Quinolines as a novel structural class of potent and selective PDE4 inhibitors: Optimisation for oral administration

Article abstract of DOI:10.1016/j.bmcl.2009.01.045

Crystallography-driven optimisation of a lead derived from similarity searching of the GSK compound collection resulted in the discovery of a series of quinoline derivatives that were highly potent and selective inhibitors of PDE4 with a good pharmacokine

Full text of DOI:10.1016/j.bmcl.2009.01.045

Bioorganic & Medicinal Chemistry Letters 19 (2009) 1380–1385
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Quinolines as a novel structural class of potent and selective PDE4 inhibitors:
Optimisation for oral administration
*
Christopher J. Lunniss , Anthony W. J. Cooper, Colin D. Eldred, Michael Kranz, Mika Lindvall , Fiona S. Lucas,
Margarete Neu, Alex G. S. Preston, Lisa E. Ranshaw, Alison J. Redgrave, J. Ed Robinson, Tracy J. Shipley,
Yemisi E. Solanke, Don O. Somers, Joanne O. Wiseman
GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire SG1 2NY, England
a r t i c l e i n f o
a b s t r a c t
Article history:
Crystallography-driven optimisation of a lead derived from similarity searching of the GSK compound
collection resulted in the discovery of a series of quinoline derivatives that were highly potent and selec-
tive inhibitors of PDE4 with a good pharmacokinetic profile in the rat. Quinolines 43 and 48 have poten-
tial as oral medicines for the treatment of COPD
Received 28 November 2008
Revised 9 January 2009
Accepted 14 January 2009
Available online 19 January 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
PDE4
Quinoline
Crystallography
COPD
The use of phosphodiesterase 4 (PDE4) inhibitors for the treat-
ment of chronic obstructive pulmonary disease (COPD) is well doc-
umented in the scientific literature.1–3 The current generation of
PDE4 inhibitors, represented by Roflumilast (DaxasTM) 1, show
moderate efficacy in COPD at tolerated doses, however, the maxi-
mum dose that can be given (and hence efficacy) is still limited
by undesired side effects such as nausea and emesis.4 Our objective
was thus to develop an oral PDE4 inhibitor with improved thera-
peutic index.
This letter describes the SAR that was discovered, culminating
in the identification of two compounds that had the required pro-
file to progress into our in-vivo models to evaluate their therapeu-
tic index with respect to nausea and emesis.
Quinoline 2 has an attractive profile and is a good start point to
initiate a lead optimisation programme. It has submicromolar
activity against PDE4B (pIC50 = 7.0). It is greater than 300-fold
selective against other PDE enzymes (PDE3 and PDE5) and has
three potential points of diversity. It also has a low clogP (2.1)
The initial lead compound and start point was trisubstituted
quinoline 2 (Fig. 1). This quinoline series was discovered from sim-
ilarity searching for alternative templates to the original pyrazolo-
pyridine template, for example, pyrazolopyridine 3 that was being
worked on in GSK.5 When the first co-crystal structure of a quino-
line 4 bound in the active site of the PDE4B enzyme was solved, it
was discovered that the quinoline series had a different binding
mode to that of the pyrazolopyridine template and that the SAR
did not transfer between the two series. An overlay of pyrazolo-
pyridine 3 and quinoline 4 bound into the active site of the PDE4
enzyme is shown in Figure 2. It shows that the two amide moieties
are binding into different parts of the active site and the two amine
substituents are binding into the same pocket.
and moderate aqueous solubility (30 lg/mL). Unfortunately, quin-
oline 2 suffers from poor oral exposure in the rat. The primary
emphasis of our SAR exploration was to increase the cellular activ-
ity and improve the rat oral pharmacokinetic (PK) profile. The bio-
logical profile that was targeted was: human whole blood (hWB)
potency pIC50 >7.5 (inhibition of LPS induced TNF
a
production)8;
>100-fold selective versus other PDEs; PK profile in the rat consis-
Cl
O
N
NH
N
HO
MeO2S
N
H
CONH2
Cl
O
O
F
F
1
2
* Corresponding author. Fax: +44 (0)1438 768301.
E-mail address: christopher@lunniss.net (C.J. Lunniss).
Present address: Novartis Institute for Biomedical Research, 4560 Horton Street,
Emeryville, CA 74607, USA.
Figure 1. Structure of Roflumilast (1) and quinoline lead 2. Profile of Roflumilast
(1): PDE4B pIC50 = 9.4; >300-fold selective versus PDEs 3 and 5; hWB pIC50 = 7.7.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.01.045

