Expedited discovery of second generation NK-1 antagonists: Identification of a nonbasic aryloxy substituent

Article abstract of DOI:10.1016/S0960-894X(01)00250-5

Solution-phase, parallel-synthesis techniques were used to optimize a series of nonbasic NK-1 antagonists, resulting in the identification of (R)-26, an orally bioavailable compound with subnanomolar potency.

Full text of DOI:10.1016/S0960-894X(01)00250-5

Bioorganic & Medicinal Chemistry Letters 11 (2001) 1643–1646
Expedited Discovery of Second Generation NK-1 Antagonists:
Identification of a Nonbasic Aryloxy Substituent
James E. Fritz,* Philip A. Hipskind, Karen L. Lobb, James A. Nixon,
Penny G. Threlkeld, Bruce D. Gitter, Carl L. McMillian and Stephen W. Kaldor
Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, IN 46285, USA
Received 7 August 2000; accepted 6 April 2001
Abstract—Solution-phase, parallel-synthesis techniques were used to optimize a series of nonbasic NK-1 antagonists, resulting in
the identification of (R)-26, an orally bioavailable compound with subnanomolar potency. # 2001 Elsevier Science Ltd. All rights
reserved.
Substance P (SP) is a member of the tachykinin neuro-
peptide family and acts through three recognized recep-
tor subtypes: NK-1, NK-2, and NK-3.1 Studies with
NK-1 receptor agonists and antagonists indicate SP may
be associated with a wide variety of disease states including
persistent pain,2a asthma,2b arthritis,2c and migraine.2d
Previously, we reported on compound 1 (Fig. 1), a potent
NK-1 antagonist currently under clinical evaluation.3
Despite its desirable features, 1 possesses suboptimal oral
bioavailability and causes irritation upon iv injection. In
an effort to improve upon the properties of 1, expedited
synthesis techniques were employed to facilitate dis-
covery of a second generation clinical candidate.
source of its shortcomings, and therefore examined
earlier SAR work surrounding this side chain.3 In
particular, we noted that compound 2, a relatively weak
NK-1 antagonist (IC50=34 nM), and others like it
without a basic side chain, did not cause irritation upon
iv administration. Furthermore, the physical properties
of the compounds were different and we hoped that this
would translate into a better pharmacokinetic profile.
As a result, database similarity searching techniques
were employed to identify a diverse and novel set of
nonbasic acylating agents analogous to the amide side
chains of 1 and 2.4
In the original SAR study, the (R)-enantiomer had
consistently been the more potent. We elected to use
racemic starting material (3, Scheme 1) to ensure that
We hypothesized that the dibasic 2-[4-(piperidin-1-
yl)piperidin-1-yl]acetyl side chain of 1 was a potential
Figure 1.
*Corresponding author. Fax: +1-317-433-1685; e-mail: j.fritz@lilly.com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00250-5

