Chiral NG-acylated hetarylpropylguanidine-type histamine H2 receptor agonists do not show significant stereoselectivity

Article abstract of DOI:10.1016/j.bmcl.2010.03.082

A set of chiral imidazolylpropylguanidines and 2-aminothiazolylpropylguanidines bearing N^G-3-phenyl- or N^G-3-cyclohexylbutanoyl residues was synthesized and investigated for histamine H₂ receptor (H₂R) agonism (guinea pig (gp) right atrium, GTPase assay on recombinant gp and human (h)H₂R) and for hH₂R selectivity compared to hH₁R, hH₃R and hH₄R. In contrast to previous studies on arpromidine derivatives, the present investigation of acylguanidine-type compounds revealed only very low eudismic ratios (1.1-3.2), indicating the stereochemistry of the acyl moiety to play only a minor role in this series of H₂R agonists.

Full text of DOI:10.1016/j.bmcl.2010.03.082

Bioorganic & Medicinal Chemistry Letters 20 (2010) 3173–3176
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Bioorganic & Medicinal Chemistry Letters
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Chiral NG-acylated hetarylpropylguanidine-type histamine H2 receptor
agonists do not show significant stereoselectivity
Prasanta Ghorai a, , Anja Kraus a, , Tobias Birnkammer a, Roland Geyer a, Günther Bernhardt a, Stefan Dove a,
Roland Seifert b, Sigurd Elz a, Armin Buschauer a,
*
a Institute of Pharmacy, Pharmaceutical/Medicinal Chemistry, University of Regensburg, D-93040 Regensburg, Germany
b Institute of Pharmacology, Medical School of Hannover, D-30625 Hannover, Germany
a r t i c l e i n f o
a b s t r a c t
A set of chiral imidazolylpropylguanidines and 2-aminothiazolylpropylguanidines bearing NG-3-phenyl-
or NG-3-cyclohexylbutanoyl residues was synthesized and investigated for histamine H2 receptor (H2R)
agonism (guinea pig (gp) right atrium, GTPase assay on recombinant gp and human (h)H2R) and for
hH2R selectivity compared to hH1R, hH3R and hH4R. In contrast to previous studies on arpromidine deriv-
atives, the present investigation of acylguanidine-type compounds revealed only very low eudismic
ratios (1.1–3.2), indicating the stereochemistry of the acyl moiety to play only a minor role in this series
of H2R agonists.
Article history:
Received 16 February 2010
Revised 22 March 2010
Accepted 23 March 2010
Available online 27 March 2010
Dedicated to Professor Dr. Dr. Dr. h.c. Walter
Schunack, Berlin, on the occasion of his 75th
birthday
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Histamine
H2 receptor agonist
Chiral
Enantiomers
Acylguanidine
Sf9 cells
Imidazolylpropylguanidines such as arpromidine and related
compounds (Fig. 1) are the most potent histamine H2 receptor
(H2R) agonists reported in the literature so far.1 Such compounds
are interesting pharmacological tools and potential drugs, for
example, for the treatment of severe congestive heart failure.2
However, their applicability is limited. The strongly basic guani-
dino group is the main reason for very low oral bioavailability,
non H2R-mediated effects and the lack of penetration across the
blood brain barrier. We previously reported on NG-acylated guan-
idines as bioisosteres of alkylguanidines with strongly reduced ba-
sicity (by 4–5 orders of magnitude) and improved pharmacokinetic
properties.3,4 For instance, the N-(3-phenylbutanoyl)guanidine 5
proved to be up to 60 times more potent than histamine (1) at
the spontaneously beating guinea pig right atrium. However, the
first generation of these acylguanidines turned out to be insuffi-
ciently selective for H2R, in particular regarding H3R and H4R.5,6
This problem was solved by replacing the privileged structure,
the imidazolylpropylguanidine moiety, with a bioisosteric 2-ami-
no-4-methylthiazol-5-yl group resulting in highly potent and
selective H2R agonists.4
In the arpromidine series, the (S)-configured enantiomers, for
example, of the 3,4-dihalogenated analogues 3 and 4, are the
eutomers with eudismic ratios of up to 40.7 This motivated us to
investigate the enantiomers of representative acylguanidines. As
the oxo analogues of arpromidine-like compounds turned out to
* Corresponding author. Tel.: +49 941 943 4827; fax: +49 941 943 4820.
