Non-peptide αvβ3 antagonists: Identification of potent, chain-shortened RGD mimetics that incorporate a central pyrrolidinone constraint

Article abstract of DOI:10.1016/j.bmcl.2003.09.055

Antagonists of the integrin receptor α_vβ₃ are expected to have utility in the treatment of osteoporosis through inhibition of bone resorption. A series of potent, chain-shortened, pyrrolidinone-containing α_vβ₃ receptor antagonists is described. Two sets of diasteromeric pairs of high-affinity antagonists demonstrated marked differences in log P values, which translated into differing dog pharmacokinetic properties. One member of this set was demonstrated to be effective in reducing bone resorption in rats.

Full text of DOI:10.1016/j.bmcl.2003.09.055

Bioorganic & Medicinal Chemistry Letters 13 (2003) 4285–4288
Non-peptide ꢀvꢁ3 Antagonists: Identification of Potent,
Chain-Shortened RGD Mimetics that Incorporate a Central
Pyrrolidinone Constraint
James J. Perkins,a,* Le T. Duong,b Carmen Fernandez-Metzler,d George D. Hartman,a
Donald B. Kimmel,b Chih-Tai Leu,b Joseph J. Lynch,d Thomayant Prueksaritanont,c
Gideon A. Rodan,b Sevgi B. Rodan,b Mark E. Duggana and Robert S. Meissnera
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, Merck Research Laboratories, West Point, PA 19486, USA
dDepartment of Pharmacology, Merck Research Laboratories, West Point, PA 19486, USA
Received 19 August 2003; accepted 27 September 2003
Abstract—Antagonists of the integrin receptor avb3 are expected to have utility in the treatment of osteoporosis through inhibition
of bone resorption. A series of potent, chain-shortened, pyrrolidinone-containing avb3 receptor antagonists is described. Two sets of
diasteromeric pairs of high-affinity antagonists demonstrated marked differences in log P values, which translated into differing dog
pharmacokinetic properties. One member of this set was demonstrated to be effective in reducing bone resorption in rats.
# 2003 Elsevier Ltd. All rights reserved.
Chemistry
Introduction
The vitronectin receptor avb3 is a member of the integrin
family of receptors and is highly expressed in osteoclasts,
the cells responsible for the resorption of bone.1,2 Anti-
bodies to avb3 and the peptide echistatin have been
shown to inhibit bone resorption in vitro and in vivo.3À8
More recently, small molecule mimetics of the RGD
amino acid triad have been reported to inhibit bone
resorption in vivo.9À16 These results suggest that an
orally active, small molecule avb3 antagonist may have
utility in the treatment of osteoporosis.
The stereoselective synthesis of the chain-shortened
pyrrolidinone antagonists are described in Scheme 1.
Commercially available 6-oxo-heptanoic acid 1 is con-
verted to the acyl oxazolidinone by addition of the
lithium anion of (S)-4-benzyl-2-oxazolidinone to
the mixed anhydride of 1. The resulting ketone is
then protected as the ketal 2 by treatment with
ethylene glycol and catalytic TsOH in refluxing toluene.
Stereoselective addition of allyl bromide to the acyl
oxazolidinone followed by ozonolysis afforded the
aldehyde intermediate 4. A facile one pot reductive
amination/cyclization of the aldehyde with the dihydro-
benzofuran (DHBF)-substituted b-alanine ester18 was
followed by unmasking of the ketal with TsOH/acetone
to afford the methyl ketone 5. Friedlander condensation
of 5 with 2-amino-3-formylpyridine produced the
adduct 6. Regioselective hydrogenation of the
napthyridine followed by hydrolysis of the ester pro-
vided the target acid 7. The alternate diastereomer
(10) was prepared with the use of the (R)-oxazolidi-
none chiral auxiliary, and the analogues 11 and 12 were
prepared with the corresponding quinoline-containing b-
alanine.18
Previously, several reports from these laboratories
described our work toward ‘chain-shortened’ RGD
mimetics as potential inhibitors of bone resorption.16,17
The chain-shortened mimetics are potent antagonists
that offer significant improvements in pharmacokinetic
profiles as compared to their ‘full-length’ analogues.
Herein, we describe pyrrolidinone-constrained versions of
these chain-shortened compounds and their comparative
in vitro potencies and pharmacokinetic profiles.
*Corresponding author. Tel.: +1-215-652-4407; fax: +1-215-652-
7310; e-mail: robert_meissner@merck.com
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.09.055

