Meclonazepam analogues as potential new antihelmintic agents

Article abstract of DOI:10.1016/j.bmcl.2008.02.077

New analogues of the potent antihelmintic meclonazepam were prepared and evaluated against Schistosoma mansoni. The biological data suggests substitution at positions 2 and 4 of meclonazepam could provide promising analogues for prophylactic and therapeut

Full text of DOI:10.1016/j.bmcl.2008.02.077

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Bioorganic & Medicinal Chemistry Letters 18 (2008) 2333–2336
Meclonazepam analogues as potential new antihelmintic agents
Aman Mahajan,a Vipan Kumar,a Nuha R. N. Mansour,b
Quentin Bickleb and Kelly Chibalea,c,
*
aDepartment of Chemistry, University of Cape Town, Private Bag, Rondebosh-7701, South Africa
bDepartment of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, KeppelStreet,
London WC1E7HT, UK
cInstitute of Infectious Disease and Molecular Medicine, University of Cape Town, Private Bag, Rondebosh-7701, South Africa
Received 25 December 2007; revised 23 February 2008; accepted 29 February 2008
Available online 6 March 2008
Abstract—New analogues of the potent antihelmintic meclonazepam were prepared and evaluated against Schistosoma mansoni.
The biological data suggests substitution at positions 2 and 4 of meclonazepam could provide promising analogues for prophylactic
and therapeutic activity against S. mansoni.
ꢀ 2008 Elsevier Ltd. All rights reserved.
Infectious diseases such as Schistosomiasis spread by
parasites are the causative agents of several highly
disabling and often fatal human diseases especially in
the tropical and subtropical areas.1 These parasitic
infections require a mammalian host which is indispens-
able for maintaining the parasite. There are limitations
associated with currently available drugs available for
the treatment of these diseases.2 Drug resistance has
further compounded the situation.3 This has necessi-
tated the continued search for new chemotherapeutic
agents.
eases. One strategy to provide new drugs is the modifica-
tion of existing ones to restore or enhance their activity
against resistant strains of parasites or to remove unac-
ceptable side effects such as the sedative properties of
meclonazepam. In the field of tropical medicine, the
development of many conventional drugs can be traced
to dye precursors. This is due to the efficacy of dye chro-
mophores against drug resistant organisms.5,6 In
addition, the dye chromophore would facilitate investi-
gations into the location of the drug in the target organ-
ism.This prompted us to synthesize some meclonazepam
dyes reported in this paper. In addition other chemical
modifications were carried out to produce non-dye ana-
logues for comparison purposes.
The benzodiazepine derivative meclonazepam (3-methyl
clonazepam) (1), showed a marked degree of therapeutic
and prophylactic activity against all stages of schistoso-
miasis caused by Schistosoma mansoni and
S. haematobium.4 This drug appears to act by binding
to a low affinity benzodiazepine receptor on the epider-
mis of the schistosome, leading to an increase in the in-
flux of extracellular calcium and subsequent spastic
paralysis of the parasite’s musculature. These findings
are relevant both to the considerable clinical significance
of meclonazepam as well as the potential usefulness of
this drug in the treatment of various other tropical dis-
H
O
N
H
1
2
9
6
8
7
3
4
5
CH3
N
O2N
Cl
1
The dye analogues were synthesized from meclonaze-
pam by variation at the 7-position of the aromatic ring.
The synthesis of the targeted compounds was achieved
by diazotization of the amine using NaNO2 and HCl
at 0–4 ꢁC followed by coupling with variously
(Scheme 1) substituted phenols.7 In addition, MACLO-
NAC, RML-1, A-151 were prepared with a standard
combination of the reagents for the thionation8 and
Keywords: Meclonazepam; Schistosomiasis.
*
Corresponding author at present address: Department of Chemistry,
University of Cape Town, Private Bag, Rondebosh-7701, South
Africa. Tel.: +27 21 650 2553; fax: +27 21 689 7499; e-mail:
Kelly.Chibale@uct.ac.za
0960-894X/$ - see front matter ꢀ 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.02.077

