2-Phenylimidazo[1,2-b]pyridazine derivatives highly active against Haemonchus contortus

Article abstract of DOI:10.1016/j.bmcl.2011.05.096

A series of 2-phenylimidazo[1,2-b]pyridazine derivatives were synthesized and evaluated for their in vitro anthelmintic activity against Haemonchus contortus. The most active compounds had in vitro LD₉₉ values of 30 nM, which is comparable to that of the benchmark commercial nematocide, Ivermectin.

Full text of DOI:10.1016/j.bmcl.2011.05.096

Bioorganic & Medicinal Chemistry Letters 21 (2011) 4160–4163
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
2-Phenylimidazo[1,2-b]pyridazine derivatives highly active against Haemonchus
contortus
Abdelselam Ali a, Teresa Cablewski a, Craig L. Francis a, , Ashit K. Ganguly b, Roger M. Sargent c,
⇑
David G. Sawutz b, Kevin N. Winzenberg a
a CSIRO Materials Science and Engineering, Bayview Avenue, Clayton, VIC 3168, Australia
b Merck Research Laboratories, Merck & Co., 2015 Galloping Hill Road, Kenilworth, NJ 07033-1300, USA
c Intervet Australia Pty Ltd, 26 Artisan Road, Seven Hills, NSW 2147, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2-phenylimidazo[1,2-b]pyridazine derivatives were synthesized and evaluated for their
in vitro anthelmintic activity against Haemonchus contortus. The most active compounds had in vitro
LD99 values of 30 nM, which is comparable to that of the benchmark commercial nematocide, Ivermectin.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 1 April 2011
Revised 24 May 2011
Accepted 25 May 2011
Available online 1 June 2011
Keywords:
Imidazo[1,2-b]pyridazine
Anthelmintic
Haemonchus contortus
In many parts of the world the viability of small-ruminant pro-
duction is under threat because of growing resistance to the lim-
ited number of current drugs used to treat infection by
endoparasites, for example control of Haemonchus contortus in
sheep.1 Two recently introduced anthelmintics, Monepantel2 and
Derquantel,3 are welcome, but new drugs are still urgently
needed.4
pyridazine 211 with a sodium alkoxide4 or a sodium alkanethiolate
salt12 to give 3-amino-6-alkoxypyridazines 3 and 3-amino-6-alkyl-
thio-pyridazines 4, respectively. Reaction of these aminopyrida-
zines with 2-bromoethanones 5 afforded the target 6-substituted
imidazo[1,2-b]pyridazines 6 and 7. Analogous reactions of 3 and
4 with 2-bromopropan-1-ones 8 provided the 3-methyl derivatives
9 and 10.
In the course of our search for new anthelmintics based upon
high-throughput screening of compounds in the commercial Nem-
aTox H. contortus (McMaster strain) larval development assay,5 we
obtained the hit compound, 2-(3,4-methylenedioxyphenyl)-6-pro-
poxy-imidazo[1,2-b]pyridazine 16,7 (Fig. 1) which afforded an LD99
comparable to the commercial nematocide Levamisole (Table 1).
Importantly, compound 1 maintained activity against resistant
The C-6 amino derivatives 13a–d were obtained by heating
6-chloroimidazo[1,2-b]pyridazine 12, prepared6 from 2 and 2-bro-
mo-1-[4-(2-methoxyethoxy)phenyl]-ethanone 11,13 with the
appropriate amine.14,15 The 6-n-butyl substituted compound 14
was obtained from 12 by a Ni-catalyzed coupling reaction with
n-BuMgBr. The 6-aryl substituted derivatives 15 were prepared
via Suzuki-coupling of aryl boronic acids with 12 using a published
procedure.16,17
strains of H. contortus: LD99 = 0.94
tant VRSG8 strain, LD99 = 0.67
M for the benzimidazole- and Lev-
amisole-resistant Lawes strain, and LD99 = 1.85 for the
Ivermectin-resistant CAVR9 strain. Furthermore, compound
showed good activity against the McMaster strains of Trichostron-
gylus colubriformis, LD99 = 0.67 M, and Ostertagia circumcincta,
LD99 = 0.33 M.
lM for the benzimidazole-resis-
l
Compounds 6 and 7 were brominated with NBS in chloroform18
to produce 3-bromo-substituted imidazo[1,2-b]pyridazine deriva-
tives 16. We also prepared 3-chloro derivatives 17 and 3-iodo
derivatives 1817 using similar chemistry. The 3-iodo compounds
were converted to the corresponding 3-cyanoimidazo[1,2-b]pyrid-
azines 19 by treatment with cuprous cyanide in hot DMF.19
l
M
1
l
l
Exploration of SAR associated with compound 1 was aided by
well-established synthetic routes to the imidazo[1,2-b]pyridazine
template.10 As outlined in Scheme 1, key steps in the preparation
of compounds of this type involved reaction of 3-amino-6-chloro-
O
3
nPrO
6
N
N
2
O
7
N
8
1
⇑
Corresponding author. Tel.: +61 3 9545 2222.
E-mail address: craig.francis@csiro.au (C.L. Francis).
Figure 1.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.05.096

