Synthesis of 2,5-thiazole butanoic acids as potent and selective αvβ3 integrin receptor antagonists with improved oral pharmacokinetic properties

Article abstract of DOI:10.1016/j.bmcl.2005.11.017

We describe a series of 2,5 thiazole containing compounds, which are potent antagonists of the integrin α_vβ₃ and show selectivity relative to the other integrins, such as α_IIbβ ₃ and α_vβ₆. These analogs were demonstrated to have high bioavailability relative to other relative heterocyclic analogs.

Full text of DOI:10.1016/j.bmcl.2005.11.017

Bioorganic & Medicinal Chemistry Letters 16 (2006) 845–849
Synthesis of 2,5-thiazole butanoic acids as potent and selective
avb3 integrin receptor antagonists with improved
oral pharmacokinetic properties
b
b
a,b
John A. Wendt,a,b, Hongwei Wu, Heather G. Stenmark, Mark L. Boys,
*
Victoria L. Downs,a,b Thomas D. Penning,b Barbara B. Chen,b Yaping Wang,b
Tiffany Duffin,c Mary Beth Finn,c Jeffery L. Keene,c V. Wayne Engleman,c
Sandra K. Freeman,c Melanie L. Hanneke,c Kristen E. Shannon,c Maureen A. Nickols,c
Christina N. Steininger,c Marissa Westlin,c Jon A. Klover,c William Westlin,c
G. Allen Nickolsc and Mark A. Russellb
aDepartment of Medicinal Chemistry, Pfizer Global Research and Development, 2800 Plymouth Road, Ann Arbor, MI 48105, USA
bDepartment of Medicinal Chemistry, Pfizer Global Research and Development, 4901 Searle Parkway, Skokie, IL 60077, USA
cDepartments of Discovery Pharmacology and Oncology, Pfizer Global Research and Development,
700 Chesterfield Village Parkway, Chesterfield, MO 63198, USA
Received 23 September 2005; revised 3 November 2005; accepted 4 November 2005
Available online 21 November 2005
Abstract—We describe a series of 2,5 thiazole containing compounds, which are potent antagonists of the integrin avb3 and show
selectivity relative to the other integrins, such as aIIbb3 and avb6. These analogs were demonstrated to have high bioavailability
relative to other relative heterocyclic analogs.
Ó 2005 Elsevier Ltd. All rights reserved.
The cell surface integrin superfamily that consists of
non-covalently linked, heterodimeric, transmembrane
receptors has attracted much scientific interest.1 A main
focus in our laboratories is the receptor, avb3. This
receptor is found on activated endothelial cells, smooth
muscle cells, osteoclasts, and many tumor cells. Its pri-
mary role in angiogenesis is described in many publica-
tions. The avb3 receptor is very promiscuous and
recognizes a variety of extracellular matrix proteins,
such as vitronectin, fibronectin, fibrinogen, thrombo-
spondin, osteopontin, bone sialoprotein, and denatured
collagen.2 The primary receptor recognition site on these
extracellular matrix proteins involves the arginine-gly-
cine-aspartic acid (RGD) tripeptide sequence contained
within these proteins. Attachment of these proteins
to the avb3 receptor results in a chain of events, that
initiates the process of new blood vessel growth from
existing vasculature. Antagonists of avb3 have been
demonstrated to inhibit angiogenesis in vivo and there-
by have potential utility in inhibiting tumor growth.3
The avb3 receptor is also the prevalent integrin found
on the surface of osteoclasts, that is responsible for cel-
lular attachment, and subsequent bone resorption.4
Antagonists of avb3 inhibit bone loss in animal models
of osteoporosis5 and appear to play a significant role
in several other pathophysiological conditions, includ-
ing restenosis after angioplasty,6 ocular neovasculariza-
tion,7 and rheumatoid arthritis.8 Antagonists of this
integrin thereby may be a beneficial therapy for the
treatment of a wide variety of diverse disease states.
Previously, we identified several new chemical series of
avb3 antagonists. The initial chemical scaffold was very
similar to the RGD sequence in vitronectin. These com-
pounds demonstrated potent anti-tumor efficacy in the
mouse Leydig cell tumor model,9 but contained several
amide moieties that limited its PK exposure. Other
series, such as the cinnamic acids, were revealed to be
potent and selective toward the integrin avb3 but also
Keywords: avb3 integrin receptor; Antagonists; Angiogenesis; Vitro-
nectin; Synthesis; SAR.
*
Corresponding author. Tel.: +1 734 622 2924; fax: +1 734 622
5165; e-mail: john.a.wendt@pfizer.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.11.017

