Non-peptide αvβ3 antagonists. Part 6: Design and synthesis of αvβ3 antagonists containing a pyridone or pyrazinone central scaffold

Article abstract of DOI:10.1016/S0960-894X(03)00254-3

Two novel series of small-molecule RGD mimetics containing either a substituted pyridone or pyrazinone central constraint were prepared. Modification of the β-alanine 3-substituent produced compounds that are potent and selective α_vβ₃ antagonists and exhibit a range of physicochemical properties.

Full text of DOI:10.1016/S0960-894X(03)00254-3

Bioorganic & Medicinal Chemistry Letters 13 (2003) 1809–1812
Non-Peptide ꢀvꢁ3 Antagonists. Part 6: Design and Synthesis of
ꢀvꢁ3 Antagonists Containing a Pyridone or Pyrazinone
Central Scaffold
Michael J. Breslin,a,* Mark E. Duggan,a Wasyl Halczenko,a Carmen Fernandez-Metzler,c
Cecilia A. Hunt,a Chih-Tai Leu,b Kara M. Merkle,c Adel M. Naylor-Olsen,e
Thomayant Prueksaritanont,c Gary Stump,d Audrey Wallace,d Sevgi B. Rodanb
and John H. Hutchinsona,*
aDepartment of Medicinal Chemistry, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
bDepartment of Bone Biology and Osteoporosis Research, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
cDepartment of Drug Metabolism, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
dDepartment of Pharmacology, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
eDepartment of Molecular Design and Diversity, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
Received 26 March 2002; accepted 9 January 2003
Abstract—Two novel series of small-molecule RGD mimetics containing either a substituted pyridone or pyrazinone central con-
straint were prepared. Modification of the b-alanine 3-substituent produced compounds that are potent and selective avb3 antago-
nists and exhibit a range of physicochemical properties.
# 2003 Elsevier Science Ltd. All rights reserved.
Osteoporosis is a disease that affects a large proportion
of the elderly population. In particular, it strikes post-
menopausal women resulting in fractures which can
lead to other significant health problems.1 One
approach to combatting this disease is to reduce osteo-
clast activity by antagonism of the integrin avb3.2 This
receptor is found predominantly in osteoclasts and is
thought to be involved in the adhesion and migration of
osteoclasts on the bone surface. avb3 has been shown to
bind to a number of proteins including osteopontin,
vitronectin and fibrinogen through the recognition of
the RGD triad.3 Small molecule mimetics of RGD have
been reported as avb3 antagonists and have been shown
to inhibit bone resorption in rats.4,5 In this paper, we
describe an extension of our work resulting in a series of
potent compounds which contain a pyridone or pyr-
azinone as a rigid central core.
antagonist.6,7 Common to compounds in this series is a
5,6,7,8-tetrahydro[1,8]naphthyridine (which functions
as a guanidine mimic), a pyrrolidinone central con-
straint and a 3-aryl b-alanine. Although these com-
pounds are potent antagonists of avb3, they generally
suffer poor pharmacokinetics due to low oral bioavail-
ability and high clearance. We sought to explore the
structure–activity relationships of new central scaffolds
with alternate b-alanines in order to modify the physical
properties and, hopefully, improve the pharmacokinetic
profiles.
Results and Discussion
Analogues of 1 containing different central cores were
prepared and evaluated for their ability to bind to the
receptor and they are described in Table 1. The binding
to avb3 was measured by the ability of the compound to
displace a non-peptide radio-ligand from Scintillation
Proximity Beads coated with the human avb3 receptor
(SPAV3 assay).8 Parenthetically, all compounds were
also tested for their ability to bind to the aIIBb3 integrin
Previous reports from this laboratory have disclosed the
1,3-disubstituted pyrrolidinone 1 as a novel avb3
*Corresponding author. Fax: +1-858-202-5752; e-mail: john_
hutchinson@merck.com
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00254-3