C. J. Lunniss et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1380–1385
1381
O
(ii)
O
N
Cl
MeO2S
O HN
N
CONH2
Cl
N
H
3
(green)
N
MeO2S
CONH2
N
9
N
(iii)
8
NH
N
O
S
N
Cl
MeO2S
MeO2S
CONH2
CONH2
4
(orange)
10
Figure 2. Overlay of pyrazolopyridine 3 and quinoline 4 bound in the active site of
the PDE4B enzyme. Shown without the peptide for clarity.6
Scheme 2. Synthesis of compounds with variation of the R4 linker. Reagents and
conditions: (ii) K2CO3, 3-chlorophenol, DMF; (iii) 3-chlorobenzenethiol, Et3N,
MeCN, 80 °C.
tent with once daily dosing (F > 30%; t1/2 > 2 h). The PDE4 enzyme
has four sub-types: A, B, C and D, but it is the PDE4B sub-type that
is believed to play a central role in inflammation,7 being the pre-
dominant subtype in monocytes and neutrophils and hence we
used this isoform for routine screening. For the exemplars from
this quinoline series tested against PDE4A, B, C and D, no isoform
selectivity was observed.
MeO
MeO2S
NH
N
MeO
MeO2S
NH
N
CONH2
CN
(i)
The quinoline template can be assembled in five steps from
commercially available starting materials in good overall yield
(Scheme 1). 4-(Methylsulphonyl)aniline 5 was condensed with
diethyl ethoxymethylenemalonate to give aniline derivative 6.
Cyclisation at 250 °C in diphenyl ether provided ester 7 which
underwent basic hydrolysis to provide the 3-carboxylic acid quin-
olone derivative. Chlorination of the quinolone and quenching of
the resultant acid chloride with aqueous ammonia, followed by
displacement of the 4-chloro group in quinoline 8 with 3-hydrox-
yaniline in refluxing acetonitrile gave quinoline 2.9–11
4
11
MeO
MeO2S
NH
N
(ii)
NH2
12
Scheme 3. Synthesis of quinolines with variation at the 3-position. Reagents and
Modifications to the linker group at the 4 position were made
by displacing the 4-chloro substituent in quinoline 8 (Scheme 2).
The oxygen linker was introduced by displacement with a phenol
and potassium carbonate in DMF. The sulphide linker was inserted
conditions: (i) TFAA: (ii) H2, Pd/C, EtOH.
O
by displacement with
triethylamine.
a thiol in refluxing acetonitrile with
MeO
NH
MeO2S
CO2Et
(i)
MeO2S
CO2Et
Modifications to the primary carboxamide were made using
standard functional group transformations (Schemes 3 and 4).
The primary carboxamide 4 was converted to nitrile 11 using tri-
fluoroacetic acid anhydride and subsequently hydrogenated to af-
ford amine 12. Quinolone ester 7 was reacted with thionyl chloride
N
H
N
13
7
(iv)
(ii)
N
N
MeO
MeO2S
NH
MeO
MeO2S
NH
O
CO2H
EtO2C
MeO2S
CO2Et
MeO2S
(i)
N
N
N
H
14
17
NH2
5
6
(v)
(iii)
O
Cl
(iii)-(v)
CO2Et
MeO2S
(ii)
MeO2S
CONH2
N
N
MeO
MeO2S
NH
N
MeO
CONHMe MeO2S
NH
N
N
H
N
O
8
7
15
16
(vi)
NH
N
HO
MeO2S
Scheme 4. Synthesis of quinolines with variation at the 3-position. Reagents and
conditions: (i) SOCl2, DMF, then 3-methoxyaniline, MeCN, 80 °C; (ii) NaOH, EtOH;
(iii) TBTU, iPr2NEt, amine, DMF; (iv) N-hydroxyethanimidamide, NaOEt, 3 Å
molecular sieves, DMF, reflux; (v) CH3C(O)NHNH2, EDC, HOBT, DMF, followed by
Burgess reagent, THF.
CONH2
2
Scheme 1. Synthesis of quinoline 2. Reagents and conditions: (i) Diethylethoxym-
ethylenemalonate, 80 °C; (ii) Ph2O, 250 °C; (iii) NaOH, EtOH; (iv) SOCl2, DMF (cat);
(v) NH4OH; (vi) 3-hydroxyaniline, MeCN, 80 °C.
and then treated with 3-methoxyaniline in refluxing acetonitrile to
give quinoline ester 13. This ester was hydrolysed with sodium

