Antimalarial Pyrido[1,2- a]benzimidazole Derivatives with Mannich Base Side Chains: Synthesis, Pharmacological Evaluation, and Reactive Metabolite Trapping Studies

Article abstract of DOI:10.1021/acsinfecdis.8b00279

A novel series of pyrido[1,2-a]benzimidazoles bearing Mannich base side chains and their metabolites were synthesized and evaluated for in vitro antiplasmodium activity, microsomal metabolic stability, reactive metabolite (RM) formation, and in vivo antimalarial efficacy in a mouse model. Oral administration of one of the derivatives at 4 × 50 mg/kg reduced parasitemia by 95% in Plasmodium berghei-infected mice, with a mean survival period of 16 days post-treatment. The in vivo efficacy of these derivatives is likely a consequence of their active metabolites, two of which showed potent in vitro antiplasmodium activity against chloroquine-sensitive and multidrug-resistant Plasmodium falciparum (P. falciparum) strains. Rapid metabolism was observed for all the analogues with <40% of parent compound remaining after 30 min of incubation in liver microsomes. RM trapping studies detected glutathione adducts only in derivatives bearing 4-aminophenol moiety, with fragmentation signatures showing that this conjugation occurred on the phenyl ring of the Mannich base side chain. As with amodiaquine (AQ), interchanging the positions of the 4-hydroxyl and Mannich base side group or substituting the 4-hydroxyl with fluorine appeared to block bioactivation of the AQ-like derivatives though at the expense of antiplasmodium activity, which was significantly lowered.

Full text of DOI:10.1021/acsinfecdis.8b00279

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Article
Antimalarial Pyrido[1,2-a]benzimidazole Derivatives with
Mannich Base Side Chains: Synthesis, Pharmacological
Evaluation and Reactive Metabolite Trapping Studies
John Okombo, Christel Brunschwig, Kawaljit Singh, Godwin Akpeko
Dziwornu, Linley Barnard, Mathew Njoroge, Sergio Wittlin, and Kelly Chibale
ACS Infect. Dis., Just Accepted Manuscript • DOI: 10.1021/acsinfecdis.8b00279 • Publication Date (Web): 04 Jan 2019
Downloaded from http://pubs.acs.org on January 6, 2019
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Antimalarial Pyrido[1,2-a]benzimidazole Derivatives with Mannich
Base Side Chains: Synthesis, Pharmacological Evaluation and
Reactive Metabolite Trapping Studies
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John Okombo,1 Christel Brunschwig,2 Kawaljit Singh,3 Godwin Akpeko Dziwornu,1 Linley
Barnard,1 Mathew Njoroge,2 Sergio Wittlin,4,5 and Kelly Chibale*1,3,6
1.
Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa.
2.
Drug Discovery and Development Centre (H3D), Division of Clinical Pharmacology,
Department of Medicine, University of Cape Town, Observatory 7925, South Africa.
3.
South African Medical Research Council Drug Discovery and Development Research Unit,
Department of Chemistry, University of Cape Town, Rondebosch, 7701, South Africa
4.
Swiss Tropical and Public Health Institute, Socinstrasse 57, 4002 Basel, Switzerland
5.
University of Basel, 4003 Basel, Switzerland
6.
Institute of Infectious Disease and Molecular Medicine, University of Cape Town,
Rondebosch, 7701, South Africa
*Corresponding Author: Kelly Chibale
Email: Kelly.Chibale@uct.ac.za: Phone: +27-21-6502553. Fax: +27-21-6505195.
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A novel series of pyrido[1,2-a]benzimidazoles bearing Mannich base side chain and their
metabolites were synthesised and evaluated for antiplasmodium activity, microsomal
metabolic stability, reactive metabolite (RM) formation and in vivo antimalarial efficacy in a
mouse model. Oral administration of one of the derivatives at 4x50 mg/kg reduced
parasitemia by 95% in Plasmodium berghei-infected mice, with a mean survival period of 16
days post treatment. The in vivo efficacy of these derivatives is likely a consequence of their
active metabolites, two of which showed potent antiplasmodium activity against chloroquine-
sensitive and multidrug resistant P. falciparum strains. Rapid metabolism was observed for
all the analogues with < 40% of parent compound remaining after 30 minutes of incubation in
liver microsomes. RM trapping studies detected glutathione adducts only in derivatives
bearing 4-hydroxyl, with fragmentation signatures showing that this conjugation occurred on
the phenyl ring of the Mannich base side chain. As with amodiaquine (AQ), interchanging the
positions of the 4-hydroxyl and Mannich base side group or substituting the 4-hydroxyl with
fluorine appeared to block bioactivation of the AQ-like derivatives though at the expense of
antiplasmodial activity, which was significantly lowered.
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KEYWORDS: Plasmodium falciparum, Pyrido[1,2-a]benzimidazoles, Mannich base,
Amodiaquine, 4-aminophenol, Reactive metabolite
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Malaria constitutes one of the most important human parasitic diseases due to its impact on
public health and economies, especially in low-income countries.1 The World Health
Organization (WHO) estimates that there were ~216 million malaria cases and ~445 000
related deaths globally in 2016.2 Though these statistics attest to successful global expansion
in control efforts in the last 15 years, they still remain troublingly high and highlight the
numerous impediments to mitigation strategies, which include reduced sensitivity to the
WHO-recommended first-line artemisinin combination therapy (ACT)3 and development of
insecticide resistance in malaria vectors.4 Furthermore, the only licensed vaccine (RTS,S) is
plagued by limited efficacy,5 lacks pan-protection and is currently introduced only as a pilot
scheme in a few settings in sub-Saharan Africa. These underscore the urgent need for novel
alternative therapeutic options.
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In an effort to explore new chemotypes for antimalarial drug development, we recently
reported on the efficacy of pyrido[1,2-a]benzimidazoles (PBIs),6, 7 which have previously
been investigated for their antitumor and antimicrobial activity.8 Interestingly, PBIs share an
architectural semblance with the 4-aminoquinoline antimalarials chloroquine (CQ) (A) and
amodiaquine (AQ) (B) (Figure 1), including an electron-dense planar heterocyclic moiety, a
basic amine side group and a putative mode of action involving inhibition of the haem
detoxification pathway,7, 9 presumably due to their ability to adopt a flat conformation.
Although AQ (in fixed dose combination with artesunate) is now an integral constituent of
ACT, its previous use as monotherapy for prophylaxis was associated with agranulocytosis
and life-threatening hepatotoxicity10 attributed to its ability to undergo oxidative metabolism
to the chemically reactive quinoneimine (C) and aldehyde quinoneimine (D) metabolites.11, 12
Within cells, this reactive metabolite (RM) is trapped by the antioxidant glutathione (GSH)
and its precursor N-acetylcysteine, and the resultant conjugates have been detected in
granulocytes as well as in the bile of experimental animals.13, 14 Upon depletion of GSH, the
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quinoneimine reacts with other soft nucleophiles in the cell leading to toxicity. Crucially, the
formation of this RM species requires the presence of a 4-aminophenol substructure that
permits formation of the electrophilic quinoneimine. A number of antimalarial agents in
development and clinical use, like pyronaridine (E), amopyroquine (F), tebuquine (G) and
cycloquine (H) (Figure 1), coincidentally contain this 4-aminophenol motif, which is likely
to impact their toxicological profiles. Additionally, incorporation of an amine group in their
side chains further classes some of these AQ-like molecules as Mannich base antimalarials.
