Synthesis, biological evaluation, and receptor docking simulations of 2-[(acylamino)ethyl]-1,4-benzodiazepines as κ-opioid receptor agonists endowed with antinociceptive and antiamnesic activity

Article abstract of DOI:10.1021/jm0307640

The synthesis and biological evaluation of a series of new derivatives of 2-substituted 5-phenyl-1,4-benzodiazepines, structurally related to tifluadom (5), are reported. Chemical and pharmacological studies on compounds 6 have been pursued with the aim o

Full text of DOI:10.1021/jm0307640

J . Med. Chem. 2003, 46, 3853-3864
3853
Syn th esis, Biologica l Eva lu a tion , a n d Recep tor Dock in g Sim u la tion s of
2-[(Acyla m in o)eth yl]-1,4-ben zod ia zep in es a s K-Op ioid Recep tor Agon ists
En d ow ed w ith An tin ocicep tive a n d An tia m n esic Activity
Maurizio Anzini,*,† Laura Canullo,† Carlo Braile,† Andrea Cappelli,† Andrea Gallelli,† Salvatore Vomero,†
M. Cristina Menziani,‡ Pier G. De Benedetti,‡ Milena Rizzo,§ Simona Collina,| Ornella Azzolina,|
Massimo Sbacchi,$,# Carla Ghelardini,∧ and Nicoletta Galeotti∧
Dipartimento Farmaco Chimico Tecnologico, Universita` degli Studi di Siena, Via A. Moro, 53100 Siena, Italy;
Dipartimento di Chimica, Universita` degli Studi di Modena e Reggio Emilia, Via Campi 183, 41100 Modena, Italy;
Dipartimento di Scienze Farmacobiologiche, Universita` degli Studi “Magna Græcia” di Catanzaro, Complesso “Nin`ı Barbieri”,
88021 Roccelletta di Borgia (CZ), Italy; Dipartimento di Chimica Farmaceutica, Universita` degli Studi di Pavia,
Via Taramelli 12, 27100 Pavia, Italy; GlaxoSmithKline, Via Zambeletti, Baranzate di Bollate, 20021 Milano, Italy;
Dipartimento di Farmacologia Preclinica e Clinica “M. Aiazzi Mancini”, Universita` degli Studi di Firenze,
Viale G. Pieraccini 6, 50139 Firenze, Italy
Received J anuary 13, 2003
The synthesis and biological evaluation of a series of new derivatives of 2-substituted 5-phenyl-
1,4-benzodiazepines, structurally related to tifluadom (5), are reported. Chemical and
pharmacological studies on compounds 6 have been pursued with the aim of expanding the
SAR data and validating the previously proposed model of interaction of this class of compounds
with the κ-opioid receptor. The synthesis of the previously described compounds 6 has been
reinvestigated in order to obtain a more direct synthetic procedure. To study the relationship
between the stereochemistry and the receptor binding affinity, compounds 6e and 6k were
selected on the basis of their evident structural resemblance to tifluadom. Since a different
specificity of action could be expected for the enantiomers of 6e and 6k , owing to the results
shown by (S)- and (R)-tifluadom, their racemic mixtures have been resolved by means of liquid
chromatography with chiral stationary phases (CSP), and the absolute configuration of the
1
enantiomers has been studied by circular dichroism (CD) and H NMR techniques. Moreover,
some new 2-[(acylamino)ethyl]-1,4-benzodiazepine derivatives, 6a -d ,f,g,j, have been synthe-
sized, while the whole series (6a -o) has been tested for its potential affinity toward human
cloned κ-opioid receptor. The most impressive result obtained from the binding studies lies in
the fact that this series of 2-[2-(acylamino)ethyl]-1,4-benzodiazepine derivatives binds the
human cloned κ-opioid receptor subtype very tightly. Indeed, almost all the ligands within
this class show subnanomolar Ki values, and the least potent compound 6o shows, in any case,
an affinity in the nanomolar range. A comparison of the affinities obtained in human cloned
κ-receptor with the correspondent one obtained in native guinea pig κ-receptor suggests that
the human cloned κ-receptor is less effective in discriminating the substitution pattern than
the native guinea pig κ-receptor. Furthermore, the results obtained are discussed with respect
to the interaction with the homology model of the human κ-opioid receptor, built on the recently
solved crystal structure of rhodopsin. Finally, the potential antinociceptive and antiamnesic
properties of compounds 6e and 6i have been investigated by means of the hot-plate and passive
avoidance test in mice, respectively.
In tr od u ction
nervous system of many species, including man, the
existence of the ꢀ-opioid receptor has been controversial
and only recently has a paper appeared supporting
evidence for ꢀ-opioid receptor mediated â-endorphin-
Opioids have attracted increasing interest, primarily
because of their powerful pain-relieving properties and
their potential tendency toward addiction. Opioids exert
their actions through four types of receptors, µ, δ, κ,1
and ꢀ,2 belonging to the G-protein coupled receptor
(GPCR) superfamily of integral proteins. Although it is
now well established that three of them (i.e. µ-, δ-, and
κ-receptors) are present in the central and peripheral
induced antinociception in some animal models.2
A
precise classification of opioid receptors followed the
discovery of their specific endogenous neuropeptides:
Met5- and Leu5-enkephalins for the µ- and δ-sites and
dynorphins for κ-sites.3
Thus, research into the mechanism of opioid actions
has focused on the function of each opioid receptor type
and on the functional interaction between the individual
receptor types. Development of selective receptor ligands
and, particularly, recent cloning of receptors have
provided efficient tools for the study of the function of
these receptors.4 A considerable interest has developed
* To whom correspondence should be addressed. Tel.: +39 0577 234
173. Fax: +39 0577 234 333. E-mail: anzini@unisi.it.
† Universita` degli Studi di Siena.
‡ Universita` degli Studi di Modena e Reggio Emilia.
§ Universita` degli Studi “Magna Græcia” di Catanzaro.
| Universita` degli Studi di Pavia.
$
GlaxoSmithKline.
Present address: Via G. Rossa, 20090 Assago, Milano.
#
∧ Universita` degli Studi di Firenze.
10.1021/jm0307640 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/06/2003

3854 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 18
Anzini et al.
enhanced by the discovery11 that simple trans-diami-
nocyclohexane derivatives, such as U-50488 (3), display
a very high selectivity for κ-opioid receptors along with
a side-effect profile more evident than benzomorphan.12
Furthermore, Toray Industries Inc. focused their efforts
on the discovery and development of TRK-820 (4), an
N-cyclopropylmethylmorphine derivative, an atypical,
potent κ-opioid agonist with a µ-partial agonist profile
as well as a weak ORL-1 antagonist component. This
compound entered phase II clinical trials for pain
treatment in both J apan and the US in 1997.13 In two
recent patent applications filed by Toray, TRK-820 and
related morphinan κ-agonists are claimed to be of
particular efficacy in the treatment of neuropathic
pain14 and as cornea and conjunctiva antipruritics.15
TRK-820 also appears to be a promising potential drug
among opiates, particularly for the apparent lack of
centrally related side effects at therapeutic doses. Selec-
tive affinity for κ-receptors has also been identified in
different structures, such as the 1,4-benzodiazepine
derivative tifluadom (5),16 which, however, was found
to interact also with CCK-A receptors.17 In a previous
paper, we described the synthesis and biological evalu-
ation of a series of 2-substituted 5-phenyl-1,4-benzodi-
azepines 6, structurally related to tifluadom (5), which
bound with nanomolar affinity and high selectivity to
κ-opioid receptors with respect to both opioid µ- and
δ-subtypes and to CCK-A receptors. In particular, the
2-thienyl derivative 6e with a Ki ) 0.50 nM represented
a clear improvement with respect to tifluadom, showing
a comparable potency but higher selectivity. Further-
more, the application of computational simulations and
linear regression analysis techniques to the complexes
of guinea pig κ(κ1) receptor and the synthesized com-
pounds allowed the identification of the structural
determinants for the recognition and quantitative elu-
cidation of the structure-affinity relationships in this
class of receptors.18
Ch a r t 1
during the past decade around the research of selective,
nonpeptidic, κ-opioid receptor agonists as safe and
effective antinociceptive agents. In particular, these
compounds are able to keep the antinociceptive potency
of morphine (1a ) (the prototype agonist at the µ-recep-
tor) in the absence of the typical side effects of morphine-
like drugs: gastrointestinal constipation, respiratory
depression, physical dependence liability, and high
addiction potential.5
Besides, some important clues on the structural
elements that confer high affinity/selectivity were de-
rived. On the basis of these results, chemical and
pharmacological studies on compounds 6 have been
pursued with the aim of expanding the SAR data and
of validating the model proposed. The synthesis of the
previously described compounds 6 has been reinvesti-
gated in order to obtain a more direct procedure.
Administration of κ-opioid receptor agonists, however,
is also associated with a spectrum of side effects
including diuresis, sedation/locomotor incoordination,
and an array of central nervous system (CNS) effects.5
Interestingly, many studies suggest that the activa-
tion of the κ-receptor suppresses the antinociceptive
effect of µ-receptor agonists. In fact, systemic (intra-
cerebroventricular or intraperitoneal) administration of
dynorphin or U-50488 (3) dose-dependently antagonizes
the antinociception induced by morphine, a weakly
selective µ-receptor agonist6,7 in tail-flick and hot-plate
tests in mice, rats, or guinea pigs.8 The anti-µ-receptor-
mediated action of κ-receptors has also been reported
in learning and memory processes in the brain. In
behavioral studies, the selective activation of the µ-re-
ceptor by Tyr-DAla-Gly-[NMePhe]-NH(CH2)2-OH
(DAMGO) impairs working memory-associated sponta-
neous alternation performance in mice, but dynorphin,
at otherwise ineffective doses on the control perfor-
mance, significantly improves the DAMGO-induced
memory impairment.9 The first two non-peptide κ-ago-
nist antinociceptives, though they have a limited selec-
tivity for the κ-receptor, were nalorphine (1b), the
N-allyl derivative of morphine, and ketocyclazocine (2),
a benzomorphan derivative (Chart 1) lacking some of
the typical effects of morphine.10 The research was also
Since a different specificity of action could be expected
for the enantiomers of 6e and 6k , owing to the results
shown by (S)- and (R)-tifluadom,19-22 their racemic
mixtures have been resolved by means of liquid chro-
matography with chiral stationary phases (CSP), and
the absolute configuration of the enantiomers has been
1
studied by circular dichroism (CD) and H NMR tech-
niques.23
Moreover, some new 2-[(acylamino)ethyl]-1,4-benzo-
diazepine derivatives, 6a -d ,f,g,j, have been synthe-
sized, while the whole series (6a -o) has been tested for
potential affinity toward human cloned κ-opioid recep-
tor. The results obtained are discussed with respect to
the interaction with the homology model of the human
κ-opioid receptor built on the recently solved crystal
structure of rhodopsin.24
Finally,the potential antinociceptive and antiamnesic
properties of compounds 6e and 6i have been investi-

