alpha1-Adrenoceptor antagonists. 5. Pyridazinone-arylpiperazines. Probing the influence on affinity and selectivity of both ortho-alkoxy groups at the arylpiperazine moiety and cyclic substituents at the pyridazinone nucleus.

Article abstract of DOI:10.1016/S0960-894X(02)00932-0

Our previous work on pyridazinone-arylpiperazine derivatives suggested some structural features that a compound should have to show high affinity and good selectivity for alpha(1) adrenoceptors (AR) with respect to alpha(2)-AR. Accordingly, two classes of new alkoxyphenylpiperazinylheptylpyridazinones were designed and synthesized to evaluate the effect of the alkoxy substituent on affinity and selectivity. As expected, affinity increased with larger alkoxy groups. Affinity values are all comparable with that of the reference compound (prazosin), with the exception of compound 1c found 4.5-fold more active than prazosin.

Full text of DOI:10.1016/S0960-894X(02)00932-0

Bioorganic & Medicinal Chemistry Letters 13 (2003) 171–173
ꢀ1-Adrenoceptor Antagonists. 5. Pyridazinone-arylpiperazines.
Probing the Influence on Affinity and Selectivity of Both
ortho-Alkoxy Groups at the Arylpiperazine Moiety and
Cyclic Substituents at the Pyridazinone Nucleusy
Laura Betti,a Monia Floridi,b Gino Giannaccini,a Fabrizio Manetti,c,*
Giovannella Strappaghetti,b,* Andrea Tafic and Maurizio Bottac
a
`
Dipartimento di Psichiatria, Neurobiologia, Farmacologia e Biotecnologie, Universita di Pisa, Via Bonanno 6, 56126, Pisa, Italy
b
`
Istituto di Chimica e Tecnologia del Farmaco, Universita di Perugia, Via del Liceo 1, 06123 Perugia, Italy
`
Dipartimento Farmaco Chimico Tecnologico, Universita degli Studi di Siena, Via Aldo Moro, 53100 Siena, Italy
c
Received 9 August 2002; revised 22 October 2002; accepted 24 October 2002
Abstract—Our previous work on pyridazinone–arylpiperazine derivatives suggested some structural features that a compound
should have to show high affinity and good selectivity for a1 adrenoceptors (AR) with respect to a2-AR. Accordingly, two classes of
new alkoxyphenylpiperazinylheptylpyridazinones were designed and synthesized to evaluate the effect of the alkoxy substituent on
affinity and selectivity. As expected, affinity increased with larger alkoxy groups. Affinity values are all comparable with that of the
reference compound (prazosin), with the exception of compound 1c found 4.5-fold more active than prazosin.
# 2002 Elsevier Science Ltd. All rights reserved.
In recent years, the search for new selective a1-AR
antagonists has intensified, mainly due to their impor-
tance in the treatment of hypertension2ꢀ4 and benign
prostatic hyperplasia.5ꢀ7 In this context, goal of our
research was to discovery and develop novel adreno-
ceptor antagonists characterized by high affinity for
a1-AR and, possibly, selectivity toward a1 receptor with
respect to a2-AR.
seven carbon atoms the polymethylene spacer between
the pyridazinone and arylpiperazine moieties of piper-
azinylalkylpyridazinone derivatives. Moreover, placing
a methoxy group at the ortho position of the phenyl-
piperazine moiety, in conjunction with a seven carbon
atom spacer, led to the best a1 affinity profile. On the
other hand, a2/a1 selectivity is mainly dependent on
the terminal molecular fragment directly linked to the
pyridazinone ring. In fact, although 2a showed high
affinity toward a1-AR (1.4 nM) without any appreciable
selectivity, 1a exhibited an affinity of 1.9 nM and an
interesting a2/a1 ratio of 274.
Building on the results from our previous work,1 new
compounds that have an ortho-substituted phenylpiper-
azinylheptyl pyridazinone moiety as a common chemical
scaffold, which allows for variations to be introduced on
the terminal moiety linked to the pyridazinone nucleus
and on the size of the ortho alkoxy substituent, were
designed and prepared for study.
