Novel in vivo active anti-malarials based on a hydroxy-ethyl-amine scaffold

Article abstract of DOI:10.1016/j.bmcl.2012.11.118

A novel series of anti-malarials, based on a hydroxy-ethyl-amine scaffold, initially identified as peptidomimetic protease inhibitors is described. Combination of the hydroxy-ethyl-amine anti-malarial phramacophore with the known Mannich base pharmacophor

Full text of DOI:10.1016/j.bmcl.2012.11.118

Bioorganic & Medicinal Chemistry Letters 23 (2013) 658–662
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel in vivo active anti-malarials based on a hydroxy-ethyl-amine scaffold
Claire-Lise Ciana a, Romain Siegrist a, Hamed Aissaoui a, Léo Marx a, Sophie Racine a, Solange Meyer a,
Christoph Binkert a, Ruben de Kanter a, Christoph Fischli b,c, Sergio Wittlin b,c, Christoph Boss a,
⇑
a Actelion Pharmaceuticals Ltd, Drug Discovery Chemistry and Biology, Hegenheimermattweg 91, CH-4123 Allschwil/BL, Switzerland
b Swiss Tropical and Public Health Institute, Parasite Chemotherapy, Socinstrasse 57, CH-4002 Basel, Switzerland
c University of Basel, CH-4003 Basel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel series of anti-malarials, based on a hydroxy-ethyl-amine scaffold, initially identified as peptidom-
imetic protease inhibitors is described. Combination of the hydroxy-ethyl-amine anti-malarial phrama-
cophore with the known Mannich base pharmacophore of amodiaquine (57) resulted in promising
in vivo active novel derivatives.
Received 23 October 2012
Revised 28 November 2012
Accepted 29 November 2012
Available online 11 December 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Malaria
Medicinal chemistry
Dual mechanism of action
Mannich base
Speed of action
Malaria is an infectious disease caused by Plasmodium parasites
and is transmitted to humans via the bite of infected female
anopheles mosquitoes. According to the World Health Organiza-
tion (WHO), 3.3 billion people were at risk of being infected with
malaria.1 Every year, about 250 million people are affected by ma-
laria resulting in 1 million deaths. Most vulnerable are children un-
der the age of five and pregnant women. Five Plasmodium parasite
species can cause malaria in humans, with Plasmodium falciparum
being responsible for most of the fatal cases. Due to the develop-
ment of resistance towards existing treatment regimens, there is
an urgent need for new anti-malarial drugs.2
O
O
O
OH
O
H
N
H
N
N
N
H
NH2
Cl
O
OH
NH
H
N
N
O
O
NH2
1
2
Figure 1. Examples of hydroxy-ethyl-amine compounds targeting plasmepsin I
and II.
Hydroxy-ethyl-amine containing compounds (Fig. 1) have pre-
viously been studied as potential anti-malarials. Since the hydro-
xy-ethyl-amine moiety represents a stabilized peptidomimetic
inhibitor of aspartic proteases, it has been suggested that these
compounds act by blocking the parasite specific aspartic proteases,
the plasmepsins.3
O
O
O
R
N
H
N
H
N
N
H
N
H
Ar
OH
OH
3
4
Food-vacuolar plasmepsins were considered viable drug targets
in plasmodium parasites, based on their role in the parasite specific
degradation of hemoglobin in the food-vacuole.4 However, it has
been recently shown that the parasite has redundant mechanisms
for hemoglobin digestion and that targeting of food-vacuolar
plasmepsins is not sufficient to kill the malaria parasites.5 In our
search for a new anti-malarial drug, compounds of type 3 (Fig. 2)
were screened with the [3H]hypoxanthine incorporation assay6
IC50 NF54 alb 72 h = 43 nM
left hand-side
acid part
right hand-side
amine part
Figure 2. Hydroxy-ethyl-amine scaffold based anti-malarials.
for the inhibition of parasite growth in red blood cells (RBC) in-
fected with the P. falciparum NF54 strain (chloroquine-sensitive
strain). The initial hit compound 4 (Fig. 2) showed promising activ-
ity against the malaria parasite with an IC50 value of 43 nM and
possesses a lower molecular weight than the previously described
⇑
Corresponding author. Tel.: +41 61 565 65 61; fax: +41 61 565 65 00.
