SAR studies of pyridazinone derivatives as novel glucan synthase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2011.03.083

A novel series of pyridazinone analogs has been developed as potent β-1,3-glucan synthase inhibitors through structure-activity relationship study of the lead 5-[4-(benzylsulfonyl)piperazin-1-yl]-4-morpholino-2-phenyl- pyridazin-3(2H)-one (1). The effect of changes to the core structure is described in detail. Optimization of the sulfonamide moiety led to the identification of important compounds with much improved systematic exposure while retaining good antifungal activity against the fungal strains Candida glabrata and Candida albicans.

Full text of DOI:10.1016/j.bmcl.2011.03.083

Bioorganic & Medicinal Chemistry Letters 21 (2011) 2890–2893
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
SAR studies of pyridazinone derivatives as novel glucan synthase inhibitors
Gang Zhou a, , Pauline C. Ting a, Robert Aslanian a, Jianhua Cao a, David W. Kim a, Rongze Kuang a, Joe F. Lee a,
⇑
John Schwerdt a, Heping Wu a, R. Jason Herr b, Andrew J. Zych b, Jinhai Yang b, Sang Lam b,
Samuel Wainhaus c, Todd A. Black d, Paul M. McNicholas d, Yiming Xu d, Scott S. Walker d
a Department of Chemical Research, Merck Research Laboratories, 2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
b Albany Molecular Research Institute, Albany, NY 12203, USA
c Department of Drug Metabolism, Merck Research Laboratories, 2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
d Department of Biological Research, Merck Research Laboratories, 2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel series of pyridazinone analogs has been developed as potent b-1,3-glucan synthase inhibitors
through structure–activity relationship study of the lead 5-[4-(benzylsulfonyl)piperazin-1-yl]-4-mor-
pholino-2-phenyl-pyridazin-3(2H)-one (1). The effect of changes to the core structure is described in
detail. Optimization of the sulfonamide moiety led to the identification of important compounds with
much improved systematic exposure while retaining good antifungal activity against the fungal strains
Candida glabrata and Candida albicans.
Received 20 January 2011
Revised 17 March 2011
Accepted 22 March 2011
Available online 30 March 2011
Keywords:
Pyridazinone
Published by Elsevier Ltd.
1,3-Glucan synthase inhibitors
Antifungal
Over the past decades, several different series of antifungal
agents, such as the azoles or polyenes, have been widely used for
the treatment of invasive fungal infections caused by Candida albi-
cans and Aspergillus fumigatus.1 However, to overcome their off-tar-
get toxicity, fungistatic activity and emerging resistance, a new
class of macrocyclic lipopeptidolactones2 represented by caspofun-
gin and anidulafungin have been developed as potent fungicidal
agents. The lipopeptidolactones2 exert their fungicidal activity
through a totally different mechanism of action from the azoles
and polyenes by disrupting the fungal cell wall polymer synthesis
through inhibition of b-1,3-glucan synthase (GS). Consequently,
they have attracted considerable attention. A major drawback of
this new class of compounds, however, is that they are only deliv-
erable via the intravenous (iv) route so an orally bioavailable small
molecule inhibitor is desirable. Unfortunately, only a few small
molecule GS inhibitors have been reported to date.3 Herein, we de-
scribe the SAR optimization of a novel pyridazinone series of small
molecule glucan synthase inhibitors.
H2N
O
NH
OH
HO
OH
HN
O
HO
OH
HN
NH
O
O
O
H3C
HO
HN
CH3
N
H
O
O
H
N
N
H2N
OH
Me
HN
O
O
OH
OH
O
H
O O
NH
OH
N
H
O
Me
Me Me
H3C
HN
HO
O
NH
N
H
H
N
HO
H
OH
HO
O
OH
HO
Anidulafungin
Caspofungin
HO
Pfizer, 2006
Merck, 2001
⇑
Corresponding author. Tel.: +1 908 740 3724; fax: +1 908 740 7152.
E-mail address: gang.zhou@merck.com (G. Zhou).
