardly disadvantageous.
Keywords: platinum; anticancer agents; synthesis; cytotoxicity; NMR spectroscopy
1. Introduction
Chemotherapy involving metal complexes is a rewarding and widely accepted mode of
treatment. Specifically, the platinum complexes cisplatin, carboplatin and oxaliplatin (Figure
1), bearing ligands of a varying nature, are often administered intravenously to patients with
different forms of neoplasms.
Figure 1. Chemical structures of cisplatin, carboplatin and oxaliplatin.
2

The platinum(II)-based agents are taken up from the blood stream by tissue and ultimately
bind to the cellular DNA (cisplatin, carboplatin, and oxaliplatin) [1] or additionally cause
ribosome biogenesis stress (oxaliplatin) [2], preferably in the cancer cell; in last
consequence, apoptotic mechanisms are initiated, leading to the death of the cell. Cisplatin
has a broad spectrum of application and is used in many chemotherapies [3]. In spite of its
success in the treatment of tumors, this drug has various side-effects such as nephrotoxicity,
nausea and neurotoxicity. Only a certain proportion of the administered drug reaches the
DNA and a significant amount binds to functional proteins, causing systemic toxicity [4] or
leading to detoxification of cisplatin. Modification of the leaving ligands leads to a different
bio-distribution profile and change of toxic side effects (e.g., carboplatin and oxaliplatin).
Carboplatin, bearing a more inert chelate ligand, causes much less nephrotoxicity than
cisplatin. Modification of the non-leaving ammine ligand as is the case in oxaliplatin,
however, leads to an altered anticancer profile. Oxaliplatin is active in colorectal carcinomas,
which are known to be resistant to cis- and carboplatin.
During the last two decades, octahedrally coordinated platinum(IV) complexes have been
developed, which are kinetically inert compared to their platinum(II) counterparts [5]. They
act as pro-drugs and have to be reduced inside or outside the tumor cell to the respective
platinum(II) complexes to be activated [6]. The reduction process may be favored by the
hypoxic medium of the tumor cell [7], so that the inert platinum(IV) complex is activated
preferably at the site of the tumor, where the active form of the drug is desired. Thus,
selectivity over healthy tissue is achieved. One positive consequence of the kinetic inertness
outside of the tumor tissue is that systemic toxicity is reduced. In addition, delivery of the
intact drug at the target site is improved as a result of decreased deactivation by binding,
e.g., to glutathione, which helps to overcome platinum resistance [8]. Further, platinum(IV)
drugs can be administered orally, which could be much more convenient for the patient. In
terms of drug design, the octahedral coordination sphere offers a broad range of possibilities
3

for the fine-tuning of pharmacological properties of the compound. Nevertheless, no Pt(IV)
complex has gained clinical approval up to now [9] .
As can be deduced from the mechanism of activation, the drug’s anticancer efficacy depends
strongly on the reduction potential [10,11]. Complexes with axial carboxylato ligands possess
an intermediate redox potential in comparison to analogs with chlorido or hydroxido ligands
[12], conferring the drug sufficient stability to reach the target site but also an appropriate
degree of therapeutic activity without causing severe systemic toxicity. In fact, the promising
results
obtained
in
clinical
evaluation
of
satraplatin,
(OC-43)-
bis(acetato)amminedichlorido(cyclohexylamine)platinum(IV), can be explained in part by this
combination of properties [13,14].
Bulky amine ligands such as isopropylamine or cyclohexylamine, however, would contribute
to faster reduction by destabilizing the platinum(IV) center [15,16]. The 1-propylamine ligand
employed here is sterically less demanding so that a positive effect in terms of stability and,
consequently, of anticancer properties is expected.
Lipophilicity is an important parameter for the bioavailability of a compound, as the passage
through cell membranes is facilitated [9]. With regard to platinum drugs, an increased
lipophilicity frequently correlates with enhanced cytotoxicity [11]. Our group has previously
investigated the influence of lipophilicity on cytotoxicity by varying axial and equatorial
ligands within a series of dicarboxylatoplatinum(IV) complexes [13,17,18,19] , some of which
have IC50 values in the micromolar range. A rough correlation was demonstrated between
the IC50 values of such complexes and their lipophilicity [17].
In this work, we present a series of novel platinum(IV) complexes with 1-propylamine and
oxalate as equatorial ligands and succinic esters of varying chain length (or acetate in the
case of unsymmetric complexes) as axial ligands. Compared to typical am(m)ine ligands, the
presence of two equatorial 1-propylamine ligands renders these compounds significantly
more lipophilic, which might allow a higher accumulation even after reduction to the
corresponding platinum(II) species.
4

2. Experimental
2.1 Materials and Methods: All chemicals were obtained from commercial suppliers and
used as received. K2[PtCl4] was purchased from Johnson Matthey (Switzerland), potassium
iodide (99%) from Alfa Aesar, oxalic acid (99%) from Fluka, silver nitrate (99.85%) from
Acros, 1-propylamine (98%) from TCI Europe, succinic anhydride (99%) from Acros Fisher,
hydrogen peroxide (30% solution) from Sigma-Aldrich, dimethyl formamide (99.8%) from
Acros Organics, propanol (99.5%) and butanol (99%) from Sigma-Aldrich. Methanol and
ethanol were dried before use. Water was purified by reversed osmosis and double
distillation before use. Reactions involving platinum compounds were performed under light
protection and with glass-coated magnetic stirring bars. NMR spectroscopy measurements
were performed with a Bruker AVANCE NEO 500 MHz spectrometer at 500.32 (1H), 125.81
(13C), 50.70 (15N), and 107.38 (195Pt) MHz at 25 °C. The solvent resonances were used as
1
internal references for H {DMSO-d6, δ = 2.51 ppm, CDCl3, δ = 7.26 ppm, DMF-d7, δ = 2.75
ppm} and 13C (DMSO-d6, 40.0 ppm, CDCl3, δ = 77.4 ppm, DMF-d7, δ = 29.76 ppm). The 195Pt
and 15N NMR resonances were referenced relative to external K2[PtCl4] or NH4Cl,
respectively. High-resolution electrospray ionization mass spectra were measured with a
Bruker maXis ESI-QqTOF (electrospray ionization quadrupole–quadrupole time-of-flight)
spectrometer in the positive mode with ACN/MeOH 1 % H2O as the solvent (ACN =
acetonitrile). Elemental analyses were performed at the microanalytical laboratory of the
Faculty of Chemistry of the University of Vienna with a Eurovector EA3000 elemental
analyzer.
2.2 Synthesis
The diiodidoplatinum(II) precursors 1 and 7 (Schemes 1 and 2) were prepared by reacting
K2[PtCl4] with an excess of KI and the respective amines, namely 1-propylamine or
ethylamine. Iodido ligands were removed with AgNO3 according to Dhara's method [20], and
replaced by adding oxalic acid to obtain the oxalatoplatinum(II) complexes 2 and 8. Oxidation
with 30% hydrogen peroxide in aqueous solution yielded the corresponding dihydroxido
5