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M. J. Breslin et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1809–1812
Table 1. Central constraints
Table 2. Pyridones
Compd
R
H
R0
SPAV3 IC50 (nM)
1.4
LogP
Compd
X
SPAV3 IC50 Compd
(nM)
X
SPAV3 IC50
(nM)
6
ꢀ1.55
1
2
3
4
0.4
1.3
5
6
7
1.2
1.4
0.8
8
Me
c-Pr
Pr
1.0
0.2
1.4
9
ꢀ1.46
ꢀ0.8
ꢀ0.48
ꢀ0.97
0.28
9
21
10
11
12
13
246
c-Pr
c-Pr
c-Pr
using a platelet aggregation assay and all were found to
have an IC50 of greater than 3 mM.9 As can be seen from
Table 1, compound 1 is very potent with an IC50 of 0.4
nM in the SPAV3 assay. Removal of the cyclic con-
straint to give the amide 2 leads to a modest 3-fold
decrease in potency while the installation of planar aro-
matic groups, such as phenyl (3) or thiophene (4), was
particularly detrimental. Clearly, simple aryl ring repla-
cements of the pyrrolidinone ring are not well tolerated.
Ring expansion of 1 provided the six-membered lactam
5 (racemic) with an IC50 of 1.2 nM.6 We hypothesized
that removal of the chiral center in 5 by utilizing a pla-
nar carbonyl-containing group would afford a potent
compound. Thus, the incorporation of a pyridone to
give 6 did afford a potent antagonist (IC50=1.4 nM) in
contrast to the reduction in potency that was seen with
compounds 3 or 4. Moreover, the pyrazinone 7 was also
shown to be very potent (IC50=0.8 nM). The carbonyl
present in the ring of both 6 and 7 appears to play a
major role in the binding to avb3. Since both the pyr-
idone and pyrazinone containing analogues are of com-
parable potency to the starting pyrrolidinone 1, but
without the additional chiral center, they were selected
for further SAR studies.
0.2
0.8
0.54
zwitterionic compounds, lipophilic b-alanine sub-
stituents were evaluated in the 6-cyclopropyl-pyridone
series. The acetylene analogue 11 was considerably less
potent (IC50=9 nM) and still very polar. The addition
of larger, more lipophilic groups was successful in rais-
ing the logP value above zero. Thus, the dihydro-
benzofuran 12 and the 3-fluorophenyl analogue 13 are
both potent avb3 antagonists with logP values of 0.28
and 0.54 compared to ꢀ1.55 for 6.
The SAR of the pyrazinone-based antagonists was
explored using a 3-fluorophenyl-b-alanine (Table 3). As
with the pyridone series, small alkyl substituents such as
methyl (14), ethyl (16) and cyclopropyl (18) are well
tolerated however, the large phenyl substituent of 20
results in a significant loss (100-fold) in potency.
Table 3. Pyrazinones
In the pyridone series, compounds were prepared to
examine the potency effects of C-6 substituents in con-
junction with a variety of b-alanine substituents.7,10 The
compounds synthesized are detailed in Table 2. Com-
parisons of compounds containing a 3-pyridyl-b-alanine
show that small alkyl substituents are tolerated in the
C-6 position of the pyridone. The methyl (8) and propyl
(10) groups are equipotent to the unsubstituted com-
pound 6. Interestingly, compound 9 containing a cyclo-
propyl substituent shows a 7-fold boost in potency
when compared to 6 (IC50s of 0.2 and 1.4 nM,
respectively, in the SPAV3 assay). It is noteworthy
that these analogues are all very polar as shown by
their negative measured logP values. In an attempt to
enhance oral absorption for these very polar and
Compd
R1
R2
SPAV3 IC50 (nM)
LogP
14
15
16
17
18
19
20
H
Cl
H
Cl
H
Me
Me
Et
1.1
2.1
1.9
9.3
ꢀ0.31
0.61
ꢀ0.47
1.3
Et
cPr
cPr
Ph
1.4
2.7
112
0.15
1.1
1.6
Cl
Cl

