Synthesis and biological evaluation of 2-aminothiazole derivatives as antimycobacterial and antiplasmodial agents

Article abstract of DOI:10.1016/j.bmcl.2013.12.022

A series of compounds derived from the 2-amino-4-(2-pyridyl) thiazole scaffold was synthesized and tested for in vitro antimycobacterial activity against the Mycobacterium tuberculosis H37Rv strain, antiplasmodial activity against the chloroquine sensitive NF54 Plasmodium falciparum strain and cytotoxicity on a mammalian cell line. Optimal antimycobacterial activity was found with compounds with a 2-pyridyl ring at position 4 of the thiazole scaffold, a substituted phenyl ring at the 2-amino position, and an amide linker between the scaffold and the substituted phenyl. The antiplasmodial activity was best with compounds that had the phenyl ring substituted with hydrophobic electron withdrawing groups.

Full text of DOI:10.1016/j.bmcl.2013.12.022

Accepted Manuscript
Synthesis and biological evaluation of 2-aminothiazole derivatives as antimy-
cobacterial and antiplasmodial agents
Faith Mjambili, Mathew Njoroge, Krupa Naran, Carmen De Kock, Peter J.
Smith, Valerie Mizrahi, Digby Warner, Kelly Chibale
PII:
S0960-894X(13)01394-2
DOI:
http://dx.doi.org/10.1016/j.bmcl.2013.12.022
BMCL 21130
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
17 October 2013
2 December 2013
4 December 2013
Please cite this article as: Mjambili, F., Njoroge, M., Naran, K., De Kock, C., Smith, P.J., Mizrahi, V., Warner, D.,
Chibale, K., Synthesis and biological evaluation of 2-aminothiazole derivatives as antimycobacterial and
antiplasmodial agents, Bioorganic & Medicinal Chemistry Letters (2013), doi: http://dx.doi.org/10.1016/j.bmcl.
2013.12.022
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Synthesis and biological evaluation of 2-
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Faith Mjambili, Mathew Njoroge, Krupa Naran, Carmen De Kock, Peter Smith, Valerie Mizrahi, Digby Warner
and Kelly Chibale

Bioorganic & Medicinal Chemistry Letters
jo u rn al ho me p ag e : w ww .e lse vie r .c om
Synthesis and biological evaluation of 2-aminothiazole derivatives as
antimycobacterial and antiplasmodial agents
Faith Mjambilia, Mathew Njorogea, Krupa Naranb, Carmen De Kockc, Peter J. Smithc, Valerie Mizrahib,d
Digby Warnerb,d, and Kelly Chibalea,d ∗
aDepartment of Chemistry, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
bMRC/NHLS/UCT Molecular Mycobacteriology Research Unit, Division of Medical Microbiology, Department of Clinical Laboratory Sciences, University of
Cape Town, Rondebosch 7701, South Africa
c Division of Pharmacology, Department of Medicine, University of Cape Town, K45, OMB, Groote Schuur Hospital, Observatory 7925, South Africa
dInstitute of Infectious Disease and Molecular Medicine, University of Cape Town, Rondebosch 7701, South Africa
ARTICLE INFO
ABSTRACT
Article history:
Received
A series of compounds derived from the 2-amino-4-(2-pyridyl) thiazole scaffold was
synthesized and tested for in vitro antimycobacterial activity against the Mycobacterium
tuberculosis H37Rv strain, antiplasmodial activity against the chloroquine sensitive NF54
Plasmodium falciparum strain and cytotoxicity on a mammalian cell line. Optimal
Revised
Accepted
Available online
antimycobacterial activity was found with compounds with a 2-pyridyl ring at position 4 of the
thiazole scaffold, a substituted phenyl ring at the 2-amino position, and an amide linker between
the scaffold and the substituted phenyl.The antiplasmodial activity was best with compounds
that had the phenyl ring substituted with hydrophobic electron withdrawing groups.
Keywords:
2-amino-4-(2-pyridyl)thiazoles
antimycobacterial
2009 Elsevier Ltd. All rights reserved.
antiplasmodial
Structure-activity relationship
Hit optimisation
Tuberculosis (TB) is the second leading cause of death from
infectious disease globally.1 It is an airborne disease caused
by Mycobacterium tuberculosis (M.tb),2 and was declared a
global public health emergency by the World Health
Organization (WHO) in 1993.1 In 2011 the WHO estimated that
TB prevalence stood at 12 million cases worldwide, with
approximately 1.4 million deaths annually.1 Although there is an
existing pipeline of drugs under development that will bring new
anti-tubercular agents into clinical use in the near future,2–4 it is
still not sufficient to address the need for new regimens that are
effective against drug-susceptible and drug-resistant TB. This
highlights the urgent need for new chemotypes with anti-
mycobacterial activity.
widespread
resistance
to
known
Like TB, malaria too requires new chemotherapeutic agents
urgently. A parasitic disease, malaria is caused by five species of
the genus Plasmodium; falciparum, vivax, malariae, ovale and
knowlesi which are transmitted by the anopheles mosquito.5
Malaria has a global distribution with an estimated 219 million
cases and 660,000 deaths in 2010 according to WHO.5 Although
malaria can effectively be treated and cured, the rapid and
———
∗ Corresponding author. Tel.: +27216505495; fax: +27216505495; e-mail: Kelly.Chibale@uct.ac.za