2334
A. Mahajan et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2333–2336
O
N
O
N
O
N
HN
CH3
H
HN
CH3
H
HN
CH3
H
ArOH, pH 9-10
HCl, NaNO2
0-4 º
N
Cl
N
N
0-4 º
C
HOAr
N
C
Cl
H2N
Cl
Cl
ArOH = phenol
ArOH = alpha napthol
ArOH = beta napthol
ArOH = 2-ethyl phenol
3
MACLOT-3
ArOH = 2-isopropyl phenol
ArOH = 2,4 dimethyl phenol
ArOH = 3,5 difluorophenol
ArOH = 3-bromo phenol
ArOH = 3,5dichloro phenol
Sn/ HCl
ethanol, reflux
O
N
S
N
HN
CH3
H
HN
CH3
H
Lawesson reagent
Dry THF
O2N
O2N
Cl
Cl
A-151
1
CH
O
S
COO
H
3
HN
CH3
H
HN
CH3
H
NaCNBH3, THF
Lawesson reagent
Dry THF
NH
NH
O2N
O2N
Cl
Cl
RML-1
MACLONAC
Scheme 1.
O
N
O
O
H
N
HN
CH3
H
HN
CH3
H
CH3
N
H
N
N
N
N
N
Cl
Cl
N
N
Cl
OH
OH
OH
MACLO4 (78 % )
MACLODY-3 ( 52 % )
MACLODY ( 78 %)
O
O
N
O
HN
CH3
H
HN
CH3
H
HN
CH3
H
N
N
N
N
N
N
Cl
N
Cl
Cl
N
HO
H3C
CH3
C2H5
OH
OH
MACLO-2,4 ( 54 % )
MACLO-ISO (71 %)
MACLOETPH ( 50 %)
O
O
O
HN
CH3
H
HN
CH3
H
HN
CH3
H
N
N
N
N
N
N
Cl
N
N
Cl
Cl
N
Cl
Cl
Br
F
F
OH
OH
OH
MACLODIF (50 %)
MACLODIC (30 %)
MACLOBR (45 %)
Figure 1. Diazo-derivatives of meclonazepam along with their respective yields.

A. Mahajan et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2333–2336
2335
reduction at positions 2 and 4 of meclonazepam. All the
prepared compounds were characterized by spectro-
scopic and analytical means (Fig. 1).9
that substitutions can be made at 2- and 4-positions
without destroying the antischistosome effects but so
far such substitutions have not enhanced the IC50 values
(Table 1). The other group of active compounds were
dyes (MACLODY-3, MACLODIC, MACLOETPH
and MACLOISO) produced by variation at the 7-posi-
tion were slower, producing signs of damage at day 3
and death by day 5, with severe to slight tegumental
damage. MACLOBR only killed a proportion of worms
by day 5 (Table 1). Some of the diazo compounds MA-
CLO-4 and MACLO-2,4, MACLODY were inactive as
all worms were found alive and moving. The diazo com-
pound MACLODIF caused severe tegumental damage
These newly synthesized analogues were tested in vitro
for their activity at a concentration of 10 lg/ml against
S. mansoni. Some compounds were effective against
the parasite by killing and paralyzing the worms within
1 h of application to day 5 of the culture. The com-
pounds produced by variation at the 2- and 4-position
were highly active. Analogues RML-1, MACLONAC,
A-151 killed all worms within 1 h of application with se-
vere damage to the teguments of the worms. This shows
Table 1. In vitro activity of meclonazepam analogues against S. mansoni
Observationa
IC50 (lg/ml)
c
Compound
MACLODY
All worms were alive, appeared dark, granular, opaque and occasionally
moved their anterior and posterior ends. One eggb
>10
MACLODY-3
All females and 67% males were dead by day 5 of culture (all alive on day
3). All worms looked dark, granular and opaque. Few worms showed
severe tegument damage. Most worms had blebs on their surfaces. No eggsb
All worms were alive, actively moving and stained red. Teguments of males
appeared wrinkled. One eggb
9.10
MACLO-4
>10
5.79
MACLOETPH
All worms were dead by day 5 of culture (all alive on day 3). All were dark,
granular, opaque and slender-shaped. Few worms showed severe tegument
damage and were covered in blebs. No eggsb
MACLOISO
All worms were dead, dark, granular and opaque by day 5 (90% dead on
day 3). All showed severe tegument damage and teguments were covered in
blebs. No eggsb
5.79
MACLO-2,4
MACLODIF
All worms were alive, very slow in movement and were stained yellow. One
eggb
>10
3.80
All worms were dead by day 5 of culture (90% dead on day 3). All looked
dark, granular, opaque, showed severe tegument damage and teguments
covered in blebs. No eggsb
MACLOBR
MACLODIC
Fifty percent of worms killed by day 5 of culture. Dead worms showed
severe tegument damage with few blebs appearing on their teguments. Live
worms occasionally moved their anterior and posterior ends. All worms
looked dark, granular and opaque. No eggsb
10
All worms were dead by day 5 of culture. All looked dark, granular and
opaque. Most worms showed severe tegument damage and their surfaces
were covered in blebs. No eggsb
No effect on worms. Some eggsb
5.79
MACLOT-3
MACLONAC
>10
1.30
All worms were dead within 1 h of application. By day 5, all worms were
dark, granular and opaque. Females appeared thin and curled-up. All
worms showed severe tegument damage and males’ teguments were covered
in blebs. No eggsb
A-151
All worms were dead within 1 h of application. On day 5, all worms were
dark, granular and opaque, showed severe tegument damage and most male
teguments were covered in blebs. No eggsb
0.35
4.75
RML-1
Some worms were killed within 1 h of application. By day 5, drug killed all
males and 50% females. Live worms occasionally moved their anterior and
posterior ends. All worms were dark, granular and opaque. Females
appeared slender and curled-up. All worms showed severe tegument
damage. Teguments of all males and most females were covered in blebs.
No eggsb
Meclonazepam (Ro11-3128)
Some worms were killed by 1 h. By day 5, all worms were dead. All showed
severe tegument damage, and looked dark, granular and opaque. No blebs
formed on the teguments of dead worms. No eggsb
0.37
In vitro screening was performed against adult S. mansoni using five male and five female worms as described in reference.10 Drug stock solutions
were prepared in 100% DMSO immediately before use.
a Microscopic observations of motility and morphology are the criteria used for establishing drug activity. The descriptions above refer to the effects
on worms of 10 lg/ml compound.
b Subtle drug effects are manifest by inhibition of egg production by adult worms in the cultures.
c IC50 values were calculated using Microsoft Excelfit software [IDBS Ltd, UK] from % mortality of worms that were cultured in four serial drug
dilutions (1:3) starting with 10 lg/ml (10, 3.33. 1.11, 0.37 and 0.12 lg/ml).

