Discovery of novel bis-oxazolidinone compounds as potential potent and selective antitubercular agents

Article abstract of DOI:10.1016/j.bmcl.2014.02.025

A variety of new mono-oxazolidinone molecules by modifying the C-ring of Linezolid, a marketed antibiotic for MRSA, were synthesized and tested for their in vitro antibacterial activities against several Staphylococcus aureus, Mycobacterium smegmatis and two Gram-negative bacteria strains (Escherichia coli and Pseudomonas aeruginosa). Among them, compounds 4-7 displayed moderate antimicrobial activities. After development of a second oxazolidinone ring in the western part of the mono-oxazolidinone compounds 4-7 by a ring closure reaction with N,N′-carbonyldiimidazole (CDI), we found thus obtained bis-oxazolidinone compounds 22-25 possess excellently inhibitory activities against H₃₇Rv but poor or no effects on other test bacteria. Among them, bis-oxazolidinone compound 22 and 24 are the most potent two compounds with a same MIC value of 0.125 μg/mL against H₃₇Rv virulent strain. Compound 22 also exhibited extremely low cytotoxicity on monkey kidney Vero cells with a selective index (IC₅₀/MIC) over 40,000, which suggested bis-oxazolidinone compound 22 is a promising lead compound for subsequent investigation in search of new antitubercular agents.

Full text of DOI:10.1016/j.bmcl.2014.02.025

Bioorganic & Medicinal Chemistry Letters 24 (2014) 1496–1501
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of novel bis-oxazolidinone compounds as potential potent
and selective antitubercular agents
Wei Ang a,b, , Weiwei Ye a, , Zitai Sang a, , Yuanyuan Liu a, Tao Yang a, Yong Deng b, Youfu Luo a,
,
⇑
Yuquan Wei a
a State Key Laboratory of Biotherapy, West China Hospital, West China Medical School, Sichuan University, Chengdu, Sichuan 610041, PR China
b Key Laboratory of Drug Targeting and Drug Delivery System of the Education Ministry, Department of Medicinal Chemistry, West China School of Pharmacy, Sichuan University,
Chengdu, Sichuan 610041, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A variety of new mono-oxazolidinone molecules by modifying the C-ring of Linezolid, a marketed anti-
biotic for MRSA, were synthesized and tested for their in vitro antibacterial activities against several
Staphylococcus aureus, Mycobacterium smegmatis and two Gram-negative bacteria strains (Escherichia coli
and Pseudomonas aeruginosa). Among them, compounds 4–7 displayed moderate antimicrobial activities.
After development of a second oxazolidinone ring in the western part of the mono-oxazolidinone com-
pounds 4–7 by a ring closure reaction with N,N0-carbonyldiimidazole (CDI), we found thus obtained
bis-oxazolidinone compounds 22–25 possess excellently inhibitory activities against H37Rv but poor or
no effects on other test bacteria. Among them, bis-oxazolidinone compound 22 and 24 are the most
Received 27 December 2013
Revised 29 January 2014
Accepted 3 February 2014
Available online 15 February 2014
Keywords:
Tuberculosis
Bis-oxazolidinone
Antitubercular
Selectivity
potent two compounds with a same MIC value of 0.125
lg/mL against H37Rv virulent strain. Compound
22 also exhibited extremely low cytotoxicity on monkey kidney Vero cells with a selective index (IC50
/
MIC) over 40,000, which suggested bis-oxazolidinone compound 22 is a promising lead compound for
subsequent investigation in search of new antitubercular agents.
Ó 2014 Elsevier Ltd. All rights reserved.
Vero cells
Tubcerculosis, or TB, is one of the most widespread infectious
diseases. The World Health Organization (WHO) has declared that
one-third of the world’s population is currently infected with TB,
while nearly 13.7 million active cases worldwide.1 The WHO also
estimates that 8 million people get TB every year and 3 million
people will die yearly if control is not further strengthened.2 The
emergence of multi-drug resistant TB strains (MDR-TB) and exten-
sively drug resistant TB strains (XDR-TB) is a great challenge for the
treatment of tuberculosis. Furthermore, the combined epidemics of
HIV and tuberculosis have aggravated this situation.
Most of first-line TB drugs were discovered in 1950s and 1960s.