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C. J. Lunniss et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1380–1385
group (e.g., pyrazolopyridine 3 PDE4B pIC50 = 7.1).16 Unfortunately,
SO2Me
SO2Me
the quinoline 32 with the tetrahydropyran in the 4-position was
inactive (Table 1). It was discovered that an aromatic substituent
was essential for potency in this position in this quinoline series.
Removal of the 4-substituent resulted in an inactive compound 33.
A large number of compounds were synthesised investigating a
wide range of substituents, a selection of which is detailed in Table
1. Some of the key SAR findings for PDE4B enzyme potency were:
(ii) - (vi)
NH
N
MeO
MeO2S
(i)
HN
NH2
CO2Et
CONH2
CO2Et
5
18
19
Scheme 5. Synthesis of 2-substituted quinolines. Reagents and conditions: (i)
diethyl 2-butynedioate, 80 °C; (ii) Ph2O, 250 °C; (iii) SOCl2, DMF (cat); (iv) 3-
methoxyaniline, MeCN, 80 °C; (v) NaOH, EtOH; (vi) TBTU, iPr2NEt, DMF, 0.880
ammonia.
ꢀ Meta substitution on the aromatic ring was preferred by the
PDE4B-binding site.
ꢀ Large and bulky substituents in the ortho and para positions
reduced PDE4B potency (data not shown).
ꢀ Ortho substitution with small substituents was tolerated (34, 35,
and 4) especially if constrained in a ring to the meta position
(36).
ꢀ Heterocyclic phenyl ring replacements and basic substituents
reduced the logP (37 clogP = 1.6 and 38 clogP = 1.4) which
was potentially advantageous for oral exposure, but they were
less potent at PDE4B.
hydroxide in ethanol to give acid 14. Acid 14 was treated with
TBTU in DMF followed by an amine to give amide 15. Acid 14
was transformed into the 1,3,4-oxadiazole 16 by treatment with
an acyl hydrazide followed by cyclo-dehydration with Burgess’ re-
agent.12 To form 1,2,4-oxadiazole 17, ester 13 was reacted with N-
hydroxyethanimidamide.
The synthesis of the compound with the primary carboxamide
group moved to the 2-position of the quinoline is shown in Scheme
5. 4-(Methylsulphonyl)-aniline 5 was condensed with diethyl 2-
butynedioate at 80 °C,13 followed by similar chemistry to that
which was used to prepare the 3-substituted analogues.
Variation of the 8-substituent was not possible at a late stage of
the synthesis so modifications were made as shown in Scheme 6.
Quinolines containing the 8-methyl and 8-methoxy substituents
were synthesised by displacement of an aromatic fluoride 20 and
21 with sodium methane sulphinate at 75 °C in DMA,14 followed
by reduction of the nitro group to give amines 22 and 23. The syn-
thesis then proceeded as described in Scheme 1. Condensation of
aryl iodides 24–27 with diethylethoxymethylene malonate and
cyclisation in diphenyl ether at 250 °C followed by ester hydrolysis,
chlorination with thionyl chloride and reaction with ammonia
gave intermediates 28–31. Amine displacement in refluxing aceto-
nitrile followed by palladium-catalysed coupling with the tribu-
tyl(methylthio)stannane in refluxing toluene under palladium
catalysis.15 Oxidation with OxoneÒ afforded the target compounds.
Initially we investigated finding a replacement for the poten-
tially metabolically vulnerable hydroxy substituent on the pendant
aromatic ring in quinoline 2. As shown in Figure 2 the group at C-4
in the quinoline binds into the same part of the active site of the
PDE4B enzyme as the cyclic substituents that gave potent com-
pounds in the pyrazolopyridine series, such as the tetrahydopyran
ꢀ Acidic substituents on the phenyl ring were not well tolerated
(39).
The replacement of the metabolically vulnerable hydroxy group
in quinoline 2 by a methoxy group 4 resulted in an increase in po-
tency and an improved rat PK profile giving 10% oral bioavailabil-
ity. Further improvements in the PK profile were observed by
replacing the methoxy group with a nitrile 35 or constraining the
methoxy group into a ring as in the dihydrobenzofuran 36. The
addition of fluorine (40 and 41) onto the aryl ring resulted in im-
proved bioavailability.
Modifications to the 4-amino linker group were very detrimen-
tal to PDE4B potency (Table 2).
This data indicates the potential importance of an intramolecu-
lar hydrogen bond between the NH and the carbonyl group of the
3-carboxamide. This hydrogen bond is not possible with any of the
other linkers that were tried, possibly contributing to their lack of
PDE4B binding potency.
Next the hydrogen bond donor/acceptor properties of the pri-
mary carboxamide were modified, small substituents placed on
the nitrogen (11, 12a, 14a and 15) and the primary carboxamide
replaced with simple aromatic heterocycles (16 and 11). Some of
the compounds described in Table 3 have a phenyl sulphone group
in the 6-position of the quinoline (e.g., 4a, 12a and 14a). This group
was under investigation as an inhaled target early in the pro-
gramme. The phenyl sulphone gave a higher PDE4B enzyme po-
tency compared to the methyl sulphone, but was detrimental for
solubility and oral PK.19 The chemistry to make the compounds
with the phenyl sulphone is analogous to the chemistry to make
compounds with the methyl sulphone. All of these modifications
reduced the potency at PDE4B significantly (Table 3). These data
showed that the 3-primary carboxamide was essential for the high
binding affinity of the quinolines in the PDE4B enzyme. This was
rationalised when the crystal structure of 4 bound to the catalytic
domain of PDE4B was determined (Fig. 3).
The primary carboxamide sits in a small binding pocket with
one of the NH’s hydrogen bonding to asparagine-395 in PDE4B
and the other NH binding to glutamine-443 via a water molecule
(Fig. 3). The carbonyl group participates in two hydrogen bonds,
one to the NH of the 4-amino group and a second to an extensive
water network. The intra-molecular hydrogen bond is believed to
lock the quinoline into the preferred conformation to bind into
the active site of the PDE4B enzyme.
HNR4
F
MeO2S
MeO2S
CONH2
(ii)
(iiii)
(i)
NO2
NH2
R8
N
R8
R8
(ix), (x)
CONH2
R8=Me (20)
OMe (21)
R8=Me (22)
OMe (23)
43-48
Cl
N
I
I
(iv) - (viii)
NH2
R8
R8
R8=Et(24), F(25)
R8=Et(28), F(29)
Cl(26), CF3(27)
Cl(30), CF3(31)
Scheme 6. Synthesis of variants at the quinoline 8-position. Reagents and
conditions: (i) MeSO2Na, DMA, 75 °C; (ii) Pd/C, H2, AcOH; (iii) see conditions in
Scheme 1; (iv) Diethylethoxymethylenemalonate, 80 °C; (v) Ph2O, 250 °C; (vi)
NaOH, EtOH; (vii) SOCl2, DMF (cat); (viii) NH4OH; (ix) amine, MeCN, 80 °C; (x)
MeSSnBu3, Pd(PPh3)4, toluene, reflux, then Oxone, DMF.
The crystal structure of quinoline 4 bound in the active site of
the PDE4B enzyme showed the presence of a small lipophilic bind-
ing pocket beneath the 8-position of the quinoline (Fig. 4). Subse-

