Contrasting chemistry of cis- and trans-platinum(II) diamine anticancer compounds: Hydrolysis studies of picoline complexes

Article abstract of DOI:10.1021/ic050763t

cis-[PtCl₂(NH₃)(2-picoline)] (AMD473) is currently on clinical trials as an anticancer drug. The trans isomer, AMD443 (1), is also cytotoxic in a variety of cancer cell lines. The X-ray crystal structure of the trans isomer (1) shows that the pyridine ring is tilted by 69° with respect to the platinum square-plane in contrast to the cis isomer in which it is almost perpendicular (103°). In the 3-picoline (2) and 4-picoline (3) trans isomers, the ring is tilted by 58°/60° (2 molecules/unit cell) and by 56°, respectively. Hydrolysis may be an important step in the intracellular activation and anticancer mechanism of action of these complexes. The first hydrolysis step is relatively fast even at 277 K, with rate constants (determined by ¹H, ¹⁵N NMR) of k₁ = 2.6 × 10^-5 s^-1, 12.7 × 10^-5 s^-1, and 5.2 × 10^-5 s^-1 (l = 0.1 M) for formation of the monoaqua complexes of 1-3, respectively. Although the hydrolysis of 3 is slower than 2, it is hydrolyzed to a greater extent. No formation of the diaqua complex was observed for any of the three complexes at 277 K, and it accounts for <3% of the platinum species at 310 K. In general the extent of hydrolysis of the trans complexes is much less than for their cis analogues. The pK _a values for the monoaqua adducts of 1-3 were determined to be 5.55, 5.35, and 5.39, respectively, suggesting that they would exist largely as the monohydroxo complex at physiological pH. The pK_a values for the diaqua adducts were determined to be 4.03 and 7.01 for 1, 3.97 and 6.78 for 2, and 3.94 and 6.88 for 3, the first pK_a being >1 unit lower than for related cis complexes.

Full text of DOI:10.1021/ic050763t

Inorg. Chem. 2005, 44, 7459−7467
Contrasting Chemistry of cis- and trans-Platinum(II) Diamine Anticancer
Compounds: Hydrolysis Studies of Picoline Complexes
Geraldine McGowan, Simon Parsons, and Peter J. Sadler*
School of Chemistry, UniVersity of Edinburgh, King’s Buildings, West Mains Road,
Edinburgh EH9 3JJ, U.K.
Received May 13, 2005
cis-[PtCl2(NH3)(2-picoline)] (AMD473) is currently on clinical trials as an anticancer drug. The trans isomer, AMD443
(1), is also cytotoxic in a variety of cancer cell lines. The X-ray crystal structure of the trans isomer (1) shows that
the pyridine ring is tilted by 69
is almost perpendicular (103 ). In the 3-picoline (2) and 4-picoline (3) trans isomers, the ring is tilted by 58
(2 molecules/unit cell) and by 56 , respectively. Hydrolysis may be an important step in the intracellular activation
°
with respect to the platinum square-plane in contrast to the cis isomer in which it
°
°
/60°
°
and anticancer mechanism of action of these complexes. The first hydrolysis step is relatively fast even at 277 K,
5
5
5
1
with rate constants (determined by 1H,15N NMR) of k1
0.1 M) for formation of the monoaqua complexes of 1
)
2.6
−
×
10- s-1, 12.7
×
10- s-1, and 5.2
×
10- s- (I
)
3, respectively. Although the hydrolysis of 3 is slower
than 2, it is hydrolyzed to a greater extent. No formation of the diaqua complex was observed for any of the three
complexes at 277 K, and it accounts for <3% of the platinum species at 310 K. In general the extent of hydrolysis
of the trans complexes is much less than for their cis analogues. The pKa values for the monoaqua adducts of 1−3
were determined to be 5.55, 5.35, and 5.39, respectively, suggesting that they would exist largely as the monohydroxo
complex at physiological pH. The pKa values for the diaqua adducts were determined to be 4.03 and 7.01 for 1,
3.97 and 6.78 for 2, and 3.94 and 6.88 for 3, the first pKa being >1 unit lower than for related cis complexes.
Introduction
Much recent research work has been aimed at the
discovery of platinum complexes which are not cross-
resistant with cisplatin, have fewer toxic side effects, are
active against a broader range of types of cancer, and can
be administered orally. In 1989, two exceptions to the classic
SAR were reported showing that the overall charge on the
complex and the cis orientation of the N-bound ligands are
not a prerequisite for antitumor activity of PtII complexes.
Hollis et al.5 reported that positively charged triaminemon-
ochloroplatinum(II) complexes of the general formula [Pt-
(NH3)2(Am)Cl]+ (where Am ) planar heterocyclic amine)
possess antitumor activity. These complexes were neither
neutral nor did they possess two cis weakly bound leaving
groups. Subsequently, Farrell et al.6 reported that trans
platinum complexes with planar amine ligands have antitu-
Cisplatin is now a widely used clinical drug for the
treatment of various neoplastic diseases, including testicular
and ovarian cancers, while its congener transplatin is
therapeutically inactive.1 This observation, which is consid-
ered a paradigm for the structure-activity relationships
(SARs) of platinum-based antitumor compounds, has domi-
nated development in this field for more than 2 decades. The
SARs have aided the design of new drugs and guided
attempts to overcome some of the drawbacks associated with
cisplatin, such as nausea, neurotoxicity, nephrotoxicity, and
ototoxicity, as well as drug resistance. These SARs led to a
second generation of platinum drugs, all of which are
analogues of cisplatin containing two cis primary or second-
ary amine groups and two anionic leaving groups. The most
successful of these is carboplatin, which has less severe side
effects than cisplatin.2,3 However, many of the second-
generation platinum drugs appear to be cross-resistant with
cisplatin.4
(2) Alberts, D. S.; Green, S.; Hannigan, E. V.; O’Toole, R.; Stock-Novack,
D.; Anderson, P.; Surwit, E. A.; Malvlya, V. K.; Nahhas, W. A.; Jolles,
C. J. J. Clin. Oncol. 1992, 10, 706-717.
(3) Harrap, K. R. Cancer Res. 1995, 55, 2761-2768.
(4) Monk, B. J.; Alberts, D. S.; Burger, R. A.; Fanta, P. T.; Hallum, A.
V.; Hatch, K. D.; Salmon, S. E. Gynecol. Oncol. 1998, 71, 308-312.
(5) Hollis, L. S.; Amundsen, A. R.; Stern, E. W. J. Med. Chem. 1989,
32, 128-136.
* To whom correspondence should be addressed. E-mail:
p.j.sadler@ed.ac.uk.
(1) Farrell, N. Transition Metal Complexes as Drugs and Chemothera-
peutic Agents; Kluwer: Dordrecht, The Netherlands, 1989.
