Substituted [(4-phenylpiperazinyl) methyl]benzamides: Selective dopamine D4 agonists [3]

Full text of DOI:10.1021/jm970021c

J . Med. Chem. 1997, 40, 1771-1772
1771
Su bstitu ted [(4-P h en ylp ip er a zin yl)-
m eth yl]ben za m id es: Selective Dop a m in e
D4 Agon ists
Shelly A. Glase,* Hyacinth C. Akunne,
Lynn M. Georgic, Thomas G. Heffner,
Robert G. MacKenzie, Peter J . Manley,
Thomas A. Pugsley, and Lawrence D. Wise
F igu r e 1.
Sch em e 1a
Departments of Chemistry and Therapeutics,
Parke-Davis Pharmaceutical Research,
Division of Warner-Lambert Company,
Ann Arbor, Michigan 48105
Received J anuary 10, 1997
The drugs currently available for treating schizophre-
nia are limited by disabling side effects such as extra-
pyramidal syndrome (EPS) and tardive dyskinesia. The
blockade of dopamine (DA) D2-like receptors, in limbic
as well as striatal regions of the brain, is thought to be
responsible for both their efficacy and neurological side
effects.1 Recent advances in molecular biology have
shown that the DA D2 family of receptors is comprised
of D2, D3, and D4 subtypes.2 The D3 and D4 receptors
are much less abundant in the brain than are D2
receptors and are much more localized in the limbic
regions that are thought to be responsible for cognitive
and emotional functions.3 The clinical potency of most
antipsychotic drugs shows a high correlation with the
DA D2 receptor affinity.4 An exception is clozapine,
which has an almost 10-fold higher clinical potency than
would be expected from its DA D2 receptor affinity. This,
and its low propensity to induce EPS, may be explained
by clozapine’s reported 2-10-fold higher affinity for the
DA D4 receptor.5 However, clozapine’s ability to bind
with high affinity to several additional nondopaminergic
receptors makes it difficult to identify the contribution
of D4 receptor function to clinical efficacy. Therefore,
identification of a D4 receptor subtype selective agonist
or antagonist is necessary to determine the biological
significance of this receptor.
In our search for a DA D4 selective antagonist, we
discovered a series of N-methylpiperazinyl benzamides
(Figure 1) that consistently showed partial agonist
activity. Optimization of the structure-activity rela-
tionship (SAR) for D4 receptor affinity, selectivity, and
agonist activity resulted in the identification of several
compounds that may be useful tools for pharmacological
testing.
Several analogs with various substituents on the
phenyl rings of the arylpiperazine and benzamide were
synthesized as shown in Scheme 1. The synthesis of
many of these analogs started with oxidative decar-
boxylation of commercially available hippuric acids. The
hippuric acids that were not commercially available
were readily prepared from substituted acid chlorides
and glycine. The acetates, resulting from oxidative
decarboxylation,6 were displaced with arylpiperazines7
giving N-methylpiperazinyl benzamides in good overall
yield.
a
(a) Glycine, 2 N NaOH, CH2Cl2, 25 °C, 4 h, 60-90%; (b)
Pb(OAc)4, Cu(OAc)2, C6H6, reflux, 4 h, 50-90%; (c) arylpiperazine,
Et3N, CH3CN, 25 °C, 4 h, 75-98%.
