Auranofin and related heterometallic gold(I)-thiolates as potent inhibitors of methicillin-resistant Staphylococcus aureus bacterial strains

Article abstract of DOI:10.1016/j.jinorgbio.2014.05.008

A series of new heterometallic gold(I) thiolates containing ferrocenyl-phoshines were synthesized. Their antimicrobial properties were studied and compared to that of FDA-approved drug, auranofin (Ridaura), prescribed for the treatment of rheumatoid arthritis. MIC in the order of one digit micromolar were found for most of the compounds against Gram-positive bacteria Staphylococcus aureus and CA MRSA strains US300 and US400. Remarkably, auranofin inhibited S. aureus, US300 and US400 in the order of 150-300 nM. This is the first time that the potent inhibitory effect of auranofin on MRSA strains has been described. The effects of a selected heterometallic compound and auranofin were also studied in a non-tumorigenic human embryonic kidney cell line (HEK-293).

Full text of DOI:10.1016/j.jinorgbio.2014.05.008

Journal of Inorganic Biochemistry 138 (2014) 81–88
Contents lists available at ScienceDirect
Journal of Inorganic Biochemistry
journal homepage: www.elsevier.com/locate/jinorgbio
Auranofin and related heterometallic gold(I)–thiolates as potent
inhibitors of methicillin-resistant Staphylococcus aureus bacterial strains
Yozane Hokai a,b, Boruch Jurkowicz b, Jacob Fernández-Gallardo a, Nuruddinkodja Zakirkhodjaev b,
Mercedes Sanaú c, Theodore R. Muth b, María Contel a,
⁎
a
Department of Chemistry, Brooklyn College and The Graduate Center, The City University of New York, Brooklyn, NY 11210, USA
Department of Biology, Brooklyn College and The Graduate Center, The City University of New York, Brooklyn, NY 11210, USA
Departamento de Química Inorgánica, Universidad de Valencia, Burjassot, Valencia 46100, Spain
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new heterometallic gold(I) thiolates containing ferrocenyl-phoshines were synthesized. Their
antimicrobial properties were studied and compared to that of FDA-approved drug, auranofin (Ridaura),
prescribed for the treatment of rheumatoid arthritis. MIC in the order of one digit micromolar were found for
most of the compounds against Gram-positive bacteria Staphylococcus aureus and CA MRSA strains US300 and
US400. Remarkably, auranofin inhibited S. aureus, US300 and US400 in the order of 150–300 nM. This is the
first time that the potent inhibitory effect of auranofin on MRSA strains has been described. The effects of a
selected heterometallic compound and auranofin were also studied in a non-tumorigenic human embryonic
kidney cell line (HEK-293).
Received 13 February 2014
Received in revised form 5 May 2014
Accepted 6 May 2014
Available online 28 May 2014
Keywords:
Antimicrobial
Auranofin
© 2014 Elsevier Inc. All rights reserved.
Heterometallic gold–thiolates
MRSA
1. Introduction
the selenium metabolism is implied in the growth inhibition observed
[15,16]. Both C. difficile and T. denticola utilize selenoproteins for energy.
During the past two decades there has been a renewed interest in
the study of the biological activities and potential medicinal applications
of gold compounds [1]. In particular, gold(I) compounds have been
thoroughly explored as cytotoxic agents [2]. Many of the compounds re-
ported contain thiolates and/or phosphines as ligands [3,4] and their
mode of action seems to be based on thioredoxin reductase inhibition
[5]. Some of these gold(I) thiolates (e.g. aurothiomalate and auranofin
(AF) in Fig. 1) are used therapeutically [6] and belong to the group of
disease-modifying antirheumatic drugs (DMARDs) used to slow down
or stop the progression of this rheumatic disorder. Auranofin (approved
by the FDA in 1985 and commercialized under the brand name of
Ridaura) has been granted orphan-drug status and its potential medic-
inal applications have been reviewed recently [7].
Recent reports indicate that AF is able to effectively kill the parasites
Schistosoma mansoni [8], Trypanosoma crucei [9], Echinococcus
granulosus [10], Plasmodium falciparum [11], Leishmania infantum [12]
and Giardia lamblia [13]. Last year, the potential of AF for the treatment
of amebiasis was described [14]. In this report it was suggested that AF
targets Entamoeba histolytica thioredoxin reductase activity [14]. In
comparison, the antibacterial activities of gold(I) derivatives have
been less explored [7]. Some reports on the effect of AF on Clostridium
difficile and Treponema denticola in vitro have shown that disruption of
The growth inhibition effect of AF on Staphylococcus aureus had been
reported before [17], but surprisingly this article has not been cited in
any of these recent articles and reviews.
While S. aureus infections (staph) can be effectively treated
with penicillinase-resistant β-lactam antibiotics (methicillin, oxacillin),
resistant strains of S. aureus, such as the methicillin-resistant S. aureus
(MRSA) strains, are very common. Hospital associated infections caused
by MRSA (HA MRSA) are a major problem in hospitals, prisons and
nursing homes around the world. Patients with open wounds, invasive
devices, and weakened immune systems are at great risk of infection.
However, in the 1990s community associated MRSA (CA MRSA) infec-
tions emerged in persons in which the healthcare associated risk factors
were generally absent. Two of the best described CA MRSA strains are
US300 and US400. The first line treatment for MRSA infections is the
glycopeptide antibiotic vancomycin, although vancomycin-resistant
strains of S. aureus (VRSA) exist and reports of these strains are on the
rise globally. Additional effective antibiotics for both HA and CA MRSA
bacteria, which are currently considered a serious public health threat
in the US [18], are urgently needed.
In our group we have explored the antimicrobial properties of differ-
ent gold(I)–phosphine compounds, including some heterometallic
gold(I)–silver(I) and gold(I)–copper(I) derivatives [19,20]. The hetero-
metallic compounds Au2–Ag were more active than the parent mono-
metallic fragments against Gram-negative (Salmonella typhimurium
and Escherichia coli) and Gram-positive bacteria (Bacillus cereus and
⁎
Corresponding author. Fax: +1 7189514607.
E-mail address: mariacontel@brooklyn.cuny.edu (M. Contel).
http://dx.doi.org/10.1016/j.jinorgbio.2014.05.008
0162-0134/© 2014 Elsevier Inc. All rights reserved.

82
Y. Hokai et al. / Journal of Inorganic Biochemistry 138 (2014) 81–88
(E. coli and Pseudomonas aeruginosa), Gram-positive bacteria (including
S. aureus and MRSA US300 and US400 strains) and yeast (Saccharomyces
cerevisiae) and compared to that of AF.