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To bypass cellular toxicity from the facile oxidation of AQ, a number of strategies have been
reported. For example, replacement of the 4’-hydroxyl group with 4’-fluorine atom yielded
4’-dehydroxy-4’-fluoroamodiaquine (I) which was shown to exhibit potent antiplasmodium
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activity.14,
Miroshnikova and colleagues also synthesized a series of isotebuquine
analogues (J) with good antimalarial activity albeit blemished by poor oral bioavailability.16
More famously, O’Neill and colleagues devised an approach involving the interchanging of
the 3-hydroxyl and 4-Mannich side-chain of AQ to afford new analogues lacking the
potential to form RMs via this quinoneimine pathway, while retaining potent antimalarial
activity.17 Among these derivatives, isoquine (K) proved efficacious in animal models and
showed no evidence of in vivo bioactivation.17 Further development of K was, however,
compromised by unacceptably high first pass metabolism to de-alkylated metabolites
detected during preclinical evaluation leading to diminished activity against CQ-resistant
(CQR) parasite strains.18 The metabolically less labile N-tert-butyl isoquine (L) (Figure 2)
was subsequently developed but, despite its comparatively superior pharmacokinetic and
pharmacodynamic profiles, inability to achieve exposures at doses considered to demonstrate
superior safety compared with CQ led to its discontinuation. As back up candidate for L, 4’-
fluoro-N-tert-butylamodiaquine (M) (Figure 2) was developed with a fluorine replacement of
the 3’-hydroxyl and 4’-N- tert-butyl instead of the 3’-N-diethyl to block the N-dealkylation
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site. This molecule, with a suitable oral bioavailability and acceptable safety profile
underwent preclinical evaluation.19 Even more recently, our research group also reported a
novel approach to block the formation of both the quinoneimine and aldehyde metabolites
based on the hypothesis that AQ analogues in which the hydroxyl group and the Mannich
base side chains are cyclized into a benzothiazole (N), benzoxazole (O) or benzimidazole (P)
ring system will not form RMs.20, 21
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Pursuant to the abovementioned strategies, this article presents data on the synthesis,
antiplasmodium activity as well as microsomal metabolic profiles of a series of PBI
derivatives bearing Mannich base side chains and/or designed to ablate their RM-forming
ability. These comprise analogues with the hydroxyl and Mannich side group either
interchanged at the C3 or C4 positions or the hydroxyl substituted with fluorine on the
background of various substitutions at C2, C3 and the left hand side (LHS) of the PBI
nucleus (Figure 3). As proof-of-concept, we also synthesised and tested the N-deethylation
metabolites of some of the derivatives to highlight their potency. To gain insight into the
ability of this series to undergo bioactivation and the compounds’ antimalarial efficacy, a
representative set of analogues was tested for RM formation and in vivo activity in a murine
malaria model.
Scheme 1: Synthesis of pyrido[1,2-a]benzimidazole (PBI) derivatives a
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O
O
O
O
R1 = 4-CF3Ph
a
EtO
1a: R1 = CF3
1b: R1 = 4-CF3Ph
R1
EtO
F
2
CF3
OH
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9
X
X
Cl
R2
R1
R2
R1
Y
Y
H
N
X
Y
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NH2
NH2
X
N
6
CN
N
b
d
1 or 2
c
N
N
5
N
Y
CN
CN
3
4
a: X = Y = H; R1 = CF3; R2 = H
a: X = Y = H
b: X = Y = F
a: X = Y = H
b: X = Y = F
b: X = Y = H; R1 = 4-CF3Ph; R2 = H
c: X = Y = F; R1 = 4-CF3Ph; R2 = H
d: X = Y = H; R1 = 4-CF3Ph; R2 = F
O
O
NH2
HN
NH2
HN
e
g
f
.2HCl
R4
R4
R4
R4
R3
7
R3
8
R3
R3
9
10
a: R3 = H; R4 = OH
b: R3 = OH; R4 = H
a: R3 = CH2N(CH2CH3)2; R4 = OH
b: R3 = OH; R4 = CH2N(CH2CH3)2
F
F
F
F
O
S
N
O
j
h
i
N
OH
+
O
O
S
O
NH2
15
NO2
13
NO2
14
NO2
11
Cl
12
OH
OH
OH
CHO
l
k
N
H
N
H
NO2
16
NO2
17
NH2
18
R4
a: R1 = 4-CF3Ph; R2 = H; R3 = OH; R4 = CH2N(CH2CH3)2; X = Y = H (KP-84)
b: R1 = CF3; R2 = H; R3 = CH2N(CH2CH3)2; R4 = OH; X = Y = H (KP-87)
c: R1 = CF3; R2 = H; R3 = OH; R4 = CH2N(CH2CH3)2; X = Y = H (KP-94)
d: R1 = 4-CF3Ph; R2 = H; R3 = OH; R4 = CH2N(CH2CH3)2; X = Y = F (LB-88)
e: R1 = 4-CF3Ph; R2 = F; R3 = OH; R4 = CH2N(CH2CH3)2; X = Y = H (LB-97)
f: R1 = CF3; R2 = H; R3 = F; R4 = CH2N(CH2CH3)2; X = Y = H (KP-98)
g: R1 = 4-CF3Ph; R2 = H; R3 = OH; R4 = CH2NH(CH2CH3); X = Y = F (GAD096)
h: R1 = 4-CF3Ph; R2 = H; R3 = OH; R4 = CH2NH(CH2CH3); X = Y = H (GAD098)
i: R1 = 4-CF3Ph; R2 = H; R3 = CH2N(CH2CH3)2; R4 = OH; X = Y = H (GAD099)
R3
X
HN
6
R2
R1
Y
10 or 15 or 18
N
m or n
N
CN
19
Scheme 1: Reagents and reaction conditions: (a) SelectfluorTM, MeCN, microwave (150 W),
82 °C, 10 min. (b) Ethyl cyanoacetate, DMF, microwave, 110 °C, 15 min. (c) NH4OAc, 150
°C, 1 h; (d) POCl3, 130 °C, 2 h. (e) (CH3CO)2O, THF, 60 °C, 30 min. (f) HCHO, Et3NH,
EtOH, microwave, 80 °C, 1 h. (g) 6N HCl, 100 °C, 2 h. (h) aq. NaOH, THF, 5-10 °C, 2 h. (i)
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Et2NH, Et3N, Dioxane, 60 °C, 4 h. (j) SnCl2, 1N HCl, THF, 60 °C, 8 h. (k) EtNH2, MeOH, 25
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o
o
oC, 1 h/0 C, NaBH4, 16 h (l) H2, Pd/C, MeOH, 25 C, 16 h. (m) Et3N, THF:DMF,
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microwave, 80 °C, 20 min. (n) EtOH, HCl, microwave, 80 °C, 1.30 h (for 19g-h).
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RESULTS AND DISCUSSION
Chemistry: The synthesis of target PBI derivatives 19a-i was relatively straightforward using
published procedures (Scheme 1).7, 15, 17 Briefly, ethyl acetoacetate 1 (R1 = 4-CF3Ph) was
treated with SelectfluorTM to afford starting material ethyl 2-fluoro-3-oxo-3-(4-
(trifluoromethyl)phenyl)propanoate, 2. Acetonitrile 4b was synthesized by reacting
commercially available 1,2-diaminobenzene 3b with ethylcyanoacetate in DMF under
microwave irradiation. Subsequent reaction of commercially available acetonitrile 4a and 4b
with ethyl acetoacetates 1a/2 in the presence of ammonium acetate afforded the tricyclic
intermediates 5, which were then converted to the key chloro intermediates 6a-d on reaction
with POCl3. The Mannich bases 10a-b were synthesized as previously described.15, 17 The
flourinated Mannich base 15 was synthesized by activating the hydroxyl group of
commercially available 11 with p-toluenesulfonylchloride to afford 13 followed by reaction
with diethylamine and subsequent nitro group reduction. Reductive amination of
commercially available 2-hydroxy-5-nitrobenzaldehyde 16 with ethylamine to yield 17,
followed by reduction of the nitro group afforded 18, the N-desethyl analogue of 10b. The
obtained Mannich bases described above were then reacted with the chloro intermediates 6a-
d to afford the desired PBI derivatives 19a-i.