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J. J. Perkins et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4285–4288
0.88 while the (S)-isomer 10 was much less lipophilic,
with a log P of À0.12. The quinoline analogues 11 and
12 were also prepared, and while both diastereomers
had higher receptor affinities than their amide parent 9,
the (R)-pyrrolidinone 11 proved to be slightly more
potent than the (S)-analogue 12. The relationship
between lipophilicity and pyrrolidinone stereochemistry
seen with the previous pair was maintained, however, as
the (R)-isomer 11 had the higher log P value of 1.03.
The pharmacokinetic profile for the avb3 antagonists
following their oral administration to dogs is detailed in
Table 1. DHBF-(R)-pyrrolidinone 7 demonstrated an
oral bioavailability (F) of 52%, a clearance (Cl) of 2.1
mL/min/kg and a plasma half-life (t1/2) of 8.0 h. Com-
pared to the amide parent 8, 7 offered similar F and Cl,
and an improved t1/2. The more polar diastereomer 10,
however, had a much poorer profile, with low F, higher
Cl, and a short t1/2. The same relationship between
relative polarity and pharmacokinetic profile was
demonstrated for the quinoline-containing diastereo-
meric pair of analogues 11 and 12. In an effort to more
closely examine the relationship between log P and
pharmacokinetic performance, an in-depth study of the
in vitro and in vivo disposition of the isomers 11 and 12
was carried out in rats.20 Among the conclusions of
these studies was the finding that the absorption, trans-
port, and metabolism of the pair was quantitatively
different and that the physicochemical properties were an
important, but not exclusive cause of these differences. In
light of these biological differences of the pyrrolidinone
diastereomers, we sought to identify physical property
differences that could explain the phenomenon. In our
hands, modeling studies did not identify preferred con-
formations which differentiated the diastereomers 7 and
10 in any profound way, and solution NMR studies of
the pair did not show any markedly different hydrogen
bonding patterns.
Scheme 1. (a) Isobutyl chloroformate, NEt3, THF, then Li(S)-(+)-
4-benzyl-2-oxazolidinone; (b) ethylene glycol, TsOH, toluene, reflux;
(c) LiHMDS, THF, then allyl bromide; (d) ozone, CH2Cl2, then
PPh3; (e) ethyl-3(S)-3-amino-3-(2,3-dihydro-1-benzofuran-6-yl)pro-
pionate, NaB(OAc)3H, CH2Cl2; (f) TsOH, acetone, reflux; (g) 2-
amino-3-formylpyridine, proline, EtOH, reflux; (h) 10% Pd/C, EtOH,
H2 (1 atm); (i) 1 N NaOH, EtOH.
Results and Discussion
The affinities of the avb3 antagonists for human avb3
were determined in a competitive binding assay19 dis-
placing a high affinity 125I-labelled nonpeptide from
purified recombinant human avb3 bound to scintillation
proximity beads (SPAV3), and are listed in Table 1 as
IC50 values. We have recently described16 a pair of
chain-shortened avb3 antagonists 8 and 9 that show high
affinity for the avb3 receptor while offering favorable
pharmacokinetic profiles in dogs. These profiles are
characterized by high oral bioavailability, low clearance
and moderate plasma half-lives. The shortening of the
chain length between the charged centers from that
contained in earlier peptide-derived ‘full-length’
antagonists was a crucial factor in obtaining the desir-
able pharmacokinetic properties. In addition, the aryl
substituent of the b-alanine portion of these antagonists
was shown to be favored in this series for achieving
desirable receptor affinity in conjunction with the good
pharmacokinetic profile. Our efforts then turned to
focus on the design of additional analogues that would
offer further enhanced receptor affinity while maintain-
ing favorable pharmacokinetic profiles.
Because compound 7 displayed excellent affinity for the
avb3 receptor and attractive pharmacokinetic para-
meters in dogs, it was selected to be evaluated in an in
vivo model of osteoporosis. Our model utilized young,
rapidly growing male rats, and measured the effect of
resorption inhibitors on the remodeling of the femur
during normal growth.21 In this assay, a twice-daily 30
mpk oral dose of 7 given for 10 consecutive days was
demonstrated to increase bone density.22
Conclusion
We have prepared a new series of chain-shortened pyr-
rolidinone-containing avb3 antagonists that have high
affinity for the avb3 receptor. An efficient and stereo-
selective route to this class of antagonists is described,
utilizing an asymmetric oxazolidinone alkylation and
Friedlander condensation as key steps. Two diastereo-
meric pairs of pyrrolidinones were prepared by varying
the b-alanine aryl substituent and pyrolidinone stereo-
chemistry, and each pair possessed enhanced receptor
affinity as compared to their amide parents. Noteworthy
was the finding that the diastereomeric pairs showed
It was quickly determined that incorporation of a pyr-
rolidine as a constraint of the amide-bond of compound
7 provided analogues with enhanced affinity for the avb3
receptor. The (R)-pyrrolidinone (7) had an SPAV3 IC50
of 0.39 nM, while the slightly more potent (S)-antipode
10 had an IC50 of 0.22 nM. While the pyrrolidinone
configuration had only modest effects on in vitro
potency, the pyrrolidinone stereochemistry had a pro-
found effect on the physical properties of the diastereo-
mers. The (R)-pyrrolidinone 7 had a measured log P of

J. J. Perkins et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4285–4288
4287
Table 1. avb3 Antagonists
SPAV3 (IC50, nM)
Log P
Dog pharmacokineticsa
Cl (mL/min/kg)
Oral F (%)
t1/2 (h)
8
9
3.0
1.6
0.53
0.77
99
1.2
5.8
2.5
65
2.4
7
0.39
0.22
0.13
0.88
À0.12
1.03
52
2.1
22.9
2.52
8
10
11
11.5
1.5
2.2
39
12
0.25
À0.42
9.5
15.7
2
aCompounds were dosed at 1 mpk po and 0.2 mpk iv to dogs (n=2).
References and Notes
similar affinities for the avb3 receptor, but markedly
different polarities as measured by log P determinations.
In each case, the (R)-pyrrolidinone was more lipophilic
and offered improved pharmacokinetics in dogs when
compared to the (S)-isomer. Compound 7 was further
demonstrated to be effective at reducing bone resorption
in a 10-day rat model of osteoporosis.
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studies were approved by the Merck Research Laboratories
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density after 10 days as compared to vehicle dosing.
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