N-Acridin-9-yl-butane-1,4-diamine derivatives: High-affinity ligands of the α2δ subunit of voltage gated calcium channels

Article abstract of DOI:10.1016/j.bmcl.2004.01.087

A series of N-acridin-9-yl-butane-1,4-diamines were found to be high-affinity ligands of the α₂δ subunit of voltage gated calcium channels. The SAR studies of butane-1,4-diamine side chain resulted in the identification of compound 10 (IC₅

Full text of DOI:10.1016/j.bmcl.2004.01.087

Bioorganic & Medicinal Chemistry Letters 14 (2004) 1913–1916
N-Acridin-9-yl-butane-1,4-diamine derivatives: high-affinity ligands
of the ꢀ2ꢁ subunit of voltage gated calcium channels
Jongwon Lim,a,*,y Nicholas Stock,a,y Richard Pracitto,a Julia K. Boueres,a
Benito Munoz,a Ashok Chaudhary,c Angelina M. Santini,b Karia Orr,b
Herve Schaffhauser,b Robert E. Bezverkov,b Jayashree Aiyarb
and Shankar Venkatramana
aDepartment of Chemistry, Merck Research Laboratories, 3535 General Atomics Court, San Diego, CA 92121, USA
bDepartment of Neurobiology, Merck Research Laboratories, 3535 General Atomics Court, San Diego, CA 92121, USA
cDepartment of Drug Metabolism, Merck Research Laboratories, Rahway, NJ 07065, USA
Received 4 December 2003; revised 26 January 2004; accepted 28 January 2004
Abstract—A series of N-acridin-9-yl-butane-1,4-diamines were found to be high-affinity ligands of the a2d subunit of voltage gated
calcium channels. The SAR studies of butane-1,4-diamine side chain resulted in the identification of compound 10 (IC50=9 nM),
which is more potent than gabapentin (IC50=27 nM). Partial saturation of the acridine ring was also pursued and provided a
compound with higher binding affinity than 1.
# 2004 Elsevier Ltd. All rights reserved.
1. Introduction
2. N4-Acridin-9-yl-N1,N1-diethyl-1-alkylbutane-
1,4-diamines
Gabapentin (Neurontin1), [1-(aminomethyl)cyclohexyl]-
acetic acid, is an anticonvulsant agent, and has been
shown to be effective against neuropathic pain both in
animal models and in human.1ꢀ5 The exact mechanism
of action of gabapentin in these therapeutic areas,
however, is still unclear. Recently, a high affinity
[3H]gabapentin-binding protein has been identified as
an a2d subunit of a voltage gated calcium channel, and
this subunit has been suggested to play an important
role in the pharmacological actions of gabapentin.6
First, we investigated the binding affinity of the enantio-
mers of 1, and found that the (S)-enantiomer 2 is the
active component (IC50=110 nM, Table 1). The stark
contrast in binding affinity between 2 and 3 suggests that
the methyl group of 1 is proximal to a binding pocket of
the protein, thereby the protein can differentiate the (S)
and (R) methyl groups. Secondly, the desmethyl analogue
4 is inactive whereas the gem-dimethyl analogue 5 is
almost as active as 2. Taken together these results
strongly suggest a specific interaction between the
ligands in this position and the protein.
Our goal was to discover novel gabapentin-mimetic
compounds, with which we can better understand the
mechanism of the pharmacological actions of gaba-
pentin and thereby develop more effective drugs. A
high-throughput screening campaign of [3H]gabapentin
binding in human brain membrane (A710 membrane)7
identified compound 1 as a high-affinity ligand of the
a2d subunit of a voltage gated calcium channel
(IC50=220 nM). Herein we report SAR studies on 1
and the discovery of highly potent ligands of the a2d
subunit of voltage gated calcium channels.
* Corresponding author. Tel.: +1-858-202-5731; fax: +1-858-202-
5743; e-mail: jongwon_lim@merck.com
Both authors contributed equally to this work.
We next sought the optimal length of the side chain.
Preliminary SAR studies showed that all three nitrogen
y
0960-894X/$ - see front matter # 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.01.087

