Exploiting a basic chemosensitizing pharmacophore hypothesis. Part 1: Synthesis and biological evaluation of novel arylbromide and bicyclic chemosensitizers against drug-resistant malaria parasites

Article abstract of DOI:10.1016/j.bmcl.2005.04.033

An exploratory series of novel arylbromide and bicyclic chemosensitizers (modulators) against chloroquine-resistant Plasmodium falciparum were designed and synthesized on the basis of a basic chemosensitizing pharmacophore hypothesis in malaria. ortho-Substituted bromo and biphenyl ether compounds displayed the best activity from the series.

Full text of DOI:10.1016/j.bmcl.2005.04.033

Bioorganic & Medicinal Chemistry Letters 15 (2005) 3024–3028
Exploiting a basic chemosensitizing pharmacophore hypothesis.
Part 1: Synthesis and biological evaluation of novel arylbromide
and bicyclic chemosensitizers against
drug-resistant malaria parasites
Franck Chouteau,a David Ramanitrahasimbola,b Philippe Rasoanaivob,* and
Kelly Chibalea,*
aDepartment of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
b
´
´
Laboratoire de Pharmacognosie Appliquee aux Maladies Infectieuses, Institut Malgache de Recherches Appliquees,
B.P. 3833, 101-Antananarivo, Madagascar
Received 4 December 2004; revised 19 April 2005; accepted 19 April 2005
This paper is dedicated to Professor Iwao Ojima on the occassion of his 60th birthday
Abstract—An exploratory series of novel arylbromide and bicyclic chemosensitizers (modulators) against chloroquine-resistant
Plasmodium falciparum were designed and synthesized on the basis of a basic chemosensitizing pharmacophore hypothesis in
malaria. ortho-Substituted bromo and biphenyl ether compounds displayed the best activity from the series.
Ó 2005 Elsevier Ltd. All rights reserved.
Multi-drug resistant strains of the causative agent of
malaria Plasmodium falciparum have rendered previ-
ously effective drugs such as chloroquine (CQ), once
the mainstay of malaria prophylaxis and chemotherapy,
essentially useless and malaria impossible to treat in cer-
tain areas.1 Drug combination therapy is an attractive
option aimed at prolonging the usefulness of CQ espe-
cially in poor countries.2 Within this context, the use
of reversing agents or chemosensitizers is a potential
new and effective treatment strategy.3 However, this
strategy has received little attention especially following
the failure of initial in vivo experiments with desipr-
amine.4 In spite of this, it is noteworthy that chlorphen-
iramine was reported to show clinical effectiveness in
reversing CQ resistance in West Africa.5 Moreover,
recent in vitro and in vivo data have provided additional
new impetus for research in this area as we march
toward the realistic possibility of potent clinically useful
CQ resistance reversal agents.6,7
In dissecting the structures of malagashanine,8 a natu-
rally occurring CQ chemosensitizer, and some tricyclic
antidepressants3 exemplified by chloropromazine, we
identified the N-phenyl-1,3-diamino-propane as a com-
mon unit. Likewise, the 1-phenyl-4-amino unit was also
identified in verapamil and some synthetic chemosensi-
tizers exemplified by amitriptyline3 (Fig. 1).
Reduction of the benzene ring of the indoline group of
malagashanine did not affect the activity of the parent
Keywords: Malaria; Chloroquine; Chemosensitizers.
*
Corresponding authors. Tel./fax: +261 20 22 304 70 (P.R.); tel.: +27
21 650 2553; fax: +27 21 689 7499 (K.C.); e-mail addresses:
rafita@wanadoo.mg; chibale@science.uct.ac.za
Figure 1. The N-phenyl-1,3-diamino-propane and 1-phenyl-4-amino
unit as a common unit in the three structures.
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.04.033