(6) Farrell, N.; Ha, T. T. B.; Souchard, J.-P.; Wimmer, F. L.; Cros, S.;
Johnson, N. P. J. Med. Chem. 1989, 32, 2240-2241.
10.1021/ic050763t CCC: $30.25
Published on Web 09/21/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 21, 2005 7459

McGowan et al.
Table 1. Cytotoxicity of Trans PtII Complexes in L1210 and
Cisplatin-Resistant (L1210R) Leukemia Cells
An understanding of the aqueous chemistry of diamine
PtII complexes is crucial for establishing their mechanism
of action. Once dissolved in water, the labile chloride ions
of cisplatin are slowly replaced by water molecules in a
stepwise manner: first forming monoaqua species and then
further hydrolysis to form diaqua species.10 The relative
amounts of all these hydrolysis species vary as a function
of pH and chloride concentration. Hydrolysis is likely to be
more extensive inside the cells where the Cl- concentration
is much lower (4-23 mM) than outside cells (∼100 mM).11
Hydrolysis is the rate-limiting step in the reaction of cisplatin
with DNA. Since water is a far better leaving group than
chloride and hydroxide,12 it is important to determine the
pKa values of the aqua adducts.
IC50 (µM)b
complexa
L1210
L1210(R)
cis-[PtCl2(py)2]
trans-[PtCl2(py)2]
cis-[PtCl2(tz)2]
4.4
1.2
2.8
1.6
0.5
0.5
3.2
4.2
0.3
15.7
3.3
1.1
7.3
7.4
2.8
trans-[PtCl2(tz)2]
cis-[PtCl2(NH3)(quin)]c
trans-[PtCl2(NH3)(quin)]c
cis-[PtCl2(NH3)(tz)]c
trans-[PtCl2(NH3)(tz)]c
cis-[PtCl2(NH3)2]
1.4
3.1
15.0
9.2
22.0
trans-[PtCl2(NH3)2]
a py ) pyridine, tz ) thiazole, and quin ) quinoline. b,c IC50 values were
determined from studies of percent growth inhibition after 72 hb or 96 hc
of drug exposure (data from refs 7 and 8).
Although both the kinetics and thermodynamics of hy-
drolysis reactions of cisplatin and related cis-PtII amine
complexes are well understood, surprisingly little is known
about analogous reactions of trans complexes. For transplatin
the first aquation step is rapid, whereas the second aquation
step is very slow.13 This is due to the stabilizing effect of
the entering trans oxygen ligand. Moreover, it has been
postulated that since transplatin is more reactive than the
cis isomer, undesired side reactions occurring on its way to
the pharmacological target are likely to contribute, at least
in part, to the lack of anticancer activity. Sterically demand-
ing ligands, like imino ethers in trans-[PtCl2{(E)-HNd
C(OMe)Me}2] (trans-EE) and substituted pyridine ligands
in cis-[PtCl2(NH3)(2-pic)], could in principle reduce the axial
accessibility of the platinum center and slow the hydrolysis
reaction and the subsequent substitution of the aqua ligand
by biologically relevant substrates.14
Here we have studied three trans isomers of cis-[PtCl2-
(NH3)(2-pic)] (2-pic ) 2-methylpyridine), AMD473, a
recently reported anticancer complex currently in phase II
clinical trials. AMD473 is active against cisplatin-resistant
cell lines and against an acquired cisplatin-resistant subline
of a human ovarian carcinoma xenograph, by injection and
oral administration.15,16 It has shown significantly reduced
cross-resistance to cisplatin in a panel of three cell lines with
known acquired platinum drug resistance mechanisms:
reduced accumulation; increased cytoplasmic detoxification
by cellular thiols; increased DNA repair/tolerance of plati-
num-DNA adducts.15,16 The toxicity of AMD473 is also
greatly reduced, with no renal toxicity observed. The trans
isomers [PtCl2(NH3)(2-pic)] (1), [PtCl2(NH3)(3-pic)] (2), and
[PtCl2(NH3)(4-pic)] (3) have been labeled with 15N, and 2D
[1H,15N] HSQC spectroscopy was used to compare their
hydrolysis behavior and to determine the pKa values for the
mor activity superior to transplatin, especially in cisplatin-
resistant cell lines. These were the first examples of platinum-
based anticancer agents that defy the classic SARs.
The presence of planar ligands in trans-[PtCl2(L)2] (L )
pyridine, thiazole) and trans-[PtCl2(NH3)(L)] (L ) quinoline)
complexes greatly enhances the cytotoxicity of such species,
with respect to their corresponding cis isomers and also to
transplatin.7 The cytotoxicity is further marked by high
activity toward both sensitive and cisplatin-resistant murine
leukemia (L1210) cells (Table 1).7,8 Thus, the activity is 1
order of magnitude greater than that of transplatin. DNA
interstrand cross-linking is enhanced relative to transplatin,
and conformational changes such as unwinding are accentu-
ated in the presence of the planar ligand. The sequence
specificity is also altered, with the trans complexes exhibiting
a higher affinity for alternating purine-pyrimidine (GC)
runs.9
Unfortunately, high antitumor activity of such trans
complexes has not yet been obtained in vivo. Nevertheless,
the observations on the enhanced cytotoxicity are important
mechanistically, because they demonstrate that there is no
restriction, per se, to development of active antitumor trans
platinum complexes. The mechanistic importance of these
findings is also emphasized by the fact that the vast majority
of studies examining the differences between cis- and
transplatin have been carried out in tissue culture or in cell-
free systems. Factors such as water solubility and pharma-
cokinetic considerations (e.g. partition coefficient, drug
metabolism, and tissue distribution) may all contribute to in
vivo deactivation of cytotoxic agents.
Two major chemical differences between NH3 and a planar
amine are evident: steric hindrance by the appropriately
positioned H atoms of the ring system reduces chemical
reactivity, and surface and stacking interactions become more
pronounced over the H-bonding associated with the NH3
groups. These structural features suggest simple chemical
solutions to the design of sequence-specific agents as well
as using steric effects to prevent metabolic deactivation.
(10) Davies, M. S.; Berners-Price, S. J.; Hambley, T. W. Inorg. Chem.
2000, 39, 5603-5613.
(11) Jennerwein, M.; Andrews, P. A. Drug Metab. Dispos. 1995, 23, 178-
184.
(12) Howe-Grant, M. E.; Lippard, S. J. Met. Ions Biol. Syst. 1980, 11, 63-
125.
(13) Lippert, B. Met. Ions Biol. Syst. 1996, 33, 105-141.
(14) Liu, Y.; Vinje, J.; Pacifico, C.; Natile, G.; Sletten, E. J. Am. Chem.
Soc. 2002, 124, 12854-12862.
(7) Beusichem, M. V.; Farrell, N. Inorg. Chem. 1992, 31, 634-639.