C. J. Lunniss et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1380–1385
1383
Table 1
PDE4B, hWB potency and rat PK profile of compounds modifying the 4-amino substituent8,17,18
NHR4
MeO2S
CONH2
N
R4
PDE4B pIC50 (n)
hWB pIC50 (n)
Rat F%
t1/2 (h)
Cl (mL/min/kg)
—
Solubility (
30
lg/mL)
2
4
7.0 (6)
—
—
—
OH
O
8.4 (7)
<4.5 (1)
<4.5 (1)
7.8 (3)
7.7 (4)
8.6 (2)
7.0 (1)
7.1 (1)
5.3 (1)
7.5 (5)
8.2 (2)
—
10
—
1.1
—
10
—
5
O
32
33
34
35
36
37
38
39
40
41
—
—
—
2
H
—
—
—
—
6.6 (4)
6.5 (8)
7.6 (4)
5.7 (2)
5.9 (2)
—
—
—
—
Cl
23
38
—
0.9
1.7
—
16
38
—
—
43
—
—
—
4
CN
O
N
—
—
—
NH2
O
—
—
OH
F
6.8 (4)
6.6 (4)
65
47
1.9
2.0
21
13
O
12
Cl
F
Table 2
quently a range of small substituents in the 8-position of the quin-
oline were investigated.
PDE4B potency of compounds modifying the linker to the 4-substituent
The 8-methyl quinoline 43 was found to be 1.3 log units more
potent than the unsubstituted compound in the hWB assay and
it had an excellent PK profile in the rat (Table 4). Increasing the size
of the 8-substituent to an ethyl or a chloro group (44 and 45) in-
creased the potency in the enzyme assay (data not shown) but
their increased lipophilicity resulted in a larger drop off into the
cellular assay, resulting in compounds that were equipotent with
the 8-methyl quinolines. The 8-chloro compounds suffered from
a lack of oral exposure and this precluded their progression. 8-Flu-
oro analogue 46 was slightly less potent than 8-methyl quinoline
43 and consequently was not progressed. Methoxy derivative 47
X
N
Cl
CONH2
MeO
2S
X
PDE4B pIC50 (n)
9
10
34
42
O
S
NH
NMe
5.5 (1)
4.7 (1)
7.8 (3)
6.0 (1)

