3H-1,2,4-Dithiazol-3-one compounds as novel potential affordable antitubercular agents

Article abstract of DOI:10.1016/j.bmcl.2012.12.065

Small molecules with oxathiazol-2-one moiety were recently reported as potent inhibitors of Mycobacterium bovis var. bacilli Calmette-Guérin (BCG), among which HT1171 was the most potent and selective proteasome inhibitor. Herein we synthesized a series of novel compounds by bioisosteric replacement of the oxathiazol-2-one ring with 3H-1,2,4-dithiazol-3-one, and also fifteen 1,3,4-oxathiazol-2-one molecules in order for potency comparison and structure-activity relationship elucidation since their antibacterial effects on the virulent strains were not evaluated before. All the compounds were assessed for antitubercular activities on the virulent H37Rv strain by a serial dilution method. Among the tested compounds, 3H-1,2,4-dithiazol-3-one compound 4n was found to be the most active with a lowest MIC₉₀ value of 1 μg/mL. Furthermore, the cytotoxicities of all the compounds against normal human liver cell line L02 were determined by an MTT method. Compound 4n displayed a lower inhibitory ratio than HT1171 at the concentration of 100 μM, indicating its better safety profile.

Full text of DOI:10.1016/j.bmcl.2012.12.065

Bioorganic & Medicinal Chemistry Letters 23 (2013) 1424–1427
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
3H-1,2,4-Dithiazol-3-one compounds as novel potential affordable
antitubercular agents
Jianzhong Yang a, , Weiyi Pi a, , Li Xiong b, , Wei Ang b, Tao Yang a, Jun He a, Yuanyuan Liu a, Ying Chang a,
Weiwei Ye a, Zhenling Wang a, , 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, Ministry of Education, West China Medical School, 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:
Small molecules with oxathiazol-2-one moiety were recently reported as potent inhibitors of Mycobacte-
rium bovis var. bacilli Calmette–Guérin (BCG), among which HT1171 was the most potent and selective
proteasome inhibitor. Herein we synthesized a series of novel compounds by bioisosteric replacement
of the oxathiazol-2-one ring with 3H-1,2,4-dithiazol-3-one, and also fifteen 1,3,4-oxathiazol-2-one mol-
ecules in order for potency comparison and structure–activity relationship elucidation since their anti-
bacterial effects on the virulent strains were not evaluated before. All the compounds were assessed
for antitubercular activities on the virulent H37Rv strain by a serial dilution method. Among the tested
compounds, 3H-1,2,4-dithiazol-3-one compound 4n was found to be the most active with a lowest MIC90
Received 5 November 2012
Revised 11 December 2012
Accepted 20 December 2012
Available online 4 January 2013
Keywords:
3H-1,2,4-Dithiazol-3-ones
H37Rv
Tuberculosis
Bioisosterism
value of 1
line L02 were determined by an MTT method. Compound 4n displayed a lower inhibitory ratio than
HT1171 at the concentration of 100 M, indicating its better safety profile.
Ó 2012 Elsevier Ltd. All rights reserved.
lg/mL. Furthermore, the cytotoxicities of all the compounds against normal human liver cell
l
Tuberculosis (TB) is an infectious disease caused by the bacillus
Mycobacterium tuberculosis. In 2010, there were 8.8 million inci-
dent cases of TB, 1.1 million deaths from TB among HIV-negative
people and an additional 0.35 million deaths from HIV-associated
TB.1 In addition, the emergence of drug-resistant tuberculosis is
worrisome, especially the increase in multidrug-resistant tubercu-
losis (MDR-TB), and most recently, extensively drug-resistant
tuberculosis (XDR-TB).2–5 On the other hand, tuberculosis inci-
dence rates correlate with poor socio-economic conditions. How-
ever, no new TB drugs have been developed in the last 40 years.6
Bedaquiline was approved by the FDA on December 28th, 2012.