For nearly half a century, TB treatment lacks new drug until Bedaq-
uiline was launched in the end of 2012, which was approved to
treat multi-drug-resistant tuberculosis.3 The successful discovery
of Bedaquiline is an exciting milestone in TB therapy with a unique
mechanism of action targeting adenosine triphosphate (ATP) syn-
thase, which Mycobacterium tuberculosis requires to generate its
energy.4 Nevertheless, case report suggested that its adverse
effects such as nausea, joint, pain and headache lead to a risk in
clinical use. The drug also has a black-box warning by FDA for
arrhythmias, as it may induce long QT syndrome by blocking the
hERG channel.5,6 Consequently it is still desirable to develop new
antitubercular drugs to address the unmet demand of anti-tuber-
culosis medical goals such as fast and long-acting, high potency
against MDR-TB and latent infections, minimal drug–drug interac-
tions and low toxicity.7
The development of new members of established antibiotics,
which already possess desirable pharmacological properties, is
one approach to add new drugs to the existing anti-TB armamen-
tarium rapidly. Linezolid (Fig. 1) was approved for the treatment
of serious infections caused by Gram-positive bacteria such as
Streptococci, vancomycin-resistant enterococci and methicillin-
resistant Staphylococcus aureus. And it was once failed in clinical
trials for TB as a result of its peripheral and optic nerve toxicity.8
However, recently research demonstrated that Linezolid is
effective with patients who suffering from treatment-refractory
extensive drug-resistant (XDR) pulmonary tuberculosis when the
Abbreviations: INH, Isoniazid; MIC, minimum inhibitory concentration; MRSA,
methicillin-resistant Staphylococcus aureus; MSSA, methicillin-sensitive Staphylo-
coccus aureus.
⇑
Corresponding author. Tel./fax: +86 28 855503817.
E-mail address: luo_youfu@scu.edu.cn (Y. Luo).
Wei Ang, Weiwei Ye and Zitai Sang contributed equally to this work.
http://dx.doi.org/10.1016/j.bmcl.2014.02.025
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

W. Ang et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1496–1501
1497
Staphylococcus aureus, Mycobacterium smegmatis and two Gram-
negative bacteria strains (Escherichia coli and Pseudomonas aerugin-
osa). Among them, compounds 4–7 displayed moderate antimicro-
bial activities. In order for the enhancement of their antibacterial
potencies, we took advantage of the bis-pharmacophore strategy,
which has been widely applied in drug design and many inspiring
progresses have been achieved with this method. Thus a second
pharmacophore oxazolidinone ring was developed in western part
of each of the oxazolidinone compounds 4–7 by a ring-closure
reaction with N,N0-carbonyldiimidazole (CDI) to obtain bis-oxazo-
lidinone compounds 22–25. Biological assays showed that thus
obtained four bis-oxazolidinone compounds possess good antitu-
bercular effects. Such good results prompt us to make subsequent
structural modifications aiming at establishment of a preliminary
body of structure–activity relationships (SARs) and elucidation of
the importance of oxazolidinone ring number within a single mol-
ecule. The in vitro antitubercular activities of the final compounds
were assessed on virulent Mycobacterium tuberculosis H37Rv strain
and the cytotoxicities were assayed towards monkey kidney Vero
cells.
To efficiently achieve the mono-oxazolidinone compounds
4–21, a synthetic route outlined in Scheme 1 was adopted. Firstly,
the starting material (S)-N-((3-(3-fluoro-4-hydroxyphenyl)-2-oxo-
oxazolidin-5-yl)methyl)acetamide 1, conveniently prepared in our
lab according to a reported procedure,14 reacted with (S)-epichlo-
rohydrin or (R)-epichlorohydrin in the presence of potassium
carbonate15 at 50 °C to get the common intermediate 2 (S configu-
ration in western part) or 3 (R-counterpart of compound 2). Sec-
ondly, the terminal epoxide in compound 2 or 3 was opened
Figure 1. Chemical structures of Linezolid, PNU-100480 and AZD5847.
adverse events can be monitored carefully.9,10 Besides, great efforts
have made to modify the C-ring of Linezolid in order to find some
promising candidates with potent antimicrobial activities, which
successfully led PNU-100480 (Fig. 1) to phase II clinical trials.11–
13 Combinational modification strategy of C-ring and C-5 side chain
of Linezolid brought another phase II clinical candidate AZD5847
(Fig. 1).
