Lactams as EP4 prostanoid receptor agonists. 3. Discovery of N-ethylbenzoic acid 2-pyrrolidinones as subtype selective agents

Article abstract of DOI:10.1021/jm049290a

Two distinct synthetic schemes were applied to access heteroatom-containing α-chain lactams or lactams terminated as aryl acids. The latter lactams were devised using a pharmacophore for EP4 receptor activity. γ-Lactams were characterized for their prostanoid EP receptor affinities and EP ₄ activity and found to be selective for the EP₂ and EP₄ receptors or selective for the EP₄ subtype. Benzoic acid 17 displayed enhanced in vivo exposure relative to 3.

Full text of DOI:10.1021/jm049290a

6124
J. Med. Chem. 2004, 47, 6124-6127
Lactams as EP4 Prostanoid Receptor
Agonists. 3. Discovery of N-Ethylbenzoic
Acid 2-Pyrrolidinones as Subtype
Selective Agents
Todd R. Elworthy,* Emma R. Brill, San-San Chiou,‡
Frances Chu, Jason R. Harris,† R. Than Hendricks,
Jane Huang, Woongki Kim, Leang K. Lach,§
Tara Mirzadegan, Calvin Yee, and
Keith A. M. Walker
Figure 1. Structures of 1-4.
Roche Palo Alto, 3431 Hillview Avenue,
Palo Alto, California 94304
vating a single prostanoid receptor subtype is borne out
of the observations that 1 and 2 (i.e., nonselective
agonists) exert many physiological effects in human.6a,7
It was hoped that by selective stimulation of only the
EP4 receptor,8 most of the undesirable effects might be
avoided.
We have previously described a functional assay for
the EP4 receptor and the optimization of the ω-chain of
3 toward high EP4 subtype selectivity.9a We next sought
to improve the in vivo half-life of active and selective
EP4 agonists.9b The focus of this report is to outline some
design criteria and syntheses and to describe selected
in vitro pharmacologic and pharmacokinetic data of
lactams related to 3.
Cyclopentane prostaglandins having an aliphatic
R-side chain are known to be metabolized by â-oxida-
tion.7 An abbreviated pharmacokinetic analysis of lac-
tam 3 was conducted in rat (Table 2, vide infra). The
products of â-oxidation and 13,14-reduction of 3 were
identified in circulation. In addition, the â-oxidation
product 4 was found upon sampling plasma from the
portal vein following oral dosing of 3 at 3 mpk. This
latter finding pointed to the high lability of heptanoic
acids to extrahepatic based metabolism, thus severely
eroding the presence of active parent in circulation.
One approach to stabilize ligands toward â-oxidation
is to prepare lactams containing a heteroatom in the
R-chain. Following some experimentation, lactam 510
was employed to produce thioether ligands via TIPS
ether 6 as shown in Scheme 1. Diol 7 was converted to
desired R-chain materials without resorting to 2° hy-
droxyl protection. The structures of the esters leading
to acids 8 and 9 were supported by 2D NMR. The
regiochemistry of mesylation of 7 was confirmed because
alkylation of 5 with methyl 7-chloro-5-thiaheptanoate
furnished the same thioether ligand. To produce 3-oxa
derivatives (see Table 2), lactam 5 was alkylated with
ethyl 7-bromo-3-oxaheptanoate11 to produce 10 and 11
according to precedence.9a
Another approach to obviate the â-oxidation process
is to prepare lactams bearing an aromatic acid. Inter-
phenylene analogues of prostaglandins are known in
which the double bond (e.g., PGE2) is replaced in the
R-chain.12 However, m-phenylenealkyl acids were found
to be inactive at the EP4.9b To our knowledge, Tani and
colleagues were the first to report a benzoic acid as a
potent prostaglandin agonist (as a butaprost derivative
selective for the mouse EP2 subtype).13 The benzoic acid
prototype 12 was prepared according to N-alkylation
Received August 30, 2004
Abstract: Two distinct synthetic schemes were applied to
access heteroatom-containing R-chain lactams or lactams
terminated as aryl acids. The latter lactams were devised using
a pharmacophore for EP4 receptor activity. γ-Lactams were
characterized for their prostanoid EP receptor affinities and
EP4 activity and found to be selective for the EP2 and EP4
receptors or selective for the EP4 subtype. Benzoic acid 17
displayed enhanced in vivo exposure relative to 3.
Osteoporosis is the loss of bone mass due to an
imbalance in the resorption and the remodeling pro-
cesses. The onset of this imbalance can lead to an
increased risk of fracture which may be anticipated for
both women and men after age 50. Postmenopausal
women in particular can present bone loss and thus
have been of primary interest for effective drug therapy.
The inhibition of bone resorption has been the estab-
lished therapy with agents such as bisphosphonates or
estrogen and estrogen-related compounds.1 While sta-
bilizing existing bone mass at best, the antiresorptives
do not give rise to a net increase in bone.
Intermittently dosed parathyroid hormone2 and re-
lated peptides3 have been reported to stimulate bone
formation.
Body and colleagues have recently reported a head-
to-head comparison of the effectiveness of either ap-
proach. The bisphosphonate alendronate (Fosamax, 10
mg/day, oral) or recombinant human parathyroid hor-
mone (Forteo, 40 µg/day, s.c.) was dosed for 14 months
to postmenopausal women with established osteoporo-
sis. The Forteo treated group responded more favorably
in increased bone mineral density and more importantly
in reduction of fracture rate.4 While the bone anabolic
approach is clinically advantageous for individuals at
risk of fracture, the peptidic nature of Forteo requires
parenteral administration.
The potential to discover an orally active bone-forming
agent was instilled by the reports that prostaglandin
(PG) E2 1 and misoprostol 2 (Figure 1) promote bone
growth in mammals.5,6 The interest in selectively acti-
* To whom correspondence should be addressed. Phone: 650-852-
3159. Fax: 650-354-2442. E-mail: todd.elworthy@Roche.com.
‡ Current address: EÄ lan Pharmaceuticals, 800 Gateway Boulevard,
South San Francisco, CA 94080.
† Current address: Department of Chemistry, University of Cali-
fornia at Irvine, 516 Rowland Hall, Irvine, CA 92697.
§ Current address: ARYx Therapeutics, 2338 Walsh Avenue, Santa
Clara, CA 95051.
10.1021/jm049290a CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/10/2004