antimalarial agents is the biggest challenge.6 Recent reports of
resistance to ACTs which are the current mainstay of malaria
chemotherapy are worrying as there are no ready alternatives.7
This heightens the need to search for new chemical entities with
antiplasmodial activity.
(5b) and 4-pyridyl (5c) analogues of 5a were synthesized starting
from commercially available 3-acetylpyridine and 4-
acetylpyridine respectively. To investigate the role of the linker,
amino (4b), urea (4c), and acylthiourea (4d) compounds were
synthesized. Compound 4b was obtained through the reaction of
2 and phenylthiourea [Scheme 1, step (iv)] while compounds 4c
Thiazole derivatives are known to exhibit a broad range of
biological activities,8 and have previously been documented to
possess antimycobacterial and antiplasmodial activity.9–12 Of
interest are compounds that possess the 2-amino-4-(2-pyridyl)
thiazole scaffold 1 (Figure 1) with an aryl or aryl alkyl
substituent on the amino group. The scaffold 1 was identified in a
high through-put screen (HTS) conducted in 2009 by the
Tuberculosis Antimicrobial Acquisition and Coordinating
Facility (TAACF).13
and 4d were obtained from the reaction of
3 with
phenylisocyanate and benzoylisothiocyanate respectively
[Scheme 1, step (v) and (vi)].
Scheme 1
To the best of our knowledge, there are no Structure-Activity
Relationship (SAR) studies reported on 2-amino-4-(2-
pyridyl)thiazoles as either antimycobacterial or antiplasmodial
agents. The objective of this work was therefore to explore the
SAR of this class of compounds and evaluate them as potential
antimycobacterial and/or antiplasmodial agents. Herein we
describe the initial results of our SAR exploration of 1.
aReagents and conditions: (i) 48% HBr, Br2, 65°C ,1 h, then rt, 1 h. (ii)
NH2CSNH2, EtOH, rt, 1 h. (iii) R2COOH, EDCI, HOBt, DCM, rt, 24 h. (iv)
EtOH, 90°C, 2h, then 20°C, 1 h. (v) Toluene, 56°C, 2 h, then rt, 24 h. (vi)
Acetone, 70°C, 2h, then 0°C, 1h.
Figure 1. The 2-amino-4-(2-pyridyl)thiazole scaffold.
The first series of target compounds 4a, 5a, 6-25 was
synthesized via a facile synthetic approach [Scheme 1, steps (i) to
(iii)] starting with the bromination of commercially available 2-
acetylpyridine, to yield the α brominated intermediate, 2, which
was then condensed with thiourea, to give the key intermediate 3.
All synthesized compounds were evaluated in vitro for their
antimycobacterial activity against the drug sensitive M.tb H37Rv
strain, antiplasmodial activity against the CQS P.falciparum
NF54 strain, and for cytotoxicity against the Chinese Hamster
Ovarian (CHO) cell line. Rifampicin, kanamycin and CQ were
used as positive controls, and the results are presented in Tables 1
and 2.
In the presence of mono substituted carboxylic acids,
intermediate
3
underwent
an
EDCI
(1-ethyl-3-(3-
dimethylaminopropyl) carbodiimide)-mediated coupling to
deliver the target. The HTS findings from antimycobacterial
evaluation suggests that only compounds with a 2-pyridyl
substituent at position 4 of the thiazole ring were active.13
Therefore, to investigate the role of the 2-pyridyl ring, 3-pyridyl