1,4-Benzodiazepines as κ-Opioid Receptor Agonists
J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 18 3855
Sch em e 1a
Sch em e 2a
a
Reagents: (i) Et3SiH, TFA; (ii) LiAlH4, Et2O; (iii) SOCl2,
CH2Cl2; (iv) potassium phthalimide, NaI, DMF; (v) NH2NH2,
MeOH; (vi) R-COCl, Et3N, THF.
gated by means of the hot-plate and passive avoidance
test in mice, respectively.25
Ch em istr y. Our efforts toward synthesis began with
the application of the previously described procedure18
for the preparation of nor-derivatives 6a -d from key
intermediate 7 (Scheme 1). Selective reduction of 7 with
triethylsilane in trifluoroacetic acid yielded the satu-
rated ester 8, which was reduced with lithium alumi-
num hydride to afford the expected 2-(2-hydroxyethyl)-
1,4-benzodiazepine 9. This compound was transformed
into target nor-derivatives 6a -d in four steps via
phthalimido derivative 10.
Since the previously described synthesis of key inter-
mediate 7 concerned a complex multistep sequence,26,27
we realized that the preparation of this class of κ-opioid
ligands 6 could be significantly improved through the
development of a more direct procedure from com-
mercially available 2-aminobenzophenone (11a ) to key
intermediate 7. The initial attempts at direct synthesis
of 7 concerned the reaction of either tert-butyl acetate
or ethyl acetate carbanions with the iminophosphonate
generated in situ from benzodiazepinone 12a and di-
ethyl chlorophosphate in the presence of sodium hy-
dride. Unfortunately, such an attractive procedure did
not give synthetically useful yields. On the other hand,
the reaction of benzodiazepinone 12a with the phos-
phoryl-stabilized anion of dimethyl malonate28 gave
good yields of diester 13a (Scheme 2), which underwent
a
Reagents: (i) NH2CH2 COOEt, pyridine; (ii) CH2(COOMe)2,
NaH, ClPO(OEt)2, DME; (iii) 3 N NaOH, MeOH; (iv) (Bu)4N+Br-,
TsOCH3, 50% NaOH, benzene; (v) Et3SiH, TFA; (vi) LiAlH4, THF;
(vii) (a) SOCl2, CH2Cl2; (b) potassium phthalimide, NaI, DMF; (c)
NH2NH2, MeOH; (d) ClCOR, Et3N, THF.
partial hydrolysis and spontaneous decarboxylation to
afford the ylidene acetic ester 14a .29 This compound was
N-methylated with methyl p-toluenesulfonate in a two-
phase system in the presence of tetrabutylammonium
bromide as a phase-transfer catalyst to give N-methyl
derivative 15a . A two-step reduction of 15a led to the
expected 2-(hydroxyethyl)-1,4-benzodiazepine 17a, which
was transformed into target derivative 6e in four steps
following the chemistry already described. This proce-
dure allowed an easier synthesis of substantial amounts
of previously described 6e (for the enantiomeric resolu-
tion and an extensive pharmacological testing of 6e) and
of its new derivatives 6f,j. Tertiary amide derivative 6g
was obtained by N-methylation of 6e with methyl iodide
in the presence of sodium hydride (Scheme 3).
Among the newly designed compounds, iodo deriva-
tives 6p ,q (Scheme 4) and the synthetically related
acetyl derivatives 6r ,s could not be synthesized by
means of the above-described procedure. In fact, when

3856 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 18
Anzini et al.
Sch em e 3a
on the corresponding chromatographic peaks of the
analytical monitoring. Very high values (g97%) were
observed for all enantiomers.
Configurational study was performed through CD and
NOESY 1H NMR spectra, by comparing the enanti-
omers (-)-6e and (-)-6k with (S)-tifluadom, the abso-
lute configuration of which was determined by X-ray
crystallographic analysis of its p-toluenesulfonic acid
salt.30 As already reported,23 on the basis of the CD
curves, the (S) absolute configuration of (+) tifluadom
has also been proposed for (-)-6e and (-)-6k , because
of the sequence of substituents around the stereogenic
center, which is the same for all three enantiomers.
Additional analysis by 1H NMR spectroscopy con-
firmed the configuration assignment. On the basis of
the cross-peaks of the NOESY spectra, we could deter-
mine which protons are close to each other in the space
for all compounds and, consequently, the configuration
of the chiral center.23
aReagents: (i) MeI, NaH, DMF.
Sch em e 4a
Biology. Binding experiments of the title compounds
to human cloned opioid receptors were performed in
membranes prepared from HEK-293 (human embryonic
kidney) or CHO (Chinese hamster ovary) cells stably
expressing κ- (HEK-293), µ-, or δ- (CHO) opioid recep-
tors, while binding studies on the native receptor were
carried out in guinea pig brain homogenate, as previ-
ously described.18
A stable expression of µ-(h-MOR), δ-(h-DOR), and
κ-(h-KOR) receptors has been performed in house, using
pCDN vectors as described by Kotzer et al.31
The highly potent and specific radioligands [3H]-[D-
Ala2,Mephe4,Gly-ol5]enkephalin ([3H]-DAMGO), [3H]-[D-
Ala2,D-Leu5]enkephalin ([3H]-DADLE), and [3H]-U-
69593 were used to label µ-, δ,- and κ-opioid receptors,
respectively.18,31 The results are reported in Table 1.
The potential antinociceptive and antiamnesic prop-
erties of compounds 6e and 6i were investigated by
means of the hot-plate and passive avoidance test in
mice, respectively.
a
Reagents: (i) 2-thienyl-COCl, Et3N, THF.
the iodoamines 19a ,b were acylated with 2-thiophen-
ecarbonyl chloride under the usual conditions, the
formation of compounds 20a ,b was observed. These
compounds appear to derive from the normal attack of
the amino group by the 2-thiophenecarbonyl chloride
and the addition of 2-thiophenecarboxylic acid to the
imino double bond. Although we have no reasonable
explanation for this result, we believe that the aromatic
iodine atom could play a peculiar role.
Resolu tion of Ra cem ic 6e a n d 6k a n d Con figu -
r a tion a l An a lysis. To investigate the relationship
between stereochemistry and receptor binding affinity,
compounds 6e and 6k were selected on the basis of their
clear structural resemblance with tifluadom. Thus, a
direct HPLC method of enantiomeric separation was
applied to the resolution of racemates, and the scaling
up of the process was performed to obtain the pure
enantiomers for the biological investigation of their
enantioselectivity.
Resu lts a n d Discu ssion
Bin d in g Stu d ies. The most impressive result ob-
tained from the binding studies is that this series of 2-[2-
(acylamino)ethyl]-1,4-benzodiazepine derivatives binds
the human cloned κ-opioid receptor subtype very tightly.
Indeed, almost all the ligands within this class show
subnanomolar Ki values, and the least potent compound
6o shows, in any case, an affinity in the low nanomolar
range. Comparison of the affinities obtained in the
human cloned κ-receptor with the corresponding ones
obtained in native guinea pig κ-receptor suggests that
human cloned κ-receptor is less effective in discriminat-
ing the substitution pattern than the native guinea pig
κ-receptor. In fact, the Ki values range from 0.25 to 13.6
nM in the case of the human cloned κ-receptor, while a
wider range (0.19-296 nM) is observed in the case of
native guinea pig κ-receptor. In particular, the human
κ-receptor appears to be more tolerant in accommodat-
ing the additional steric bulk. For example, 7-chloro
derivatives 6k -m show κ-opioid affinities about an
order of magnitude lower than the corresponding un-
substituted derivatives 6e,h ,i when the native guinea
pig receptor is considered, while they show comparable
affinities when the human cloned receptor is considered.
Baseline separation of both racemates was obtained
by the analytical columns (250 × 4.6 mm, 5 µm)
Chiralcel OJ -R and Chiralpak AD (Daicel Chemical
Industries). Therefore, the semipreparative column
Chiralpak AD (250 × 20 mm, 10 µm) was used to
separate suitable amounts of the pure enantiomers. The
enantiomeric excess of optical antipodes was calculated