M. J. Breslin et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1809–1812
1811
The addition of a chloro substituent at C-5 has only a
modest 2- to 4-fold negative effect on avb3 binding affi-
nity but was found to raise the logP by a full log unit
(compare 14 to 15, 16 to 17 and 18 to 19). Different
3-substituted b-alanines were incorporated into the
6-cyclopropyl-pyrazinone series (see Table 4). These
were made with and without a 5-chloro substituent to
obtain compounds with high intrinsic avb3 binding
potency and a range of polarity. The 3-pyridyl sub-
stitution on the b-alanine produced potent compounds
(21, IC50=0.2 nM and 22, IC50=0.3 nM) with low logP
values. The 6-benzodioxazolyl derivative 23 had a simi-
lar potency and polarity to the pyridine analogues.
However, the non-polar b-alanines such as 3-fluoro-
phenyl (19), 3-quinolinyl (24) and 6-dihydrobenzofur-
anyl (25) in conjunction with a 5-chloro substituent
afforded lipophilic antagonists with logP values ranging
from 0.9 to 1.12.
arranged in order of increasing lipophilicity with the start-
ing pyrrolidinone 1 given for reference. As can be seen
from Table 5, increases in protein binding for both the
pyridone and pyrazinone series mirror increases in logP.
These protein binding values are higher than the value for
the pyrrolidinone 1. Unfortunately, all of the compounds
tested exhibited half-lives of approximately 1 h with mod-
erate to high clearance values. The oral bioavailability
values were low, and in some cases (e.g., 8 and 14) the
plasma levels following oral dosing were undetectable.
Chemistry
Compounds illustrated in Table 1 were prepared as
previously described.6,11 Pyridone containing analogues
were prepared as shown in Scheme 1 commencing with
the appropriate 6-alkyl-3-nitro-2-(1H)-pyridone I.12
N-Alkylation was achieved by heating I at 55 ꢁC for 15
h with NaH and ethylbromoacetate. The nitro group
was reduced to give II using PtO2 and H2 in ethanol.
This amine was subjected to reductive amination with
5,6,7,8-tetrahydro[1,8]naphthyridine-2-aldehyde (THN-
CHO) followed by a basic hydrolysis to yield III.11 EDC
coupling with the appropriate b-alanine ester was then
followed by a second hydrolysis to give the final com-
pound IV. Scheme 2 describes the synthesis of the
Compounds representing a range of lipophilicities were
selected for protein binding determinations using
human plasma and for PK evaluation in the dog. The
compounds chosen are given in Table 5 and they are
Table 4.
Compd
R1
R2
SPAV3 IC50 (nM)
LogP
18
19
H
Cl
1.4
2.7
0.15
1.1
21
22
H
Cl
0.2
0.3
ꢀ0.25
ꢀ0.26
23
24
25
H
Cl
Cl
0.2
0.7
0.2
ꢀ0.1
1.12
0.9
Scheme 1.
Table 5. Physical properties and in vivo parameters
Compd
LogP
%PB
Dog PKa
t1/2
Cl (mg/mL/kg)
%F
1
8
ꢀ0.61
ꢀ1.46
ꢀ0.47
ꢀ0.11
0.15
55.4
91.2
97.2
97.5
97.5
99.4
98.5
1.0
0.8
1.4
1.1
1.0
0.8
2.1
42
21
22
26
18
20
28
—
—
4.3
0.3
1.5
3
16
23
18
13
15
0.54
0.61
—
aDosed at 0.2 mpk iv and 1 mpk po in water.
Scheme 2.