2. Experimental
2.1. Materials and methods
All manipulations involving air-free syntheses were performed using
standard Schlenk-line techniques under a nitrogen atmosphere or in a
glove-box MBraun MOD System. Solvents were purified by use of a
PureSolv purification unit from Innovative Technology, Inc. Auranofin
was purchased from Enzo Life Sciences, thiophenol (a) was purchased
from Alfa Aesar, 2-propanethiol (b) and 2-mercaptothiazoline
(c) were purchased from Acros Organic, 1-thio-beta-D-glucose tetraacetate
(d) was purchased from Sigma-Aldrich and 6-mercaptopurine
monohydrate (e) was purchased from MP Biochemicals. The compound
[(Cp-PPh2)2Fe] 1,1′-bis-diphenylphosphino-ferrocene (DPPF) was
purchased from Strem chemicals and used without further purification.
[AuCl(tht)] [23], [{AuCl}2(μ-DPPF)] (1) [24] and [(Cp-PPh2)Fe(Cp)]
diphenylphosphino-ferrocene (MPPF) [25] were prepared by reported
methods. [{AuSPh}2(μ-DPPF)] (3a) was prepared by a modification of
a previously reported procedure [26], more detailed spectroscopic and
crystallographic data for this published compound are included here.
Purity of the compounds is based on elemental analysis and in all
cases is ≥99.5%. Elemental analyses were performed on a Perkin
Elmer 2400 CHNS/O Analyzer, Series II. NMR spectra were recorded in
Fig. 1. Auranofin and aurothiomalate used therapeutically in the treatment of rheumatoid
arthritis.
S. aureus) with MIC ranging from 1 to 10 μg/mL [20]. More recently, we
have described the potential of gold(III) and palladium(II) hetero-
metallic complexes (FeAu2 and FePd2) based on ferrocenyl imino-
phosphorane ligands as anticancer agents through zinc finger protein
poly-(adenosine diphosphate (ADP)-ribose) polymerase 1 (PARP-1)
inhibition [21]. Ferrocene has special properties such as low toxicity,
high lipophilicity and unique electrochemical behavior which makes it
an attractive motif in drug design [22].
In this context, we report here on the synthesis, characterization and
antimicrobial properties of heterometallic FeAu2 and FeAu thiolate
gold(I) complexes containing ferrocenyl phosphines (Scheme 1). Their
antimicrobial properties have been studied on Gram-negative bacteria
Scheme 1. Synthesis of heterometallic gold(I) thiolate complexes based on DPPF (3) or MPPF (4) ferrocenyl phosphines. Base = K2CO3 in CH2Cl2 or KOH/MeOH.

Y. Hokai et al. / Journal of Inorganic Biochemistry 138 (2014) 81–88
83
a Bruker AV400 (1H NMR at 400 MHz, 13C NMR at 100.6 MHz, 31P NMR
at 161.9 MHz). Chemical shifts (δ) are given in ppm using CDCl3
or DMSO-d6 as solvent, unless otherwise stated. 1H and 13C chemical
shifts were measured relative to solvent peaks considering TMS
(tetramethylsilane) = 0 ppm; 31P{1H} was externally referenced to
H3PO4 (85%). MNova was the report software package used for NMR
spectra analysis. Infrared spectra (4000–250 cm−1) were recorded on
a Nicolet 6700 FT-IR spectrophotometer on KBr pellets. Mass spectra
(ESI-MS and HR-ESI MS) were performed on an Agilent Analyzer or a
Bruker Analyzer. X-ray collection was performed at room temperature
on a Kappa CCD diffractometer using graphite monochromated Mo-
Kα radiation (λ = 0.71073 Å).
tetraacetate), δ 21.2 (s, Ch-β-D-glucose tetraacetate), δ 20.8 (s, Ch-β-D-
glucose tetraacetate), δ 20.7 (s, Ch-β-D-glucose tetraacetate), δ 20.6
(s, Ch-β-D-glucose tetraacetate).
3d: Yield: 0.184 g, 61%. Anal. Calc. for C40H36Au2FeN2P2S4 (1184.71):
C, 40.55; H, 3.06; N, 2.36; S, 10.82. Found: C, 40.59; H, 3.21, N, 2.45;
S, 10.56. MS(ESI+) [m/z]: 751.1 [DPPF + Au]+, 1066.1 [M −
S(thiazoline)]+ 31P{1H} NMR (DMSO-d6): δ 32.21. 31P{1H} NMR
.
(CDCl3): δ 32.15. 1H NMR (plus COSY, plus NOESY-2D, CDCl3): δ 7.52
(20H, m, C6H5), δ 4.81 (4H, s, C5H4), δ 4.37 (4H, s, C5H4), δ 4.29 (4H,
m, CH2N), δ 3.45 (4H, m, CH2S). 13C{1H} NMR (plus HSQC, CDCl3):
δ 173.1 (s, CS(S)(N)). δ 133.6 (d, JPC = 13.9 Hz, C6H5), δ 131.7
(s, C6H5), δ 130.7 (d, JPC = 59.1 Hz, C6H5), δ 129.1 (d, JPC = 11.6 Hz,
C6H5), δ 75.4 (d, JPC = 8.3 Hz, C5H4), 74.9 (d, JPC = 13.3 Hz, C5H4)
71.6 (d, JPC = 66.8 Hz, C5H4), δ 64.6 (s, CH2N), δ 37.5 (s, CH2S).
3e: Yield: 0.128 g, 52%. Anal. Calc. for C44H34Au2FeN8P2S2 (1250.66):
C, 42.26; H, 2.74; N, 8.96; S, 5.13. Found: C, 41.89; H, 3.06, N, 8.65;
S, 5.37. MS(ESI+) [m/z]: 751.1 [DPPF + Au]+, 1099.1 [M − S(6-mer-
2.2. Synthesis and characterization of the new compounds
2.2.1. [{AuCl}(MPPF)] (2)
MPPF (0.738 g, 2 mmol) was added to a solution of [AuCl(tht)]
(0.641 g, 2 mmol) in 40 mL of dichloromethane at 0 °C. The mixture
was stirred for 20 min at room temperature and then the solvent was
removed to ca. 3 mL. The addition of 5 mL of cold diethyl ether afforded
a yellow-brown precipitate that was isolated by filtration. Yield: 1.142 g,
95%. Anal. Calc. for C22H19AuClFeP (602.63): C, 43.85; H, 3.18. Found: C,
44.19; H, 3.28. MS(ESI+) [m/z]: 567.0 [M − Cl]+. 31P{1H} NMR (CDCl3):
δ 28.46. 1H NMR (CDCl3): δ 7.60 (4H, m, C6H5-orto), δ 7.48 (6H, m,
C6H5-meta/para), δ 4.60 (2H, s, C5H4), δ 4.38 (2H, s, C5H4), δ 4.22 (5H,
s, C5H5). 13C{1H} NMR (CDCl3): δ 133.5 (d, JPC = 13.5 Hz, C6H5),
captopurine)]+ 31P{1H} NMR (DMSO-d6): δ 27.17. 31P{1H} NMR
.