(8) Bierbach, U.; Qu, Y.; Hambley, T. W.; Peroutka, J.; Nguyen, H. L.;
Doedee, M.; Farell, N. Inorg. Chem. 1999, 38, 3535-3542.
(9) Farrell, N. Met. Ions Biol. Syst. 1996, 32, 603-639.
(15) Holford, J.; Sharp, S. Y.; Murrer, B. A.; Abrams, M.; Kelland, L. R.
Br. J. Cancer 1998, 77, 366-373.
(16) Holford, J.; Raynaud, F.; Murrer, B. A.; Grimaldi, K.; Hartley, J. A.;
Abrams, M.; Kelland, L. R. Anti-Cancer Drug Des. 1998, 13, 1-18.
7460 Inorganic Chemistry, Vol. 44, No. 21, 2005

cis- and trans-Pt(II) Diamine Anticancer Compounds
clear solution. The acidic mixture was refluxed under nitrogen for
6 h. The resulting solution was bright yellow and was placed on
ice for several hours to induce precipitation. The yellow precipitate
was filtered off, washed with ice-cold water, followed by diethyl
ether, and dried in vacuo overnight. Yield: 80.0 mg, 33.9%. Anal.
Calcd for C6H10Cl2N2Pt: C, 19.16; H, 2.68; N, 7.45. Found: C,
mono- and diaqua complexes. The data reveal notable
differences between the chemistry of these trans complexes
(1-3) and that of cis-[PtCl2(NH3)(2-pic)] and cisplatin.
Experimental Section
Materials. 2-, 3-, and 4-Picoline and 15NH4Cl (>98% 15N) were
purchased from Aldrich, and 15NH4OH (>98% 15N) was purchased
from Isotec. cis-[PtCl2(NH3)2] and cis-[PtCl2(15NH3)2] were prepared
according to a reported procedure.17 Complexes 2 and 3 were
prepared by a general procedure described in the literature for
natural abundance, mixed-ligand ammine/amine PtII complexes.18
Preparation of trans-[PtCl2(NH3)(2-pic)] (AMD443) (1). cis-
[PtCl2(2-pic)2] (0.392 g, 0.866 mmol), prepared by literature
methods,19 was dissolved in deionized water (7 mL), and the
solution was heated at reflux; then NH4OH (2.77 mL, 2.5 M) was
added to give a yellow suspension, which was heated at reflux and
stirred overnight. This produced a clear solution, which was filtered
and then rotary evaporated. The cis-[Pt(NH3)2(2-pic)2]Cl2 intermedi-
ate was dissolved in water (2 mL), and the solution was heated
and stirred under reflux. HCl (289 µL, 11.98 M) was added to the
solution, and the mixture allowed to stir under reflux for 5 days
under nitrogen, during which time it was necessary to add additional
aliquots of water. The mixture was then placed on an ice bath to
induce precipitation. The yellow product was then filtered off,
washed with ice-cold water, and dried in vacuo overnight. The
product was recrystallized from 0.1 M HCl, giving small yellow
crystals. Yield: 258.5 mg, 79%. Anal. Calcd for C6H10Cl2N2Pt:
C, 19.16; H, 2.68; N, 7.45. Found: C, 18.87; H, 2.54; N, 7.11. 1H
NMR (acetone-d6): δ ) 8.81 (d, J ) 5.7 Hz, 1H, H-6), 7.78 (t,
1H, H-4), 7.43 (d, J ) 7.9 Hz, 1H, H-3), 7.27 (t, 1H, H-5), 3.69
(broad, NH3), 3.14 (s, 3H, CH3). MS: m/z 399.3 corresponding to
[PtCl2(NH3)(2-pic)]Na+. trans-[PtCl2(15NH3)(2-pic)] (15N-1) was
prepared by the same method using 15NH4OH.
1
19.35; H, 2.66; N, 8.42. H NMR (acetone-d6): δ ) 8.66 (d, J )
6.3 Hz, 2H, H-2), 7.26 (d, J ) 5.65 Hz, 2H, H-3), 3.70 (broad,
NH3), 2.41 (s, 3H, CH3). MS: m/z ) 399.1 corresponding to
[PtCl2(NH3)(4-pic)]Na+. trans-[PtCl2(15NH3)(4-pic)] (15N-3) was
prepared by the same method using 15N-cisplatin as the starting
material.
X-ray Crystallography. Diffraction data were collected with
Mo KR radiation on a Bruker Smart Apex CCD diffractometer
equipped with an Oxford Cryosystems low-temperature device
operating at 150 K. Data were corrected for absorption using the
SADABS20 procedure. The structure of 1 was solved by direct
methods (SHELXS),21 while those of 2 and 3 were solved using
Patterson methods (DIRDIF22 and SHELXS, respectively). The
structures were refined against F2 using all data (SHELXL).23 All
non-H atoms were modeled with anisotropic displacement param-
eters, and H-atoms were placed in calculated positions. CH3 and
NH3 groups were treated according to the Sheldrick rotating rigid
group model.23
Crystal Data for 1. The sample was a pale-yellow needle of
dimensions 1.00 × 0.14 × 0.14 mm3; the crystals tended to split
into smaller needles when cut: orthorhombic, space group Pccn;
a ) 10.5710(12), b ) 22.826(3), c ) 7.9743(9) Å; V ) 1924.1(7)
Å3; Z ) 8; Dcalc ) 2.597 Mg m-3. The final conventional R-factor
[R1, based on |F| and 2018 data with F > 4σ(F)] was 0.0431, and
wR2 (based on F2 and all 2374 unique data to θmax ) 28.7°) was
0.0921. The final ∆F synthesis extremes were +2.32 and -1.82 e
Å-3
.
Crystal Data for 2. The sample was a yellow block of
dimensions 0.70 × 0.25 × 0.25 mm3: monoclinic, space group
P21/c; a ) 8.5867(2), b ) 22.8583(3), c ) 9.8514(3) Å; V )
1930.00(13) Å3; Z ) 8 (there are two molecules in the asymmetric
unit); Dcalc ) 2.589 Mg m-3. The final conventional R1-factor
(based on 4034 data) was 0.0386, and wR2 (based on 4576 unique
data to θmax ) 28.6°) was 0.0865. The final ∆F synthesis extremes
Preparation of trans-[PtCl2(NH3)(3-pic)] (2). Cisplatin (0.233
g, 0.775 mmol) was suspended in 23 mL of water, which had been
bubbled with argon, and 2 mol equiv of 3-picoline (151 µL, 1.55
mmol) was added. The mixture was stirred at 363 K for 3 h, allowed
to cool to room temperature, and filtered to remove any traces of
precipitate, and then concentrated HCl (1.6 mL) was added to the
clear solution. The acidic mixture was refluxed under nitrogen
overnight. The resulting solution was bright yellow and was placed
on ice for several hours to induce precipitation. The yellow
precipitate was filtered off, washed with ice-cold water, followed
by diethyl ether, and dried in vacuo overnight. Yield: 188.3 mg,
64.6%. Anal. Calcd for C6H10Cl2N2Pt: C, 19.16; H, 2.68; N, 7.45.
were +2.31 and -2.22 e Å-3
.
