Lead optimization of a sulfonylurea-based piperazine pyridazinone series of glucan synthase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2012.04.127

The structure-activity relationship studies of a novel sulfonylurea series of piperazine pyridazine-based small molecule glucan synthase inhibitors is described. The optimization of PK profiles within the series led to the discovery of several compounds with improved pharmacokinetic profiles which demonstrated in vitro potency against clinically relevant strains. However, the advancement of compounds from this series into a non-lethal systemic fungal infection model failed to show in vivo efficacy.

Full text of DOI:10.1016/j.bmcl.2012.04.127

Bioorganic & Medicinal Chemistry Letters 22 (2012) 4896–4899
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Lead optimization of a sulfonylurea-based piperazine pyridazinone series
of glucan synthase inhibitors
Andrew J. Zych a, Sang Q. Lam a, David M. Jenkins a, R. Jason Herr a, , Pauline C. Ting b, , Joe F. Lee b,
Rongze Kuang b, Heping Wu b, David W. Kim b, Robert G. Aslanian b, Samuel Wainhaus c, Todd A. Black d,
Anthony Cacciapuoti d, Paul M. McNicholas d, Yiming Xu d, Scott S. Walker d
⇑
⇑
a Medicinal Chemistry Department, Albany Molecular Research, Inc. (AMRI), 26 Corporate Circle, Albany, NY 12203, USA
b Department of Chemical Research, Merck Research Laboratories, 126 E. Lincoln Ave., Rahway, NJ 07065, USA
c Department of Drug Metabolism, Merck Research Laboratories, 126 E. Lincoln Ave., Rahway, NJ 07065, 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:
The structure–activity relationship studies of a novel sulfonylurea series of piperazine pyridazine-based
small molecule glucan synthase inhibitors is described. The optimization of PK profiles within the series
led to the discovery of several compounds with improved pharmacokinetic profiles which demonstrated
in vitro potency against clinically relevant strains. However, the advancement of compounds from this
series into a non-lethal systemic fungal infection model failed to show in vivo efficacy.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 28 March 2012
Accepted 27 April 2012
Available online 17 May 2012
Keywords:
b-Glucan synthase inhibitor
Candidiasis
Antifungal
Pathogenic fungi
Echinocandin
Fungal cells are unique structures as the cellular membrane is
enclosed within a porous cell wall. This cell wall serves as an impor-
tant structure for fungi, as the metabolism of this shape-defining
scaffold is directly linked with growth and mitosis. A major compo-
highly desired. To this end, a few small molecule GS inhibitors aris-
ing from early stage programs have already been reported.3
Recently we communicated the discovery of a novel series of
pyridazine-based glucan synthase inhibitors as candidates for anti-
fungal treatment,4 and further described compounds from the ser-
ies that exhibit oral efficacy in a non-lethal fungal infection
model.5 Preliminary SAR studies identified some structural
requirements for good antifungal potency (cf. compounds 2 and
3), including an alkoxyl moiety at the C-4 position, and a sulfonyl
element on the C-5 piperazine subunit.
nent of this cell wall is 1,3-b-
D-glucan, which is synthesized under
catalysis of the membrane-bound enzyme complex 1,3-b-
D-glucan
synthase (GS). Inhibition of GS-mediated cell wall synthesis leads to
a disruption of the fungal cytoskeleton, which is a lethal event in
pathogenic fungi such as Saccharomyces cerevisiae and Candida albi-
cans. As mammalian cells lack this comparable cell wall structure,
therapies for the treatment of systemic fungal infections through
a mechanism that inhibits GS function are highly selective for fun-
gal cells.1
O
O
N
S
O
N
Several 1,3-b-D-glucan synthase inhibitors have been identified,
O
N
O
N
O
O
N
including several recently launched drugs from the echinocandin
class of compounds.2 Characterized as cyclic hexapeptides bearing
a lipophilic side chain, this chemical series shows potent candicidal
activity and acceptable safety profiles in use. Nevertheless, the ech-
inocandins are only available for dosing by iv administration, and
thus the development of orally available GS inhibitor drugs is
Cl
N
preliminary
SAR
S
2
O
N
µg/mL
IC50 = 0.2
N
N
H3C
O
O
CH3
CH3
O
S
N
1
screening hit
IC50 = 4.6 µg/mL
O
N
Cl
N
N
3
⇑
Corresponding authors. Tel.: +1 518 512 2954; fax: +1 518 512 2079 (R.J.H.);
tel.: +1 732 594 9894 (P.C.T.).