(CDCl3): δ 27.67. 1H NMR (plus COSY, plus NOESY-2D, CDCl3): δ 8.81
(2H, s, 2-purine), δ 8.23 (2H, s, 8-purine), δ 7.52 (8H, m, C6H5), δ 7.46
(12H, m, C6H5), δ 4.68 (4H, s, C5H4), δ 4.35 (4H, s, C5H4), δ 1.62 (2H,
s, 9-purine). 13C{1H} NMR (plus HSQC, CDCl3): δ 154.1 (s, 4-purine),
δ 150.8 (s, 8-purine), δ 133.4 (d, JPC = 14.0 Hz, C6H5), δ 132.2 (d, JPC
=
77.6 Hz, C6H5), δ 131.9 (s, C6H5), δ 129.1 (d, JPC = 11.9 Hz, C6H5),
δ 128.4 (s, 2-purine), δ 126.3 (s, 5-purine), δ 77.2 (s, C5H4), 75.3
(s, C5H4), C6-purine not observed.
δ 131.6 (s, C6H5), δ 130.9 (d, JPC = 63.5 Hz, C6H5), δ 128.8 (d, JPC
=
4a: Yield: 0.078 g, 58%. Anal. Calc. for C28H24AuFePS (676.35): C,
49.72; H, 3.58; S, 4.74. Found: C, 49.62; H, 3.55, S, 4.88. MS(ESI+)
[m/z]: 567.16 [M − SPh]+, 677.04 [M + H]. 31P{1H} NMR (DMSO-d6):
δ 32.40. 31P{1H} NMR (CDCl3): δ 33.87. 1H NMR (plus COSY, plus
NOESY-2D, CDCl3): δ 7.70 (2H, d, 3JHH = 7.4 Hz, P(C6H5)), δ 7.63 (4H,
dd, 3JHH = 7.4 Hz, 3JHH = 12.5 Hz P(C6H5)), δ 7.48 (4H, m, P(C6H5)), δ
7.26 (2H, m, S(C6H5)), δ 7.16 (2H, m, S(C6H5)), δ 7.03 (1H, m,
S(C6H5)), δ 4.58 (2H, s, C5H4), δ 4.39 (2H, s, C5H4), δ 4.17 (5H, s, C5H5).
13C{1H} NMR (plus HSQC, CDCl3): δ 133.6 (d, JPC = 13.8 Hz, P(C6H5)),
δ 132.8 (s, S(C6H5)), δ 132.0 (s, S(C6H5)), δ 131.3 (m, P(C6H5)), δ 131.1
(d, JPC = 68.8 Hz, P(C6H5)), δ 128.8 (d, JPC = 11.3 Hz, P(C6H5)), δ
128.1 (s, S(C6H5)), δ 123.5 (s, S(C6H5)), δ 73.5 (d, JPC = 13.8 Hz, C5H4),
72.4 (d, JPC = 8.5 Hz, C5H4) 70.0 (s, C5H4).
11.8 Hz, C6H5), δ 73.4 (d, JPC = 14.07 Hz, C5H4), δ 72.5 (d, JPC = 9.6 Hz,
C5H4), δ 70.2 (s, C5H5). IR (cm−1): ν 335 (Au–Cl).
2.2.2. [{AuSR}2(DPPF)] (3a–3e) and [{AuSR}(MPPF)] (4a–4e)
A mixture of the thiols (a–e) (0.2 mmol) and of K2CO3 (2.0 mmol)
in 30 mL of dichloromethane was stirred for 10 min at room tempera-
ture. The corresponding gold complex, either [{AuCl}2(DPPF)] (1)
(0.1 mmol) or [AuCl(MPPF)] (2) (0.2 mmol), was added to the solution.
The reaction mixture was stirred during 2 h at room temperature. The
solution was subsequently filtered through celite to remove any excess
reagents and undesired side-products. The solvent was removed to
dryness to afford the expected gold thiolate complex.
3a (ref. [4]): Yield: 0.132 g, 57%. 31P{1H} NMR (DMSO-d6): δ 33.0.
3b: Yield: 0.146 g, 66%. Anal. Calc. for C40H42Au2FeP2S2 (1098.62): C,
43.73; H, 3.85; S, 5.84. Found: C, 43.29; H, 3.78, S, 5.91. MS(ESI+) [m/z]:
4b: Yield: 0.130 g, 76%. Anal. Calc. for C25H26AuFePS (642.33): C,
46.75; H, 4.08; S, 4.99. Found: C, 47.14; H, 4.05, S, 4.79. MS(ESI+)
[m/z]: 567.15 [M − SiPr]+, 643.05 [M + H]. 31P{1H} NMR (DMSO-d6):
δ 34.32. 31P{1H} NMR (CDCl3): δ 34.50. 1H NMR (plus COSY, plus
NOESY-2D, CDCl3): δ 7.61 (4H, m, C6H5), δ 7.45 (6H, m, C6H5), δ 4.56
(2H, s, C5H4), δ 4.37 (2H, s, C5H4), δ 4.20 (5H, s, C5H5), δ 3.76 (1H,
1024.1 [M − SiPr]+ 31P{1H} NMR (DMSO-d6): δ 32.21. 31P{1H} NMR
.
(CDCl3): δ 33.58. 1H NMR (plus COSY, plus NOESY-2D, CDCl3): δ 7.48
(20H, m, C6H5), δ 4.78 (4H, s, C5H4), δ 4.25 (4H, s, C5H4), δ 3.73 (2H, s,
3
3
3
CH(CH3)2), δ 1.51 (12H, d, JHH = 6.4 Hz, CH(CH3)2). 13C{1H} NMR
sept, JHH = 6.5 Hz, CH(CH3)2), δ 1.55 (6H, d, JHH = 6.5 Hz,
(plus HSQC, CDCl3): δ 133.5 (d, JPC = 14.0 Hz, C6H5), δ 131.4 (d, JPC
=
CH(CH3)2). 13C{1H} NMR (plus HSQC, CDCl3): δ 133.6 (d, JPC
=
56.4 Hz, C6H5), δ 131.4 (s, C6H5), δ 128.9 (d, JPC = 11.3 Hz, C6H5), δ
13.9 Hz, C6H5), δ 131.4 (d, JPC = 57.5 Hz, C6H5), δ 131.0 (s, C6H5),
δ 128.7 (d, JPC = 11.0 Hz, C6H5), δ 73.4 (d, JPC = 13.6 Hz, C5H4), 72.2
(d, JPC = 8.3 Hz, C5H4) 69.9 (s, C5H4), δ 34.2 (s, CH(CH3)2), ), δ 32.5
(s, CH(CH3)2).
