Synthesis and in vitro evaluation of N,N'-diphenyl and N-naphthyl-N'-phenylguanidines as N-methyl-D-aspartate receptor ion-channel ligands.

Article abstract of DOI:10.1016/S0960-894X(02)00235-4

A series of N,N'-diphenyl and N-naphthyl-N'-phenyl guanidine derivatives was synthesized as potential N-methyl-D-aspartate (NMDA) receptor positron emission tomography (PET) ligands. The affinity of the different compounds was determined using in vitro receptor binding assays, and their log P values were estimated using HPLC analysis. The effect of N'-3 and N'-3,5 substitution on affinity and lipophilicity was examined. The K(i) values ranged from 1.87 to 839nM, while log P values between 1.22 and 2.88 were observed.

Full text of DOI:10.1016/S0960-894X(02)00235-4

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1583–1586
Synthesis and In Vitro Evaluation of N,N0-Diphenyl and
N-Naphthyl-N0-phenylguanidines as N-Methyl-D-aspartate
Receptor Ion-Channel Ligands
Filip Dumont,* Abida Sultana and Rikki N. Waterhouse
Division of Functional Brain Mapping, Columbia University, 1051 Riverside Drive, NewYork, NY 10032, USA
Received 9 November 2001; accepted 12 April 2002
Abstract—A series of N,N0-diphenyl and N-naphthyl-N0-phenyl guanidine derivatives was synthesized as potential N-methyl-d-
aspartate (NMDA) receptor positron emission tomography (PET) ligands. The affinity of the different compounds was determined
using in vitro receptor binding assays, and their log P values were estimated using HPLC analysis. The effect of N0-3 and N0-3,5
substitution on affinity and lipophilicity was examined. The Ki values ranged from 1.87 to 839 nM, while log P values between 1.22
and 2.88 were observed. # 2002 Elsevier Science Ltd. All rights reserved.
The ionotropic N-methyl-d-aspartate (NMDA) receptor
is a major site of action for glutamate, the most pre-
valent neurotransmitter in the mammalian brain. The
receptor plays a role in many biological functions, such
as long-term potentiation (LTP), memory, and cogni-
tion. In addition, evidence exists for a role in glutamate
toxicity, which is thought to be involved in a variety of
neurodegenerative processes, including stroke, Parkin-
son’s disease, Alzheimer’s disease and epilepsy, and in
schizophrenia.1ꢀ9 The NMDA receptor is a receptor
complex, and in addition to binding sites for glutamate
and glycine (co-agonist), there are also several mod-
ulatory binding sites, including the PCP site that lies
within the ion channel.10,11 Since the PCP site is located
within the NMDA ion channel, a PET PCP site radio-
ligand could prove useful for reporting on the activation
state of the channel in vivo.
nidine derivatives have shown that these compounds act
at the NMDA receptor ion channel.23ꢀ25
An iodinated derivative, N-(1-naphthyl)-N0-(3-iodophe-
nyl)guanidine (CNS 1261), was synthesized as a poten-
tial tracer for single photon emission computerized
tomography (SPECT) studies of the NMDA receptor.26
Our interest in using a high affinity ligand for in vivo
positron emission tomography (PET) studies prompted
us to synthesize a series of N,N0-diphenyl and N-naph-
thyl-N0-phenyl guanidines containing functional groups
amenable to labeling PET isotopes (O- and S-methyla-
tion reactions with C-11 methyl iodide are commonly
used in radiochemistry to form labeled methoxy or
thiomethyl groups). The affinity (Ki) of the new com-
pounds was determined using in vitro receptor binding
assays, and their lipophilicity (log P value) was esti-
mated using HPLC analysis. The effect of N0-3 and
N0-3,5 substitution on the affinity and lipophilicity was
examined.
To date, several radioligands for this site have been
synthesized and labeled with C-11, F-18, or I-123.
Although these compounds showed promising results in
vitro, their in vivo use was restricted due to fast meta-
bolism or a high lipophilicity (and thus, a high degree of
nonspecific binding).12ꢀ22 Structure–activity relationship
(SAR) studies of symmetrical (N,N0-diphenyl) and
unsymmetrical (N-phenyl-N0-naphthyl) N,N0-diarylgua-
N,N0-Diphenyl and N-naphthyl-N0-phenyl guanidines
were synthesized by reacting N-methyl-N-arylcyana-
mides with the requisite amine hydrochloride salts in
refluxing chlorobenzene (N,N0-diphenyl guanidines) or
neat (N-naphthyl-N0-phenyl guanidines) (Scheme 1).24,25
The cyanamides were prepared from a reaction of cya-
nogen bromide (CAUTION: highly toxic) with the pri-
mary amine in diethyl ether, followed by alkylation of
the arylcyanamide with sodium hydride/methyl iodide
*Corresponding author at current address: Labo Radiofarmacie,
Universiteit Gent, Harelbekestraat 72, 9000 Gent, Belgium. Tel.: +32-
9-264-8065; fax: +32-9-264-8071; e-mail: filip.dumont@rug.ac.be
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00235-4