E-mail address: armin.buschauer@chemie.uni-regensburg.de (A. Buschauer).
These authors contributed equally to this work.
Figure 1. Structures of selected histamine H2 receptor agonists.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.03.082

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P. Ghorai et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3173–3176
be less potent than their corresponding parent compounds,3
branched butanoylguanidines were preferred as model com-
pounds. The present study is focused on the synthesis and the
pharmacological characterisation of the enantiomers of 5 and the
corresponding cyclohexyl (6) and aminothiazole analogues (7
and 8).
The pertinent chiral building blocks were obtained via asym-
metric synthesis. The building block (R)-3-phenylbutanoic acid
((R)-11) was synthesized from the achiral precursor, methyl (E)-
but-2-enoate (9), via asymmetric addition of phenylboronic acid
(10) as reported by Hayashi et al.,8 using a catalytic amount of rho-
dium catalyst and (S)-binap ligand. As the enantiomeric excess (ee)
of this reaction did not exceed about 80%, the methyl ester (R)-11a
was recrystallized to obtain a sufficiently pure enantiomer (99% ee)
which was then hydrolysed to give (R)-11 (Scheme 1).
(S)-3-Phenylbutanoic acid was prepared according to the same
procedure except for using (R)-binap instead of (S)-binap as the
chiral ligand. Analytical enantioseparation of the esters (R)- and
(S)-11a was performed by chiral HPLC, whereas the acids (R)-
and (S)-11 were analysed by means of capillary electrophoresis
using (2-hydroxypropyl)-b-cyclodextrin as chiral selector. The con-
figurations of (R)- and (S)-11 were assigned according to the liter-
ature.8,9 The 3-cyclohexylbutanoic acids (R)- and (S)-12 were
prepared by hydrogenation of (R)- and (S)-11 (Scheme 1).
Coupling of the acids (R)-11, (S)-11, (R)-12 and (S)-12 with tri-
tyl-protected imidazolylpropylguanidine or di-Boc-protected 2-
amino-4-methylthiazolylpropylguanidine, followed by cleavage
of the protecting groups under acidic conditions gave the corre-
sponding enantiomers of the NG-acylated products 5–8 (Scheme
2).10 For enantiomeric excess (determined by means of CE by anal-
ogy with a previously reported method11), HPLC purity and specific
optical rotation cf. Supplementary data.