species 3 and 9. Oxidation of 2 in a mixture of acetic acid and 30% hydrogen peroxide in
aqueous solution yielded the asymmetric precursor (OC-6-44)-acetatohydroxidooxalatobis(1-
propylamine)platinum(IV) (5). The respective noncyclic anhydride of the succinic monoester
(2-4 eq) in absolute DMF was added to the respective platinum(IV) precursor (3, 5 or 9) in an
oven-dried nitrogen flask flushed with argon and the suspension was stirred at 70 °C until a
clear solution formed. Afterwards the solution was stirred at room temperature overnight.
DMF was removed under reduced pressure and the crude product was purified by HPLC.
General procedure for the synthesis of succinic acid monoesters L1-L4
Succinic anhydride was refluxed in an excess of the respective alcohol under anhydrous
conditions and the remaining alcohol was removed under reduced pressure. Chloroform was
added to precipitate remaining succinic acid, which was subsequently removed by filtration.
The succinic acid mono ester was dried in vacuo and was used without further purification.
3-(Methoxycarbonyl)propanoic acid (L1). Methanol (10 mL, 0.25 mol, 10 eq), succinic
anhydride (2.5 g, 0.025 mol), 2 h at 70 °C. The colorless liquid was cooled to -18 °C for 15
1
min to give a white crystalline solid which was dried in vacuo. Yield 3.237 g (98%), H NMR
(CDCl3): δ 3.74 (s, 3H, H5), 2.74–2.71 (m, 2H, H2/H3), 2.68–2.65 (m, 2H, H2/H3) ppm.
3-(Ethoxycarbonyl)propanoic acid (L2). Ethanol (15 mL, 0.25 mol, 10 eq), succinic
1
anhydride (2.5 g, 0.025 mol), 2 h at 80 °C. Yield 2.885 g (79%), colorless liquid. H NMR
3
(CDCl3): δ = 4.19 (q, 2H, H5, JH,H = 7.1 Hz), 2.72–2.70 (m, 2H, H2/H3), 2.66–2.63 (m, 2H,
H2/H3), 1.28 (t, 3H, H6, 3JH,H = 7.1 Hz) ppm.
3-(Propoxycarbonyl)propanoic acid (L3). Propanol (18.70 mL, 0.25 mol, 10 eq), succinic
1
anhydride (2.5 g, 0.025 mol), 2 h at 100 °C. Yield 3.323 g (83%), colorless liquid. H NMR
3
(CDCl3): δ = 4.06 (t, 2H, H5, JH,H = 6.7 Hz), 2.70–2.68 (m, 2H, H2/H3), 2.64–2.61 (m, 2H,
H2/H3), 1.65 (m, 2H, H6), 0.93 (t, 3H, H7, 3JH,H = 7.5 Hz) ppm.
3-(Butoxycarbonyl)propanoic acid (L4). Butanol (22.87 mL, 0.25 mol, 10 eq), succinic
1
anhydride (2.5 g, 0.025 mol), 2 h at 127 °C. Yield 3.715 g (85%), colorless liquid. H NMR
6

3
(CDCl3): δ = 4.10 (t, 2H, H5, JH,H = 6.7 Hz), 2.70–2.68 (m, 2H, H2/H3), 2.64–2.61 (m, 2H,
H2/H3), 1.61 (m, 2H, H6), 1.37 (m, 2H, H7), 0.92 (t, 3H, H8, 3JH,H = 7.1 Hz) ppm.
General procedure for the synthesis of anhydrides L5-L8 of succinic acid mono esters
Acetic anhydride and the corresponding succinic acid mono ester were heated to 110 °C for
1.5 h under anhydrous conditions. The clear solution was allowed to cool down to room
temperature and acetic acid and the unreacted excess of acetic anhydride were distilled off
in vacuo.
3-(Methoxycarbonyl)propanoic anhydride (L5). Acetic anhydride (6.0 mL, 63.60 mmol,
4.55 eq), L1 (1.850 g, 14.00 mmol). Yield 1.568 (91%) colorless liquid. 1H NMR (CDCl3): δ =
3.74 (s, 6H, H5), 2.83 (t, 2H, H2/H3, 3JH,H = 6.6 Hz), 2.70 (t, 2H, H2/H3, 3JH,H = 6.6 Hz) ppm.
3-(Ethoxycarbonyl)propanoic anhydride (L6). Acetic anhydride (7.0 mL, 74.2 mmol, 4 eq),
L2 (2.711 g, 18.55 mmol). Yield 2.490 g (98%) colorless liquid. 1H NMR (CDCl3): δ = 4.19 (q,
4H, H5, 3JH,H = 7.1 Hz), 2.82 (t, 2H, H2/H3, 3JH,H = 6.6 Hz), 2.69 (t, 2H, 2H/3H, 3JH,H = 6.5 Hz),
1.29 (t, 6H, H6, 3JH,H = 7.1 Hz) ppm.
3-(Propoxycarbonyl)propanoic anhydride (L7). Acetic anhydride (9.0 mL, 95.40 mmol, 4.6
1
eq), L3 (3.310 g, 20.66 mmol). Yield 3.885 g (97%) colorless liquid. H NMR (CDCl3): δ =
4.06 (t, 4H, H5, 3JH,H = 6.7 Hz), 2.79 (t, 2H, H2/H3, 3JH,H = 6.6 Hz), 2.67 (t, 2H, 2H/3H, 3JH,H
6.5 Hz), 1.65 (m, 4H, H6), 0.93 (t, 6H, H7, 3JH,H = 7.5 Hz) ppm.
=
3-(Butoxycarbonyl)propanoic anhydride (L8). Acetic anhydride (8.1 mL, 85.5 mmol, 4 eq),
1
L3 (3.700 g, 21.24 mmol). Yield 3.885 g (97%) colorless liquid. H NMR (CDCl3): δ = 4.10 (t,
3
3
3
4H, H5, JH,H = 6.73 Hz), 2.79 (t, 2H, H2/H3, JH,H = 6.6 Hz), 2.66 (t, 2H, 2H/3H, JH,H = 6.5
Hz), 1.61 (m, 4H, H6), 1.37 (m, 4H, H7), 0.92 (t, 6H, H8, 3JH,H = 7.1 Hz) ppm.
(SP-4-2)-Diiodidobis(1-propylamine)platinum(II) (1): Potassium tetrachloroplatinate (900
mg, 2.17 mmol) was dissolved in 20 mL H2O and potassium iodide (1800 mg, 10.84 mmol, 5
eq) was added. The mixture was stirred at rt for 20 min. A solution of 1-propylamine (513 mg,
7

