Synthesis and anti-cancer activities of a water soluble gold(III) porphyrin

Article abstract of DOI:10.1142/S1088424615500236

Gold(III) compounds continue to be explored for their potential utility as anticancer agents. A recognized limitation is the reactivity of gold(III), which is typically reduced to the more labile gold(I) state under physiological conditions. The use of po

Full text of DOI:10.1142/S1088424615500236

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2015; 19: 1–6
DOI: 10.1142/S1088424615500236
Published at http://www.worldscinet.com/jpp/
Synthesis and anti-cancer activities of a water soluble
gold(III) porphyrin
Aaron D. Lammer, Melissa E. Cook and Jonathan L. Sessler*◊
Department of Chemistry, The University of Texas at Austin, 105 E. 24th Street-A5300, Austin, TX 78712-1224, USA
Dedicated to Professor Shunichi Fukuzumi on the occasion of his retirement
Received 6 December 2014
Accepted 19 December 2014
ABSTRACT: Gold(III) compounds continue to be explored for their potential utility as anticancer
agents. A recognized limitation is the reactivity of gold(III), which is typically reduced to the more
labile gold(I) state under physiological conditions. The use of porphyrins can overcome this problem.
However, to date the stabilization provided by the use a strongly chelating porphyrin is offset by the poor
solubility of the resulting complex in aqueous media. In this work, we describe the synthesis and in vitro
anti-cancer activity of a gold(III)porphyrin complex with relatively good aqueous solubility. As judged
from standard antiproliferation assays, this complex displays an IC50 of 9 mM for the A2780 human
ovarian cancer cell line. This is a higher level of potency than displayed by two related control systems.
KEYWORDS: gold, porphyrin, anti-cancer, solubility.
in aqueous media. Various synthetic strategies have been
INTRODUCTION
utilized to overcome this latter deficiency, including the
As cancers continue to develop resistance to traditional
use of methylated pyridine-, sulfate-, and polysaccharide-
chemotherapies, such as cisplatin and its derivatives, there
modified porphyrins. However, these modifications affect
isacorrespondingneedtodevelopnoveldrugsthatoperate
the biological properties of the core porphyrin species,
by different mechanisms of action [1]. One approach
including if or how it triggers cell death [5]. In this study,
involves testing other metal complexes. In this context,
we report the synthesis of a water soluble, hydroxyl
significant effort has been devoted towards use of gold(III)
modified tetraphenyl gold porphyrin and show that it
species for the treatment of cancer, as well as many other
has antiproliferative activity against the human ovarian
illnesses [2]. However, in most ligand environments
cancer cell line, A2780.
gold(III) is unstable under physiological conditions, and
Goldcompoundshavebeenusedsince2500B.C.forthe
is reduced quickly to the more labile gold(I). A number
treatment of smallpox and measles [6]. Gold complexes
of stabilizing ligand have been explored in order to avoid
were incorporated into the modern pharmacopeia in the
reduction and retain the presumably more active Au(III)
1920’s by Koch when he used them for the treatment of
oxidation state, a number of stabilizing ligand have
tuberculosis, with an eventual shift to gold(I) thiolates to
been explored [3]. A particular attractive complexing
mediate toxicity [2a and 2b]. Since then, gold-containing
agent would be a porphyrin due to its known ability to
agents have been extensively studied for the treatment of
stabilize otherwise labile metal centers, as well as the
rheumatoid arthritis, and recently HIV [2h–2j]. In 1965
recognized cancer targeting capabilities demonstrated by
Lorber and coworkers reported the anti-cancer activity
many porphyrin derivatives [4]. One major drawback of
most easy-to-prepare porphyrins is their poor solubility
of auranofin (1) against strains of cancer derived from
the HeLa cell line Fig. 1 [7]. Berners–Price and Sadler
studied the ligand exchange of gold(I) and gold(III)
complexes in 1996 [8]. In that same year, Kelland and
◊SPP full member in good standing
co-workers reported in vivo studies of 2-[(dimethylamino)
methyl] phenylgold(III) against a panel of human cancer
*Correspondence to: Jonathan L. Sessler, email: sessler@
cm.utexas.edu
Copyright © 2015 World Scientific Publishing Company