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J. E. Fritz et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1643–1646
we identified active compounds in the event the diverse
side chains caused an inversion of stereochemical pref-
erence by the receptor.3 Initially, in vitro binding affi-
were utilized to remove any unreacted starting material
or byproducts as necessary.8 To ensure meaningful
results, only compounds of purity ꢀ90% as determined
by HPLC analysis at two wavelengths were submitted
for bioassay.9
nities were obtained in triplicate at
a
single
concentration of 10 nM using human IM-9 cells. This
allowed us to more readily assay larger sets of analo-
gues.5 Full IC50 determinations were performed on
compounds exhibiting >40% inhibition at 10 nM.
Expedited solution-phase synthesis was employed to
construct amides, sulfonamides, carbamates, and ureas
wherein polymer-supported scavengers (e.g., amino-
methylpolystyrene) were used to facilitate removal of
excess acylating agent.6 Amides were also prepared
using polymer-supported 1-(3-pyrrolidinylpropyl)-3-
ethylcarbodiimide and carboxylic acids, thus expanding
the number and diversity of possible side-chain replace-
ments.7 Additional scavengers or ion-exchange resins
Representative SAR highlights from the initial synthesis
run are shown in Table 1. Compounds with urea,
sulfonamide, or carbamate side chains were generally
less potent than 2 (results not shown). However, amides
11–14 showed significant improvement over the lead.
Aromatic side chains, where the aromatic group was
separated from the carboxamide by a 1–3 atom spacer,
provided compounds with greater potency. A heteroatom
in the spacer and polar groups on the aromatic ring
appeared to contribute to the increased potency. A second
synthesis run focusing on substituted benzoylpropionic
Scheme 1. Method A: CHCl3, (3) (1 equiv), acylating agent (acid chloride, sulfonyl chloride, isocyanate, carbamoyl chloride) (1–1.5 equiv),
(piperidinomethyl)-polystyrene (3.5 equiv if required), rt, 24 h followed by aminomethyl polystyrene; method B: CHCl3, (3) (1 equiv), (piperidino-
methyl) polystyrene (3 equiv), polymer-supported 1-(3-pyrrolidinylpropyl)-3-ethylcarbodiimide (4.5 equiv), carboxylic acid (1.5 equiv), 65 ꢁC, 24 h
followed by methylisocyanate polystyrene (1–2 equiv).
Table 1. In vitro activity for representative products from synthesis run 1
Entry
R
% Inhibition
@ 10 nM
IC50 (nM)
Entry
R
% Inhibition
@ 10 nM
IC50 (nM)
2
4
5
6
7
8
9
20
0
34
—
—
—
—
—
—
10
11
12
13
14
15
16
20
46
41
59
68
19
4
—
8.3
10.8
3.2
3.5
—
0
13
18
30
25
—

J. E. Fritz et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1643–1646
Table 2. In vitro activity for products from synthesis run 2
1645
Entry
R
% Inhibition
@ 10 nM
IC50 (nM)
Entry
R
% Inhibition
@ 10 nM
IC50 (nM)
17
63
49
3.3
22
52
23
10.2
18
19
20
21
—
6.0
24
23
24
25
26
—
66
42
3
22
81
87
—
9.7
0.82
—
acids, phenylacetic acids, and phenoxyacetic acids was
performed: representative SAR results are highlighted in
Table 2. A systematic study of the aromatic ring revealed
a strong preference for the para position (22 and 25).
acid side chains. Unfortunately less than 20 aryloxy-
acetic acids with polar o-, m-, or p-substituents are
commercially available.
The solution came from earlier work on the SAR
wherein amine 3 was acylated with bromoacetyl bro-
mide and alkylated with amines in the presence of base.
Using the appropriately substituted phenols (>2000
commercially available) in the presence of potassium t-
butoxide we were able to construct the desired sub-
stituted aryloxyacetic acid side chains (Scheme 2).
Similar substitution enhanced the potency of the ben-
zoylpropionic acids as well (17 and 19). To our surprise,
an additional increase in potency was obtained with 4-
(hydroxymethyl)phenoxy acetic acid as the side chain
(26). At this point, the (R)- and (S)-enantiomers of 13 of
the more potent compounds were independently syn-
thesized to examine NK-1 receptor stereopreference. In
accord with previous SAR work, the receptor exhibited
a uniform and overwhelming preference for the (R)-
enantiomers (data not shown).10 In further SAR work,
the active enantiomer (R) of 3 was used as starting
material. Compound (R)-26, in contrast to 1, proved to
be non-irritating in an ocular irritation assay.11 Addi-
tionally, (R)-26 showed a modest improvement over 1 in
both bioavailability (17% vs 9%) and peak blood levels
(Cmax 426 ng/mL vs 302 ng/mL).12 Finally, we desired a
route to further explore the 4-substituted phenoxyacetic
To our satisfaction, we have been able to identify addi-
tional 4-substituted aryloxyacetic acid side chains with
subnanomolar potency (data not shown). Three of the
better examples from the third iteration are illustrated
above.
Evaluation of the third generation of the SAR shows
that these compounds remain nonirritating, but show
further improvements in bioavailability and microsomal
stability (Table 3).
Scheme 2. Step 1: THF, DIEA, bromoacetyl bromide (1.1 equiv), 0 ꢁC; step 2: THF, phenol (3 equiv), potassium t-butoxide (3 equiv), 2 h, 80 ꢁC.

