Structure-activity relationships of non-imidazole H(3) receptor ligands. Part 2: binding preference for D-amino acids motifs.

Article abstract of DOI:10.1016/S0960-894X(02)00310-4

Structure-activity relationship studies on novel non-imidazole, D-amino acid containing ligands of histamine 3 receptors are presented. A-304121 is a D-alanine piperazine amide with high affinity at the rat H(3) receptor.

Full text of DOI:10.1016/S0960-894X(02)00310-4

Bioorganic & Medicinal Chemistry Letters 12 (2002) 2035–2037
Structure–Activity Relationships of Non-imidazole H3 Receptor
Ligands. Part 2: Binding Preference for D-Amino Acids Motifs
Ramin Faghih,* Wesley Dwight, Larry Black, Huaqing Liu, Robert Gentles,
Kathleen Phelan, Timothy A. Esbenshade, Lynne Ireland, Thomas R. Miller,
Chae-Hee Kang, Kathy M. Krueger, Gerard B. Fox, Arthur A. Hancock
and Youssef L. Bennani
Neuroscience Research, Global Pharmaceutical Research and Development, Abbott Laboratories, Abbott Park, IL 60064-6123, USA
Received 2 November 2001; accepted 11 March 2002
Abstract—Structure–activity relationship studies on novel non-imidazole, d-amino acid containing ligands of histamine 3 receptors
are presented. A-304121 is a d-alanine piperazine amide with high affinity at the rat H3 receptor. # 2002 Elsevier Science Ltd. All
rights reserved.
Histamine mediates its action both in the CNS and the
periphery through four distinct receptors (H1–H4)
known to date.1 Both agonists and antagonists of the
histamine-3 receptor (H3R) offer therapeutic potential
since the receptor is involved in the presynaptic regula-
tion of the release of not only histamine itself, but
acetylcholine, gamma-aminobutyric acid, dopamine,
noradrenaline and serotonin2 as well. Potential ther-
apeutic indications for novel H3 ligands include cogni-
tive disorders such as ADHD and Alzheimer’s disease,
or obesity.3 In the preceding paper, we described efforts
to delineate a pharmacophore for H3 receptor blockade
starting from a high throughput screening lead A-923, a
non-imidazole lipophilic arylketone.4 Early studies
revealed the following: (1) the hydrophobic ketone
region can be expanded somewhat both in size and
substitution, (2) ‘small-size’ N-acyl substitutions with a
basic site are tolerated and preferred, and (3) the piper-
azine basic site in A-923 is mandatory for binding/
recognition of the ligand at the rat H3R. In order to add
the desired physicochemical features to improve drug-like
characteristics, we explored this system further and
introduced amino-acid attachments with the goal of
increasing basicity towards a better balance of basicity/
lipophilicity. We wish to report herein the SAR leading to
the discovery of A-304121, a d-alanine derivative, which
is a highly selective, potent ligand for the rat H3 receptor.
Ketone 1 (R=Boc; Fig. 1) was prepared as described in
the preceding paper (Scheme 1): the N-Boc group was
removed under TFA/CH2Cl2 conditions. The free
piperazine underwent N-Boc-amino-acid coupling using
EDCI/DIEA/DMAP conditions to give compounds of
the general structure in Figure 1. Table 1 displays bind-
ing results on human H1, H2, and rat cortical H3R for a
select number of N-Boc and free amino-acid derivatives.
These data are consistent with a trend of enhanced
receptor affinity for N-Boc-d-amino acid containing
compounds versus their natural (l) counterparts by a
ratio of ꢀ4–5. This trend was less pronounced when an
additional basic site on the molecule was present (i.e., 8/
9 and 16/17; free amino amides). The enhanced affinity
was indicative of the potential to find potent and selec-
tive compounds. From our studies, we found the corre-
sponding cyclopropyl ketone to be well tolerated both
in radioligand binding and in PK.
Intermediate 22 was prepared by treating commercially
available chloro-40-hydroxybutyrophenone 18 with con-
centrated NaOH and heat to induce cyclopropanation
*Corresponding author. Fax: +1-847-937-4143; e-mail: ramin.
faghih@abbott.com
Figure 1.
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00310-4

