Aplysamine-1 and related analogs as histamine H3 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2005.11.003

Aplysamine-1 (1), a marine natural product, was synthesized and screened for in vitro activity at the human and rat histamine H₃ receptors. Aplysamine-1 (1) was found to possess a high binding affinity for the human H₃ receptor (K_i = 30 ± 4 nM). Synthetic analogs of 1, including des-bromoaplysamine-1 (10) and dimethyl-{2-[4-(3-piperidin-1-yl- propoxy)-phenyl]-ethyl}-amine (13), were potent H₃ antagonists.

Full text of DOI:10.1016/j.bmcl.2005.11.003

Bioorganic & Medicinal Chemistry Letters 16 (2006) 897–900
Aplysamine-1 and related analogs as histamine H3
receptor antagonists
Devin M. Swanson,* Sandy J. Wilson, Jamin D. Boggs, Wei Xiao, Richard Apodaca,
Ann J. Barbier, Timothy W. Lovenberg and Nicholas I. Carruthers
Johnson & Johnson Pharmaceutical Research and Development, L.L.C., 3210 Merryfield Row, San Diego, CA 92121, USA
Received 19 July 2005; revised 31 October 2005; accepted 1 November 2005
Available online 21 November 2005
Abstract—Aplysamine-1 (1), a marine natural product, was synthesized and screened for in vitro activity at the human and rat his-
tamine H3 receptors. Aplysamine-1 (1) was found to possess a high binding affinity for the human H3 receptor (Ki = 30 4 nM).
Synthetic analogs of 1, including des-bromoaplysamine-1 (10) and dimethyl-{2-[4-(3-piperidin-1-yl-propoxy)-phenyl]-ethyl}-amine
(13), were potent H3 antagonists.
Ó 2005 Elsevier Ltd. All rights reserved.
The histamine H3 receptor is a G-protein-coupled recep-
O
N
tor belonging to the family of histamine receptor sub-
types (H1, H2, H3, and H4). The H3 receptor is located
presynaptically in the peripheral and central nervous
systems, on both histaminergic neurons, as an autore-
ceptor, and other neuronal systems, as a heterorecep-
tor.1 In this capacity, it functions as a negative
modulator, inhibiting the release of histamine and other
neurotransmitters such as acetylcholine, GABA, norepi-
nephrine, and serotonin.2,3 Histamine H3 antagonists
enhance levels of cerebral histamine and, therefore,
may be useful for the treatment of neurological disor-
ders affecting memory, appetite, and sleep.4
S
Br
Br
N
N
NH2
Me
H
N
N
HN
N
H
N
1, aplysamine-1
2 (R)-α-methyl
histamine
3, thioperamide
H
N
Cl
N
N
The histamine H3 receptor was first characterized in
19831 and subsequently cloned and expressed some 15
years later.5 Early ligand design centered around the
well-known affinity of 4(5)-substituted imidazoles for
the H3 receptor.6 Representative examples of this class
of ligands include the first potent and selective agonist,
(R)-a-methylhistamine (2), and the antagonist, thio-
peramide (3) (Fig. 1).7 Historically, imidazole-based
H3 compounds have suffered from poor drug-like prop-
erties, including metabolic degradation by histamine
N-methyltransferase (HMT), cytochrome P450 inhibi-
N
N
4, phencyclidine
5, clozapine
Figure 1. Early imidazole and non-imidazole-based H3 ligands.
tion, and inability to penetrate the blood–brain barrier
in high concentration.8 Few imidazole-based H3 ligands
have advanced into human clinical development and to
date, no selective H3 receptor ligand has been approved
for therapeutic use.8,9
Keywords: Histamine; Histamine H3 receptor; Aplysamine-1; Hista-
mine H3 receptor antagonists; Neurotransmitter; H3 ligand; Marine
natural product.
