Non-peptide alpha(v)beta(3) antagonists. Part 5: identification of potent RGD mimetics incorporating 2-aryl beta-amino acids as aspartic acid replacements.

Article abstract of DOI:10.1016/S0960-894X(02)00743-6

A series of novel, highly potent alpha(v)beta(3) receptor antagonists with favorable pharmacokinetic profiles has been identified. In this series of antagonists, 2-aryl beta-amino acids function as potent aspartic acid replacements.

Full text of DOI:10.1016/S0960-894X(02)00743-6

Bioorganic & Medicinal Chemistry Letters 12 (2002) 3483–3486
Non-Peptide ꢀvꢁ3 Antagonists. Part 5: Identification of
Potent RGD Mimetics Incorporating
2-Aryl ꢁ-Amino Acids as Aspartic Acid Replacements
Karen M. Brashear,a,* Cecilia A. Hunt,a Brian T. Kucer,a Mark E. Duggan,a
George D. Hartman,a Gideon A. Rodan,b Sevgi B. Rodan,b Chi-Tai Leu,b
Thomayant Prueksaritanont,c Carmen Fernandez-Metzler,c Andrea Barrish,c
Carl F. Homnick,a John H. Hutchinsona and Paul J. Colemana
aDepartment of Medicinal Chemistry, Merck Research Laboratories, West Point, PA 19486, USA
bDepartment of Bone Biology and Osteoporosis Research, Merck Research Laboratories, West Point, PA 19486, USA
cDepartment of Drug Metabolism and Pharmacology, Merck Research Laboratories, West Point, PA 19486, USA
Received 19 May 2002; accepted 5 August 2002
Abstract—A series of novel, highly potent avb3 receptor antagonists with favorable pharmacokinetic profiles has been identified. In
this series of antagonists, 2-aryl b-amino acids function as potent aspartic acid replacements.
# 2002 Published by Elsevier Science Ltd.
Introduction
RGD tripeptide sequence. A critical potency-enhancing
feature identified in an earlier series of GPIIbIIIa
antagonists6 was the sulfonamide substitiuent alpha to
the carboxylic acid terminus. Indeed, a-sulfonamyl avb3
antagonists were known to be potent inhibitors
although oral pharmacokinetic profiles as a whole were
poor.7 We postulated that a-aryl substituted analogues
might also function as potent avb3 antagonists, while
maintaining similar physical properties and pharmaco-
kinetic profiles to their b-aryl counterparts. We exam-
ined the 3-aryl versus 2-aryl substitution SAR in the
chain-shortened series and in particular for substituted
and cyclized tetrahydronaphthyridine analogues (Fig. 1).
Osteoporosis is a systemic skeletal disease characterized
by low bone mass and associated with an increased risk
of fractures.1 In post-menopausal women, osteoporotic
bone loss results from a net increase in the number and
activity of bone-resorbing osteoclast cells. The integrin
receptor avb3 is highly expressed in osteoclasts and plays
a critical role in both the adhesion and migration of
osteoclasts on the bone surface.2 Antibodies to avb3,
and more recently non-peptide RGD (arg-gly-asp)
mimetics, have been reported to inhibit bone resorption
in vitro and prevent bone loss in vivo.3 Our laboratories
have been active in the search for non-peptide avb3
antagonists that could be utilized as novel therapies in
the prevention and treatment of osteoporosis.
Chemistry
In a previous communication,4 we described a novel,
potent class of ‘chain-shortened’ avb3 antagonists that
display favorable pharmacokinetic profiles. We have
also reported antagonists to the integrin receptor GPII-
bIIIa (also known as aIIbb3),5 that also recognizes the
The synthesis of specific 2-aryl b-amino esters and the
final products are shown in Schemes 1–4. This chemistry
*Corresponding author. Fax: +1-215-652-7310; e-mail: karen_
brashear@merck.com
Figure 1. 2-Aryl versus 3-aryl substitution. For 2-aryl series, R1=H,
R2=aryl; For 3-aryl series, R1=aryl, R2=H.
0960-894X/02/$ - see front matter # 2002 Published by Elsevier Science Ltd.
PII: S0960-894X(02)00743-6