1,4-Benzodiazepines as κ-Opioid Receptor Agonists
J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 18 3857
Ta ble 1. Receptor Binding Affinities of 2-[(Acylamino)ethyl]-1,4-benzodiazepine Derivatives 6a -o
Ki (nM) ( SEMa
κ1
µ
δ
compd
X
Y
R1
R2
R3
guinea pigb
humanc
guinea pigb
humanc
guinea pigb
humanc
6a
6b
6c
6d
6e
(S)-6e
(R)-6e
6f
6g
6h
6i
6j
6k
(S)-6k
(R)-6k
6l
6m
6n
H
H
H
H
H
H
H
H
H
H
H
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
H
H
H
H
H
H
H
H
H
H
CH3
CH3
CH3
H
thiophen-2-yl
phenyl
1.74 ( 0.46 0.45 ( 0.05
0.25 ( 0.05
H
H
H
H
H
H
H
2′- fluorophenyl
4′-fluorophenyl
thiophen-2-yl
thiophen-2-yl
thiophen-2-yl
thiophen-2-yl
0.53 ( 0.14
0.28 ( 0.04
0.50 ( 0.10 0.58 ( 0.20 6.42 ( 0.96
0.19 ( 0.03 0.43 ( 0.09
9.4 ( 1.6
9.2 ( 3.6
54.8 ( 11
105 ( 48
93.9 ( 12
63.2 ( 9.0
544 ( 36
2.46 ( 0.13 1.28 ( 0.21
OCH3 CH3
116 ( 15.9 3.77 ( 0.73
H
H
H
H
H
H
H
H
H
H
H
CH3 CH3 thiophen-2-yl
25.7 ( 2.2
4.34 ( 0.66
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
H
H
H
H
H
H
H
H
H
H
2′-fluorophenyl 1.03 ( 0.42 0.36 ( 0.02
4′-fluorophenyl 0.56 ( 0.10 0.30 ( 0.04
4′-nitrophenyl
thiophen-2-yl
thiophen-2-yl
thiophen-2-yl
2′-fluorophenyl 17.9 ( 6.44 1.52 ( 0.18
4′-fluorophenyl 4.74 ( 1.33 0.55 ( 0.03
4′-chlorophenyl 35.0 ( 5.1
indol-2-yl 296 ( 37
0.47 ( 0.04
8.84 ( 1.47 0.70 ( 0.08 69.4 ( 15
4.52 ( 0.86 0.69 ( 0.21
26.3 ( 2.5
27.4 ( 1.2
33.2 ( 3.0
>1000
494 ( 34
451 ( 6.0
750 ( 42
6.55 ( 1.3
0.79 ( 0.21
1.31 ( 0.17
13.6 ( 2.56
6o
5
0.78 ( 0.19 0.37 ( 0.02 1.93 ( 0.08
0.17 ( 0.04
3.0 ( 0.2
1.5 ( 0.3
24.8 ( 2.0
15.3 ( 1.71
69.0 ( 4.8
27.8 ( 4.7
438 ( 42
(S)-5
(R)-5
U-69593
morphine
naltrindole
0.71 ( 0.26
1.89 ( 0.29 1.11 ( 0.13 1880 ( 324 >1000
>10 000
456 ( 57
0.16 ( 0.04
>1000
151 ( 10.2
10.2 ( 1.5
64 ( 10
4.4 ( 0.5
3.0 ( 0.3
14.7 ( 3.6
3.0 ( 0.3
7.2 ( 1.7
299 ( 8
0.24 ( 0.02
a
b
Each value is the mean ( SEM of three independent experiments. Native receptor from guinea pig. c Human cloned receptor.
From another viewpoint, 7-chloro derivatives 6k -o
show the human κ-receptor affinities to be about an
order of magnitude higher than those shown for the
native guinea pig κ-receptor, while unsubstituted de-
rivatives 6e,h ,i show very similar affinities for both
receptor subtypes. In a similar way, the introduction of
either a methoxy substituent in position 6 of the 1,4-
benzodiazepine nucleus or a methyl group on the amide
nitrogen atom has more dramatic effects on the native
guinea pig κ-receptor affinity than those shown on the
human cloned κ-receptor. Interestingly, nor-derivatives
6a ,c,d show human κ-receptor affinities very similar to
those shown by the corresponding 1-methyl derivatives
6e,h ,i, suggesting that the methyl group in position 1
of 1,4-benzodiazepine nucleus of this class of κ-receptor
ligands does not play a key role in the interaction with
the receptor and the most potent ligand of the class
F igu r e 1. Antinociceptive effect of 6e (100 mg kg-1 po) and
6i (100 mg kg-1 po) in the mouse hot-plate test. The licking
latency values reported in the figure were recorded 15 min
after administration of 6e and 6i. Vertical lines represent sem;
*P < 0.05 in comparison with saline-group; °P < 0.05 as
compared with the corresponding antinociceptive compound.
(compound 6b) belongs to this small subseries of nor-
derivatives.
Ster eoselectivity. Finally, the binding studies reveal
that compound 6e interacts in the same stereoselective
manner (the eutomer shows (S)-configuration as in the
case of tifluadom) with all the opioid receptor subtypes
evaluated showing eudismic ratios in the range of
12.95-2.97 (κ guinea pig, 12.95; κ human, 2.97; µ
human, 5.95; δ human, 8.61). On the other hand,
7-chloro analogue 6k binds the same opioid receptor
subtypes with apparently absent stereoselectivity, sug-
gesting that the 7-chloro substituent plays an important
role in the interaction modalities.
In Vivo Biologica l Activity. Compounds 6e and 6i
produced an increased pain threshold in the mouse hot-
plate test after po (100 mg kg-1) administration, as
shown in Figure 1. The antinociceptive effect of 6e and
6i showed a peak 15 min after injection and then slowly
diminished (see Supporting Information). The antinoci-
ceptive effect of 6e and 6i (100 mg kg-1 po) was
antagonized by pretreatment with the κ-opioid antago-
nist n-BNI (1 nmol per mouse icv), injected 4 h before
the test (Figure 1).
These results emphasize the importance of the κ-
opioid receptor activation in the inhibition of pain
perception. Previous literature data reported that the
κ-opioid agonists U-50,488H, TRK-820, and dynorphin
A-(1-13) can induce antinociception in rats against
mechanical, thermal, and chemical noxious stimuli.32-35
The κ-opioid receptor antagonist n-BNI (0.49 µg per
mouse icv), injected immediately after the training
session, produced amnesia in the passive-avoidance
test.36 This effect was obtained after intracerebroven-
tricular administration, indicating that the blockade of
κ-opioid receptors causes a centrally mediated memory
impairment.

3858 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 18
Anzini et al.
In conclusion, the experimental data obtained by the
use of two new and selective κ-opioid agonists, 6e and
6i, indicate that activation of κ-opioid receptors in-
creases the pain threshold as well as improving phar-
macologically induced cognitive deficits. These results
confirm the important role played by κ-opioid receptors
in the regulation of both pain perception and memory
processes. On these bases, the κ-opioid agonists could
be clinically useful for the relief of algic pathologies or
for the treatment of cognitive disorders.
Com p u t a t ion a l Sim u la t ion s. Com p a r ison b e-
tw een th e “Old ” a n d “New ” Mod el of th e K-Op ioid
Recep tor . The results obtained in our previous paper,18
published in 1996, have been obtained by a quantitative
analysis of receptor docking simulations. The model of
the guinea pig transmembrane (TM) receptor domain
was based on both the bacteriorhodopsin structure42 and
on the helical wheel projection model proposed by
Baldwin.43 As can be noted from the sequence alignment
reported in Figure 3, the differences in the TM domain
between the human and guinea pig sequences consist
only of four conservative mutations in helices 4, 5, and
6, while most of the differences are located in the
N-terminal tail.
F igu r e 2. Antiamnesic effect of 6e (100 mg kg-1 po) and 6i
(50 mg kg-1 po) in the mouse passive avoidance test. nor-
Binaltorphimine hydrochloride (n-BNI) was injected icv, at the
dose of 0.49 µg per mouse immediately before the training
session. Vertical lines represent sem; *P < 0.05 in comparison
with the saline group.
The zones selected as TM portions in the “old” model
are highlighted in Figure 3 and can be easily compared
to the TM domain of bovine rhodopsin, as shown by the
crystal structure, and to those of the “new” model,
evaluated after molecular dynamics and energy refine-
ment of the averaged receptor structure. In the old
model the helices were, in general, shorter, and by
taking into account the uncertainty (4 aa) manifested
by secondary structure prediction algorithms in locating
the starting and ending points of R-helices,44 the only
significant differences are observed in the C-term por-
tion of helix 3, and the N-term portion of helices 5 and
6.
The deficits in passive avoidance behavior induced by
n-BNI were prevented by pretreatment with the κ-opioid
receptor agonists 6e (100 mg kg-1 po) and 6i (50 mg
kg-1 po), administered 20 min before the training
session, as shown in Figure 2. Both of the κ-opioid
agonists enhanced the entrance latency up to a value
comparable to that produced in control animals. Com-
pounds 6e and 6i were completely ineffective at 50 and
10 mg kg-1 po, respectively (Figure 2).
In addition, the packing and mutual positions of the
TMs are significantly different for the new and old
models. However, the residues that were considered to
be part of the binding site in our previous paper overlap
very well, as can be appreciated in Figure 4 (top). They
are D138, N141, and M142, in helix 3; W287, I290,
H291, and I294, in helix 6; G319, Y320, N322, and S323
in helix 7. Some of these residues (reported in bold
capitals in the text and in Figure 3) have been mutated
into the opioid receptor superfamily. In particular, D138
has been demonstrated to be involved both in agonist/
antagonist binding and in signal transduction in κ-opioid
receptor and several other GPCRs (GPCR family; a
point mutation database, GRAP).45 Mutation of the
residue corresponding to N141 in the δ-receptor (N150A)
suggests that it may normally hinder the binding of
opioid agonists.46 The conserved H291 in helix 6 has
been found to be critical in both agonist and antagonist
binding in the µ-receptor.46 Mutation of Y320 in the
µ-receptor (Y326F) showed that it may be critical for
opioid receptor binding, affecting the binding of agonists
and antagonists, both opioid peptides and alcaloids.46
Compounds 6e and 6i, when given alone at the
highest doses, had no effect on mouse passive avoidance
test in comparison with saline-treated mice (Figure 2).
No statistically significant difference among the en-
trance latencies for each compound tested in the train-
ing session of the passive avoidance test was observed
(Supporting Information).
These experimental results are in agreement with
previous data reporting that the κ-opioid agonist dynor-
phin A-(1-13) reverts the scopolamine- and pirenzepine-
induced memory impairment of step-down type passive
avoidance response and spontaneous alternation per-
formance.37 Dynorphin A-(1-13) also reduces the am-
nesia induced by â-amyloid peptide-(25-35)38 and cy-
cloheximide,39 a protein synthesis inhibitor in a mouse
passive avoidance paradigm. Furthermore, U-50,488H,
another κ-opioid agonist, has been reported to revert
mecamylamine-induced memory impairment in mice
and rats.40-41
The κ-opioid receptor agonists investigated, at the
highest doses used, did not modify the animals’ gross
behavior. Nor did these compounds impair motor coor-
dination or modify locomotor and inspection activity (see
Supporting Information). We can, thus, assume that the
effects produced by κ-opioid receptor agonists were not
imputable to compromised viability.
K1-Liga n d Com p lexes a n d Lin ea r Regr ession
An a lysis vs Bin d in g. The energies for the κ1-ligand
complexes obtained as described in the Computational
Methodology section, are reported in Table 2, together
with the experimental binding affinities (pKi). The