E-mail address: christoph.boss@actelion.com (C. Boss).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.11.118

C.-L. Ciana et al. / Bioorg. Med. Chem. Lett. 23 (2013) 658–662
659
hydroxy-ethyl-amine plasmepsin inhibitors. This chemotype was
also described in the TCAMS dataset released by GSK during the
O
O
course of this work.7
R
N
N
H
N
H
As a starting point 4 displayed a good in vitro activity against P.
falciparum in the RBC assay, therefore we set out to optimize its
activity against the parasite as well as its physico-chemical
properties.
OH
13
Figure 4. Structure of compounds in Table 1.
Two synthetic routes were developed to access the compounds
of class 3 Figure 3. The first one allowed easy modification of the
right hand-side of the molecule while the second route permitted
variation of the left hand-side. Route 1 started with the homologa-
tion of Boc-Phe with nitromethane. After reduction of the ketone,
recrystallization afforded the desired diastereomer 6.8 Subsequent
Boc-deprotection resulted in amine 7. At this point different acids
were introduced by standard amide coupling reactions. The nitro-
group was subsequently hydrogenated and the right hand-side
substituent introduced by reductive amination affording the de-
sired hydroxy-ethyl-amine compounds 3.9 In route 2, epoxide
opening of 1010 afforded Boc-protected amine 11. The target com-
pounds of structure 3 were obtained by Boc-deprotection followed
by amide-coupling with carboxylic acid derivatives.
All compounds were tested for inhibition of parasite growth in
red blood cells (RBC) infected with the P. falciparum NF54 parasite
strain which was cultured in vitro according to Trager and Jensen.6
IC50 values were determined by measuring incorporation of the nu-
cleic acid precursor [3H]hypoxanthine after 72 h of incuba-
tion.11a,11b IC50 values were also determined in the presence of
50% serum (no albumax), since a large activity shift between the
assay with 0.5% albumax and the assay with 50% serum has previ-
ously led to inactive compounds in vivo, most likely due to high
protein binding.
We first optimized the right hand-side of the molecule by
replacing the phenyl-propylamine substituent with a range of
amines (Fig. 4 and Table 1). Both alkyl and aromatic substituents
were tolerated. The best alkyl substituent was a cyclopentyl (18)
with an IC50 in the NF54 albumax assay of 7.1 nM. Reducing the
chain length of the original phenylpropylamine in 4 resulted in a
loss of activity (22–24). However, introducing one (26 and 27) or
two methyl groups (25) at the benzylic position of 24 led to very
active compounds with IC50 values below 10 nM. The chirality of
the methyl group was important, with the R-epimer 26 being more
active. Substituents were then introduced to the aromatic ring of
25 and it was found that meta-substitution resulted in the most ac-
tive compounds, for example derivative 29 which has an IC50 be-
low 1.5 nM.
Removal of the OH group (44) or replacing it with two fluorine
atoms (46) resulted in inactive compounds as did exchanging the
amino-acid core to valine (47).
Having identified active substituents for the right hand-side of
the molecule, we turned our attention to the modification of the
left hand-side of the molecule (Table 2 and Fig. 6). Compound 26
was taken as starting point. First, the position of the amide substi-
tuent was varied. The para-isomer 49a was also highly active with
an IC50 value of 1.6 nM. The ortho-isomer 49b was inactive against
the parasite with an IC50 >500 nM. Changing the N-substituent of
the amide slightly improved the activity (49c and 49f) with IC50
values below 1 nM. Compound 49i suggests that both alkyl chains
are necessary for high activity. To further investigate the influence
of the left-hand side of the molecule on the anti-malarial activity,
the amide group of 48 was replaced by heterocycles and alkyl sub-
stitutents (Table 1, Supplementary data). Unfortunately this re-
sulted in
a loss of activity against the malaria parasite.
Introducing an N-methyl-piperazine unit as the R group led to
compound 49j with an IC50 below 10 nM. Further modifications al-
lowed us to identify 4-amino-piperidine as in 49m, n and 4-amino-
pyridine as in 49o–r as active substituents.