Isolated examples of weakly binding non-imidazole H3
ligands were also reported prior to the cloning of the
H3 receptor. These include the stimulant phencyclidine
*
Corresponding author. Tel.: +1 858 320 3306; fax: +1 858 450
2049; e-mail: dswanso1@prdus.jnj.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.11.003

898
D. M. Swanson et al. / Bioorg. Med. Chem. Lett. 16 (2006) 897–900
OH
OH
OH
O
N
Br
Br
c or d
Br
Br
N
O
a
b
N
nBu
nBu
N
N
N
NH2
Tyramine (7)
N
6
8
9
1
Figure 2. HTS hit.
c or d
NR1R2
O
n = 1, 2
(4)10, the antipsychotic clozapine (5)11, and the marine
natural product aplysamine-1 (1) (Fig. 1).12
10 n = 2, NR1R2 = dimethylamine
11 n = 1, NR1R2 = dimethylamine
12 n = 1, NR1R2 = piperidine
13 n = 2, NR1R2 = piperidine
Aplysamine-1 (1) was isolated in 1989 from an Austra-
lian sponge of the Verongidae family, Aplysina sp.13
The natural product is a bromotyrosine derived metab-
olite consisting of two tertiary alkyl amines connected
by a dibromo-phenol and has been previously synthe-
sized.14 Structural relatives of 1 include the marine nat-
ural products molokaÕiamine,15 ceratinamine,16 and
turbotoxins A and B.14 These compounds possess a wide
range of biological activities including acetylcholinester-
ase inhibition,14 anti-HIV activity,17 and use as antifoul-
ing agents.16 In 1994, aplysamine-1 (1) was reported to
have weak H3 binding affinity in guinea pig brain and
to behave as an H3 functional antagonist in a guinea
pig tissue strip assay.12
N
Scheme 1. Synthesis of aplysamine-1 and analogs. Reagents and
conditions: (a) formaldehyde, NaBH(OAc)3, MeOH, rt, 18 h, 70% (b)
bromine, AcOH, rt, 3 h, 84% (c) 3-(dimethylamino)propyl chloride
HCl or 2-(dimethylamino)ethyl chloride HCl, NaH, DMF, 50 °C, 30–
40% (d) N,N-dimethylethanolamine, or 3-dimethylamino-1-propanol,
or 1-piperidinepropanol, polymer supported PPh3, DBAD, DCM, rt,
18 h, 30–40%.
NR3R4
OH
OH
O
X
X
X
X
X
X
a
b or c,d
Aplysamine-1 (1) contains a structural motif similar to
that of a series of 2-phenyl-imidazo[1,2-a]pyridines that
were identified via high-throughput screening (HTS) ef-
forts using the cloned human H3 receptor. The 2-phenyl-
imidazo[1,2-a]pyridines were first prepared as calcium
channel blockers and subsequently found to be weak li-
gands for the H3 receptor (e.g., RWJ-22085, 6, H3
Ki = 4 lM) (Fig. 2).18–20 Recognizing the similarities be-
tween the natural product and the HTS hit, we chose to
explore the structure–activity relationship (SAR) of
aplysamine-1-based H3 ligands. Herein the synthesis
and biological activities of aplysamine-1 (1) and related
analogs are reported.
NR1R2
16-19
NR1R2
O
14 X = Br
15 X = H
20-24
16 X = Br, NR1R2 = dimethylamine. 17 X = Br, NR1R2 = piperidine
18 X = H, NR1R2 = dimethylamine. 19 X = H, NR1R2 = piperidine
20 X = Br, NR1R2 = dimethylamine, NR3R4 = dimethylamine
21 X = H, NR1R2 = dimethylamine, NR3R4 = dimethylamine
22 X = Br, NR1R2 = piperidine, NR3R4 = piperidine
23 X = H, NR1R2 = piperidine, NR3R4 = piperidine
24 X = H, NR1R2 = piperidine, NR3R4 = dimethylamine
Scheme 2. Synthesis of aplysamine-1 analogs. Reagents and condi-
tions: (a) HNR1R2, NaBH(OAc)3, DCE, rt, 18 h, 50–70% (b) 3-
(dimethylamino) propyl chloride HCl, NaH, DMF, 50 °C, 30% (c) 1-
bromo-3-chloropropane, K2CO3, acetone, reflux, 18 h (d) piperidine,
KI, Na2CO3, 1-butanol, 95 °C, 18 h, 50% over steps c and d.