IC50 = 0.2 µg/mL
E-mail addresses: rjason.herr@amriglobal.com (R.J. Herr), pauline.ting@merck.
com (P.C. Ting).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.04.127

A. J. Zych et al. / Bioorg. Med. Chem. Lett. 22 (2012) 4896–4899
4897
O
O
Although compound 8h exhibited reasonable in vitro antifungal
activity, it produced no measurable blood levels in the rat upon
oral dosing in several vehicles (LOQ was 10 ng/mL). However, upon
applying several of the substitution strategies previously worked
out to address the low plasma exposure challenges for the analo-
gous sulfonamide series,4,5 benzyl sulfonylurea 11 was synthe-
sized. This compound was typical of this series in that excellent
oral exposure was generally obtainable only at the expense of
diminished glucan synthase inhibition.
O
O
O
O
a
TfO
N
b, a
TfO
S
S
N
N
S
R1
N
N
N
R2
N
N
N
CH3
N
CH3
N
5
6
4
O O
S
R3
H
R3
O
N
N
R1
c
O
N
N
O
N
O
N
R2
N
•HCl
N
Ar
N
Ar
8
7
Scheme 1. Reagents and conditions: (a) TfOCH3, CH2Cl2, 0 °C–rt; (b) R1R2NH,
CH3CN, rt; (c) 6, iPr2NEt, CH2Cl2, rt.
O O
S
O O
S
F
NH2
CH3
N
O
N
N
N
O
N
N
O
N
O
N
H
H
F
N
O
S
Cl
N
O
H
R3
O
N
N
R3
a
N
N
O
N
O
N
F
N
N
8h
11 (1:1 cis/trans mixture)
GS IC50 = 0.31 µg/mL
O
N
GS IC50 = 0.048 µg/mL
C. glabrata MIC100 < 0.02 µg/mL
C. albicans MIC100 = 0.030 µg/mL
AUC(0-6h) < LOQ µg•h/mL (10 mpk po)
cLogP = 5.6
N
•HCl
N
Ar
C. glabrata MIC100 = 0.26 µg/mL
Ar
C. albicans MIC100 = 0.52 µg/mL
7
9
b
UC(0-6h) = 2.58 µg•h/mL (10 mpk po)
A
cLogP = 3.9
O
O O
S
O
TfO
CH3
R3
S
R1
R3
c
N
O
N
N
O
N
O
N
O
N
R2
N
N
Table 1
N
N
Antifungal profile of cyclopentyl ether sulfonamides 2 versus sulfamides 8a
Ar
Ar
10
8
O
O
O O
Scheme 2. Reagents and conditions: (a) 5, iPr2NEt, CH3CN, rt; (b) TfOCH3, CH2Cl2,
0 °C–rt; (c) R1R2NH, iPr2NEt, CH2Cl2, rt.
R2
S
S
N
O
N
N
R
O
N
N
O
N
O
N
R3
Unfortunately, this class of compounds in general suffers from
poor plasma exposure in the rat (10 mg/kg po) and in other ani-
mals primarily due to both metabolism and absorption issues. In
order to address both of these PK concerns, we set about to explore
a series of compounds bearing the sulfamide moiety on the termi-
nal piperazine nitrogen.
N
N
Cl
Cl
2
8
b
Compd R or NR2R3
GS IC50
g/mL)
C. glabrata
C. albicans
c
d
(l
MIC100
(
l
g/mL)
MIC100
(lg/mL)
CH3
CH3
Unsymmetrical sulfonylurea analogs 8 were synthesized accord-
ing to the method of Beaudoin et al.,6 as depicted in Schemes 1 and 2.
In one sequence, N,N0-sulfuryldiimidazole (4) was mono-alkylated
with methyl triflate to provide the active imidazolium salt 5
(Scheme 1). This reagent was treated with a variety of primary
and secondary amines to provide the corresponding imidazolyl sul-
fonylureas, which were in turn treated with a second equivalent of
methyl triflate to afford activated unsymmetrical sulfonylurea salts
6. Coupling of adducts 6 with our piperazine scaffold 74,5 provided
the requisite unsymmetrical sulfonyl ureas 8. Similarly, but in a re-
verse order, scaffolds 7 were reacted with imidazolium salt 5 to pro-
vide imidazolyl sulfonylureas 9 (Scheme 2). Adducts 9 were in turn
activated by alkylation with methyl triflate followed by reaction
with primary and secondary amines to produce sulfonyl ureas 8.