In order to prioritize candidates for further investigations, the
in vitro metabolic stability of the compounds was evaluated using
a mouse liver microsomal preparation.12 Unfortunately, highly ac-
tive compounds where R is an amide group (compounds 26, 49a
and 49f) mostly showed only moderate in vitro metabolic stability,
which may be a potential indicator for high in vivo clearance and
for a non-optimal pharmacokinetic profile. However, 49c together
with 49j, 49m, n and 49p had better in vitro metabolic stability
(below 100 ll/(min mg)).
A fast onset of action is highly desirable for an anti-malarial
drug as it suggests that it is active against all three asexual eryth-
rocytic stages of the parasite. The widely used artemisinins for
example exhibit a fast onset of action.13 In view of selecting com-
pounds for in vivo testing, this property was assessed by measur-
ing the IC50 values of the compounds against the parasite after
an incubation time of 24, 48 and 72 h. Together with the evaluation
of the onset of action, the potency of the compounds against the
rodent parasite Plasmodium berghei was as well measured, being
a prerequisite to test the compounds in vivo in the P. berghei
The phenyl-alanine core was studied, too (Fig. 5). The OH group
was found to be crucial for the activity of the compounds against
the parasite. Indeed, the S,S-diastereomer 42 was totally inactive.
A. Route 1
O
a.
RCOOH
HCl 4 M
Pd/C
R1
R2
Ti(OiPr)4, 65°C, 16 h
NH2
b. NaBH4, r.t. 3 h
NH4OOCH
HATU, NEM
in dioxane
ref 8
O
O
O
R1
OH
DMSO, r.t., 16 h
MeOH
50°C, 16 h
DCM, r.t., 20 h
NO2
R
N
H
NO2
R
N
H
R
N
H
N
H
R2
BocHN
BocHN
H2N
HCl
NO2
OH
OH
OH
O
OH
OH
5
6
7
8
9
3
B. Route 2
R1NH2 , MeOH
65°C, 48 h
HCl 4 M in dioxane
DCM, r.t., 16 h
R2COOH
O
R
R1
R1
HATU, NEM
DMF, r.t., 16 h
R2
N
H
N
BocHN
N
H
H2N
HCl
N
BocHN
H
H
O
OH
11
OH
12
OH
10
3
Figure 3. (A) Route 1: synthesis of hydroxy-ethyl-amine compounds with variations on the left hand-side. (B) Route 2: synthesis of hydroxy-ethyl-amine compounds with
variations on the right hand-side.

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C.-L. Ciana et al. / Bioorg. Med. Chem. Lett. 23 (2013) 658–662
Table 1
In vitro anti-malarial activity of hydroxy-ethyl-amine compounds of structure 13: optimization of the amine part (IC50 values are the mean of at least three independent
experiments)
O
R
NH
=
N
H
N
H
N
H
N
H
N
H
18
7.1 nM
15 nM
N
H
N
H
14
24 nM
66 nM
15
16
17
19
20
78 nM
14 nM
28 nM
16 nM
34 nM
11 nM
48 nM
324 nM
466 nM
IC50 NF54 alb
IC50 NF54 ser
=
=
150 nM
N
N
H
N
N
H
N
H
N
R
H
N
H
N
H
=
H
N
H
21
22
47 nM
126 nM
23
186 nM
-
24
> 500 nM
-
25
7.6 nM
40 nM
26
27
21 nM
-
IC50 NF54 alb (nM) =
IC50 NF54 ser (nM) =
>500 nM
>500 nM
2.9 nM
15 nM
F
OMe
CN
F
N
H
N
H
N
H
N
H
N
H
N
H
N
R
=
H
N
H
F
OMe
CN
28
55 nM
188 nM
29
< 1.5 nM
8.6 nM
30
31
9.6 nM
50 nM
32
24 nM
110 nM
33
< 7.8 nM
25 nM
34
13 nM
44 nM
IC50 NF54 alb (nM) =
IC50 NF54 ser (nM) =
21 nM
142 nM
N
N
HN
N
N
N
N
H
