1,2,4-Benzothiadiazine derivatives as α1 and 5-HT 1A receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2004.12.004

A series of new 1,2,4-benzothiadiazine derivatives with an arylpiperazine mojety linked at position 3 of the heterocyclic ring were synthesized and assessed for their pharmacological profiles at α₁-adrenoceptor subtypes (α_1A, α₁

Full text of DOI:10.1016/j.bmcl.2004.12.004

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1185–1188
1,2,4-Benzothiadiazine derivatives as a1 and 5-HT1A
receptor ligands
Annalisa Tait,a,* Amedeo Luppi,a Silvia Franchini,a Elisa Preziosi,a
Carlo Parenti,a Michela Buccioni,b Gabriella Marucci,b Amedeo Leonardi,c
Elena Poggesic and Livio Brasilia
a
`
Dipartimento di Scienze Farmaceutiche, Universita degli Studi di Modena e Reggio Emilia, Via Campi 183, 41100 Modena, Italy
b
`
Dipartimento di Scienze Chimiche, Universita degli Studi di Camerino, Via S. Agostino 1, 62032 Camerino, Italy
cDivisione Ricerca e Sviluppo, Recordati S.p.A., Via Civitali 1, 20148 Milano, Italy
Received 24 September 2004; revised 30 November 2004; accepted 2 December 2004
Available online 24 December 2004
Abstract—A series of new 1,2,4-benzothiadiazine derivatives with an arylpiperazine mojety linked at position 3 of the heterocyclic
ringwere synthesized and assessed for their pharmacological profiles at a1-adrenoceptor subtypes (a1A, a1B and a1D) by functional
experiments and by in vitro bindingstudies at human cloned 5-HT 1A receptor. Compound 1 was identified as a novel a1D antagonist
(pKba1D = 7.59; a1D/a1A > 389; a1D/a1B = 135) with high selectivity over 5-HT1A receptor (5-HT1A/a1D < 0.01), while compound 6,
a 3,4-dihydro-derivative, was characterized as a novel 5-HT1A receptor ligand, highly selective over a1D-adrenoceptor subtype (pKi5-
HT1A = 8.04; 5-HT1A/a1D = 1096). Further pharmacological studies demonstrated that 6 is a partial agonist at 5-HT1A receptor
(Emax = 23, pD2 = 6.92).
Ó 2004 Elsevier Ltd. All rights reserved.
The existence of multiple adrenergic receptors is now
firmly established and this has given new impulse to
medicinal chemists for the search of new and more selec-
tive ligands.1 The a1A subtype has received much atten-
tion as a potential target for symptomatic treatment of
benign prostatic hyperplasia (BPH) and several urose-
lective agents have been disclosed.2 On the contrary a
potential therapeutic use for a1B and a1D subtype selec-
tive ligands has not been found yet.
lical transmembrane structure which shows some com-
6
mon features in their bindingsites.
As a consequence, a large number of compounds show
high affinity for both receptors and consequently poor
selectivity.
A widely accepted model of adrenergic and serotoniner-
gic ligands provides the presence of an arylpiperazine
moiety, a linker and a heterocycle system.
Receptor for serotonin (5-HT) represent also an hetero-
geneous population and seven distinct classes (5-HT1 to
5-HT7) have been identified accordingto structural
diversity and preferred effector mechanism.3 During
the last decade, the 5-HT1A subtype has been the most
studied as a major target for neurological research and
R
Het (CH2)n
N
N
The arylpiperazine represents the pharmacophore moi-
ety, while the selectivity 5-HT1A/a1-adrenergic receptors
is modulate by the type and the position of the substitu-
ent on the aromatic ringof the arylpiperazine moiety, by
the length of the spacer and by the nature of the hetero-
cycle system.7
4,5
drugdevelopment.
The a1 and 5-HT1A receptors belongto the class of G-
protein coupled receptors, characterized by an heptahe-
Keywords: 5-HT1A agonist; a1D antagonist; Benzothiadiazine;
Arylpiperazine.