F. Chouteau et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3024–3028
3025
compound,9 indicating that this functional group was
not directly involved in the chemosensitizing activity
of the concerned indole alkaloid. This led us to addition-
ally propose a 1,4-diamino unit as another bioactive
unit, which is also found in chlorpheniramine (Fig. 2).
arylbromides (1), biphenyl (2, 3), and biaryl ether (4)
chloroquine potentiating agents (Fig. 4). The biphenyl
and biaryl ether aromatic templates present in these
compounds were selected for exploratory studies due
to their ease of synthesis and amenability of the chemis-
try (Suzuki palladium-catalyzed and copper-mediated,
respectively) to chemical library generation. This is ex-
pected to facilitate structure–activity relationship stud-
ies. Results described in this paper give the first
confirmation of the proposed hypothesis, thus providing
a preliminary clue for further design and synthesis.
These observations led us to postulate a unifying chemo-
sensitizing pharmacophore in malaria as outlined in Fig-
ure 3.10 Previously, we and others have unwittingly
exploited this basic chemoreversal pharmacophore
hypothesis in the recent design and synthesis of pheno-
thiazine- and xanthene-based chloroquine potentiating
agents.11,12
Schemes 1 and 2 depict the straightforward synthesis of
target compounds 1–4 from commercially available di-
bromo compounds. The synthesis starts with a Suzuki13
cross-coupling of a large excess of the respective o-, m-,
and p-dibromobenzenes 5 with phenylboronic acid to
deliver a small amount (7–12%) of the triphenyl by-
products 6 along with the target intermediate biphenyl
bromides 7 in moderate to good yield. Finally, palla-
dium-catalyzed arylamination of dibromobenzenes 5
and biphenyl bromide 7 afforded the respective target
compounds in moderate yield. The alternative route of
In an ongoing and continuing programme aimed at val-
idating the aforementioned hypothesis, we report the
synthesis and biological evaluation of exploratory novel
Figure 2. The 1,4-diamino unit in hexahydro-malagashanine and
chlorpheniramine.
Scheme 1. Synthesis of compounds 1 and 2. Reagents and conditions:
(a) (i) 1.2 equiv of PhB(OH)2, n-propanol, rt, 15 min; (ii) 0.003 equiv of
Pd(OAc)2, 0.01 equiv of Ph3P, 2.0 equiv of Na2CO3, H2O, reflux, 6 (7–
12%) and 7 (58–67%); (b) 1.0 equiv of H2N(CH2)3NEt2, 2.0 equiv of
NaOtBu, 2 mol % of Pd2(dba)3, 6 mol % of BINAP, dioxane, reflux,
18 h, 50–58%.
Figure 3. Proposed unifying chemosensitizing pharmacophore in
malaria.
Figure 4. Structures of new bicyclic chloroquine potentiating agents.
Scheme 2. Synthesis of compounds 3 and 4. Reagents and conditions: (a) (i) 1.2 equiv of PhB(OH)2, n-propanol, rt, 15 min; (ii) 0.003 equiv of
Pd(OAc)2, 0.01 equiv of Ph3P, 2.0 equiv of Na2CO3, H2O, reflux, 90–98%; (b) (i) 1.2 equiv of NaH, 0 °C to rt, 30 min; (ii) 2.0 equiv of 1,4-
dibromobutane, DMF, 0 °C, 18 h; (c) 2.0 equiv of pyrrolidine, CH3CN, 18 h, 82–90%; (d) (i) 2.2 equiv of PhB(OH)2, 1.5 equiv of Cu(OAc)2,
2.5 equiv of pyridine, CH2Cl2, rt, 18 h 89%; (e) 1.0 equiv of H2N(CH2)3NEt2, 2.0 equiv of NaOtBu, 2 mol % of Pd2(dba)3, 6 mol % of BINAP,
dioxane, reflux, 18 h, 50–54%.