N
H
H
N
H
R
=
O
N
H
35
116 nM
435 nM
36
45 nM
143 nM
37
48 nM
-
38
11 nM
-
39
12 nM
168 nM
40
> 500 nM
-
41
> 500 nM
-
IC50 NF54 alb (nM) =
IC50 NF54 ser (nM) =
O
O
nPr2N
R1
R1
(CH2)3Ph
(CH2)3Ph
R2
R1
=
N
H
N
N
H
N
H
N
H
N
H
H
F
F
OH
OH
4
42
46
IC50 NF54 alb =44 nM
IC50 NF54 ser =126 nM
IC50 NF54 alb> 500 nM
IC50 NF54 ser > 500 nM
IC50 NF54 alb > 500 nM
IC50 NF54 ser > 500 nM
R2
N
N
H
O
H
OH
45
R2
=
IC50 NF54 alb =81 nM
IC50 NF54 ser = 75 nM
R1
R1
R2
N
N
H
N
H
N
H
N
H
N
H
N
H
N
OH
15
OH
47
44
IC50 NF54 alb =78 nM
IC50 NF54 ser = 150 nM
IC50 NF54 alb> 500 nM
IC50 NF54 ser > 500 nM
IC50 NF54 alb > 500 nM
IC50 NF54 ser > 500 nM
Figure 5. Variation of the core of hydroxy-ethyl-amine compounds.
Table 2
In vitro anti-malarial activity of hydroxy-ethyl-amine compounds: optimization of the acid part
Entry
Compound
R
IC50 NF54 alb
72 h (nM)
IC50 NF54 ser
72 h (nM)
IC50 NF54 alb
24 h (nM)
IC50 NF54 alb
48 h (nM)
IC50 P. berghei
24 h (nM)
MLM (ll/
(min mg))
1
2
3
4
5
6
7
8
26
3-CONnPr2
2.0
1.6
>500
0.6
98
10
>500
>500
—
>500
—
—
>500
—
—
—
>500
—
—
>500
>500
198
173
169
249
<3.1
—
—
—
—
—
—
—
—
—
—
—
—
<7.8
<7.8
11
—
26
27
>500
>500
—
>500
—
—
>500
—
—
—
>500
—
—
—
—
329
228
307
>500
>1250
>1250
—
75
—
908
860
—
—
—
80
—
—
76
71
—
69
112
180
49a
49b
49c
49d
49e
49f
49g
49h
49i
49j
49k
49l
49m
49n
49o
49p
49q
49r
4-CONnPr2
6.5
>500
<0.6
102
138
1.3
9.3
13
2-CONnPr2
4-CO-Me-piperazine
3-CO-Me-piperazine
3-SO2-Me-piperazine
4-CO-piperidine
3-CO-pyrolidine
3-CO-azepane
98
0.9
4.9
3.8
12
8.7
190
21
9
10
11
12
13
14
15
16
17
18
19
3-CONHnPr
30
4-Me-piperazine
3-Me-piperazine
4-Bn-piperazine
4-NH-Me-piperidine
4-NMe-Me-piperidine
4-NH-2,6-diMe-pyridine
4-NH-2-Me-pyridine
4-NH-pyridine
8.5
300
39
12
14
23
29
36
35
4
4.4
8.7
9.8
20
4-NMe-pyridine
21

C.-L. Ciana et al. / Bioorg. Med. Chem. Lett. 23 (2013) 658–662
661
dose of 100 mg/kg did not result in a significant reduction of para-
sitemia (33% of control) and mouse survival time was similar to
that of non-treated animals. Despite low nM anti-parasitic activity
and reasonable metabolic stability in mice, this series of com-
pounds exhibited poor behavior in in vivo experimental settings,
possibly due to the slow onset of action.
We expected that improving the speed of action of this series
would permit to obtain an efficient anti-malarial drug. Inspired
by the work of Meunier16a and O’Neill and co-workers16b on hybrid
anti-malarial drugs in which two pharmacophores were combined
to obtain molecules exhibiting both modes of action, we set out to
introduce a Mannich base pharmacophore into our compounds. We
hoped to improve the speed of action by combining the mechanism
of the Mannich base pharmacophore with the mechanism of the
hydroxy-ethyl-amine pharmacophore. The Mannich base motif is
part of the amodiaquine 57 pharmacophore, a drug used for the
treatment of acute malaria, and was also used by O’Neill and
co-workers16b in hybrid anti-malarial 58 (Fig. 8).
