2
C. Kim et al. / Bioorganic & Medicinal Chemistry Letters xxx (2018) xxx–xxx
128 mg/mL for NRS100 and NRS70 and 256 mg/mL for ATCC29213
and NRS123. The results are given in Table 1.
The potentiation of AMP and OXA by farnesol was different
from what Kuroda et al. described.3 They reported dramatic
improvement in susceptibilities to AMP and OXA by farnesol.
MIC values of AMP and OXA were reported decreased by 24- and
>64-fold for NRS100 and 256- and 96-fold for NRS70, respectively.
In our hands, the improvement in susceptibility by farnesol was
modest, with MICs for AMP and OXA attenuated by 4- and >32-fold
for NRS100, and a mere 4- and 2.7-fold for NRS70, respectively.
Farnesol reduced MICs of AMP and OXA as much as 4- and 12-fold
for NRS123 as well. We attributed the difference between our
determinations and those of Kuroda et al. by our observation of
slow growth of all strains, especially NRS70, on CAMHB-T agar
plates containing 1000 mg/mL of farnesol. The reported large effect
of farnesol on susceptibility to b-lactams might be the result of not
taking into account the slow growth of bacteria in the presence of
farnesol. That is to say that the MIC values determined after the 24-
hour incubation in the presence of farnesol were similar to those
reported by Kuroda and colleagues (Figure 1). However, the MIC
values were significantly larger against the MRSA strains when
read after a 48-hour incubation period (Figure 1). Chemical stabil-
ity of compounds 2 and 6 was checked by LC-MS.14 A total of 88
( 1)% of compound 2 remains after 48-hour incubation in the
growth medium. However, compound 6 was not detected beyond
24-hour of incubation. Compound 6 is a prodrug of compound 2.
Compound 2 produced larger improvement in susceptibility of
strain NRS123 to both b-lactams—12-fold for AMP and 32-fold
for OXA—compared to the case of farnesol. As for compound 6, it
exhibited the largest effect with OXA against strains NRS100
(>128-fold) and NRS70 (32-fold); compared to that of farnesol
(>32-fold for NRS100 and 2.7-fold for NRS70). When concentration
of farnesol was reduced further (256 and 512 mg/mL) and then
strains NRS100 and NRS70 were challenged by AMP and OXA, we
saw no effect on bacterial growth. The MIC values for the b-lactams
under these conditions were the same as those in the absence of
farnesol. Compounds 2 and 6 exhibited similar effects on b-lactam
susceptibility as the parental farnesol, except they were larger. The
MIC drop or potentiation by farnesol and the synthetic derivatives
on AMP and OXA against the strains NRS100 and NRS70 was due to
attenuation of the rate of the growth.
Scheme 1. (A) Lipid II, anchored to the membrane by the undecaprenyl moiety, is
the precursor of peptidoglycan. (B) Chemical structures of farnesyl derivatives used
in this study. Overall synthetic yields are in parentheses. (C, D) Syntheses of farnesyl
amine (2) and thiophosphoryl derivative.
strains ATCC29213 and NRS123, whereas compound 6 exhibited
MIC of 512 mg/mL to all strains. The MIC values of farnesol com-
pounds could not be determined without Tween-80 since they
are immiscible with the media. Tween-80 at 1% of concentration
did not affect the MICs of b-lactam antibiotics.
In order to investigate potentiation of b-lactam antibiotics—we
used ampicillin (AMP) and oxacillin (OXA)—by farnesol and its
derivative, the MIC determination was performed using the Etest
(bioMereux, USA) on CAMHB-T agar plates containing farnesol
derivatives at half-MIC concentrations: farnesol, 1000 mg/mL for
all strains; compound 6, 256 mg/mL for all strains; compound 2,
The potential for synergism between OXA and farnesol (or its
derivatives) was checked in the checkerboard assay with measure-
ments of the fractional inhibitory concentration index (FICI).15–17
We could not observe any synergy between the b-lactams and far-
nesol or its derivatives with 0.53 ꢀ FICI ꢀ 1.13 (Fig. 2). Farnesol
may best be characterized as a potentiator of b-lactam antibiotics,
rather than a synergistic effector.
Table 1
Effect of farnesol and its derivatives on b-lactam susceptibility.a,b,c
MIC (µg/mL) of ampicillin
MIC (µg/mL) of oxacillin
S. aureus
SCCmec
2d
6
None
Farnesol
2d
6
strain
type
None
Farnesol
ATCC29213
NRS100 (COL)
0.5
16
0.5 (1)
4 (4)e
0.2 (2.5)
8 (2)
0.5 (1)
3 (5.3)e
0.4
>256
0.5 (0.8)
8 (>32)e
0.5 (0.8)
>256 (–)
0.4 (1)
2 (>128)e
I
NRS70 (N315)
II
16
4 (4)e
3 (4)e
16 (1)
6 (2.7)e
32
12 (2.7)e
32 (1)
1 (32)e
NRS123 (MW2)
IV
12
1 (12)e
8 (1.5)
24
2 (12)e
0.75 (32)e
16 (1.5)
aMIC was determined by Etest on CAMHB-T agar without/with half-MIC of farnesol or its derivatives: 1000 mg/mL of farnesol; 256 mg/mL of 2 and 6.
bFold ratios of increased susceptibility are in parentheses.
cEntry with fold increases of 4 or higher are in bold.
dCompound 2 was used at 128 mg/mL for NRS70 (N315) and NRS100 (COL).
eThe bacterial growth got slower at the half-MICs of farnesol and its derivatives; highlighted. The MIC determination was performed in triplicate; each experiment exhibited
similar results.
Kim, Choon
