Spirocyclic nonpeptide glycoprotein IIb-IIIa antagonists. Part 1: Design of potent and specific 3,9-diazaspiro[5.5]undecanes

Article abstract of DOI:10.1016/S0960-894X(01)00215-3

The synthesis and biological activity of novel glycoprotein IIb-IIIa antagonists containing the 3,9-diazaspiro[5.5]undecane nucleus are described. The potent activity of these compounds as platelet aggregation inhibitors demonstrates the utility of the spirocyclic structures as a central template for nonpeptide RGD mimics.

Full text of DOI:10.1016/S0960-894X(01)00215-3

Bioorganic & Medicinal Chemistry Letters 11 (2001) 1289–1292
Spirocyclic Nonpeptide Glycoprotein IIb–IIIa Antagonists.
Part 1: Design of Potent and Specific
3,9-Diazaspiro[5.5]undecanes
M. S. Smyth,a J. Rose,a M. M. Mehrotra,a J. Heath,a G. Ruhter,b T. Schotten,b
J. Seroogy,a D. Volkots,a A. Pandeya and R. M. Scarborougha,*
aCOR Therapeutics, Inc., Department of Medicinal Chemistry and Biology, South San Francisco, CA 94080, USA
bLilly Research Laboratories, Hamburg, Germany
Received 17 January 2001; accepted 16 March 2001
Abstract—The synthesis and biological activity of novel glycoprotein IIb–IIIa antagonists containing the 3,9-diazaspiro[5.5]-
undecane nucleus are described. The potent activity of these compounds as platelet aggregation inhibitors demonstrates the
utility of the spirocyclic structures as a central template for nonpeptide RGD mimics. # 2001 Elsevier Science Ltd. All rights
reserved.
The activation and aggregation of platelets is a critical
component of arterial thrombosis and leads to a num-
ber of cardiovascular disease states including unstable
angina, myocardial infarction, and arterial re-occlusion
following coronary angioplasty procedures.1,2 By pre-
venting the final common pathway of platelet aggrega-
tion, fibrinogen receptor antagonists [also termed
platelet glycoprotein (GP) IIb–IIIa antagonists] have
been demonstrated to be more potent antiplatelet agents
than other classes of drugs that inhibit specific platelet
activation pathways such as aspirin, or the ADP receptor
antagonists ticlopidine or clopidogrel.3,4
(Fig. 1). Although the discovery of structurally diverse
templates leading to potent antagonists has been quite
extensive, one type of constrained template that has
received little attention contains spirocyclic ring sys-
tems. One example of a spirocyclic template in the
GPIIb–IIIa antagonist field has been disclosed by the
Dupont-Merck group.8 Their efforts employing a spiro-
isoxazolinylimide central template resulted in weakly
In the design of potent GPIIb–IIIa antagonists, the tri-
peptide sequence RGD has been utilized as the starting
motif for generating many structurally diverse inhibi-
tors.4,5 A vast majority of the synthetic efforts have
focused on developing novel templates onto which an
acidic and a basic pharmacophore are appended which
mimic the RGD sequence. Many of the reported
antagonists contain constrained templates consisting of
monocyclic and/or fused bicyclics ring structures.4,6,7
Oral antagonists with some of these features have pro-
ceeded into large phase III clinical trials including
xemilofiban, orbofiban, sibrafiban, and lotrafiban
*Corresponding author. Tel.: +1-650-244-6822; fax: +1-650-244-
9287; e-mail: rscarborough@corr.com
Figure 1. Representative oral nonpeptide GPIIb–IIIa antagonists in
clinical development.
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00215-3