O
N
H
N
H
R
OH
48
Figure 6. Structure of compounds in Table 2.
In vitro properties
IC50 NF54 alb 72 h = 9.8 nM
IC50 NF54 ser 72 h = 29 nM
IC50 NF54 24 h = 173 nM
IC50 P.berghei 24 h = 228 nM
O
N
N
H
N
H
MLM =
69 µl/min.mg
OH
49p
N
H
In vivo properties
Dose: 1x100 mg/kg
33% reduction of parasitemia
Survival days : 3 days
Figure 7. In vivo activity of 49p in the P. berghei infected mouse model.
The Mannich base pharmacophore was introduced into our
compounds as depicted in Table 3. Replacing the phenethyl-amine
by a Mannich base led to slightly less active compounds (50a–52d)
however for all compounds no difference in activity was detected
for 24–72 h of incubation, confirming the hypothesis that combin-
ing the hydroxy-ethyl-amine scaffold and a Mannich base pharma-
cophore accelerates the onset of action. For example, compound
50a exhibited an IC50 below 20 nM against P. falciparum NF54 at
all time points. It was also active against the rodent parasite with
an IC50 of 143 nM. Moreover, the introduction of the Mannich base
pharmacophore did not affect the in vitro metabolic stability. In
contrast to previous observations, the presence of a methyl group
at the benzylic position did not have a dramatic effect on the activ-
ity against NF54 (e.g. 51b vs 52d). The nature of the R group on the
infected mouse model of malaria. The IC50 against P. berghei was
measured after 24 h of incubation (IC50 P. berghei 24 h).14 Unfortu-
nately, most compounds were inactive against P. falciparum as well
as P. berghei after 24 h of incubation time. This seems to be a major
drawback as it suggests that many of our compounds suffer from a
slow onset of action. However, compounds bearing a 4-amino-pyr-
idine 49o–r showed efficacy against the parasite after 24 h of incu-
bation time albeit with a significant shift between the IC50 at 24 h
and the IC50 at 48 and 72 h.
Compound 49p (Fig. 7) which exhibited the best combination of
activities so far and the best in vitro metabolic stability was tested
for efficacy in the P. berghei infected mouse model.15 A single oral
Table 3
In vitro anti-malarial activity and in vitro metabolic stability of hybrid compounds
OH
O
O
O
NR2
N
H
N
H
N
H
N
H
N
H
N
H
OH
OH
52
OH
50
OH
OH
N
N
N
N
N
N
51
R2N
R2N
O
NR2
R2N
HO
O
NR2 OH
O
O
N
H
N
H
HO
N
H
N
H
N
H
N
H
N
H
N
H
OH
53
HO
OH
OH
OH
54
N
H
N
H
NR2
55
56
Entry
Compound
NR2
IC50 NF54 alb
72 h (nM)
IC50 NF54 ser
72 h (nM)
IC50 NF54 alb
24 h (nM)
IC50 NF54 alb
48 h (nM)
IC50 P.berghei
24 h (nM)
MLM (ll/
(min mg))
1
2
3
4
5
6
7
50a
51a
51b
52a
52b
52c
52d
NnPr2
19
203
89
>500
333
120
65
—
125
62
295
167
57
20
—
90
>500
408
158
69
19
—
75
>500
314
102
50
143
—
—
—
—
74
—
—
—
—
Piperidine
NnPr2
Pyrolidine
NEt2
Piperidine
—
140
—
90
NnPr2
42
8
9
10
11
53a
53b
53c
54a
NEt2
357
118
261
4.9
>500
334
386
5.9
380
151
—
305
83
—
—
>500
—
—
—
—
—
NnPr2
Pyrolidine
Piperidine
235
<7.8
>500
12
13
14
15
16
17
55a
55b
55c
56a
56b
56c
NEt2
8.3
9.4
17
<7.8
22
3.6
13
4.9
9
20
3.3
133
77
174
65
77
25
<7.8
11
<7.8
15
—
<7.8
—
—
—
—
—
172
—
—
—
—
—
415
NnPr2
Pyrolidine
NEt2
NnPr2
Piperidine
3.9

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C.-L. Ciana et al. / Bioorg. Med. Chem. Lett. 23 (2013) 658–662
OH
mouse model.15 The activity of the compound was measured after
OH
a single oral dose of 1 Â 100 mg/kg. Compound 52d showed a
reduction of 87% of parasitemia compared to the non-treated mice
(Fig. 9). Taking into account the modest IC50 of 52d against P. berg-
hei, this result confirms the potential of this hybrid series in the
quest for new anti-malarials.