Aplysamine-1 (1) was prepared in three steps from
tyramine (7) in an approach similar to the literature
precedent (Scheme 1).14 Treatment of tyramine (7) with
excess aqueous formaldehyde in the presence of sodium
triacetoxyborohydride (NaBH(OAc)3) gave dimethyl-
tyramine (8). Bromination of 8 with bromine in acetic
acid afforded the dibromo-phenol, 9. Subsequent alkyl-
ation using 3-(dimethylamino) propylchloride hydro-
chloride and sodium hydride gave the marine natural
product, aplysamine-1 (1).
ethyl}-amine (13), respectively.22 O-alkylation of 8
using sodium hydride and 2-(dimethylamino)ethyl
chloride hydrochloride in dimethylformamide at
50 °C gave compound 11.
Benzylic amines were prepared starting with several 4-
hydroxybenzaldehydes (Scheme 2). Reductive amina-
tion of 3,5-dibromo-4-hydroxybenzaldehyde (14) using
piperidine and NaBH(OAc)3 in 1,2-dichloroethane pro-
vided 2,6-dibromo-4-piperidin-1-ylmethyl-phenol (17).23
O-alkylation of 17 with 1-bromo-3-chloropropane in
refluxing acetone followed by installation of the second-
ary amine using potassium iodide, Na2CO3, and piperi-
dine gave 22.
In order to investigate the SAR of the natural prod-
uct, a series of analogs, including two compounds that
were previously prepared in our laboratories (23,
24),21 were synthesized (Schemes 1 and 2). Mitsunobu
etherification of dimethyltyramine (8) using dimethyl-
propanolamine or 1-piperidinepropanol, di-tert-butyl
diazodicarboxylate (DBAD), and polymer supported
triphenylphosphine gave des-bromoaplysamine-1 (10)
and dimethyl-{2-[4-(3-piperidin-1-yl-propoxy)-phenyl]-
The human and rat binding affinities were determined
for aplysamine-1 (1) and a series of analogs (10–13,

D. M. Swanson et al. / Bioorg. Med. Chem. Lett. 16 (2006) 897–900
899
20–24) (Table 1). To explore the SAR of 1, three regions
were examined: (1) the bromo-substituent effect; (2) the
alkoxy and alkyl amine chain lengths; and (3) size of the
two amine groups. Thus, removal of the aryl bromines
afforded a 5-fold increase in H3 affinity (10). The pres-
ence of the ortho-substituents presumably induced an
unfavorable conformational or steric interaction.
Reduction of the alkoxy amine chain length reduced
H3 affinity (11, 12), while shortening the alkyl amine
portion of the molecule had little impact (20, 21).
Replacement of the dimethylamine on the alkoxy chain
with a piperidine resulted in a 5- to 10-fold increase in
H3 affinity (12, 13), while replacement of the dimethyl
benzylamine with piperidine had little effect (22–24).
Previously, it was established that both basic nitrogens
are required to retain high affinity for the H3 receptor
in this class of phenyl diamines.21 Hence, a summary
of the SAR to date is depicted in Figure 3.
Table 1. In vitro H3 receptor dataa
NR3R4
n = 1, 2
O
N
X
X
O
N
nBu
nBu
m = 1, 2
NR1R2
N
6
1, 10-13, 20-24
Compound
n
m
NR1R2
X
NR3R4
H3 Ki(nM)b
Functionalc pA2
Human
rat
6
4000 1000
1
10
11
12
13
20
21
22
23
24
2
2
1
1
2
2
2
2
2
2
2
2
2
2
2
1
1
1
1
1
Br
H
H
H
H
Br
H
Br
H
H
30
6
4
1
5
4
249 54
89 14
345 48
50 10
—
N
N
N
N
N
N
N
N
7.77
—
N
N
40
14
—
N
0.5 0.2
5
1
9.30
—
N
N
36
18
8
4
3
1
319 61
255 22
57 12
—
N
N
8.14
9.08
8.03
N
N
N
0.4 0.1
1
0.6
3
N
N
3
1
45
a Displacement of N-[3H]methylhistamine from human H3 receptors expressed in SK-N-MC cells. For determination of binding to the rat receptor,
the same procedure was employed, except frozen rat cortical hemispheres were used instead of cell pellets.
b Value reported as means of three determinations.
c Human pA2 values are derived from Schlid regression analysis of the compound-induced rightward shifts in dose–response curves of histamine-
induced inhibition of forskolin-stimulated cAMP accumulation in SK-N-MC cells overexpressing the histamine H3 receptor.