On the bases of this model our research group has
undertaken a research project aimed at developingnew
and selective ligands in order to get new insight into
*
Correspondingauthor. Tel.: +39 059 2055133; fax: +39 059
371590; e-mail: tait.annalisa@unimore.it
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.12.004

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A. Tait et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1185–1188
SO2NH2
O
ide (1) and for the corresponding3,4-dihydro-derivative
(4).8
SO2NH2
NH2
(i)
(ii)
R
R
N
H
C
R
CH2Br
The compounds were converted into the corresponding
di-hydrochloride salts for pharmacological tests.
O
S
O
O
S
O
(iii)
NH
NH
N
The intermediates 2H- and 3,4-dihydro-2H-3-bromom-
ethyl-1,2,4-benzothiadiazines were prepared from the
corresponding2-aminobenzenesolfonamides by conden-
sation with bromoacetyl bromide and 2-bromomethyl-
1,3-dioxolane, respectively.
Br
N
OCH3
R
N
N
1 - 3
Scheme 1. Reagents and conditions: (i) bromoacethylbromide, THF
anhydrous, room temperature; (ii) 1,4-dioxane, H2SO4 (few drops),
reflux; (iii) 2-methoxyphenylpiperazine, acetonitrile, reflux.
Receptor subtype selectivity of compounds 1–6 was
determined at a1-adrenoceptors on different isolated tis-
sues usingBMY-7378 as standard compound. Blocking
activity was assessed by antagonism of (À)-noradrena-
O
S
O
SO2NH2
NH2
line-induced contraction of rat prostatic vas deferens
NH
(i)
(ii)
9
Br
(a1A
(a1D
)
[pD2 = 5.89 0.01 (n = 30)] or thoracic aorta
[pD2 = 7.85 0.01 (n = 30)] and by antagonism
R
R
R
N
H
10
)
of (À)-phenylephrine-induced contraction of rat spleen
O
S
O
11
(a1B
)
[pD2 = 4.95 0.01 (n = 30)]. The same com-
NH
N
pounds were also assessed for in vitro bindingaffinity
at 5-HT1A subtype receptor in cell membranes of HeLa
cells transfected with human cloned 5-HT1A receptor.12
N
OCH3
N
H
4 - 6
Scheme 2. Reagents and conditions: (i) 2-bromomethyl-1,3-dioxolane,
1,4-dioxane, H2SO4 (few drops), reflux; (ii) 2-methoxyphenylpiper-
azine, acetonitrile, reflux.
All compounds behave as competitive a1-adrenoceptor
antagonists, as they cause a parallel shift of agonist
dose–response curves, and 5-HT1A-ligands with varying
degrees of potency and selectivity (Table 1). The excep-
tions are compounds 1 and 2 at a1A-adrenoceptor for
which any effect was seen at the maximum concentration
tested (10À5 M). The best profile of activity at adrenergic
system resides in compound 1, which exhibits high a1D
the structural features that address selectivity toward
one receptor or the other. In this work we present the
synthesis and the pharmacological profile of a new series
of 1,2,4-benzothiadiazine-arylpiperazine and the effect
of some structural modifications on the affinity and
selectivity for a1-adrenergic and 5-HT1A serotoninergic
receptor subtypes.
affinity (pKb = 7.59) and relevant a1D selectivity (a1D
1A > 389; a1D/a1B = 135). Moreover it shows weak
affinity at 5-HT1A receptor (pKi < 6.0) and selectivity
at a1D-subtype of up to more than 30 folds (5-HT1A
1D < 0.03). The introduction of chlorine atom in posi-
/
a
/
a
The 2H-1,2,4-benzothiadiazine 1,1-dioxide derivatives
1–6 were synthesized, as outlined in Schemes 1 and 2,
by nucleophilic substitution with 2-methoxyphenylpip-
erazine from the opportune 2H-3-bromomethyl-1,2,4-
benzothiadiazines.