Encouraged by the mentioned clinic success and research
advancement, we undertook novel structural modification of the
western part (C-ring) of Linezolid as a continuing effort of our
group to find novel antibacterial agents. At beginning, a variety
of mono-oxazolidione Linezolid analogues 4–21 (Table 1) with 3-
phenyloxazolidin-2-one scaffold remained, were synthesized and
tested for their in vitro antibacterial activities against several
Table 1
Structures of the new target compounds 4–30 and their calculated LogP (CLogP), LogP and aqueous solubility values
Compd
R
S/R
CLogP
LogP
Solubility (lmol/L)
4
5
6
7
8
9
p-Methoxyphenyl
p-Methoxyphenyl
Phenyl
Phenyl
p-Cyanophenyl
p-Cyanophenyl
3,5-Di(trifluoro-methyl) phenyl
3,5-Di(trifluoro-methyl) phenyl
2-Pyridyl
2-Pyridyl
3-Pyridyl
3-Pyridyl
4-Pyridyl
4-Pyridyl
Cyclopropyl
Cyclopropyl
3-Methylbutyl
3-Methylbutyl
Phenyl
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
—
—
—
—
—
1.56
1.56
1.64
1.64
1.07
1.07
3.40
3.40
0.14
0.14
0.14
0.14
0.14
0.14
0.89
0.89
2.29
2.29
1.70
1.70
2.14
2.14
0.59
0.78
1.28
2.69
3.13
0.95
1.15
1.42
0.59
2.12
1.29
3.10
3.01
1.37
1.60
0.02
-0.18
-0.04
0.02
0.02
-0.10
1.17
1.19
1.78
1.86
1.91
1.73
0.01
0.59
0.43
NTa
56
1.3
124
296
47
69
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
NTa
22
22,207
35,080
3042
10,138
17,724
24,631
96,015
2202
385
7276
14
164
132
211
Phenyl
p-Methoxyphenyl
p-Methoxyphenyl
—
—
—
2720
81
333
—
—
NTa
3.19
441
a
Not tested.

1498
W. Ang et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1496–1501
Scheme 1. General synthetic route of compounds 4–21. Reagents and conditions: (a) (S)-epichlorohydrin or (R)-epichlorohydrin, potassium carbonate, acetonitrile, 50 °C,
20 h; (b) R-NH2, calcium trifluoromethanesulfonate, acetonitrile, 45 °C, 16 h.
with appropriate amine in warmed acetonitrile solution of calcium
trifluoromethanesulfonate by a well-known internal SN2 reaction
mechanism. After completion of the reaction evidenced by thin
layer chromatography, the resultant was concentrated under re-
duced pressure and thus obtained residue was purified by flash
column chromatography to furnish the target mono-oxazolidinone
compounds. Notably the possible regional isomers in this step
were not found in the crude products, which is logically due to
the steric hindrance of the phenyloxymethyl group in the common
intermediate 2 or 3.
Bis-oxazolidinone compound 26 was made by reaction of com-
pound 2 with ethyl carbamate in the presence of triethylamine
(Scheme 3).17
As illustrated in Scheme 4, compound 27, with a second oxazolid-
inone ring at different site from bis-oxazolidinones 22–26, was pre-
pared in a ring-opening reaction of terminal epoxide ring of the
common intermediate 2, similarly to the method for mono-oxazo-
lidinone compounds. Compound 28, with three oxazolidinone rings,
was made from compound 27 by a ring-closure reaction with CDI.
The mono-oxazolidinone compound 30 with relatively simple
structure was also prepared in a similar way to investigate the
structure–activity relationship (Scheme 5).
The bis-oxazolidinone compounds 22–25 was readily synthe-
sized through a ring-closure reaction between the hit compounds
4–7 and N,N0-carbonyldiimidazole in dichloromethane as de-
scribed in the Scheme 2.16
Before biological activities assessment, all the compounds were
fully characterized by NMR and mass spectrometry, whose purities
Scheme 2. General synthetic route of compounds 22–25. Reagents and conditions: (a) N,N0-carbonyldiimidazole, triethylamine, dichloromethane, room temperature, 20 h.
Scheme 3. Synthetic route of compound 26. Reagents and conditions: (a) ethyl carbamate, triethylamine, 130 °C, 15 h.
Scheme 4. Synthetic route of compound 28. Reagents and conditions: (a) (S)-N-((3-(4-amino-3-fluorophenyl)-2-oxooxazolidin-5-yl)methyl) acetamide, calcium
trifluoromethanesulfonate, acetonitrile, 80 °C, 16 h; (b) N,N0-carbonyldiimidazole, triethylamine, dichloromethane, room temperature, 20 h.