846
J. A. Wendt et al. / Bioorg. Med. Chem. Lett. 16 (2006) 845–849
did not have optimal PK.10 This paper follows the newly
published discovery of a class of peptidomimetic b-
substituted 1,2,4-oxadiazole butanoic acids, that were
potent and selective antagonists toward the integrin
avb3 and possessed promising rat PK.11 The replacement
of the hetrocyclic core was the focus of this SAR study
targeting the optimization of affinity, selectivity, and
pharmacokinetic properties. This investigation revealed
the 2,5-thiazole analogs (1) yielded avb3 affinity, integrin
selectivity, and enhanced pharmacokinetic properties,
relative to the 1,2,4-oxadiazole series.
dride. These anhydride intermediates were then convert-
ed to both the racemic and S-isomer a-chloro-ketones
(3) by the route described in Scheme 2.12
A series of b-substituted 2,5-thiazole-butanoic acids
were prepared (Table 1) and tested in the avb3, aIIbb3,
and avb6 solid-phase receptor binding assays (SPRA).4
Most analogs demonstrated at least 100-fold selectivity
over the related platelet integrin, aIIbb3, and 1000-fold
selectivity over the integrin avb6. We sought selectivity
from the avb6 integrin due to the physiological deficits
in the B6 knockout mouse.13 Variation of a b-substituent
with substituted aromatic rings, and bicyclic aromatic
and heterocyclic rings resulted in good affinity with
dioxolane analog (8) affording the best affinity.
Aryl
H
N
N
N
CO2H
S
1
While the 1,8-tetrahydronaphthyridine moiety was an
effective guanidine mimetic for these 2,5-thiazole con-
taining avb3 antagonists, we sought to explore other
guanidine surrogates such as to optimize avb3 antagonist
affinity. A selection of guanidine mimetic variants is
shown in Table 2. These mimetics were selected based
on their calculated increased basicity relative to the
1,8-tetrahydronaphthyridine moiety. In addition, varia-
tion of the b-substituent, as either a 3,4-methylenedioxy-
phenyl (S-enantiomer) or a 4-methoxypyridyl group
(racemic), was utilized to further explore the SAR.
The 2-amino-4-cycloaminopyridinyl containing S-iso-
mer analogs (17–19) revealed a common theme of
possessing comparable affinity to the rac-1,8-tetra-
hydronaphthyridine analog (8). Further data analysis
reveals that variation of the b-position from the S-iso-
meric 3,4-methylenedioxyphenyl group (17–19) to the
rac-4-methoxypyridyl group (21–23) reveals compara-
ble, but not improved, avb3 antagonist affinity. The final
guanidine surrogate analyzed was the 1-methyl-1,2,3,4-
tetrahydropyrido[2,3-b]pyrazine (20, 24). These 2,5-thia-
The synthesis of the 2,5 thiazole ring exploited a thioam-
ide intermediate (Scheme 1). The 1,8-tetra-
2
hydronaphthyridine moiety was utilized due to its
effectiveness in generating potency in our 1,3,4-oxadiaz-
ole series.11 A subsequent reaction of thioamide 2 with
a-chloro-ketone 3 afforded 2,5-thiazole analogs in high
yield. This overall route interfaced well with 1,2,4-
oxadiazole analogs, such that, many common intermedi-
ates were utilized. This methodology increased output
and simplified the overall SAR. The limiting factor
was accessibility of the glutaric anhydrides. However,
various established methods were used to generate these
common intermediates (Scheme 2).
Aromatic aldehyde was condensed with ethyl acetoace-
tate in the presence of piperidine. The resulting carbocy-
clic intermediate was saponified to give 3-aryl glutaric
diacids. These diacids were then readily converted to
cyclic anhydrides by heating with excess acetic anhy-
H
N
NH2
N
N
NH2
O
a,b,c
+
CN
S
CHO
2
Aryl
H
O
Aryl O
N
N
d,e
N
Cl
2
+
OEt
CO2H
S
3
1
Scheme 1. Reagents and condition: (a) (i) L-Proline, EtOH; (ii) chromatographic isomer separation; (b) H2/Pd; (c) H2S, pyridine, triethylamine;
(d) 1,4-dioxane, D; (e) LiOH, THF.
Aryl
O
Aryl
3
O
d, e, f, g, h
a, b, c
Aryl CHO
Cl
OEt
O
O
O
Scheme 2. Representative methods for forming a-chloroketo c-esters. Racemic 3. Reagents and conditions: (a) ethyl acetoacetate, piperidine; (b) (i)
NaOH, H2O, EtOH, D; (ii) HCl; (c) Ac2O, D; (d) EtOH, reflux; (e) oxalyl chloride, CH2Cl2; (f) TMSCHN2, THF; (g) HCl in Et2O. S-isomer 3.
Reagents and conditions: (a) Ethyl acetoacetate, piperidine; (b) (i) NaOH, H2O, EtOH, D; (ii) HCl; (c) Ac2O, D; (d) EtOH, HCl, reflux; (e) Chirazyme
L-2, pH 7.4; (f) oxalyl chloride, CH2Cl2; (g) TMSCHN2, THF; (h) HCl in Et2O.