These findings led us to conclude that the ortho position
is a crucial key for improving the a1-AR antagonist
properties in terms of affinity and selectivity, which are
also strictly dependent on the polymethylene chain
length and on the terminal cyclic fragment.1,8
It was recently reported by us1 that a gradual increase in
affinity may be obtained by lengthening from two up to
Based on these considerations, compounds 1a and 2a
(Table 1 and Scheme 1) were in turn used as a template
to design the remaining compounds 1b,c and 2b–e. As a
consequence, a heptyl spacer was maintained in the
piperazine-pyridazinone system, and alkoxy moieties
*Corresponding authors. Tel.: +39-0577-234307; fax: +39-0577-234333;
e-mail: manettif@unisi.it (F. Manetti); tel.: +39-075-585-5136; fax: +39-
075-585-5129; e-mail: noemi@unipg.it (G. Strappaghetti).
yA part of our research in this field was previously reported in ref 1.
0960-894X/03/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00932-0

172
L. Betti et al. / Bioorg. Med. Chem. Lett. 13 (2003) 171–173
Scheme 1.
larger than a methoxy group were substituted at the ortho
position of the phenyl ring, in agreement with a pharma-
cophore model for a1-AR antagonists1 suggesting that
hydrophobic groups larger than a methoxy substituent
can be accommodated by a hydrophobic pocket where
the substituted phenyl ring bound to the piperazine lies.
Moreover, to probe the influence of the terminal cyclic
substituent on a1-AR affinity and selectivity, a fur-
oylpiperazine or a phenoxyethylpiperazine moiety was
placed at the 5-position of the pyridazinone nucleus.
place [3H]prazosin, [3H]rauwolscine, and [3H]8-OH-
DPAT from a1-AR, a2-AR, and 5-HT1A, respectively)
on rat cerebral cortex. Moreover, in order to determine
the intrinsic activity of 1c and 2e (the furoyl and
phenoxyethyl derivatives found to have the best affinity
profile toward a1-AR), competitive binding studies were
performed in the presence and in the absence of 1 mM
GTP using the radiolabeled antagonist prazosin. The
GTP shift values of the selected compounds (0.8 for 1c,
1.7 for 2e, and 1.2for the reference compound prazosin)
are indicative of an antagonist profile as prazosin.
Compounds 1b,c and 2b–e were synthesized as outlined
in Scheme 1. A mixture comprised of 4,5-dichloropyr-
idazin-3(2H)-one and the appropriate 1-substituted
piperazine was refluxed in ethanol and Et3N for 15 h to
afford intermediates 3a–c that were in turn transformed
into 4a–c by treating with 1,7-dibromoheptane in ace-
tone and potassium carbonate (method A). Compounds
1b,c were obtained from 4c with 1-(2-ethoxy-
phenyl)piperazine or 1-(2-isopropoxyphenyl)piperazine,
respectively,9,10 in isoamyl alcohol and sodium carbon-
ate (method B). Following method B, compound 2c was
prepared from 4a with 1-(2-ethoxyphenyl)piperazine.
Similarly, compounds 2b,d,e were obtained starting from
4b with 1-(2-methoxyphenyl)piperazine, 1-(2-ethoxy-
phenyl) piperazine, and 1-(2-isopropoxyphenyl)-
piperazine, respectively. Chemical and physical data of
compounds 1a–c and 2b–e are reported in Table 1.11
All the newly synthesized compounds were found to
have a subnanomolar affinity (data listed in Table 2)
toward a1-AR that is comparable to the affinity of
prazosin. As expected, replacing the ortho methoxy
substituent on the phenylpiperazine moiety of 1a with
larger alkoxy groups, enhanced affinity. In fact, while
compound 1b showed an improvement of about 4-fold
in affinity with respect to the methoxy counterpart, affi-
nity of 1c was more than 40-fold higher than affinity of
1a. Interestingly, compound 1c, bearing the isopropoxy
substituent, showed an affinity about 4.5-fold higher