0960-894X/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2011.03.083

G. Zhou et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2890–2893
2891
Table 1
In the course of our program pursuing orally active antifungal
SAR on core modification with pyridazinone
compounds, we have previously reported the identification of com-
pound 1 as a GS inhibitor with modest activity against the yeast
strain C. albicans via high throughput screening of the legacy Scher-
ing–Plough compound collection.4 Initial SAR studies towards
improving the biological activity of this series have been de-
scribed.5 In this Letter, we would like to provide further informa-
tion on our efforts to explore alternative core structures and to
optimize both the antifungal and pharmacokinetic profiles.
Preliminary examples of modification of the original pyridazi-
none core are summarized in Table 1. From our previous report,
introducing an ether or aryl chain at the C-4 position in place of
the morpholine ring in 1 produces a 5- to 10-fold improvement
in activity against C. glabrata and C. albicans. (compounds 2 and
8). Further introduction of an alkyl or ether chain at the C-6 posi-
tion (compounds 3 and 4) reduces antifungal activity dramatically.
While replacement of the pyridazinone core with a simple benzene
or pyridazine ring (compounds 5 and 6) removes all antifungal
activity, the pyrimidin-4-one and pyridinone (compounds 7 and
9) showed antifungal activity against fungal strain C. glabrata. A
comparisons of several different core series demonstrated that
the C-6 unsubstituted pyridazinone was the best core structure
in terms of antifungal activity.
In our previous report on the SAR of pyridazinone series, a
cyclopentyloxy or cyclohexyloxy group at C-4 shows relatively
optimal activity in combination with a 3-F/Cl substituted phenyl
group at the N-2 position.5 Since the sulfonamide moiety on the
right side of the structure was shown to be essential for antifungal
activity after screening of a variety of different moieties, we next
decided to optimize the sulfonamide tail. A large variety of sulfon-
amide groups have been introduced into the pyridazinone (10) and
are summarized in Table 2. Simple and substituted alkyl sulfona-
mides (compounds 10a–d) and aryl sulfonamides (compounds
10f–h) displayed relatively low activity against C. albicans except
for the thienyl compound (10e). The benzyl compound (10i) and
benzothiazolmethyl compound (10m) exhibited the best anti-
fungal activity. Activity against fungal strains C. glabrata and C.
albicans was increased 20- to 50-fold in comparison to the mor-
pholine compound (1). Extension of the benzyl sulfonamide to
the phenethyl (10k and l), or phenylpropyl (10n) result in a de-
crease in antifungal activity.
a
Compd Structure
GS IC50
C. glabrata
MIC100
C. albicans
b
b
(
l
g/mL)
MIC100
g/mL)
(
lg/mL)
(l
O
O
O
O
S
O
N
2
0.9
71
0.10
25
3.13
>50
O
N
N
N
N
O
O
S
O
N
3
O
N
N
O
O
O
O
S
N
O
N
O
4
4.2
6.25
>50
N
N
O
EtO
O
S
O
N
O
5
N
138
37
>50
> 50
>25
>25
EtO
O
S
O
N
N
6
>25
O
N
N
O
O
S
N
7
1.31
9.00c
O
N
Cl
N
N
F
O
S
O
N
8
9
NA
2.4
0.05
0.78
6.25
>50
O
O
N
N
N
F
O
O
The biggest challenge remaining for the sulfonamide series was
the very low plasma exposure shown in the rat upon oral dosing
mainly due to low solubility and high clearance.5 Therefore, signif-
icant optimization was still necessary in order to obtain in vivo
activity.
S
N
N
N
a
GS activity (C. albicans membranes, strain BWP17) IC50 values are the average of
at least two independent determinations.