HN
O
O
O
NEt2
NEt2
O
Cl
N
Hybrid compound 5816b
57
Amodiaquine
In conclusion, we have developed a new series of compounds
based on a hydroxy-ethyl-amine scaffold. The compounds are
highly potent against P. falciparum parasites in the 72 h assay,
show only a low reduction in activity upon addition of plasma to
the assay medium and the potential for low in vivo clearance.
The incorporation of a Mannich base pharmacophore resulted in
improved speed of action. Further results will be reported in due
course.
Figure 8. Anti-malarial compounds containing a Mannich base pharmacophore.
IC50 NF54 alb 72 h = 65 nM
IC50 NF54 ser 72 h = 42 nM
IC50 NF54 24 h =
IC50 K1 alb 72h =
IC50 K1 ser 72h =
69 nM
13 nM
9.5 nM
N
O
N
H
N
H
IC50 P.Berghei 24 h = 140 nM
OH
MLM =
90 µl/min.mg
N
OH
52d
N
Dose: 1x100 mg/kg po
87% reduction of parasitemia
7 days of survivals
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.
11.118.
Figure 9. In vivo activity of 52d in the P. berghei infected mouse model.
benzyl amine had however a noticeable effect on the IC50, going
from an inactive compound 52a with a pyrrolidine as a side chain
to 52d with an IC50 NF54 alb 72 h = 65 nM bearing a dipropylamine
side chain. In addition compound 52d showed activity against P.
References and notes
1. World Malaria Report 2011, WHO, 2011.
2. Global Report on Antimalarial Efficacy and Drug Resistance 2000–2010, WHO,
2010.
berghei and an in vitro metabolic stability below 100 ll/(min mg).
3. For examples see: (a) Liu, K.; Shi, H.; Xiao, H.; Chong, A. G. L.; Bi, X.; Chang, Y.-
T.; Tan, K. S. W.; Yada, R. Y.; Yao, S. Q. Angew. Chem., Int. Ed. 2009, 48, 8293; (b)
Haque, T. S.; Skillman, A. G.; Lee, C. E.; Habashita, H.; Gluzman, I. Y.; Ewing, T. J.
A.; Goldberg, D. E.; Kuntz, Irwin D.; Ellman, J. A. J. Med. Chem. 1999, 2, 1428; (c)
Noeteberg, D.; Hamelink, E.; Hulten, J.; Wahlgren, M.; Vrang, L.; Samuelsson, B.;
Hallberg, A. J. Med. Chem. 2003, 46, 734.
4. (a) Berry, C. Curr. Opin. Drug Disc. Dev. 2000, 3, 624; (b) Boss, C.; Richard-
Bildstein, S.; Weller, T.; Fischli, W.; Meyer, S.; Binkert, C. Curr. Med. Chem. 2003,
10, 883.
The replacement of the N,N-dipropylbenzamide in 13 by a Man-
nich base resulted in poorly active compounds 53a–c. Changing the
substitution pattern of the Mannich base significantly improved the
IC50 in the 72 h assay. However, despite the presence of the Man-
nich base pharmacophore, compound 54a showed a considerable
shift between the 24 and the 72 h assay.