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M. S. Smyth et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1289–1292
active compounds leading them to conclude that the
steric bulk and/or rigidity of the spirocyclic template
resulted in poor complimentarity with the receptor.
There has been one additional report wherein the 3,9-
diazaspiro[5.5]undecane ring system has been utilized as
the basic pharmacophore unit, rather than as a central
template to prepare GPIIb–IIIa antagonists.9
The synthesis of 3,9-diazaspiro[5.5]undecane nucleus
follows published procedures.9 A benzamidine moiety
was incorporated using an amide or the direct linkage as
shown in Schemes 1 and 2. The preparation of the
required carboxylic acid side chains investigated in this
study are shown in Schemes 5 and 6. Conversion of
cyano to amidino functionality was carried out via the
thiomethylimidate protocol or the hydroxyamidino
method. The unsubstituted side-chain carboxylic acids
used in compounds 1–4 (Scheme 1) and 6–9 (Scheme 3)
were derived from the commercially available acid
chloride esters or the appropriate anhydrides as shown
in Schemes 1 and 3. Incorporation of the direct-linked
benzamidines (Scheme 2) and 4-pyridyl groups (Scheme 3)
into the spirocyclic template was accomplished utilizing
a Pd-mediated coupling reaction.11 Other more con-
strained carboxylic acid containing groups such as the
piperidine-4-oxyacetic acid (Scheme 5 for synthesis)
were coupled to the spiropyridine nucleus via the p-nitro-
phenylcarbamate intermediate as shown in Scheme 4.
As part of our effort to discover novel antagonists with
improved bioavailability and duration of action, we
initially investigated the 3,9-diazaspiro[5.5]undecane
ring system as a suitable template for preparing potent
and selective GPIIb–IIIa antagonists. We initiated our
investigation of spirocycles by choosing the 3,9-diaza-
spiro[5.5]undecane template, and also chose the benz-
amidine ring system as the basic group to append to this
nucleus. However, we also wished to study less basic
groups such as the 4-aminopyridyl residue found in the
Zeneca inhibitor, ZD2496 (Fig. 1).10 The lower pKa of
the pyridine moiety has been postulated to be a major
reason for the excellent bioavailability reported for
ZD2496.
The biological activity of the 3,9-diazaspiro[5.5]unde-
cane-containing analogues prepared in this study were
evaluated in vitro by measuring their ability to inhibit
the binding of fibrinogen to purified human GPIIb–IIIa
in a 96-well format.12 The compounds were also eval-
uated in functional assays, which determined their
Scheme 1. Synthesis of compounds 1–4: (a) 4-cyanobenzoyl chloride,
TEA, DCM, DMAP; (b) TFA/DCM; (c) ClCO(CH2)1,4CO2Et, TEA,
DCM; (d) 1 N NaOH; (e) (i) H2S, Et3N, Pyr; (ii) MeI, acetone, rt; (iii)
NH4OAc, EtOH, reflux; (f) succinic or glutaric anhydride, Et3N,
DCM.
Scheme 3. Synthesis of compounds 6–9; (a) 4-bromopyridine,
ꢀ
.
Pd2(dba)3 CHCl3, S-BINAP, t-BuONa, toluene, 85 C; (b) TFA/
DCM; (c) ClCO(CH2)3À6CO2Et, DIEA, DCM; (d) 2 M HCl; (e) 20–
24, HOBt, DIEA, DMF; (f) 2 M HCl.
Scheme 2. Synthesis of compound 5: (a) 4-bromobenzonitrile,
ꢀ
.
Pd2(dba)3 CHCl3, S-BINAP, t-BuONa, toluene, 85 C; (b) TFA/
DCM; (c) Br(CH2)2CO2Et, DIEA, DCM; (d) LiOH, THF, H2O; (e) (i)
.
NH2OH HCl, Et3N, EtOH; (ii) Ac2O, AcOH, rt; (iii) 1 atm H2, 10%
Pd/C, EtOH.
Scheme 4. Synthesis of compounds 15–17; (a) DMF, DIEA, 60 ꢀC,
DMF; (b) 2 M HCl.