Scheme 2. Coupling of chiral acids with guanidine building blocks. Reagents and
conditions: (i) CDI (1.1 equiv), NaH (60% dispersion in mineral oil) (2 equiv), THF,
5 h, rt; (ii) TFA (20%), DCM, 5 h, rt; (iii) EDCꢀ ꢀ ꢀHCl (1 equiv), HOBtꢀ ꢀ ꢀH2O (1 equiv),
DIPEA (1 equiv), DCM, 15 h, rt; (iv) TFA (20%), DCM, 3–5 h, rt.10
Table 1
Histamine H2 receptor agonism at the spontaneously beating guinea pig right atrium
a
c
Compd
pEC50
Relative potencyb
Emax (%)
(R):(S)d
Histamine
( )-53
(R)-(+)-5
(S)-(ꢁ)-5
( )-63
(R)-(+)-6
(S)-(ꢁ)-6
( )-74
(R)-(ꢁ)-7
(S)-(+)-7
( )-84
6.00 0.10
7.80 0.07
7.03 0.12
7.60 0.08
7.17 0.07
6.99 0.07
7.48 0.12
7.31 0.17
6.99 0.10
7.40 0.11
7.16 0.06
7.24 0.14
7.04 0.11
1
100
99
93
102
101
82
103
89
73
87
74
80
2
2
1
2
3
4
2
3
4
3
8
4
3
63.50 17.85
10.71 3.85
39.81 11.74
14.70 4.13
9.77 2.75
30.20 10.86
20.42 9.27
9.77 3.18
25.12 8.60
14.45 3.88
17.38 6.89
10.96 3.75
1:3.7 1.2
1:3.1 1.0
1:2.6 0.9
1.6:1 0.7
The synthesized compounds were investigated for H2R agonistic
activity on the isolated spontaneously beating guinea pig right at-
rium (positive chronotropic response)3,4 (Table 1) and in a steady
state GTPase assay using membrane preparations of Sf9 insect cells
(R)-(+)-8
(S)-(ꢁ)-8
78
a
pEC50 was calculated from the mean shift
D
pEC50 of the agonist curve relative
pEC50; sum-
expressing guinea pig (gp) or human (h) H2R-Gs S fusion proteins12
a
to the histamine reference curve by equation: pEC50 = 6.00 + 0.13 +
D
(Table 2). Furthermore, a selection of compounds was investigated
for H2R selectivity versus hH1R, hH3R and hH4R in GTPase assays
using recombinant human histamine receptors3,4,13 (Table 3).
On the spontaneously beating guinea pig right atrium (S)-5
proved to be the eutomer with a eudismic ratio of 3.7. (S)-5 was
a full agonist, whereas the intrinsic activity of (R)-5 was slightly re-
duced. This is in agreement with the results for the corresponding
2-aminothiazole analogues: (S)-7 was found to be more active than
(R)-7 by a factor of 2.6. The result is in accordance with the data
mand 0.13 represents the mean desensitization observed for control organs when
two successive curves for histamine were performed (0.13 0.02, N = 16). The SEM
given for pEC50 is the SEM calculated for
DpEC50 for 3–6 experiments.
b
Potency, relative to histamine = 1%.
Efficacy, maximal response (%) relative to the maximal increase in heart rate
induced by the reference compound histamine.
c
d
SEM of the eutomer/distomer ratio due to error propagation.
was two times more active than its optical antipode. The efficacy
was also higher for the (S)-enantiomer. The enantiomers of the
3-cyclohexylbutanoyl- and the 3-phenylbutanoylguanidine deriva-
tives 6 and 5 show the same preference for H2Rs: higher potency
resides in the (S)-configured cyclohexyl-substituted enantiomer
((S)-6) with a eudismic ratio of about three (guinea pig atrium
from the GTPase assay on guinea pig H2R-Gs S fusion protein (Table
a
2). Here also (S)-5 was found to be 3.2 times more potent than the
corresponding (R)-enantiomer. Moreover, similar to the results
from guinea pig right atrium, the 2-aminothiazole analogue (S)-7
and GTPase assay on human and guinea pig H2R-Gs S). The amino-
a
thiazolyl enantiomers (R)-8 and (S)-8 apparently show an inverse
preference ((R)-8 > (S)-8) compared to both the corresponding imi-
dazolylpropyl analogue ((S)-6 > (R)-6) and 3-phenylbutanoyl
substituted analogue ((S)-7 > (R)-7), but the activity ratios of the
enantiomers are not significantly different from one (guinea pig at-
rium and GTPase assays). The eudismic ratios of the 2-amino-4-
methylthiazol-5-ylpropylguanidines were lower by trend than
those of the imidazolylpropylguanidines. Taken together, the data
suggest that the H2R binding modes of the present stereoisomers
are very similar.