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Cytotoxicity and In Vitro Antiplasmodium Activity: All compounds were evaluated for in
vitro antiplasmodium activity against the CQ-sensitive (PfNF54) and the multidrug resistant
(PfK1) strains as well as mammalian cytotoxicity against a Chinese hamster ovary (CHO)
cell line. PfNF54 and PfK1 were chosen as controls to offer insight into the full spectrum of
compound activity against P. falciparum as they broadly represent drug-sensitive and –
resistant strains, respectively. The results are summarized in Table 1. In summary, these
compounds showed a favourable cytotoxicity profile with an S.I range between 16 and 1718
(Table 1). Emetine, a potent alkaloid inhibitor of eukaryotic protein synthesis, was used as
control and, expectedly, exhibited cellular toxicity at extremely low concentrations.
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Antiplasmodium activity ranged from 0.07 – 5.14 µM and 0.07 - 6.62 µM against PfNF54
and PfK1, respectively, with 4/9 compounds exhibiting submicromolar potency against both
strains. Introduction of fluorine atoms at C7 and C8 positions on the LHS of the PBI core
while maintaining the para-trifluoromethyl phenyl (4-CF3Ph) substitution at the C3 on the
PBI core appeared to afford the most active compounds (19d and 19g) with an equally good
cytotoxicity profile, thus confirming previous observation of significant improvement in
activity upon mono- or di- halo-substitution on the LHS.7 On the other hand, replacing the
para-trifluoromethyl phenyl in 19a with CF3 at C3 (as in 19c) led to an eight-fold drop in
potency albeit with retention of some activity. Nevertheless, the same replacement while
interchanging the positions of the hydroxyl and Mannich base side group (19b) or
substituting the 4-hydroxyl with fluorine (19f) was significantly detrimental to activity.
Microsomal Metabolic Stability and Metabolite Identification Studies: Microsomal
metabolic stability was investigated using a single point metabolic stability assay involving
incubation at 37 °C with human and/or mouse liver microsomes in the presence of phosphate
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buffer and NADPH as co-factor.22 All compounds were rapidly metabolized, with < 40% of
the parent compounds remaining after 30 minutes of incubation in microsomes (Table 2).
This was characterized by short degradation half-lives, suggesting significant susceptibility to
hepatic metabolism regardless of the modification on the aromatic ring of the Mannich side
group. Di-substitution with fluorine at C7 and C8 positions of the LHS (19d) did not improve
metabolic stability. No metabolism occurred in the absence of NADPH or microsomes,
implying that the observed biotransformation was primarily a function of cytochrome p450
enzyme-mediated phase 1 metabolism with N-deethylation seemingly the predominant route
of metabolism.
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Bioactivation Experiments: To investigate RM formation, five compounds comprising those
in which the 4-aminophenol substructure is maintained (19a, 19c and 19d) and others bearing
modification on this substructure (19b and 19f) were screened by LC-MS/MS in rat liver
microsomes. Using GSH as a trapping agent, GSH-drug adducts were detected only in
samples incubated with 19a, 19c and 19d (Figure 4). Analysis of the fragmentation patterns
of the GSH conjugates in positive electrospray ionisation mode revealed [M+H-73]+ ions,
corresponding to the cleavage of the diethylamino group. Subsequent neutral loss of 129 Da
was characteristic of a GSH conjugate, while a neutral loss of 273 Da indicated that the GSH
conjugation likely occurred on the phenyl ring of the Mannich base side chain23 (Supporting
Information Figure S1 and S2, Table S1). Expectedly, modification of the structure to
circumvent formation of the reactive quinoneimine appeared successful in 19b (positions of
the hydroxyl and Mannich side group interchanged) and 19f (4’-fluoro replacement of the 4’-
hydroxyl).
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In vivo Antimalarial Efficacy: On the basis of good antiplasmodium activity and cytotoxicity
profile, 19a was chosen for proof-of-concept study - specifically the assessment of in vivo
efficacy in P. berghei-infected mice at an oral dose of 4x50 mg/kg (Table 3). Although no
mice were cured of infection (0/3), parasitemia was reduced by 95% and the animals survived
slightly longer (MSD: 16) compared to KP4 (MSD: 14), a prototype PBI molecule with
previously confirmed in vivo efficacy.7 Noteworthy, AQ itself is poorly stable in human liver
microsomes (CLint of 608 µL/min/mg), with a short elimination half-life of ~5 hours in
humans24 and its pharmacological activity is dominantly sustained by its stable longer-
circulating N-deethylation metabolite, desethylamodiaquine.25 Curiously, the main
metabolites of all the derivatives were analogous to the active metabolite of AQ, possibly
explaining the significant reduction in parasitemia observed for 19a, regardless of the
anticipated high in vivo metabolism. We accordingly synthesized and evaluated 19h, the N-
deethylation metabolite of 19a. It is likely that this metabolite persists in systemic circulation
long enough to suppress parasitemia even after the concentration of the parent compound has
abated below the therapeutic threshold. Indeed, the potent in vitro antiplasmodium activity
data of 19h (Table 1) and the extended elimination half-life of desethylamodiaquine (6 - 18
days) support this assertion. Compound 19c, tested alongside 19a, only elicited <40%
reduction in parasitemia in the mice, and we speculate that this could be due to poor oral
bioavailability possibly occasioned by any one or more of its physicochemical attributes
compounded by its low metabolic stability.
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In conclusion, modifications around the reactive quinoneimines of PBI derivatives with
Mannich base side chains led to identification of compounds with good antiplasmodium
activity, with retention of the para-trifluoromethyl phenyl group at the C3 position of the PBI
core critical to biological activity. Strategies to circumvent bioactivation were successful,
though at the expense of antiplasmodium activity. Amongst the most active compounds, 19a
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showed good in vivo activity in the mouse P. berghei model. Biotransformation data suggest
that the in vivo activity of these compounds could be a consequence of their metabolites, two
of which were in fact endowed with potent antiplasmodium activity against both CQS and
multidrug resistant parasite strains. These metabolites can form the basis of a future lead
optimization effort.
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Experimental Procedures: General Method: All commercially available chemicals were
purchased from either Sigma-Aldrich or Combi-Blocks. All solvents were dried by
1
appropriate techniques. Unless otherwise stated, all solvents used were anhydrous. H NMR
spectra were recorded on a Varian Mercury Spectrometer at 300 MHz or a Varian Unity
Spectrometer at 400 MHz. 13C NMR spectra were recorded at 75 MHz on a Varian Mercury
Spectrometer or at 100 MHz on Varian Unity Spectrometer. High-resolution mass spectra
were recorded on a VG70 SEQ micromass spectrometer. Melting points (m.p.) were
determined by Differential Scanning Calorimetry (DSC) using TA Q200/Q2000 DSCfrom
TA Instruments. Analytical thin-layer chromatography (TLC) was performed on aluminium-
backed silica-gel 60 F254 (70-230 mesh) plates. Column chromatography was performed with
Merck silica-gel 60 (70-230 mesh). Chemical shifts (δ) are given in ppm downfield from
TMS as the internal standard. Coupling constants, J, are recorded in Hertz (Hz). Purity was
determined by HPLC and all compounds were confirmed to have > 95% purity. Any data that
is not shown below is supplied in the Supporting Information.
Ethyl 2-fluoro-3-oxo-3-(4-(trifluoromethyl)phenyl)propanoate 2:
Ethyl(4-trifluromethylbenzoyl)acetate 1b (1.0 equiv.) was added to Selectfluor fluorinating
agent (2.0 equiv.) in acetonitrile (3.0 mL). The mixture was heated in the microwave at 82°C
for 10 minutes. The black precipitate was filtered off and the solvent was removed in vacuo.