2336
A. Mahajan et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2333–2336
6.30–6.31 (1H, d, J = 2.61 Hz, ArH); 6.75–6.78 (1H, dd,
J = 2.64, 8.54 Hz, ArH); 6.89–6.90 (1H, d, J = 8.54 Hz,
ArH); 7.29–7.31 (3H, m, ArH); 7.44–7.45 (1H, m, ArH);
8.21 (NH, D2O, exchangeable). 13C NMR: (100.6 MHz,
DMSO-d6) 16.2, 59.3, 114.1, 118.3, 122.1, 126.9, 128.5,
129.3, 129.9, 131.7, 133.1, 138.3, 143.1, 168.2, 171.9. Anal.
required for C16H14ClN3O C, 64.11 H, 4.71 N, 14.02.
Found: C, 63.04 H, 4.63 N, 13.55. FAB MS 300 (M+1)+;
(b) 4E-5-(2-Chlorophenyl)-7-(4-hydro-3-isopropylphenyl
diazenyl)-3-methyl-1H-benzo(e)(1,4)diazepin-2(3H)-one
(MACLO-ISO). Yield 71%, mp 261 ꢁC, IR: (cmÀ1) 3471
(OH), 3298 (NH), 3043–2870 (aromatic), 1660 (C@O),
to females, whereas MACLOT-3 had no effect on worms
at all. The low level of activity of dye analogues and
MACLOT-3 indicate that substitution at the 7-position
is not favorable as the nitro group seems crucial to antis-
chistosomal activity.
In conclusion, the data suggest that these new meclo-
nazepam analogues could prove more promising for
the future development of antischistosomal agents by
introducing substituents at the 2- and 4-positions.
1
1592 (azo). H NMR (400 MHz, DMSO-d6) d 1.16–1.17
(6H, d, J = 6.9 Hz 1, 2· CH3), 1.53–1.55 (3H, d,
J = 6.4 Hz, CH3), 3.20–3.25 (1H, m, J = 6.89 Hz, CH);
3.78–3.82 (1H, q, J = 6.36, CH); 6.89–6.92 (1H, d,
J = 8.57 Hz, ArH); 7.34–7.36 (1H, d, J = 8.75, ArH);
7.38–7.39 (1H, d, J = 2.14, ArHd); 7.45–7.49 (3H, m,
ArH), 7.51–7.53 (dd, J2.42, 8.52, 1H, ArH); 7.57–7.58
(1H, m, ArH); 7.62–7.59 (1H, J = 2.37, ArH); 7.94–7.96
(1H, d, J = 2.16, 8.71 Hz, ArH); 10.2 (NH, D2O,
exchangeable); 11.0 (1H, D2O, exchangeable OH). 13C
NMR: (100.6 MHz, DMSO-d6) 17.7, 22.8, 22.9, 27.2,
59.5, 116.1, 121.6, 122.7, 123., 124.1, 125.1, 128.0, 128.4,
130.4, 131.7, 132.1, 132.6, 135.9, 139.1, 140.7, 145.5, 147.5,
159.1, 167.5, 171.1. Anal. required for C25H23ClN4O2 C,
67.18 H, 5.19 N, 12.54. Found: C, 67.58 H, 5.03 N, 12.01.
FAB MS 447 (M+1)+; (c) 5-(2-Chlorophenyl)-3-methyl-
7-nitro-4,5-dihydro-1H-benzo(e)(1,4)diazepin-2(3H)-one
Acknowledgement
Work was supported by grants from the South African
National Research Foundation.
References and notes
1. (a) Report of the Scientific working group on Schistoso-
miasis, 2005, TDR/SWG/07. http://www.who.int/tdr; (b)
