Quinoline-4-acetamides as sPLA2 inhibitors

Article abstract of DOI:10.1016/S0960-894X(01)00229-3

Quinoline-4-acetamides were designed as potential phospholipase A₂ inhibitors by structural based method and synthesized. The chemical structures of the obtained compounds were confirmed by elemental analyses, ¹H NMR and MS. Preliminary bioassay study shows that quinoline-4-acetamides display certain inhibition to sPLA₂.

Full text of DOI:10.1016/S0960-894X(01)00229-3

Bioorganic & Medicinal Chemistry Letters 11 (2001) 1639–1641
Quinoline-4-acetamides as sPLA2 Inhibitors
Ying Liu, Yabing Feng, Renxiao Wang, Ying Gao and Luhua Lai*
Institute of Physical Chemistry & College of Chemistry and Molecular Engineering, Peking University,
State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing 100871, PR China
Received 22 January 2001; accepted 31 March 2001
Abstract—Quinoline-4-acetamides were designed as potential phospholipase A2 inhibitors by structural based method and synthe-
1
sized. The chemical structures of the obtained compounds were confirmed by elemental analyses, H NMR and MS. Preliminary
bioassay study shows that quinoline-4-acetamides display certain inhibition to sPLA2. # 2001 Elsevier Science Ltd. All rights
reserved.
Phospholipase A2 (PLA2) specifically hydrolyzes the
fatty acyl ester bond at the sn-2 position of glyceropho-
spholipid, producing rate-limiting precursors of inflam-
matory free fatty acids and lysophospholipids. PLA2
plays an important pathophysiological role in arthritis,1
septic shock,2 acute pancreatitis,3 and other inflamma-
tory diseases. It is clear that a potent and selective inhi-
bitor of PLA2 would have great potential for the
treatment of a wide range of common clinical disorders.
With reference to the literature,5,6 we obtained 2-chloro-
4-methyl-6-methoxyquinoline 3 from p-methoxyaniline
via three-step reactions of ethyl acetoacetate, oil of
vitriol and phosphoryl chloride. Compound 4, 2-phen-
oxy-4-methyl-6-methoxyquinoline, was obtained by
nucleophilic substitution of 3 with phenol in the pre-
sence of potassium hydroxide. Condensation of 4 with
benzaldehyde in the presence of anhydrous zinc chloride
gave 2-phenoxy-4-cinnamyl-6-methoxyquinoline 5. Com-
pound 5, when treated with osmium tetroxide in moderate
conditions, yielded 2-phenoxy-6-methoxyquinoline-4-
caraldehyde 6. Oxidation of 6 with silver nitrate at room
temperature gave 2-phenoxy-6-methoxyquinoline-4-car-
boxylic acid 7. Compound 7 when treated with oxalyl
chloride at room temperature gave 2-phenoxy-6-ethoxy-
quinoline-4-carbonyl chloride 8. Compound 8 was used
as the intermediate to synthesize the target,7 when 8
reacted with ammonia, the title compound 12, 2-phen-
oxy-6-methoxyquinoline-4-carboxamide, was obtained
in 93.2%, mp 202–204 ꢀC; while 8 reacted with diazo-
methane (diazo-reaction) to give 2-phenoxy-6-meth-
oxyquinoline-4-carbonyl iazoparaffinyl 9, followed by
Arndt–Eistert reaction to give the other title compound
2-phenoxy-6-methoxyquinoline-4-acetamide 10 in 76.4%
yield, mp 186–188 ꢀC, demethylation of 10 with boron
tribromide yielded the third title compound 2-phenoxy-
6-hydroxyquinoline-4-acetamide 11 in 28.4% yield.
Two major groups of PLA2 related to arthritis have
been reported, cytosolic PLA2 (cPLA2) and the secre-
tory PLA2 (sPLA2). Currently, the most widely studied
PLA2 is the latter, low molecular weight (14 kDa), Ca2+
dependent, extra-cellular enzymes which have been iso-
lated from such sources as bee venom, snake venom,
and mammalian pancreas and which are also secreted
by other mammalian cell types such as platelets.4
Human non-pancreatic secretory phospholipases A2
(hnps-PLA2) is considered to be a member of this class.
We focus on designing and synthesizing non-analogues
of the phospholipidic substrate of sPLA2 in order to
find new inhibitors. Herein, the quinoline ring was
selected and novel quinoline-4-acetamides were synthe-
sized and tested as sPLA2 inhibitors.
Chemistry
Biological Evaluation
Our general synthetic approach to the title compounds
is depicted in Scheme 1.
The SIBLINKS8,9 method was used to evaluate bioac-
tivity of the compounds. Naja naja sPLA2 was used as
the target. The reaction was monitored by the absor-
bance at 405 nm with a BIO-RAD plate reader. Test
compounds were added to each well as DMSO solution
*Corresponding author. Tel.: +86-10-6275-6833; fax: +86-10-6275-
1725; e-mail: lai@ipc.pku.edu.cn
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00229-3