We selected the most active benzylsulfonamide (compound 10i,
AUC = 0 lg h/mL) for further optimization. Since cold metabolite ID
b
C. galbrata (strain C624) and C. albicans (strain C693) MIC100 values are the
average of at least two independent determinations.
has shown the benzylsulfonamide moiety is a major site for metab-
olism, our initial approach was to block potential metabolic sites of
the benzylic position by F or Me, which did not improve the PK
profile and resulted in a deleterious effect on antifungal activity
(data not shown). Functional groups such as F, Me, CF3, CN, and
CO2Et were also introduced into the benzene ring (compounds
11a–e), but did not show any improvement over compound 10i
in rat pharmacokinetic studies (Table 3). An alternative approach
is to improve the hydrophilicity of the benzylsulfonamide series
by introducing polar group on the sulfonamide benzene ring since
this type of substitution on other parts of the molecule is not tol-
c
C. albicans (strain C693) MIC50 values are the average of at least two indepen-
dent determinations.
erated. Substitution on the phenyl ring with hydrophilic groups
such as a boronic acid in 11f, or a nitro group in 11h all displayed
no improvement in rat blood levels. Introduction of a carboxylic
acid group in 11g gave some improvement in rat PK but lost anti-
fungal activity. Replacement of the phenyl ring with the pyridine
as in 11i showed a small but measureable rat AUC (0–6 h) of
0.06 lg h/mL at 10 mg/kg po (MC vehicle) while retaining similar
antifungal activity to the parent. Most interesting, a hydrogen bond
donor amino group in the ortho, meta, or para position (compounds
11j–l) exhibited a slight improvement in rat AUC (0–6 h) at 10 mg/
O
O
S
O
N
N
kg po (MC vehicle): 0.29
lg h/mL for 11j, 0.05 lg h/mL for 11k, and
O
N
4
0.08 g h/mL for 11l. Only substitutions at the ortho and para posi-
l
5
2
N
N
tion retain good antifungal activity. Furthermore, by replacing the
cyclopentyloxy with the 1-ethyl-1-methylcyclopropanyloxy at the
1
1

2892
G. Zhou et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2890–2893
Table 2
Table 3
SAR of sulfonamide substitution on pyridazinones
SAR of substitution on pyridazinone benzylsulfonamide
O
S
O O
S
O
O
N
N
O
N
N
R
R
O
N
O
N
Cl
N
Cl
N
10
11
a
Compd
R
GS IC50
C. glabrata
C. albicans
Compd
R
C. glabrata
C. albicans
Rat AUC(0–6 h)
b
b
b
b
(l
g/mL)
MIC100
(l
g/mL)
MIC100
0.78c
3.13
(lg/mL)
MIC100
mL)
(
l
g/
MIC100
mL)
(l
g/
(
l
g h/mL)
(MC)c
10a
10b
0.41
0.62
0.10
10i
0.02
0.05
0.39
1.56
0
0.10
11a
0
10c
10d
10e
0.43
109.3
0.42
0.20
>50.0
0.05
3.13
>50.0
0.78
F
O
11b
11c
0.08
0.78
0.31
0
CO2H
CF3
>50.0
NT
S
11d
0.02
0.13
0
10f
10g
10h
NA
0.30
1.04
0.20
3.1
CN
0.074
0.16
33.4
0.78c
11e
11f
11g
0.78
0.42
25.0
6.25
13.4
>50.0
NT
0
CO2Et
N
S
B(OH)2
10i
10j
0.20
0.22
0.02
0.02
0.39
2.10
0.13
CO2H
NO2
10k
10l
0.80
0.29
0.08
0.10
5.00
1.56
11h
11i
0.05
0.78
0.20
0
O
<0.02
0.06
N
10m
10n
0.08
23.0
0.05
3.13
0.78
N
NH2
11j
0.03
0.39
0.35
3.13
0.29
0.05
>50.0
a
GS activity (C. albicans membranes, strain BWP17) IC50 values are the average of
at least two independent determinations.
11k
NH2
b
C. galbrata (strain C624) and C. albicans (strain C693) MIC100 values are the
average of at least two independent determinations.
C. albicans (strain C693) MIC50 values are the average of at least two indepen-
dent determinations.