8.67 mmol, 4 eq) in 5 mL of H2O were added and stirred at rt for 3h. The precipitate was
collected by filtration. Yield: 1073.5 mg (87%), yellow solid.
(SP-4-2)-Oxalatobis(1-propylamine)platinum(II)
(2):
(SP-4-2)-Diiodidobis(1-
propylamine)platinum(II) (1) (1070.0 mg, 1.88 mmol) and AgNO3 (609 mg, 3.58 mmol) were
suspended in 20 mL of H2O and stirred overnight at rt. AgI was filtered off twice using a MN-
GF-3 filter paper and oxalic acid (169.2 mg, 1.88 mmol) was added. The mixture was stirred
for 2h at rt, cooled and the precipitate collected by filtration. The filtrate was concentrated
and the precipitate filtered two more times. Yield: 531.6 mg (70%), pale yellow solid. HRMS:
(pos) m/z 424.0796 [M+Na+]+.
(OC-6-33)-Dihydroxidooxalatobis(1-propylamine)platinum(IV) (3): (SP-4-2)-Oxalatobis(1-
propylamine)platinum(II) (2) (505 mg, 1.31 mmol) was suspended in 15 mL of a 30%
hydrogen peroxide solution and stirred at 50°C for 2 h, whereas the solution became clear.
1
Then, the solution was lyophilized. Yield: 545.1 mg (95%), pale yellow solid. H NMR (DMF-
3
d7): δ = 6.69 (bs, 4H, NH2), 2.85 (m, 4H, H1), 1.79 (m, 4H, H2), 0.94 (t, JH,H = 7.4 Hz, 6H,
H3). 13C NMR (DMF-d7): δ = 162.1 (C4), 46.3 (C1), 22.2 (C2), 10.7 (C3) ppm. 195Pt NMR
15
(DMF-d7): δ = 3081 ppm. N NMR (DMF-d7): δ = -31.6 ppm. HRMS (ACN/MeOH 1% H2O):
calcd. 435.10, found (pos) m/z 458.0859 [M+Na+]+.
(OC-6-33)-Bis((4-methoxy)-4-oxobutanoato)oxalatobis(1-propylamine)platinum(IV) (4a):
In an oven-dried 50 mL nitrogen flask flushed with argon, 3 (510.4 mg, 1.17 mmol) and L5
(1155.0 mg, 4.69 mmol, 4 eq.) were suspended in 9 mL of DMF. The suspension was stirred
at 70 °C for 15 min and a clear solution was obtained, which was stirred at 45 °C overnight.
DMF was removed in vacuo and the residue was purified via prep-HPLC (20-25%
1
ACN:H2O). Yield: 241.7 mg (31%), white solid. H NMR (DMSO-d6): δ = 7.23 (bs, 4H, NH2),
3.56 (s, 6H, H9), 2.61 (m, 4H, H1), 2.55 (m, 4H, H6), 2.45 (m, 4H, H7), 1.61 (m, 4H, H2),
0.92 (t, 3JH,H = 7.4 Hz, 6H, H3) ppm. 13C NMR (DMSO-d6): δ = 179.1 (C5), 173.1 (C8), 163.9
(C4), 51.8 (C9), 46.6 (C1), 30.6 (C6), 29.8 (C7), 22.4 (C2), 11.4 (C3) ppm. 195Pt NMR
(DMSO-d6): δ = 3371 ppm. 15N NMR (DMSO-d6): δ = -33.8 ppm. HRMS (ACN/MeOH 1%
8