tion 6 of benzothiadiazine ring( 3) do not significantly
change the affinity at a1-adrenoceptor subtypes, while
a more than 10-fold increase is observed at 5-HT1A
receptor (pKi 5-HT1A = 7.33) and, as a consequence,
the selectivity 5-HT1A/a1 is strongly reduced. The intro-
duction of a methyl group in position 6 of benzothiadi-
azine ring( 2) is detrimental for a1D-subtype affinity
(pKb = 5.50), with consequent loss of selectivity among
A detailed synthetic procedure and final characteriza-
tion by 1H NMR, infrared spectroscopy and C,H,N
analysis are presented for 3-[4-(2-methoxy)phenylpi-
perazin-1-yl]methyl-2H-1,2,4-benzothiadiazine 1,1-diox-
a1-adrenoceptors (a1D/a1A > 3;
a1D/a1B = 0.28; a1B/
Table 1. Antagonist potency, expressed as pKb SEM values,a and selectivitiesb of compounds 1–6 at a1-adrenoceptors in isolated rat prostatic vas
deferens (a1A), spleen (a1B) and thoracic aorta (a1D
)
Compd
R
pKba1A
pKba1B
pKba1D
a
1D/a1A
a
1D/a1B
135
0.28
38
0.23
a
1B/a1A
pKi5-HT1A
5-HT1A/a1D
1
H
<5.0
<5.0
5.46 0.06
6.05 0.11
6.17 0.16
5.97 0.12
6.24 0.15
5.09 0.03
(7.48 0.09)
7.59 0.15
5.50 0.07
7.75 0.22
5.34 0.34
5.98 0.17
5.00 0.18
(8.40 0.09)
>389
>3
295
>3
>11
8
<6.0
6.97
7.33
7.78
<7
<0.03
29.5
0.38
2
CH3
Cl
3
5.28 0.23
5.81 0.09
6.00 0.10
5.05 0.06
(7.01 0.08)
4
H
0.35
1.5
2
275
<10
5
CH3
Cl
0.95
1
0.55
1
6
1
0.3
8.04
8.90
1096
3.16
BMY-7378c
25
8
Bindingdata, expressed as p Ki values, of the same compounds at human cloned 5-HT1A receptor and selectivity ratio 5-HT1A/a1D
.
17
a pKb values were calculated accordingto van Rossum at one or two concentrations.
b Antilogof the difference between the p Kb values for a1A, a1B and a1D adrenergic receptor subtypes.
c The antagonist potency of standard compound BMY-7378 is expressed as pA2 values.

A. Tait et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1185–1188
1187
a
1A > 11) and also respect to serotoninergic system (5-
outstandingfor its affinity and selectivity at a1D being
one of the most selective antagonist for this subtype,
at least in functional studies. Conversely compound 6
is outstandingfor its affinity and selectivity for 5-
HT1A receptor. More extensive structure–activity rela-
tionship studies are in progress and will be reported in
due course.
HT1A/a1D = 29.5). The 3,4-dihydro derivatives (4–6) ex-
hibit a weak affinity at a1-adrenoceptor with a signifi-
cant decrease of pKb values at a1D subtype, but they
show an interestingaffinity and selectivity at 5-HT
1A
receptor. This findingcould be explained considering
the minor steric restriction of 5-HT1A respects to a1
receptor;13,14 this might favour the interaction of more
distorted structure of 3,4-dihydroderivatives (4–6) re-
spects to unsaturated ones (1–3). Respect to its analogue
1, the unsubstituted 3,4-dihydro-compound 4 presents
lower affinity at a1D-subtype (pKb = 5.34) and higher
affinity at 5-HT1A receptor (pKi = 7.78) with a selectivity
ratio 5-HT1A/a1D of 275 folds.
Acknowledgments
This work was supported by grants from the University
of Modena and Reggio Emilia, the University of Cam-
erino and MIUR.
Introduction of methyl group in position 6 of benzothi-
adiazine ring( 5) causes a decrease of affinity and selec-
References and notes
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´
´
´
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8. Selected compounds were prepared as follows.