11l
0.20
0.13
0.26
0.78
0.52
1.04
0.08
0.70
8.10
c
NH2
11ma
11na
OH
C-4 position, a dramatic increase in drug plasma exposure was ob-
served with the p-NH2 benzylsulfonamide (11n) while the p-OH
compound (11m) showed little effect on the rat PK profile. Com-
NH2
pound 11n exhibited rat hepatocyte clearance of 6.6
cells, minimal hERG activity (12% inhibition at 5 g/mL in the
rubidium assay), and no CYP450 2D6 and 2C9 liver enzyme issues
with IC50 >30 although CYP3A4 was inhibited with an
IC50 = 0.3 M.
lL/min/M
a
l
1-Ethyl-1-methylcyclopropanyloxy instead of cyclopentyloxy at the C-4
position.
C. galbrata (strain C624) and C. albicans (strain C693) MIC100 values are the
average of at least two independent determinations.
b
l
M
l
c
Rat PK at 10 mg/kg po with methylcellulose vehicle.
A common synthetic route to the synthesis of pyridazinone ana-
logs 10a–n and 11a–n has been depicted previously.5
Benzene sulfonamides 5 were prepared using the approach de-
scribed in Scheme 1 via a six step sequence starting from dichloro-
nitrobenzene 12 (Scheme 1).6 The chlorine ortho to the nitro group
in 12 was first transformed to phenol 13 then the ether chain was
installed at the C3 position via the Mitsunobu alkylation to give 14.
Stille coupling of 14 provided the necessary intermediate 15. A fur-
ther three-step manipulation converted the nitro compound 15
into the target 5.
18 with keto acid 19 gave the core pyrimidine 20. Treatment of
20 with POCl3 gave the dichloride 21. Subsequent displacement
of the two chlorines first with N-benzylsulfonyl and alcohol
respectively at the 5- and 6-position generated the desired pyrida-
zine 6.
The conversion of thiourea 23 to the final primidin-4-one 7 was
straightforward using a general synthetic process depicted in
Scheme 3. Thiourea 23 was first condensed with malonic methyl
ester 24 to generate 25. After conversion of 25 to the triflate 26,
The construction of pyridazine 6 followed the synthetic route
depicted in Scheme 2.7 Condensation of a substituted hydrazine

G. Zhou et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2890–2893
2893
EtO
EtO
Cl
H
N
O
b
O
a
O2N
Cl
c
a, b
O2N
HO
c
NH2
NO2
NO2
COOMe
COOMe
O
O
O
OTf
S
Cl
Cl
Ph
N
N
N
N
Cl
15
12
13
14
SMe
SMe
26
24
23
25
Cl
Cl
d
EtO
Ph
d
EtO
Ph
O
O
NH
e
H2N
O
O
f
EtO
S
O
N
N
S
f, g
e
NBoc
O
N
NBoc
N
O
Ph
O
N
N
O
N
O
N
N
16
N
N
N
N
17
5
SMe
28
7
Cl
27
Cl
Cl
Scheme 1. Reagents and conditions: (a) tBuOH, KOH,
D, 10 h; (b) DEAD, Ph3P,
EtOCH2CH2OH, THF, rt, 12 h; (c) Bu3SnPh, Pd(Ph3P)2Cl2, dioxane, 80 °C, 18 h; (d) Fe,
2 N HCl in water, ethanol, 3.5 h; (e) (ClCH2CH2)2NH, K2CO3, diglyme, 180 °C, 72 h;
(f) RSO2Cl, Et3N, CH2Cl2, rt, 10 h.
Scheme 3. Reagents and conditions: (a) NaOMe, MeOH, reflux; (b) MeI, MeOH,
50 °C; (c) Tf2O, 2,6-lutidine, DCM, À78 °C; (d) N-Boc-piperazine, DIPEA, chloroben-
zene,
lw, 140 °C; (e) NaBH4, NiCl2, MeOH, THF, 0 °C, 1 h; (f) 4 N HCl in dioxane,
CH2Cl2; (g) RSO2Cl, Et3N, CH2Cl2, rt, 10 h.
Cl
O
O
a
b
NHNH2
+
Cl
Following the seven step synthetic sequence as shown below,
pyridinone analog 9 has been synthesized using 2,4-dihydroxy-
pyridine 29 as starting material (Scheme 4).