F igu r e 3. Sequence alignment of the bovine rhodopsin (b-OPSD), human k-opioid (h-OPRk), guinea pig κ-opioid (gp-OPRk), human δ-opioid (h-OPRδ), and human µ-opioid (h-
OPRµ) receptors. The zones of the sequences selected as transmembrane domains are boxed, while the R-helices in the b-OPSD, as detected by the crystal structure, are shadowed.
The residues of the h-OPRk that are believed to be part of the binding site in this study are highlighted and among these those that have been demonstrated to be important for
ligand interaction by point mutation are reported in bold.

3860 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 18
Anzini et al.
F igu r e 5. Correlation between the binding affinity values and
the calculated binding energies for the minimized ligand-
receptor complexes. The linear regression is pKi ) 6.65 ((0.19)
- 0.07 ((0.005)BE, n ) 16, r2 ) 0.92, s ) 0.15, where n is the
number of the compounds, r is the correlation coefficient, s is
the standard deviation and the values in parentheses give the
95% confidence intervals.
As expected from the similarity observed in the
spatial position of the old and new receptor binding site
residues, the binding modalities of the ligands involve
the same residues spotted in our previous model.18 This
is shown in Figure 4 (bottom), where the details of the
interaction of the (S)-6k ligand with the human receptor
are reported.
In agreement with our previous findings,18 the changes
in the potential energy of the receptor upon formation
of the complexes are mainly responsible for the correla-
tion obtained (Table 2). In fact, inspection of the
interaction energy values reveals that their variation
is disorganized and varies within a small interval. This
implies that repulsion forces play the most important
role in discriminating between the compounds consid-
ered, and a reorganization of the receptor architecture
is needed in order to accommodate the ligands.
F igu r e 4. Superimposition of the amino acids that are
believed to constitute the binding site in the “old” (blue) and
“new” (in colors) models are reported at the top of the figure.
Details of the interaction of the (S)-6k ligand with the human
receptor (bottom) binding sites residues: the residues belong-
ing to helix 3, helix 5, helix 6, and helix 7 are reported in green,
blue, white and pink, respectively. The F214 (light green)
belongs to the loop connecting helix 4 with helix 5.
Ta ble 2. Experimental Binding Constants and Computed
Energy Descriptors (kcal/mol) for the Ligand-κ1 Receptor
Interactions
pKi
κ1-human
Analysis of the molecular dynamics trajectories of the
ligand-receptor complexes shows that the receptor
reorganization is mainly triggered by Pro15 in helix 6,
positioned just below the putative active site, which
allows the kinking of the C-terminal part of the helix
away from the core of the receptor. The extent of this
structural change depends on the structural features
of the ligands and influences the overall packing of the
bundle. This finding is in full agreement with the results
obtained by mapping of the residue in the sixth TM
domain of the κ-, δ-, and µ-opioid receptor subtypes,
which are accessible in the binding-site crevices by the
substituted cysteine accessibility method.47
The flexibility of this region involves also the H-
bonding observed between residue E298 of helix 6 and
Y312 of helix 7. The E297K mutation48 has demon-
strated the importance of this residue for the binding
of the nor-BNI ligand. Since the ligands used in our
study are of much smaller size, they bind deeply in the
TM domain and do not interact directly with residue
E298, which, however, might be the second key point
for the regulation of the overall shape of the binding
pocket and might contribute to determine the receptor
selectivity. In fact, E298 and Y312 are not conserved
in the δ- and µ- receptors, being W (δ) and K (µ) and L
(δ) and W (µ), respectively (Figure 3).
compda
BE
IEk-L
Ed
Ed
k
L
6a
9.35
9.60
9.27
9.55
9.37
8.42
8.36
9.44
9.52
9.32
9.16
8.81
9.26
8.88
7.87
9.77
-37.54
-41.89
-41.19
-40.33
-38.77
-25.48
-25.11
-39.40
-43.08
-41.49
-37.23
-28.45
-38.73
-29.76
-20.58
-41.44
-70.27
-73.94
-74.49
-73.82
-75.27
-70.44
-69.71
-72.19
-74.40
-76.55
-71.56
-74.58
-75.55
-76.78
-70.55
-74.31
4.25
3.68
4.25
3.84
5.74
6.32
4.54
6.11
4.32
4.94
5.47
5.86
6.33
5.79
6.33
5.79
28.48
28.37
29.05
29.65
30.76
38.64
40.06
26.68
27.01
30.12
28.86
40.27
30.49
41.23
43.64
27.08
6b
6c
6d
(S)-6e
6f
6g
6h
6i
6j
(S)-6k
6l
6m
6n
6o
(S)-5
a
For racemic compounds 6a -d ,f-j,l-o, configuration S was
used in the docking studies.
variation in the measured binding constants is well-
explained by the computed binding energies (BE), which
take into account the ligand-receptor interaction en-
ergy (IEk-L) and the mutual conformational adjustment
of the ligand (EdL) and of the receptor (Edk) upon
binding. The quantitative relationship obtained between
the binding affinity data values and the computed
binding energies for the ligands considered is shown in
Figure 5.

