3-(4-Aminobutyn-1-yl)pyridines: Binding at α4β2 nicotinic cholinergic receptors

Article abstract of DOI:10.1016/j.bmcl.2003.10.033

The binding of a series pyridylbutynylamines 6 was examined at α4β2 nACh receptors. Structural modifications, comparing 6 with pyridyl ethers 2, did not consistently result in parallel effects on receptor affinity, suggesting possible differences in their modes of binding. Furthermore, the binding of amine 6a seemed to be accounted for by the newer vector pharmacophore models.

Full text of DOI:10.1016/j.bmcl.2003.10.033

Bioorganic & Medicinal Chemistry Letters 14 (2004) 523–526
3-(4-Aminobutyn-1-yl)pyridines: binding at ꢀ4ꢁ2 nicotinic
cholinergic receptors§
Deniz Dogruer,a Mase Lee,a Malgorzata Dukat,a M. Imad Damaj,b Billy R. Martinb and
Richard A. Glennona,b,
*
aDepartment of Medicinal Chemistry, School of Pharmacy, Virginia Commonwealth University, Richmond, VA 23298-0540, USA
bDepartment of Pharmacology and Toxicology, School of Medicine, Virginia Commonwealth University, Richmond, VA 23298, USA
Received 29 August 2003; accepted 1 October 2003
Abstract—The binding of a series pyridylbutynylamines 6 was examined at a4b2 nACh receptors. Structural modifications, com-
paring 6 with pyridyl ethers 2, did not consistently result in parallel effects on receptor affinity, suggesting possible differences in
their modes of binding. Furthermore, the binding of amine 6a seemed to be accounted for by the newer vector pharmacophore
models.
# 2003 Elsevier Ltd. All rights reserved.
Nicotine (1) produces many of its effects via interaction
with nicotinic cholinergic (nACh) receptors.1,2 Several
populations of nACh receptors have been identified, but
the most prevalent population in mammalian brain are
a4b2 receptors.1 Novel structure types with ‘extended’
side chains have been shown to bind at a4b2 receptors
including pyridyl ethers 2, and a metabolite of 1: trans-
metanicotine (also known as RJR-2403, TC-2403, or
simply as ‘metanicotine’, 3).3,4 Shortening the length of
the alkyl chain of 3 (Ki ca. 25 nM)5,6 by one methylene
unit results in dramatically decreased affinity (4; Ki
ꢀ6000 nM).7 Alkyne 5 (Ki=2285 nM) also binds in the
micromolar range.7 Chain extension of 5 results in 6a
(Ki=58 nM) which binds with higher affinity than 5,
and with an affinity comparable to 3.6
at the 6-position of either 1 or 2 has little effect on
nACh receptor affinity whereas introduction of a meth-
oxy group and replacement of the pyridine nitrogen
atom by CH are not well tolerated,8 we examined the 6-
chloro, 6-methoxy, and de-aza derivatives of 6a.
Another reason to examine analogues of 6a is from a
theoretical perspective. That is, several pharmacophore
models have been proposed for nicotinic action. In one
model, an internitrogen (N–N) distance of 4.8 A is sug-
gested to be important.9 Nicotine (1) possesses such a
distance. However, compound 2, although longer in
length relative to nicotine (1), is much more con-
The purpose of this investigation was several fold.
Compound 6a has been reported (as its free base and
fumarate salt) only in an abstract5 and in the patent lit-
erature;6 its structure–affinity requirements have not
been studied in detail. Because tertiary amines of 1 and
2 are typically favored over secondary amines for a4b2
receptor binding,8 it was of interest to determine how
variation of the terminal amine substituents of 6a would
influence affinity. Also, because introduction of halogen
§Presented in part at the 4th International Symposium on Pharma-
ceutical Chemistry, Istanbul, Turkey, 17–19 September, 2003
* Corresponding author. Tel.: +1-804-828-8487; fax: +1-804-828-
7404; e-mail: glennon@hsc.vcu.edu
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.10.033