2
A.D. LAMMER ET AL.
R
N
N
N
N
Cl-
R
Au
R
R
2
R =
R =
R =
R =
R =
(2a)
Cl-
R =
N
(2f)
O
Cl (2b)
R =
R =
SO3
O
(2g)
Na
O
O
O
OH
O
O
S
P
(2h)
OAc (2c)
OCH3 (2d)
Ph (2e)
Au
O
O
O
HO
OH
1
OH
Fig. 1. Aurofin 1 [7] and several known gold(III) porphyrin systems [3, 4]
cell lines including colon, breast, ovarian and bladder.
Unfortunately, this complex was characterized by limited
activity, an effect ascribed to poor aqueous solubility [9].
Since that time, a number of groups have developed a
wide range of gold complexes and have reported varying
degrees of efficacy against a variety of cancer cell lines,
both in vivo and in vitro [2, 10].
Many researchers have been attracted to gold(III)
porphyrins as possible drugs due to the high stability
and relative ease of synthesis typical of porphyrins [3]. A
commonly studied system is meso-tetraphenylporphyin
gold chloride (2a) [3, 4]. Among other benefits complex
2a binds non-covalently to human serum albumin (HSA)
[4n]. Many metal-based drugs, including oxaliplaten
and auranofin, bind strongly with plasma proteins,
predominately HAS, which makes gold(III) porphyrins
further attractive in the context of pharmaceutical lead
development.
Compound 2a has shown significant cytotoxicity
against numerous cell lines, including KB-3-1 and
the multi-drug resistant variant, KB-V1 [4i]. The
effectiveness against KB-V1 was taken as evidence
that the P-glycoproteins, proteins that remove foreign
substances including drugs from the cell, have little effect
on 2a. These studies also revealed a ten-fold increase
in the cytotoxicity towards tumor cells as compared to
healthy cells. Additionally, it is worth noting that the
zinc derivative (ZnTPP) proved at least 100 fold less
cytotoxic than the corresponding gold porphyrin, 2a [4].
In vivo studies involving the human ovarian A2780 and
A2780cis (cisplatin resistant) cell line-derived xenografts
in nude mice revealed severely retarded tumor growth,
and higher concentration of apoptotic cells [4o]. Similar
results were seen with 2a in the case of other studies, as
noted in several recent reviews [4a, 4b and 5].
Although 2a displays many attributes that make it
attractive as a potential anti-cancer agent, it is severely
limited by its poor solubility in aqueous media. To address
this deficiency, a number of other gold porphyrins (e.g.
2b–2h) have been prepared [3, 4].Among the most widely
studied of these are those containing methylated pyridine-
(2f), phenyl sulfate-(2g), and polysaccharide subunits.
While showing greatly increased solubility, as a general
rule, the cytotoxicity of these compounds is generally
diminished relative to 2a [4n]. In fact, the cytotoxicity
of the charged species is all but eliminated, a finding
attributed to the decreased lipopholicity of the complexes
in question [4n]. On the other hand, the polysaccharide
functionalized system (2h) displays attractive anti-
angiogenic properties [4n]. This leads us to suggest that
new gold(III) systems with good aqueous solubility may
show significant anticancer activity. We report here one
such system, the gold(III) porphyrin complex 6 derived
from the polyhydroxylated porphyrin 3.
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J. Porphyrins Phthalocyanines 2015; 19: 2–6