3-4-(2-Methoxy)phenylpiperazin-1-ylmethyl-2H-1,2,4-
benzothiadiazine-1,1-dioxide (1). To a solution of 3-
bromomethyl-2H-1,2,4-benzothiadiazine 1,1-dioxide (0.20 g,
0.7 mmol) in acetonitrile (5 mL) 2-methoxyphenylpiper-
azine (0.27 g, 1.4 mmol) was added under stirring at the
temperature of 80 °C and the reaction mixture was
refluxed for 1 h. After coolingat room temperature the
residual 2-methoxyphenylpiperazine hydrobromide was
removed and acetonitrile was evaporated in vacuo giving a
residue which was purified by crystallization from ace-
tone–petroleum ether 60–80 °C (0.24 g, yield 89% d.t.), mp
The maximal stimulation of [35S]GTPcS binding( Emax
)
achieved for 6 was 23 and the concentration required to
obtain 50% of Emax (pD2) was 6.92. The results obtained
in the [35S]GTPcS bindingstudies suegst that com-
pound 6, with a potency lower than that of the reference
compound BMY 7378, reflectingtherefore the moderate
affinity values found on bindingstudies, can be consid-
ered a partial agonist at 5-HT1A receptor.
1
205–206 °C. H NMR (DMSO-d6, 200 MHz) d 11.8 (1H,
s, D2O changeable), 7.79 (1H, dd, J = 7.5, 1.4 Hz), 7.68
(1H, ddd, J = 7.7, 7.0, 1.4 Hz), 7.50 (1H, dd, J = 7.7,
1.2 Hz), 7.45 (1H, ddd, J = 7.5, 7.0, 1.2 Hz), 6.95–6.85
(4H, m), 3.76 (3H, s), 3.47 (2H, s), 3.02 (4H, m), 2.70 (4H,
m); IR(Nujol): 3193, 1514, 1172, 1137, 759 (cmÀ1). Anal.
(C19H22N4O3S) C, H, N.
In conclusion, we have discovered a new class of a1-
adrenoceptor and 5-HT1A ligands bearing a benzothi-
adiazine-arylpiperazine structure. Adequate structural
modifications address the selectivity toward the receptor
subtype a1D or 5-HT1A. In particular compound 1 is
The free amine was transformed into the corresponding
di-hydrochloride salt, which was crystallized from meth-
anol/petroleum ether 60–80 °C; mp 230 °C.
3-[4-(2-Methoxy)phenylpiperazin-1-yl]methyl-3,4-dihydro-
2H-1,2,4-benzothiadiazine-1,1-dioxide (4) was prepared
with the same procedure of compound 1, startingfrom 3-
bromomethyl-3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-
dioxide and it was purified by silica gel column chroma-
tography using cyclohexane/ethyl acetate (1:1, v/v) and
then chloroform/acetone (9.5:0.5, v/v) as eluant; (0.052 g,
yield 19% d.t.), mp 132–135 °C (acetone–petroleum ether
60–80 °C), 1H NMR (DMSO-d6, 200 MHz) d 7.51 (1H, s,
Table 2. Potency (pD2) and relative effectiveness (Emax)a values in the
agonist-induced [35S]GTPc S-bindingassay at human 5-HT 1A receptor
Compd
pD2
Emax
6
BMY-7378
6.92
9.27
23
26
a Maximal stimulation expressed as a percentage of the maximal 5-HT
response.

1188
A. Tait et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1185–1188
D2O changeable), 7.44 (1H, dd, J = 7.3, 1.6 Hz), 7.28 (1H,
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(1H, ddd, J = 8.0, 7.3, 1.0 Hz), 4.87 (1H, m), 3.77 (3H, s),
3.00 (4H, m), 2.60 (6H, m); IR(Nujol): 3342, 3232, 1605,
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di-hydrochloride salt, which was crystallized from meth-
anol/petroleum ether 60–80 °C; mp 105 °C.
´
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