To be note, bedaquiline is not really an ideal anti-TB drug since it
has shortcomings of adverse effects and high cost. Thus there is
still an urgent need to develop novel affordable antitubercular
drugs with high potency and lower toxicity that can provide treat-
ment options for all forms of TB.
proteasome has gained considerable attention for their species
selectivity. In 2009, two novel 1,3,4-oxathiazol-2-one compounds
HT1171 and GL5 (Fig. 1) which could kill non-replicating Mycobacte-
rium tuberculosis were identified as potent and selective proteasome
inhibitors of Mycobacterium tuberculosis with low inhibitory effects
on the proteasomes of human.7 There is no difficulty finding that
1,3,4-oxathiazol-2-one compounds are easily synthesized and have
the potential to be developed as affordable antitubercular agents.
However, all their antibacterial activities tests were not fully
explored, especially not conducted on virulent strains, which are
more useful for antitubercular evaluation. Thus, there is a need to
re-synthesize some of the 1,3,4-oxathiazol-2-one compounds for
activities and toxicity reevaluation. Furthermore, exploring their
analogs may lead to identify novel chemical entities with higher
potencies or lower toxicities as potential antitubercular agents.
In medicinal chemistry, bioisosterism strategy is widely used as
an efficient approach to reduce toxicity and/or enhance the potency
of a lead compound. Some 3H-1,2,4-dithiazol-3-one compounds
Proteasomes degrade necessary proteins that serve in signaling
during adaptation and provide an attractive biological target for
drug design.7 Bortezomib,
a proteasome inhibitor, has been
launched in the UK to treat multiple myeloma.8,9 Mycobacterium
tuberculosis is so far the only known bacterial pathogen possessing
proteasomes. In recent years, antituberculosis therapy targeting
N
S
N
O
S
S
S
O
O
O2N
O
S
⇑
Corresponding authors. Tel./fax: +86 28 85503817.
E-mail addresses: wangzhenling2007@126.com (Z. Wang), luo_youfu@scu.
edu.cn (Y. Luo).
HT1171
GL5
Figure 1. Chemical structures of HT1171 and GL5.
These authors equally contributed.
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.12.065

J. Yang et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1424–1427
1425
O
Cl
Cl
O
Lawesson’s reagent. Eleven out of fifteen 1,3,4-oxathiazol-2-one
compounds (3a–e, 3g–j, 3m and HT1171) and three out of fourteen
3H-1,2,4-dithiazol-3-one compounds(4a, 4c and 4g), albeit already
known, were also synthesized herein in order to test their antitu-
bercular effects on virulent H37Rv strain since their antibacterial
effects were not evaluated or only on attenuated BCG before and
elucidate the structure–activity relationship between the two
structural classes of molecules. Other 15 compounds are new.
Chemical reactions were monitored by thin layer chromatography
(TLC) and the crude product was purified by silica-gel column
chromatography using ethyl acetate–hexane as the eluent solvent
system. All the final products 3a–n, HT1171 and 4a–n were char-
acterized by 1H and 13C NMR and mass spectrometry. The relative
positions of two sulfur atoms in the 3H-1,2,4-dithiazol-3-one com-
pounds have been proved to be consecutive in the five-member
ring in our previous study.18
O
N
a
c
R
R
NH2
O
O
S
Cl
S
1a-1n
3a-3n
b
S
S
S
R
S
Cl
R
NH2
N
O
2a-2n
4a-4n
Scheme 1. Synthetic route of compounds 3a–n and 4a–n. Reagents and conditions:
(a) chlorocarbonyl sulfenyl chloride, THF, ambient temperature, 16 h; (b) Lawes-
son’s reagent, THF, refluxed; (c) chlorocarbonyl sulfenyl chloride, THF, ambient
temperature, 16 h.
The compounds were assessed for their anti-mycobacterial
activities against Mycobacterium tuberculosis H37Rv by a microdi-
lution method in 96-wells plates according to the literatures19,20
with appropriate revision. MIC90 and MIC99 are designated as the
minimum concentration of a compound that inhibits growth by
90% and 99%, respectively. The results of 1,3,4-oxathiazol-2-one
compounds are shown in Table 1 while that of 3H-1,2,4-dith-
iazol-5-one compounds shown in Table 2.
As shown in Table 1, compounds 3a–n with 1, 3, 4-oxathiazol-
2-one ring exhibited potent activities on the virulent H37Rv strain.