than the reference compound prazosin. Analogous con-
siderations could be made for compounds 2. In fact, 2c
Table 2. a1 And a2-adrenoceptor binding affinities of compounds 1–2
Compd R1
R2
Ki, nMa
a2-AR
The pharmacological profile of the new compounds was
evaluated by radioligand binding assays (ability to dis-
a1-AR
a2/a1 5-HT1A
c
1ab
1b
1c
MeO
EtO
iPrO
MeO MeO
MeO EtO
1.9ꢂ0.1
520.1ꢂ47.422
ꢂ0.28
ND
ND
c
0.50ꢂ0.024.0
Table 1. Chemical and physical data of compounds 1 and 2
0.052ꢂ0.007 0.56ꢂ0.19
11 0.80ꢂ0.23
2ab
2b
2c
1.4ꢂ0.1
4.6ꢂ0.5
1.6ꢂ0.1
8.2ꢂ0.3
2.0ꢂ0.25
3.2ꢂ0.1
3
3
NDc
0.16ꢂ0.06
NDc
Compd
R1
R2
Formula
Mp (ꢁC)
Yield (%)
0.55ꢂ0.10
1aa
1b
1c
MeO
EtO
iPrO
MeO
MeO
EtO
C31H41ClN6O4
C32H43ClN6O4
C33H45ClN6O4
C35H49ClN6O4
C36H51ClN6O4
C36H51ClN6O4
C37H53ClN6O4
C38H55ClN6O4
128–130b
150–155c
125–129d
125–128d
65–70c
70
40
30
75
40
45
60
40
EtO MeO 0.58ꢂ0.15
14
2 0.2
2d
2e
Pd
Rd
Dd
EtO EtO
iPrO EtO
0.43ꢂ0.08
0.26ꢂ0.07
0.24ꢂ0.05
120.82 ꢂ0.18
2aa
2b
2c
MeO
EtO
MeO
EtO
EtO
4.0ꢂ0.3
71–73d
2.0ꢂ0.2
2d
2e
EtO
iPrO
50–55e
61–66b
aValues are means ꢂ standard deviation of three binding experiments,
calculated according to the equation Ki=IC50/(1+[radioligand]/Kd).12
bCompounds reported elsewhere by our research group.1 Compounds
1b,c and 2b–e have been submitted to an Italian patent.13
cND: not determined.
dP, R, and D represent prazosin, rauwolscine, and 8-OH-DPAT,
respectively.
aCompounds described elsewhere by our research group.1
bAs dihydrochloride.
cAs trihydrochloride dihydrate.
dAs trihydrochloride.
eAs trihydrochloride monohydrate.

L. Betti et al. / Bioorg. Med. Chem. Lett. 13 (2003) 171–173
173
is about 2-fold more active with respect to the corre-
sponding methoxy counterpart 2a. Finally, compounds
2b and 2d,e, bearing an ethoxy substituent on the phenyl
ring opposite to the phenylpiperazine moiety, also
showed improved affinity with respect to 2a and com-
parable with that of 2c. As expected, compound 2e, with
the largest R1 substituent, was characterized by the best
a1-AR affinity profile among compounds 2. Structure–
activity relationships (SARs) of such compounds, in
addition to validate the pivotal role of the alkoxy sub-
stituent in influencing affinity toward a1-AR, suggested
that variation on the size of the terminal aryl substituent
attached to the pyridazinone nucleus affects affinity
toward a1-AR, in agreement with previous findings
reported by our group.1,8
be the optimal spacer to bring both the pyridazinone and
the piperazine ring at the right distance to interact with the
receptor.
Additional studies are ongoing to further evaluate the
influence of the terminal molecular portions on a1-AR
affinity and selectivity and will be reported in due time.
Acknowledgements
Financial support provided by the Italian Ministero
dell’Istruzione, dell’Universita e della Ricerca Scientifica
(Project ‘Progettazione e Sintesi di Agenti Neuro-
protettivi’) and Italian Research National Council
(CNR) ‘Progetto Finalizzato Biotecnologie’ (CNR Tar-
get Project on ‘Biotechnology’) is gratefully acknowl-
edged. One of us (M.B.) thanks the Merck Research
Laboratories for the 2002 Academic Development Pro-
gram (ADP) Chemistry Award.
Regarding the a2-AR affinity profile, compounds 1
showed a trend similar to that found for a1 affinity. In fact,
higher affinity was associated with bulkier alkoxy sub-
stituent at the ortho position of the arylpiperazine system.
However, the opposite trend was found for compounds 2.
For example, 2a showed an affinity (4.6 nM) of about
2-fold higher than the corresponding ethoxy derivative 2c
(8.2nM). Similarly, a decrease in a2 affinity was observed
by replacing the methoxy substituent of 2b (1.6 nM) with
an ethoxy (2d, 2.0 nM) or isopropoxy group (2e, 3.2nM).