CO2H
N
N
N
N
18
19
20
21
c
In summary, we have identified a new series of GS inhibitors
with good in vitro antifungal activity. The effect of core structure
modifications on antifungal activity has been studied in detail. Fur-
ther optimization of the sulfonamide moiety led to the identifica-
tion of key aniline compounds with much improved systematic
exposure while retaining good antifungal activity. Further in vivo
study and SAR optimization of compound 11n to improve both
pharmacokinetic and biological activity will be discussed in the
near future.
O
O
EtO
S
O
d
S
O
Cl
N
N
N
N
O
N
N
N
N
22
6
Scheme 2. Reagents and conditions: (a) NaOAc, EtOH, reflux; (b) POCl3, PCl5;
(c) N-benzylsulfonylpiperazine, DIPEA, DMSO, MeOH,
THF,
D
; (d) EtOCH2CH2OH, NaH,
D
.
References and notes
NBoc
1. Maertens, J. A.; Boogaerts, M. A. Curr. Pharm. Des. 2000, 6, 225.
2. (a) Moudgal, V.; Sobel, J. Expert Opin. Pharmacother. 2010, 11, 2037; (b)
Kitamura, A. Expert Opin. Drug Disc. 2010, 5, 739; (c) Matejuk, A.; Leng, Q.;
Begum, M. D.; Woodle, M. C.; Scaria, P.; Chou, S.-T.; Mixson, A. J. Drugs Future
2010, 35, 197.
3. (a) Kondoh, O.; Inagaki, Y.; FuKuda, H.; Mizuguchi, E.; Ohya, Y.; Arisawa, M.;
Shimma, N.; Aoki, Y.; Sakatani, M.; Watanabe, T. Biol. Pharm. Bull. 2005, 28, 2138;
(b) Vargas, L. Y.; Castelli, M. V.; Kouznetsov, V. V.; Urbina, J. M.; Lopez, S. N.;
Sortino, M.; Enriz, R. D.; Ribas, J. C.; Zacchino, S. Bioorg. Med. Chem. Lett. 2003, 11,
1531.
b
c
N
a
O
N
N
N
HO
F
N
N
HO
OTf
f, g
HO
OH
NBoc
29
32
31
30
d
F
O
S
O
Br
NBoc
e
NBoc
N
O
N
O
N
O
N
N
N
N
4. Ting, P. C.; Aslanian, R. G.; Cao, J.; Kim, D. W.; Kuang, R.; Zhou, G.; Herr, R. J.;
Zych, A. J.; Yang, J.; Wu, H.; Zorn, N. World Patent 2008115381 September 25,
2008.
33
34
9
5. Ting, P. C.; Kuang, R.; Wu, H.; Aslanian, R. G.; Cao, J.; Kim, D. W.; Lee, J. F.;
Schwerdt, J.; Zhou, G.; Wainhaus, S.; Black, T. A.; Cacciapuoti, A.; McNicholas, P.
M.; Xu, Y.; Walker, S. S. Bioorg. Med. Chem. Lett. 2011, 21, 1819.
6. Glennon, R. A.; McKenney, J. D.; Lyon, R. A.; Titeler, M. J. Med. Chem. 1986, 29,
194.
Scheme 4. Reagents and conditions: (a) CF3SO3NPh, NaH, DMF, 5 h; (b) N-Boc-
piperazine, DMF, 80 °C, 6 h; (c) PhB(OH)2, Cu(OAc)2, Et3N, DCM, rt, 72 h; (d) NBS,
AcOH, rt, 3 h; (e) Bu3SnPhF, Pd(PPh3)4, LiCl, THF, reflux, 42 h; (f) 4 N HCl in dioxane,
CH2Cl2; (g) RSO2Cl, Et3N, CH2Cl2, rt, 10 h.
7. Sircar, I. J. Heterocycl. Chem. 1983, 20, 1473.
Boc-piperazine was used to displace the triflate to produce 27. Fur-
ther reduction of 27 gave intermediate 28. Removal of the Boc pro-
tecting group and sulfonylation produced the target compound 7.