75.1 (d, JPC = 8.1 Hz, C5H4), 74.5 (d, JPC = 13.1 Hz, C5H4) 72.1 (d, JPC
63.3 Hz, C5H4), δ 34.4 (s, CH(CH3)2), ), δ 30.9 (s, CH(CH3)2).
=
3c: Yield: 0.206 g, 61%. Anal. Calc. for C62H66Au2FeO18P2S2
(16,743.57): C, 44.46; H, 3.97; S, 3.83. Found: C, 44.93; H, 4.20, S, 3.30.
4c: Yield: 0.106 g, 55%. Anal. Calc. for C36H38AuFeO9PS (930.54): C,
46.47; H, 4.12; S, 3.45. Found: C, 45.97; H, 4.23, S, 3.29. MS(ESI+)
MS(ESI+) [m/z]: 1311.1 [M − S(β-D-glucose tetraacetate)]+ 31P{1H}
.
NMR (DMSO-d6): δ 32.52. 31P{1H} NMR (CDCl3): δ 32.94. 1H NMR
(plus COSY, plus NOESY-2D, CDCl3): δ 7.50 (20H, m, C6H5), δ 5.20 (8H,
m, Hb,c,d,e-β-D-glucose tetraacetate), δ 4.93 (2H, s, C5H4), δ 4.87 (2H, s,
C5H4), δ 4.36 (2H, s, C5H4), δ 4.30 (2H, m, Hg-β-D-glucose tetraacetate),
δ 4.28 (2H, s, C5H4), δ 4.13 (2H, m, Hg-β-D-glucose tetraacetate), δ 3.81
(2H, m, Hf-β-D-glucose tetraacetate), δ 2.10 (6H, s, Hh-β-D-glucose
tetraacetate), δ 2.04 (6H, s, Hh-β-D-glucose tetraacetate), δ 1.98
(6H, s, Hh-β-D-glucose tetraacetate), δ 1.89 (6H, s, Hh-β-D-glucose
tetraacetate). 13C{1H} NMR (plus HSQC, plus HMBC, CDCl3): δ 170.7,
170.2, 169.8, 169.5 (s, C_O), δ 133.6 (m, C6H5), δ 131.5 (m, C6H5), δ
131.0 (m, C6H5), δ 129.0 (m, C6H5), δ 83.4, 77.9, 74.2, 69.0 (s, Cb,c,d,e-β-
D-glucose tetraacetate), δ 75.8 (s, Cf-β-D-glucose tetraacetate), δ 75.1
(m, C5H4), δ 71.7 (d, JPC = 65.05 Hz, C5H4), δ 62.9 (s, Cg-β-D-glucose
[m/z]: 567.1 [M − S(β-D-glucose tetraacetate)]+, 931.1 [M + H]. 31
P
{1H} NMR (DMSO-d6): δ 33.56. 31P{1H} NMR (CDCl3): δ 34.13. 1H
NMR (plus COSY, plus NOESY-2D, CDCl3): δ 7.64 (4H, m, C6H5), δ 7.49
(6H, m, C6H5), δ 5.21 (4H, m, Hb,c,d,e-β-D-glucose tetraacetate), δ 4.59
(2H, s, C5H4), δ 4.45 (1H, m, Hg-β-D-glucose tetraacetate), δ 4.27 (7H,
m, C5H4, C5H5), δ 4.16 (1H, m, Hg-β-D-glucose tetraacetate), δ 3.82
(1H, m, Hf-β-D-glucose tetraacetate), δ 2.06 (12H, m, Hh-β-D-glucose
tetraacetate). 13C{1H} NMR (plus HSQC, plus HMBC, CDCl3): δ 170.9,
170.8, 170.3, 169.5 (s, C_O), δ 133.7 (d, JPC = 13.9 Hz, C6H5), 131.2
(s, C6H5), δ 130.7 (d, JPC = 58.1 Hz, C6H5), δ 128.8 (d, JPC = 11.5 Hz,
C6H5), δ 83.4, 77.8, 73.6, 69.0 (s, Cb,c,d,e-β-D-glucose tetraacetate), δ
77.8 (s, Cf-β-D-glucose tetraacetate), δ 72.4 (s, C5H4), δ 70.1 (s, C5H4),
δ 62.8 (s, Cg-β-D-glucose tetraacetate), δ 21.2 (s, Ch-β-D-glucose

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Y. Hokai et al. / Journal of Inorganic Biochemistry 138 (2014) 81–88
tetraacetate), δ 20.8 (s, Ch-β-D-glucose tetraacetate), δ 20.7 (s, Ch-β-D-
glucose tetraacetate), δ 20.6 (s, Ch-β-D-glucose tetraacetate).
and YPD broth for S. cerevisiae) and grown overnight at 37 °C (30 °C for
S. cerevisiae). The overnight cultures were diluted to an OD600 b 0.01
(Thermo Spectronic, Genesys 8 spectrophotometer) in 2 mL of fresh
media in sterile culture tubes. The compounds (auranofin, 3a–3e and
4a–4e) were brought up in DMSO to a concentration of 1 mg/mL and
then diluted into the cell culture tubes at the specified concentrations.
Control tubes for each cell type were inoculated with an equivalent con-
centration of DMSO alone. The cultures were then transferred as 200 μL
volumes into a 96-well plate (typically 8 replicates were performed for
each condition). The plate was incubated at the appropriate tempera-
ture and shaken in a Biotek ELx808 plate reader. Growth measurements
(OD600) were automatically taken every hour for 24 h. The minimal
inhibitory concentration (MIC) was determined to be the concentration
at which there was negligible increase in the OD600 value from the initial
reading after 24 h. All samples were independently tested two or three
times as described here.