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The residue was purified by flash column chromatography (EtOAc-Hex) to acquire
compound as a yellow oil (66%); Rf 0.33 (10 % EtOAc-Hex); 1H NMR (400 MHz, CDCl3) δ
8.17 (d, J = 7.6, 2H), 7.79 (d, J = 8.2, 2H), 5.86 (d, J = 48.8 Hz, 1H), 4.33 (q, J = 7.1 Hz,
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2H), 1.29 (t, J = 7.1 Hz, 3H); C NMR (100 MHz, CDCl3) δ 188.8, 164.3, 136.0, 129.9,
129.8, 125.8, 125.8, 121.9, 91.2, 89.3, 62.9 and 13.8; 19F NMR (377 MHz, CDCl3) δ -63.49, -
190.44; MS: m/z 277.0 [M-H]-.
2-(5,6-difluoro-1H-benzo[d]imidazol-2-yl)acetonitrile 4b:
A mixture of 4,5-difluorobenzene-1,2-diamine 3b (1.0 equiv.) and ethyl-2-cyanoacetate (2.0
equiv.) was heated in the microwave at 110°C for 15 minutes. The residue was purified by
flash column chromatography (EtOAc-Hex) to obtain compound as a red/brown solid (60%);
m.p. 267 - 269 οC; Rf 0.38 (70 % EtOAc-Hex); 1H NMR (400 MHz, DMSO) δ 12.80 (s, 1H),
7.67 – 7.59 (m, 2H), 4.39 (s, 2H); 13C NMR (101 MHz, DMSO) δ 148.5, 148.3, 147.4, 146.1,
146.0, 116.8, 106.1, 100.6 and 18.9; MS: m/z 194.1 [M+H]+.
General procedure for the synthesis of compound 5:
A mixture of benzimidazole acetonitrile 4 (1.0 equiv.), NH4OAc (2.0 equiv.), and the
ο
appropriate β-keto ester 1/2 (1.20 equiv.) was heated to reflux at 150 C for 1 h and then
allowed to cool to 100 οC. MeCN (10 mL) was added and the resulting mixture stirred for 15
min, allowed to cool to room temperature and then cooled on ice. The cold mixture was
filtered and the residue washed with cold MeCN (4×10 mL), dried in vacuo, and used in the
next step without further purification.
7,8-difluoro-1-hydroxy-3-(4-(trifluoromethyl)phenyl)benzo-[4,5]imidazo[1,2-a]pyridine-4-
carbonitrile 5c: Grey/brown solid (62 %); m.p. 289 - 294 °C; Rf 0.61 (70 % EtOAc-Hex); 1H
NMR (400 MHz, DMSO) δ 8.52 (dd, J = 10.0, 7.0 Hz, 1H ), 7.91 (d, J = 8.1 Hz, 2H), 7.84 (d,
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J = 8.1 Hz, 2H), 7.63 (dd, J = 10.0, 7.0 Hz, 1H), 6.11 (s, 1H). 13C NMR (101 MHz, DMSO) δ
158.1, 151.8, 149.1, 141.3, 130.5, 130.2, 129.6, 129.1, 129.0, 126.1, 126.1, 123.7, 123.6,
116.7, 105.7, 105.5, 105.1, 101.2 and 101.0; MS: m/z 388.0 [M-H]-.
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General procedure for the synthesis of compound 6:
ο
A mixture of compound 5 (1.0 equiv.) and POCl3 (20 equiv.) was heated to reflux at 130 C
for 2 h. Excess POCl3 was removed under reduced pressure and ice-cold water (20 mL)
added to the residue, stirring to yield a precipitate. The mixture was neutralized with
saturated NaHCO3 and filtered. The resultant solid was washed with ice-cold water (4×15
mL), dried in vacuo, and used without further purification.
1-chloro-7,8-difluoro-3-(4-(trifluoromethyl)phenyl)benzo[4,5]imidazo[1,2-a]pyridine-4-
carbonitrile 6c: Brown solid (87 %); m.p. 237 - 242 °C; Rf 0.77 (70 % EtOAc-Hex); 1H NMR
(400 MHz, DMSO) δ 8.74 (dd, J = 11.0, 7.6 Hz, 1H), 8.13 (dd, J = 11.0, 7.6 Hz, 1H), 8.02 (s,
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4H), 7.64 (s, 1H); C NMR (150 MHz, DMSO) δ 148.5, 141.2, 141.1, 139.3, 134.8, 130.4
(2C), 126.4, 126.4, 125.8, 125.1, 125.0, 115.1, 113.3, 107.4, 107.2, 105.0, 104.8 and 97.6;
MS: m/z 408.0 [M+H]+.
General procedure for the synthesis of compound 8
A mixture of corresponding aminophenol 7a-b (1.0 equiv.) and acetic anhydride (1.0 equiv.)
in THF was heated to reflux at 60 οC for 30 min. After completion of reaction (TLC), solvent
was removed under reduced pressure and residue was washed with diethyl ether to obtained
desired product 8.
N-(3-Hydroxyphenyl)acetamide 8a: Light pink solid (72%); Rf 0.4 (5% MeOH-DCM); m.p.
177-178οC; 1H NMR (400 MHz, DMSO-d6) δ 9.74 (br s, 1H), 9.30 (br s, 1H), 7.18 (d, J = 2.1
Hz, 1H), 7.05 (t, J = 8.0 Hz, 1H), 6.92 (d, J = 8.1 Hz, 1H), 6.43 (ddd, J = 8.1, 2.4, 1.0 Hz,
1H), 2.02 (s, 3H); 13C NMR (100 MHz, DMSO-d6) δ 168.6, 158.0, 140.8, 129.7, 110.5,
110.2, 106.7 and 24.5; MS: m/z 152 [M+H]+.
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N-(4-Hydroxyphenyl)acetamide 8b: Light pink solid (78%); Rf 0.2 (5% MeOH-DCM); m.p.
166-167οC; 1H NMR (400 MHz, DMSO-d6) δ 9.61 (br s, 1H), 9.10 (br s, 1H), 7.35 (d, J = 8.9
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Hz, 2H), 6.69 (d, J = 8.9 Hz, 2H), 1.99 (s, 3H); C NMR (100 MHz, DMSO-d6) δ 167.9,
153.6, 131.5, 121.3 (2C), 115.4 (2C) and 24.1; MS: m/z 152 [M+H]+.
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General procedure for the synthesis of compound 9:
Diethylamine (2.0 equiv.) and formaldehyde (2.0 equiv.) were added to a stirred mixture of
compound 8a-b (1.0 equiv.) in EtOH (5 mL) in microwave tube. The mixture was irradiated
in microwave at 80οC for 1 h (TLC), solvent was removed under reduced pressure and
residue was diluted with DCM (20 ml) and washed with 1N HCl (1×15ml). This aqueous
layer was basified with saturated solution of NaOH up to pH 12 and extracted with DCM
(3×15ml). Organic phase was dried over MgSO4 and solvent was removed under reduced
pressure to obtain compound 9a-b.
N-(4-((diethylamino)methyl)-3-hydroxyphenyl)acetamide 9a: Yellow oil (77%); Rf 0.2 (5%
1
MeOH-DCM); H NMR (400 MHz, DMSO-d6) δ 9.72 (br s, 1H), 7.04 (d, J = 1.7 Hz, 1H),
6.91 (m, 2H), 3.65 (s, 2H), 2.54 (q, J = 8.0 Hz, 4H), 2.01 (s, 3H), 1.02 (t, J = 7.1 Hz, 6H); 13C
NMR (100 MHz, DMSO-d6) δ 168.4, 158.1, 139.7, 128.9, 117.9, 109.9, 106.8, 55.3 (2C),
46.4, 46.1, 24.4 and 11.5; MS: m/z 237 [M+H]+.