SYNTHESIS AND ANTI-CANCER ACTIVITIES OF A WATER SOLUBLE GOLD(III) PORPHYRIN
3
O
O
O
O
O
O
O
O
O
O
O
O
NH
N
N
N
N
N
N
KAuCl4
Cl-
Au
HN
O
O
O
O
4
5
BBr3
BBr3
X
HO
OH
HO
OH
OH
OH
OH
OH
HO
HO
HO
HO
KAuCl4
NH
N
N
N
N
N
N
Cl-
Au
X
HN
HO
OH
HO
OH
3
6
Scheme 1. Attempted synthesis of octahydroxy gold(III) tetraphenylporphyrin 6
corresponding acetoxy protected metalated porphyrin
was observed after the reaction.
EXPERIMENTAL
Octahydroxy tetraphenylporphyrin (3) was synthe-
sized from octamethoxytetraphenyl porphyrin (4) by
subjecting the methoxy group to boron tribromide-
mediated cleavage [11]. Carrying out this bond
scission using a pre-formed gold(III) octamethoxy
tetraphenylporphyrin resulted in demetalation. Equally
unsuccessful was a strategy wherein the cleavage step
was carried out prior to metalation and then metalation
was attempted using with potassium tetrachloride
aurate. Under these conditions, only starting material
was recovered (Scheme 1). However, the desired
gold(III) complex could be obtained by protecting the
hydroxyl groups by treating with acetic anhydride prior
to metalation. This protection was found to proceed in
near quantitative yield. Metalation, concomitant with
deprotection, was then accomplished by heating the
porphyrin with potassium tetrachloroaurate at reflux in
propionic acid [12]. The desired, deprotected product
was isolated in 20% yields, along with the octahydroxy
freebase porphyrin (3) (Scheme 2). No evidence of the
Gold porphyrin complexes were characterized by
high-resolution mass spectrometretry, in addition to UV/
Vis and NMR spectroscopy. Compared to the metal-
free form 3, the Soret band of the gold complex 6 is
hypsochromically shifted by 6 nm compared in ethyl
acetate solution. This shift has been explained by Jamin
and Iwamoto in terms of a stabilization of the HOMO
orbitals as the result of strong Au-N interactions[13]. The
1H NMR spectrum of the Au(III) complex 6 recorded in
CD3OD showed all the expected CH signals. However,
the hydroxyl protons proved difficult to identify;
presumably, this reflects –OH to –OD exchange. Analysis
by HPLC shows one peak with a slight shoulder. This
peak is thought to be due to ligand exchange with acetate
under the conditions of HPLC analysis.
In vitro cytotoxicity studies
The cytotoxicity of octahydroxy gold(III) porphyrin
(6), octamethoxy gold(III) porphyrin (5) and the
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4
A.D. LAMMER ET AL.
HO
OH
AcO
OAc
OH
OH
OAc
OAc
HO
HO
AcO
AcO
NH
N
N
NH
N
N
AcOAc
KAuCl4
CH3CH2CO2H
HN
HN
HO
HO
OH
OH
AcO
HO
OAc
OH
3
OH
OH
OH
OH
HO
HO
HO
HO
NH
N
N
N
N
Cl-
+
Au
HN
N
N
HO
OH
HO
OH
3
6
Scheme 2. Synthesis of Au(III) octahydroxytetraphenylporphyrin. Also shown is the demetalated freebase porphyrin (3) obtained
as a side product during the reaction sequence
The above cell studies revealed an IC50 value of 9 mM
in the A2780 cell line for the octahydroxy gold(III)
porphyrin (6). Under analogous conditions, IC50 values
of 29 mM and 28 mM were found for the octamethoxy (5)
and freebase (3) derivatives respectively. These results are
shown in Fig. 2, which includes the error bars reflecting
reproducibility (standard deviation) across at least three
separate measurements.
RESULTS AND DISCUSSION
The mechanism by which the gold porphyrins trigger
cell death is not completely understood. It was initially
thought to be similar to cisplatin in that it involves
dative binding to DNA [3]. However, more recently
it has been proposed that gold complexes interact with
proteasomes and thioredoxin reductase [4a, 4d]. More
generally, lipophilic planar cations have been proposed as
Fig. 2. A2780 cells dosed with either compounds 3, 5, and 6 at
100 mM, followed by 50% serial dilutions. Antiproliferative
mitochondria targeting agents [4n]. Gold(III) porphyrins
bearing only neutral substituents (and thus bearing only a
single cationic charge in the case of a poorly coordinating
axial ligand) are thus of particular interest since they
would allow the limits of this latter postulate to be tested
in an operational sense within the context of anticancer
drug discovery efforts. The system we report here,
complex6, wasdesignedwithsuchconsiderationsinmind.
It incorporates multiple hydroxyphenyl substitutents.
activity was determined from MTT assays. Error bars are
the standard deviations for measurements performed in
triplicate
metal-free octahydroxy porphyrin (3) were evaluated
using the human ovarian cancer cell line A2780. Cells
were incubated with varying dosages at 37°C for five
days before treatment with thianol blue for 4 h.
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SYNTHESIS AND ANTI-CANCER ACTIVITIES OF A WATER SOLUBLE GOLD(III) PORPHYRIN
5
Gratifyingly, it displays good solubility in aqueous
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Acknowledgements
This work was supported by the National Institutes of
Health (grant CA 68682 to J.L.S.).
Supporting information
Supplementary material is available free of charge via
the Internet at http://www.worldscinet.com/jpp/jpp.shtml.
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