M. S. Smyth et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1289–1292
1291
ability to inhibit ADP-induced platelet aggregation in
human platelet-rich plasma (PRP). Both benzamidine
or pyridyl-containing analogues with simple carboxylic
acid chains afforded modest potency (Tables 1 and 2).
However, neither series displayed submicromolar IC50
values in PRP. From the initial series of pyridines, 6
appeared to have the optimal distance between the car-
bon atoms of the carboxylate and the basic nitrogen
functionality, and this template was chosen for further
structural modifications.
potency enhancement in the plasma based assay relative
to compound 6. However, incorporation of a-amino
substitution at the carboxylic acid segment significantly
enhances the potency as seen for compounds 11–14 and
17 versus 6 (250-fold, Table 4). Incorporation of the 3,5-
dimethylisoxazol-4-yl-sulfonamide moiety (14 and 17)
stands out relative to other a-amino group substituents
that were examined, affording inhibitors of ADP-
induced aggregation in PRP with IC50 values <70 nM.
These observations are consistent with the results pub-
lished by Egbertson et al.13 where it was proposed that
the a-substituent of their tyrosine-derived GPIIb–IIIa
antagonists may be interacting with an unexploited ‘exo-
site’ within GPIIb–IIIa leading to significant enhance-
ment of the binding interactions. All active compounds of
our spirocyclic series were found to have IC50 values
>100 mM against the vitronectin receptor, avb3, the
most closely related integrin to GPIIb–IIIa. Thus, both
benzamidine and pyridine-containing analogues from
Incorporation of a rigid carboxylic acid moiety (analo-
gues 15 and 16, Table 3) did not provide additional
Table 2. In vitro activity for compounds with pyridines as the basic
function
Scheme 5. Synthesis of carboxylic acid segments 18 and 19. (a) BnOH,
DCC, DMAP, DCM; (b) TFA, DCM; (c) ClCOCH2CO2Et, DIEA,
DCM; (d) 10% Pd/C, H2; (e) ethyl diazoacetate, Rh(OAc)2-dimer; (f)
p-nitrophenylchloroformate, DIEA, DMF.
Compds
R
ELISA
Fg/GPIIb–IIa
IC50 (mM)a
Human PRP
IC50 (mM)a
6
7
8
9
(CH2)3CO2H
(CH2)4CO2H
(CH2)5CO2H
(CH2)6CO2H
0.24
0.26
0.27
1.16
5.08
4.54
4.55
17.8
aSee Table 1.
Table 3. Analogues with rigid carboxylic acid moiety
Compds
X
Y
R
ELISA
Fg/GPIIb–IIa
IC50 (mM)a
Human PRP
IC50 (mM)a
Scheme 6. Synthesis of carboxylic acid segments 20–24. (a) EDC,
HOBt, EtOH; (b) TFA, DCM; (c) Pd(OH)2, 1 atm H2, EtOH; (d)
RSO2Cl or ROCOCl, DIEA, DMF.
15
16
N
CH
CH
N
OCH2CO2H
COCH2CO2H
0.21
0.043
2.65
1.17
Table 1. In vitro activity for compounds with benzamidine group
aSee Table 1.
Table 4. Modifications of the carboxylic acid function
Compds
R
ELISA
Fg/GPIIb–IIa
IC50 (mM)a
Human PRP
IC50 (mM)a
Analogues
R
X
ELISA
Fg/GPIIb-IIa
IC50 (mM)a
Human PRP
IC50 (mM)a
1
2
3
4
5
COCH2CO2H
CO(CH2)2CO2H
CO(CH2)3CO2H
CO(CH2)4CO2H
CH2CH2CO2H
9.7
15
3.5
>100
0.16
9.3
13.8
10.3
18.8
1.15
10
11
12
13
14
17
NHCBz
CH2
CH2
CH2
CH2
0.007
0.004
0.023
0.002
0.002
0.001
4.58
NHCO2nBut
NHSO2nBut
NHSO2Tos
NHSO2isoxazole CH2
NHSO2isoxazole NH
0.722
0.768
0.078
0.021
0.039
aIC50 values expressed as the average of at least two determinations.
The average error for the determinations was Æ15%.
aSee Table 1.

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M. S. Smyth et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1289–1292
this study were found to be highly selective towards
GPIIb–IIIa.
Chem. 1995, 38, 2378. (b) Weller, T.; Alig, L.; Beresini, M.;
Blackburn, B.; Bunting, S.; Hadvary, P.; Muller, M. H.;
Knopp, D.; Levet-Trafit, B.; Lipari, M. T.; Modi, N. B.;
Muller, M.; Refino, C. J.; Schmitt, M.; Schonholzer, P.; Weiss,
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The pharmacokinetic properties of analogue 14
(CT51688) as its free acid and as its ethyl ester prodrug
were evaluated in Sprague–Dawley rats. The oral bio-
availability of 14 was found to be 7.3% but was quite
rapidly cleared. The oral bioavailability of the ethyl
ester prodrug of 14 was even less (3.2%) than its free
acid form. Although we were not successful in identify-
ing oral GPIIb–IIIa antagonists with good pharmaco-
kinetic properties, the 3,9-diazaspiro[5.5]undecane
template has afforded potent and specific antagonists of
GPIIb–IIIa. Further exploration of this template and
other spirocyclic templates will be the subject of additional
communications from our laboratories.
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