Crystal Data for 3. The sample was a pale-yellow needle of
dimensions 1.06 × 0.08 × 0.04 mm3: orthorhombic, space group
Pccn; a ) 11.5185(4), b ) 22.3413(7), c ) 7.6232(3) Å; V )
1961.74(12) Å3; Z ) 8; Dcalc ) 2.547 Mg m-3. The final
conventional R1-factor (based on 2193 data) was 0.0307, and wR2
(based on 2828 unique data to θmax ) 30.5°) was 0.0668. The final
1
Found: C, 19.30; H, 2.63; N, 7.39. H NMR (acetone-d6): δ )
8.66 (s, 1H, H-2), 8.64 (d, J ) 5.49 Hz, 1H, H-6), 7.78 (d, J )
7.68 Hz, 1H, H-4), 7.32 (t, 1H, H-5), 3.73 (broad, NH3), 2.37 (s,
3H, CH3). MS: m/z ) 399.0 corresponding to [PtCl2(NH3)(3-pic)]-
Na+. trans-[PtCl2(15NH3)(3-pic)] (15N-2) was prepared by the same
method using 15N-cisplatin as the starting material.
Preparation of trans-[PtCl2(NH3)(4-pic)] (3). Cisplatin (0.188
g, 0.627 mmol) was suspended in 19 mL of water, which had been
bubbled with argon, and 2 mol equiv of 4-picoline (122 µL, 1.25
mmol) was added. The mixture was stirred at 363 K for 3 h, allowed
to cool to room temperature, and filtered to remove any traces of
precipitate, and then concentrated HCl (1.6 mL) was added to the
∆F synthesis extremes were +1.70 and -0.81 e Å-3
.
NMR Spectroscopy. NMR spectra were recorded at 298 K,
unless otherwise stated, on Bruker DMX500 (1H 500.13 MHz) or
Bruker AVA600 (1H 599.81 MHz) spectrometers using 5 mm NMR
tubes. 1H NMR chemical shifts were referenced to TSP via dioxane
(δ 3.76), and 15N chemical shifts, to 1 M 15NH4Cl in 1.5 M HCl
(external). Water suppression was achieved by presaturation. Spectra
were processed using XWINNMR (version 3.5, Bruker U.K. Ltd).
(20) Sheldrick, G. M. SADABS; University of Go¨ttingen: Go¨ttingen,
Germany, 2004; version 2004/1.
(21) Sheldrick, G. M. SHELXS; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(22) Beurskens, P. T.; Beurskens, G.; Gelder, R. d.; Garcia-Granda, S.;
Gould, R. O.; Israel, R.; Smits, J. M. M. Crystallography Laboratory;
University of Nijmegen: Nijmegen, The Netherlands, 1999.
(23) Sheldrick, G. M. SHELXL; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(17) Dhara, S. C. Indian J. Chem. 1970, 8, 193-194.
(18) Kelland, L. R.; Barnard, C. F. J.; Evans, I. G.; Murrer, B. A.; Theobald,
B. R. C.; Wyer, S. B.; Goddard, P. M.; Jones, M.; Valenti, M.; Bryant,
A.; Rogers, P. M.; Harrap, K. R. J. Med. Chem. 1995, 38, 3016-
3024.
(19) Tessier, C.; Rochon, F. D. Inorg. Chim. Acta 1999, 295, 25-38.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7461

McGowan et al.
Table 2. X-ray Crystal Structure Data for Complexes 1-3
param
1
2
3
empirical formula
Mr
space group
a (Å)
b (Å)
c (Å)
â (deg)
V (Å3)
Z
C6H10Cl2N2Pt
376.15
Pccn
10.5710 (12)
22.826 (3)
7.9743 (9)
90
1924.1 (4)
8
C6H10Cl2N2Pt
376.15
P21/c
8.5867 (2)
22.8583 (7)
9.8514 (3)
93.506 (2)
1929.99 (9)
8
C6H10Cl2N2Pt
376.15
Pccn
11.5185 (4)
22.3413 (7)
7.6232 (3)
90
1961.74 (12)
8
λ (Å)
0.710 73
150 (2)
2.597
15.080
0.0431
0.710 73
150 (2)
2.589
15.034
0.0386
0.710 73
150 (2)
2.547
14.791
0.0307
0.0668
T (K)
F
calcd (g cm-3
µ
)
calcd (mm-1
)
R1 (F0 > 4σ(F0))
wR2 (all data)
0.0921
0.0865
Table 3. Selected Bond Lengths (Å) and Angles (deg) for Complexes
1-3
param
Pt-N(1)
Pt -N(2)
Pt-Cl(1)
Pt-Cl(2)
1
2
3
2.028(7)
2.037(8)
2.313(2)
2.307(2)
2.014(6)
2.047(6)
2.293(2)
2.292(3)
2.014(4)
2.043(4)
2.2918(11)
2.3002(12)
N(1)-Pt-N(2)
N(1)-Pt-Cl(1)
N(2)-Pt-Cl(1)
N(1)-Pt-Cl(2)
N(2)-Pt-Cl(2)
Cl(2)-Pt-Cl(1)
178.9(3)
90.1(2)
89.6(2)
92.0(2)
88.4(2)
176.9(3)
179.22(15)
90.95(11)
88.32(12)
90.63(11)
90.08(12)
177.40(5)
90.61(19)
88.35(19)
91.26(19)
89.8(2)
177.48(8)
178.12(7)
Sample Preparation. NMR samples for hydrolysis reactions
were prepared by dissolving the platinum complex in an aliquot of
DMF, followed by dilution with H2O/D2O to the required concen-
tration. For pKa determinations, the platinum complexes were
reacted with 1.96 equiv of AgNO3 (24 h, room temperature) and
then filtered to remove AgCl. All samples were prepared in 90%
H2O/10% D2O unless otherwise stated.
Kinetic Analyses. The concentration/time data for each complex
were computer-fitted to the following first-order rate equation:
A ) C0 + C1e-kt
(1)
Figure 1. Space-filling models of the X-ray crystal structures of complexes
1-3 showing the steric hindrance caused by the 2-methyl group in complex
Here C0 and C1 are computer-fitted constants and A is the
concentration corresponding to time t. All kinetic data were
computer-fitted using the program Microcal Origin 7.5 to give the
1.
Results
hydrolysis rate constant kH O (k).