H2O): calcd. 663.16, found (pos) m/z 686.1480 [M+Na+]+, 664.1661 [M+H+]. C18H32N2O12Pt:
calcd. C 32.58, H 4.86, N 4.22; found C 32.62, H 5.00, N 4.06.
(OC-6-33)-Bis((4-ethoxy)-4-oxobutanoato)oxalatobis(1-propylamine)platinum(IV) (4b) In
an oven-dried 50 mL nitrogen flask flushed with argon, 3 (545.0 mg, 1.25 mmol) and L6
(1030 mg, 3.75 mmol, 3 eq.) were suspended in 9 mL of DMF. The suspension was stirred at
70 °C for 15 min and a clear solution was obtained, which was stirred at 45 °C overnight.
DMF was removed in vacuo and the crude was purified via prep-HPLC (20-33% ACN:H2O).
Yield: 275.5 mg (32%), white solid. 1H NMR (DMSO-d6): δ = 7.24 (bs, 4H, NH2), 4.02 (q, 3JH,H
= 7.1 Hz, 4H, H9), 2.61 (m, 4H, H1), 2.54 (m, 4H, H6), 2.44 (m, 4H, H7), 1.61 (m, 4H, H2),
1.16 (t, , 3JH,H = 7.1 Hz, 6H, H10), 0.92 (t, , 3JH,H = 7.4 Hz, 6H, H3) ppm. 13C NMR (DMSO-d6):
δ = 179.6 (C5), 172.7 (C8), 163.9 (C4), 60.3 (C9), 46.6 (C1), 30.5 (C6), 29.8 (C7), 22.3 (C2),
14.4 (C10), 11.3 (C3) ppm. 195Pt NMR (DMSO-d6): δ = 3371 ppm. 15N NMR (DMSO-d6): δ = -
33.5 ppm. HRMS (ACN/MeOH 1% H2O): calcd. 691.19, found (pos) m/z 692.1990 [M+H+]+.
C20H36N2O12Pt: calcd. C 34.73, H 5.25, N 4.05; found C 34.77, H 4.99, N 3.99.
(OC-6-33)-Oxalatobis((4-propoxy)-4-oxobutanoato)bis(1-propylamine)platinum(IV) (4c):
In an oven-dried 50 mL nitrogen flask flushed with argon, 3 (285.1 mg, 0.655 mmol) and L7
(396,0 mg, 1,31 mmol, 2 eq.) were suspended in 7 mL of DMF. The suspension was stirred
at 70 °C for 1 h and a clear solution was obtained, which was stirred at rt overnight. DMF
was removed in vacuo and the residue purified via preparative HPLC (23-49% ACN:H2O).
1
The combined product fractions were lyophilized. Yield: 163.0 mg (34%), white solid. H
3
NMR (DMSO-d6): δ = 7.25 (bs, 4H, NH2), 3.94 (q, JH,H = 6.7 Hz, 4H, H9), 2.62 (m, 4H, H1),
3
2.54 (m, 4H, H6), 2.44 (m, 4H, H7), 1.61 (m, 4H, H2), 1.57 (m, 4H, H10), 0.92 (t, JH,H = 7.3
3
Hz, 6H, H3), 0.87 (t, JH,H = 7.4 Hz, 6H, H11) ppm. 13C NMR (DMSO-d6): δ = 179.6 (C5),
172.7 (C8), 163.9 (C4), 65.8 (C9), 46.5 (C1), 30.6 (C6), 29.9 (C7), 22.4 (C2), 21.9 (C10),
11.4 (C3), 10.7 (C11) ppm. 195Pt NMR (DMSO-d6): δ = 3372 ppm. 15N NMR (DMSO-d6): δ = -
34.1 ppm. HRMS (ACN/MeOH 1% H2O): calcd. 719.22, found (pos) m/z 742.2113 [M+Na+]+.
C22H40N2O12Pt: calcd. C 36.72, H 5.60, N 3.89; found C 36.58, H 5.78, N 3.97.
9

(OC-6-44)-Acetato(hydroxido)oxalatobis(1-propylamine)platinum(IV)
(5):
(SP-4-2)-
Oxalatobis(1-propylamine)platinum(II) (2) (676,0 mg, 1,68 mmol) was suspended in 10 mL of
acetic acid and 500 µl of a 30% H2O2 solution were added. The solution was stirred for 20
min before another 500 µl portion of 30% H2O2 was added. After stirring for 20 min, further
1.4 mL of 30% H2O2 were added and the mixture was stirred at rt overnight. Acetic acid and
hydrogen peroxide were removed under reduced pressure. The crude product was used
1
without further purification. Yield: 365.5 mg (45%), pale yellow solid. H NMR (DMF-d7): δ =
3
7.67 (bs, 4H, NH2), 2.87 (m, 4H, H1), 1.99 (s, 3H, H6), 1.73 (m, 4H, H2), 0.97 (t, JH,H = 7.4
Hz, 6H, H3). 13C NMR (DMF-d7): δ = 179.1 (C5), 162.1 (C4), 46.5 (C1), 22.3 (C2), 21.9 (C6),
10.7 (C3) ppm. 195Pt NMR (DMF-d7): δ = 3370 ppm. 15N NMR (DMF-d7): δ = -35.8 ppm.
HRMS: (ACN/MeOH 1% H2O): calcd. 477.11, found (pos) m/z 500.0961 [M+Na+]+.
General procedure for the synthesis of complexes 6a-6d
In an oven-dried 100 mL flask flushed with argon, (OC-6-44)-acetato(hydroxido)oxalatobis(1-
propylamine)platinum(IV) (5) and the respective anhydride were suspended in DMF. The
suspension was stirred at 70 °C for several hours and a clear solution was obtained, which
was stirred at rt overnight. DMF was removed in vacuo and the residue purified via
preparative HPLC (13-35% ACN/H2O). The combined product fractions were lyophilized.
(OC-6-44)-Acetatooxalato(4-methoxy-4-oxobutanoato)bis(1-propylamine)platinum(IV)
(6a): 5 (245.1 mg, 0.514 mmol), L5 (253.5 mg, 1.026 mmol, 2 eq.) 5 mL of DMF, 70 °C for 7
1
h. Yield: 174.0 mg (57%) , white powder. H NMR (DMSO-d6): δ = 7.38 (bs, 2H, NH2), 7.17
(bs, 2H, NH2), 3.56 (s, 6H, H11), 2.63 (m, 4H, H1), 2.55 (m, 2H, H8), 2.45 (m, 2H, H9), 1.96
(s, 3H, H6), 1.63 (m, 4H, H2), 0.92 (t, 3JH,H = 7.4 Hz, 6H, H3) ppm. 13C NMR (DMSO-d6): δ =
179.4 (C7), 178.8 (C5), 173.2 (C10), 163.9 (C4), 51.8 (C11), 46.6 (C1), 30.6 (C8), 29.8 (C9),
23.1 (C6), 22.4 (C2), 11.5 (C3) ppm. 195Pt NMR (DMSO-d6): δ = 3373 ppm. 15N NMR
(DMSO-d6): δ = -34.1 ppm. HRMS (ACN/MeOH 1% H2O): calcd. 591.14, found (pos) m/z
10