Table 1. In vitro antimycobacterial activity and solubility of synthesized analogues.
Entry
R
Solubility
(µM)
Entry
R
Solubility
(µM)
M.tb
H37Rv
MIC99
>160
M.tb
H37Rv
MIC99
5
3
2-
2-
2-
2-
2-
2-
3-
4-
2-
2-
2-
2-
2-
2-
>200
80
12
13
14
15
16
17
18
19
20
21
22
23
24
25
2-
2-
2-
2-
2-
2-
2-
2-
2-
2-
2-
2-
2-
2-
10
10
4a
4b
4c
4d
5a
5b
5c
6
5
160
*10
*20
1.25
>160
>160
5
5
2.5
2.5
20
5
160
80
10
20
O
S
O
10
10
N
H
10
20
40
5
20
40
5
40
80
160
160
10
80
10
5
160
>200
80
7
2.5
5
40
8
80
9
10
40
>200
160
>200
O
10
11
10
20
N
5
40
The unsubstituted scaffold 1 showed no antimycobacterial
activity at the highest test concentration of 160 µM. Substitution
In addition, compound 4a which had an amide linker at
position 2 had superior antimycobacterial activity compared to
compounds 4b, 4c, and 4d which had an amino, urea, and
acylthiourea linkers respectively.
on
1
yielded varying antimycobacterial activities with
compounds 20, 21, and 23 with a thiazole, imidazole, and 2-
pyridyl ring respectively remaining inactive while compounds
22, 24, 25, and 4a, 5a, 6-19 with thiophene, 3-pyridyl, 4-pyridyl
and phenyl rings respectively having enhanced activity.
Compounds 5a, 16-19 with substituted phenyl rings had
comparable activity to 4a with an unsubstituted phenyl.
The unsubstituted scaffold 1 was found to have antiplasmodial
activity with an IC50 of 47.8µM. Substitutions on 1 yielded
compounds with improved antiplasmodial activity with activity
of thiophene (22) > unsubstituted phenyl (4a) > 4-pyridyl (25) >
thiazole (20) > imidazole (21) > 2-pyridyl (23) and 3-pyridyl(24).
Compounds with substituted phenyls had activity ranging from
0.8µM (11) to 18.6µM (13).
The position of the substitution on the phenyl had an influence
on activity as demonstrated by the bromo-substituted compounds
with activity of the para (5a) > meta (7) > ortho (6).
Replacing the 2-pyridyl group at position 4 of 1 (5a) with a 3-
pyridyl (5b) or 4-pyridyl (5c) group resulted in loss of
antimycobacterial activity.

Table 2. In vitro antiplasmodial activity and cytotoxicity of synthesized analogues
H
N
N
N
R
S
CHO
IC50
(µM)
P.falciparum
IC50 (µM)
P.falciparum
IC50 (µM)
CHO
IC50
(µM)
SI
SI
Entry
R
Entry
R
(µM)
(µM)
3
4a
4b
4c
4d
5a
5b
5c
6
2-
2-
2-
2-
2-
2-
3-
4-
2-
2-
2-
2-
2-
2-
47.8
1.0
3.2
1.7
2.7
5.3
15.9
3.5
2.2
1.6
3.9
5.9
14.8
0.8
55.0
3.0
1.3
2.2
4.3
3.6
89.0
34.4
3.4
2.8
7.9
19.7
2.1
3.1
1.2
12
13
14
15
16
17
18
19
20
21
22
23
24
25
2-
2-
2-
2-
2-
2-
2-
2-
2-
2-
2-
2-
2-
2-
6.1
18.6
5.5
2.2
2.0
23.3
6.5
3.4
4.5
3.2
3.1
14.3
1.4
2.8
9.2
0.7
0.0
0.4
0.1
3.0
0.4
1.3
1.6
0.7
5.6
9.8
1.5
1.8
2.0
3.3
0.1
3.9
4.2
6.5
1.0
7.4
0.5
7.6
0.6
1.3
2.5
1.8
1.7
13.8
35.0
0.9
1.0
7
0.04
3.1
8
9
>35.4
>35.4
1.9
0.3
10
11
0.02
0.005
The position of the substitution on the phenyl also had an
influence on antiplasmodial activity as seen from the bromo-
substituted compounds with activity of the meta (7) > ortho (6) >
para (5a).
Furthermore, all compounds showed significant toxicity
against the CHO cell line and thus had low selectivity indices. It
is therefore possible that the antiplasmodial activity could be due
to the cytotoxic nature of the compounds and may not be
intrinsic.
Replacing the 2-pyridyl group at position 4 of 1 (5a) with a 3-
pyridyl (5b) led to a decrease in antiplasmodial activity.
However, a 4-pyridyl (5c) group resulted in improved
antiplasmodial activity.
Finally representative samples of the active compounds were
selected for microsomal metabolic stability in human, rat, and
mouse liver microsomes, using a single point metabolic turnover
method.15 The results are presented in Table 3.
As with antimycobacterial activity, compound 4a which had
an amide linker at position 2 had better antiplasmodial activity
compared to compounds 4b, 4c, and 4d which had amino, urea,
and acylthiourea linkers respectively.
Table 3. Microsomal Metabolic Stability
The solubility of the compounds was determined via the
turbidimetric solubility method at a pH of 7.4.14 The compounds
had low to moderate solubility limits (Table 1) based on the
nature of the substituents attached.