4d: Yield: 0.100 g, 72%. Anal. Calc. for C25H23AuFeNPS2 (685.37): C,
43.81; H, 3.38; N, 2.04; S, 9.36. Found: C, 43.46; H, 3.76, N, 2.08; S,
9.87. MS(ESI+) [m/z]: 567.15 [M − S(thiazoline)]+, 686.0 [M + H].
31P{1H} NMR (DMSO-d6): δ 32.36. 31P{1H} NMR (CDCl3): δ 33.1. 1H
NMR (plus COSY, plus NOESY-2D, CDCl3): δ 7.64 (4H, m, C6H5), δ 7.48
(6H, m, C6H5), δ 4.59 (2H, s, C5H4), δ 4.42 (2H, s, C5H4), δ 4.32 (2H, d,
3
3JHH = 8.0, CH2N), δ 4. 24 (5H, s, C5H5), δ 3.47 (2H, d, JHH = 7.9,
CH2S). 13C{1H} NMR (plus HSQC, CDCl3): δ 173.3 (s, CS(S)(N)). δ 133.7
(d, JPC = 13.8 Hz, C6H5), δ 131.3 (s, C6H5), δ 130.5 (d, JPC = 56.6 Hz,
C6H5), δ 128.8 (d, JPC = 11.8 Hz, C6H5), δ 73.6 (d, JPC = 8.5 Hz, C5H4),
72.4 (d, JPC = 13.5 Hz, C5H4) 70.17 (s, C5H4), δ 68.1 (s, CH2N), δ 30.4
(s, CH2S).
4e: Yield: 0.079 g, 55%. Anal. Calc. for C27H22AuFeN4PS (718.34): C,
45.14; H, 3.09; N, 7.80; S, 4.46. Found: C, 45.35; H, 3.13, N, 7.71; S,
4.59. MS(ESI+) [m/z]: 567.0 [M − S(6-mercaptopurine)]+, 719.0 [M +
H]. 31P{1H} NMR (DMSO-d6): δ 29.27. 31P{1H} NMR (CDCl3): δ 28.32. 1H
NMR (plus COSY, plus NOESY-2D, CDCl3): δ 8.74 (2H, s, 2-purine),
δ 8.25 (2H, s, 8-purine), δ 7.66 (4H, m, C6H5), δ 7.41 (6H, m, C6H5), δ
4.53 (2H, s, C5H4), δ 4.39 (2H, s, C5H4), δ 4.16 (5H, s, C5H5), δ 1.95 (1H,
s, 9-purine). 13C{1H} NMR (plus HSQC, CDCl3): δ 152.1 (s, 4-purine),
δ 150.7 (s, 8-purine), δ 133.7 (d, JPC = 13.9 Hz, C6H5), δ 133.5 (s, 2-
purine), δ 131.9 (d, JPC = 56.8 Hz, C6H5), δ 131.2 (s, C6H5), δ 129.1 (s, 5-
purine), δ 128.8 (d, JPC = 11.2 Hz, C6H5), δ 73.51 (d, JPC = 14.0 Hz,
C5H4), δ 72.3 (d, JPC = 8.55 Hz, C5H4), 70.1 (s, C5H4), C6-purine not
observed.
2.6. Toxicity of compounds on human embryonic kidney cells (HEK-293)
The human embryonic kidney 293 cells (HEK-293), a non-tumoral
immortalized human cell line (obtained from American Type Culture
Collection, Manassas, Virginia, USA) were cultured in DMEM (Dulbecco's
Modified Eagle Medium, F-12 50/50 mix with L-glutamine and phenol
red) (from Corning Cellgro) supplemented with 10% FBS (from Gibco
Sera, Life Technology) and 1% penicillin/streptomycin, at 37 °C in a hu-
midified atmosphere of 95% of air and 5% CO2 (University of Hawaii
Cancer Center, Honolulu, Hawaii, USA). For evaluation of cell viability,
cells were seeded at a concentration of 5 × 103 cells/well in 90 μL
DMEM complete medium without phenol red, into tissue culture
grade 96-wellflat bottom microplates (Thermo Scientific BioLite
Microwell Plate, Fisher Scientific, Waltham, Massachusetts, USA) and
grown for 24 h. Solutions of the compounds were prepared by diluting
a freshly prepared stock solution (in DMSO) of the corresponding com-
pound in DMEM complete medium without phenol red. Afterwards, the
intermediate dilutions of the compounds were added to the wells
(10 μL) to obtain a final concentration ranging from 0.1 to 200 μM,
and the cells were incubated for 24 h. DMSO at comparable concentra-
tions did not show any effects on cell cytotoxicity. Following 24 h drug
exposure, 50 μL of 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-
tetrazolium-5-carboxanilide (XTT) (Roche Diagnostics, Indianapolis,
Indiana, USA) labeling mixture per well was added to the cells at a
final concentration of 0.3 mg/mL and incubated for 4 h at 37 °C in a hu-
midified atmosphere of 95% of air and 5% CO2. The optical density of
each well (96-well plates) was quantified using EnVision Multilabel
Plate Readers (Perkin Elmer, Waltham Massachusetts, USA) at 450 nm
wavelength to measure absorbance. The percentage of surviving cells
was calculated from the ratio of absorbance of treated to untreated
cells. The IC50 value was calculated as the concentration reducing the
proliferation of the cells by 50% and is presented as a mean ( SE) of
at least two independent experiments each with triplicates.
2.3. Crystallographic data for compounds 3a and 3d
Single crystals of 3a and 3d (see details below) were mounted on a
glass fiber in a random orientation. Data collection was performed
at room temperature on a Kappa CCD diffractometer using graphite
monochromated Mo-Ka radiation (l = 0.71073 Å). Space group assign-
ments were based on systematic absences, E statistics and successful
refinement of the structures. The structures were solved by direct
methods with the aid of successive difference Fourier maps and were
refined using the SHELXTL 6.1 software package. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were assigned to
ideal positions and refined using a riding model. Details of the crystallo-
graphic data are given in Table S1 (Supporting information section).
These data can be obtained free of charge from The Cambridge Crystal-
lographic Data Center via www.ccdc.cam.ac.uk/data_request/cif (CCDC
977988 for compound 3a, and 977989 for compound 3d). 3a: Crystals
of 3a (yellow prisms with approximate dimensions 0.25 × 0.23 ×
0.23 mm) were obtained from a solution of 3a in CH2Cl2 by slow diffu-
sion of Et2O at RT. 3d: Crystals of 3d (yellow prisms with approximate
dimensions 0.25 × 0.24 × 0.22 mm) were obtained from a solution of
3d in CH2Cl2 by slow diffusion of Et2O at RT.
2.4. DFT studies for compounds 4a and 4d
3. Results and discussion
The calculations have been performed using the hybrid density
functional method B3LYP, [27,28] as implemented in Gaussian09 [29].
Geometries were optimized with the 6-31G(d,p) basis set for the C, N,
P, S, and H elements and the SDD pseudopotential for the iron and
gold centers [30,31]. Frequency calculations have been done at the
same level of theory as the geometry optimizations to confirm the
nature of the stationary points.