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F. Chouteau et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3024–3028
Table 1. In vitro antiplasmodial activity and chloroquine-enhancing
action of test compounds against the chloroquine-resistant Plasmo-
dium falciparum strain FCM29
amination followed by Suzuki coupling resulted in much
lower yields of the final target compounds.
Concentrations
of test compounds
(lg/ml)
IC50/IC90
Chloroquine activity (nM)
Compounds 2 were synthesized from the o-, m-, and p-
bromophenols 8 as shown in Scheme 2. The Suzuki
cross-coupling reaction of phenyl boronic acid with 8
proceeded in excellent yields (90–98%) of biphenyl 9,
which upon alkylation with 1,4-dibromobutane in the
presence of sodium hydride in DMF at 50 °C, followed
by reaction of intermediate bromides 10 with pyrrolidine
gave the final compounds in generally high yields of 82–
90%. On the other hand, the synthesis of biarylether 4
was achieved via Cu(OAc)2-mediated14,15 arylation of
p-bromophenol with phenyl boronic acid in the presence
of pyridine to afford 10 in 89% yield. Arylamination
with 3-(diethylamino)-propylamine led to 4 in yield of
52%.
IC50
IC90
1-para
6.1 2.2/
66.7 9.8
00a
0.5
160.9 10.1
145.5 22.1
95.9 8.7
69.3 8.4
28.7 6.1
842.8 174.7
745.3 91.4
362.5 84.7
271.5 61.6
273.1 50.3
1
2
4
2-para
2.9 1.8/
18.5 2.5
0.375
0.75
1.5
98.2 21.0 367.6 42.1
54.0 6.2
25.9 8.1
198.1 17.5
113.7 20.5
1-ortho
3.7 0.4/
57.4 10.5
It is noteworthy that although synthesized, compounds
7, 9, and 11 are commercially available.
1
1.5
58.2 12.4
40.3 7.9
28.8 4.3
6.1 0.7
290.3 42.6
194.9 31.7
134.0 15.6
153.4 14.7
All new compounds gave 1H NMR, FAB-MS, and pur-
ity consistent with their structures.
2
3
2-ortho
3.1 0.9/
94.6 19.2
Both the precursor bromo compounds (1-para, 1-meta,
and 1-ortho) and the target bicyclic compounds 2–4 were
screened for intrinsic antiplasmodial activity and CQ-
potentiating effect using the CQ-resistant P. falciparum
FCM29, and the results are shown in Table 1. The in
vitro antiplasmodial and CQ-enhancing tests based on
the inhibition of [3H]-hypoxanthine uptake by P. falci-
parum cultured in human blood were conducted as
previously described.16
0.62
1.25
138.4 11.5
72.2 10.2
11.5 2.0
820.0 60.7
400.2 71.2
372.7 20.3
2.50
2-meta
2.3 0.7/
20.8 7.9
0.68
1.36
159.8 15.7
108.8 12.5
780.8 104.1
630.4 97.8
1-meta
7.6 0.9/
80.3 11.0
0.62
1.25
2.5
109.9 13.4
68.5 5.6
38.5 4.8
7.2 0.9
808.0 101.0
391.7 57.2
442.7 50.8
210.0 40.7
Based on the IC50 values presented in Table 1, most
compounds generally displayed moderate (1 < IC50
<
5
3-para
5 lg/ml) to weak (IC50 > 5 lg/ml) antiplasmodial activ-
ity, which may be attributed to the side chain basic ter-
minal nitrogen. Compounds 2-para, 2-meta, 3-para, and
3-ortho having both low IC50 and IC90 values were com-
paratively the most active. The nitrogen presumably al-
lows accumulation of the compounds within the acidic
compartments of the parasite via the weak base effect.
Concerning the chloroquine resistance modulation ef-
fects (Table 1), Figure 5A–C shows isobologrammes of
in vitro interaction between test compounds and chloro-
quine against the chloroquine-resistant strain FCM29.
Regarding the shape of the curves, if the isobologramme
graph is a line following roughly the diagonal, it repre-
sents an additive effect of the two drugs. If the graph
forms a concave curve below the diagonal, it indicates
synergy while a curve above the diagonal indicates
antagonism between the two drugs. Thus compounds 1
(ortho), 1 (meta), 1 (para), 2 (para), 3 (ortho), and 4 dis-
played synergistic effects since they exhibited concave
curves. Since the more concave the curve exhibited, the
more effective the chemosensitizer, compounds 1 (meta),
1 (para), and 3 (ortho) displayed the best chemosensiti-
zing effects. The brominated compounds showed chemo-
sensitizing activities, but the substitution of the bromine
did not significantly improve the activity. Noteworthy
was compound 3-para having the most antiplasmodial
activity, which exhibited simple additive effects in drug
1.7 0.9/
24.4 6.3
0.3
0.6
169.3 21.0
101.4 18.7
655.3 87.4
310.8 56.4
3-ortho
3.3 1.3/
22.3 2.7
1
1.5
53.9 8.7
42.9 3.5
7.6 2.3
3.0 0.6
534.4 71.3
301.4 49.2
190.6 23.7
157.3 21.8
2
3
3-meta
3.1 1.3/
49.5 8.7
1
2
4
118.6 27.1
77.7 13.0
744.7 97.6
556.1 101.0
3.1 0.7/
73.2 3.2
0.5
1
151.9 19.1
100.9 13.4
70.9 8.7
736.4 97.4
254.9 81.3
205.4 40.3
2
a For each compound, control plate with chloroquine alone was made
and the mean was calculated as IC50 = 160.9 10.1 nM and IC90
842.8 174.7 nM.
combination. This observation is in agreement with
our previous work.17 Remarkably, isobologramme
curves (Fig. 5C) depict the shift of the activity from syn-
ergistic, additive to slight antagonistic effect according

F. Chouteau et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3024–3028
3027
to the relative position, ortho, meta, para, of the benzene
ring of the biphenyl ether compounds 3.
In conclusion, we have provided the first confirmation
of the proposed chemoreversal hypothesis for drug resis-
tant malaria parasites, thus providing a preliminary clue
for further design and synthesis. The Suzuki and copper-
catalyzed chemistry utilized to construct the biphenyl
biaryl ether templates, respectively, is amenable to
chemical library generation due to the commercial avail-
ability of structurally diverse boronic acid monomers,
which can also be readily synthesized. This is expected
to facilitate structure–activity relationship studies to fur-
ther validate our working hypothesis. Work in this con-
text is currently underway in our laboratories.
Acknowledgments
This material is based upon work supported by the
National Research Foundation of South Africa under
Grant number 2053362 (KC).
Supplementary data
Supplementary data associated with this article can be
found in the online version at doi:10.1016/j.bmcl.2005.
04.033.
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Figure 5. Isobologrammes of in vitro interaction between test
compounds and chloroquine against the chloroquine-resistant strain
FCM29. (A): 1-para (s), 1-ortho (j), 1-meta ( ); (B): 2-para (m), 2-
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