5.
WHO World Malaria Report 2012; World Health Organisation:
Geneva, Switzerland, 2012.
6.
7.
Schlitzer, M. Arch. Pharm. (Weinheim). 2008, 341, 149.
% remaining
Entry
11
R
WHO Guidelines for the treatment of malaria; second.; World
Health Organisation, 2010.
HLM RLM MLM
8.
Siddiqui, N.; Kumar, S.; Ahsan, W.; Azad, B. Int. J. Drug Dev. Res.
2011, 3, 55.
24.1
64.6
1.61
9.
Roy, K. K.; Singh, S.; Sharma, S. K.; Srivastava, R.; Chaturvedi,
V.; Saxena, A. K. Bioorg. Med. Chem. Lett. 2011, 21, 5589.
18
19
8.40
14.2
42.1
72.1
1.33
3.61
10.
11.
12.
Turan-Zitouni, G.; Kaplancikli, Z. A.; Ozdemir, A. Eur. J. Med.
Chem. 2010, 45, 2085.
Cohen, A.; Verhaeghe, P.; Crozet, M. D.; Hutter, S.; Rathelot, P.;
Vanelle, P.; Azas, N. Eur. J. Med. Chem. 2012, 55, 315.
González Cabrera, D.; Douelle, F.; Feng, T.-S.; Nchinda, A. T.;
Younis, Y.; White, K. L.; Wu, Q.; Ryan, E.; Burrows, J. N.;
Waterson, D.; Witty, M. J.; Wittlin, S.; Charman, S. A.; Chibale,
K.; Gonz, D. J. Med. Chem. 2011, 54, 7713.
4a
7
27.5
>99
53.8
54.5
>99
47.8
7.60
82.4
3.71
13.
Ananthan, S.; Faaleolea, E. R.; Goldman, R. C.; Hobrath, J. V;
Kwong, C. D.; Laughon, B. E.; Maddry, J. a; Mehta, A.;
Rasmussen, L.; Reynolds, R. C.; Secrist, J. a; Shindo, N.; Showe,
D. N.; Sosa, M. I.; Suling, W. J.; White, E. L. Tuberculosis (Edinb).
2009, 89, 334.
25
14.
15.
Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J. 2001,
46, 3.
The compounds were generally unstable across all three
species, with the exception of compound 7. In conclusion
preliminary SAR studies on 1 therefore suggest that the 2-pyridyl
group at position 4 of the thiazole ring is essential for
antimycobacterial activity as reported,13 and the presence of a
substituted phenyl group at the 2-amino position is also important
for activity. In addition the linker between the groups plays a role
in the antimycobacterial activity. Substitution on 1 are beneficial
for antiplasmodial activity as activity is enhanced to varying
degrees by substitution with different groups. However, the
compounds are generally cytotoxic and metabolically unstable.
Metabolite identification studies for these compounds are in
progress, as part of lead optimization, and will be communicated
in future work.
Di, L.; Kerns, E. H.; Gao, N.; Li, S. Q.; Huang, Y.; Bourassa, J. L.;
Huryn, D. M. J. Pharm. Sci. 2004, 93, 1537.
Supplementary material
Experimental procedures, characterization of final compounds
and biological assays protocols.
ACKNOWLEDGMENT
F.M., M.N., K.N., V.M., D.W. and K. C are grateful to the
University of Cape Town, the South African Medical Research
Council, and the South African Research Chairs Initiative and
Centres of Excellence program of the Department of Science and
Technology administered through the NRF, for funding support.
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Palomino, J. C.; Martin, A. 2013, 275.

Graphical abstract