As reported recently,4 the NG-acylated aminothiazolylpropyl-
guanidines are devoid of any relevant agonistic or antagonistic
activity on hH1R, hH3R and hH4R. This is also true for the racemates
and the enantiomers of compounds 7 and 8 (Table 3). In contrast,
Scheme 1. Synthesis of (R)-3-phenylbutanoic acid ((R)-11) and (R)-3-cyclohexylb-
utanoic acid ((R)-12). Reagents and conditions: (i) [Rh(acac)(C2H4)2]/(S)-binap,
dioxane/H2O (10/1), 100 °C, 16 h; (ii) LiOH, THF, rt; (iii) H2, Rh/C or Rh/Al2O3 (cat.),
AcOH, 8 bar, 48 h, rt.

P. Ghorai et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3173–3176
3175
Table 2
Agonistic activities on hH2R-Gs and gpH2R-Gs (GTPase assay)
a
S
aS
a
a
Compd
hH2R-Gs
gpH2R-Gs
EC50 hH2R-Gs /
aS
a
S
a
S
EC50 gpH2R-Gs
a
S
Emax
pEC50
Rel. pot.
(R):(S)b
Emax
pEC50
Rel. pot.
(R):(S)b
Hisc
1.00
5.90 0.09
7.17 0.01
6.92 0.01
7.10 0.03
7.64 0.06
7.73 0.05
8.19 0.03
7.30 0.13
7.27 0.03
7.55 0.12
8.03 0.13
7.69 0.14
7.54 0.09
1
—
1.00
5.92 0.09
7.92 0.04
7.12 0.12
7.62 0.09
8.05 0.05
7.72 0.13
8.11 0.10
7.65 0.01
7.77 0.18
8.11 0.12
8.12 0.17
7.88 0.22
7.85 0.19
1
—
1.05
5.58
1.58
3.32
2.55
0.98
0.84
2.22
3.14
3.60
1.24
1.53
2.05
( )-53,13
(R)-5
(S)-5
0.87 0.01
1.01 0.16
1.07 0.17
0.87 0.05
0.85 0.02
0.99 0.01
0.82 0.01
0.65 0.04
0.87 0.01
0.56 0.05
0.62 0.04
0.71 0.05
18.81 3.77
10.48 2.08
15.87 3.36
54.78 13.01
67.02 15.44
193.85 40.73
25.15 8.83
23.59 4.99
44.84 15.56
134.04 47.97
62.07 23.38
43.60 12.63
1.03 0.06
1.01 0.10
1.19 0.17
1.11 0.16
0.90 0.02
0.93 0.00
0.98 0.00
0.86 0.00
0.99 0.02
0.82 0.03
0.80 0.04
0.90 0.14
100.00 21.61
15.83 5.24
50.21 14.53
133.33 30.50
62.83 22.30
155.84 46.42
53.10 10.68
70.59 31.94
153.85 51.60
157.89 69.87
90.22 48.27
85.11 40.56
1:1.5
0.1
1:3.2
1.1
( )-63,13
(R)-6
(S)-6
1:2.9
0.4
1:2.8
1.1
( )-74
(R)-7
(S)-7
1:1.9
0.5
1:2.2
1.1
( )-84
(R)-8
(S)-8
1.4:1
0.2
1.1:1
0.7
a
Steady state GTPase activity in Sf9 membranes expressing hH2R-Gs and gpH2R-Gs was determined as described.12 Reaction mixtures contained ligands at concen-
a
S
aS
trations from 1 nM to 10
l
M as appropriate to generate saturated concentration–response curves. Data were analysed by nonlinear regression and were best fit to sigmoidal
M) ranged
concentration–response curves. Typical basal GTPase activities ranged between ꢂ0.5 and 2.5 pmol/mg/min, and activities stimulated by histamine (His) (100
l
between ꢂ2 and 13 pmol/mg/min. The efficacy (Emax) of histamine was determined by nonlinear regression and was set to 1.0. The Emax values of other agonists were referred
to this value. Data shown are the mean SEM of two to three experiments or one experiment* performed in duplicate each. The relative potency of histamine was set to 1, and
the potencies of other agonists were referred to this value. The ratio of the EC50 values of H2R agonists for hH2R-Gs and gpH2R-Gs were also calculated.
a
S
aS
b
c
SEM of the eutomer/distomer ratio due to error propagation.