References and Notes
1. Barbaro, R.; Betti, L.; Botta, M.; Corelli, F.; Giannaccini,
G.; Maccari, L.; Manetti, F.; Strappaghetti, G.; Corsano, S.
J. Med. Chem. 2001, 44, 2118.
It is interesting to note that 1a was the sole compound
characterized by a good a2/a1 selectivity profile. In fact,
none of the reported compounds showed significant
selectivity for a1-AR with respect to a2-AR, the highest
a2/a1 ratio being 14 in compound 2c. This last finding
suggested that the bulkiness of the alkoxy group, while
positively affects the affinity toward both a1 and a2-AR,
leads in any case to a2/a1 unselective compounds. As an
example, the enhanced a1 and a2-AR affinity of 1b and
1c with respect to 1a, produces compounds with very
low selectivity (8 and 11 for 1b and 1c, respectively).
2. Russell, R. K.; Press, J. B.; Rampulla, R. A.; McNally, J. J.;
Falotico, R.; Keiser, J. A.; Bright, D. A.; Tobia, A. J. Med.
Chem. 1988, 31, 1786.
3. Chern, J.-W.; Tao, P.-L.; Yen, M.-H.; Lu, G.-Y.; Shiau,
C.-Y.; Lai, Y.-J.; Chien, S.-L.; Chan, C.-H. J. Med. Chem.
1993, 36, 2196.
4. Chern, J.-W.; Tao, P.-L.; Wang, K.-C.; Gutcait, A.; Liu,
S.-W.; Yen, M.-H.; Chien, S.-L.; Rong, J.-K. J. Med. Chem.
1998, 41, 3128.
5. Bock, M. G.; Patane, M. A. In Annual Reports in Medicinal
Chemistry; Doherty, A. M., Ed.; Academic: New York, 2000;
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6. Forray, C.; Noble, S. A. Exp. Opin. Invest. Drugs 1999, 8,
2073.
7. Kenny, B.; Ballard, S.; Blagg, J.; Fox, D. J. Med. Chem.
1997, 40, 1293.
Similarly, all compounds evaluated for their affinity
toward 5-HT1A exhibited values in the subnanomolar
range without 5-HT1A/a1 selectivity, 15 being the most
interesting 5-HT1A/a1 ratio found for compound 1c.
8. Betti, L.; Botta, M.; Corelli, F.; Floridi, M.; Giannaccini,
G.; Maccari, L.; Manetti, F.; Strappaghetti, G.; Tafi, A.;
Corsano, S. J. Med. Chem. 2002, 45, 3603.
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C. B.; Kesslick, J. M.; Baldy, W. J.; Shank, R. P.; DiStefano,
D. L.; Fedde, C. L.; Scott, M. K. J. Med. Chem. 1989, 32,
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Jordan, A. D.; Maryanoff, B. E.; Maryanoff, C. A.; McDon-
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11. C, H, N analysis of the new compounds gave results
within ꢂ0.4% of theoretical values. 1H NMR spectra are
consistent with the assigned structures of compounds 1b,c and
2b–e.
12. Cheng, Y.-C.; Prusoff, W. H. Biochem. Pharmacol. 1973,
22, 3099.
In conclusion, based on suggestions derived from our
previous work in the field of a1-AR antagonists, a
number of novel arylpiperazine-pyridazinone-containing
compounds were designed, synthesized and evaluated
for their biological properties. As a result, each of them
was found to have a high affinity for a1-AR. Moreover,
the hypothesis that an ortho substituent larger than a
methoxy group (up to a isopropoxy moiety) may sig-
nificantly improve affinity toward a1-AR in the arylpiper-
azine series, was confirmed by SAR studies. On the other
hand, replacement of the methoxy group with ethoxy or
even larger substituents differently affected a2 affinity in the
furoyl and phenoxyethyl series of compounds, in any case
leading to molecules without appreciable selectivity with
respect to a2-AR and 5-HT1A. Moreover, taking into
account the excellent affinity data of all the new com-
pounds prepared, a seven-carbon atom chain appeared to
13. Betti, L.; Botta, M.; Floridi, M.; Giannaccini, G.; Man-
etti, F.; Strappaghetti G. Italian Patent RM2002A000362, July
5, 2002.