J. A. Wendt et al. / Bioorg. Med. Chem. Lett. 16 (2006) 845–849
847
Table 1. SPRA data for racemic b-substituted analogs
Table 2. SPRA data for alternate guanidine mimetic analogs
Ar
Ar
H
N
N
N
N
R
CO2H
CO2H
S
S
1
Compound
R
Ar
IC50 (nM)aavb3
SPRA
Compound
Ar
IC50 (nM)a
H
N
avb3
SPRA
aIIbb3
SPRA
avb6
HT29
N
O
O
17c
1.26
F
N
N
4
5
2.80
3.49
559
484
1308
2119
H
N
O
O
F
18c
1.56
1.70
F
N
O
N
H
N
OMe
6
7
1.54
3.81
1.18
2.17
1.23
11.06
1.77
2.09
2.82
1.22
1.20
350
636
263
1743
2444
2390
3263
1323
5057
2671
4301
>10,000
313
O
O
19c
MeO
Cl
N
S
H
N
N
O
O
O
O
20
21
0.61
3.00
8
N
Me
H
N
F
N
MeO
9
405
H3CO
H3CO
N
N
N
N
H3CO
Me
b
10
11
12
13
14
15
16
H
N
N
22
3.54
2004
252
N
O
N
N
H
N
S
H3CO
H3CO
23
24
3.73
N
N
N
N
S
559
S
Cl
S
H3C
H
N
95.1
N
N
1.24b
N
N
Me
b
S
a Average of at least three determinations.
b Average of two determinations.
c S-isomer.
N
b
841
S
OMe
a Average of at least three determinations.
b Not tested.
Rat PK was determined on selected analogs illustrated in
Table 3. It was apparent that the 2,5-thiazole class of
antagonists possessed a different pharmacokinetic profile
than the 1,2,4-oxadiazole series (8 vs 2515). The significant
improvement of % F, t1/2, Cmax, and AUCs of 8 versus 25
may be due to the lower clearance of 8. While we did see
enterohepatic recirculation in an earlier series, it was
not determined as to what role enterohepatic recircula-
tion played in these higher %F values. The S-isomer of
8 also possessed a similar PK profile in rat and exhib-
ited good bioavailability in other species such as dog
(t1/2 = 13 h, % F = 60) and cynomonkey (t1/2 = 11.2 h,
zole analogs revealed the greatest affinity of the guani-
dine mimetic investigated but only comparable affinity
to the 1,8-tetrahydronaphthyridine analogs (8, 10).
Representative synthetic protocols for analogs 17–19
and 21–23 are shown in Scheme 3. For analogs 20 and
24 alternate thioamides were prepared (Scheme 4) and
combined with their respective a-chloro-ketones as
shown in Scheme 1.