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D. Dogruer et al. / Bioorg. Med. Chem. Lett. 14 (2004) 523–526
formationally flexible and a number of low-energy con-
formations with varying N–N distances are possible.10
Compound 6a, although still conformationally flexible,
possesses a certain amount of constraint due to the
presence of the sp-hybridized carbon atoms in the side
chain. It is unlikely that 6a can achieve an N–N distance
of 4.8 A. More recent pharmacophore models place less
importance on N–N distance.11ꢁ13 Thus, an exami-
nation of analogues of 6a also allows this concept to be
addressed.
Table 1. Physicochemical properties and a4b2 nACh receptor bind-
ing affinities for pyridylbutynylamines 6a–g
Z
X
Mp (ꢂC) Empirical formulaa
Ki, nMb
.
.
6a –NH–CH3
6b –N(CH3)2
6c –N(CH3)C2H5 –H
–H 156–157 C10H12N2 C2H2O4
113 (16)
510 (50)
90–91 C12H16N2 1.5C2H2O4 1770 (900)
–H 152–153 C11H14N2 C2H2O4
.
Several derivatives of 6a were prepared and examined
(Table 1).14 Physicochemical and spectral data for 6a
were similar to that in the literature, and its nACh
receptor affinity (Ki=113 nM) was not inconsistent with
what was reported earlier (Ki=58 nM).6
.
–H 141–142 C13H16N2 C2H2O4
6d
>10,000
6e –N(CH3)+3
6f
–H 177 C12H17IN2
.
470 (110)
–Cl 196–197 C10H11ClN2 C2H2O4 154 (40)
–NH–CH3
6g –NH–CH3 –OMe 177–178 C11H14N2O C2H2O4 >10,000
.
The novel N,N-dimethyl and N-ethyl-N-methyl deriva-
tives 6b and 6c (Ki=510 and 1770 nM, respectively)
displayed 5- and 15-fold lower affinity than the second-
ary amine 6a. The pyrrolidinyl analogue 6d lacked affi-
nity whereas the N,N,N-trimethyl quaternary amine 6e
(Ki=470 nM) displayed an affinity comparable to that
of 6b. Introduction of a 6-chloro group had little impact
on affinity (i.e., 6f Ki=154 nM) whereas a methoxy
group at this position was not tolerated (i.e., 6g Ki
>10,000 nM).
a Compounds, isolated as their oxalate salts except for 6e iodide, ana-
lyzed within 0.4% of theory. Compounds were recrystallized from
absolute EtOH except for 6a (MeOH), 6c (acetone/hexane), 6d, 6e
(acetone).
b Ki values were determined at least in triplicate using [3H](ꢁ)nicotine
as radioligand and are followed, except where Ki >10,000 nM, by
ꢃSEM in parentheses.15 For reference, the Ki value for (ꢁ)nico-
tine=2.1 nM.
little effect on affinity18 whereas quaternization of the
N,N-dimethyl analogue of 2 increased affinity by about
40-fold.19 In the present study, N-methylation of 6b had
essentially no effect on affinity. Although additional
compounds might need to be studied, a general conclu-
sion that can be tentatively reached is that alteration of
the terminal amine group of 6-series compounds, more
so than alterations in the pyridyl portion, results in
greater disparity of results when compared with the
pyridyl ether (i.e., 2-) series (or the aminomethylpyr-
idine series) of compounds.
Compound 716 (Ki >10,000 nM), the de-aza counter-
part of 6a, lacked affinity for nACh receptors.
The influence of the 6-position substituents on affinity
generally follows that seen with nicotine and pyridyl
ether 2;17 that is, halogen has little effect but a methoxy
group decreases affinity. The presence of the pyridyl
nitrogen atom is important for the binding of 6a (as
seen with 7) as it is for the binding of nicotine and the
pyridyl ethers.10 However, amine substituents seem to
influence these ligands differently. Demethylation of
nicotine to its corresponding secondary amine, norni-
cotine, decreases affinity by approximately 15-fold.8
Table 2 compares the influence of amine substituents on
6a relative to 2 and a series of ring-opened analogues of
nicotine: 3-(aminomethyl)pyridines. The N-mono-
methyl, N,N-dimethyl and N-ethyl-N-methyl analogues
of 2 bind with comparable affinity (Ki=21–35 nM).
With the aminomethylpyridines (see B, Table 2) optimal
affinity is associated with the N-ethyl-N-methyl and
pyrrolidinyl derivatives. In contrast, the highest affinity
analogue of 6 is associated with the secondary amine 6a;
the affinities of the tertiary amines are somewhat lower.
Also, the pyrrolidinyl analogue of 2 binds with low
affinity whereas that of 6 (i.e., 6d) lacks affinity. One of
the most striking differences is seen upon quaterniza-
tion. Quaternization (via methylation) of nicotine has
Do pyridylbutynylamines 6 meet the requirements of
any of the three currently prevailing nicotinic pharma-
cophore models? The Sheridan et al.9 model requires an
N–N distance of 4.8 A. Molecular modeling studies
performed on 6a identified numerous low-energy con-
formers typically displaying N–N distances of >7.7 A.20
The shortest possible N–N distance achievable is 5.7 A
(energy within 0.3 kcal/mol of the lowest-energy con-
former: 2.9 kcal/mol). On this basis, it is unlikely, as
might have been expected, that 6a can meet the Sher-
idan et al.9 pharmacophore N–N distance requirement
of 4.8 A.
Olesen and co-workers11 have proposed that the dis-
tance between two points, a and b, at the ends of vectors
(each 2.9 A in length) drawn in the direction of the lone-
pair electrons of nicotinic ligands, should be between 7
and 8 A. In a second study where a 2.9-A vector was
attached to each nitrogen atom of 6a and calculations
were repeated using an a–b constraint of 7–8 A, about
24 low-energy conformers (Eꢀ3.35 kcal/mol; N–N dis-
tance ꢀ7.2 A) with an a–b distance of about 7.8 A were
identified (Fig. 1). Thus, 6a seemingly meets the vector
pharmacophore requirements.