Centron, M. S.; Chitsulo, L.; Sullivan, J. J.; Pilcher, J.;
Wilson, M.; Noh, J.; Tsang, V. C.; Hightower, A. W.;
Addiss, D. G. The Lancet 1996, 348(Suppl. 1), 1274; (c)
Ross, A. G. P.; Bartley, P. B.; Sleigh, A. C.; Olds, G. R.;
Li, Y.; Williams, G. M.; McManus, D. P. N. Eng. J. Med.
2002, 346, 1212; (d) Dan, L. D.; Han, H. G.; Ji, Z. S. Acta
Trop. 2005, 96, 242.
(MACLONAC). Yield 90 %, mp 110–111 ꢁC, IR: (cmÀ1
)
1
3688 (NH), 1692 (CN). Diasteriomeric mixture H NMR
(400 MHz, CDCl3) d 1.34–1.36 (3H, d, J = 6.54, CH3),
1.45–1.47 (3H, d, J = 6.93 Hz, CH3), 3.44–3.44 (1H, q,
J = 6.54 Hz, 1H), 3.93–3.99 (1H, q, J = 6.92 Hz, 1H), 5.64
(1H, s, CH), 5.72 (1H, s, CH), 7.08–7.06 (1H, J = 8.79,
ArH), 7.13–7.15 (1H, d, J = 8.69, ArH), 7.28–7.51 (9H, m,
ArH), 7.80–7.82 (1H, m, ArH); 8.02–8.04 (1H, dd,
J = 2.53, 8.76 Hz, ArH); 8.12–8.14 (1H, dd, J = 2.53,
8.67, ArH); 8.51 (NH, D2O, exchangeable), 8.71 (NH,
D2O, exchangeable). presup13C NMR: (100.6 MHz,
CDCl3) 16.3, 18.8, 52.3, 57.1, 58.4, 61.2, 110.8, 111.1,
120.8, 121.0, 123.7, 124.1, 124.2, 124.6, 127.3, 127.7, 129.6,
129.7, 129.9, 130.0, 130.1, 134.2, 134.2, 138.4, 138.6, 142.2,
143.2, 168.0, 168.1. Anal. required for C16H14ClN3O3 C,
57.93 H, 4.25 N, 12.67. Found: C, 56.65 H, 4.15 N, 11.97.
FAB MS 332 (M+1)+; (d) (E)-5-(2-Chlorophenyl)-
3-methyl-7-nitro-1H-benzo(e)[1,4]diazepine-2(3H)-thione
(A-151). Yield 10%, mp 244 ꢁC, IR: (cm)À1 3400 (NH),
3051–3026 (aromatic). 1H NMR (400 MHz, CDCl3) d
1.89–1.91 (3H, d, J = 6.33 Hz, CH3), 4.0–4.07 (1H, q,
J = 6.3, 1H), 7.24–7.27 (1H, d, J = 8.87, ArH), 7.34–7.44
(3H, m, ArH), 7.57–7.61 (1H, m, ArH), 7.97–7.98 (1H,
d = 2.51, J = ArH), 8.31–8.35 (1H, dd, J = 2.53, 8.86,
ArH), 9.89 (NH, D2O, exchangeable). 13C NMR:
(100.6 MHz, CDCl3) 20.6, 62.8, 111.2, 118.9, 121.3,
125.7, 126.6, 127.3, 127.4, 127.7, 130.4, 131.4, 143.2,
152.1, 170.5. Anal. required for C16H12ClN3O2S C, 55.57
H, 3.50 N, 12.15 S, 9.27. Found: C, 53.04 H, 3.33 N, 12.07
S, 7.46. FAB MS 346 (M+1)+.
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9. (a) 7-Amino-5-(2-chloro-phenyl)-3-methyl-1,3–dihydro-
benzo(e)(1,4)diazepin-2-one (MACLOT-3). Yield 50%,
mp 239–241 ꢁC, IR: (cmÀ1) 3384 (NH2), 3014–2988
(aromatic). 1H NMR (400 MHz, DMSO-d6) d 1.69–1.7
(3H, d, J = 6.56 Hz, CH3), 3.58–3.59 (2H, broad, NH,
D2O exchangeable), 3.79–3.81 (1H, q, J = 6.57 Hz, 1H),
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