Synthesis and antiplasmodial and antimycobacterial evaluation of new nitroimidazole and nitroimidazooxazine derivatives

Article abstract of DOI:10.1021/ml300362a

The synthesis and antiplasmodial and antimycobacterial evaluation of two new series of nitroimidazole and nitroimidazooxazine derivatives is described. The majority of these compounds, especially hybrids 9d, 9f, and 14b, exhibited potent activity against the chloroquine-resistant K1 strain of Plasmodium falciparum. Furthermore, a notable number from the tetrazole series were significantly more active against M. tuberculosis than kanamycin, a standard TB drug.

Full text of DOI:10.1021/ml300362a

Letter
pubs.acs.org/acsmedchemlett
Synthesis and Antiplasmodial and Antimycobacterial Evaluation of
New Nitroimidazole and Nitroimidazooxazine Derivatives
Matshawandile Tukulula,† Rajni-Kant Sharma,† Maïa Meurillon,† Aman Mahajan,† Krupa Naran,‡,§
Digby Warner,‡,§ Jianxing Huang,∥ Belew Mekonnen,∥ and Kelly Chibale*,†,§
†Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
‡MRC/NHLS/UCT Molecular Mycobacteriology Research Unit, DST/NRF Centre of Excellence for Biomedical TB Research,
University of Cape Town, Rondebosch 7701, South Africa
§Institute of Infectious Disease and Molecular Medicine, University of Cape Town, Rondebosch 7701, South Africa
∥Hager Biosciences LLC, 116 Research Drive, Suite 265, Bethlehem, Pennsylvania 18015, United States
S
* Supporting Information
ABSTRACT: The synthesis and antiplasmodial and anti-
mycobacterial evaluation of two new series of nitroimidazole
and nitroimidazooxazine derivatives is described. The majority
of these compounds, especially hybrids 9d, 9f, and 14b,
exhibited potent activity against the chloroquine-resistant K1
strain of Plasmodium falciparum. Furthermore, a notable
number from the tetrazole series were significantly more
active against M. tuberculosis than kanamycin, a standard TB
drug.
KEYWORDS: Nitroimidazoles, nitroimidazooxazines, antiplasmodial, antimycobacterial activity
ccording to the World Health Organization (WHO),
metabolic activation of the nitro group, which leads to the
Anearly one-third of the world’s population harbors generation of nitric oxide as an active species.13 The main
drawbacks of 1 are its poor aqueous solubility and its
propensity to bind to proteins in human plasma.3,14 Various
measures have been undertaken in addressing these short-
comings, such as the synthesis of biphenyl analogues of 1,15 the
use of urea, carbamate, and amide linkers in the place of the
benzyl ether,16 and the hybridization of 1 with oxazolidinone,17
among others. The most promising results from these studies
were achieved when the benzyl group of 1 was replaced with
various (hetero)biaryl side-chains and amide groups. It was
noted that compounds that contain these scaffolds exhibited
better in vitro and in vivo potencies and improved absorption,
distribution, metabolism, and excretion (ADME) properties
compared to those of 1.18−21
In this context, we desired to investigate the antiplasmodial
and antimycobacterial properties of new analogs that contain
the key nitroimidazole pharmacophore. The first series was
designed to contain a tetrazole moiety in the place of the
lipophilic trifluoromethoxy group of 1, as it was hypothesized
that the inclusion of this moiety will significantly aid in
improving the physicochemical properties. These tetrazole-
containing compounds were synthesized in three-steps; the first
involved the synthesis of aralkyl halides 7a−c from known
literature methods.22 These aralkyl halides were then reacted
Mycobacterium tuberculosis (Mtb), the causative agent of
tuberculosis (TB).1 TB is the second leading cause of death
due to an infectious organism:2 current WHO estimates stand
at 9.2 million new cases and 1.8 million deaths annually.1,3
Malaria, on the other hand, infects about 255 million people
worldwide, resulting in 781 000 deaths, mostly to children
under 5 years and pregnant women.4 Currently, there are no
effective vaccines against these pathogens and treatment success
in some areas remains low due to poor management and
patient noncompliance.5 This situation is further exacerbated
by the increasing prevalence of multi- (MDR) and extensive-
drug resistant (XDR) strains of Mtb.6 Therefore, there is an
urgent need for new, fast acting, and more efficacious