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F. Dumont et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1583–1586
Table 1. Calculated and found exact masses (FAB+) of guanidine
derivatives
Compd
Molecular formula
Found (and calculated)
exact masses
1
2
3
4
5
6
7
8
10
C16H18N3S2Cl
C17H20N3SCl
C16H18N3OSCl
C15H15N3FSCl
C15H15N3ISCl
C17H20N3SCl
C19H19N3O
352.0728 (352.0709)
334.1143 (334.1145)
336.0953 (336.0937)
324.0747 (324.0737)
431.9776 (431.9800)
334.1143 (334.1145)
306.1583 (306.1606)
294.1385 (294.1406)
304.1812 (304.1814)
C18H16N3F
C20H21N3
Table 2. Inhibition constant (Ki) and log P values of N,N0-diphenyl
guanidine derivatives
Compd
R1
R2
Ki (nM)a,b
Log Pc
MK-801—
— .3
1
1
2
3
4
5
6
SCH3
C2H5
OCH3
F
I
CH3
H
H
H
H
H
1.87 (ꢂ0.25)
6.28 (ꢂ0.28)
5.20 (ꢂ0.30)
19.0 (ꢂ3.2)
3.48 (ꢂ0.60)
14.0 (ꢂ3.9)
2.68
3.00
2.34
2.34
2.91
3.12
Scheme 1. Synthesis of N,N0-diphenyl and N-naphthyl-N0-phenyl
guanidines. Reagents and conditions: (a) BrCN (CAUTION—highly
toxic), Et2O, ambient temperature, overnight; (b) NaH, THF, 65 ꢁC,
30 min; CH3I, ambient temperature, overnight; (c) C6H5Cl, 140–
150 ꢁC, 4–12 h; (d) neat, 140–150 ꢁC, 4–12 h.
CH3
aValues are means of at least three experiments, standard deviation is
given in parentheses.
bValues obtained by method of Cheng and Prusoff.32
cValues obtained by method of Waterhouse et al.27ꢀ30
in tetrahydrofuran. The primary amines were commer-
cially available. All of the compounds were purified by
column chromatography.
1
The structures were determined by H NMR and mass
spectrometry. Melting point analysis was performed for
each solid. The purity of the compounds was checked
by H NMR and by a HPLC system, used for the esti-
mation of the lipophilicity.27ꢀ30
stitution. Seven novel and three known N,N0-diarylgua-
nidine derivatives were synthesized. The calculated and
found exact masses of the various derivatives are listed
in Table 1.
1
In vitro radioligand binding assays for the NMDA
receptor ion-channel sites were performed according to
reported methods using [3H] MK-801(1nM) and rat
brain membrane suspensions.31 Relative affinities of
compounds were determined as IC50 values from dis-
placement curves. Ki values were calculated from the
IC50 values using the method of Cheng and Prusoff.32
In the N,N0-diphenylguanidine series, the inhibition
constants (Ki vs [3H]MK-801—listed in Table 2) of the
3-iodo, 3-fluoro, 3-methoxy, and 3,5-dimethyl deriva-
tives were 3.48, 19, 5.2, and 14 nM, respectively. The Ki
values for the 3-thiomethyl and the 3-ethyl compounds
were similar to these given in literature: 1.87 versus 1.87
and 6.28 versus 5.10, respectively.25
The lipophilicity of the compounds was examined by
determination of the log P value using a HPLC method
previously described.27ꢀ30 Briefly, the lipophilicity of
each compound was estimated by a comparison of its
retention time to that of standards having known log P
values. Relative retention times, RRT (to catechol),
were calculated, and a calibration curve of log P versus
log RRT was generated.
In the N-naphthyl-N0-phenyl guanidine series, the inhi-
bition constants (Ki vs [3H]MK-801—listed in Table 3) of
the 3-fluoro, 3-methoxy and 3,5-dimethyl derivatives
were 839, 43.5, and 21.9 nM, respectively. The Ki value
we obtained for the 3-iodo compound (4.65 nM) was
similar to the one given in literature (4.21nM). 26
Comparison of the affinities of the two groups reveals
that the compounds of the N,N0-diphenylguanidine ser-
ies are more potent PCP/NMDA antagonists than the
N-naphthyl-N0-phenyl guanidine derivatives. Within
both groups it is observed that for halogen substitution
a larger, less electronegative halogen is preferable. This
is in accordance with previous results where the Ki value
Since it was observed that, for the N,N0-diphenyl deri-
vatives, the N-(2-chloro-5-thiomethylphenyl) combina-
tion is one of the most favorable for PCP site binding,25
we only focused on the effect of the substitution on the
N0-phenyl ring, particularly on N0-3 and N0-3,5 sub-

F. Dumont et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1583–1586
1585
Table 3. Inhibition constant (Ki) and log P values of N-naphthyl-
N0-phenyl guanidine derivatives
Acknowledgements
Funding for this work was provided for by grants from
the National Institutes of Health (NIAAA IP50AA-
12870–01 and NIMH MH59342–01).
Compd
R1
R2
Ki (nM)a,b
Log Pc
References and Notes
MK-801—
7
— .3
OCH3
1
H
H
H
43.5 (ꢂ5.6)
839 (ꢂ423)
4.65 (ꢂ0.38)
21.9 (ꢂ10.1)
2.26
2.31
2.94
2.94
8
F
I
CH3
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ranging from 1.94 to 2.88. With the exception of N-(1-
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