HT1171 was shown to be the most active compound with MIC90
and MIC99 values of 2 and 4 lg/mL, respectively. When the nitro
group of HT1171 was removed from the thiophene ring there
were reported to possess antibacterial activities against some bacte-
ria and fungus strains including Staphylococcus aureus, Pasteurella
piscicida and Botlytis cinerea.10 However, this kind of structural class
has not yet been screened for its antitubercular activities. Moreover,
the nucleus structure of this class of molecules has the same number
of outermost electrons, similar shape and size to 1,3,4-oxathiazol-2-
one. Thus, similar antitubercular biological activities of 3H-1,2,4-
dithiazol-3-ones may be rationally expected based on the rule of
bioisosterism. As a part of our ongoing program11 on the discovery
of novel antitubercular agents, herein we describe the synthesis of
29 compounds from two structural classes, 1,3,4-oxathiazol-2-ones
and 3H-1,2,4-dithiazol-3-ones. The in vitro antitubercular activities
of all the final compounds against virulent Mycobacterium tuberculo-
sis H37Rv were assessed and the safety profile was evaluated to-
wards normal human liver L02 cells.
was a fourfold decrease in antitubercular activity (compound 3i,
MIC99 = 16
lg/mL). However, the substituted thiophene ring of
HT1171 can tolerate structural alteration to 2-chlorophenyl group
Chemically, the target compounds were readily prepared
according to the literatures7,12–17 with appropriate revision as out-
lined in Scheme 1. Briefly, the 1,3,4-oxathiazol-2-one compounds
3a–n were prepared respectively by treating appropriate amides
with chlorocarbonyl sulfenyl chloride in anhydrous tetrahydrofu-
ran at room temperature and the 3H-1,2,4-dithiazol-3-one com-
(3c) with
a
minor decrease in activity for compound 3c
(MIC99 = 8
lg/mL). Other substituted aromatic ring replacement
(benzene, naphthalene, pyridine and piperazine ring) resulted in
4- to 16-fold decreases in activities. To note, compound 3m has
been reported to have an MIC value of 4.5
which does not offer much of a comparison to our data (16
mL for MIC90 or 32
definition of MIC are different from ours.12
lg/mL against H37Rv,
lg/
pounds 4a–n were prepared in
a similar way from the
l
g/mL for MIC99) because the test system and
intermediate thioamides 2, which were obtained from the corre-
sponding amides by a heterocyclic atom exchange reaction with
Table 1
Antitubercular activities on virulent H37Rv strain and cytotoxicity on human normal liver L02 cells of 1,3,4-oxathiazol-2-one compounds
MICa
(l
g/mL)
IRb (%)
a
Compd
R
MW
MIC90
(l
g/mL)
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
4-Chlorophenyl
4-Fluorophenyl
2-Chlorophenyl
2-Methylphenyl
3-Methoxyphenyl
2,5-Dimethoxyphenyl
Naphthalen-2-yl
4-Aminophenyl
5-Methylthiophen-2-yl
2-Nitrophenyl
2-Chloro-4-nitrophenyl
2-Chloropyridin-3-yl
Pyrazin-2-yl-
2,6-Dichloro-5-fluoropyridin-3-yl
5-Methyl-4-nitrothiophen-2-yl
214
220
214
194
210
240
252
195
200
225
297
215
204
268
267
32
16
8
8
8
16
16
16
8
8
16
8
64
32
8
39.6
21.7
20.6
18.1
15.0
25.3
26.2
39.5
25.1
32.4
54.6
31.4
31.4
15.1
53.8
16
16
32
32
16
16
16
32
32
32
32
4
3m
3n
HT1171
16
16
2
a
MIC90 and MIC99 are the minimum concentration of a compound that inhibits growth by 90% and 99%, respectively.
IR (%) is the mean inhibitory ratio against L02 cell line measured at 48 h after treatment with the test compound at the concentration of 100 lM by MTT method.
b
IR ð%Þ ¼ ODðCÞꢀODðSÞ. OD(C) denotes the mean optical density value of control group treated with DMSO calculated from three independent experiments. OD(S) denotes the
ODðCÞ
mean optical density value of sample groups treated with the compounds calculated from three independent experiments.