N-(3-((Diethylamino)methyl)-4-hydroxyphenyl)acetamide 9b: Yellow oil (77%); Rf 0.2 (5%
1
MeOH-DCM); H NMR (400 MHz, DMSO-d6) δ 9.61 (br s, 1H), 7.29 (d, J = 2.4 Hz, 1H),
7.26 (dd, J = 8.6, 2.5 Hz, 1H), 6.62 (d, J = 8.5 Hz, 1H), 3.67 (s, 2H), 2.55 (q, J = 7.2 Hz,
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4H), 1.98 (s, 3H), 1.03 (t, J = 7.1 Hz, 6H); C NMR (100 MHz, DMSO-d6) δ 167.9, 153.7,
131.3, 123.0, 120.5, 119.9, 115.5, 55.7 (2C), 46.5, 46.3, 24.1 and 11.5; MS: m/z 237 [M+H]+.
General procedure for the synthesis of compound 10
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Compound 9a-b (3 mmol) was refluxed in 6N HCl (3 ml) at 100οC for 2h (TLC). Solvent
was removed under reduced pressure and residue was dissolved in EtOH (2×15 ml) and
solvent was removed under reduced pressure to obtain product 10a-b as a semisolid.
5-Amino-2-((diethylamino)methyl)phenol 10a: Viscous solid (93%); Rf 0.2 (20% MeOH-
DCM); 1H NMR (400 MHz, DMSO-d6) δ 10.40 ( br s, 2H), 10.11 ( br s, 1H), 7.56 (d, J = 2.6
Hz, 1H), 7.34 (dd, J = 8.7, 2.7 Hz, 1H), 7.17 (d, J = 8.7 Hz, 1H), 4.23 (s, 2H), 3.08 (q, J =
8.0 Hz, 4H), 1.28 (t, J = 7.1 Hz, 6H); 13C NMR (100 MHz, DMSO-d6) δ 156.9, 128.0, 126.4,
123.1, 117.8, 117.0, 49.6, 46.8 (2C) and 9.0 (2C); MS: m/z 195 [M+H]+.
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4-Amino-2-((diethylamino)methyl)phenol 10b: Viscous solid (88%); Rf 0.2 (20% MeOH-
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DCM); H NMR (400 MHz, DMSO-d6) δ 10.40 (s, 1H), 7.71 (s, 2H), 7.56 (d, J = 2.6 Hz,
1H), 7.34 (dd, J = 8.7, 2.7 Hz, 1H), 7.16 (d, J = 8.7 Hz, 1H), 4.23 (s, 2H), 3.06 (q, J = 8.0 Hz,
4H), 1.27 (t, J = 7.1 Hz, 6H); 13C NMR (100 MHz, DMSO-d6) δ 156.9, 128.0, 126.4, 123.1,
117.8, 117.0, 56.4 (2C), 46.8, 18.9 and 9.0; MS: m/z 195 [M+H]+.
2-Fluoro-5-nitrobenzyl 4-methylbenzenesulfonate 13: To a solution of NaOH (0.27g, 6.72
mmoles) in H2O (2 ml) was added a solution of 2-fluoro-5-nitro-phenyl)-methanol 11 (0.5g,
2.92 mmoles) in THF (10 ml) at 0°C. A solution of TsCl (0.95g, 4.97 mmoles) in THF (5 ml)
is then added drop wise. The reaction mixture was stirred for 2h at 5-10 °C. 25 ml of water
was added and the compound was extracted with DCM (3×20 mL). The organic layer was
dried over MgSO4, filtered, evaporated. Expected compound was purified by column
chromatography using hexane:ethylacetate as a eluent and title product was obtained as a
white solid (0.9g, 95%); m. p. 89-91 °C; Rf 0.5 (30% EtOAc-Hexane); 1H NMR (300 MHz,
CDCl3) δ 8.22 (m, 2H), 7.82 (d, J = 8.5 Hz, 2H), 7.37 (d, J = 8.6 Hz, 2H), 7.21(d, J = 8.7 Hz,
1H), 5.19 (s, 2H), 2.46 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 165.1, 145.5 (2C), 144.2,
132.6, 130.0 (2C), 128.0 (2C), 126.6, 126.2, 116.8, 64.0 and 21.6; MS: m/z 326 [M+1]+.
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N-ethyl-N-(2-fluoro-5-nitrobenzyl)ethanamine 14: To a solution of compound 13 (0.5g, 1.54
mmoles) in 1 ,4-dioxane (5 ml) was added TEA (0.32 ml, 2.31 mmoles) and diethylamine
(0.24 ml, 2.31 mmoles). The reaction mixture was stirred for 4h at 60 °C, concentrated and
purified by column chromatography using hexane:ethylacetate as a eluent. The expected
compound was obtained as yellow oil (0.28g, 82% yield); Rf 0.7 (50% EtOAc-Hexane); 1H
NMR (400 MHz, CDCl3) δ 8.44 (dd, J = 6.3, 2.9 Hz, 1H), 8.12 (d, J = 2.9 Hz, 1H), 7.15 (d, J
= 8.9 Hz, 1H), 3.67 (s, 2H), 2.58 (q, J = 7.1 Hz, 4H), 1.08 (t, J = 7.1 Hz, 6H); 13C NMR (100
MHz, CDCl3) δ 167.4, 143.2, 126.8, 126.3, 124.0, 116.1, 54.6, 49.0 (2C) and 13.3 (2C); MS:
m/z 227 [M+1]+.
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3-((Diethylamino)methyl)-4-fluoroaniline 15: To a solution of compound 14 (0.25 mg, 1.10
mmoles) in THF (20 ml) was added a solution of SnCl2 (0.84 mg, 4.42 mmoles) in THF (10
ml) and 1N HCl (3.4 mL). The reaction mixture was heated at 60 °C for 8h and concentrated.
Saturated solution of NaHCO3 was added until pH8. Aqueous layer was extracted with DCM
(3×15 mL). The organic layer was dried over MgSO4, filtered, evaporated and obtained as
yellow oil (0.2 mg, 92% yield); Rf 0.4 (20% MeOH-DCM); 1H NMR (400 MHz, CDCl3) δ
6.80 (dd, J = 2.7, 8.7 Hz, 1H), 6.75 (d, J = 3.0 Hz, 1H), 6.49 (d, J = 8.7 Hz, 1H), 3.57 (s, 2H),
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2.57 (q, J = 7.2 Hz, 4H), 1 .07 (t, J = 7.2 Hz, 6H); C NMR (100 MHz, CDCl3) δ 150.3,
142.6, 125.2, 116.4, 115.3, 115.0, 55.6, 48.6 (2C) and 13.6 (2C); MS: m/z 197 [M+1]+.
2-((ethylamino)methyl)-4-nitrophenol 17: To a solution of 2-hydroxy-5-nitrobenzaldehyde
16 (1.0g, 5.98 mmol) in dry MeOH (50 ml) at 25oC was added ethylamine (0.67 ml, 11.96
mmoles) dropwise. The mixture was allowed to stir for 1 h after which the temperature was
reduced to 0oC in an ice bath and NaBH4 was added in small portions over 45 min. The
reaction mixture was allowed to stir for 16 h. Water (15 ml) was added and the solvent was
removed under reduced pressure. The residue was dissolved in DCM and filtered through
ο
celite. The solvent was removed in vacuo to obtain orange solid (89%); m.p. 182-184 C; Rf
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0.4 (20% MeOH-DCM); 1H NMR (400 MHz, DMSO) δ 8.40 (d, J = 2.9 Hz, 1H), 8.13 (dd, J
= 2.9, 9.1 Hz, 1H), 7.18 (d, J = 9.1 Hz, 1H), 4.11 (s, 2H), 2.96 (q, J = 7.2 Hz, 2H), 1.24 (t, J =
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7.2 Hz, 3H); C NMR (101 MHz, DMSO) δ 163.6, 138.4, 127.7, 126.4, 119.4, 115.9, 43.9,
41.8 and 10.8; MS: m/z 197.1 [M+H]+.