antimalarial and anti-TB therapies to replace the existing drug
regimens.
Although there has been no introduction of a new anti-TB
drug over the last 40 years, a number of different classes of
compounds are undergoing clinical development,2 namely
nitroimidazooxazines (PA-824, 1),7 diarylquinolines
(TMC207, 2),8 oxazolidinones (eperesol, 3, and linezolid,
4),9 and ethylenediamines (SQ109, 5)10 (Figure 1). The use of
the antibiotic metronidazole in the management of anaerobic
bacterial and protozoan infections has reinvigorated interest in
the nitroimidazole scaffold over the past decade.11,12 Nitro-
imidazoles (e.g., 1) are pro-drugs that are highly effective
against both the replicating and nonreplicating persistent forms
of Mtb, and their activity is believed to arise as a result of
Received: October 24, 2012
Accepted: November 26, 2012
© XXXX American Chemical Society
A
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ACS Medicinal Chemistry Letters
Letter
Figure 1. Chemical structures of lead TB compounds in clinical development.
with the commercially available nitroimidazooxazine-alcohol
(6) in the presence of sodium hydride7,23 to afford aldehydes
8a−b (meta and para) (Scheme 1). The reaction failed in the
a
Scheme 1
Figure 2. Structures and yields of MCR intermediates and target
compounds.
a
Reagents and conditions: (i) NaH, DMF, N2, 0 °C to rt, 12 h; (ii)
TMSN3, MeOH, 40 °C, 12 h.
methyl-4-nitro-1H-imidazole in the presence of anhydrous
potassium carbonate to afford the isomeric mixture of 14a and
14b (Scheme 2).
All synthesized compounds were evaluated in vitro for their
antiplasmodial (against the multidrug-resistant K1 strain) and
antimycobacterial activity (against the drug-sensitive H37Rv
Mtb strain). Chloroquine, primaquine, kanamycin, and
streptomycin were used as positive controls, and the results
are tabulated in Table 1.
case of the ortho-aldehyde, presumably due to steric hindrance.
Aldehydes 8a−b were then subjected to the modified TMSN3−
Ugi multicomponent reaction (MCR)24 involving amines and
the convertible tert-butyl isocyanide. Amine inputs included
primaquine and 4-aminoquinoline diamines. Target com-
pounds 9a−h were obtained in moderate to excellent yields
1
(Figure 2) and diastereoselectivity (based on the H NMR);
compounds 9a−f were obtained exclusively as single diaster-
eomers while 9g and 9h were obtained as 1:1 diastereomeric
mixtures, owing to the fact that the commercial primaquine salt
used in this study is racemic.
Hybrid compounds 9d−h and 14a−16 showed antiplasmo-
dial IC50 values in the low micromolar range. More specifically,
the PA-824−chloroquinoline hybrids 9d (IC50 = 0.100 μM)
and 9f (IC50 = 0.164 μM) and the methylnitroimidazole−
chloroquinoline hybrid 14b (IC50 = 0.094 μM) were the most
active, endowing superior activity than both chloroquine (IC50
= 0.213 μM) and primaquine (IC50 = 0.643 μM). Moreover,
hybrids 9d, 9f, and 14b demonstrated an improved activity over
their intermediates, exemplified by 8b and quinoline diamine
17a, and the equimolar combination of the individual
components, an indication of a synergistic effect. Additionally,
the PA-824−chloroquinoline hybrids were found to be more
The synthesis of the second series of compounds, 14a−b,
15a−b, and 16, began with the synthesis of intermediates 10a
and 10b as described by Chauhan and co-workers25 (Scheme
2). These intermediates were then mesylated to the
corresponding methanesulfonic acid esters 11a and 11b,
which in turn underwent nucleophilic substitution reaction
with 2-methyl-4-nitro-1H-imidazole to yield hybrids 15a−b in
reasonable yields. Hybrid 16 was obtained via the sodium
hydride mediated reaction of 11b with alcohol 6. In addition,
the methanesulfonic acid esters 11a−b were also reacted with
N-(methyl)ethanolamine to furnish intermediates 12a−b,
which on chlorination using thionyl chloride yielded 13a−b.
These chloride derivatives were subsequently reacted with 2-
efficacious than the PA-824−primaquine hybrids [9g (IC50
=
2.042 μM) and 9h (IC50 = 0.985 μM)], and the latter hybrids
showed an antagonistic effect. The covalent attachment of
methylnitroimidazole to intermediates 11a and 11b induced a
significant reduction in antiplasmodial activity, as seen in 15a
B
dx.doi.org/10.1021/ml300362a | ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX

