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SAR17 observed in these molecular sets might be either an indica-
tive of the same behavior (agonist vs antagonist) or represent
common pharmacophoric regions to agonist and antagonist. In
both cases, it is important to have the same binding mode and
absence of activity cliffs.18 As will be described, some molecules
that do not follow the proposed models were observed. We report
here the preparation of one triamine and two piperazine positional
scanning libraries and their screening against the three opioid
receptors. The solid phase synthesis of piperazine derivatives
from resin-bound acylated dipeptides was previously described.19
Following deconvolution of each library, novel triamines and
matic). Compounds in set D had only aliphatic groups at R1 and R2
and were generally inactive, except compound 20, vide supra.
Therefore, based on the activities of the compounds in sets A, B
and C and the lack thereof for all the compounds in set D, the aro-
matic groups appears to play a role in binding for both receptors.
Between sets A and B, the overall trend was a reduction in activ-
ity to the
group was aliphatic, whereas the same substitution resulted in an
increase in activity toward the -receptor. Comparing the activities
of set A with those of set C revealed an overall preference for the S-
4-hydroxybenzyl group at R1 to the -receptor. Sets A and C were
all active with the -receptor, albeit clear trends could not be
l-receptor when the R1 group was aromatic and the R2
j
j
piperazines with good affinity and selectivity for the
receptor were identified, as well as good binders for the
j
l
-opioid
-opioid
l
established. The substitution patterns suggested that when R1
receptor with modest selectivity. Structure–activity relationships
(SAR) are discussed. A binding model for selected active com-
pounds is proposed based on similarity-based molecular alignment
and pharmacophore modeling.
and R2 were both aromatic groups (set A), these compounds were
active with both the
groups at R1 and R2, respectively, (set B) tended to yield
tive ligands. On the other hand, an aliphatic group at R1 and an
l- and
j-receptors. Aromatic and aliphatic
j
-selec-
aromatic group at R2 generally resulted in modest
gands, set C.
l-selective li-
2. Results and discussion
2.1. Biological evaluation
All compounds in this study derived from library 761 were inac-
tive to the -receptor, while only compounds 46, 54 and 56 were
active to the -opioid receptor. A possible explanation of the inac-
l
j
The N-methylated 1,3,4-trisubstituted piperazine (TPI 760), N-
tivity of these compounds is discussed below.
benzylated 1,3,4-trisubstituted piperazine (TPI 761), and N-methyl
triamine (TPI 762) libraries were screened against the l, j, and d-
opioid receptors. A total of 84 compounds was selected from the
library screening results and synthesized for testing against the
three opioid receptors. All assays contain a standard curve on every
plate using standard mu, delta or kappa ligands and IC50s are rou-
tinely determined. Overall, the 84 compounds derived from these
The N-methyl piperazines (TPI 762) were classified as shown
in Table 1, based on differences of their R3 substituents. In addi-
tion, sets I and J, and sets K and L differed at their R2 substitu-
ents. Sets M and N varied at their R1 substituents, for example,
compounds (73, 79) and (75, 81), while sets (J, L) and (I, K) dif-
fered in the stereochemistry of their R1 substituents. When the
R2 substituent was a hydrogen atom all the compounds were ac-
tive with both receptors regardless of the stereochemistry of the
R1 substituents and the R3 group, with the exception of com-
libraries had good activity in the
ity in the d-receptor, Table 1. The N-methyl triamines were clearly
more active in the -receptor than in the -receptor, and the N-
methyl piperazines were more active than the N-benzylated piper-
azines for both and receptors. From the 84 compounds tested a
l and j-receptors, but little activ-
j
l
pound 66 that was inactive with the j-receptor. Among this class
of compounds (R2 = hydrogen) 66 was the only compound whose
R3 substituent was neither aromatic nor cyclic. It was also the
l
j
subset of 43 compounds (Table 1) was further selected to propose a
binding mode with the opioid receptors based on molecular align-
ment and pharmacophore modeling.
weakest binder for the
l-receptor among the actives. Set L could
be generated from set K by substituting the hydrogen atom at the
R2 position with an S-isopropyl group. The activity trends sug-
gested that the presence of a more bulky isopropyl group at R2
would decrease the activity of the compounds in set L relative
to set K. The general preference of a less bulky R2 substituent
was also manifested, though to a lesser extent, when 60 and 64
were compared. It was also observed that the combination of
an R-hydroxybenzyl group at R1 with S-stereochemistry at R2 re-
sulted in compounds that were inactive with both receptors, evi-
dent in the activity profiles of the compounds in set L (69–72).
The S-hydroxybenzyl group at R1 with S-stereochemistry at R2
showed better activity, manifested in sets J and L. Sets J and L also
varied in the stereochemistry of the R1 position, with the com-
pounds in sets J and L bearing the S- and R-stereochemistries,
2.2. Structure–activity relationships
To describe the structure–activity relationship, the compounds
were classified into 14 sets, as shown in Table 1. Compounds from
library TPI 762 were partitioned into four sets (A–D) depending on
whether the R1 and R2 substituents were aromatic or aliphatic.
The activities of the compounds in sets A, B and C suggested that
the nature of the cyclic group at R3 might play a role in binding.
In set A, for example, with reference to the
l-receptor, changing
R3 from a cycloheptyl (1: IC50 = 54 nM) to 4-methyl-1-cyclohexyl
group (2: IC50 = 313 nM) decreased the activity by approximately
sixfold. The substitution in the R3 position of cycloheptyl (1: IC50
54 nM) to norbornylmethyl (4: IC50 270 nM) resulted in a fivefold
respectively. Switching the R1 stereochemistry of two
compounds, 61 (206 nM) and 64 (223 nM) in set J, to generate
l-active
decrease, while the cyclopentylmethyl substituent (5: IC50
=
compounds 69 (1000 nM) and 72 (1000 nM) in set L, respectively,
170 nM) reduced binding affinity by only threefold. It has been
noted that for SAR data, a modification that results in at least a
fivefold change in activity is considered significant.20 A similar
led to a loss of activity. The affinities for the
exhibited a similar trend, except for compound 61 that was inac-
tive in both scenarios.
j-receptor also
trend was observed with the
j-receptor, wherein the cycloheptyl
Sets (I, K) could be differentiated by the stereochemistry of the
R1 position: S-stereochemistry in set I and R-stereochemistry in set
K. There was a slight preference for the R-stereochemistry for com-
to 4-methyl-1-cyclohexyl, norbornylmethyl and cyclopentylmeth-
yl substitutions resulted in a four-, twenty- and eightfold decrease
in activity, respectively. In contrast, most of the compounds in set
D were inactive regardless of the group at the R3 position, except
compound 20 (IC50 = 102 nM) that was active only with the
pounds in set K for the
were generally inactive against the
81 (336 nM). For the -receptor, in addition to compound 81
l-receptor. Compounds in sets M and N
l
-receptor, except compound
j
j
-receptor.
(230 nM), compounds 73 (58 nM) and 74 (119 nM) were both ac-
tive. Interestingly, compounds 73 and 74 did not possess any aro-
matic groups, reminiscent of compound 20 from library 762, yet
In general, compounds in set B (R1: aromatic; R2: aliphatic)
were more active with the
j-receptor than the l-receptor, while
the reverse was true for compounds in set C (R1: aliphatic; R2: aro-
were active only with the j-receptor.