2
X-ray Crystal Structures. X-ray crystallographic data and
details of the refinement of the structures of the trans
ammine/2-picoline, 3-picoline, and 4-picoline complexes (1-
3, respectively) are listed in Table 2, and selected bond
lengths and angles are given in Table 3. All complexes have
a square-planar configuration with angles close to the ideal
values of 90 and 180°, as shown in Figure 1. Pt-Cl bond
lengths range from 2.292 to 2.313 Å and are close to the
expected values. The Pt-N bond lengths of 2.014-2.047 Å
are comparable to those of related structures. The most
notable feature of the structures is the orientation of the
picoline ring with respect to the platinum square plane. In
complex 1, the 2-picoline ligand is tilted by 69.2°, in complex
2 3-picoline is tilted by 60.0 and 57.8° (2 molecules/unit
cell), and for 4-picoline in 3 is tilted by only 56.2°. The tilt
angle therefore decreases in the order 2-pic > 3-pic, 4-pic.
In complex 1, the 2-methyl group lies over the square plane
(H3C‚‚‚Pt: 3.207 Å), whereas in complexes 2 and 3 the
pH Measurements. The pH values of solutions were determined
using a Corning 145 pH meter equipped with a micro combination
electrode, calibrated with Aldrich buffer solutions at pH 4, 7, and
10. The pH was adjusted with dilute solutions of HClO4 and NaOH.
No correction has been made for deuterium isotope effects on the
glass electrode.
pKa Values. These were determined by fitting the NMR pH
titration curves for samples in 90% H2O/10% D2O to the Hender-
son-Hasselbalch equation using the program Kaleidagraph (Syn-
ergy Software, Reading, PA).
Mass Spectrometry. Ion electrospray mass spectrometry was
performed on a Platform II mass spectrometer (Micromass,
Manchester, U.K.). The samples were infused at 8 µL/min and the
ions produced in an atmospheric pressure ionization (API)/ESI ion
source. The source temperature was 383 K, and the drying gas flow
rate was 300 L/h. A potential of 3.5 kV was applied to the probe
tip, and cone voltage gradients of 20-40 V over 200-1000 Da
were used. Data acquisition was performed on a Mass Lynx (V2.5)
Windows NT PC data system. All samples were prepared in water.
7462 Inorganic Chemistry, Vol. 44, No. 21, 2005

cis- and trans-Pt(II) Diamine Anticancer Compounds
Figure 3. 2D [1H,15N] HSQC NMR spectra of a 5 mM aqueous solution
of trans-[PtCl2(15NH3)(2-pic)] (1) at 310 K after (A) 0.4 h and (B) 21.4 h.
Peak a is assigned to 1, peak b to the monoaqua complex, and peak c to
the diaqua complex; / ) 195Pt satellites.
to three Cl ligands from neighboring molecules. Similarly
for 1 and 3, chains of complexes are connected by H-bonds
and also interact with one another through interleaving
π-stacks. Such intermolecular H-bonds are common in
chloroplatinum(II) am(m)ine complexes. However, there
appears to be an additional H-bonding interaction in complex
1 between the aromatic H5 proton of the 2-picoline ring and
a Cl ligand (2.853 Å). For complex 2, H-bonds of similar
strength are formed but only to two Cl ligands, giving infinite
chains composed of channels lined by Cl and NH3 along
the c-direction.
Hydrolysis of Complexes 1-3. The hydrolysis of 15NH3-
labeled 1-3 in aqueous solutions containing 0.1 M NaClO4
was monitored by [1H,15N] 2D NMR for a period of over
20 h at 277 K. This low temperature was chosen because
the rates were too fast at ambient temperature to allow
determination by NMR. Initially for 1, a single cross-peak
was observed at δ ) 3.63, -70.11, which was assigned to
the dichloro complex 1. After 2 h, a weak cross-peak was
detected at δ ) 3.69, -68.30, which was assigned to the
monoaqua species. For trans complexes, the 15NH3 ligand
remains trans to the picoline N during substitution reactions
in the cis position and only small changes in chemical shift
are observed due to the cis effect. The peak for 1 decreased
in intensity as the monoaqua peak increased in intensity over
a period of 21 h. No peak assignable to a diaqua species
was observed within the time the reaction was followed (48
h). At 310 K an additional peak at δ 4.07, -65.99 ppm
assignable to the diaqua adduct was observed after 4 h, but
its intensity was low and accounted for <3% of the Pt present
in the solution after 21 h (Figure 3).
For the 3-picoline complex 2, initially, two cross-peaks
were observed. The most intense peak at δ ) 3.64, -70.60
was assigned to the dichloro complex 2, and the less intense
peak at δ ) 3.77, -69.21, to the monoaqua species. Again,
no diaqua species was observed. The time dependence of
the [1H,15N] 2D NMR spectrum was similar to that of
complex 1, except that equilibrium was reached more quickly
(<7 h at 277 K).
The hydrolysis of the 4-picoline complex 3 followed a
similar time dependence but, interestingly, reached equilib-
rium significantly more slowly than for the 3-picoline
complex 2. The chemical shifts for the dichloro complex
Figure 2. Intermolecular H-bonding in crystals of complexes 1-3.
N-H‚‚‚Cl distances range from 2.46-2.85 Å for complex 1, 2.49-2.55 Å
for complex 2, and 2.49-2.70 Å for complex 3.
methyl group is much further away from Pt (H3C‚‚‚Pt: 5.452,
5.458 Å for 2 and 6.312 Å for 3). The space-filling models
in Figure 1 demonstrate that in complex 1 there is signifi-
cantly more steric hindrance to an axial approach to Pt from
above than in either complexes 2 or 3.
Strong intermolecular hydrogen bonds are involved in the
crystal packing of all complexes (Figure 2). For complexes
1 and 3, the three H atoms of the NH3 ligand are H-bonded
Inorganic Chemistry, Vol. 44, No. 21, 2005 7463

McGowan et al.
Figure 4. 2D [1H,15N] HSQC NMR spectrum of 5 mM aqueous solution
of trans-[PtCl2(15NH3)(4-pic)] (3) at 277 K after 21.3 h. Peak a is assigned
to 3, and peak b to the monoaqua complex.
and monoaqua species are similar to those of 1 and 2, with
the dichloro complex 3 having a cross-peak at δ ) 3.63,
-70.60 and the monoaqua species at δ ) 3.68, -68.89
(Figure 4). No diaqua species was observed under these
1
experimental conditions. It is noteworthy that the H NMR
peaks, in particular, tended to shift downfield throughout the
course of the reaction. This is a consequence of a decrease
in pH caused by deprotonation of coordinated water to
produce the monohydroxo complex.