Representative examples of sulfamide modification SAR are
summarized in Table 1. Replacing the isopropyl sulfonamide from
lead compound 2a5 with the corresponding isosteric7 dimethyl
sulfamide 8a retained antifungal activity against Candida glabrata
and C. albicans.8 Modulation of the alkyl substituents did little to
improve the overall potency (compounds 8b–c), including cyclic
amine analogs such as 8d, in which antifungal activity was dramat-
ically reduced. Removal of one of the amine alkyl groups led to po-
tent compound 8e, although the primary sulfamide 8f was inactive
against the yeast strains tested. Preparation of the aniline-based
sulfonylurea 8g, the direct amine congener of benzyl sulfonamide
lead compound 2d5 led to reasonable potency against C. glabrata
C624, but was inactive for inhibition of C. albicans C693. The benzyl
variant 8h, however, was found to be an excellent inhibitor of both
2a
0.52
0.92
0.10
3.13
CH3
N
8a
0.10
0.39
6.25
10.0
CH3
CH3
CH3
N
8b
0.74
CH3
CH3
N
8c
0.32
1.51
0.20
0.39
6.25
25
CH3
N
8d
O
CH3
CH3
8e
0.18
0.030
0.52
N
H
H
N
H
8f
19.2
16.7
>33
2b
0.20
<0.02
0.39
8g
8h
0.062
0.048
0.16
>25
N
H
N
H
<0.02
0.03
a
Assay values are the average of at least two independent determinations. In
general, 100% inhibition of fungal growth is observed for MIC100 determination.
b
GS activity (C. albicans membranes, strain BWP17).
c
yeast strains with IC50 values of <0.02 and 0.03
C693, respectively.
lg/mL for C624 and
C. glabrata (strain C624).
C. albicans (strain C693).
d

4898
A. J. Zych et al. / Bioorg. Med. Chem. Lett. 22 (2012) 4896–4899
Table 2
2). While the unsubstituted N-benzyl compound 12a was sur-
prisingly much less active versus its cyclopentyl ether congener
8h, it was quickly found that the addition of polar functionality
onto the aryl subunit led to analogs of generally satisfactory po-
tency (12b–d). The benzylic pyridines 12e–f were also found to
be comparable in activity against both the C. glabrata and C.
albicans fungal strains, although only a few heterocyclic counter-
parts 12g–h exhibited GS inhibition in the requisite potency
range.
Expansion of this SAR into benzyl sulfonylureas bearing an ami-
no group at C-3 or C-4 of the aryl moiety led to compounds of even
greater potency that were also effective against isolated patho-
genic clinical strains of C. albicans as well as against the mold
Aspergillus fumigatus (Table 3). Although the ortho-fluoroaniline-
derived compounds 12i and 12m exhibited good antifungal po-
tency, neither resulted in measurable blood levels in the 6-h rat
oral exposure assay (10 mg/kg).
Antifungal profile for methyl(cyclopropyl)-methyl ether sulfamides 12 preliminary
SARa
H3C
O
O O
S
N
O
N
N
R
N
H
Cl
N
12a-g
b
Compd Benzylic
substitution
GS IC50
g/mL)
C. glabrata
C. albicans
c
d
(
l
MIC100
mL)
(
l
g/
MIC100
mL)
(lg/
12a
12b
2.90
0.32
2.08
0.13
>33
0.52
CO2CH3
OCH3
12c
12d
0.18
0.06
0.06
0.06
0.26
0.52
More promising PK profiles were realized with ortho-methylan-
ilines 12j and 12n as well as ortho-methoxyaniline derivatives 12k
and 12o, all of which were the first analogs to show moderate rat
OCH3
S
CH3
N
AUC(0–6h) levels close to 1 lg h/mL (HPBCD vehicle). In contrast, the
N
N
4-[(2-aminopyridyl)methyl]amino derivative 12l revealed a sub-
stantially lowered activity profile and close to no plasma exposure
upon oral dosing. Interestingly, the alpha-methyl-substituted var-
iant 13, which also showed a marked decrease in antifungal po-
tency against clinically-relevant strains, exhibited one of the best
results for this in vivo PK assay.
A few in vitro experiments helped to rationalize the difficulty in
achieving useful blood levels for this series (Table 3). Moderate he-
patic clearance was observed in either human or murine liver cells.
When compounds were tested in the C. albicans clinical isolate
strain C43 with and without the presence of 25% human serum, a
fourfold shift in observed MIC100 was observed, indicating that
binding to plasma proteins likely contributed to the low compound
availability in vivo.
12e
12f
0.11
0.05
2.08
2.08
1.04
0.52
N
N
12g
12h
0.24
0.81
0.26
0.52
0.26
2.08
S
O
Cl
a
Assay values are the average of at least two independent determinations. In
general, 100% inhibition of fungal growth is observed for MIC100 determination.
b
GS activity (C. albicans membranes, strain BWP17).