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J. E. Fritz et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1643–1646
Phebus, L. A.; Regoli, D.; Simmons, R. M.; Threlkeld, P. G.;
Table 3.
Waters, D. C.; Gitter, B. D. J. Med. Chem. 1996, 39, 736. (b)
Hipskind, P. A.; Howbert, J. J.; Cho, S. S. Y.; Cronin, J. S.;
Fort, S. L.; Ginah, F. O.; Hansen, G. J.; Huff, B. E.; Lobb,
K. L.; Martinelli, M. J.; Murray, A. R.; Nixon, J. A.; Staszak,
M. A.; Copp, J. D. J. Org. Chem. 1995, 60, 7033.
4. Structure searches were performed using the ACD provided
by MDL information systems, Inc., 14600 Catalina St., San
Leandro, CA 94577, USA.
Compound
IC50
(nM)
Microsome stability
dog liver (% remaining)
%F (dog)
(R)-26
27
28
0.43
0.48
0.65
0.98
28
87
96
67
17
14
36
47
29
5. (a) NK-1 receptor binding affinities (IC50s) for all com-
pounds were determined in [125I]Bolton-Hunter SP binding
experiments using a human IM-9 cell line expressing NK-1
receptors. See: Gitter, B. D.; Bruns, R. F.; Howbert, J. J.;
Waters, D. C.; Threlkeld, P. G.; Cox, L. M.; Nixon, J. A.;
Lobb, K. L.; Mason, N. R.; Stengel, P. W.; Cockerham, S. L.;
Silbaugh, S.; Gehlert, D. R.; Schober, D. A.; Phebus, L. A.;
Iyengar, S.; Calligaro, D. O.; Regoli, D.; Hipskind, P. J.
Pharmacol. Exp. Ther. 1995, 275, 737. (b) Payne, D. G.;
Brewster, D. R.; Goetzl, E. J. J. Immunol. 1984, 133, 3260.
IC50 values were determined from 11-point concentration
response curves with each concentration in triplicate.
6. Kaldor, S. W.; Siegel, M. G.; Fritz, J. E.; Dressman, B. A.;
Hahn, P. J. Tetrahedron Lett. 1996, 37, 7193.
7. Kaldor, S. W.; Siegel, M. G. Curr. Opin. Chem. Biol. 1997,
1, 101.
8. Siegel, M. G.; Hahn, P. J.; Dressman, B. A.; Fritz, J. E.;
Grunwell, J. R.; Kaldor, S. W. Tetrahedron Lett. 1997, 38,
3357.
9. Reverse-phase HPLC analysis using a 8Â200 mm C18 prep
Nova pak cartridge 1% NH4OAc in 10% H2O, 45% CH3CN,
45% MeOH monitored at 254 and 280 nm was used to estab-
lish purity. All compounds provided satisfactory MS, 1H
NMR, and TLC.
In summary, solution-phase, combinatorial techniques
have been used to rapidly identify potent, nonbasic side-
chain replacements for the dibasic side chain of the
clinical candidate 1. Further studies to improve the in
vitro and in vivo properties of these leads using expe-
dited synthesis will be reported in subsequent commu-
nications.
Acknowledgements
The authors would like to thank Process Research at Eli
Lilly and Company for supplying us with sufficient
quantities of 3 including both enantiomers, and Dr.
Dennis A. Laska for his work with the ocular irritation
assay.
References and Notes
10. For example, (R)-26 provided an IC50 of 0.4 nM, whereas
its corresponding (S)-enantiomer was greater than 100-fold
less potent.
11. Compound 1 had an ADC (in mm) score of 48, which
classifies it as a severe irritant while (R)-26 had an ADC score
of 10 classifying it as nonirritating. For experimental detail
see: Laska, D. A.; Hoffman, W. P.; Reboulet, J. T.; Poole,
J. W. In Vitro Tox. 1996, 9, 201.
12. Male F344 rats were administered (R)-26 as both an oral
acacia suspension (10% w/v) 25 mg/kg and an intravenous 1
mg/kg in 10% Emulphor1 solution. Plasma samples were col-
lected at 0.5, 1, 2, 4, 6, 8, and 12 h after oral dosing with mea-
surable concentrations persisting for 4 h. Reverse-phase HPLC
was use to quantify the concentration of (R)-26 in the plasma.
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