The time dependence of the concentrations of species
detected during hydrolysis of complexes 1-3 is shown in
Figure 5. The assignments of the peaks of the aqua
complexes were confirmed by pH titrations (Figure 6), using
aqua adducts of complexes 1-3 which had been generated
by reactions with AgI to produce a mixture of monoaqua,
diaqua, and dichloro species. These allowed the determina-
tion of pKa value for each monoaqua complex as well as
two pKa values for each diaqua complex.
The NMR data allow the determination of the rates for
monoaquation of complexes 1-3 at 277 K, and these are
listed in Table 4. It is notable that the rate of monoaquation
of the 3-picoline complex 2 is more than twice as fast as
that of complex 3 and ca. five times as fast as complex 1.
However, the extent of hydrolysis of complex 2 was ca. 75%
that of complexes 1 and 3. An estimate of the half-lives for
hydrolysis of complex 1 at 310 K is included in Table 6 and
compared with related complexes. Half-lives for hydrolysis
(310 K) and pKa values (298 K) for complex 1 are shown in
Scheme 1.
Figure 5. Time dependence of the concentrations of the dichloro
complexes (A) 1, (B) 2, and (C) 3 and their aqua adducts during hydrolysis
at 277 K. Concentrations are based on integration of 2D NMR cross-peaks.
Labels: 1-3 (9); monoaqua complexes (b). The full lines represent
computer fits giving the first-order rate constants listed in Table 4. Note
the longer time scale for the hydrolysis of the 2-pic complex 1 in (A).
overcomes cisplatin resistance in the resistant cell lines tested,
but complex 3 is cross-resistant, implying that complex 1
may have a mechanism of action different from that for
complex 3. IC50 values for these and related compounds are
listed in Table 5.24-26 Interestingly, trans-[PtCl2(NH3)(pip)]
(where pip ) piperidine) and trans-[PtCl2(NH3)(pz)]+ (where
pz ) piperazine) are less cytotoxic than both trans-[PtCl2-
(NH3)(2-pic)] (1) or trans-[PtCl2(py)2] in the cell lines tested.
Moreover, trans-[PtCl2(py)2] overcomes cisplatin resistance
while trans-[PtCl2(NH3)(2-pic)] has a small average resis-
tance factor of ca. 1.5.
15N-labeling of 1-3 and use of [1H,15N] NMR spectros-
copy has allowed study of the speciation of these complexes
in aqueous solution. The [15N,1H] NMR method has the
advantage that the concentrations of aquated species can
usually be measured directly at low (micromolar) concentra-
tions, even when other species are present in the solution.
Discussion
cis-[PtCl2(NH3)(2-picoline)] is currently on clinical trial
as an anticancer drug. Its trans isomer, complex 1, shows in
vitro cytotoxicity24 even higher than that of cisplatin in a
panel of human tumor cell lines (mean IC50 ) 3.5 and 33.7
µM, respectively), whereas the trans 4-picoline complex 3,
tested by Kasparkova et al.,25 exhibits a lower activity (mean
IC50 ) 80.3 µM), Table 5. The 3-picoline complex 2 has
not yet been evaluated for its cytotoxic activity. Complex 1
(24) Harrap, K. ICR Rep.; AnorMED Inc.: Langley City, Vancouver, 1992.
(25) Kasparkova, J.; Marini, V.; Najajreh, Y.; Gibson, D.; Brabec, V.
Biochemistry 2003, 42, 6321-6332.
(26) Farrell, N.; Kelland, L. R.; Roberts, J. D.; Beusichem, M. V. Cancer
Res. 1992, 52, 5065-5072.
7464 Inorganic Chemistry, Vol. 44, No. 21, 2005

cis- and trans-Pt(II) Diamine Anticancer Compounds
detected, although a peak for the diaqua species was detected
after ca. 4 h of hydrolysis of complex 1 at 310 K.
The weaker basicity of 3-picoline appears to influence both
the extent and rate of hydrolysis of complex 2 in comparison
with complexes 1 and 3. The pKa value of 3-picoline is ca.
0.3 units lower than those of 2- or 4-picoline.27 At equilib-
rium, the monoaqua species of complexes 1 and 3 account
for only ca. 24% of all platinum species present and the
monoaqua species of complex 2 accounts for only ca. 18%
of all platinum species. The t1/2 values for the first hydrolysis
step at 277 K follow the order 1 (7.43 h) > 3 (3.68 h) > 2
(1.52 h). Since the steric hindrance due to the picoline ligand
in complexes 2 and 3 is similar, it seems likely that the faster
hydrolysis of complex 2 relative to complex 3 is due to
electronic factors. A methyl substituent at the 2- and
4-positions (complexes 1 and 3, respectively) increases
electron density on the platinum center, making nucleophilic
attack by the incoming water molecule less favorable.
However, at the 3-position (complex 2), the methyl sub-
stituent directs less electron density to the platinum center
and thus promotes the hydrolysis reaction. Complex 1 is the
slowest to hydrolyze due to both steric and electronic effects.
The more sterically demanding 2-picoline ligand hinders
axial approach of the incoming water molecule from above.
Substitution reactions of square-planar PtII complexes (i.e.
PtL2XY + Z f PtL2YZ + X) usually occur through
formation of a five-coordinate intermediate (PtL2XYZ) in
an associative process.28 The use of a more sterically
hindering nonleaving group (i.e. Y or L) compared to NH3
raises the energy of the five-coordinate intermediate (i.e.
increases the activation energy associated with entry of Z
into the coordination sphere).
When hydrolysis of complex 1 was monitored at 310 K,
approximate values of t1/2 were determined as 0.9 h for the
monoaquation and 70.2 h for the diaquation steps. These
values compare well with t1/2 ) 0.92 h and t1/2 ) 77.0 h (T
) 298 K) for mono- and diaquation, respectively, of trans-
[PtCl2{E-HNC(OMe)Me}2].29 Farrell et al.30 have obtained
a t1/2 value of 0.33 h for trans-[PtCl2(NH3)(4-pic)] (3) at T
) 298 K. Table 6 lists t1/2 values of various related
compounds.29-34 Only a few studies of transplatin hydrolysis
have been reported, and there appears to be a large disparity
between the reported half-lives. Aprile and Martin33 reported
a t1/2 value of 1.96 h (I ) 0.318 M, T ) 298 K) for
monoaquation of transplatin, whereas Miller et al.34 calcu-
lated a t1/2 value of 10.13 h (I ) 0.1 M, T ) 298.2 K) on the
basis of the activation parameters, obtained from the tem-
perature-dependent intercepts of plots of kobs versus [Cl-].
1
Figure 6. pH dependence of the H NMR chemical shifts of NH3 in the
monoaqua and diaqua complexes (A) 1, (B) 2, and (C) 3. The curves
represent computer fits to the Henderson-Hasselbalch equation giving the
pKa values listed in Table 7. Labels: monoaqua complexes (b); diaqua
complexes ([).