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4-amino-2-((ethylamino)methyl)phenol 18: To a solution of 17 (0.5g, 2.55 mmol) in dry
MeOH (100 ml) at 25oC was added palladium on carbon catalyst in small portions. The
mixture was allowed to stir under hydrogen gas atmosphere for 16 h. The reaction mixture
was filtered through celite and the solvent removed under reduced pressure to obtain brown
viscous oil (85%); Rf 0.2 (20% MeOH-DCM); 1H NMR (400 MHz, DMSO) δ 6.70 (d, J = 8.5
Hz, 1H), 6.60 (d, J = 2.7 Hz, 1H), 6.52 (dd, J = 2.7, 8.5 Hz, 1H), 3.90 (s, 2H), 2.90 (q, J = 7.2
Hz, 2H), 1.21 (t, J = 7.2 Hz, 3H); 13C NMR (101 MHz, DMSO) δ 146.9, 140.4, 118.4, 117.0,
116.5, 116.4, 44.8, 41.4 and 10.7; MS: m/z 167 [M+H]+.
General procedure for the synthesis of compound 19a-f and 19i
Compound 10a-b/15/18 (1.0 equiv.) was added to a stirred mixture of compound 6a-d (1.1
equiv.) and triethylamine (4.0 equiv.) in THF:DMF (4.5:0.5) in microwave tube. The mixture
was irradiated in microwave at 80οC for 20 min. The solvent was removed under reduced
pressure and the residue was purified through column chromatography using MeOH:DCM as
a eluent. The compound was further recrystallized from ethanol.
General procedure for the synthesis of compound 19g-h
Compounds 6b-c (1.0 equiv.) was added to a stirred mixture of 18 (1.5 equiv.) in EtOH in a
microwave tube followed by HCl (50 µL, 4M HCl in dioxane). The reaction mixture was
irradiated in microwave at 80oC for 1.3 h. The solvent was evaporated in vacuo and the
residue was purified by flash chromatography using 5 – 20% MeOH/DCM as eluent to obtain
the desired compounds.
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(trifluoromethyl)phenyl)benzo[4,5]imidazo[1,2-a]pyridine-4-carbonitrile 19a:
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Yellow solid (10%); m. p. 151-153 C; Rf 0.3 (6% MeOH-DCM); H NMR (600 MHz,
DMSO) δ 8.71 (d, J = 8.2 Hz, 1H), 7.85 (d, J = 8.2 Hz, 2H), 7.76 (d, J = 8.2 Hz, 2H), 7.71
(d, J = 7.8 Hz, 1H), 7.46 (t, J = 7.5 Hz, 1H), 7.25 (t, J = 7.6 Hz, 1H), 7.12 (s, 1H), 7.08 (d, J
= 8.4 Hz, 1H), 6.84 (d, J = 8.4 Hz, 1H), 5.79 (s, 1H), 3.92 (s, 2H), 2.77 (q, J = 7.1 Hz, 4H),
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1.09 (t, J = 7.1 Hz, 6H); C NMR (150 MHz, DMSO) δ 150.9, 150.8, 150.5 (2C), 148.4
(2C), 144.8, 144.7, 142.9 (2C), 129.8, 129.3 (2C), 125.9 (2C), 125.8, 125.4, 124.9, 123.6,
121.8, 119.9, 117.2, 116.7 (2C), 93.5, 53.3, 46.7 (2C) and 10.5 (2C); MS: m/z 530 [M+H]+;
HPLC purity 99% (tr = 3.44).
1-((4-((Diethylamino)methyl)-3-hydroxyphenyl)amino)-3-
(trifluoromethyl)benzo[4,5]imidazo[1,2-a]pyridine-4-carbonitrile 19b:
Yellow solid (0.2g, 53%); m. p. 314-316 οC; Rf 0.3 (8% MeOH-DCM); 1H NMR (400 MHz,
CD3OD) δ 8.78 (d, J = 8.3 Hz, 1H), 7.61 (d, J = 7.9 Hz, 1H), 7.40 (t, J = 7.6 Hz, 1H), 7.26 (d,
J = 7.9 Hz, 1H), 7.19 (t, J = 7.3 Hz, 1H), 6.64 (d, J = 1.9 Hz, 1H), 6.61 (dd, J = 7.9, 2.0 Hz,
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1H), 6.03 (s, 1H), 4.25 (s, 2H), 3.20 (q, J = 7.3 Hz, 4H), 1.37 (t, J = 7.3 Hz, 6H); C NMR
(100 MHz, CD3OD) δ 157.7, 153.0, 151.5, 150.2, 135.9, 132.3, 130.9, 130.2, 128.4, 124.6,
121.8, 119.6, 117.4, 116.6, 114.6, 113.8, 110.8, 108.7, 90.8, 67.7, 52.2 (2C) and 12.9 (2C);
MS: m/z 454 [M+H]+; HPLC purity 99% (tr = 3.50).
1-((3-((Diethylamino)methyl)-4-hydroxyphenyl)amino)-3-
(trifluoromethyl)benzo[4,5]imidazo[1,2-a]pyridine-4-carbonitrile 19c:
Yellow solid (0.12g, 26%); m. p. 172-174 οC; Rf 0.3 (6% MeOH-DCM); 1H NMR (400 MHz,
DMSO) δ 8.81 (d, J = 7.6 Hz, 1H), 7.56 (d, J = 7.9 Hz, 1H), 7.34 (t, J = 8.0 Hz, 1H), 7.11 (t,
J = 8.0 Hz, 1H), 7.01 (d, J = 2.0 Hz, 1H), 6.92 (m, 2H), 5.78 (s, 1H), 4.19 (s, 2H), 3.09 (q, J =
13
7.1 Hz, 4H), 1.24 (t, J = 7.2 Hz, 6H); C NMR (100 MHz, DMSO) δ 152.0, 151.8, 150.9,
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145.0, 142.8, 134.7, 134.4, 130.9, 125.7, 125.3, 124.8, 124.5, 122.6, 119.1, 118.6, 118.2,
117.0, 116.9, 116.1, 88.7, 51.4, 47.1 and 9.2 (2C); MS: m/z 454 [M+H]+; HPLC purity 99%
(tr = 3.29).
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1-((3-((diethylamino)methyl)-4-hydroxyphenyl)amino)-7,8-difluoro-3-(4-
(trifluoromethyl)phenyl)benzo[4,5]imidazo[1,2-a]pyridine-4-carbonitrile 19d:
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Yellow solid (26 %); m.p 284-288 °C; Rf 0.65 (20 % MeOH-DCM); H NMR (400 MHz,
DMSO) δ 8.76 (dd, J = 11.5, 8.7 Hz, 1H), 7.80 (d, J = 8.2 Hz, 2H), 7.69 (d, J = 8.2 Hz, 2H),
7.55 (dd, J = 11.5, 8.7 Hz, 1H), 7.06 (d, J = 2.6 Hz, 1H), 6.99 (dd, J = 8.5, 2.6 Hz, 1H), 6.86
(d, J = 8.5 Hz, 1H), 5.59 (s, 1H), 4.08 (s, 2H), 2.97 (q, J = 7.2 Hz, 4H), 1.16 (t, J = 7.2 Hz,
13
6H); C NMR (101 MHz, DMSO) δ 153.8, 151.0, 149.3, 149.2, 147.8, 146.8, 144.9, 143.8,
141.8, 141.6, 129.4, 129.2, 126.2, 126.0, 125.8, 125.8, 125.1, 123.3, 120.3, 116.7, 105.0,
104.8, 103.5, 103.3, 92.6, 55.4, 51.9, 47.0, 25.6 and 9.6; HPLC-MS (APCI/ESI): Purity=
97%, tR = 4.15 min, MS: m/z 566.2 [M+H]+.