2036
R. Faghih et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2035–2037
Scheme 1. (a) NaOH, heat, 19; (b) Cl–(CH2)3–Br, K2CO3, 2-butanone, reflux 24 h; (c) N-Boc-piperazine, KI/K2CO3, 2-butanone-reflux 72 h; (d)
TFA, CH2Cl2, 0 ꢁC to rt, 12 h.
followed by O-alkylation with 1-bromo-3-chloro-
propane to give 20 (65% yield for two steps). Further
displacement with N-Boc-piperazine and removal of the
protecting group gave piperazine derivative 22. Alter-
natively, piperazine 22 can be prepared directly from
chloride 20 and 3 mol/equivalents of piperazine under
KI/K2CO3 conditions.
pounds is shown in Table 2, along with their affinities at
H1, H2 and H3 receptors.
Data from Table 2 highlights several pharmacophore
features: (1) Preference for d-amino acid (AA’s) sub-
stituents is clear for most examples (e.g., 24 vs 25; 53 vs
54); (2) small AA’s enhance receptor affinity (e.g., 24,
25, 43, 47, 55, and 57), whereas large aromatic groups
(e.g., 31, 34, 35, and 38) are poorly tolerated; exception
can be made for groups with increased chain flexibility
(e.g., 29) as well as groups containing a basic site (33),
(3) b-alanine (e.g., 55) provides the highest affinity
ligand at rat H3 receptors in this series; (4) chain sub-
stitutions (e.g., 57–60) do not enhance binding affinity
Further acylations with various N-Boc-amino-acids,
according to standard coupling procedures (see above)
gave the corresponding desired products. TFA treat-
ment produced the unprotected amino-acid derivatives,
which were characterized and tested as either TFA or
l-tartaric acid salts. A representative subset of com-
Table 1. Binding affinitiesa (pKi) at rat cortical H3 receptors and human H1 and H2 receptors5
Compdb: R
H3
H1
H2
Compd: R
H3
H1
H2
2: N-Boc-d-Ala
8.91
8.62
7.83
7.01
7.84
6.95
7.63
7.27
5.17
4.77
5.48
4.97
5.33
5.09
4.77
5.03
4.97
4.64
4.87
4.90
5.35
5.46
4.92
4.83
10: d-Ala
8.72
8.15
7.77
7.10
7.21
6.77
8.04
7.99
4.45
4.33
4.78
4.72
4.84
5.01
5.48
5.64
4.39
4.46
4.50
4.42
4.85
4.78
4.43
4.46
3: N-Boc-d-Ser
11: d-Ser
4: N-Boc-d-Val
12: d-Val
5: N-Boc-l-Val
13: l-Val
6: N-Boc-d-Leu
14: d-Leu
7: N-Boc-l-Leu
15: l-Leu
8: N-Boc-d-4-Pyridyl-Ala
9: N-Boc-l-4-Pyridyl-Ala
16: d-4-Pyridyl-Ala
17: l-4-Pyridyl-Ala
aValues were estimated from at least three separated competition experiments (SEM ꢂ0.2).
1
bSatisfactory H NMR, MS spectra and elemental analyses were obtain for all new compounds.
Table 2. Binding affinitiesa (pKi) at rat cortical H3 receptors and human H1 and H2 receptors
Compd: R
H3
H1
H2
Compd: R
H3
H1
H2
23: N-Boc-d-Ala
24: d-Ala
8.68
8.90
7.69
6.92
7.52
8.40
8.34
7.64
7.53
8.24
8.34
6.67
6.54
7.52
7.56
6.88
7.58
7.54
8.07
5.09
4.16
4.27
4.33
4.43
5.26
5.00
5.01
5.03
4.00
4.02
5.70
5.36
5.01
5.34
4.92
4.97
4.34
4.79
4.08
4.15
4.38
4.30
4.27
5.00
5.00
4.33
4.40
4.01
4.05
4.68
4.82
4.48
4.61
4.69
4.72
4.14
4.34
42: d-b-tert-Butyl-Ala
43: d-2-Abu
7.28
8.80
7.92
8.40
6.79
8.64
8.06
7.36
7.12
8.36
7.37
8.88
7.89
9.30
8.33
8.71
8.60
8.19
8.05
4.66
4.15
4.99
4.33
4.84
4.00
4.33
4.87
4.46
5.00
4.66
4.90
5.00
4.55
4.02
5.68
5.33
5.82
5.76
4.72
4.00
4.24
4.22
4.69
4.10
4.30
4.75
4.31
5.00
4.57
4.85
4.91
4.00
4.05
4.96
4.97
4.99
4.97
25: l-Ala
44: d-tert-Leu
26: N-Boc-d-Pro
27: d-Pro
28: N-Boc-d-Phe
29: d-HPhe
30: d-Thi
31: d-4-F-Phe
32: d-Thiazole-Ala
33: d-His
45: d-Nva
46: d-b-Cyclohexyl-Ala
47: Sarcosine
48: N-Isopropyl-d-Ala
49: N-Methyl-d-Phe
50: N-Methyl-d-Val
51: (R)-4-Oxo-2-Azetidine
52: (S)-4-Oxo-2-Azetidine
53: d-Azetidine-2-CO
54: l-Azetidine-2-CO
55: b-Ala
56: d-b-Hpro
57: d-Abu
58: l-Abu
59: d-a-Methyl-b-Ala
60: l-a-Methyl-b-Ala
34: d-3-(1-Naphthyl)-Ala
35: d-3-Benzothienyl-Ala
36: d-Try
37: d-Phg
38: d-Biphenyl-Ala
39: d-(2-INDA)-Gly
40: d-Cyano-Ala
41: d-O-tert-Butyl-Ser
aSee corresponding footnotes in Table 1.