614.1266 [M+Na+]+, 1205.2631 [2M+Na+]+. C15H28N2O10Pt: calcd. C 30.46, H 4.77, N 4.74;
found C 30.17, H 4.75, N 4.62.
(OC-6-44)-Acetato(4-ethoxy-4-oxobutanoato)oxalatobis(1-propylamine)platinum(IV)
(6b): 5 (360.1 mg, 0.754 mmol), L6 (310.5 mg, 1.131 mmol, 1.5 eq.), 7 mL of DMF, 70 °C for
1
2 h. Yield: 347.28 mg (76%), white powder. H NMR (DMSO-d6): δ = 7.38 (bs, 2H, NH2),
3
7.17 (bs, 2H, NH2), 4.02 (q, , JH,H = 7.1 Hz, 2H, H11), 2.63 (m, 4H, H1), 2.54 (m, 2H, H8),
2.44 (m, 2H, H9), 1.96 (s, 3H, H6), 1.61 (m, 4H, H2), 1.16 (t, , 3JH,H = 7.1 Hz, 3H, H12), 0.92
3
(t, JH,H = 7.4 Hz, 6H, H3) ppm. 13C NMR (DMSO-d6): δ = 179.5 (C7), 178.8 (C5), 172.7
(C10), 163.9 (C4), 60.3 (C11), 46.6 (C1), 30.6 (C8), 29.9 (C9), 23.1 (C6), 22.3 (C2), 14.5
(C12), 11.5 (C3) ppm. 195Pt NMR (DMSO-d6): δ = 3373 ppm. 15N NMR (DMSO-d6): δ = -
34.20 ppm. HRMS (ACN/MeOH 1% H2O): calcd. 605.15, found (pos) m/z 628.1435
[M+Na+]+, 1233.2971 [2M+Na+]+. C16H30N2O10Pt: calcd. C 31.74, H 4.99, N 4.63; found C
31.44, H 4.96, N 4.54.
(OC-6-44)-Acetatooxalato(4-propoxy-4-oxobutanoato)bis(1-propylamine)platinum(IV)
(6c): 5 (120.1 mg, 0.251 mmol), L7 (114.0 mg, 0.377 mmol, 1.5 eq.), 7 mL of DMF, 70 °C for
1
3 h. Yield: 53 mg (34%), white solid. H NMR (DMSO-d6): δ = 7.38 (bs, 2H, NH2), 7.17 (bs,
2H, NH2), 3.94 (q, 3JH,H = 6.7 Hz, 2H, H11), 2.63 (m, 4H, H1), 2.55 (m, 2H, H8), 2.45 (m, 2H,
3
H9), 1.96 (s, 3H, H6), 1.61 (m, 4H, H2), 1.57 (m, 2H, H12), 0.92 (t, JH,H = 7.3 Hz, 6H, H3),
0.87 (t, 3JH,H = 7.4 Hz, 3H, H13) ppm. 13C NMR (DMSO-d6): δ = 179.6 (C7), 178.8 (C5), 172.7
(C10), 163.9 (C4), 65.7 (C11), 46.5 (C1), 30.6 (C8), 29.9 (C9), 23.1 (C6), 22.4 (C2), 21.9
15
(C12), 11.4 (C3), 10.7 (C13) ppm. 195Pt NMR (DMSO-d6): δ = 3373 ppm. N NMR (DMSO-
d6): δ = -33.9 ppm. HRMS (ACN/MeOH 1% H2O): calcd. 619.17, found (pos) m/z 642.1917
[M+Na+]+. C17H32N2O10Pt: calcd. C 32.96, H 5.21, N 4.52; found C 32.78, H 5.07, N 4.49.
(OC-6-44)-Acetato(4-butoxy-4-oxobutanoato)oxalatobis(1-propylamine)platinum(IV)
(6d): 5 (365.1 mg, 0.764 mmol), L8 (505.12 mg, 0.377 mmol, 2 eq.), 7 mL of DMF, 70 °C for
3 h. Yield: 280.5 mg (58%), white powder. 1H NMR (DMSO-d6): δ = 7.38 (bs, 2H, NH2), 7.17
(bs, 2H, NH2), 3.98 (s, 2H, H11), 2.63 (m, 4H, H1), 2.55 (m, 2H, H8), 2.45 (m, 2H, H9), 1.96
3
(s, 3H, H6), 1.62 (m, 4H, H2), 1.53 (m, 2H, H12), 1.32 (m, 2H, H13), 0.93 (t, JH,H = 7.4 Hz,
11

3
6H, H3), 0.89 (t, JH,H = 7.5 Hz, 3H, H14) ppm. 13C NMR (DMSO-d6): δ = 179.5 (C7), 178.8
(C5), 172.7 (C10), 163.9 (C4), 64.1 (C11), 46.6 (C1), 30.6 (C8), 30.2 (C12), 29.9 (C9), 23.1
(C6), 22.4 (C2), 19.08 (C13), 14.0 (C14), 11.5 (C3) ppm. 195Pt NMR (DMSO-d6): δ = 3373
ppm. 15N NMR (DMSO-d6): δ = -34.1 ppm. HRMS (ACN/MeOH 1% H2O): calcd. 633.19,
found (pos) m/z 656.1746 [M+Na+]+, 1289.3606 [2M+Na+]+. C18H34N2O10Pt: calcd. C 34.12, H
5.41, N 4.42; found C 33.81, H 5.14, N 4.34.
(SP-4-2)-Bis(ethylamine)diiodoplatinum(II) (7): Potassium tetrachloroplatinate (1300.6 mg,
3.13 mmol) was dissolved in 20 mL of H2O and potassium iodide (3000 mg, 15.65 mmol, 5
eq) was added. The mixture was stirred at rt for 20 min. A solution of ethylamine (564.4 mg,
12.52 mmol, 4 eq) in 5 mL of H2O were added and stirred at rt for 3h. The precipitate was
collected by filtration. Yield: 1575.5 mg (93%), yellow solid.
(SP-4-2)-Bis(ethylamine)oxalatoplatinum(II)
(8):
(SP-4-2)-
Diiodobis(ethylamine)platinum(II) (1484.5 mg, 2.75 mmol, 1 eq) and AgNO3 (887.54 mg, 5.22
mmol, 1.9 eq) were suspended in 20 mL of H2O and stirred overnight at rt. AgI was filtered
off twice using a MN-GF-3 filter paper and oxalic acid (247.58 mg, 2.75 mmol, 1 eq) was
added. The mixture was stirred for 2h at rt, cooled and the precipitate collected by filtration.
The filtrate was concentrated and the precipitate filtered two more times. Yield: 719.2 mg
(70%), yellow solid.
(OC-6-33)-Bis(ethylamine)dihydroxidooxalatoplatinum(IV)
(9):
(SP-4-2)-
Bis(ethylamine)oxalatoplatinum(II) (705 mg, 1,75 mmol) was suspended in 15 mL of a 30%
hydrogen peroxide solution and stirred at rt for 4 h, whereas the solution became clear.
Then, the solvent was removed in vacuo. Yield: 679.4 mg (95%), pale yellow solid.
(OC-6-33)-Bis(ethylamine)oxalatobis((4-propoxy)-4-oxobutanoato)platinum(IV) (10): In
an
oven-dried
50
mL
nitrogen
flask
flushed
with
argon,
(OC-6-33)-
bis(ethylamine)dihydroxidooxalatoplatinum(IV) (415.4 mg, 1.02 mmol) and 4-propoxy-4-
oxobutanoic anhydride (1233.5 mg, 4.08 mmol, 2 eq.) were suspended in 8 mL DMF (dry).
The suspension was stirred at 70 °C for 3 h and a clear solution was obtained, which was
stirred at rt overnight. DMF was removed in vacuo and the residue purified via prep-HPLC
12