ACS Medicinal Chemistry Letters
Letter
a
Scheme 2
Table 1. In Vitro Antimycobacterial and Antiplasmodial
Activity of the Synthesized Compounds
P. falciparum M. tuberculosis
cytotoxicity
IC50 (μM)
IC50 (μM)
MIC99 (μM)
a
entry
K1
H37Rv
L6
SI
6
>270
40
330.6
nd
b
8a
nd
0.25
0.625
0.625
0.313
0.313
0.313
nd
8b
62.62
15.79
7.95
34.3
0.54
11.2
14.2
8.65
246
85
9a
176.9
112.7
75.23
24.6
9b
9c
8.70
9d
0.100
0.485
0.164
2.042
0.985
1.468
1.086
0.078
0.391
0.812
0.0943
11.063
4.25
9e
41.52
29.3
9f
0.625
0.625
5
175
37
9g
75.9
9h
142.8
233.17
178.3
77.57
24.75
33.69
17.6
145
159
164
995
63
10a
10b
12a
12b
14a
14b
15a
15b
16
>160
>160
>160
>160
>160
>160
>160
>160
1.25
>160
0.25
0.25
0.25
0.5
41.5
186.6
13
140.8
136.8
134.5
21.22
20.41
21.89
11.81
29.00
22.94
32.2
12.8
71.2
67.4
11.7
4.69
32.2
31.8
10.52
0.298
0.303
1.874
2.514
0.900
0.721
0.213
0.643
c
17a
d
8a/17a
8a/17b
d
d
8a/17c
8a/primaquine
c
c
a
Reagents and conditions: (i) MsCl, TEA, THF, 0 °C, N2, 1 h; (ii) 2-
8b/primaquine
chloroquine
primaquine
0.625
>160
80
methyl-4-nitro-1H-imidazole, K2CO3, DMF, 80 °C, 6 h; (iii) 2-
nitroimidazo[2,1-b][1,3]oxazine, NaH, DMF, −50 °C to rt, 12 h; (iv)
N-(methyl)ethanol amine, TEA, 0−60 °C, 3 h; (v) SOCl2, DMF,
toluene, 0 to rt, 14 h; (vi) 2-methyl-4(5)-nitroimidazole, K2CO3,
DMF, 100−110 °C, 6 h.
podophyllotoxin
kanamycin
0.0193
5.40
0.27
streptomycin
a
b
Selective indices [(IC50 L6 cell-line)/IC50 (K1)]. nd: not
c
determined. Structures of quinoline diamines 17a−c can be found
in the Supporting Information (section 1.10). Equimolar combina-
d
and 15b. Interestingly, the methylnitroimidazole derivative 14b
displayed a 4-fold increase in potency compared to
intermediate 12b whereas the opposite was observed for
hybrid 14a, which had a 10-fold reduction in potency compared
to 12a. The 2- and 4-carbon spacer appeared to be more
favored in PA-824-chloroquinoline hybrids whereas the 3-
carbon spacer was favorable in the methylnitroimidazole−
chloroquinoline hybrids. The selective indices of the most
active compounds were closer to or greater than 100,
suggesting that these compounds are more selective toward
the chloroquine-resistant Plasmodium falciparum parasite.
Against replicating H37Rv M. tuberculosis, the MCR series
intermediates and target tetrazoles (8 and 9) exhibited MIC99
values ranging between 0.25 and 1.25 μM, except for 9e and 9h,
while from the second series only hybrid 16 showed potent
antimycobacterial activity. Also, compounds 9a−d, 9f−g, and
16 were more active than the standard TB drug, kanamycin
(MIC99 = 5.40 μM), and the two antimalarial drugs,
tion of individual components.
In summary, we have designed and synthesized new
nitroimidazole and nitroimidazooxazine derivatives, and these
were screened for antiplasmodial and antimycobacterial activity.
The majority of these compounds, especially hybrids 9d, 9f,
and 14b, exhibited potent activity against the K1 strain of P.
falciparum, with IC50 values in the low micromolar range.
Furthermore, compounds from the MCR series possessed
superior antimycobacterial activity, with MIC99 values in the
region of 0.25−125 μM while only one compound, 16, from
the second series showed an appreciable activity. Furthermore,
the majority of the active compounds were more efficacious
than kanamycin, a standard TB drug, in these assays
chloroquine (MIC99 > 160 μM) and primaquine (MIC99
=
ASSOCIATED CONTENT
■
80 μM). Interestingly, hybridization was found to be less
beneficial for antimycobacterial activity, as evidenced by the
antagonistic effect in both the PA-824-aminoquinoline and PA-
824-primaquine hybrids. Notably, an increase in the length of
the alkyl side-chain of the PA-824-chloroquinoline hybrids
resulted in reduced activity.
S
* Supporting Information
Synthetic experimental procedures, characterization of final
compounds, and details regarding biological assays. This
material is available free of charge via the Internet at http://
pubs.acs.org.
C
dx.doi.org/10.1021/ml300362a | ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX

ACS Medicinal Chemistry Letters
Letter
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AUTHOR INFORMATION
Corresponding Author
*E-mail: Kelly.Chibale@uct.ac.za. Tel: +27 21 650 2553. Fax:
+27 21 650 5195.
■
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Author Contributions
All authors have given approval to the final version of the
manuscript.
Funding
We thank the University of Cape Town Department of
Chemistry Equity Development Program scholarship and the
South African National Research foundation (NRF) for
financial support. The University of Cape Town, South African
Medical Research Council, and South African Research Chairs
Initiative of the Department of Science and Technology
administered through the NRF is also gratefully acknowledged
(K.C.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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