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C. J. Lunniss et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1380–1385
Table 3
Table 4
PDE4B potency of compounds modifying the primary carboxamide
PDE4B, hWB potency and rat PK profile of compounds modifying the quinoline 8-
substituent
OMe
F
OMe
NH
N
O
NH
N
R6
O
O
O
R3
R2
S
S
NH2
O
R8
R3
R2
R6
PDE4B pIC50 (n)
4
–CONH2
–CONH2
–CN
–CH2NH2
–CO2H
H
H
H
H
H
H
Me
Ph
Me
Ph
Ph
Me
8.4 (7)
8.8 (2)
6.3 (1)
6.6 (1)
4.9 (1)
5.5 (1)
R8
H
43 Me
44 Et
45 Cl
PDE4B
pIC50 (n)
hWB pIC50 Oral AUC
F% t1/2
Cl (mL/
min/kg)
4a
11
12a
14a
15
(n)
(ng h/mL)
(h)
40
7.5 (5)
9.5 (3)
—
—
—
6.8 (4)
8.1 (10)
7.9 (2)
8.1 (5)
7.8 (2)
6.6 (3)
530
770
—
16
—
65 1.9
82 1.6
21
16
—
—
—
—
—
—
—
—
—
—
–CONHMe
46
F
N N
O
16
H
Me
4.7 (1)
47 OMe 8.3 (1)
81
N
17
19
H
Me
Me
4.8 (1)
5.1 (1)
N
O
Table 5
H
–CONH2
Structure and profile of compounds 43 and 48
CN
OMe
F
O
NH
N
O
NH
N
O
O
O
O
S
S
NH2
NH2
OMe
43
48
NH
MeO2S
CONH2
PDE4B
pIC50 (n)
hWB pIC50 F% t1/2
Cl (mL/
min/kg)
clogP Solubility
g/mL)
(n)
(h)
(l
N
43 9.5 (3)
48 9.4 (2)
8.1 (10)
7.6 (11)
82 1.6
27 2.3
16
19
3.1
3.5
5
18
4
Figure 3. Picture of quinoline 4 in the PDE4B active site showing the hydrogen
bond network around the primary carboxamide. Water is represented by red
spheres. The green lines represent hydrogen bonds.
These compounds are very potent inhibitors of PDE4B, they are
>100-fold selective over other PDEs (PDE1, 2, 3, 5, 6 and 7), have
some aqueous solubility, a reasonable clogP and have a good PK
profile in the rat. They were progressed to our in-vivo models for
further profiling to determine a therapeutic index versus emetic
side effects and these results will be reported in due course.
Acknowledgments
The authors thank Magali Gillet, Fanny Loustau-Chartez and Lu-
cie Krzackowski for their synthetic contributions whilst on place-
ment at GSK.
References and notes
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6. Methods as described in Ref.16, except compound 4 was soaked into an apo-
PDE4B crystal at 0.25 mM for ꢁ16 h. The final R-factor and R-free achieved for
the refined crystal structure of complex 4 were 20.9% and 24.3%, respectively,
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Figure 4. Crystal structure of quinoline 4 in the active site of the PD4B enzyme
showing the small binding pocket (A) beneath the 8-position of the quinoline.
showed a reduction in potency that was rationalised by the 8-
methoxy group being unfavorably twisted out of the plane of the
quinoline ring to fit into the PDE4B-binding site.
In summary the quinoline series has been optimised to improve
its potency and oral PK profile resulting in the discovery of key
compounds 43 and 48 (Table 5).

C. J. Lunniss et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1380–1385
1385
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centrifugation and compound extracted from 50
precipitation with acetonitrile. Samples were then evaporated under nitrogen
and re-suspended in 100 of 25:75 acetonitrile/water. Analysis was
performed using LC–MSMS with 5-min fast gradient comprising 0.1%
formic acid in water and 0.1% formic acid in acetonitrile (mobile phases),
20 injection volume, flow rate 4mL/min and LunaC18 column
(5 cm  2.1 mm, m) with detection on Sciex API365 instrument.
lL plasma using protein
l
l
a
ll
a
5
l
a
Pharmacokinetic data was generated using a non-compartmental approach.
19. Woodrow, M. D.; in preparation.