848
J. A. Wendt et al. / Bioorg. Med. Chem. Lett. 16 (2006) 845–849
Cl
CO2Et
O
Ar
S
O
S
Ar
d
a,b,c
O
CO2Et
TBDPSO
NH2
TBDPSO
N
100 oC
N
NH2
S
S
N
Ar
Ar
R
e
17-19
21-23
CO2Et
CO2Et
HO
O
f,g
N
N
NH2
N
NH2
h
Cl
NR2
Scheme 3. Synthesis of compounds 17–19 and 21–23. Reagents and conditions: (a) NH3, MeOH; (b) t-butyldimethylsilyl chloride, imidazole; (c)
LawessonÕs reagent, benzene, 60–70 °C; (d) TBAF, THF; (e) TPAP, NMO, sieves; (f) NaBH(OAc)3, R = pyrrolidine, azepine, morpholine,
thiomorpholine; (g) LiOH; (h) NR2 = pyrrolidine, azepine, morpholine, thiomorpholine; dimethylacetamide, 200 °C, 5 min, CEM microwave.
Boc
H
O
N
N
O2N
HO
N
O2N
N
N
N
N
d,e
a,b
c
f,g
N
EtO2C
N
Boc
N
H
N
H
N
N
NH2
N
OH
N
h,i,j
k
CN
S
N
N
N
Scheme 4. Synthesis of alternate thioamides for the preparation of compounds 20 and 24. (a) Reagents and conditions: (CF3SO2)2O, Et3N; (b)
MeNHCH2CO2Et; (c) H2, Pd/C; (d) LiAlH4; (e) Boc2O, Et3N; (f) LDA, diethylcarbonate; (g) LiBH4; (h) Ph3P, imidazole, I2; (i) ethylcyanoacetate,
NaH, DMF; (j) KOH, ethylene glycol, D; (k)H2S, Pyridine, Et3N.
Table 3. Rat pharmacokinetic data
Compound % F t1/2 (h) AUC-PO/dose (lg h/ml/mg/kg) AUC-IV/dose (lg h/ml/mg/kg) Cmax (lg/mL) CL (mL/min/kg) Vz (mL/kg)
6
9
98
95
6.1
4.3
2.2
1.8
2.5
1.6
1.7
6.8
1.5
2.2
1.9
2.4
1.4
1.9
5.2
1.6
6.1
9.1
7.3
9.0
2040
1290
1020
2950
2680
897
15
10
16
8
>100 3.4
>100 6.3
22.0
6.6
6.8
12.3
8.7
90
>100 7.0
95 1.4
7.1
3.0
19.6
12
3.3
10.9
25
733
% F = 64). Although not shown, most b-substituted
2,5-thiazoles listed in this table also revealed greater
rat pharmacokinetic properties when compared directly
to those of other b-substituted heterocyclic analogs,
such as 1,3,4-thiadiazoles and 1,3,4-oxadiazoles.14
thiazole derivatives possessed an increased oral rat phar-
macokinetic profile relative to similar 1,2,4-oxadiazole
analogs. Future communications from our laboratories
will be reported in time on the variation of the heterocy-
clic core as well as the in vivo pharmacology of this class
of molecules.
In summary, synthetic routes of a series of readily acces-
sible 2,5-thiazole butanoic acids possessing potent avb3
antagonist activity was developed. The overall SAR of
this series illustrates the modification of the b-aryl ring
and incorporation of an appropriate guanidine mimetic
yields novel analogs with low to sub-nanomolar avb3
affinity possessing a 100-fold selectivity over aIIbb3 and
a 100-fold selectivity over avb6. In addition, these 2,5-
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12. General procedure for the isolation of S-isomer half ester:
diethyl ester in phosphate buffer (pH 7.4) is stirred with
Chirazyme L-2 for 140 h followed by filtration of catalyst,
addition of brine, acidification, and ethyl acetate extrac-
tion to afford of the desired single enantiomer. The
absolute configuration was assigned from the X-ray
analysis of (S)-8.
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15. Compound 25 is rac-3-benzo[1,3]dioxol-5-yl-4-{3-[3-
(5,6,7,8-tetrahydro-[1,8]naphthyridin-2-yl)-propyl]-[1,2,4]-
oxadiazol-5-yl}-butyric acid.