1,4-Benzodiazepines as κ-Opioid Receptor Agonists
J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 18 3861
agent. After purification by flash chromatography (EtOAc-
MeOH, 90:10, v/v) an orange glassy solid was obtained. 1H
NMR (CDCl3): δ 1.80-2.05 (m, 2H), 3.31-3.49 (m, 1H), 3.72-
3.97 (m, 3H), 4.05-4,14 (m, 1H), 4.68 (bs, 1H, disappeared with
D2O), 6.72-6.89 (m, 2H), 7.03 (d, 1H, J ) 8.3), 7.21-7.58 (m,
10H), 7.82 (d, 2H, J ) 7.05). MS (FAB): m/z 370 (M + 1). Anal.
(C24H23N3O): C, H, N.
2,3-Dih yd r o-2-[[(2-flu or oben zoyl)a m in o]eth yl]-5-p h en -
yl-1H-1,4-ben zod ia zep in e (6c). This compound was pre-
pared in 21% yield starting from 10 according to the general
procedure, using 2-fluorobenzoyl chloride in the second step
as acylating agent and purifying by flash chromatography
(EtOAc-MeOH, 90:10, v/v) to obtain a yellow solid. A recrys-
tallization from cyclohexane gave an analytical sample melting
at 97-98 °C. 1H NMR (CDCl3): δ 1.81-2.05 (m, 2H), 3.31-
3.46 (m, 1H), 3.76-4.16 (m, 4H), 4.65 (bs, 1H, disappeared with
D2O), 6.73 (t, 1H, J ) 7.4), 6.85 (d, 1H, J ) 7.9), 6.98-7.55
(m, 10H) 8.03-8.11 (m, 1H). MS (EI): m/z 387 (M+, 22). Anal.
(C24H22FN3O): C, H, N.
The results obtained showed that the “new” compu-
tational model satisfactorily explains the structure-
activity relationships observed and provides additional
structural interpretation for the results of published
mutagenesis experiments.
Exp er im en ta l Section
Melting points were determined in open capillaries on a
Gallenkamp apparatus and are uncorrected. Microanalyses
were carried out using a Perkin-Elmer 240 C elemental
analyzer. 1H NMR spectra were recorded on a Bruker AC 200
spectrometer; the values of the chemical shift are expressed
in ppm and the coupling constants (J ) in hertz. Merck silica
gel 60 (230-400 mesh) and Chromabond Florisil 6 mL/1000
mg (Superchrom) were used for column chromatography.
Merck DC-Platten F 254 was used for TLC. Mass spectra were
recorded on a VG 70-250 S spectrometer. Microanalyses were
performed by the Dipartimento Farmaco Chimico Tecnologico
dell’Universita` di Siena, while mass spectra were performed
by Centro di Analisi e Determinazioni Strutturali, Universita`
di Siena. Commercial chemicals were used without further
purification, except for solvents, which were purified and dried,
where appropriate, before use by standard methods.
HPLC was performed on a Gilson (model 319) with an
injection valve (Reodyne, model 7125) and a UV detector
(Perkin-Elmer, model LC 95). Experimental data were ana-
lyzed with HPLC software (Gilson 715). UV spectra were
recorded on a spectrophotometer (Perkin-Elmer, model 552).
CD curves were measured on a dichrograph (J asco, model
J -710).
2,3-Dih yd r o-2-[[(4-flu or oben zoyl)a m in o]eth yl]-5-p h en -
yl-1H-1,4-ben zod ia zep in e (6d ). This compound was pre-
pared in 58% yield starting from 10 according to the general
procedure, using 4-fluorobenzoyl chloride in the second step
as acylating agent and purifying by flash chromatography
1
(EtOAc-n-hexane, 70:30, v/ v) to obtain a light yellow oil. H
NMR (CDCl3): δ 1.85-1.97 (m, 2H), 3.36-3.48 (m, 1H), 3.73-
3.81 (m, 2H), 3.93 (dd, 1H, J ) 3.9, 11.7), 4.08-4.21 (m, 1H),
4.64 (bs, 1H, disappeared with D2O), 6.72-6.83 (m, 2H), 6.99-
7.17 (m, 2H), 7.21-7.45 (m, 6H), 7.50-7.56 (m, 2H), 7.77-
7.84 (m, 2H). MS (EI): m/z 387 (M+, 19). Anal. (C24H22FN3O):
C, H, N.
(Z)-2,3-Dih ydr o-2-[(eth oxycar bon yl)m eth ylen e]-5-ph en -
yl-1H-1,4-ben zod ia zep in e (7). This compound was prepared
as previously described18 and its analytical and spectral data
were consistent with those reported therein.
Gen er a l P r oced u r e for t h e P r ep a r a t ion of 2-[(Acyl-
a m in o)eth yl]-1,4-ben zod ia zep in e 6a -d . The 2-[(acylami-
no)ethyl]-1,4-benzodiazepines 6a -d were synthesized follow-
ing a previously described procedure.18 Briefly, a suspension
of the phthalimido derivative 10 (2.5 mmol) in MeOH (30 mL)
treated with hydrazine 98% (1.21 mL, 25 mmol) was stirred
at room temperature for 4 h, diluted with MeOH (20 mL), and
filtered. The precipitate was discarded, and the filtrate was
concentrated under reduced pressure. The residue was parti-
tioned between CH2Cl2 (50 mL) and ice-cold aqueous dilute
hydrochloric acid (10 mL). The organic layer was discarded,
and the acidic aqueous solution was made alkaline with ice-
cold 3 M sodium hydroxide solution. The reaction product was
then extracted with CH2Cl2 and the organic phase washed with
brine until neutral and dried over MgSO4. Evaporation under
reduced pressure gave pure 2,3-dihydro-2-(2-aminoethyl)-5-
phenyl-1H-1,4-benzodiazepine in 70% yield as a colorless oil,
which was dissolved into dry THF (12 mL) and treated in
succession, after cooling at 0 °C, with Et3N (245 µL) and the
appropriate acyl chloride (1.76 mmol) dissolved into dry THF
(3 mL). The ice bath was removed, and the reaction mixture
was stirred overnight at room temperature and then diluted
with ethyl acetate (100 mL). The organic layer was washed
with a saturated NaHCO3 solution (2 × 30 mL) and brine,
dried over MgSO4, and concentrated under reduced pressure
to give a crude product which, after purification by column
chromatography, afforded the respective acylamino derivative.
2,3-Dih ydr o-2-[[(2-th ien ylcar bon yl)am in o]eth yl]-5-ph en -
yl-1H-1,4-ben zod ia zep in e (6a ). This compound was pre-
pared in 35% yield starting from 10 according to the procedure,
using 2-thienylcarbonyl chloride in the second step as acylating
agent and purifying by flash chromatography (EtOAc-n-
hexane, 70:30, v/v) to obtain a light yellow oil. A further flash
chromatography afforded an analytical sample. 1H NMR
(CDCl3): δ 1.77-1.99 (m, 2H), 3.26-3.41 (m, 1H), 3.72-3.93
(m, 3H), 4.05-4,14 (m, 1H), 4.65 (bs, 1H, disappeared with
D2O), 6.74 (t, 1H, J ) 7.1), 6.82 (d, 1H, J ) 7.8), 6.95-7.07
(m, 2H), 7.19-7.55 (m, 9H). MS (EI): m/z 375 (M+, 25). Anal.
(C22H21N3OS): C, H, N.
Eth yl (2,3-Dih yd r o-5-p h en yl-1H-1,4-ben zod ia zep in e)-
2-a ceta te (8). This compound was prepared following the
procedure previously described.18 A solution of compound 7 (1.0
g, 3.2 mmol) in TFA (15 mL) was stirred under N2 atmosphere
and cooled in an ice bath. The solution was treated with
triethylsilane (0.74 g, 6.4 mmol) added over a period of 5 min.
The ice bath was removed, and the stirring was continued for
30 min. The mixture was then poured onto crushed ice, made
alkaline by addition of ammonia, and extracted with CH2Cl2.
The extracts were combined, dried (Na2SO4), and evaporated
under reduced pressure. The residue was purified by column
chromatography (CH2Cl2-EtOAc, 95-5 v/v), and 8 was ob-
tained as a thick yellow oil. Further purification by chroma-
1
tography afforded an analytical sample as a colorless oil. H
NMR (CDCl3): δ 1.25 (t, 3H, J ) 7.4), 2.60-2.85 (q, 2H), 3.65-
3.91 (m, 2H), 4.05-4.25 (m, 2H), 4.38-4.57 (bs, 2H, 1H
disappeared with D2O), 6.74-6.89 (m, 2H), 7.0-7.10 (m,1H),
7.20-7.28 (m, 1H), 7.30-7.42 (m, 3H), 7.51 (dd, 2H, J ) 1.5,
7.5). Anal. (C19H20N2O2): C, H, N.
2,3-Dih yd r o-2-(2-h yd r oxyet h yl)-5-p h en yl-1H -1,4-b en -
zod ia zep in e (9). To a stirred suspension of lithium aluminum
hydride (3.2 mmol) in anhydrous THF (15 mL) was added
dropwise a solution of 8 (1.5 mmol) in dry THF (15 mL). After
stirring under argon atmosphere for 15 min, the excess of
lithium aluminum hydride was decomposed by careful addition
of H2O (1 mL). The organic material was filtered off and
washed with THF. The filtrate was dried over Na2SO4, filtered,
and evaporated under reduced pressure. The crude product
was purified by flash chromatography (EtOAc) to give 9 in 83%
yield, as a yellow oil. An analytical sample was obtained by
1
further column chromatography. H NMR (DMSO): δ 1.72-
1
1.98 (m, 2H), 3.49 (bs, H, disappeared with D2O), 3.67-3.80
1
(m, 2H), 3.88 (dd, 2H, J ) 3.1; 11,0), 4.17-4.21 (m, H), 4.28
(bs, 1H, disappeared with D2O), 6.70-6.81 (m, 2H), 7.01 (d,
1H, J ) 7.4), 7.18-7.39 (m, 4H), 7.53 (dd, 2H, J ) 1.5, 7.5).
Anal. (C17H18N2O): C, H, N.
2,3-Dih yd r o-2-[[(b en zoyl)a m in o]et h yl]-5-p h en yl-1H -
1,4-ben zod ia zep in e (6b). This compound was prepared in
54% yield starting from 10 according to the general procedure
and using benzoyl chloride in the second step as acylating
2,3-Dih yd r o-2-(2-p h t h a lim id oet h yl)-5-p h en yl-1H -1,4-
ben zod ia zep in e (10). This compound was prepared in 87%
yield as previously described18 and purified by flash chroma-