3.1. Synthesis and characterization of the heterometallic compounds
The synthesis and partial characterization of compound 3a had been
described before [26]. The standard method employed for the synthesis
of all the heterometallic complexes described here is based on the
deprotonation of thiols with a base and subsequent reaction with
[{AuCl}2(μ-DPPF)] (1) [24] or [AuCl(MPPF)] (2) to obtain compounds
3a–e or 4a–e respectively (Scheme 1). The synthesis of the gold(I)-
chlorophosphino starting materials with DPPF (1) and MPPF (2) is
by displacement of the labile ligand tetrahydrothiophene (tht) in
[AuCl(tht)] [23] by the ferrocenyl phosphines. Compound 1 has been
reported previously and 2 was synthesized in a similar manner
(Scheme 1) by reaction of [AuCl(tht)] and MPPF [26].
2.5. Microbial toxicity assays
Bacteria and yeast were stored as glycerol stocks at −80 °C and
streaked onto Mueller–Hinton or YEPD plates prior to each set of exper-
iments. Colonies from newly prepared plates were inoculated into 5 mL
of media (tryptic soy broth for E. coli, P. aeruginosa and S. aureus strains

Y. Hokai et al. / Journal of Inorganic Biochemistry 138 (2014) 81–88
85
All new compounds are air-stable, yellow solids that are obtained in
moderate yields. The structures proposed are based on NMR and IR
spectroscopy, elemental analysis, and mass spectrometry (see spectra
in Supporting information). The heterometallic derivatives are soluble
in DMSO and in mixtures DMSO:H2O (1:99) at micromolar concentra-
tions. Compound 3c is more hydrophilic and is soluble in mixtures of
50:50 at millimolar concentrations. All complexes are stable for at
least 8 days at RT in DMSO solutions as determined by 31P{1H} and 1H
NMR spectroscopy (no more than 2–3% phosphine oxide generated,
see selected spectra in DMSO-d6 overtime in the SI). DMSO solutions
of the compounds can be kept for several weeks or months in the freez-
er without noticeable decomposition. The spectroscopic data 31P{1H}
NMR in CDCl3 and DMSO-d6 is collected in Table 1. The main peaks
observed in the MS spectra (ESI in CH2Cl2) are also collected in
Table 1. It has been reported that compounds of the type [Au(X)(PR3)]
can give rise to species [Au(PR3)2][Au(X)2] (e.g. X = SR−, SeR−, CN−
)
by ligand scrambling and dissociation [32,33]. This is relevant in order
to understand the metabolism of drugs like AF [34]. AF is known to
bind an SH fragment from cysteine 34 to form AlbSAuPEt3 in vivo and
subsequently displacing PEt3 which oxidizes to PEt3O (a process favored
by increasing the affinity of the anion to bind gold(I) or retarded by the
bulk and basicity of the phosphine) [34]. We have found by NMR
spectroscopy and MS (HR ESI) spectrometry that the trimetallic FeAu2
species [{Au(SR)}2(DPPF)] (3a–3e) do not seem to undergo this type
of ligand scrambling easily. No noticeable decomposition is noticed at
RT in DMSO-d6 after 8 days (Figs. S36–39 in SI) and sharp peaks were
found in the 31P{1H} NMR spectra in CDCl3 and DMSO-d6 solution (SI).
The ESI MS spectra for 3a–3e (Table 1 and Figs. S44–56 in the SI) do
not show peaks that can be assigned to species [Au(DPPF)2]+.
The bimetallic FeAu species [Au(SR)(MMPF)] 4b–4e give rise to
broader signals in the 31P{1H} NMR spectra (see Figs. S20, S23, S26,
S29, S32, S41 and S43 in SI). It is worth noting that these compounds
are less soluble in DMSO than compounds 3a–3e or 4a (completely sol-
uble) and the solutions prepared to get the 31P{1H} are almost saturated.
As for compounds with DPPF their MS spectra (ESI+, Figs. S57–S65 in
SI) do not show peaks which could be assigned to species [Au(MPPF)2]+
indicative of ligand scrambling. Dissociation of the SR is evident by the
peaks observed that correspond to [Au(MPPF)]+ (Table 1) but this is
common for the ESI + or FAB + MS spectra of gold complexes of the
Fig. 2. Ortep view of the crystal structure of 3d. Structure of 3a in Supplementary material.
type [AuX(PR3)]. However with the data we have so far we cannot ex-
clude the possibility of ligand scrambling in solution for compounds
4b–4e.
The structures of the trimetallic FeAu2 compounds 3a and 3d were
further confirmed by X-ray single crystal diffraction (structure of 3d
shown in Fig. 2). Selected bond lengths and angles for both compounds
are collected in Table 2 while the drawing for the crystal structure of 3a
and crystallographic tables for 3a and 3d (Table S1) are in the Supple-
mentary material. A more complete table including more values for
bond lengths and angles for complexes 3a and 3d is also included in
the SI (Table S2).
Both crystal structures are very similar. The cyclopentadienyl rings
are staggered by 36.09° (3a) and 36.88° (3d) around the Cp…Cp axis
(Cp = center of the cyclopentadienyl ring) defined by the torsion
angle C11–Cp–Cp–C13. The phosphorus atoms are located at the 1 and
3′ carbon atoms of the ferrocene rings. The molecular structures display
a typical linear geometry around the gold centers with a P1–Au1–S1
angle of 171.33(5) (3a) and 177.75(11) (3d). The distances Au\S and
Au\P are within the usual range of other gold(I) thiolates containing
phosphine ligands [17,35]. Since we could not obtain crystals of the
bimetallic complexes 4a–e of enough quality for a single crystal X-ray
determination, good estimates for the structural parameters of 4a and
4d were obtained from density functional calculations (see figures in
SI and Table S3). The distances Au\S and Au\P are similar to those in
3d and 3d and the main difference is the eclipsed disposition of the
two Cp rings (one containing the phosphine group attached to the
gold–thiolate group) for the ferrocene moiety as well as the smaller
size of the overall molecule.
Table 1
31P{1H} NMR data (CDCl3 and DMSO-d6) and main peaks in MS spectra (HR ESI) for the
new gold compounds. 31P{1H} δ in DMSO-d6 remain the same after 8 days at RT. All spec-
tra are collected in the SI section.