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J. Lim et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1913–1916
Table 1. a2d Binding affinity of N4-acridin-9-yl-N1,N1-diethyl-1-
alkylbutane-1,4-diamines
observed in rat liver microsomes, which presumably
accounts for the rapid clearance of 1 in this species. To
improve the poor pharmacokinetic properties of 1, SAR
studies on the terminal diethylamino group were carried
out. Both acyclic and cyclic amine analogues with various
steric and electronic demand at the C1 position of the
side chain were synthesized (Table 2). These studies
revealed a very tight SAR; minor modification at the C1
center of the side chain furnished completely inactive
analogues. Neither bulkier alkyl amine analogues (13
and 14) nor cyclic amine analogues (15–18) showed high
binding affinity to the a2d subunit of voltage gated calcium
channels. Analogues having aromatic heterocyclic
amino groups such as imidazole, triazole and tetrazole
at the C1 center of the side chain were also inactive in
the binding assay (data not shown).
Compd
R1
a2d Binding IC50 (nM)
1
220
2
110
11,000
13,000
270
3
4
2.2. Partial saturation of N4-acridin-9-yl-N1,N1-diethyl-
pentane-1,4-diamines
5
Since planar aromatic dyes such as 9-aminoacridine
have been known to be mutagenic,8 potential gene-
toxicity of these compounds was one of our initial
concerns. To address this issue, we pursued analogues
having different ring structures. The quinoline and tetra-
hydroquinoline analogues 19 and 20 turned out to be
much less potent than 1 suggesting that a tricyclic ring
skeleton is required for high binding affinity (Table 3).
To avoid planarity of the acridine ring, partial saturation
of the aromatic ring system was pursued. Although the
tetrahydro analogue 21 is not as potent as 1, its potency
indicates that there is room for further SAR studies on
the tricyclic ring. Interestingly, further saturation of the
ring to the octahydro tricyclic ring system provided a
more potent compound (22, IC50=160 nM) than 1. The
potency trend of enantiomer pairs also applied to 22;
the (S)-enantiomer 23 has much higher binding affinity
than the (R)-enantiomer 24 (IC50=43 nM and 760 nM
respectively).
6
>10,000
6000
38
7
8
9
18
10
11
12
9
52
1100
Table 2. a2d Binding affinity of N4-acridin-9-yl-N1,N1-dialkyl-pent-
ane-1,4-diamines
atoms of 1 are essential for high binding affinity (data
not shown). Thus, spacers with different numbers of
carbon atoms between the two nitrogen atoms of the
side chain were synthesized. With the crucial methyl
group in place, neither the three carbon spacer analogue
6 nor the five carbon spacer analogue 7 showed binding
activity.
SAR studies on the C4 center of the side chain were
then explored, keeping the optimal four carbon spacer.
The ethyl analogue 8 is five times as potent as 1 and the
propyl analogue 9 is more than ten times as potent
as 1 (IC50=18 nM). The cyclopropyl analogue 11 is
comparable to 8 but the phenyl analogue 12 is less
potent than 1. As expected from the potency pattern of
enantiomers 2 and 3, the (S)-enantiomer 10 was indeed
the active enantiomer (IC50=9 nM).
Compd
R2
a2d Binding IC50 (nM)
13
14
–NHiPr
–NHiPr2
5100
>10,000
15
16
17
9900
>10,000
>10,000
2.1. N4-Acridin-9-yl-N1,N1-dialkylpentane-1,4-diamines
18
5900
Pharmacokinetic studies showed rapid clearance of 1 in
rats. Both des-ethyl and bis-des-ethyl analogues of 1 were