C. glabrata (strain C624).
C. albicans (strain C693).
c
d
Compound 12o was tested in a four-day non-lethal mouse mod-
el of systemic fungal infection. Normal mice were infected with C.
albicans strain C43 at 1.2 ꢀ 105 colony-forming units (CFU) per
As significant optimization was still required to create an
efficacious sulfamide series, we turned greater efforts to the re-
lated methyl(cyclopropyl)methyl ether scaffold series 12 (Table
Table 3
Expanded antifungal profile of selected methyl(cyclopropyl)methyl ether sulfamides 12i–o and 13a
H3C
O
H3C
H3C
O O
S
O O
S
O O
S
CH3
NH2
NH2
R
R
N
N
N
O
N
N
O
N
N
O
N
N
N
O
N
O
N
H
H
H
NH2
N
N
Cl
N
Cl
Cl
12i-l
12m-o
13
A.f. 158 MECg
g/mL)
Hep CLh
M cells)
(l
L/min/ Rati AUC(0–6h)
g h/mL)
b
c
d
e
f
Compd
R
GS IC50
C.g. 624 MIC100
g/mL)
C.a. 693 MIC100
g/mL)
C.a. 43 MIC100
g/mL)
C.a. 375 MIC100
g/mL)
(
l
g/mL)
(
l
(
l
(
l
(
l
(
l
(l
12i
12j
12k
12l
13
12m
12n
12o
F
CH3
OCH3 0.14
(N)
—
F
CH3
OCH3 0.093
0.172
0.072
0.52
0.26
0.52
2.08
1.04
0.13
0.52
0.52
0.52
0.26
0.26
8.35
1.04
0.06
0.26
0.52
2.08
0.52
4.17
2.08
4.17
>33
8.35
0.52
2.08
4.17
2.08
1.04
4.17
16.7
16.7
1.04
2.08
4.17
N/T
0
19.6 (h)
15.0 (r)
N/T
1.41
0.44
0.05
1.30
0
0.52 (2.08)j
16.7
0.23
0.30
0.064
0.062
4.17
N/T
0.13 (1.04)j
0.52
12.7 (r)
13.4 (h)
12.6 (r)
0.90
0.86
1.04 (4.17)j
a
b
c
d
e
f
Assay values are the average of at least two independent determinations. In general, 100% inhibition of fungal growth is observed for MIC100 determination.
GS activity (C. albicans membranes, strain BWP17).
C. glabrata (strain C624).
C. albicans (strain C693).
C. albicans (strain C43).
C. albicans (strain C375).
A. fumigatus (strain ND158).
Hepatic clearance in rat (r) or human (h) cells or not tested (N/T).
Oral dosing at 10 mg/kg in 20% HPBCD formulation.
MIC100 in the presence of 25% human serum.
g
h
i
j

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mouse and subsequently treated daily with sulfamide 12o at
100 mg/kg (ip). Following treatment, compound efficacy was mea-
sured as a function of fungal burden reduction in the kidney. In this
assay, compound 12o was found not to have significantly reduced
fungal accumulation relative to vehicle-treated mice or to the
known GS inhibitor caspofungin (CSP, administered at 5 mg/kg
ip). Analysis of plasma levels suggested that the concentration of
12o at this dose was not above the C. albicans MIC for sufficient
time for efficacy.
4. (a) Black, T. A.; McNicholas, P. M.; Walker, S. S.; Xu, Y.; Ting, P. C. PCT Int. Appl.
WO 2008115385, September 25, 2008.; (b) 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.
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Patel, R.; Bauman, N.; Norris, C.; Antonacci, B.; Gurnani, M.; Cacchiapuoti, A.;
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P. C.; Aslanian, R.; Cao, J.; Kim, D. W.; Kuang, R.; Lee, J. F.; Schwerdt, J.; Wu, H.;
Herr, R. J.; Zych, A. J.; Yang, J.; Lam, S. Q.; Wainhaus, S.; Black, T. A.; McNicholas,
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Bioorg. Med. Chem. Lett. 2012 accepted for publication.
In conclusion, a new series of sulfonylurea-based 1,3-b-D-glucan
synthase inhibitors has been discovered during a campaign for the
optimization of the original lead compounds 2 and 3. While com-
pounds of series 12 exhibit superior potency against clinically rel-
evant fungal strains, efficacy against an in vivo systemic C. albicans
infection model was not achieved. Further SAR and PK profile opti-
mization of the lead scaffold series will be reported in due course.
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