Ta ble 1. Effect of Substitution on Binding Affinity and hD4
Receptor Mitogenesis
[3H]thymidine
uptakec
binding affinitya
(Ki, nM)b
% max EC50
,
compd
R1
R2
hD4
hD3
hD2
effectd
nMe
1
2
3
4
5
6
7
8
9
H
H
H
2-OMe 22
2470 1880
177 1500
60
100
92
50
50
80
49
45
54
31
106
24
13
16
2-Cl
2-CN
32
27
>3000 4180
3-Me 2-OMe
3-Me 2-Cl
3-Me 2-CN
4-Me 2-OMe
4-Me 2-Cl
4-Me 2-CN
3-Cl
3-Cl
3-Cl
4-Cl
4-Cl
4-Cl
6.7
27
8.7
7.7
12
6.5
2480
835
464 1330
2810 3740
17
174
49
291
466
3.4
218
1.2
15
377 1150
10
11
12
13
14
15
2-OMe 15
1450
451
100
2-Cl
2-CN
41
34
364 2420
2270
268
66
897
349
495
929
98
41
47
5.3
3.5
8.4
2-OMe 14
2-Cl
2-CN
5.3
5.5
600
g70
g3.9
a
[3H]Spiperone as ligand; human D4, D3, and D2 receptors
b
expressed in CHO K1 transfected cells. Ki values were obtained
from four to six concentrations, run in triplicate, by a nonlinear
regression analysis. c Stimulation of [3H]thymidine uptake on CHO
d
pro-5 cells expressing human D4 receptors. Percent maximal
effect compared to that of the full DA agonist quinpirole (100%).
e EC50 values (95% confidence intervals) were generated from 10
concentrations, n ) 4.
the cellular uptake of [3H]thymidine and comparing the
response to the full agonist quinpirole (defined as 100%).
When the benzamide phenyl was unsubstituted (R1
) H) it was found that only ortho substitution on the
phenylpiperazine (R2) gave compounds with at least
moderate D4 affinity. Of the ortho substituents inves-
tigated methoxy (1), chloro (2), and cyano (3) resulted
in compounds with the best D4 receptor affinity, selec-
tivity, and agonist activity. To further optimize these
properties various substituents were introduced on the
benzamide phenyl (R1). Of the substituents examined
at R1, m- and p-chloro and m- and p-methyl afforded
the most significant improvement.
The binding affinities for the DA D4, D3, and D2
receptors8 and the agonist activity9 at the D4 receptor
are shown in Table 1. Agonist activation of dopamine
receptors stimulates mitogenesis in D4-transfected CHO
pro-5 cells. This response was estimated by measuring
These functional group manipulations resulted in
compound 6 (R1 ) 3-Me, R2 ) 2-CN), an agonist with
S0022-2623(97)00021-6 CCC: $14.00 © 1997 American Chemical Society

1772 J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 12
Communications to the Editor
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Cloning and Characterization of a Novel Dopamine Receptor (D3)
as a Target of Neuroleptics. Nature 1990, 347, 72-76. (c) Van
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Seeman, P.; Niznik, H. B.; Civelli, O. Cloning of the Gene for a
Human Dopamine D4 Receptor with High Affinity for the
Antipsychotic Clozapine. Nature 1991, 350, 610-614.
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Damask, S. P.; Little, K. Y.; Civelli, O.; Watson, S. J . Dopamine
Receptor Gene Expression in the Human Medial Temporal Lobe.
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Receptors for Antipsychotic Drugs and Dopamine. Direct Bind-
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of Neuroleptics Occupy D2 Receptors, Clozapine Occupies D4.
Neuropsychopharmacology 1992, 7, 261-284.
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by Lead Tetraacetate. In Organic Reactions; Dauben, W. G.,
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Shih, Y. H.; Damsma, G.; Wikstro¨m, H.; Whetzel, S. Z.; Georgic,
L. M.; Cooke, L. W.; DeMattos, S. B.; Corbin, A. E.; Glase, S. A.;
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Heterologously expressed D3 dopamine receptors: Comparison
with D2 dopamine receptors. Mol. Pharmacol. 1994, 45, 51-
60. (b) CHO pro-5 cells transfected with the human D4 receptor
were plated on 96-well plates in MEM-Alpha with 10% fetal calf
serum containing penicillin (100 U/mL) and streptomycin (100
g/mL). Forty-eight hours later, cells were serum deprived by
washing and maintained in serum-free media. Twenty-four
hours later, vehicle, standards, or test compounds were added.