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D. M. Swanson et al. / Bioorg. Med. Chem. Lett. 16 (2006) 897–900
3. Schlicker, E.; Malinowska, B.; Kathmann, M.; Gothert,
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Lecomte, J.-M.; Pollard, H.; Robba, M.; Schunack, W.;
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Stark, H.; Schunack, W.; Schwartz, J.-C. J. Pharmacol.
Exp. Ther. 1997, 281, 1085.
A
rigid
three carbon
"spacing" is optimal
H
N
O
X = Br results in
X
X
small, tapered,
hydrophobic
a five fold loss in
H3 activity (vs. H)
N
1 or 2 carbons
tolerated
H
A
large,
hydrophobic
flexible
Figure 3. SAR summary.
9. Halpern, M. T. Curr. Opin. Cent. Peripher. Nerv. Syst.
Invest. Drugs 1999, 1, 524.
10. Arrang, J.-M.; Defontaine, N.; Schwartz, J.-C. Eur. J.
Pharmacol. 1988, 157, 31.
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macology 1994, 116, 464.
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Coval, S. U.S. Patent 5,352,707, 1994.
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14. Kigoshi, H.; Kanematsu, K.; Yokota, K.; Uemura, D.
Tetrahedron 2000, 56, 9063.
15. Hamann, M. T.; Scheur, P. J.; Kelly-Borges, M. J. Org.
Chem. 1993, 58, 6565.
16. Tsukamoto, S.; Kato, H.; Hirota, H.; Nobuhiro, F. J.
Org. Chem. 1996, 61, 2936.
17. Schoenfied, R.; Lumb, J.; Fantini, J.; Ganem, B. Bioorg.
Med. Chem. Lett. 2000, 10, 2679.
To explore the selectivity of 1, it was screened against a
panel of 50 monoamine and hormone receptors, ion
channels, and neurotransmitter uptake sites (CEREP,
ExpresProfile, data not shown). Aplysamine-1 (1) was
shown to be selective for the H3 receptor, possessing
low affinity (>1 lM) for the other histamine receptor
types (H1, H2, and H4). A 10-fold reduction in the bind-
ing affinities of aplysamine-1 (1) and analogs is consis-
tently observed across species (human to rat). This
speciation effect can be attributed to crucial structural
differences between the rat and human H3 receptors.24
Compounds with
a
high H3 binding affinity
(Ki < 25 nM) were further evaluated in a cell-based
model of human H3 receptor activation (Table 1,
pA2). All were found to function as competitive antago-
nists in good agreement with the observed H3 binding
affinities.
18. Sanfilippo, P. J.; Urbanski, M.; Press, J. B.; Dubinsky, B.;
Moore, J. B., Jr. J. Med. Chem. 1988, 31, 2221.
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Tobia, A. J.; Rosanthale, M. E. Drug Dev. Res. 1990, 21,
277.
20. Shah, C.; McAtee, L.; Breitenbucher, J. G.; Rudolph, D.;
Xiaobing, L.; Lovenberg, T. W.; Mazur, C.; Wilson, S. J.;
Carruthers, N. I. Bioorg. Med. Chem. Lett. 2002, 12,
3309.
21. Apodaca, R.; Dvorak, C. A.; Xiao, Wei; Barbier, A. J.;
Boggs, J. D.; Wilson, S. J.; Lovenberg, T. W.; Carruthers,
N. I. J. Med. Chem. 2003, 46, 3938.
In conclusion, the marine natural product, aplysamine-1
(1), is a non-imidazole, high affinity, and selective hu-
man H3 receptor ligand synthesized in three steps from
tyramine (7). Modifications of 1 provide potent H3
receptor antagonists at the human and rat H3 receptors
(10, 13).
22. Tunoori, A. R.; Dutta, D.; Georg, G. I. Tetrahedron Lett.
1998, 39, 8751.
References and notes
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3849.
24. Lovenberg, T. W.; Pyati, J.; Chang, H.; Wilson, S. J.;
Erlander, M. G. J. Pharmacol. Exp. Ther. 2000, 293,
771.
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