1-((3-((diethylamino)methyl)-4-hydroxyphenyl)amino)-2-fluoro-3-(4-
(trifluoromethyl)phenyl)benzo[4,5]imidazo[1,2-a]pyridine-4-carbonitrile 19e:
1
Yellow solid (11 %); m.p 275-280 °C; Rf 0.49 (20 % MeOH-DCM); H NMR (400 MHz,
DMSO) δ 8.66 (d, 1H), 7.88 (d, J = 8.1 Hz, 2H), 7.74 (d, J = 8.1 Hz, 2H), 7.59 (d, J = 7.8 Hz,
1H), 7.37 (t, J = 7.8 Hz, 1H), 7.10 (t, J = 7.8 Hz, 1H), 6.92 – 6.81 (m, 2H), 6.75 (d, J = 8.5
Hz, 1H), 4.02 (s, 2H), 2.92 (q, J = 8.0 Hz, 4H), 1.15 (t, J = 8.0 Hz, 3H); 13C NMR (101 MHz,
DMSO δ 152.4, 151.4, 149.3, 144.8, 144.7, 142.8, 137.8, 131.5, 130.9, 129.7, 129.4, 126.0,
125.6 (2C), 124.9, 123.5, 123.3, 122.5, 119.8, 118.9, 117.1, 116.2, 115.7, 106.5, 98.3, 52.1,
46.7, 31.1, 29.8 and 9.6; HPLC-MS (APCI/ESI): Purity= 99%, tR = 3.98 min, MS: m/z 548.1
[M+H]+.
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(trifluoromethyl)benzo[4,5]imidazo[1,2-a]pyridine-4-carbonitrile 19f:
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Yellow solid (33%); m. p. 209-211 C; Rf 0.2 (5% MeOH-DCM); H NMR (400 MHz,
DMSO) δ 8.81 (d, J = 7.6 Hz, 1H), 7.58 (d, J = 7.8 Hz, 1H), 7.37 (t, J = 8.0 Hz, 1H), 7.31 (t,
J = 8.0 Hz, 1H), 7.21 (dd, J = 6.7, 2.5 Hz, 1H), 7.12 (m, 2H), 5.76 (s, 1H), 4.29 (s, 2H), 3.11
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(q, J = 7.1 Hz, 4H), 1.24 (t, J = 7.2 Hz, 6H); C NMR (100 MHz, DMSO) δ 155.7, 151.4,
150.7, 135.0, 134.9, 130.6, 126.2, 125.4, 125.2, 124.7 (2C), 119.3 (2C), 118.4, 117.1 (2C),
116.9, 116.1, 88.3, 49.3, 47.3 (2C) and 8.9 (2C); MS: m/z 456 [M+H]+; HPLC purity 99% (tr
= 4.23).
1-((3-((ethylamino)methyl)-4-hydroxyphenyl)amino)-7,8-difluoro-3-(4-
(trifluoromethyl)phenyl)benzo[4,5]imidazo[1,2-a]pyridine-4-carbonitrile 19g: Yellow solid
ο
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(11%); m. p. 232-234 C; Rf 0.2 (15% MeOH-DCM); H NMR (400 MHz, DMSO) δ 8.76
(dd, J = 11.5, 8.7 Hz, 1H), 7.80 (d, J = 8.2 Hz, 2H), 7.69 (d, J = 8.2 Hz, 2H), 7.55 (dd, J =
11.5, 8.7 Hz, 1H), 7.06 (d, J = 2.6 Hz, 1H), 6.99 (dd, J = 8.5, 2.6 Hz, 1H), 6.86 (d, J = 8.5 Hz,
13
1H), 5.59 (s, 1H), 4.08 (s, 2H), 2.97 (q, J = 7.2 Hz, 2H), 1.16 (t, J = 7.2 Hz, 3H); C NMR
(101 MHz, DMSO) δ 153.8, 151.0, 149.3 (2C), 147.8, 146.8, 144.9, 143.8, 141.8 (2C), 141.6,
129.4, 129.2 (2C), 126.0, 125.8 (2C), 125.1, 123.3, 120.3, 116.7, 105.0, 104.8, 103.5, 103.3,
92.6, 55.4, 47.0 and 9.6; MS: m/z 538 [M+H]+; HPLC purity 99% (tr = 2.70).
1-((3-((ethylamino)methyl)-4-hydroxyphenyl)amino)-3-(4-
(trifluoromethyl)phenyl)benzo[4,5]imidazo[1,2-a]pyridine-4-carbonitrile 19h: Yellow solid
(14%); m. p. 230-232 οC; Rf 0.2 (15% MeOH-DCM); 1H NMR (600 MHz, DMSO) δ 8.67 (d,
J = 8.3 Hz, 1H), 7.85 (m, 4H), 7.78 (d, J = 8.1 Hz, 1H), 7.53 (t, J = 7.8 Hz, 1H), 7.43 (m,
1H), 7.33 (t, J = 7.8 Hz, 1H), 7.22 (m, 1H), 7.07 (d, J = 8.5 Hz, 1H), 5.94 (s, 1H), 4.03 (s,
2H), 2.90 (q, J = 7.3 Hz, 2H), 1.21 (t, J = 7.3 Hz, 3H); 13C NMR (151 MHz, DMSO) δ 150.9,
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150.8, 150.5 (2C), 148.4 (2C), 144.8 (2C), 142.9 (2C), 129.8, 129.3 (2C), 125.9 (2C), 125.8,
125.4, 124.9, 123.6, 121.8, 119.9, 117.2, 116.7 (2C), 93.5, 53.3, 46.7 and 10.5; MS: m/z 502
[M+H]+; HPLC purity 99% (tr = 2.60).
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1-((4-((diethylamino)methyl)-3-hydroxyphenyl)amino)-3--(4-
(trifluoromethyl)phenyl)benzo[4,5]imidazo[1,2-a]pyridine-4-carbonitrile 19i: Yellow solid
ο
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(20%); m. p. 232-234 C; Rf 0.2 (10% MeOH-DCM); H NMR (600 MHz, CD3OD) δ 8.57
(d, J = 8.3 Hz, 1H), 7.75 (m, 4H), 7.72 (d, J = 7.9 Hz, 1H), 7.46 (t, J = 7.6 Hz, 1H), 7.26 (d, J
= 7.9 Hz, 1H), 7.25 (t, J = 7.3 Hz, 1H), 7.04 (d, J = 7.8 Hz, 1H), 6.65 (m, 2H), 6.11 (s, 1H),
3.86 (s, 2H), 2.75 (q, J = 7.3 Hz, 4H), 1.15 (t, J = 7.3 Hz, 6H); 13C NMR (151 MHz, CD3OD)
δ 159.8, 149.7, 149.5, 145.5 (2C), 144.2, 131.8, 131.3, 131.2, 130.5, 129.8 (2C), 126.1 (2C),
124.6, 123.2, 121.8, 120.1, 117.6, 117.1, 116.7, 114.5, 110.8, 108.7, 90.8, 56.6, 47.2 (2C),
and 11.1 (2C); MS: m/z 530 [M+H]+; HPLC purity 98% (tr = 2.62).
MATERIALS AND METHODS:
In Vitro P. falciparum Assay and In Vivo Antimalarial Efficacy Studies: Compounds
were screened against multidrug resistant (PfK1) and sensitive (PfNF54) strains of P.
falciparum in vitro using the modified [3H]-hypoxanthine incorporation assay.26 In vivo
efficacy against P. berghei was performed using mice (n = 3) which were infected with a
GFP-transfected P. berghei ANKA strain (donated by A. P. Waters and C. J. Janse, Glasgow
and Leiden Universities respectively).27 Parasitemia was determined using standard flow
cytometry techniques. Briefly, 3 µL of tail blood from mice in the treated and control groups
was pipetted into staining solution of 100 nM MitoTracker Deep Red and 1× SYBR green in
1× phosphate-buffered saline (pH 7.4) and incubated at 37 °C for at least 20 min in the dark.