W. Ang et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1496–1501
1499
exhibited more potent inhibitory activities to Staphylococcus aureus
with MIC values range from 4 to 32 g/mL (compounds 4–10). The
l
above results of antibacterial activities indicated that the aromatic-
ity of substituents in western part and the moderately soluble
physicochemical properties are both important factors to keep
antibacterial activities. Meanwhile, it is no difficulty to find that
compounds with electron-rich substituents (compounds 4–5)
possessed better activities than those electron-deficient ones
(compounds 8–11). It is also noteworthy to point out that the abso-
lute configuration of the chiral center in the western part of the
molecule is somewhat of importance. In most cases, compounds
with S configuration at the secondary alcohol group are more po-
tent than their R-counterparts. A second oxazolidinone ring was
further incorporated into the western part of the promising
compounds 4–7, respectively to furnish the corresponding bis-oxa-
zolidinone compounds 22–25, which did not shown any improve-
ment on activities towards the test bacteria strains (not including
M. tuberculosis strains) listed in Table1.
The experimental LogP data listed in Table 1 showed that com-
pounds with LogP less than 0.59 did not inhibit any tested strains.
Compounds with LogP range from 0.59 to 3.1 exhibited moderate
antimicrobial activities except for those lack phenyl substituents in
the western part (compounds 12, 13, 20, 21). Consequently it can
be deduced that lipophility and phenyl substitution in the western
part of the test compounds may both play roles for their antibacte-
rial effects. Compounds 11 (LogP: 1.37) and 12 (LogP: 1.60) with
pyridyl substituents with ideal aqueous solubility did not dis-
played any antibacterial effects because of its lack of phenyl sub-
stituents in their western parts.
Scheme 5. Synthetic route of compound 30. Reagents and conditions: (a) N,N0-
carbonyldiimidazole, triethylamine, dichloromethane, room temperature, 20 h.
were confirmed to be over 99% by HPLC area normalization meth-
od (see Supplementary material). We also determined their aque-
ous solubilities and experimental LogP values by HPLC method
(see Supplementary material) and the data are listed in Table 1.
The antimicrobial activities were determined with a serial dilution
method with Linezolid and Isoniazid as positive control (see Sup-
plementary material) and the results are listed in Table 2. Herein
MIC is defined as the minimum concentration of a test compound
that inhibits bacterial growth by 99%.
As shown in Table 2, most of the novel mono-oxazolidinone
compounds merely displayed low to moderate antimicrobial activ-
ities against the tested strains. The compounds did not show any
notable activities at our highest tested concentration (32 lg/mL)
when the western substituents are aliphatic groups (compounds
18–21) or pyridyl rings (compounds 12–17) which tends to be
more soluble in aqueous medium as proved by the aqueous solu-
bility data in Table 1. However, compounds bearing phenyl group
Some of above compounds with moderate antibacterial activi-
ties were further evaluated for their antitubercular activity against
M. tuberculosis H37Rv (Table 3) and drug-resistant M. tuberculosis
strains (Table 4) by a microdilution method in 96-wells plates
Table 2
Antibacterial activity data of target compounds 4–25
Compd
MICa
(
lg/mL)
S.ab
S.ac
S.ad
S.ae
S.af
S.ag
M.sh
E.ci
P.aj
Table 3
4
5
6
7
8
9
8
16
8
8
16
8
4
8
4
4
4
8
8
32
>32
>32
>32
32
>32
>32
>32
>32
>32
>32
8
4
8
4
8
8
8
16
>32
>32
>32
>32
32
>32
>32
>32
>32
>32
>32
16
4
16
16
8
8
16
16
32
>32
>32
>32
>32
32
>32
>32
>32
>32
>32
>32
16
8
16
4
8
8
64
64
>64
>64
16
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
—
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
—
Antitubercular activities of compounds 4, 6, 22–28 and 30 against M. tuberculosis
H37Rv
Compd
MICa
(lg/mL)
Compd
MICa
(lg/mL)
8
8
16
16
16
>32
>32
>32
>32
32
>32
>32
>32
>32
>32
>32
16
8
16
16
16
>32
>32
>32
>32
32
>32
>32
>32
>32
>32
>32
16
8
4
6
22
23
24
Linezolid
8
8
25
26
27
28
30
INH
0.25
8
32
16
32
16
16
>32
>32
>32
>32
32
>32
>32
>32
>32
>32
>32
16
8
32
16
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Linezolid
INH
0.125
0.25
0.125
0.25
>64
>64
>64
>64
>64
>64
>64
32
>64
>64
>64
>64
32
0.0625
a
MIC is definded as minimal drug concentration of the well in which bacterial
growth is similar to that with only 10 CFU of bacteria inoculated. Identical values
were obtained for each compound in three replicates by visual investigation as
stated in the experimental section.