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K. M. Brashear et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3483–3486
Scheme 3. (a) Where R=H, EDC, HOBT, Et3N, DMF; (b) where
R=CH3, N,N,N0,N0-bis(tetramethylene)-chloroformamidinium hexa-
fluorophosphate, CH2Cl2, DIPEA; (c) 1 N NaOH, MeOH/THF.
Scheme 1. (a) Pd2(dba)3, dppf, LiOAc; (b) HCl/EtOH; (c) LDA,
TMS-Cl, THF; (d) CH2NOBn, TMS-OTf (cat), CH2Cl2; (e) Pd(OH)2,
EtOH, H2.
Scheme 4. (a) l-Proline, ethanol, reflux; (b) Pd/C, ethanol H2, 1 atm;
(c) chiral resolution: chiral OD column, 95% hexane, 5% 2-propanol,
0.1% diethylamine added, slower-moving enantiomer was isolated in
94% ee; (d) 1 N NaOH, THF/H2O.
with the silyl ketene acetal 2 derived from tert-butyl
acetate provided the aryl acetate 3. Transesterification
of 3 was accomplished in ethanolic HCl to furnish 4.
Reaction of the silyl ketene acetal of 4 with the benzy-
loxyimine, in the presence of a catalytic amount of
TMS-triflate, gave the benzyloxyamine 5 in good yield.9
Hydrogenolysis/hydrogenation of 5 with palladium
hydroxide in ethanol provided the desired 2-aryl
b-amino ester 6 as a racemic mixture.
Scheme 2. (a) p-Nitrophenylsulfonyl chloride, aq NaHCO3, CH2Cl2;
(b) DIAD/DEAD, PPh3, THF/MeOH; (c) mercaptoacetic acid, Et3N,
CH2Cl2.
was applied to prepare the other 2-aryl analogues
shown in Table 1.
The preferred route for synthesis of the 2-aryl b-amino
esters utilized a Mannich-type addition of the lithio
enolate of an aryl acetate to benzyloxime of formalde-
hyde (Scheme 1). Heck coupling of the aryl triflate8 1
The N-methyl b-amino ester 9 was prepared as shown in
Scheme 2. Sulfonylation of 6 followed by a Mitsunobu
reaction with methanol yielded compound 8. Desulfo-
nylation of 8 with mercaptoacetic acid and triethyla-
mine in methylene chloride provided the N-methyl
b-amino ester 9.
Table 1. SPAV3 binding data for 2-aryl versus 3-aryl analogues
As described in Scheme 3, the b-amino ester 6 was cou-
pled to the tetrahydronaphthyridinyl-pentanoic acid 10
using standard carbodiimide conditions, followed by
saponification to provide 12. The N-methyl b-amino
ester 9 was coupled using PYCLU (N,N,N0,N0-bis(te-
tramethylene)-chloroformamidinium hexafluorophos-
phate) conditions, and also underwent saponification to
yield the desired 13.
R
Entry SPAV, IC50 (nM) Entry SPAV3, IC50 (nM)
1-1A
1-2A
1-3A
1-4A
57.4
1-1B
1-2B
1-3B
1-4B
84
The tetrahydronaphthyridinyl tricyclic side chain 17 was
prepared as described in Scheme 4. The known cyclo-
hexone propionic acid ethyl ester10 14 underwent a
Friedlander condensation with 2-amino-3-formyl pyri-
dine11 15 to yield the naphthyridine product 16. Reduc-
tion of the naphthyridine to tetrahydronaphthyridine
was accomplished using palladium hydroxide on carbon
in ethanol. Resolution on a chiral OD column isolated
the more slowly eluting enantiomer, and subsequent
10.9
22.8
0.82
1.07
1.01
3.03

K. M. Brashear et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3483–3486
3485
saponification provided the tetrahydronaphthyridinyl
tricyclic acid 17.
Table 3. Pharmacokinetic data for selected compounds following
oral and iv dosing in dogs
Entry
F (%)
CI (mL/min/kg)
T1/2
Syntheses of the tetrahydronaphthyridinyl-pentanoic
acid side chain and its 3-cyclopropyl derivative, as well
as the b-substituted b-alanines, have been previously
described.12
1-4B
1-4A
2-3
2-5
2-6
99
47
40
1.1
18
2.0
6.6
13.7
18.9
10.3
3.5
4.7
0.7
1.1
1.6
Results and Discussion
Compounds were evaluated for their ability to inhibit
the binding of a high affinity radioligand to human
avb3 immobilized on scintillation proximity beads
(SPAV3).13 Table 1 depicts a comparison of the IC50
values in this assay for the corresponding 2- and 3-aryl-
substituted chain-shortened analogues. With the excep-
tion of 1-1B, all of the 3-aryl analogues were tested as
the single (S)-enantiomer. The racemic 2-aryl analogues
exhibited in vitro potencies that were comparable to or
better than the corresponding 3-aryl analogues. Inter-
estingly, the rank order for the aryl substituents is the
same in both series, suggesting that in both cases the
aryl substituents might be accessing the same binding
site in the receptor.
Table 3. Entry 1-4B, the 3-substituted dihy-
drobenzofuran analogue, demonstrated excellent oral
bioavailability with low clearance and a moderate
half-life. The 2-aryl analogue 1-4A also demonstrated
good oral bioavailability, with low clearance and an
improved half-life. The cyclopropyl analogue 2–3 dis-
played good oral bioavailability, but also possessed a
greatly reduced half-life. The tricyclic analogues 2–5
and 2–6 displayed low oral bioavailability and short
half-lives.
Conclusion
In summary, we have identified a new class of highly
potent, non-peptide avb3 receptor antagonists, with
favorable pharmacokinetic profiles, where a 2-aryl
b-amino acid functions as a potent aspartic acid repla-
cement. In particular, analogue 1-4A shows improved
(3-fold) binding affinity for the avb3 receptor versus 1-4B,
while maintaining a good pharmacokinetic profile. Fur-
ther improvements in potency were realized in this series
through substitution on the tetrahydronaphthyridine
moiety, incorporation of a tricyclic N-terminus, or
methylation of the amide moiety.
Further potency enhancements were investigated in the
2-aryl series (Table 2). The N-methyl amide analogue 2-2
did not significantly increase SPAV3 potency over the
N–H analogue. The 3-cyclopropyl analogue 2-3 gave a
significant 3-fold increase in potency. However, incor-
poration of these two substitutions (2-4) did not provide
a further increase in potency. Although the constrained
tricyclic analogue 2-5 did not provide an increase in
potency over 1-4A, the corresponding N-methyl amide
tricyclic did afford a 3-fold increase in SPAV3 potency.
Pharmacokinetic data following oral and iv dosing in
dogs for selected compounds are summarized in
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