A. Ali et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4160–4163
4161
Table 1
In vitro anthelmintic activity of 2-phenylimidazo[1,2-b]pyridazines 1, 6, 7, 9, 10, 13–22, 24 against H. contortus
R3
R1
R4
N
R2
N
N
R5
a
Compound
R1
R2
R3
R4
R5
LD99 (lM)
1
6a
n-PrO
n-PrO
3,4-Methylenedioxy
H
H
H
H
H
H
H
0.77
na
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
6l
6m
6n
n-PrO
n-PrO
n-PrO
n-PrO
n-PrO
n-PrO
n-PrO
EtO
n-BuO
Cyclopropylmethoxy
Cyclopropylmethoxy
Cyclopropylmethoxy
4-Cl
3-OMe
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
13.03
0.81
4.15
2.60
0.29
0.32
2.27
1.30
0.64
0.11
0.11
0.09
na
3,4-diOMe
3,4-Ethylenedioxy
4-OCH2CH2OMe
4-OCH2CH2OEt
3-OMe–4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OEt
4-OEt
Cyclopropylmethoxy
4-CF3
H
H
H
6o
6p
6q
6r
6s
7a
7b
7c
7d
7e
7f
7g
7h
7i
Cyclopropylmethoxy
Cyclopropylmethoxy
Cyclopentyloxy
i-PrO
MeOCH2CH2O
n-PrS
n-PrS
n-PrS
n-PrS
n-BuS
2,4-diCl
4-Et
4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
4-OEt
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
na
0.27
5.31
4.96
17.47
1.21
2.10
2.36
2.17
0.26
0.59
0.59
0.87
na
3-OMe
4-OCH2CH2OMe
3-OMe-4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OEt
4-OCH2CH2OMe
4-OCH2CH2OMe
4-CF3
n-BuS
i-PrS
i-BuS
n-BuS
7j
n-PrS
2,4-diCl
H
H
H
na
9a
9b
9c
10
13a
13b
13c
14
15a
15b
16a
16b
16c
17a
17b
18a
18b
19a
19b
20a
20b
21a
21b
22
n-PrO
n-PrO
i-PrO
i-PrS
Pyrrolidino
n-PrNH
n-Bu(Me)N
n-Bu
Ph
3-Thienyl
n-PrO
Cyclopropylmethoxy
n-PrS
n-PrO
BnO
n-PrO
n-PrO
n-PrO
n-PrO
n-PrO
4-OCH2CH2OMe
3-OCH2CH2OMe
3-OCH2CH2OMe
3-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
3-OMe-4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
4-OCH2CH2OMe
3-OMe–4-OCH2CH2OMe
4-OCH2CH2OMe
3-OMe–4-OCH2CH2OMe
4-OCH2CH2OMe
4-OEt
Me
Me
Me
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
Me
Fused
Fused
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
0.32
5.49
14.65
19.58
9.60
2.11
2.29
1.92
8.11
1.96
0.03
0.10
1.66
0.06
na
0.03
10.35
0.11
na
0.32
0.60
0.91
1.80
9.29
na
H
Br
Br
Br
Cl
Cl
I
I
H
H
H
H
CN
CN
H
H
H
H
Ph
H
n-PrO
n-PrO
n-PrO
n-PrO
H
4-OCH2CH2OMe
4-OEt
4-OCH2CH2OMe
4-OCH2CH2OMe
Phenyl
Phenyl
H
24
H
H
H
Levamisole
Ivermectin
0.66–2.00
0.006–0.031
a
na = not active (LD99 >15 lg/mL).
The 7-methyl compounds 20 (Fig. 2) were synthesized by reac-
tion of 3-amino-5-methyl-6-propoxypyridazine (prepared from
3-amino-6-chloro-5-methylpyridazine20
with the appropriate
The 3-phenyl derivative 22 (Fig. 2) was obtained from the cor-
responding 3-iodo compound 18a by
a
Suzuki-coupling
)
reaction.16,17
2-bromo-1-(4-alkoxyphenyl)ethanone. Analogous chemistry with
The results from the screening of compound 1 and a range of
examples of compounds of type 6, 7, 9, 10, 13–22, and 24 in the
1-amino-4-chlorophthalazine20 gave compounds 21 (Fig. 2).