3862 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 18
Anzini et al.
tography (n-hexanes-EtOAc, 65:35, v/v) to obtain a light
with no external perturbation was run. Time-averaged struc-
tures were then determined over the last 200 ps of each
simulation. Data were collected every 0.5 ps.
yellow oil. Further column chromatography afforded an ana-
1
lytical sample. H NMR (CDCl3) δ 1.94-2.04 (m, 2H), 3.71-
4.01 (m, 5H), 4.75 (bs, 1H, disappeared with D2O), 6.76 (t, 1H,
J ) 7.7), 6.88 (d, 1H, J ) 8.1), 7.0 (d, 1H, J ) 7.6), 7.20-7.29
(m, 1H), 7.32-7.38 (m, 3H), 7.52 (dd, 2H, J ) 1.3, 6.6), 7.69-
7.75 (m, 2H), 7.80-7.87 (m, 2H). Anal. (C25H21N3O2) C, H, N.
Bu ild in g a n d R efin em en t of t h e Liga n d -R ecep t or
Com p lexes. The ligands were manually docked into the
minimized average structure of the receptor, the main criteria
being the formation of a charge-reinforced hydrogen bond
between the protonated nitrogen atom of the ligands and the
D138 residue of helix 3 and the formation of a hydrogen bond
between the carbonyl oxygen atom of the ligand and the H291
residue of helix 6. The procedure used is described in detail
in our previous paper.18
Ch ir a l Ch r om a togr a p h ic Resolu tion . All chemicals and
solvents were of analytical grade or HPLC grade. UV spectra
were measured on a Beckman DU-7000 spectrometer. Specific
rotations (R) were measured on a J asco DIP-1000 photoelectric
polarimeter (0.5 dm cell, sodium lamp). Circular dichroism was
recorded on a J asco J -710 dichrograph. The analytical chiral
resolutions were performed by liquid chromatography on
Chiralcel OJ -R and Chiralpak AD columns (0.46 × 25 cm) from
Daicel Chemical Industries, Tokyo, J apan. The chiral station-
ary phases of these columns were, respectively, cellulose tris-
4-methylbenzoate or amylose tris-3,5-dimethylphenylcarbam-
ate, coated on a 5 µm silica gel substrate. Semipreparative
resolution was accomplished by a Chiralpak AD column (2 ×
25 cm, 10 µm). The HPLC system consisted of two Gilson
pumps (model 306), a Reodyne injector (model 7125, 20 µL or
3 mL sample loop, respectively, for analytical or semiprepara-
tive resolutions), and a Gilson (model 119) UV detector at
double wavelength. Experimental data were analyzed with
Gilson 715 software. All chromatographic separations were
performed at room temperature. The enantiomers of 6e and
6k were completely resolved on Chiralpak AD column by
means of n-hexane/isopropyl alcohol 85/15 as eluent. The
enantioselectivity (R) and the resolution (Rs) were 1.87 and
1.26 for 6e and 1.18 and 2.18 for 6k . A semipreparative process
was carried out with the same chiral stationary phase on a
250 × 20 mm column, eluting with n-hexane/isopropyl alcohol
95/5. The chromatographic process was repeated every 45 min
by injection of 2 mL of the sample solution (c ) 2-3 mg solute/
mL of n-hexane 70/isopropyl alcohol 30 mixture). Pure enan-
tiomers were properly recovered by partition of the eluate
according to the profile of the chromatogram. In this way about
25 mg of the chiral compounds was obtained.
Com p u ta tion a l Meth od ology. Com p a r a tive Mod elin g
of th e Hu m a n K-Op ioid Recep tor . The recently determined
2.8 Å X-ray structure of rhodopsin24 was used to build the
human κ-opioid receptor model. The sequence alignment used
as input for the Modeler49 program is reported in Figure 3 and
was obtained by Clustalw50 multiple sequence alignment of
bovine rhodopsin and human κ-, δ-, and µ-opioid receptor
sequences. Among the 50 models obtained by randomizing the
Cartesian coordinates, allowing a deviation of (4 Å, we
selected the one showing the lowest restraint violations and
the lowest number of main chain and side chain bad confor-
mations. The quality of the models was checked by using the
protein health module of the program Quanta98 (Accelrys, San
Diego, CA). Polar hydrogens were added to the selected model,
and energy minimization and dynamics were carried out by
means of the CHARMM program51 following a standard
procedure consisting of 50 steps of steepest descent, followed
by a conjugate gradient minimization until the root mean
square gradient of the potential energy was lower than 0.001
kcal/mol Å. The united atom force field parameters, a distance-
dependent dielectric term (ꢀ ) 4r), and a 12 Å nonbonded cutoff
were employed. During dynamics the lengths of the bonds
involving hydrogen atoms were constrained according to the
SHAKE algorithm52 allowing an integration time step of 0.001
ps. Moreover, weak harmonic constraints (30 kJ /mol per Å)
were applied between the backbone oxygen atoms of residue i
and backbone nitrogen atom of residue I + 4 by means of the
NOE facility in the CHARMM program51 in order to preserve
the helical structure.
The binding energies (BE) of the minimized ligand-receptor
complexes were calculated according to the following equation:
53 BE ) IEk-L + EdL + Ed where, IEk-L is the total interaction
k
energy between the receptor and the ligand, Ed is the
L
distortion energy of the ligand calculated with respect to the
optimized energy of the free molecule, and Ed is a measure of
k
the conformational energy change in the receptor induced by
ligand binding. Ligand and receptor distortion energies were
obtained by minimizing the ligand and the receptor separately,
resulting in the same geometry in each minimized complex.
In Vitr o Bin d in g Assa ys. Binding experiments of the title
compounds to human cloned opioid receptors were performed
in membranes prepared from CHO (Chinese hamster ovary)
or HEK-293 (human embrionic kidney) cells stably expressing
µ- or δ-(CHO) or κ- (HEK-293) opioid receptors. Native receptor
studies were carried out in guinea pig brain homogenate, as
previously described.18
A stable expression of µ- (h-MOR), or δ- (h-DOR), and κ-
(h-KOR) receptors has been performed in house, using pCDN
vectors as described by Kotzer et al.31
Membranes were prepared by lysis in hypotonic phosphate-
buffer according to the method described by Scheideler and
Zukin.54 The highly potent and specific radioligands [3H]-[D-
Ala2,Mephe4,Gly-ol5]enkephalin ([3H]-DAMGO, 50 Ci/mmol,
New England Nuclear, Bruxelles, Belgium), [3H]-[D-Ala2,D-
Leu5]enkephalin ([3H]-DADLE, 50 Ci/mmol, New England
Nuclear, Bruxelles, Belgium), and [3H]-U-69593 (63 Ci/mmol,
Amersham) were used to label µ-, δ- and κ-opioid receptors,
respectively.18,31
Nonspecific binding was determined in the presence of 10
µM naloxone. Binding experiments to cloned receptors were
performed in 25 mM monobasic potassium phosphate buffer
containing 3 mM MgCl2, pH 7.4, in polypropylene 96-deep-
well plates.
Incubation was carried on for 60 min at 25 °C at the final
volume of 0.5 mL (µ-assay) or 0.7 mL (δ- and κ-assay). The
reaction was terminated by filtration using a Packard Filter-
mate harvester containing a GF/B Unifilter plate. After
filtration the Unifilter plates were dried, each well was filled
with 50 µL of Packard Microscint 30, and radioactivity counted
by a Packard Topcount NXT.
An a lysis of Bin d in g Da ta . The binding parameters deriv-
ing from competition experiments (IC50 values) were calculated
by nonlinear regression analysis using the RS/1 software.55
The inhibition constants (Ki) were calculated from IC50 values
using the Cheng and Prusoff equation.56
In Vivo Biologica l Tests. The potential antinociceptive
and antiamnesic properties of compounds 6e and 6i were
investigated by means of the hot-plate and passive avoidance
test in mice, respectively.
Male Swiss albino mice (23-25 g) from the Morini (San Polo
d’Enza, Italy) breeding farm were used. Fifteen mice were
housed per cage (26 × 41 cm). The cages were placed in the
experimental room 24 h before the test for acclimatization. The
animals were fed a standard laboratory diet and tap water ad
libitum and kept at 23 ( 1 °C with a 12 h light/dark cycle,
light on at 7 a.m. All experiments were carried out in
accordance with the European Community Council Directive
of 24 November, 1986 (86/609/EEC) for experimental animal
care. All efforts were made to minimize animal suffering and
to reduce the number of animals used.
The structures were thermalized to 300 K with 5 °C rise
per 6000 steps by randomly assigning individual velocities
from the Gaussian distribution. After heating, the systems
were allowed to equilibrate until the potential energy versus
time was approximately stable (34 ps). Velocities were scaled
by a single factor. An additional 10 ps period of equilibration