Table 4. Rate Constants (k) and Equilibrium Constants (K) for the
Hydrolysis Reactions of Complexes 1-3 and AMD473
a
complex
T (K)
k (10-5 s-1
)
K (10-4 M)a
(27) Brown, H. C.; Mihm, X. R. J. Am. Chem. Soc. 1955, 77, 1723-1726.
(28) Tobe, M. L.; Burgess, J. In Inorganic Reaction Mechanisms; Addison-
Wesley Longman Inc.: New York, 1999; pp 70-79.
(29) Liu, Y.; Intini, F. P.; Natile, G.; Sletten, E. J. Chem. Soc., Dalton.
Trans. 2002, 3489-3495.
1
2
3
277
277
277
310
2.6 ( 0.2
12.7 ( 0.8
5.2 ( 0.3
3.2,d 2.2e
3.9 (3.7)b
2.0
4.0
AMD473c
12.1,d 21.4e
(30) Farrell, N.; Kozma, E. S. Unpublished results.
(31) Bancroft, D. P.; Lepre, C. A.; Lippard, S. J. J. Am. Chem. Soc. 1990,
112, 6860-6871.
a Based on concentrations derived from integration of 2D [1H,15N] NMR
cross-peaks. b 310 K. c Data from ref 32. d Trans to 2-pic. e Trans to NH3.
(32) Chen, Y.; Guo, Z.; Parsons, S.; Sadler, P. J. Chem.sEur. J. 1998, 4,
672-676.
(33) Aprile, F.; Martin, D. S., Jr. Inorg. Chem. 1962, 1, 551-557.
(34) Miller, S. E.; Gerard, K. J.; House, D. A. Inorg. Chim. Acta 1991,
190, 135-144.
However, due to the extremely fast rate of the monoaquation
step, rates were measured at lowered temperature (277 K).
At 277 K, only peaks for the monoaqua species were
Inorganic Chemistry, Vol. 44, No. 21, 2005 7465

McGowan et al.
Table 5. Mean IC50 Values for Trans Pt Compounds against Various Cell Lines24-26
IC50 (µM)b
complexa
A2780
A2780(R)
CH1
CH1(R)
41M
41M(R)
cisplatin
transplatin
trans-[PtCl2(py)2]
2.2
>200
38
>200
6
23
>200
1.7
26
>200
107
>200
2.0
>200
1.6
2.2
trans-[PtCl2(NH3)(pip)]
trans-[PtCl2(NH3)(pz)]+
trans-[PtCl2(NH3)(2-pic)] (1)
trans-[PtCl2(NH3)(4-pic)] (3)
5
5
1.6
7
20
44
6
15
12
3.8
22
94
34
5.2
154
27
52
2.8
32
150
155
1.7
187
80
a py ) pyridine, pip ) piperidine, and pz ) piperazine. b R ) cisplatin-resistant cell line.
Table 6. Half-Lives [t1/2 (h)] for Hydrolysis of Various PtII Complexes
[1H,15N] NMR allowed also the determination of the pKa
values of the aqua ligands in the monoaqua and diaqua
species of complexes 1-3. Movement of the methyl group
from the 2- to the 3- to the 4-position has little affect on the
pKa values of the aqua ligands. A comparison of pKa values
for monoaqua adducts, Table 7,37-42 shows those for
complexes 1-3 are fairly typical for trans-{Pt(NH3)(amine)}-
type complexes (5.3-5.9) and significantly lower than those
of cis complexes such as cisplatin and AMD473. Also, the
first pKa value for the trans diaqua complexes is ca. 1 unit
lower than those for related cis complexes (Table 7).
In recent years, there has been an increasing number of
investigations of trans platinum complexes.9,13,43 One of the
most striking differences between cis and trans isomers
apparent from such investigations is the variation in forma-
tion and reactivity of the diaqua species. However, Ar-
palahti44 reported that, for cisplatin and transplatin, the second
complexation step in the formation of the bis(inosine) adduct
appears to be kinetically similar. In the case of multinuclear
platinum am(m)ine complexes, which hydrolyze only to a
small extent and yet have a high DNA affinity (rapid DNA
binding), it has been suggested that formation of aqua species
may not be a necessary step in DNA adduct formation of
such compounds.40 Moreover, the high cytotoxicity of the
trans-[PtCl2(NH3)(planar amine)] class of compounds has
been attributed to an alteration of the mode of DNA binding
in comparison to cisplatin, resulting in an altered spectrum
of antitumor activity and enhanced reactivity in cisplatin-
resistant cell lines. For example, an intercalative manner of
interaction of the quinoline or thiazole moiety with the duplex
has been proposed for all or a significant fraction of the DNA
adducts of trans-[PtCl2(NH3)(quin)] and trans-[PtCl2(NH3)-
(tz)]45 (where quin ) quinoline, tz ) thiazole). Also, it has
been reported that the mechanism of antitumor activity of
complex
dichloro t1/2 monoaqua t1/2
ref
trans-[PtCl2(NH3)(2-pic)] (1)a
trans-[PtCl2(NH3)(3-pic)] (2)a
trans-[PtCl2(NH3)(4-pic)] (3)a
transplatinc
7.43 (0.9)b
1.52
3.68
(70.2)b
3.85
present work
present work
present work
1.96
33
34
30
29
transplatind
10.13
0.33
trans-[PtCl2(NH3)(4-pic)]
trans-[PtCl2{EHNC(OMe)Me}2]c 0.92
77.02
cis-[PtCl2(NH3)(2-pic)]b
cis-[PtCl2(NH3)(3-pic)]b
cisplatinb
6.04, 8.71
2.64, 55.01 32
1.86, 4.31
1.9
2.47, 5.50
2.1
32
31
a 277 K. b 310 K,. c 298 K. d 298.2 K, calculated from the activation
parameters, which were determined from the temperature-dependent
intercepts of plots of kobs vs [Cl-].
Scheme 1. Half-Lives for Hydrolysis (310 K) of Complex 1 and pKa
Values (298 K) of Its Aqua Adducts
The equilibrium constants for monoaquation determined
for complexes 1-3, (2-4) × 10-4 M-1 (Table 4), are 1 order
of magnitude less than reported values (298 K) for
cisplatin33-35 of (33-68) × 10-4 M-1. The equilibrium
constants reported for transplatin,33,34 3.2 × 10-4 and 6.2 ×
10-4 M-1, are also 1 order of magnitude smaller than those
of cisplatin, and our values for complexes 1-3 fall within
this range. Thus, the amount of (reactive) monoaqua species
formed by the trans complexes at intracellular chloride
concentrations (4-23 mM)11 is likely to be <10%, much
smaller than for the cis analogues (Table 4). A consequence
of the higher stability of the trans chloro complexes is that
little of the diaqua adduct is formed during the hydrolysis.