1640
Y. Liu et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1639–1641
Scheme 1. Synthesis of quinoline-4-acetamide derivatives: (i) CH3COCH2CO2C2H5 (4 eq), 140 ꢀC, 30 min, 73.8%; (ii) H2SO4, 90–110 ꢀC, 2 h,
62.4%; (iii) POCl3, 110–120 ꢀC, 30 min, 89.3%; (iv) PhOH/KOH, 160 ꢀC, 4 h, 80.8%; (v) PhCHO, ZnCl2, 160 ꢀC, 3 h, 20.1%; (vi) OsO4/NaIO4, 24–
26 ꢀC, 24 h, 74.6%; (vii) AgNO3/NaOH, rt, 12 h, 95.6%; (viii) (COCl)2, rt, 24 h, ꢁ100%; (ix) CH2N2/Et2O, 0–15 ꢀC, 3 h, 92.0%; (x) AgNO3/NH3,
100–110 ꢀC, 2 h, 76.4%; (xi) BBr3/CH2Cl2, rt, 4 h, 28.4%; (xii) NH3, rt, 6 h, 93.2%.
with pure DMSO in the control wells. Reaction was
initiated by addition of the enzyme. Enzyme inhibition
data are the mean of two samples at each concentration.
Reported IC50 values, determined by plotting log con-
centration versus inhibition values, are the mean of
three or four separate experiments.10
hydrogen bond (HB) acceptor and one HB donor. In
the model, the quinoline ring was adopted as molecular
framework at the pocket, 2-phenoxy was selected to be
the hydrophobic center, 4-acetamide and 6-substituent
used to HB acceptor and HB donor, respectively. The
superimposition of pharmacophore and one quinoline
inhibitor is shown in Figure 1.
We encountered some difficulties in the experiments of
synthesis: (a) The 2-substituent of quinoline influenced
the reactive activity of the 4-substituent. Benzoxy at the
2-position which directs the quinoline ring N atom to
the ortho position, not only was itself a very strong
leaving group, but also had electron-withdrawing abil-
ity. It deactivated the reactive activity of the methyl at
the 4-position of the quinoline ring. The activation of 4-
methyl was the key step for the target. Through trial
and error, using the condensation with benzaldehyde
and the subsequent breaking of the double bond under
the mild conditions, the 4-methyl was activated success-
fully. (b) How to change 2-phenoxy-6-methoxyquinoline-
4-carboxylic acid into 2-phenoxy-6-methoxyquinoline-4-
acetamide? In our experiment, only using Arndt–Eistert
reaction,16 4-carboxyl could be translated to 4-acetamide.
Discussion
Our study is based on the structure of hnps-PLA2. The
crystal structure of hnps-PLA2 was obtained from the
Brookhaven Protein Data Bank (PDB). The ligand
associated to the enzyme was a phosphonate transition
state analogue (TSA) (PDB code: 1POE).11 The programs
DOCK12,13 and SCORE14 were used to predict how
these compounds bind with the enzyme. After analyzing
known inhibitors and using pocket module in Lig-
Builder,15 the pharmacophore was derived and a series
of heterocyclic compounds was designed. All com-
pounds had similar characteristics according to the
pharmacophore—one hydrophobic center which was
suggested to be present at the pocket of hnps-PLA2; one
Figure 1. Superimposition of pharmacophore and one quinoline
inhibitor, compound 10. The pharmacophore elements are shown in
gray, red and blue for hydrophobic moiety, HB acceptor and HB
donor, respectively.
Figure 2. The concentration–velocity curve of the compound 10:
à standard; ~ inhibitor (10À6 M).