Letters
Journal of Medicinal Chemistry, 2004, Vol. 47, No. 25 6125
Scheme 1. Preparation
8-Aza-5-thia-11-deoxyprostaglandins
Figure 2. Predicted active conformation of 9 (brown-yellow)
and 16 (blue) using a model of EP4 activity. Features are
negative ionizable (blue), hydrogen bond acceptor (green),
hydrophobe (cyan).
chemistry shown in Scheme 1 using ethyl 3-(3-bro-
mopropyl)benzoate. Acid 12 displayed measurable af-
finity at the target receptor (Table 1), and thus we
elected to pursue related aryl acids using pharmaco-
phore searching.
Scheme 2. Syntheses of Alcohol 15 and Benzoic Acid
22
The R-chain conformers of 1, 3, and 9 were sampled
via a Monte Carlo simulation using the Schro¨dinger
software to identify a predictive model of activity for
the EP4 receptor. The lower side-chain features were
simplified to a small hydrophobe (-CHdCH2) and thus
approximated not to perturb the upper chain conforma-
tions.
Figure 2 displays an R-chain pharmacophore that was
generated using Catalyst software based on the con-
formers of 1, 3, and 9. The carbonyl oxygen of the lactam
and the carboxylate are highlighted with a distance of
8.6 Å. A number of candidate structures varying linker
and benzoic acid isomers were analyzed with the model.
A minimum chain length of two methylenes was pre-
dicted by this model to place the carboxylate of the
p-benzoic acid to populate the conformers readily acces-
sible to 1, 3, and 9. A pharmacophore overlay of one
predicted active R-chain conformation of 9 and the
proposed benzoic acid 16 is shown in Figure 2.
Not surprisingly, the treatment of the salt of 5 with
an electrophile to generate the desired phenethyl ester
was problematic. N-Ethyl-linked benzoic acids were
prepared in a highly optically enriched form as shown
in Scheme 2. The hydroxyl of (S)-5-hydroxymethylbu-
tyrolactone was protected as the benzyl ether, and the
lactone was condensed with 4-(2-aminoethyl)benzoic
acid. The amidoalcohol 14 was sequentially treated with
a single equivalent of MsCl and excess tert-butoxide to
generate the core lactam.14 The crystalline alcohol 15
resulted upon hydrogenolysis.
Hydroxymethyl lactam 15 served as the common
intermediate for all the ethyl-linked benzoic acid lac-
tams in this report. The synthesis of benzoic acid 22 is
typical for this class and is exemplified in Scheme 2.
The completion of 16, 18 [from methyl 5-(2-bromoethyl)-
thiophene 2-carboxylate],15 19, and 21 is analogous to
results from our earlier report.9a Saturated ω-chain
ligands 17 and 20 were produced from the esters,
leading to 16 and 19, respectively, by reduction (1 atm
of H2, 10% Pd-C, alcohol, 2-3 h) prior to saponification.
Pharmacologic data are presented in Table 1 for
compounds containing an aryl feature in the R-chain.
Ligands were first assessed for their functional activity
at the human EP4 receptor and then profiled for their
affinity to the hEP subtypes9a as warranted. The data
suggest that the extended carboxylate conformers (to
the lactam ring) presented by propyl-linked benzoate
12 do not stimulate the EP4 receptor. The R-chain
presented by the ethyl-linked benzoate 16 and thiophene
2-carboxylate 18 confers high potency at the EP4
subtype, with the latter showing good subtype selectiv-
ity. However, 16 and the ω-cyclobutyl 19 possess modest
subtype selectivity because these compounds show
affinity for the EP2 subtype. This selectivity is further
eroded by the increased flexibility presented in the
ω-chain of ligands 17 and 20. The affinity data of 17
and 20 reveal that these ligands are coselective for the
EP2 and EP4 subtypes. High subtype selectivity for the
R-chain benzoates is restored when the ω-chain contains