J. Lim et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1913–1916
1915
Table 3. a2d Binding affinity of partially saturated N4-acridin-9-yl-
N1,N1-diethylpentane-1,4-diamines
followed by coupling (PhOH, TEA, 120 ꢁC)11 to 9-chloro-
acridine proceeded smoothly to provide 8 in high yield.
The synthesis of side chains with various amino groups
at the C1 center (see Table 2) started from 5-methyl-2-
pyrrolidinone. Hydrolysis followed by Boc protection
and reduction of the resultant amino acid provided
alcohol 27. By following the same protocol utilized for
the preparation of 8, the isopropylamine analogue 13
was prepared in good overall yield.
Compd
R3
R4
a2d Binding IC50 (nM)
19
rac-CH3
960
As an example of the synthesis of partially saturated
analogues (see Table 3), the synthesis of octahydro
analogue 22 is shown in Scheme 2. Reduction of 1,2,3,4-
tetrahydro-9-acridanone followed by chlorination provided
9-chloro-1,2,3,4,5,6,7,8-octahydroacridine (31). Buchwald
amination of 31 proceeded smoothly to provide the
octahydro analogue 22 in good yield.12
20
21
22
rac-CH3
rac-CH3
rac-CH3
>10,000
420
160
23
24
(S)-CH3
(R)-CH3
43
760
4. [13H]-10: A potent and selective radioligand
Compound 10, the most potent ligand identified in this
study, was radiolabeled to measure specific versus non-
specific binding and to provide a tool for in vitro binding
assays. Binding studies with [3H]-10 using gabapentin as
cold displacer showed high specific binding of [3H]-10 to
the a2d subunit of human calcium channels (about 90%
of total binding, Fig. 1). This result supports the use of
[3H]-10 as a potent and selective radioligand for in vitro
binding assays.
3. Chemistry
All the analogues of 1 herein studied were synthesized in
a convergent fashion via coupling 9-chloroacridine9
with the relevant side chains as the last step. For example,
the synthesis of ethyl analogue 8 is depicted in Scheme 1.
Aldehyde 25 was prepared by following the known lit-
erature procedure10 on a large scale and coupled to the
ethyl Grignard reagent. The resultant secondary alcohol
was converted to the corresponding amine via the phtha-
limide and the amine was protected with a Boc group to
provide 26. After deprotection of the TBDPS group, the
diethylamino moiety was introduced at the C1 center
by displacement of the corresponding mesylate with
diethylamine to afford 28. Deprotection of the Boc group
Scheme 2. (a) H2, PtO2, HCl; (b) POCl3, reflux; (c) 2-amino-5-
diethylaminopentane, Pd(OAc)2, NaOtBu, biphenyl-2-yl-di-tert-butyl-
phosphane, toluene.
Scheme 1. (a) EtMgBr, THF; (b) phthalimide, PPh3, DEAD, THF; (c)
N2H4, EtOH; (d) Boc2O, TEA, CH2Cl2; (e) TBAF, THF; (f) 6 N HCl,
reflux; (g) ClCO2Et, NMM, THF; NaBH4, MeOH; (h) MsCl, TEA,
THF; (i) Et2NH or iPrNH2, C HCN, 50 ꢁC; (j) 4 N HCl, 1,4-dioxane;
Figure 1. Specific binding of [3H]-10 to a2d subunit of human voltage
3
(k) 9-chloroacridine, PhOH, TEA, 120 ꢁC.
gated calcium channels.7

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J. Lim et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1913–1916
2. Rosner, H.; Rubin, L.; Kestenbaum, A. Clin. J. Pain
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5. Conclusion
3. Singh, L.; Field, M. J.; Ferris, P.; Hunter, J. C.; Oles,
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Ligand 10 which binds potently (IC50=9 nM) to the
a2d subunit of calcium channels was identified through
optimization of the side chain of the lead compound 1.
To the best of our knowledge, 10 is the first high affi-
nity, non amino acid gabapentin-mimetic ligand repor-
ted to date. A potent octahydroacridine was also
prepared. Binding studies with [3H]-10 validated it as a
specific radioligand.
7. [3H]Gabapentin binding assay in human brain membrane
(A710 membrane) was adopted from the method in the
literature: Suman-Chauhan, N.; Webdale, L.; Hill, D. R.;
Woodruff, G. N. Eur. J. Pharmacol., Mol. Pharmacol.
Sect. 1993, 244, 293.
8. Topal, M. D. Biochemistry 1984, 23, 2367.
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10. Spino, C.; Barriault, N. J. Org. Chem. 1999, 64, 5292.
11. Phanstiel, O., IV; Price, H. L.; Wang, L.; Juusola, J.;
Kline, M.; Shah, S. M. J. Org. Chem. 2000, 65, 5590.
12. Wolfe, J. P.; Tomori, H.; Sadighi, J. P.; Yin, J.;
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Acknowledgements
The authors thank Bill Bray for technical assistance.
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