It has been suggested for cisplatin that the diaqua complex
plays a key role in the attack on DNA.36
(37) Appleton, T. G.; Bailey, A. J.; Barnham, K. J.; Hall, J. R. Inorg. Chem.
1992, 31, 3077-3082.
(38) Pizarro, A. M.; Munk, V. P.; Navarro-Ranninger, C.; Sadler, P. J.
Angew. Chem., Int. Ed. 2003, 42, 5339-5342.
(39) Pizarro, A. M. Ph.D. Thesis, Universidad Auto´noma de Madrid,
Madrid, 2004.
(40) Davies, M. S.; Cox, J. W.; Berners-Price, S. J.; Barklage, W.; Qu, Y.;
Farrell, N. Inorg. Chem. 2000, 39, 1710-1715.
(41) Barton, S. J.; Barnham, K. J.; Habtemariam, A.; Sue, R. E.; Sadler,
P. J. Inorg. Chim. Acta 1998, 273, 8-13.
(42) Berners-Price, S. J.; Frenkiel, T. A.; Frey, U.; Ranford, J. D.; Sadler,
P. J. J. Chem. Soc., Chem. Commun. 1992, 789-791.
(43) Natile, G.; Coluccia, M. Coord. Chem. ReV. 2001, 216-217, 383-
410.
(35) Miller, S. E.; House, D. A. Inorg. Chim. Acta 1989, 161, 131-137.
(36) Legendre, F.; Bas, V.; Kozelka, J.; Chottard, J.-C. Chem.sEur. J. 2000,
6, 2002-2010.
(44) Mikola, M.; Arpalahti, J. Inorg. Chem. 1994, 33, 4439-4445.
(45) Brabec, V.; Neplechova, K.; Kasparkova, J.; Farrell, N. J. Biol. Inorg.
Chem. 2000, 5, 364-368.
7466 Inorganic Chemistry, Vol. 44, No. 21, 2005

cis- and trans-Pt(II) Diamine Anticancer Compounds
Table 7. Comparison of pKa Values for trans-(Diam(m)ine)platinum(II) Aqua Complexes and Aquated Cisplatin
complex
pKa
5.55
ref
trans-[PtCl(OH2)(15NH3)(2-pic)]+
present work
trans-[Pt(OH2)2(15NH3)(2-pic)]2+
4.03, 7.01
5.35
3.97, 6.78
5.39
3.94, 6.88
5.63
4.35, 7.40
5.90
present work
present work
present work
present work
trans-[PtCl(OH2)(15NH3)(3-pic)]+
trans-[Pt(OH2)2(15NH3)(3-pic)]2+
trans-[PtCl(OH2)(15NH3)(4-pic)]+
trans-[Pt(OH2)2(15NH3)(4-pic)]2+
present work
trans-[PtCl(OH2)(15NH3)2]+
37
37
38
38
39
39
40
40
41
32
32
32
32
42
42
trans-[Pt(OH2)2(15NH3)2]2+
trans-[PtCl(OH2)(15NH3)(2-Me-(15N)butylamine)]+
trans-[Pt(OH2)2(15NH3)(2-Me-(15N)butylamine)]2+
trans-[PtCl(OH2)(isopropylamine)((S)-2-methyl(15N)butylamine)]+
trans-[Pt(OH2)2(isopropylamine)((S)-2-methyl(15N)butylamine)]2+
[{trans-PtCl(15NH3)2}{trans-Pt(OH2)(15NH3)2}(µ-15NH2(CH2)615NH2)]3+
[{trans-Pt(OH2)(15NH3)2}2(µ-15NH2(CH2)615NH2)]4+
trans-[PtCl(OH2)(15NH3)(c-C6H1115NH2)]+
cis-[PtCl(OH2)(15NH3)(2-pic)]+
4.16, 7.17
5.86
4.21, 7.33
3.9
5.62a
5.40
6.13, 6.49
5.22, 7.16
5.98, 6.26
5.07, 6.94
6.41
cis-[Pt(OH2)2(15NH3)(2-pic)]2+
cis-[PtCl(OH2)(15NH3)(3-pic)]+
cis-[Pt(OH2)2(15NH3)(3-pic)]2+
cis-[PtCl(OH2)(15NH3)2]+
cis-[Pt(OH2)2(15NH3)2]2+
5.37, 7.21
a Average value.
trans-[PtCl2(NH3)(pip)] does not involve recognition of its
DNA adduct by HMG domain proteins as a crucial step,46
in contrast to the proposals for cisplatin and its analogues.
Thus, further studies are warranted to develop the potential
of such trans platinum anticancer complexes which act by a
different mechanism and exhibit activity complementary to
agents such as cisplatin.
monoaqua complexes 1-3 (5.3-5.6) implies that they will
be largely in the less reactive hydroxo forms at physiological
pH. The limited extent of hydrolysis (<10%) at intracellular
chloride concentrations (ca. 4 and 23 mM in the nucleus and
cytoplasm, respectively11) implies that reactions with nucleo-
bases will be slow. This, together with the faster rate of
hydrolysis, due to the higher trans effect of Cl- compared
with NH3, is likely to have a major influence on the
biological activity (including detoxification) of trans com-
pared to cis complexes.
Conclusions
1H,15N NMR studies of the aquation of the trans-ammine-
picoline-dichloro complexes 1-3 (2-pic, 3-pic, and 4-pic,
respectively) showed that the monoaquation step is relatively
rapid (t1/2 ) 1.5-7.5 h at 277 K). However, aquation occurs
only to a limited extent (18-24% hydrolysis, 5 mM Pt, 277
K), the equilibrium favoring the dichloro species, with little
formation of the diaqua complex. Increased steric hindrance
to an axial approach to PtII in the 2-picoline complex 1 was
apparent in the X-ray crystal structure. This hindrance has
the effect of destabilizing the expected trigonal bipyramidal
transition state, an effect well-known in substitution reactions
of square-planar PtII complexes. The low pKa values of the
Acknowledgment. We thank AnorMED and AstraZeneca
for their support for this work and the Wellcome Trust for
provision of NMR facilities in the Edinburgh Protein
Interaction Centre (EPIC). We are grateful to N. Farrell and
E. S. Kozma (Richmond, VA) for sharing with us unpub-
lished data on the hydrolysis of a series of their trans
compounds, J. Bella (Edinburgh, U.K.) for advice and
assistance with NMR spectroscopy, and colleagues in the
EC COST Action D20 for stimulating discussions.
Supporting Information Available: X-ray crystallographic data
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
(46) Kasparkova, J.; Novakova, O.; Marini, V.; Najajreh, Y.; Gibson, D.;
Perez, J.-M.; Brabec, V. J. Biol. Chem. 2003, 278, 47516-47525.
IC050763T
Inorganic Chemistry, Vol. 44, No. 21, 2005 7467