(20-37% ACN:H2O). The combined product fractions were lyophilized. Yield: 236.3 mg
(33%), white powder. 1H NMR (DMSO-d6): δ = 7.20 (bs, 4H, NH2), 3.94 (t, 3JH,H = 6.7 Hz, 4H,
3
H8), 2.68 (m, 4H, H9), 2.55 (m, 4H, H5), 2.46 (m, 4H, H6), 1.57 (m, 4H, H1), 1.19 (t, JH,H
=
7.3 Hz, 6H, H10), 0.87 (t, 3JH,H = 7.4 Hz, 6H, H2) ppm. 13C NMR (DMSO-d6): δ = 179.4 (C4),
172.7 (C7), 164.0 (C3), 65.8 (C8), 30.6 (C5), 29.9 (C6), 21.9 (C1), 14.4 (C10), 10.7 (C2).
195Pt NMR (DMSO-d6): δ = 3374 ppm. 15N NMR (DMSO-d6): δ = -30.3 ppm. HRMS
(ACN/MeOH 1% H2O): calcd. 691.19, found (pos) m/z 714.1906 [M+Na+]+. C20H36N2O12Pt:
calcd. C 34.73, H 5.25, N 4.05; found C 34.52, H 4.90, N 3.93.
2.3 Determination of lipophilicity
Lipophilicity of all compounds was determined via their chromatographic retention factors k´
using reverse-phase HPLC and expressed as kw (the retention time in 100% aqueous
solvent). The analysis was performed on an Ultimate 3000 Dionex system controlled by the
Dionex Chromeleon 7.0 software. A reversed phase Acquity BEH C18 column (3.0 × 50 mm,
1.7 μm) from Waters served as stationary phase. The mobile phase consisted of different
ratios of methanol and water (Milli-Q, 18.2 MΩ cm, Milli-Q Advantage A10, Darmstadt,
Germany) with 0.1% TFA, ranging from 20% MeOH for the most hydrophilic to 70% MeOH
for the most lipophilic compound. Samples were prepared as 0.5 mM solutions in a 10%
mixture of MeOH and H2O and filtered through 0.45 μm nylon filters (Minisart RC 25,
Sartorius AG, Göttingen, Germany) before injection. A solution of potassium iodide (0.3 mM)
was used as external dead volume marker. The chromatographic conditions were as follows:
column temperature 25 °C, flow rate 0.6 mL/min, injection volume 10 μL, UV–vis detection at
210, 230, 252 and 318 nm. The capacity factor k′ is defined as the partition of a compound
between a stationary (apolar) and a mobile phase (polar) and can be determined via k′ = (tR −
t0) / t0 where tR denotes the retention time of the analyte and t0 the retention time of the
external dead volume marker. Retention time of each analyte was determined for at least 3
different mobile phase compositions to obtain capacity factors k′ in the linear range of 0.5 <
log k′ < 1.5. A linear plot of logk´ versus mobile phase composition follows the equation log k′
13

=logkw− SMeOH·ɸ where S is a specific constant for an analyte and a certain HPLC system,
and ɸ is the percentage of the organic component of the mobile phase [21]. Comparison of
different substances analyzed at various amounts of organic phase is possible by
extrapolation of the equation to the same mobile phase concentration. The y-intercept of the
linear log k′ correlation affords logkw values, which are the retention factors in 100% aqueous
phase.
2.4 Reduction behavior
Reduction of complexes 4a-c, 6a-d and the ethylamine complex 10 by ascorbic acid was
monitored by 1H NMR spectroscopy at ambient temperature. 0.5 mM (0.25 mM in case of 4c
and 10) solutions of the compounds were prepared in 50 mM phosphate buffer (in D2O, pD
1
=7.4) and H NMR spectra were measured over a period of 24 h and the stability of the
complexes in aqueous solution was confirmed. Then, ascorbic acid (12.5 mM or 6.25 mM, 25
1
equiv.) was added and H NMR spectra were recorded for several days. Reduction was
monitored either by following the decrease of intensity of triplets of the terminal methyl
groups of the propylamine moieties in Pt(IV) and Pt(II) species (0.86 ppm versus 0.82 ppm)
in case of complexes 4a, 4b and 6b or by the ratio of CH2C(O)O protons of the coordinated
or free succinato ligand (2.49 ppm versus 2.33 ppm) in case of complexes 4c, 6a, 6c, 6d, 10.
2.5 Cytotoxicity tests in human cancer cell lines
CH1(PA-1) (provided by L. R. Kelland, CRC Centre for Cancer Therapeutics, Institute of
Cancer Research, Sutton, UK; confirmed by STR profiling as PA-1 ovarian teratocarcinoma
cells at Multiplexion, Heidelberg, Germany), as well as SW480 colon carcinoma and A549
non-small cell lung cancer cells (both provided by B. Marian, Institute of Cancer Research,
Department of Medicine I, Medical University of Vienna, Austria) were grown as monolayer
cultures in 75 cm2 culture flasks (Starlab) in complete medium, i.e., minimal essential
medium (MEM) supplemented with 1 mM sodium pyruvate, 4 mM L-glutamine, 1% (v/v)
nonessential amino acids from 100-fold stock solution (all purchased from Sigma-Aldrich)
14