Table 4
Antitubercular activities of compounds 22 and 24 against drug-resistant TB
4
8
4
1
16
16
8
2
Compd
MICa
(lg/mL)
16
8
2
16
8
2
8
4
2
—
16
4
2
>64
>64
—
Strain
1b
Strain
2c
Strain
3d
Strain
4e
Strain
5f
Strain
6g
Strain
7h
—
—
—
—
—
4
22
24
INH
8
4
2
4
4
4
4
2
0.25
4
4
2
1
4
4
4
0.25
2
2
4
0.25
8
8
8
0.25
a
MIC is defined as the minimum concentration of a compound that inhibits
growth by 99%. Identical values were obtained for each compound in three repli-
cates by visual investigation as stated in the experimental section.
8
Linezolid 0.25
NTi
b
MSSA ATCC25923.
MSSA ATCC29213.
MSSA ATCC6538.
MSSA CMCC26003.
MRSA ATCC33591.
MRSA ATCC43300.
Mycobacterium smegmatis.
Escherichia coli ATCC25922.
c
a
MIC is defined as minimal drug concentration of the well in which bacterial
d
growth is similar to that with only 10 CFU of bacteria inoculated. Identical values
were obtained for each compound in three replicates by visual investigation as
stated in the experimental section.
Clinical isolated Isoniazid-resistant M. tuberculosis H37Rv.
Clinical isolated MDR-TB.
e
f
g
b
h
c,d
i
e–h
Clinical isolated XDR-TB.
Not tested.
j
i
Pseudomonas aeruginosa ATCC27853.

1500
W. Ang et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1496–1501
Table 5
according to the literatures with appropriate revision (see Supple-
mentary material).18,19 Obviously, compound 22 and 24 are shown
to be highly potent against M. tuberculosis H37Rv.
Antimycobacterial activities (MIC), cytotoxicities (IC50) on vero cells andselectivity
index (SI) values for the bis-oxazolidinone compounds 22–25
MICa
(
lg/mL)
IC50
(lg/mL)
SIc
b
Compd
In some literatures, two oxazolidinone rings in a single mole-
cule are reported to exhibit non-selective antibacterial activity to
a panel of Gram-positive bacteria (not including M. tuberculosis
strains).20 In their work, the second oxazolidinone was tethered
to the aromatic C-ring part of Linezolid scaffold, which resulted
in rigid structure moieties. To the contrary, in our work a flexible
linker was involved in the connection of the second oxazolidinone
ring to the B-ring of Linezolid, expecting to possess better solubil-
ities for thus obtained bis-oxazolidinones. Thus obtained bis-oxa-
zolidinone compounds 22–25 exhibited excellent activities
against M. tuberculosis H37Rv with MIC values ranging from 0.125
22
23
24
25
0.125
0.25
0.125
0.25
>5000
63.62
19.08
>40,000
254
152
107.10
428
a
MIC is defined as minimal drug concentration of the well in which bacterial
growth is similar to that with only 10 CFU of bacteria inoculated. Identical values
were obtained for each compound in three replicates by visual investigation as
stated in the experimental section.
b
IC50 denotes half maximal inhibitory concentration.
SI is defined as the ratio of IC50 in mammalian cells/MIC against Mtb.
c
to 0.25 lg/mL. The encouraging results from the antitubercular
studies indicated that introduction of a second oxazolidinone ring
can significantly improve their selectivity and potency against M.
tuberculosis. Introduction of a third oxazolidinone ring does not
seem to take effect on enhancement of antitubercular activities,
which was illustrated by compound 28 with three oxazolidinone
in vitro cytotoxicity assays using monkey kidney Vero cells
(Table 5). All the tested drugs showed good safety profile, particu-
larly compound 22 with an IC50 value over 5000 lg/mL and selec-
tivity index over 40,000 (IC50 on Vero cells/MIC against H37Rv). The
relationship of cytotoxicity data and the compounds’ structure
suggest that even a minor structrual alteration, herein with or
without a methoxy group in the phenyl substituent at N-position
of the second oxazolidinone ring in the western part of the mole-
cules, or absolute configuration conversion, may severely affect
their biological effects.