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M. J. Breslin et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1809–1812
5-chloropyrazinone and the pyrazinone containing
antagonists both of which start with a common inter-
mediate V.13 The chlorine at position 3 of V can be dis-
placed readily by amines. Thus chloropyrazinones were
produced by displacement with THNCH2NH2 to give
VI. After saponification, an EDC coupling with the
desired b-alanine ester followed by a second hydrolysis
gave the desired acid VII. The synthesis of compounds
in the pyrazinone series began with a similar displace-
ment with p-methoxy benzylamine and then hydrolysis
of the ester. Treatment with Raney–Ni in equal parts
methanol and 1 N NaOH removed the chlorine in the
5-position and then the PMB group was removed with
TFA. Standard methodology (as described above) was
used to convert VIII into the target compounds IX.
2. Rodan, S. B.; Rodan, G. A. J. Endocrinol. 1997, 154, S47.
3. Duong, L. T.; Rodan, G. A. Front. Biosci. 1997, 3, d757.
4. (a) Hartman, G. D.; Duggan, M. E. Exp. Opin. Invest.
Drugs 2000, 9, 1281. (b) Duggan, M. E.; Hutchinson, J. H.
Exp. Opin. Ther. Pat. 2000, 10, 1367.
5. (a) Yamamoto, M.; Fisher, J. E.; Gentile, M.; Seedor, J. G.;
Leu, C.-T.; Rodan, S. B.; Rodan, G. A. Endocrinology 1998,
139, 1411. (b) Crippes, B. A.; Engleman, V. W.; Settle, S. L.;
Delarco, J.; Ornberg, R. L.; Helfrich, M. H.; Horton, M. A.;
Nickols, G. A. Endocrinology 1996, 137, 918. (c) Hoffman,
S. J.; Vasko-Moser, J.; Miller, W. H.; Lark, M. W.; Gowen,
M.; Stroup, G. J. Pharm. Exp. 2002, 302, 205.
6. Meissner, R. S.; Perkins, J. J.; Duong, L. T.; Hartman,
G. D.; Hoffman, W. F.; Huff, J. R.; Ihle, N. C.; Leu, C.-T.;
Nagy, R. M.; Naylor-Olsen, A. M.; Rodan, G. A.; Rodan,
S. B.; Whitman, D. B.; Wesolowski, G. A.; Duggan, M. E.
Bioorg. Med. Chem. Lett. 2002, 12, 25.
7. Coleman, P. J.; Brashear, K. M.; Hunt, C. A.; Hoffman,
W. F.; Hutchinson, J. H.; Breslin, M. J.; McVean, C. A.;
Askew, B. C.; Hartman, G. D.; Rodan, S. B.; Rodan, G. A.;
Leu, C.-T.; Prueksaritanont, T.; Fernandez-Metzler, C.; Ma,
B.; Libby, L. A.; Merkle, K. M.; Stump, G. L.; Wallace, A. A.;
Lynch, J. J.; Lynch, R.; Duggan, M. E. Bioorg. Med. Chem.
Lett. 2002, 12, 31.
In conclusion, we have been successful in finding new
scaffolds to use as central constraints to the replace
pyrrolidinone present in our initial series of small-
molecule avb3 integrin antagonists. 1,3-Disubstituted
pyridones and pyrazinones containing small alkyl sub-
stituents at the 6-position were found to be equipotent
to the pyrrolidinone 1. All the compounds prepared
were selective when tested against the related platelet
receptor aIIBb3. Combination of different 3-substituted
b-alanines and ring substituents on the central scaffold
were found to give highly potent compounds with a
range of physical properties. However, as with com-
pound 1 from the pyrrolidinone series, these antagonists
suffer from low t1/2, moderate to high clearance and low
oral bioavailability in dog.
8. Hamill, T. G.; Duggan, M. E.; Perkins, J. J. J. Label.
Compd. Radiopharm. 2001, 44, 55.
9. Inhibition of ADP-stimulated aggregation of human gel-
filtered platelets, see: Hutchinson, J. H.; Cook, J. J.; Brashear,
K. M.; Breslin, M. J.; Glass, J. D.; Gould, R. J.; Halczenko,
W.; Holahan, M. A.; Lynch, R. J.; Sitko, G. R.; Stranieri,
M. T.; Hartman, G. D. J. Med. Chem. 1996, 39, 4583.
10. Coleman, P. J.; Askew, B. C.; Hutchinson, J. H.; Whit-
man, D. W.; Perkins, J. J.; Hartman, G. D.; Rodan, G. A.;
Leu, C.-T.; Prueksaritanont, T.; Fernandez-Metzler, C.; Mer-
kle, K. M.; Lynch, R.; Lynch, J. J.; Rodan, S. B.; Duggan,
M. E. Bioorg. Med. Chem. Lett. 2002, 12, 2463.
11. Merck and Co. US Patent 5 981 546.
12. (a) Merck and Co. US Patent 5 668 289, 1997. (b) Merck
and Co. US Patent 5 981 546, 1999.
13. Sanderson, P. E.; Lyle, T. A.; Cutrona, K. J.; Dyer, D. L.;
Dorsey, B. D.; McDonough, C. M.; Naylor-Olsen, A. M.;
Chen, I.; Chen, Z.; Cook, J. J.; Cooper, C. M.; Gardell, S. J.;
Hare, T. R.; Krueger, J. A.; Lewis, S. D.; Lin, J. H.; Lucas,
B. J.; Lyle, E. A.; Lynch, J. J.; Straneri, M. T.; Vastag, K.;
Yan, Y.; Shafer, J. A.; Vacca, J. P. J. Med. Chem. 1998, 41,
4466.
Acknowledgements
We thank Robert Lynch for the aIIBb3 IC50 determinations.
References and Notes
1. Sato, M.; Grese, T. A.; Dodge, J. A.; Bryant, H. U.;
Turner, C. H. J. Med. Chem. 1999, 42, 1.