and 10% heat-inactivated fetal bovine serum (BioWest). Cultures were maintained at 37 °C
in a humidified atmosphere containing 5% CO2 in air.
Cytotoxicity of the compounds was determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-
diphenyl-2H-tetrazolium bromide (MTT) assay in at least three independent experiments as
follows: Cells were harvested from culture flasks by using trypsin-EDTA (Sigma-Aldrich), and
the following cell numbers were seeded into 96-well microculture plates (Starlab): 1 × 103
(CH1(PA-1)), 2 × 103 (SW480), 3 × 103 (A549) cells per well (each in 100 µL). Plates were
incubated for 24 h, then the test compounds were dissolved and serially diluted in complete
MEM and each dilution added to plates in triplicate (100 µL per well). After exposure for 96 h,
the medium was replaced with 100 µL of drug-free RPMI 1640 medium (Sigma-Aldrich) and
20 µL of MTT solution (5 mg/mL) in phosphate-buffered saline (Sigma-Aldrich). After
incubation for 4 h, these mixtures were removed, and the formazan products were dissolved
in 150 µl of DMSO per well. Optical densities at 550 nm (and at 690 nm as a reference) were
measured with a microplate reader (Biotek ELx808), and IC50 values were calculated from
concentration–effect curves by interpolation.
3. Results and Discussion
3.1 Synthesis
Platinum(IV) complexes were synthesized as shown in Schemes 1 and 2. The platinum(II)
precursor (SP-4-2)-diiodobis(1-propylamine)platinum(II) (1) was prepared by reacting [PtCl4]2-
with an excess of KI forming [PtI4]2− and further addition of 1-propylamine (Scheme 1).
According to Dhara's method [20], iodide ligands in 1 were removed with AgNO3;
subsequently, oxalic acid was added to obtain the oxalato complex 2.
Oxidation of 2 with 30% hydrogen peroxide in aqueous solution yielded the
dihydroxidoplatinum(IV) complex (3). Oxidation of 2 in a mixture of acetic acid and 30%
hydrogen peroxide in aqueous solution, however, gave the unsymmetric
acetatohydroxidoplatinum(IV) species 5.
15

Noncyclic anhydrides used for carboxylation of hydroxidoplatium(IV) complexes were
synthesized by mono-esterification of succinic anhydride with the corresponding anhydrous
alcohols (methanol, ethanol, 1-propanol or 1-butanol) and subsequent heating under reflux in
acetic anhydride. Carboxylation reactions were performed according to the following general
procedure to obtain final compounds 4a-4c, 6a-6d. The anhydride (2-3 eq) in absolute DMF
was added to the respective platinum(IV) precursor (3, 5 or 9) and the suspension was
heated until a clear solution was formed and stirred at room temperature overnight. DMF was
removed under reduced pressure and the crude product was purified via preparative HPLC
and dried in vacuo. The bis(ethylamine) analog 10 was synthesized in the same manner
(Scheme 2).
Scheme 1. Synthesis of novel bis(carboxylato)oxalatobis(1-propylamine)platinum complexes together
with the NMR numbering scheme. i) KI, 5 eq.; ii) 1-propylamine, 4 eq.; iii) AgNO3, 1.90 eq.; iv) C₂H₂O₄,
1 eq.; v) 30% H2O2/H2O; vi) CH3COOH/H2O2; vii) (ROCOCH2CH2CO)2O/DMF, 3 eq.; viii)
(ROCOCH2CH2CO)2O/DMF, 1.5 eq.
16

Scheme
2.
Synthesis
of
(OC-6-33)-bis(ethylamine)oxalatobis((4-propoxy)-4-
oxobutanoato)oxalatoplatinum(IV). i) KI, 5 eq.; ii) ethylamine, 4 eq.; iii) AgNO3, 1.90 eq.; iv) C₂H₂O₄, 1
eq.; v) 30% H2O2/H2O; vi) (CH3CH2CH2OCOCH2CH2CO)2O/DMF, 2 eq.
3.2 Characterization
The novel complexes were fully characterized by elemental analysis, one and two
dimensional multinuclear NMR spectroscopy (1H, 13C, 15N, 195Pt) and ESI-MS. The 195Pt
resonance for the platinum(II) complex 2 was detected at −263 ppm (measured relative to
K2PtCl4). Upon oxidation to the corresponding hydroxidoplatinum(IV) analogue 3 or 5, the
chemical shift was found at 3081 or 3370 ppm, respectively. Carboxylation of 3 or 5 resulted
in signals for complexes 4a-c and 6a-d at 3372 ppm, the ethylamine complex 10 resonated
at 3374 ppm. The symmetric complexes 4a–c displayed NH2 proton resonances as a broad
multiplet at around 7.24 ppm, whereas unsymmetric complexes 6a-d featured a marked
splitting of this signal, giving two broad multiplets at 7.38 and 7.17 ppm.
3.3 Lipophilicity
Determining major pharmacokinetic processes such as membrane and cell permeation,
biotransformation and clearance of a drug, lipophilicity is an essential physico-chemical
parameter [22].
In addition to its role in solubility as well as tissue distribution and metabolism, lipophilicity
can be a relevant factor modulating the cytotoxic properties of a platinum drug. As has been
shown for various types of platinum complexes, a good correlation exists between IC50 and
log P values [19,23]. Herein, we have determined the lipophilicity of all new compounds by
RP-HPLC, using isocratic methods and varying compositions of water/MeOH as the mobile
17

phase. In contrast to the classical shake-flask method, modern HPLC instrumentation is
sensitive, and providing fast results in parallel. Further, chromatographic retention factors as
a measure of partitioning between the mobile phase and the stationary phase (C18) can be
used as a direct measure of lipophilicity, without the need of converting them into log P
values [21]. The obtained retention factors are summarized in Table 1 as log kw values
(retention factor extrapolated to purely aqueous mobile phase). There is an unambiguous
correlation between IC50 and log kw values: the higher the lipophilicity, the higher the
cytotoxicity (the lower the IC50 value).
18