6126 Journal of Medicinal Chemistry, 2004, Vol. 47, No. 25
Letters
Table 1. Prostanoid E-Type Receptor Profile of Lactams Bearing an Arene R-Chain
a nd) not determined. b The value is the average of three determinations except where noted in parentheses.
Table 2. Select Data of Modified Lactam Prostanoids
an appropriately substituted benzyl (e.g., 21) or biphenyl
(e.g., 22) as observed earlier.9a The substituted benzyl
feature of 21 was reported by Maruyama and colleagues
(as a PGE1 derivative) to impart high murine EP4
subtype selectivity.16
The lactam prostanoids appear to undergo in vivo
metabolism analogous to their cyclopentanone counter-
parts. The pharmacokinetics data of Table 2 show that
lactam 3 is subject to rapid degradation. Of note, the
i.v. clearance rate of 3 was measured to be faster
(∼150%) than hepatic blood flow (HBF). The products
of both â-oxidation and 13,14-reduction of 3 were
identified in circulation. The 3-oxa acid 10 displayed a
marked decrease in clearance rate (∼40% of HBF)
compared to acid 3 but with no improvement in t1/2. This
can be clarified by noting their difference in volume of
distribution (i.e., Vâ for 3 was 6.9 L/kg and 10 was 1.5
a The value is the average of at least three determinations
expressed in nanomolar. b The average value following i.v. dosing
with 1.0 mg/kg of lactam and the parent compound quantified with
LC-MS/MS and detected to 0.1 ng/mL. c nd ) not determined.
d Additional binding data (Ki) for 9: EP1 nd; EP2 4700 nM; EP3
1900 nM. e Estimate due to nonlinear terminal phase.
L/kg).
Further insight was gained upon assessing the extent
of protein binding of the two lactams. Ligand 3 dis-
played much higher plasma protein binding than 10.
The potency of 3-oxa ligands could be improved but at
the expense of in vivo exposure (e.g., 11 presents CL at
60% of HBF). The benzoic acids 16 and 17 are moderate
to high-clearance compounds with i.v. rates estimated
at 65-80% HBF. Finally, the saturated ω-chain ligand
17 displayed a prolonged t1/2 mainly due to its high Vâ
of 15.3 L/kg.

Letters
The introduction of an ether or a thioether feature in
Journal of Medicinal Chemistry, 2004, Vol. 47, No. 25 6127
(5) Ke, H. Z.; Shen, V. W.; Qi, H.; Crawford, D. T.; Wu, D. D.; Liang,
X. G.; Chidsey-Frink, K. L.; Pirie, C. M.; Simmons, H. A.;
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affinity measurements. Significantly, benzoic acid 16
was predicted to be active at the EP4 subtype based on
a pharmacophore model. This differs from the classical
modified PG upper side chains prior to identification of
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clearance rate compared to 3, and the “13,14-dihydro
ligand” 17 illustrates the effect of the ω-chain alkene
on in vivo PG metabolism.
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Lactams as EP4 Prostanoid Receptor Subtype Agonists. Part 1.
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Acknowledgment. We are grateful to Dr. J. M.
Muchowski, Dr. E. B. Sjogren, and Prof. E. J. Corey for
their insights and to Dr. N. Illangasekare for his NMR
expertise. We are also grateful to the staff of Analytical
Chemistry and Laboratory Animal Technology depart-
ments at Roche Palo Alto for their assistance in char-
acterizing the compounds described herein.
Supporting Information Available: Detailed experi-
mental information for the compounds prepared according
Schemes 1 and 2 and for the protocol for rat pharmacokinetic
analysis. This material is available free of charge via the
Internet at http://pubs.acs.org.
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JM049290A