Complex 31P{1H} NMR (δ)
HR ESI-MS
2
3a
28.46 (b) 29.31 (b) [M-Cl]+ or [MPPF + Au]+ m/z = 567.01 (100%)
32.94 (s) 33.00 (s) [DPPF − Ph]+ m/z = 477.17 (80%)
[DPPF + Au]+ m/z = 751.24 (100%)
[M − SR]+ m/z = 1057.05 (1%)
3b
3c
33.58 (s) 32.21 (s) [M − SR]+ m/z = 1024.05 (100%)
[M + Au]+ m/z = 1295.08 (10%)
32.94 (s) 32.52 (s) [DPPF + Au]+ m/z = 751.10 (10%)
[DPPF + 2Au]2+ m/z = 981.10 (35%)
3.2. Biological activity
[M − SR]+ m/z = 1311.10 (100%)
[M + Au]+ m/z = 1871.10 (5%)
The antimicrobial activities of the gold(I) trimetallic Fe–Au2 (3a–e)
and bimetallic (4a–e) compounds were evaluated against Gram-
negative (E. coli and P. aeruginosa), Gram-positive (S. aureus) bacteria
and yeast (S. cerevisiae) (Table 2). A number of the compounds analyzed
in this work exhibited minimum inhibitory concentration (MIC) values
in the 1 μg/mL–100 μg/mL range (Table 3). For selected compounds the
antimicrobial activities against MRSA strains US300 and US400 and two
S. cerevisiae yeast mutants lacking thioredoxin reductase TR1 or TR2
were also evaluated (Table 3 and data not shown). The antimicrobial ac-
tivity of AF was also studied against all the above mentioned microbes
(Table 3). MIC values for AF against Gram-negative (E. coli and
P. aeruginosa) and Gram-positive (S. aureus) are similar to those report-
ed before [17] (see Table 3). To the best of our knowledge the
3d
3e
4a
4b
4c
4d
4e
32.15 (s) 32.21 (s) [DPPF + Au]+ m/z = 751.10 (100%)
[M − SR]+ m/z = 1066.10 (57%)
27.67 (s) 27.17 (s) [DPPF + Au]+ m/z = 751.10 (58%)
[M − SR]+ m/z = 1099.10 (100%)
33.87 (s) 32.40 (s) [MPPF + Au]+ or [M − SR]+ m/z = 567.01 (75%)
[M + H]+ m/z = 677.04 (0.1%)
34.50 (b) 34.32 (b) [MPPF + Au]+ or [M- SR]+ m/z = 567.01 (75%)
[M + H]+ m/z = 643.05 (0.5%)
34.13 (b) 33.56 (b) [MPPF + Au]+ or [M − SR]+ m/z = 567.01 (95%)
[M + H] + m/z = 931.10 (2%)
33.17 (b) 32.36 (b) [MPPF + Au]+ or [M − SR]+ m/z = 567.01 (67%)
[M + H]+ m/z = 686.17 (50%)
28.32 (b) 29.27 (b) [MPPF + Au]+ or [M − SR]+ m/z = 567.01 (10%)
[M + H]+ m/z = 719.03 (2%)

86
Y. Hokai et al. / Journal of Inorganic Biochemistry 138 (2014) 81–88
Table 2
Selected bond lengths [Å] and angles [°] for complexes 3a and 3d.
3a
3b
Au(1)–P(1)
Au(1)–S(1)
Fe–C
S–C(41)
2.2546(14)
2.3000(16)
2.032(4)–2.063(5)
1.783(6)
Au(1)–P(1)
Au(1)–S(1)
Fe–C
S–C(1)
2.259(3)
2.307(3)
2.025(15)–2.050(15)
1.761(14)
P(1)–Au(1)–S(1)
C(14)–Fe(1)–C(14A)
C(41)–S(1)–Au(1)
171.33(5)
180.0(3)
100.93(19)
P(1)–Au(1)–S(1)
C(14)–Fe(1)–(C14A)
C(1)–S(1)–Au(1)
177.75(11)
179.999(2)
104.0(5)
antimicrobial activity of AF against MRSA strains US300 and US400 and
S. cerevisiae has not been reported before.
genes complement one another and that a double mutant in these
two genes would need to be tested to see any decreased sensitivity to
AF. Or it could be simply that the effects of AF work on yeast through
a different pathway unrelated to thioredoxin reductase. Additional test-
ing in the future will be required to characterize the mechanism of ac-
tivity against yeast.
Some of the new heterometallic compounds, specially the bimetallic
FeAu derivatives incorporating the MPPF phosphine (4b–e) and
trimetallic FeAu2 compounds with DPPF (4b and 4c) have shown
activity against the Gram-positive bacterium S. aureus with MIC in the
low micromolar range (2.4–6.6 μM or 2–6 μg/mL). None of the new
heterometallic complexes or AF [17] have a MIC below the 100 μg/mL
range for E. coli or P. aeruginosa. Remarkably, AF inhibits the growth of
S. aureus and the MRSA strains US300 and US400 in the nanomolar
range (150–300 nM or 0.1 to 0.20 μg/mL). The most active
heterometallic compound 4e inhibits the growth of S. aureus and the
MRSA strains US300 and US400 at 2.8 μM. In addition, compound 4d
containing 2-thiazoline and AF were equally effective against yeast
S. cerevisiae with MIC of 100 μg/mL.
Recent studies have reported that the mechanism of AF toxicity
against the eukaryotic microbial pathogens Entamoeba histolytica [14],
and Giardia lamblia [13] is linked to the inhibition of thioredoxin reduc-
tase enzyme activity. We sought to test if there were a role for AF toxic-
ity in yeast through a similar mechanism by testing mutants of
S. cerevisiae lacking thioredoxin reductase genes, TRR1 or TRR2. The ex-
pectation being that these mutants lacking the target of AF would be
less sensitive to the drug. However, upon testing AF we found no signif-
icant difference in activity between the wild type yeast and the
thioredoxin reductase mutants. It could be that the TRR1 and TRR2
The activity is not solely correlated to the gold content. The bimetal-
lic compounds have in general lower content of gold than the trimetallic
derivatives (range 21.2–30.6% 4a–e versus 23.5–35.8% 3a–e) but display
in general a better activity against the MRSA strains than three of the
trimetallic compounds (3a, 3d and 3e). Effects such as ligand dissocia-
tion or ligand scrambling and subsequent formation of different metab-
olites in vivo will play a more important role for the biological
activity [34]. The most active compounds are 3b, 3c and 4e (3.6 μM
(3b, 3c) and 2.8 μM (4e) against MRSA strains). It is well established
that MRSA strains carry the MecA gene, often in combination with
other resistance genes, which inactivates β-lactam antibiotics such as
methicillin. In the CA MRSA strains tested, it is unlikely that the presence
of the MecA gene would influence the activity of AF on these cells, so
that any differences in toxicity seen with AF, or AF-related compounds,
is likely to be linked to differences in the genetic background of the CA
MRSA strains and the routine laboratory S. aureus strains.
