M. Michaut, et al.
Bioorganic&MedicinalChemistryLetters30(2020)127098
Staphylococcus species and several Enterococcus strains than ampicillin.
Surprisingly, however, the use of a trimethylphosphine ligand (com-
pound 17) did not further increase antibacterial activity against Gram-
positive bacteria but resulted in moderate activity against E. coli. This
finding is consistent with recently reported data for antibacterial gold(I)
derivatives bearing trimethylphosphine as a ligand.24,25 Conjugates 18,
21 and 22 had similar antibacterial profiles, demonstrating that the
nature of the linker attaching the phosphine-Au(I)-triazole moiety to
the penam scaffold does not greatly influence biological activity. Fi-
nally, purely organic penam derivatives 14 and 25 were less active than
ampicillin, highlighting the negligible contribution of the triazole
moiety and the major importance of the gold(I) center for antibacterial
activity (Table 1). Chryso-antibiotic 18 thus appeared to the most
sphine gold(I) derivatives, such as auranofin, have already found uses
in human medicine.25,26 The MICs of compound 18 were low for wild-
type susceptible strains of S. aureus, S. epidermis and E. faecium. More
importantly, they were not affected by the most prevalent mechanisms
of resistance to β-lactams (e.g., methicillin and oxacillin) in Staphy-
loccus sp. or known mechanisms of resistance to glycopeptides (e.g.,
vancomycin) in Enterococcus sp. These data suggest that cross-resistance
between chryso-lactams and drugs classically used against Gram-posi-
tive bacteria should not emerge easily. The activity of compound 18
was similar to that of ampicillin with reference strains of E. faecalis.
The toxicity of gold(I) is a critical issue for the further development
of molecules of therapeutic potential, such as chryso-antibiotics. The
cytotoxicity of compound 18 was assessed on healthy human hepato-
cytes and compared with that of ampicillin 13, used as a control.
Scheme 1. Synthesis of phosphine gold(I) azides 5, 6, 7 and 8, and synthesis of
ampicillin-Au(I) conjugates 17 to 22. i. Tl(acac), toluene, 20 °C. ii. TMSN3,
toluene/MeOH, 20 °C. iii. C6F5OH, EDCI.HCl, CH2Cl2, 20 °C. iv. pNO2PhOCOCl,
NEt3, THF 0 °C to 20 °C. v. Propargyl chloroformate, 10 or 12, NEt3, THF/H2O,
0 °C to 20 °C. vi. 5, 6, 7 or 8, THF, 20 °C.
Compound 18 exhibit an EC50 of 16.39
0.67 µM. In detail com-
pound 18 did not affect cell viability at concentrations of up to 10 μM
(7.9 µg.mL−1), but was toxic at higher concentrations, with only 40%
viable cells at 22.5 μM (17.7 µg.mL−1
) and 10% at 25 μM
(19.7 µg.mL−1). By contrast, ampicillin remained non-cytotoxic over
the entire range of concentrations tested (see ESI). These data clearly
characterize an interesting therapeutic window for compound 18 even
the strong antibacterial activity of this molecule is counterbalanced by
a peripheral toxicity to host cells.
The current antibiotic arsenal mostly consists of purely organic
compounds, with carbon, hydrogen, sulfur, oxygen and nitrogen atoms
involved in the inhibitory processes. However, the emergence and rise
of increasingly resistant bacterial strains, has led to the use of last-resort
antibiotics known to have significant adverse effects.27,28
Compounds containing other elements from the periodic table have
been developed against cancer, but other applications of these com-
pounds in the treatment of bacterial infections have been limited by
peripheral toxicity issues, particularly for molecules administered in-
travenously. Organometallic compounds should be more and more
competitive in terms of the benefit vs risk balance for the patient.
Chryso-lactams proved to have a specific activity on Gram-positive
microorganisms and the low permeability of Gram-negative bacteria
outer envelope seems to impair the antibacterial efficiency of many
gold(I) compounds.19 Therefore, the vectorization of organometallic
drugs using nutrient uptake systems, should increase their therapeutic
potential (antibacterial activity vs cytotoxicity).29,30
Scheme 2. Synthesis of ampicillin analog 25. i. C6F5OH, EDCI.HCl, CH2Cl2,
20 °C. ii. NEt3, THF/H2O, 20 °C.
Most of the pathogenic bacterial species used in the evaluation assays
were Gram-positive (Staphylococcus aureus, Staphylococcus epidermidis,
Enterococcus faecalis and Enterococcus faecium). Minimal inhibitory
concentrations (MIC) were also determined for Escherichia coli, a Gram-
negative bacterium (Table 1). However, this bacterium turned out to be
tested. The resistance of Gram-negative bacteria to gold(I) derivatives
has been reported to be due mostly to the low permeability of the
bacterial outer membrane. Indeed, the presence of a membrane-per-
meabilizing agent greatly increases the antibacterial activity of gold(I)
drugs against Gram-negative pathogens.19 By contrast, compounds 17
to 22 proved to be more effective than their non-metallic parental
compounds against several pathogenic Gram-positive strains. The size
of the phosphine ligand of the gold(I) derivative of ampicillin appeared
to be crucial for biological activity. Indeed, compound 20, with its
triphenylphosphine ligand, generally had lower activity than ampi-
cillin. A decrease in ligand size (dimethylphenylphosphine) increased
antibiotic activity, but compound 19 had a level of activity similar to
that of ampicillin for only a subset of bacterial strains. The optimal
ligand was triethylphosphine, as compound 18 was more active against
This article describes the synthesis of the first conjugates of β-lactam
antibiotics with phosphine-gold(I) complexes. In these conjugates,
ampicillin was connected to the metal complex through a 1,2,3-triazole
linker. Triethylphosphine derivative 18 proved to be an effective mo-
lecule against Gram-positive bacteria, with an activity 120 times higher
than ampicillin with certain strains of S. aureus, S. epidermidis and E.
faecium. This compound escapes the most common mechanisms of re-
sistance to β-lactams and glycopeptides. With, for certain bacterial
strains, a selectivity index ca 130, compound 18 will serve as lead
molecule in the development of the next generation of chryso-lactams.
However, the mode of action of compound 18 and its ability to select
resistant isolates should now be investigated. This information will be
2
Contreras, Jean-Marie