4162
A. Ali et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4160–4163
O
MeO
Br
OMe
R1
N
Cl
N
Cl
N
(i)
11
O
N
1
N
N
3
4
R =RO
O
1
N
R =RS
(ii)
NH2
NH2
2
12
O
R2
Br
(ii)
5
8
R3=H
(iii), (iv)
or (v)
3
R3
R2
R =Me
OMe
R3
X
R1
N
R1
N
R1
N
R2
(vi) or (vii)
R3=H
N
N
N
O
N
N
18
N
1
1
16
13
6
R1=RO, R3=H
7
R1=RS, R3=H
X=Br
X=I
R =RR
4
R =n-Bu
N
(viii)
1
3
1
3
15 R1=aryl
9
10
R =RO, R =Me
R =RS, R =Me
17 X=Cl 19 X=CN
Scheme 1. Reagents and conditions: (i) ROH, Na, 130 °C or RSH, NaOH, H2O, 130 °C; (ii) NaHCO3, EtOH, reflux; (iii) R4R5NH, heat; (iv) n-BuMgBr, cat. [Ph2P(CH2)3PPh2ÁNiCl2],
THF, rt; (v) R1B(OH)2, cat. Pd(PPh3)4, NaOH, DME, H2O, 75 °C; (vi) NBS or NCS, CHCl3, reflux; (vii) NIS, CH3CN, rt; (viii) CuCN, DMF, 100 °C.
nPrO
N
nPrO
N
N
N
OR
OR
NemaTox H. contortus larval development assay are shown in Table
1. Removal of the 3,4-methylenedioxyphenyl substituent at posi-
tion 2 in 1 led to loss of activity, since compound 23 (Fig. 2), pre-
pared from reaction of 3-amino-6-propoxypyridazine6 and
bromoacetaldehyde diethyl acetal,21 was inactive. Replacement of
the 3,4-methylenedioxyphenyl substituent by unsubstituted phe-
nyl (compound 6a) or 4-chlorophenyl (6b) or 4-trifluoromethyl-
phenyl (6n) likewise led to striking loss of activity, indicating
that an electron-rich C-2 phenyl substituent was desirable. Re-
moval of the propoxy substituent at position 6 led to dramatic loss
of activity (compound 24, Table 1).
N
N
20a
20b R = Et
R = (CH2)2OMe
21a R = (CH2)2OMe
21b R = Et
OMe
nPrO
N
nPrO
N
N
N
O
N
N
23
22
Figure 2.
Replacement of the potentially metabolically- and chemically-
labile methylenedioxy moiety of compound 1 by 3,4-dimethoxy
substitution (6d) or an ethylenedioxy moiety (6e) resulted in
weaker activity; however, replacement of the methylenedioxy
moiety by non-classical isosteric alkoxyethoxy substituents22,23
(compounds 6f and 6g) or an ethoxy group (6m) led to a significant
improvement in activity.
Further improvement in activity was observed when the pro-
poxy substituent at C-6 was replaced by a cyclopropylmethoxy
group (6k). Shortening or lengthening the C-6 alkoxy group re-
sulted in lesser activity (6i, 6j) and incorporation of secondary alk-
oxy groups (6q, 6r) led to poor activity. The activity of the 6-
alkylthio-imidazo[1,2-b]pyridazines 7 followed a similar trend
(though generally slightly less potent) to that of the 6-alkoxy
analogues.
the 3-bromo compound 16a and the 3-iodo derivative 18a is com-
parable to the level of activity associated with the Ivermectin class
of commercial endoparasiticides.
The 3-cyano derivative 19a also showed good activity while the
activity of 3-phenyl derivative 22 was relatively poor. The results