Histamine.
Table 3
Agonist/antagonist activities on recombinant human histamine H1, H3 and H4 receptors in GTPase assaysa
Compd
hH1R + RGS4
hH3R + G i2 + b1
c
2 + RGS4
Emax
hH4R-GAIP + G i2 + b1c2
a
a
pEC50 (pKB)
Emax
pEC50 (pKB)
pEC50 (pKB)
Emax
Histamine
( )-5
(R)-5
(S)-5
( )-6
(R)-6
(S)-6
( )-7
(R)-7
(S)-7
( )-8
(R)-8
(S)-8
6.72 0.02
4.88b 0.17
5.21 0.09
6.08 0.20
6.82 0.16
7.05 0.33
7.18 0.16
(<5)
(<5)
(<5)
(<5)
(<5)
1.00
0.35 0.05
0.34 0.03
0.53 0.05
0.54 0.05
0.41 0.04
0.51 0.05
—
—
—
—
—
—
7.89 0.07
(7.71)
(7.34 0.34)
(8.73 0.02)
(8.44)
(8.54 0.17)
(8.23)
(<5)
(<5)
(<5)
(5.35 0.04)
(<5)
(5.56 0.01)
1.00
—
—
—
—
—
—
—
—
—
—
—
—
8.04 0.18
7.82 0.20
7.64 0.24
8.58 0.23
8.03 0.11
8.08 0.14
8.34 0.05
(<5)
(<5)
(<5)
(5.22 0.04)
(<5)
(5.86 0.07)
1.00
0.89 0.02
0.85 0.08
0.82 0.04
0.51 0.09
0.57 0.03
0.59 0.01
—
—
—
—
—
—
(<5)
a
Membrane preparations of Sf9 insect cells expressing hH1R (co-expressed with RGS4),14 hH3R (co-expressed with G i2, Gb1 2 and RGS4),15 or hH4R-GAIP fusion protein co-
a
c
expressed with G and Gb1 were used.16 Data shown are means SEM of two to three experiments in duplicates each.
a
i2
c2
b
Data from Ref. 3.
the investigated imidazolylpropylguanidines turned out to be
weak partial hH1R agonists, potent hH3R antagonists as well as po-
tent partial hH4R agonists (Table 3). As for the results at the H2Rs,
there is a distinct preference of (S)-5 versus (R)-5 at the hH1R, hH3R
and hH4R, respectively, whereas the stereoselectivity of the cyclo-
hexyl-substituted analogues is weak and subtype-dependent.
In summary, there was no significant stereoisomeric preference
at histamine receptors detectable in this set of acylguanidine-type
H2R agonists. In contrast to the arpromidine series, where eudismic
ratios up to 40 (guinea pig atrium) were determined in favour of
the (S)-enantiomers, the present acylguanidines show very low
some compounds at recombinant histamine receptors. This work
was supported by the Graduate Training Program (Graduiertenkol-
leg) GRK 760, ‘Medicinal Chemistry: Molecular Recognition—Li-
gand–Receptor Interactions’, of the Deutsche Forschungsgeme-
inschaft.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.03.082.
eudismic ratios in the range of 1.1–3.2 (gpH2R-Gs S). Thus, the ste-
reochemistry of the acyl moiety plays only a minor role in this ser-
ies of H2R agonists.
a
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1H NMR (CD3OD), d (ppm): 8.76 (s, 1H, Im-2-H), 7.34 (s, 1H, Im-5-H), 7.30–7.10
(m, 5H, Ph-H), 3.32 (m, 3H, Ph-CHCH3 and NHCH2), 2.78 (m, 4H, CH2CO and Im-
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ꢃ
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