The cells were then quantified on an Accuri C6 flow cytometer (Becton Dickinson) with
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approximately 10, 000 events read per sample. The detection limit was 1 parasite per 1,000
erythrocytes (that is, 0.1%). Activity was calculated as the difference between the mean %
parasitemia for the control and treated groups expressed as a % relative to the control group.
Compounds were dissolved or suspended in 90/10 Tween80/ethanol (v/v), diluted 10 times
with water and administered orally as four consecutive daily doses (4, 24, 48 and 72 h after
infection). Animals were considered cured if there were no detectable parasites on day 30
post-infection as determined by light microscopy.
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Cytotoxicity Testing: Compounds were screened for in vitro cytotoxicity against Chinese
Hamster Ovarian (CHO) mammalian cell-lines, using the 3-(4,5-dimethylthiazol-2-yl)-2,5-
diphenyltetrazoliumbromide (MTT)-assay. The reference standard, emetine, was prepared to
2 mg/ml in distilled water while stock solutions of test compounds were prepared to 20
mg/ml in 100% DMSO with the highest concentration of solvent to which the cells were
exposed having no measurable effect on the cell viability. The initial concentration of the
drugs and control was 100 μg/mL, which was serially diluted in complete medium with 10-
fold dilutions to give 6 concentrations, the lowest being 0.001 μg/mL. Plates were incubated
for 48 h with 100 μL of drug and 100 μL of cell suspension in each well and developed
afterwards by adding 25 μL of sterile MTT (Thermo Fisher Scientific) to each well and
followed by 4 h incubation in the dark. The plates were then centrifuged, medium aspirated
off and 100 μL of DMSO added to dissolve crystals before reading absorbance at 540 nm.
IC50 values were then obtained from dose-response curves, using a nonlinear dose-response
curve fitting analysis via GraphPad Prism v.4.0 software (La Jolla, USA). The assay was
conducted in triplicate and conducted on two separate occasions.
Single Point In Vitro Metabolic Stability Assay: The test compound (1 µM) was incubated
at 37 °C in a solution containing 0.35 mg/mL microsomes (MLM; male Mouse BALB/c,
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Xenotech, or HLM, mixed gender, Xenotech) and metabolic reactions were initiated by the
addition of NADPH (1 mM) in phosphate buffer (100 mM, pH 7.4), and incubated for 30
min. The samples were then prepared by cold-ice acetonitrile precipitation containing 0.1 µM
carbamazepine (internal standard), centrifuged and filtered for LC-MS analysis. The
incubation of compounds and controls (midazolam and MMV390048) were performed in
triplicate. The relative loss of parent compound over the course of the incubation was
monitored by LC-MS/MS and results reported as % remaining after 30 min incubation.
LC-MS/MS analyses were performed on a 4000 QTRAP® mass spectrometer (AB Sciex)
equipped with a Turbo V™ ion source and coupled to an Agilent 1200 Rapid Resolution
HPLC system (600 bar, Agilent technologies, USA). 2 µL of samples stored in a sample tray
set at 8°C were injected onto a Kinetex C18 column, 2.1 mm x 50 mm, 2.6 µm particles
(Phenomenex) or Kinetex PFP column, 2.1 mm x 50 mm, 2.6 µm particles (Phenomenex) at
40 °C. Compounds were separated using a gradient solvent system consisting of 0.1% formic
acid (A) and 0.1% formic acid in acetonitrile (B). MRM scans were operated under
electrospray positive ionization mode and the operation parameters were as follows: curtain
gas, 30 psi; nebulizer gas (GS1), 50 psi; turbo gas (GS2), 60 psi; source temperature, 500 °C;
ion spray voltage, 5000 V; declustering potential and collision energy optimized from
infusion of the compounds; collisional activated dissociation (CAD) gas setting: medium.
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Metabolite Identification and Glutathione Trapping Experiments: Metabolites formed in
the incubations to determine single point microsomal stability were identified by comparison
of their respective chromatograms with chromatograms at T=0 and in the buffer only control
using Lightsight v2.3. The tentative identity of the metabolites was deduced by comparison
of the product ion spectra of the [M+H]+ ions of the metabolites with that of parent
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compound. A confirmatory identification was done by comparing the retention and
fragmentation of the detected metabolite to that of a synthetic metabolite.
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For the glutathione (GSH) trapping experiments, the test compounds (10 μM) were incubated
at 37 °C in solutions containing HLM (1 mg/mL; mixed gender, pool of 50 individuals,
Xenotech, Lot. 1210153), reduced glutathione (5 mM, Sigma Aldrich) and NADPH (1 mM,
Calbiochem) in sodium phosphate buffer (50 mM, pH 7.4). Control incubations in which
either GSH, NADPH, test compound or HLM were individually excluded were also prepared.
For each test and control preparation, T0 incubations were terminated by addition of
acetonitrile at the start of the incubation. T90 incubations were terminated after 90 minutes
by addition of ice-cold acetonitrile. All samples were subsequently centrifuged at 10,000 rpm
for 10 min and the resultant supernatant solutions filtered for LC-MS analysis. Diclofenac (5
μM) was incubated concomitantly as a positive control. The supernatant was analyzed by LC-
MS/MS (5500 Q-TRAP AB Sciex) with a Supelco Ascentis C18, 4.6x150 mm, 2.6 µm
particles column, using EMS-IDA-EPI methods.
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ASSOCIATED CONTENT
Supporting Information
Additional details of the structures of all derivatives assessed are provided as Supplementary
material. Excel file with the compounds SMILES format is also provided
ACKNOWLEDGEMENTS
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 South African National Research Foundation are gratefully acknowledged for
support (K.C). At Swiss TPH, we thank Christoph Fischli, Ursula Lehmann, Sibylle Sax and
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Christian Scheurer for assistance in performing the [3H]-hypoxanthine incorporation assay
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FIGURES AND TABLES
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O
OH
O
7
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9
NEt2
NEt2
N
N
N
CHO
HN
N
N
HN
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Cl
Cl
N
Cl
Cl
N
B
( )
A
( )
C
( )
D
( )
Amodiaquine
Chloroquine
N
OH
OH
N
HN
N
N
HN
N
N
O
Cl
F
( )
Cl
Amopyroquine
E
( )
Pyronaridine
Cl
N
OH
N
OH
HN
H
N
HN
Cl
N
H
( )
Cycloquine
Cl
N
G
( )
Tebuquine
Figure 1: Chemical structures of chloroquine (A), amodiaquine (B), reactive amodiaquine
metabolites (C and D) and examples of antimalarials in clinical use or under development (E
– H) that bear the 4-aminophenol motif (highlighted in red).
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R
4
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F
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NEt2
HN
N
N
H
HN
N
OH
Cl
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I
( )
Cl
4'-dehydroxy-4'-fluoroamodiaquine
J
( )
Isotebuquine derivatives
R
F
H
HN
N
OH
N
HN
Cl
Cl
N
M
)
(
K
=
( ) Isoquine R NEt2
4'-fluoro-N-tert-butyl amodiaquine
L
=
( ) N-tert-butyl isoquine R NHtBu
X
N
NHR
HN
N
Cl
N
=
( ) Benzothiazole X S; R = -C(CH3)3 or -CH2CH3
O
=
( ) Benzoxazole X O; R = -C(CH3)3 or -CH2CH3
P
=
( ) Benzimidazole X NH; R = -C(CH3)3 or -CH2CH3
Figure 2: Chemical structures of representative examples of compounds (I – P) from
different studies designed to bypass AQ-like bioactivation.
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