from compounds 16–19 and 22 indicate that for strong activity
the 3-substituent should be electron-withdrawing and not too
large.
In conclusion, we have discovered a class of compounds that are
highly active in a H. contortus larval development assay. For high
activity, 6-n-propoxy or 6-cyclopropylmethoxy, and 2-(4-[2-
methoxyethoxy])phenyl substituents appear preferable: see (in Ta-
ble 1) compounds 6f, 6k, 9a, 16a, 16b, 17a, 18a and 20a, which
show a level of activity superior to Levamisole. The potent activity
of the 3-halo derivatives 16a and 18a is comparable to that of the
benchmark commercial product, the macrocyclic lactone
Ivermectin.
The relatively good activity of the 3-methyl imidazo[1,2-b]
pyridazine derivative 9a compared with analogues 9b, 9c and 10
indicated that having the 2-methoxyethoxy substituent at the
4-position of the C-2 phenyl moiety generally resulted in better
activity than when the substituent is at the 3-position.
There remains scope for further exploration of the imidazo[1,2-
b]pyridazine template which may lead to compounds useful for
treatment of helminth infections in animals.
The 6-amino derivatives 13 showed reasonable activity (though
less potent than the 6-alkoxy or 6-alkylthio compounds), with the
n-propylamino derivative 13b being the most active.
The 6-butyl compound 14, the direct C-linked analogue of the
highly active propoxy derivative 6f, showed good activity against
H. contortus, as did the 6-(3-thienyl) substituted derivative 15b.
The 6-phenyl analogue 15a was much less potent, suggesting that
the presence of the heteroatom may be significant.
Compounds 20 and 21 also showed good activity with the 7-
methyl compounds 20 being superior to the corresponding phthal-
azine derivatives 21, indicating that in order to maintain high
activity, minimal steric bulk should be incorporated at positions
7 and 8 of the imidazo[1,2-b]pyridazine core.
Acknowledgments
We thank Marianne Bliese (CSIRO) for technical assistance with
chemistry and Andrew Smith (Intervet Australia Pty Ltd) for man-
agement of biological screening.
Supplementary data
Supplementary data (representative synthetic procedures and
spectral data) associated with this article can be found, in the on-
line version, at doi:10.1016/j.bmcl.2011.05.096.
The 3-substituted imidazo[1,2-b]pyridazine derivatives 16–19
showed very potent activity. The LD99 value of 30 nM for each of

A. Ali et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4160–4163
4163
7. Winzenberg, K. N.; Francis, C. L.; Sawutz, D. G.; Ganguly, A. U.S. Patent
7,759,349, 2010; Chem. Abstr., 2005, 143, 153390.
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