Eighteen hours later, [3H]thymidine (5 µCi/well) was added for
2 h, then trypsin (100 µL of 0.25%) was added for 1 h, and the
assay was terminated by filtration using a Brandel 96-well
harvester. The filters were counted for radioactivity using the
LKB-plate counting system. Ten-point dose-response curves
were determined for each test compound, and the drug concen-
tration necessary for 50% stimulation (EC50) was calculated from
the resulting curve. Intrinsic agonist activity was assessed by
comparing the maximal effect of each test compound relative to
the effect obtained with a maximally effective concentration of
quinpirole in each experiment. Compounds with low intrinsic
activity were tested for their ability to block the stimulation of
[3H]thymidine uptake caused by a submaximal concentration
of quinpirole (10 nM).
F igu r e 2. Compound 6 (100 nM) increases [3H]thymidine
uptake in CHO pro-5 cells expressing the human dopamine
D4 receptor. This effect is antagonized by 1000 nM haloperidol
and L-745,870.
8.7 nM affinity for the DA D4 receptor with >300-fold
selectivity over D3 and >400-fold selectivity over D2. The
binding affinities (Ki) for compound 6 for the R-1
adrenergic, R-2 adrenergic, 5-HT1A, and 5-HT2A recep-
tors are 168, 177, 385, and 4010 nM, respectively.10 We
would therefore conclude that the activity of compound
6 would not be exerted through these receptors. Figure
2 shows that the agonist effect was blocked by the
nonselective DA antagonist haloperidol (16 ( 12%) and
the DA D4 selective antagonist L-745,87011 (20 ( 10%).
It has been demonstrated that N-Mannich bases of
amides and other compounds containing acidic NH
groups may be useful prodrugs. These N-Mannich
bases were shown to decompose very rapidly in aqueous
solution with formation of formaldehyde, amine, and
parent compound (amide).12 Since compounds 1-15
closely resemble these prodrugs, several were examined
for their aqueous stability and were found to be stable
in water for up to 24 h. They were also found to be
stable in dilute acid (0.1 N HCl) for several hours but
began to degrade in 3 N HCl within 10 min. This acid
lability may exclude their use for orally administered
in vivo experiments.
In conclusion, optimization of R1 and R2 functional
groups resulted in 6, a D4 agonist with >400-fold
selectivity over the D2 receptor and >300-fold selectivity
versus D3. To our knowledge compound 6 is the first
D4 selective agonist reported in the literature. This
compound may provide a useful tool in determining the
contribution of D4 receptors to schizophrenia.
(10) The reference agents for the R-1 adrenergic, R-2 adrenergic,
5-HT1A, and 5-HT2A assays had Ki values of 0.18 nm (prazosin),
1 nM (idazoxan), 0.65 nM (8-OH-DPAT), and 3.39 nM (ketan-
serine), respectively.
(11) Kulagowski, J . J .; Broughton, H. B.; Curtis, N. R.; Mawer, I.
M.; Ridgill, M. P.; Baker, R.; Emms, F.; Freedman, S. B.;
Marwood, R.; Patel, S.; Patel, S.; Ragan, C. I.; Leeson, P. D. 3-[[4-
Ack n ow led gm en t . We would like to thank Y.-H.
Shih, K. Zoski, and S. Yee for the binding data.
(4-Chlorophenyl)piperazin-1-yl]-methyl]-1
H-pyrrolo[2,3-b]-
Su p p or tin g In for m a tion Ava ila ble: Experimental data
for compounds in this paper (4 pages). Ordering information
is given on any current masthead page.
pyridine: An Antagonist with High Affinity and Selectivity for
the Human Dopamine D4 Receptor. J . Med. Chem. 1996, 39,
1941-1942.
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Systems XV. Bioreversible Derivatization of Phenytoin, Acet-
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pounds by N-aminomethylation to Effect Enhanced Dissolution
Rates. Int. J . Pharm. 1980, 7, 129-136. (b) Bundgaard, H.;
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Mannich Bases as Potential Novel Prodrugs for Amides, Ureides,
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