1,4-Benzodiazepines as κ-Opioid Receptor Agonists
J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 18 3863
Hot-P la te Test. The method adopted was described by
O’Callaghan and Holzman.57 Mice were placed inside a stain-
less steel container, thermostatically set at 52.5 ( 0.1 °C in a
precision water-bath from KW Mechanical Workshop (Siena,
Italy). Reaction times (s), were measured with a stop-watch
before and at regular intervals up to a maximum of 60 min
after treatment. The endpoint used was the licking of the fore
or hind paws. Those mice scoring below 12 and over 18 s in
the pretest were rejected (30%). An arbitrary cutoff time of 45
s was adopted.
P a ssive-Avoid a n ce Test. The test was performed accord-
ing to the step-through method described by J arvik and
Kopp.58 The apparatus consists of a two-compartment acrylic
box with a lighted compartment connected to a darkened one
by a guillotine door. As soon as they entered the dark
compartment, the mice received a punishing electrical shock
(0.15 mA, 1 s). The latency times for entering the dark
compartment were measured in the training test and after 24
h in the retention test. For memory disruption, mice were ip
injected with amnesic drugs immediately after termination of
the training session. Compounds 6e and 6i were administered
20 min before the training session. Those mice scoring more
than 60 s in the training session were rejected. The maximum
entry latency allowed in the retention session was 180 s. The
degree of memory recall of received punishment (electric shock)
was expressed as the difference (∆) between retention and
training latencies.
In tr a cer ebr oven tr icu la r In jection Tech n iqu e. Intra-
cerebroventricular (icv) administration was performed under
ether anaesthesia with isotonic saline as a solvent, according
to the method described by Haley and McCormick.59 During
anaesthesia, mice were grasped firmly by the loose skin behind
the head. A hypodermic needle (0.4 mm external diameter)
attached to a 10 µL syringe was inserted perpendicularly
through the skull and no more than 2 mm into the brain of
the mouse, where 5 µL of drug was then administered. The
injection site was 1 mm to the right or left from the midpoint
on a line drawn through to the anterior base of the ears.
Injections were performed randomly into the right or left
ventricle. To ascertain that the drugs were administered
exactly into the cerebral ventricle, some mice were injected
with 5 µL of 1:10 diluted India ink and their brains were
examined macroscopically after sectioning. The accuracy of the
injection technique was evaluated and the percentage of
correct injections was 95.
Refer en ces
(1) Reisine, T.; Pasternak In Goodman and Gilman’s The Pharma-
cological Basis of Therapeutics; Hardman, J . G., Limbird, L. E.,
Eds.; McGraw-Hill: 1996; pp 521-525.
(2) Tseng, L. F. Evidence for ꢀ-Opioid Receptor-Mediated â-Endor-
phin-Induced Analgesia. Trends Pharm. Sci. 2001, 22, 623-629.
(3) Simon, E. J .; Gioannini, T. L. Opioid Receptor Multiplicity:
Isolation, Purification and Chemical Characterization of Binding
Sites. In Handbook of Experimental Pharmacology, 104/1; Herz
A., Ed.; Springer-Verlag: Berlin, 1993; pp 3-26.
(4) B. N.; Cesselin, R.; Raghubir, R.; Reisine, T.; Braedley, P. B.;
Portoghese, P. S.; Hamon M. International Union of Pharmacol-
ogy. XII. Classification of Opioid Receptors. Pharmacol. Rev.
1996, 48, 567-592.
(5) Zimmerman, D. M.; Leander, J . D. Selective Opioid Receptor
Agonists and Antagonists: Research Tools and Potential Thera-
peutic Agents. J . Med. Chem. 1990, 33, 895-902.
(6) Matthes, H. W. D.; Maldonado, R.; Simonin, F.; Valverde, O.;
Slowe, S.; Kitchen, I.; Befort, K.; Dierich, A.; Le Meur, M.; Dolle´,
P.; Tzavara, E.; Hanoune, J .; Roques, B. P.; Kieffer, B. L. Loss
of Morphine-Induced Analgesia, Reward Effect and Withdrawal
Symptoms in Mice Lacking the µ-Opioid-Receptor Gene. Nature
1996, 383, 819-823.
(7) Sora, I.; Takahashi, N.; Funada, M.; Ujike, H.; Revay, R. S.;
Donovan, D. M.; Miner, L. L.; Uhl, G. R. Opiate Receptor
Knockout Mice Define µ Receptor Roles in Endogenous Nocice-
ptive Responses and Morphine-induced Analgesia. Proc. Natl.
Acad. Sci. U.S.A. 1994, 94, 1544-1549.
(8) Tao, P. L.; Hwang, C. L.; Chen, C. Y. U-50,488 Blocks the
Development of Morphine Tolerance and Dependence at a Very
Low Dose in Guinea Pigs. Eur. J . Pharmacol. 1994, 256, 281-
286.
(9) Itoh, J .; Ukai, M.; Kameyama, T. Dynorphin A-(1-13) Potently
Improves the Impairment of Spontaneous Alternation Perfor-
mance Induced by the µ-Selective Opioid Receptor Agonist
DAMGO in Mice. J . Pharm. Exp. Ther. 1994, 269, 15-21.
(10) Martin, W. R.; Eades, C. G.; Thompson, J . A.; Huppler. R. E.;
Gilbert, P. The Effects of Morphine- and Nalorphine-like Drugs
in the Non-Dependent and Morphine-Dependent Chronic Spinal
Dog. J . Pharm. Exp. Ther. 1976, 197, 517-532.
(11) Szmuszkovicz, J .; Von Voightlander, P. F. Benzeneacetamide
Amines: Structurally Novel Non-µ-Opioids. J . Med. Chem. 1982,
25, 1125-1126.
(12) Von Voigtlander, P. F.; Lahti, R. A.; Ludens, J . H. U-50,488: a
Selective and Structurally Novel non-Mu (kappa) Opioid Agonist.
J . Pharm. Exp. Ther. 1983, 224, 7-12.
(13) Endoh, T.; Tajima, A.; Izumimoto, N.; Suzuki, T.; Saitoh, A.;
Suzuki, T.; Narita, M.; Kamei, J .; Tseng, L. F.; Mizoguchi, H.;
Nagase, H. TRK-820, a Selective kappa Opioid Agonist, Produces
Potent Antinociception in Cynomologus monkeys. J pn. J . Phar-
macol. 2001, 85, 282-290.
(14) Endo, T.; Nagase, H.; Suzuki, T.; Suzuki, T.; Tanaka, T.;
Kawamura, K.; Kuraishi, Y.; Shiraki, K. Remedies for Neuro-
pathic Pain and Model Animals of Neuropathic Pain. 2001, WO-
00114383.
(15) Nagase, H.; Uchiumi, J .; Okano, K.; Kawamura, K.; Inada, H.;
Togashi, H. Antipruritic for Cornea or Conjunctiva. 2001, J P-
163784.
(16) Romer, D.; Buscher, H. H.; Hill, R. C.; Maurer, R.; Petcher, T.
J .; Zeugner, H.; Benson, W.; Finner, E.; Milkowsky, W.; Thies,
P. W. An Opioid Benzodiazepine. Nature (London) 1982, 298,
759-760.
(17) Chang, R. S. L.; Lotti, V. J .; Chen, T. B.; Keegan, M. E.
Tifluadom, a k-Opiate Agonist, Acts as a Peripheral Cholecys-
tokinin Receptor Antagonist. Neurosci. Lett. 1986, 72, 211-214.
(18) Cappelli, A.; Anzini, M.; Vomero, S.; Menziani, M. C.; De
Benedetti, P. G.; Sbacchi, M.; Clarke, G. D.; Mennuni, L.
Synthesis, Biological Evaluation, and Quantitative Receptor
Docking Simulations of 2-[(Acylamino)ethyl]-1,4-benzodiazepines
as Novel Tifluadom-like Ligands with High Affinity and Selec-
tivity for κ-Opioid Receptors. J . Med. Chem. 1996, 39, 860-872.
(19) Petrillo, P.; Amato, M.; Tavani, A. The Interaction of the Two
Isomers of the Opioid Benzodiazepine Tifluadom with µ, κ and
δ Binding Sites and their Antinociceptive and Intestinal Effects
in Rats. Neuropeptides 1985, 5, 403-406.
Dr u gs. nor-Binaltorphimine hydrochloride (n-BNI, pur-
chased from RBI) was used.
The drug was dissolved into isotonic (NaCl 0.9%) saline
solution (for icv administration) or dispersed into sodium
carboxymethylcellulose 1% (for po administration) immediately
before use. Drug concentrations were prepared so that the
necessary dose could be administered in a volume of 5 µL per
mouse by intracerebroventricular (icv) injection and 10 mL
kg-1 by per os (po) injection.
Sta tistica l An a lysis. All experimental results are given
as the means ( sem. The analysis of variance (ANOVA),
followed by Fisher’s protected least significant difference
(PLSD) procedure for post-hoc comparison, was used to verify
the significance between two means. The data were analyzed
with the StatView software for the Macintosh (1992). P values
of less than 0.05 were considered significant.
Ack n ow led gm en t. We acknowledge the contribu-
tion of Dr G. Giorgi (C.I.A.D.S., University of Siena,
Italy) for the recording of mass spectra. Prof. S. D’Agata
D’Ottavi’s careful reading of the manuscript is also
acknowledged. Financial support from CNR and Italian
MIUR is acknowledged.
(20) Freye, E.; Boeck, G.; Schaal, M.; Ciaramelli, F. The Benzodiaz-
epine (+)-Tifluadom (KC-6128), but not its Optical Isomers (KC-
5911) Induces Opioid kappa Receptor-Related EEG Power
Spectra and Evoked Potential Changes. Pharmacology 1986, 33,
241-248.
(21) Kley, H.; Scheidemantel, U.; Bering, B.; Muller, W. E. Reverse
Stereoselectivity of Opiate and Benzodiazepine Receptors for the
Opioid Benzodiazepine Tifluadom. Eur. J . Pharmacol. 1983, 87,
503-504.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails for the synthesis and the characterization of compounds
6 and their intermediates (chemistry, NMR, MS). This mate-
rial is available free of charge via Internet at http://
pubs.acs.org.