IC50 [µM]
log kw
reduction half-life [h]
A549
CH1(PA-1)
SW480
4a
4b
4c
6a
6b
6c
6d
10
> 400
291 ± 32
153 ± 35
> 300
270 ± 107
202 ± 77
119 ± 42
173 ± 32
12 ± 4
188 ± 33
119 ± 15
52 ± 15
103 ± 25
81 ± 11
61 ± 29
42 ± 15
73 ± 3
1.5
150
120
78
116
111
53
8.1 ± 1.1
4.0 ± 0.1
13 ± 1
9.1 ± 1.0
5.9 ± 2.2
2.8 ± 0.9
2.0 ± 0.3
2.42
2.88
1.82
2.13
2.57
2.84
2.73
55
72
Table 1. In vitro cytotoxicity, log kw and reduction half-lives of the new platinum(IV)
complexes. IC50 values (in µM, means ± standard deviations) of 4a–c, 6a–d and 10 were
determined by the MTT assay (exposure time: 96 h) in three human cancer cell lines.
3.4 Reduction by ascorbic acid
Since activation of platinum(IV) compounds by reduction is critical for their activity, the rate of
reduction was studied by incubating the synthesized platinum(IV) complexes with a 25-fold
excess of ascorbic acid in phosphate buffered D2O solution (pD 7.4). At that point it should
be emphasized, that in living systems a series of reductants is available, one of which is
ascorbic acid, which was chosen for the present study. A clear trend is visible for the
symmetrical as well as the unsymmetrical complexes: With increasing chain length of the
succinic ester, the reduction of the platinum(IV) compound to platinum(II) is facilitated.
Reduction half-lives of the presented platinum(IV) compounds are in the range of 2.5 to 5
days, which is slow but not unexpected for platinum(IV) compounds with such a type of N2O4
coordination sphere [24].
3.5 Cytotoxicity in cancer cell lines
Compounds 4a–c, 6a–d and 10 were tested for their capacity of inhibiting proliferation of the
human cancer cell lines A549 (non-small cell lung cancer), CH1(PA-1) (ovarian
teratocarcinoma) and SW480 (colon carcinoma) by the MTT assay, with CH1(PA-1) cells
19

being the most sensitive, the P-glycoprotein-expressing SW480 cells intermediately and the
highly multidrug-resistant A549 cells the least sensitive of these cell lines, as expected.
Concentration–effect curves (Figures 2 and 3) and the corresponding IC50 values (listed in
Table 1) reveal the following structure–activity relationships: Within each of the two
compound series, cytotoxic potency increases with the length of the terminal alkyl chain of
the axial ligand(s) (Figure 4). For each CH2 group subtracted, the IC50 values increase on
average 1.8–1.9 times within series 4a–c (containing two identical variable ligands) and 1.4–
1.7 times within series 6a–d (containing just one variable ligand), depending on the cell line
(and except for two cases where the IC50 value is either higher than the tested range or
varies around the maximum concentration applied). In each cell line, the ranges of IC50
values of the two series overlap largely. Hence, replacing one of the identical axial ligands of
4a–c with an acetato ligand is not necessarily disadvantageous and often has negligible
effect. Overall, 6d is the most cytotoxic within these series of compounds, reaching an IC50
value as low as 2.8 µM in CH1(PA-1) cells, second only to compound 10 (IC50 = 2.0 µM),
which, however, shows only intermediate potency in the other two cell lines. Despite its
higher potency in CH1(PA-1) cells, the overall activity of compound 10 is quite well
comparable with that of its analog 4c, from which it differs only by the presence of two
equatorial ethylamine instead of propylamine ligands.
20

120
100
80
60
40
20
0
A
4a
4b
4c
10
0.1
0.1
0.1
1
1
1
10
100
100
100
1000
Concentration (µM)
120
100
80
60
40
20
0
B
4a
4b
4c
10
10
1000
Concentration (µM)
120
100
80
60
40
20
0
C
4a
4b
4c
10
10
1000
Concentration (µM)
Figure 2. Concentration–effect curves based on the MTT assay (exposure time: 96 h) of
compounds 4a–c and 10 in A549 (A), CH1(PA-1) (B) and SW480 (C) cells.
21

A
120
100
80
60
40
20
0
6a
6b
6c
6d
0.1
0.1
0.1
1
1
1
10
100
100
100
1000
Concentration (µM)
120
100
80
60
40
20
0
B
6a
6b
6c
6d
10
1000
Concentration (µM)
120
100
80
60
40
20
0
C
6a
6b
6c
6d
10
1000
Concentration (µM)
Figure 3. Concentration–effect curves based on the MTT assay (exposure time: 96 h) of
compounds 6a–d in A549 (A), CH1(PA-1) (B) and SW480 (C) cells.
22

1000
100
10
SW480
CH1(PA-1)
1
4a 4b 4c
6a 6b 6c 6d 10
Figure 4. Structure–activity relationships in CH1(PA-1) and SW480 cells. Dependence of
IC50 values (means ± standard deviations) on the axial ligands for compound series 4a–4c
and 6a–6d, compared to compound 10.
5. Conclusion
Seven novel oxalatobis(1-propylamine)platinum(IV) complexes comprising three symmetrical
and four unsymmetrically coordinated complexes were synthesized and fully characterized;
additionally,
a
bis(ethylamine)oxalatoplatinum(IV) analogue was synthesized and
investigated for comparison in order to judge the influence of the length/lipophilicity of the
equatorial amine ligand. Lipophilicity of the compounds in either series as determined by
reverse-phase HPLC corresponds to the length of the alkyl chain of the succinic ester. In
general, there is a good correlation between the lipophilicity and cytotoxic potency: the longer
the terminal alkyl chain, the lower the IC50 value. As expected for such type of complexes
featuring a N2O4 coordination sphere, an overall slow rate of reduction was found, which was
further influenced by the length of the axial ligands (esters). However, the cytotoxic
properties of the complexes could not further be increased since higher lipophilicity results in
a lack of solubility in aqueous media.
23

R = methyl, ethyl, propyl or butyl
R' = CH2CH3 or CH3
Highlights
Diaminetetracarboxylatoplatinum(IV) complexes are potential anticancer prodrugs.
Novel complexes exhibited IC50 values down to the low micromolar range.
Lipophilicity and reduction for two compound series was determined.
Lipophilicity as well as reduction were found to parallel the cytotoxic properties.
24

6. References
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