R. Faghih et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2035–2037
2037
(note that these latter enantiomeric derivatives have
minimal affinity advantage over one another).
References and Notes
1. Hough, B. L. Mol. Pharmacol. 2001, 59, 415.
From the above studies, we selected A-304121 and A-
308830 (e.g., 24 and 55) based on their high affinity and
selectivity for the rat H3 receptor. d-Alanine derivative
24 and b-alanine 55 were tested at a number of biogenic
amine receptors and transporters for the neuro-
transmitters norepinephrine, dopamine, and serotonin
including different subtypes. Compounds 24 and 55
demonstrated low affinity at all of these sites generally
with pKi <5.00. In order to test the hypothesis behind
these studies, we evaluated the bioavailability of
compounds 24 and 55 in rat after a dose of 5 mg/kg
po. A-304121 showed an excellent oral bioavailability
of 83% compared with 46 and 0%, respectively, for
A-308830 (55) and A-923. Based on its bioavailability,
potency and selectivity compound 24 (A-304121) was
selected for further evaluation in models of in vitro
functional activity. In the guinea pig isolated ileum
assay6 in which histamine H3 receptors mediate inhi-
bition of neurogenic contractions, A-304121 demon-
strated a pA2 value of 6.98. Compound 24 was also
tested for H3 receptor antagonist activity in an assay
with K+-evoked depolarization- induced release of
[3H]-histamine from rat synaptosomes7 with pKb of
8.75.
2. (a) Hill, S. J.; Gannelin, C. R.; Timmerman, H.; Schwartz,
J. C.; Shankley, N. P.; Young, J. M.; Schunack, W.; Levi, R.;
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In summary, we have discovered highly potent H3
receptor antagonists. Additionally A-304121 (24) and
compound 55 were shown to be more than 1000-fold
selective for the H3 receptor subtype over the H1 and H2
receptor subtypes. Thus, A-304121 and several analo-
gues represent highly potent antagonists of the H3
receptor, with potential clinical utility for treating CNS-
related diseases.