The MIC values obtained for the new heterometallic complexes on
Gram-positive S. aureus are similar, or below, those described for
some bimetallic aminothiol gold compounds containing phosphines
[16]. Monometallic aminothiolates containing the triethyl phosphine
ligand (like auranofin) and aminothiols had a MIC below 1 μg/mL
(like AF) on this Gram-positive bacterium [17]. It is clear that the subtle
interplay of the ligands coordinated to gold may be responsible for the
biological activity observed on this type of compounds [17,19]. In this
context, although the new heterometallic compounds described here
are very potent they have not been as efficient as AF against S. aureus
and the CA MRSA strains. We believe that the design of complexes
with ferrocenyl-based alkyl phosphines may render new heterometallic
complexes with improved antimicrobial and solubility properties.
An explanation for the behavior displayed by these new thiolate gold
complexes and AF could be as follows: if these compounds interfere
with respiration (a mitochondrial function), then the fungi should be
the most resistant, because electron transport is shielded inside internal
organelles, whereas in bacteria electron transport occurs at the plasma
membrane. Gram-negative bacteria have a protective outer membrane;
therefore the compounds may not have ready access to the electron-
transport chain. These compounds should be more active in general
against Gram-positive bacteria such as S. aureus which is what we
have observed here. The testing of these compounds against selected
mutant strains of Gram-negative bacteria that have a compromised
outer membrane function may help to elucidate the mechanism of
these gold-based compounds and the apparent reduced activity against
Gram-negative bacteria relative to Gram-positive bacteria.
Table 3
Minimum inhibitory concentration MIC (in μg/mL, +/−0.1) of compounds 3a–e, 4a–e
and auranofin against microbial organisms.a
Compound
Gram-negative
P. aeruginosa
N100
Gram-Positive
Yeast
US400
–
E. coli
S. aureus
US300
S. cerevisiae
3a
N100
N10
(N9)b
4
–
N100
3b
N100
N100
N100
N100
N100
N100
N100
N100
N100
N100
N100
N100
N100
N100
N100
N100
N100
N100
N100
N100
4
4
N100
N100
N100
N100
N100
N100
N100
100
(3.6)b
4
(3.6)b
6
(3.6)b
6
3c
(2.4)b
N10
(N8)b
N10
(N8)b
2
(3.6)b
–
(3.6)b
–
3d
3e
–
–
4a
4
4
(3)b
2
(6)b
4
(6)b
2
4b
(3.2)b
2
(6.4)b
6
(3.2)b
6
4c
(2.2)b
2
(6.6)b
4
(6.6)b
4
4d
(3)b
2
(6)b
2
(6)b
2
4e
N100
100
(2.8)b
0.1
(2.8)b
0.2
(2.8)b
0.2
Studies of the toxicity of selected compounds (AF and compound 4e)
against a non-tumorigenic human embryonic kidney cell line (HEK-
293) were performed to assess the toxicity on human “normal” or
healthy cells. The IC50 values obtained after 24 h of incubation
Auranofin
(0.15)b
(0.3)b
(0.3)b
a
Auranofin and the heterometallic compounds were dissolved in DMSO. When com-
pounds were not active in S. aureus below 10 μM they were not tested on US300 or US400.
b
MIC (micromolar units) in parentheses.
with these compounds (XTT assay) were 0.549
0.022 μM (AF) and

Y. Hokai et al. / Journal of Inorganic Biochemistry 138 (2014) 81–88
87
0.403 0.004 μM 4e. Auranofin was slightly less toxic than compound
4e to the immortalized healthy cell lines but it was 5 times or 2.5 times
less toxic to the human cell line than to S. aureus or the resistant strains
US300 and US400 respectively.
Cordone (Contel's lab at Brooklyn College) for their assistance with
some experiments.
Appendix A. Supplementary data
Crystallographic tables for compounds 3a and 3d, drawing for the
crystal structure of 3a, DFT calculations on molecules 4a and 4d, 1H,
4. Conclusion
31P{1H} and 13C{1H} NMR (CDCl3) spectra for all new compounds, 1
H
In conclusion, we have prepared a number of new heterometallic
gold–thiolate complexes highly active against Gram-positive bacteria
S. aureus and CA MRSA strains US300 and US400 (MIC in the order of
one digit micromolar). Remarkably, auranofin inhibited S. aureus,
US300 and US400 in the order of 150–300 nM or 0.1–0.2 μg/mL.
Therapeutic treatment of rheumatoid arthritis with AF results in sta-
ble serum concentrations of 0.5–0.7 μg/mL [36,37]. The fact that an oral,
FDA approved, drug is able to inhibit CA MRSA strains in nanomolar
concentrations that are less (3.5 to 5 times) than those known to be
well tolerated by patients suggests that treatment of MRSA with AF,
and related gold-based compounds, could represent an important and
novel option for patients.
and 31P{1H} NMR (DMSO-d6) spectra overtime for selected compounds
(4a, 3b, 3c, 4d and 4e), and MS spectra for all new compounds. http://
dx.doi.org/10.1016/j.jinorgbio.2014.05.008.
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Abbreviations
AF
auranofin
COSY
DPPF
DFT
correlation spectroscopy
1,1′-bis-diphenylphosphino-ferrocene
density functional theory
ESI-MS electrospray ionization-mass spectrometry
FDA
Food and Drug Administration or USFDA, an agency of the
United States Department of Health and Human Services
HEK-293 inmortalized human embryonic kidney cells
HMBC
HSQC
MIC
heteronuclear multiple-bond correlation spectroscopy
heteronuclear single-quantum correlation spectroscopy
minimum inhibitory concentration
MPPF
MRSA
diphenylphosphino-ferrocene
methicillin-resistant S. aureus
NOESY nuclear overhauser effect spectroscopy
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Acknowledgment
Research at Brooklyn College was supported by a grant from the Na-
tional Institute of General Medical Sciences (NIGMS), SC2GM082307
and a grant from the National Cancer Institute (NCI) 1SC1CA182844
(M.C.). We thank the New York Louis Stokes Alliance for Minority
Participation (Fellowship to Masters' student Y.H.) for funding. This
research was supported, in part, by a grant of computer time from the
City University of New York High Performance Computing Center
under NSF Grants CNS-0855217, CNS-0958379 and ACI-1126113. We
thank Prof. Joe W. Ramos (University of Hawaii Cancer Center) for the
evaluation of the toxicity of auranofin and compound 4e on HEK-293
cells. We also thank research assistants Benelita T. Elie and Pierpaolo

88
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