3864 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 18
Anzini et al.
(22) Meurisse, R. L.; Hevlen, L.; Leysen, J . E.; Meunier, J . C.; De
Ranter, C. Radioligand Binding Profile of Enantiomers of
Tifluadom Analogues for Opioid, CCK, and Central Benzodiaz-
epine Receptors: Versatility of Benzodiazepine Skeleton. Regul.
Pept. 1984, 1, 5347-5248.
(23) Azzolina, O.; Collina, S.; Linati, L.; Anzini, M.; Cappelli, A.;
Scheideler M. A.; Sbacchi, M. Enantiomers of 2[(Acylamino)-
ethyl]-1,4-benzodiazepiones, Potent Ligands of k-Opioid Recep-
tor: Chiral Chromatographic Resolution, Configurational As-
signment, and Biological Activity. Chirality 2001, 13, 606-612.
(24) Palczewski, K.; Kumasaka, T.; Hori, T.; Behnke, C. A.; Mo-
toshima, H.; Fox, B. A.; Le Trong, I.; Teller, D. C.; Okada, T.;
Stenkamp, R. E.; Yamamoto, M.; Miyano, M. Crystal Structure
of Rhodopsin: A G-Protein-Coupled Receptor. Science 2000, 289,
739-745.
(25) Ghelardini, C.; Galeotti, N.; Bartolini, A.; Anzini, M.; Cappelli,
A.; Vomero, S. Antiamnesic Effect of the Two Novel k-Opioid
Agonists, VA 100 and VA 101, in the Mouse Passive Avoidance
Test. Drug Dev. Res. 2001, 54, 12-18.
(26) Anzini, M.; Cappelli, A.; Vomero, S. Synthesis and Receptor
Binding Studies of 2-Functionalized 1,4-Benzodiazepine Deriva-
tives as Potential Metaclazepam-like Antianxiety Agents. Far-
maco 1993, 48, 897-905.
(27) Anzini, M.; Garofalo, A.; Vomero, S. Syntheses of Functionalized
Derivatives of Quinazolines and 1,4-Benzodiazepines. Hetero-
cycles 1989, 29, 1477-1487.
(28) Fryer, R. I.; Pinto, J . C.; Upasani, R. B. Studies Related to
Alkylations of Enolate Anions of 1,4-Benzodiazepinones. J .
Heterocyclic Chem. 1993, 30, 945-951.
(29) Walser, A.; Flynn, T.; Fryer, R. I. Quinazolines and 1,4-
Benzodiazepines. Lxxxv (I). Sintheses of 3-Substituted Imidazo-
[1,5-a][1,4]benzodiazepines. J . Heterocyclic Chem. 1978, 15, 577-
583.
(30) Petcher, T. J .; Widmer, A.; Maetzel, U.; Zeugner, H. Structures
of Tifluadom [5-(2-fluorophenyl)-1-methyl-2-(3-thenoyl-amino-
ternation Performance in the Mouse. Eur. J . Pharmacol. 1995,
81, 173-178.
(41) Hiramatsu, M.; Murasawa, H.; Nabeshima, T.; Kameyama, T.
Effects of U-50,488H on Scopolamine-, Mecamylamine- and
Dizocilpine-Induced Learning and Memory Impairment in Rats.
J . Pharmacol. Exp. Ther. 1998, 284, 858-867.
(42) Henderson, R.; Baldwin, J .; Ceska, T. H.; Zemlin, F.; Beckmamn,
E.; Downing, K. Model of the Structure of Bacteriorhodopsin
Based on High-Resolution Electron Cryomicroscopy. J . Mol. Biol.
1990, 213, 899-919.
(43) Baldwin, J . M. The Probable Arrangement of the Helices in G
Protein-Coupled Receptors. The EMBO J . 1993, 12, 1693-1703.
(44) Ballesteros, J . A.; Weinstein, H. Analysis and Refinement of
Criteria for Predicting the Structure and Relative Orientations
of Transmembranal Helical Domains. Biophys. J . 1992, 62, 107-
109.
(45) Beukers, M. W.; Kristiansen, K.; Ijzerman, A. P.; Edvardsen,
Ø. TinyGRAP Database:
a Bioinformatics Tool to Mine G
Protein-Coupled Receptor Mutant Data. TiPS 1999, 20, 475-
477.
(46) Mansour, A.; Taylor, L. P.; Fine, J . L.; Thompson, R. C.;
Hoversten, M. T.; Mosberg, H. I.; Watson, S. J .; Akil, H. Key
Residues Defining the µ-Opioid Receptor Binding Pocket: a Site-
Directed Mutagenesis Study. J . Neurochem. 1997, 68, 344-353.
(47) Xu, W.; Li, J .; Chen, C.; Huang, P.; Weinstein, H.; J avitch, J .
A.; Shi, L.; De Riel, J . K.; Liu Chen, L. Y. Comparison of the
Amino acid Residues in the Sixth Transmembrane Domains
Accessible in the Binding-Site Crevices of mu, delta, and kappa
Opioid Receptors. Biochemistry 2001, 40, 8018-8029.
(48) Metzger, T. G.; Paterlini, M. G.; Ferguson, D. M.; Portoghese,
P. S. Investigation of the Selectivity of Oxymorphone- and
Naltrexone-Derived Ligands via Site-Directed Mutagenesis of
Opioid Receptors: Exploring the “Address” Recognition locus.
J . Med. Chem. 2001, 44, 857-62.
(49) Sali, A.; Blundell, T. L. Comparative Protein Modeling by
Satisfaction of Spatial Restraints. J . Mol. Biol. 1993, 234, 779-
815.
methyl)-2,3-dihydro-1H- 1,4-benzodiazepine]Hydrochloride, C22H21
-
FN3OS+‚Cl-,and of(+)-Tifluadom p-Toluenesulphonate,C22H21FN3-
OS+‚C7H7O3S-, and the Absolute Configuration of the Latter.
Acta Cryst. 1985, C41, 909-912.
(50) Thompson J . D.; Higgins D. G.; Gibson T. CLUSTAL W:
Improving the Sensitivity of Progressive Multiple Sequence
Alignment Through Sequence Weighting, Position-Specific Gap
Penalties and Weight Matrix Choice. J . Nucleic Acids Res. 1994,
22, 4673-4680.
(31) Kotzer, C. J .; Hay, D. W. P.; Dondio, M.; Giardina, G. A. M.;
Petrillo, P.; Underwood, D. The Antitussive Activity of Delta
Opioid Receptor Stimulation in Guinea Pigs. J . Pharmacol. Exp.
Ther. 2000, 292, 803-809.
(32) Leighton, G. E.; Rodriguez, R. E.; Hill, R. G.; Hughes, J . kappa-
Opioid Agonists Produce Antinociception after i.v. and icv but
not Intrathecal Administration in the Rat. Br. J . Pharmacol.
1988, 93, 553-560.
(33) Pelissier, T.; Paeile, C.; Soto-Moyano, R.; Saavedra, H.; Her-
nandez, A. Analgesia Produced by Intrathecal Administration
of the kappa Opioid Agonist, U-50, 488H, on Formalin-Evoked
Cutaneous Pain in the Rat. Eur J . Pharmacol. 1990, 190, 287-
293.
(34) Endoh, T.; Matsuura, H.; Tajima. A.; Izumimoto, N.; Tajima, C.;
Suzuki, T.; Saitoh, A.; Suzuki, T.; Marita, M.; Tseng, L.; Nagase,
H. Potent Antinociceptive Effects of TRK-820, a Novel kappa-
Opioid Receptor Agonist. Life Sci. 1999, 65, 1685-1694.
(35) Stevens, C. W.; Yaksh, T. L. Dynorphin A and Related Peptides
Administered Intrathecally in the Rat: A Search for Putative
kappa Opiate Receptor Activity. J . Pharmacol. Exp. Ther. 1986,
238, 833-838.
(36) Colombo, P. J .; Martinez, J . L.; Bennet, E. L.; Rosenzweig, M.
R. k-Opioid Receptor Activity Modulates Memory for Peck-
Avoidance Training in the 2-Day-Old Chick. Psycopharmacology
1992, 108, 235-241.
(37) Ukai, M.; Itoh, J .; Kobayashi, T.; Shinkai, N.; Kameyama, T.
Effects of the k-Opioid Dynorphin A-(1-13) on Learning and
Memory in Mice. Behav. Brain Res. 1997, 83, 169-172.
(38) Hiramatsu, M.; Inoue, K.; Kameyama, T. Dynorphin A-(1-13)
and (2-13) Improve â-Amyloid Peptide-Induced Amnesia in
Mice. Neuroreport 2000, 11, 431-435.
(39) Ukai, M.; Shan-Wu, Y.; Kobayashi, T.; Kameyama, T. Systemic
Administration of Dynorphin A-(1-13) Markedly Improves
Cycloheximide-Induced Amnesia in Mice. Eur. J . Pharmacol.
1996, 313, 11-15.
(40) Ukai, M.; Shinkai, N.; Kameyama, T. k-Opioid Receptor Agonists
Improve Pirenzepine-Induced Disturbance of Spontaneous Al-
(51) Brooks, B. R.; Bruccoleri, R. E.; Olafson, B. D.; States, D. J .;
Swaminathan, S.; Karplus, M. CHARMM:
a Program for
Macromolecular Energy Minimization and Dynamics Calcula-
tions. J . Comput. Chem. 1983, 4, 187-217.
(52) van Gunsteren, W. F.; Berendsen, J . C. Algorithms for Macro-
molecular Dynamics and Constraint Dynamics. Mol. Phys. 1977,
34, 1311-1327.
(53) Menziani, M. C.; De Benedetti, P. G.; Gago, F.; Richards, W. G.
The Binding of Benzensulphonamides to Carbonic Anhidrase
Enzyme. A Molecular Mechanics Study and Quantitative Struc-
ture-Activity Relationships. J . Med. Chem. 1989, 32, 951-956.
(54) Scheideler, M. A.; Zukin, R. S. Reconstitution of Solubilized
Delta-Opiate Receptor Binding Sites in Lipid Vesicles. J . Biol.
Chem. 1990, 265, 15176-15182.
(55) Baron, B. M.; Siegel, B. W.; Slone, A. L.; Harrison, B. L.;
Palfreyman, M. G.; Hurt, S. D. [3H]5,7-Dichlorokynurenic Acid,
a Novel Radioligand Labels NMDA Receptor-Associated Glycine
Binding Sites. Eur. J . Pharmacol. 1991, 206, 149-154.
(56) Cheng, Y. C.; Prusoff, W. H. Relationship Between the Inhibition
Constant (Ki) and the Concentration of Inhibitor Which Causes
50 Per Cent Inhibition (I50) of an Enzymatic Reaction. Biochem.
Pharmacol. 1973, 22, 3099-3108.
(57) O’Callaghan, J . P.; Holtzman, S. G. Quantification of the
Antinociceptive Activity of Narcotic Antagonists by a Modified
Hot-Plate Procedure. J . Pharmacol. Exp. Ther. 1975, 192, 497-
505.
(58) J arvik, M. E.; Kopp, R. An Improved One-Trial Passive Avoid-
ance Learning Situation. Psychol. Rep. 1967, 21, 221-224.
(59) Haley, T. J .; McCormick, W. G. Pharmacological Effects Pro-
duced by Intracerebral Injection of Drugs in the Conscious
Mouse. Br. J . Pharmacol. Chemother. 1957, 12, 12-15.
J M0307640