Y. Liu et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1639–1641
1641
We obtained the designed title compounds, but the
synthetic approach was rather long and had a ‘bottle-
neck’ step [compound 5 was obtained in lower yield
(almost 20%)]. Further improvements are underway.
4. Kramer, R. M.; Johansen, B.; Hession, C.; Pepinsky, R. B.
Adv. Prostaglandin Thromboxane Leuktriene Res. 1990, 20,
79.
5. Conrad, M.; Limpach, L. Ber. 1887, 20, 944.
6. Kaslow, C. E.; Lauer, W. H. Org. Synth. Coll. 1955, 3,
194.
A preliminary bioassay study in vitro showed that
compounds 11 and 12 displayed inhibition to Naja naja
sPLA2 with the IC50 of 10 mMin the system we used.
The IC50 of compound 10 was 1 mM, and it was also
shown to be active on an animal model (data not
shown). The concentration–velocity (maximum) curve
of compound 10 compared to a standard curve is shown
in Figure 2.
7. New compounds gave satisfactory structural data. Selected
1
physical data and H NMR data are as follows: 10: M S:m/z
308 (M+) calcd for C18H16N2O3 308. 34; 1H NMR (400 MHz,
CD3COCD3): 3.94 (s, 3H, –OCH3), 4.05 (s, 2H, –CH2–), 6.52
(br, 1H, –NH, disappears after D2O exchange), 7.15 (br, 1H,
–NH, disappears after D2O exchange), 7.22–7.28 (m, 4H, 3-H,
Ph-H), 7.32 (dd, 1H, J=9.4, 2.0 Hz 7-H), 7.44–7.52 (m, 2H,
Ph-H), 7.54 (d, J=2.0 Hz, 5-H), 7.59 (d, J=9.4 Hz, 8-H);
Anal (calcd) %: C: 70.11 (70.13); H: 5.23 (5.19); N: 9.11 (9.09).
12. m/z 294 (M+) calcd for C17H14N2O3 294.32; 1H NM R
(400 MHz, CD3COCD3): 3.84 (s, 3H, –OCH3) 7.22–7.28 (m,
3H, Ph-H), 7.29 (s, 1H, 3-H), 7.35 (dd, 1H, J=8.2, 1.9 Hz, 7-
H), 7.45–7.49 (m, 2H, Ph-H), 7.57 (d, 1H, J=1.9 Hz, 5-H),
7.60 (d, 1H, J=8.2 Hz, 8-H), 7.89 (br, 1H, –NH weakens after
D2O exchange), 8.29 (br, 1H, –NH2, weakens after D2O
exchange); Anal (calcd)%: C: 69.48 (69.39); H: 4.71 (4.76); N:
9.48 (9.52).
Conclusions
We have designed and synthesized novel quinoline-4-
acetamide compounds as non-structural analogues of
the phospholipidic substrate of PLA2s. Bioassay results
indicate that the compounds are potential inhibitors of
sPLA2.
8. Washburn, W. N.; Dennis, E. A. J. Am. Chem. Soc. 1990,
112, 2040.
9. Washburn, W. N.; Dennis, E. A.. J. Biol. Chem. 1991, 266,
5042.
10. Test condition in vitro: Re=1.6; CMC=0.29;
[SIBLINKS]=0.768 M; [Triton]=1.49 mM; T=22 ꢀC;
[PLA2]=7.5 mg/mL.
11. Scott, D. L.; White, S. P.; Browning, J. L.; Rosa, J. J.;
Gelb, M. H.; Sigler, P. B. Science 1991, 254, 1007.
12. Kuntz, I. D.; Blaney, J. M.; Oatley, S. J.; Langridge, R.;
Ferrin, T. E. J. Mol. Biol. 1982, 161, 269.
Acknowledgements
This work was supported by the 863 High Technology
Program of China (863-103-13-04-02) and the National
Natural Science Foundation of China.
13. Meng, E. C.; Kuntz, I. D.; Abraham, D. J.; Kellogg, G. E.
J. Comput.-Aided. Mol. Des. 1994, 8, 299.
14. Wang, R. X.; Liu, L.; Lai, L. H.; Tang, Y. Q. J. Mol.
Model. 1998, 4, 379.
15. Wang, R. X.; Gao, Y.; Lai, L. H. J. Mol. Model. 2000, 6,
498.
16. Arndt, F.; Eistert, B. Ber. 1935, 1, 38.
References and Notes
1. Bomalaski, J. S.; Clark, M. A. Arthritis Rheum. 1993, 36, 190.
2. Vadas, P. Biochim. Biophys. Acta 1997, 1346, 193.
3. Buchler, M.; Malfertheiner, P.; Schadlich, H.; Nevalainen,
T. J.; Friess, H.; Berger, H. G. Gastroenterology 1989, 97, 1521.