1426
J. Yang et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1424–1427
Table 2
Antitubercular activities on virulent H37Rv strain and cytotoxicity on human normal liver L02 cells of 3H-1,2,4-dithiazol-3-one compounds
MICa
(l
g/mL)
IRb (%)
a
Compd
R
MW
MIC90
(l
g/mL)
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4-Chlorophenyl
4-Fluorophenyl
2-Chlorophenyl
2-Methylphenyl
3-Methoxyphenyl
2,5-Dimethoxyphenyl
Naphthalen-2-yl
4-Aminophenyl
5-Methylthiophen-2-yl
2-Nitrophenyl
231
214
231
210
248
256
246
211
216
263
276
232
236
284
16
8
32
16
16
32
32
32
16
32
32
32
16
16
16
4
46.0
33.7
40.1
18.6
38.6
23.7
47.2
46.3
36.3
33.0
28.6
34.4
56.7
24.3
16
16
16
16
16
16
16
16
8
2-Chloro-4-nitrophenyl
2-Chloropyridin-3-yl
Pyrazin-2-yl-
4
8
1
2,6-Dichloro-5-fluoropyridin-3-yl
a
MIC90 and MIC99 are the minimum concentration of a compound that inhibits growth by 90% and 99%, respectively.
IR (%) is the mean inhibitory ratio against L02 cell line measured at 48 h after treatment with the test compound at the concentration of 100
b
lM by MTT method.
IR ð%Þ ¼ ODðCÞꢀODðSÞ. OD(C) denotes the mean optical density value of control group treated with DMSO calculated from three independent experiments. OD(S) denotes the
ODðCÞ
mean optical density value of sample groups treated with the compounds calculated from three independent experiments.
As illustrated in Table 2, 3H-1,2,4-dithiazol-3-one compounds
with electron-deficient aromatic rings such as pyridine or pyrazine
(4l, 4m, 4n) were generally shown more potent than those with
electron-rich benzene or thiophene (4a, 4d, 4e, 4f, 4h, 4i). In spe-
cific, compounds with chlorine atom (4a, 4k, 4l and 4n) or fluorine
atom (4b) were more potent than those without chlorine or fluo-
rine except 4C, which indicates that chlorine and/or fluorine atom
may play a part on the potency in this structural class of com-
pounds. Actually removal of two halogen atoms (5-fluro and 6-
chloro) of the most potent compound 4n produced compound 4l,
which led to a fourfold decrease in antitubercular activities, from
human normal liver L02 cells were evaluated. The SARs between
these two classes of molecules and the discovery of the potent
3H-1,2,4-dithiazol-3-one compound with better safety profile pro-
vide us a novel promising scaffold to develop potential affordable
antitubercular agents. Meanwhile our results demonstrated a suc-
cessful scaffold hopping strategy by taking advantage of bioisoster-
ic replacement of 1,3,4-oxathiazol-2-one ring with 3H-1,2,4-
dithiazol-3-one.
Acknowledgments
1 to 4
l
g/mL of MIC90 value and from 4 to 16
l
g/mL of MIC99 value.
This work was supported by the National S&T Major Special
Project on Major New Drug Innovation (2012ZX09103101-036).
We also thank Dr. Ruiliang Jin in Shanghai Pulmonary Hospital
for antitubercular activity evaluation and fruitful discussion.
As compared Table 2 with Table 1, 3H-1,2,4-dithiazol-3-one
compounds 4a–n demonstrated similar antibacterial activities to
1,3,4-oxathiazol-2-one compounds 3a–n as expected, which coin-
cides the rule of bioisosterism. Our results showed a successful
scaffold hopping by taking advantage of bioisosteric replacement
of 1,3,4-oxathiazol-2-one ring in the lead compound with 3H-
1,2,4-dithiazol-3-one cycle.
Supplementary data
Supplementary data associated with this article can be found, in
The toxic profile of all final compounds was evaluated by an
MTT assay on L02 cells. The inhibitory ratios at concentration of
the
online
version,
at
http://dx.doi.org/10.1016/
j.bmcl.2012.12.065.
100
iazol-3-one compounds are listed in Table 1 and Table 2. The inhib-
itory ratios at concentration of 100 M of 1,3,4-oxathiazol-2-one
lM of 1,3,4-oxathiazol-2-one compounds and 3H-1,2,4-dith-
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