D. Dogruer et al. / Bioorg. Med. Chem. Lett. 14 (2004) 523–526
525
Table 2. Comparison of a4b2 affinities for several series of 3 -pyridyl-substituted ligandsa
–NRR0
Ki (nM)b
A
B
C
–NH–CH3
–N(CH3)2
–N(CH3)C2H5
35
21
22
>10,000
540
28
113
510
1770
>10,000
470
212
30
–N(CH+3
)
0.5
110
3
a Data for the pyridyl compounds A and B have been previously reported;7,10 data for pyridylalkynes C are from Table 1. 3-Py=3-pyridyl.
as 2 (or 1), but different terminal amine sites; alter-
natively, if a common amine site is used, the amine
substituents must be oriented differently to account for
the lack of parallel affinity shifts between members of
the series. Compound 6a was shown to be too long
to conform to the internitrogen distance requirements of
the Sheridan et al.9 pharmacophore model but, see-
mingly, might meet the requirements of the newer vec-
tor-based pharmacophore models proposed by Olesen
and colleagues.11ꢁ13
Figure 1. Computational studies with 6a revealed that it meets the 7–8 A
a–b distance requirement of the vector pharmacophore model where a
and b are hypothetical receptor points located 2.9 A from each of the
two nitrogen atoms.
Acknowledgements
Olesen and co-workers12,13 have also proposed a modi-
fication of their vector model by introducing an aryl
centroid feature c. In the modified vector pharmaco-
phore, optimal affinity is associated with an a–b distance
of 7.3–8.0 A, an a–c distance of 6.5–7.4 A, and a b–a–c
angle of 30.4–35.8ꢂ. Several conformers of 6a were
identified with an a–b distance of about 7.9 A, an a–c
distance of about 6.6 A, and a b–a–c angle of 32.6–
32.9ꢂ; however, these conformers possessed energies
(ꢀ8.1 kcal/mol) about 5 kcal/mol above the lowest
energy conformer. Thus, it is possible, at least in theory,
for 6a to meet the modified pharmacophore require-
ments, and the higher energy associated with the neces-
sary conformers might offer an explanation for the
reduced affinity of 6a relative to nicotine.
The present study was supported in part by DA 05274.
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The results of the present investigation confirm that
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6-methoxy group is tolerated, that the N-methyl sec-
ondary amine (i.e., 6a) binds with higher affinity than
simple tertiary amine analogues, that quaternization has
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20. Modeling studies. Studies were conducted using SYBYL
(SYBYL Molecular Modeling Package, Version 6.8; Tri-
pos Inc.: St. Louis, MO, USA, 2001). The structure of 6a
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the BUILD/SKETCH molecule command followed by
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calculation of charges by the Gasteiger-Huckel algorithm.
Conformational search was performed using the SYS-
TEMATIC SEARCH command; the four rotatable
bonds were rotated in 30ꢂ-increments starting at 0ꢂ. For
evaluation of vector models, an a–b distance constraint of
7–8 A was introduced. Results were ANALYZED with
the SYSTEMATIC SEARCH command. A total of 791
conformers were identified, of which only the 24 lowest
energy conformers were considered.