The toxicity of amodiaquine 57 (Fig. 8) is believed to come from
the 4-hydroxyanilino moiety, which can be oxidized by enzymes to
the quinoneimine. Nucleophilic addition of proteins to this reactive
intermediate can occur, affecting cellular function.17 Placing the
OH group meta to the nitrogen would block this oxidation pathway
and solve the potential toxicity problem. Therefore, in case the pyr-
idine was replaced by a Mannich base, two different substitution
patterns were investigated using 4-hydroxy-aniline (55a–c) and
3-hydroxy-aniline (56a–c). Both substitution patterns resulted in
compounds with low nM activity in the 72 h assay against P. falci-
parum. Despite significant improvement in the activity of these hy-
brid compounds compared to the original series in the 24 h assay,
they still exhibited a difference between their IC50 after 24 and
72 h of incubation.
To assess the potency towards a resistant strain of P. falciparum,
the IC50 values of a set of compounds was determined against the P.
falciparum K1 strain (chloroquine-resistant). All compounds mea-
sured did not show an activity difference between the NF54 and
the K1 strain (Table 2, Supplementary data). This series therefore
has the potential to be active against chloroquine resistant
parasites.
5. Liu, J.; Istvan, Eva S.; Gluzman, I. Y.; Gross, J.; Daniel, E.; Goldberg, D. E. Proc.
Natl. Acad. Sci. U.S.A. 2006, 103, 8840.
6. Trager, W.; Jensen, J. B. Science 1976, 193, 673.
7. Gamo, F.-J.; Sanz, L. M.; Vidal, J.; de Cozar, C.; Alvarez, E.; Lavandera, J.-L.;
Vanderwall, D. E.; Green, D. V. S.; Kumar, V.; Hasan, S.; Brown, J. R.; Peishoff, C.
E.; Cardon, L. R.; Garcia-Bustos, J. F. Nature 2010, 465, 305.
8. Yuasa, Y.; Yuasa, Y.; Tsuruta, H. Synth. Commun. 1998, 28, 395.
9. Dei, S.; Teodori, E.; Garnier-Suillerot, A.; Gualtieri, F.; Scapecchi, S.; Budriesi, R.;
Chiarini, A. Bioorg. Med. Chem. 2001, 9, 2673.
10. Synthesized according to: Wang, D.; Nugent, W. A. Org. Synth. 2007, 84, 58. or
commercially available.
11. (a) Desjardins, R. E.; Canfield, C. J.; Haynes, J. D.; Chulay, J. D. Antimicrob. Agents
Chemother. 1979, 16, 710; (b) Snyder, C.; Chollet, J.; Santo-Tomas, J.; Scheurer,
C.; Wittlin, S. Exp. Parasitol. 2007, 115, 296.
12. Obach, R. S.; Baxter, J. G.; Liston, T. E.; Silber, B. M.; Jones, B. C.; MacIntyre, F.;
Rance, D. J.; Wastall, P. J. Pharmacol. Exp. Ther. 1997, 283, 46.
13. White, N. J. Science 2008, 320, 330.
14. For method see: Brunner, R.; Aissaoui, H.; Boss, C.; Bozdech, Z.; Brun, R.;
Corminboeuf, O.; Delahaye, S.; Fischli, C.; Heidmann, B.; Kaiser, M.; Kamber, J.;
Meyer, S.; Papastogiannidis, P.; Siegrist, R.; Voss, T.; Welford, R.; Wittlin, S.;
Binkert, C. J. Infect. Dis. 2012, 206, 735.
15. For methods see Ref. 14.
16. (a) Meunier, B. Acc. Chem. Res. 2008, 41, 69; (b) Chadwick, J.; Amewu, R. K.;
Marti, F.; Bousejra-El Garah, F.; Sharma, R.; Berry, N. G.; Stocks, P. A.; Burrell-
Saward, H.; Wittlin, S.; Rottmann, M.; Brun, R.; Taramelli, D.; Parapini, S.;
Ward, S. A.; O’Neill, P. M. ChemMedChem 2011, 6, 1357.
17. (a) Maggs, J. L.; Kitteringham, N. R.; Park, B. K. Biochem. Pharmacol. 1988, 37,
303; (b) Harrison, A. C.; Kitteringham, N. R.; Clarke, J. B.; Park, B. K. Biochem.
Pharmacol. 1992, 43, 1421.
In order to evaluate the potential of the hybrid series in vivo,
compound 52d was selected for testing in the P. berghei infected