In conclusion, twenty-six novel oxazolidinone or bis-oxazolidi-
none compounds have been synthesized, fully characterized and
evaluated for their antitubercular effects against the virulent stan-
dard H37Rv strain as well as the drug-resistant clinic isolates. Four
novel bis-oxazolidinone compounds were identified as potent
rings (MIC: 16
the N-position of the second oxazolidinone ring, also showed a
apparent loss of antitubercular activity (MIC: 8 g/mL) compared
lg/mL). Compound 26, lacked a phenyl group on
l
to compounds 22 and 24, which suggested that phenyl or substi-
tuted phenyl groups were essential to remain antitubercular activ-
ities. The removal of the first oxazolidinone ring in the eastern part
of compound 22 led to severe loss of antitubercular activity, where
compound 30 merely possessed an MIC value of 32 lg/mL. As a re-
sult, a preliminary body of structure–activity relationships (SARs)
was established (Fig. 2). Besides, it is also easy to find that more
lipophilic compounds with higher LogP values showed better anti-
tubercular activities except for the simplest mono-oxazolidione
compound 30.
agents with MIC values of 0.125–0.25 lg/mL to H37Rv. A prelimin-
ary body of structure–activity relationships (SARs) and property–
structure relationship was established. To date, researchers have
put great efforts on the expansion of antibacterial spectrum to
gram-negative bacteria of oxazolidinone compounds and obtained
some positive achievement. The agents with broad antimicrobial
agents surely can attenuate the symptoms of the patients who
have been infected with unknown microorganism, but these
agents may also bring the potential risk of aggravating antibiotic-
resistant bacteria epidemics. Here we found several bis-oxazolidi-
nones displayed excellent and highly selective activities to virulent
M. tuberculosis H37Rv with excellent safety profile on monkey kid-
ney Vero cells, especially for compound 22 with a selectivity index
over 40,000. Our results also envisioned an alternative consider-
ation to add new drug candidates to the well-established antibiotic
structural type with good species selection.
Listed in Table 4 are the MIC values of bis-oxazolidinone
compounds 22–25 against several clinical drug-resistant TB iso-
lates, including Isoniazid-resistant M. Tuberculosis, multi-drug
resistant M. Tuberculosis (MDR-TB) and extensive-drug resistant
M. Tuberculosis (XDR-TB). The major disappointment is that the
activity decreases 20 fold or more against MDR and XDR TB. These
relatively frustrating results against MDR and XDR TB of com-
pounds 22 and 24 compared to Linezolid drove us to go deeply into
the mechanism of resistance of the strains. As shown in Table 4,
Linezolid, an inhibitor of ribosomal protein synthesis, remained
its activities against the tested drug-resistant TB strains, which
suggest that the natural resistance of the MDR and XDR TB strains
to our bis-oxazolidinone compounds may be correlated to a mech-
anism of efflux pumps or other non-ribosomal alterations. This
non-ribosomal mechanism has been shown for Linezolid in M.
smegmatis mutants.21
Acknowledgments
To obtain insight into potential toxicities of the final bis-oxazo-
lidinone compounds, bis-oxazolidinone 22–25 were evaluated in
This work was supported by the National Major Program of Chi-
na during the 12th Five-Year Plan Period (2012ZX09103-101-036)
and New-Century Excellent Talent Fund by Ministry of Education
(NCET-13-0388). The authors are grateful to Dr. Zhenling Wang
in National Key Lab of Biotherapy and technician Jianying Tang in
Good doctor Pharmaceutical Company for the in vitro preliminary
screening of antibacterial activities on Staphylococcus aureus, Esch-
erichia coli and Pseudomonas aeruginosa. The authors also express
their thanks to Dr. Ruiliang Jin in Shanghai Pulmonary Hospital
for in vitro antitubercular screening.
Supplementary data
Supplementary data (experiment section and spectrum data)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.bmcl.2014.02.025.
Figure 2. SAR of target compounds.

W. Ang et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1496–1501
1501
Kim, C. T.; Dartois, V.; Park, S. K.; Cho, S. N.; Barry, C. E. N. Engl. J. Med. 2012, 367,
1508.
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