1,2,5-Thiadiazole derivatives are potent and selective ligands at human 5-HT1A receptors

Article abstract of DOI:10.1016/S0960-894X(01)00151-2

Amino acid derivatives of 1,2,5-thiadiazol-3-yl-piperazine related to (+)-WAY-100135 and WAY-100635 are potent 5-HT_1A receptor agonists and antagonists, which have selective affinity for 5-HT_1A receptors versus α and dopamine (D₂, D₃, and D₄) receptors.

Full text of DOI:10.1016/S0960-894X(01)00151-2

Bioorganic & Medicinal Chemistry Letters 11 (2001) 1069–1071
1,2,5-Thiadiazole Derivatives Are Potent and Selective Ligands
at Human 5-HT1A Receptors
Annmarie L. Sabb,a,* Robert L. Vogel,a Michael G. Kelly,b Yvette Palmer,a
c
Deborah L. Smith,c Terrance H. Andreec andLee E. Schechter
aMedicinal Chemistry, Chemical Sciences, Wyeth-Ayerst Research, CN8000, Princeton, NJ 08543, USA
bMedicinal Chemistry, Amgen Inc., Thousand Oaks, CA 91320, USA
cWyeth Neuroscience, Wyeth-Ayerst Research, CN8000, Princeton, NJ 08543, USA
Received5 December 2000; accepted23 February 2001
Abstract—Amino acid derivatives of 1,2,5-thiadiazol-3-yl-piperazine related to (+)-WAY-100135 and WAY-100635 are potent
5-HT1A receptor agonists andantagonists, which have selective affinity for 5-HT 1A receptors versus a1 anddopamine (D 2, D3, and
D4) receptors. # 2001 Elsevier Science Ltd. All rights reserved.
The serotonin 5-HT1A receptor has been implicatedin
the regulation of cognition, psychosis, feeding/satiety,
temperature regulation, anxiety, depression, sleep, pain
perception, andsexual activity. The edvelopment of
non-benzodiazepine anxiolytics, such as buspirone 1, a
partial agonist at 5-HT1A receptors, has substantiated
the correlation between serotonin andanxiety. In addi-
tion, there has been a recent report that flesinoxan 2, a
5-HT1A full agonist, is effective in generalizedanxiety
disorder in humans. On the contrary, the 5-HT1A
receptor antagonists, (+)-WAY-100135 3 andWAY-
100635 4, show limitedanxiolytic activity. Insteadthese
compounds have demonstrated robust activity in a
number of preclinical models of cognitive impairment
associated with Alzheimer’s disease. Harder et al.1
reportedthat WAY-100635 alleviates impairments
causedby idzocilipine (MK-801), a noncompetitive
NMDA antagonist, in monkeys. Similarly, Boast et al.2
foundthat WAY-100635 significantly reducedthe cog-
nitive impairment causedby MK-801, in a edlayed
nonmatch to sample radial arm maze task in rats. Fur-
thermore, Carli et al.3 showedthat post-training
administration of WAY-100635 was able to reverse a
learning deficit induced in rats by administration of
scopolamine, a cholinergic antagonist, in an autoshaping
learning task (Fig. 1).
In an effort to discover novel 5-HT1A receptor antago-
nists in the arylpiperazine family having improved
bioavailability anda longer biological half-life than
(+)-WAY-100135 andWAY-100635, it was proposed
that compounds containing a 1,2,5-thiadiazolepiper-
azine wouldserve as a bioisosteric replacement for those
containing the ortho-methoxyphenylpiperazine (OMPP)
moiety andhence retain 5-HT
affinity. It was also
1A
reasonedthat such compounds wouldbe less susceptible
to oxidative metabolism of the thiadiazole ring than the
metabolism typically seen with those containing OMPP.
We now report that unlike OMPP, 1,2,5-thiadiazole-
piperazine, itself, has no affinity for the human 5-HT1A
receptor transfectedinto Chinese hamster ovary (CHO)
cells. However, amino acid2-aryl substitutedethyla-
mide derivatives of 1,2,5-thiadiazol-3-yl-piperazines are
potent 5-HT1A receptor agonists andantagonists, which
demonstrate selective affinity for 5-HT1A receptors ver-
sus a1 anddopamine (D 2, D3, andD ) receptors.
4
*Corresponding author. Tel.: +1-732-274-4411; fax: +-1-732-274-
4505; e-mail: sabba@war.wyeth.com
Figure 1. Known 5-HT1A partial agonsits, agonists, andantagonists.
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00151-2

1070
A. L. Sabb et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1069–1071
In this paper we describe the synthesis and structure–
activity relationships of a series of phenylalanine and3-
pyridylalanine derivatives of 4-substituted [1,2,5]thia-
diazolepiperazines and discuss their structure–activity
relationships in relation to their affinity andintrinsic
activity at human 5-HT1A receptors. In addition, the
selectivity versus a1 anddopamine receptors for specific
compounds will be reported.
as DCC, to give [1,2,5]thiadiazolepiperazine derivatives
10 and 11.
Compounds 10a–d were preparedusing R and S phe-
nylalanine. The compounds were evaluated for 5-HT1A
binding affinity by measuring their ability to displace
[3H]8-OH-DPAT from CHO cells stably transfected
with human 5-HT1A receptors according to the proce-
dure of Dunlop et al.5 Their intrinsic activity was
determined by measuring the compounds’ ability to
reverse the forskolin-stimulatedturnover of cAMP in
CHO cells.5 (% Emax=0 for antagonists in this assay)
Results of these studies are summarized in Table 1.
The [1,2,5]thiadiazol-3-yl-piperazine compounds shown
in Tables 1–3 were preparedas outlinedin Scheme 1.
Table 1. 5-HT1A in vitro profile of 4-chloro[1,2,5]thiadiazol-3-yl
piperazines
2
CompdR
R3
Stereo
5-HT1A
Ki (nM)
c-AMP
%Emax
10a
10b
H
Cyclohexyl
S
0.8
4.5
93.0
EC50=4.6 (nM)
0
IC50=49.3 (nM)
NTa
CH3 Cyclohexyl
R
10c
10d
H
H
4-Pyridyl
2-Pyridyl
S
S
425
47%@1 (mM)
NT
aNT=not tested.
Compounds 11a–d were preparedusing
N-Boc-N-
methyl-R-phenylalanine or N-Boc-R-3-pyridylalanine.
Results of their evaluation for 5-HT1A binding affinity
andintrinsic activity are summarizedin Table 2.
Scheme 1. 1-Benzyl-2-[4-chloro-[1,2,5]thiadiazol-3-yl-piperazin-1-yl]-
ethylamides 10 and3-pyryidl-2-[4-methoxy-[1,2,5-thiaiadzol-3-yl]-
ethylamides 11.4 (a) Na metal, MeOH, N2, reflux; (b) DMF, N2,
reflux; (c) 4 N HCl, dioxane; (d) N-Boc-amino acid, TEA, DCC,
HOBt, CH2Cl2; (e) 4 N HCl, dioxane; (f) 1 M BH3 in THF; (g)
R3COCl or R3COOH plus a coupling reagent. Intermediates 7, 8, and
9 can be usedwithout purification. Yields are averages.
Table 2. 5-HT1A affinity andantagonist activity of 4-methoxy[1,2,5]-
thiadiazol-3-yl piperazines
Commercially available 3,4-dichloro-[1,2,5]thiadiazole
was treatedwith N-Boc-piperazine in refluxing DMF
under a nitrogen atmosphere to provide the protected 4-
chloro-3-piperazinyl-[1,2,5]thiadiazole 5. Alternatively,
treatment of 3,4-dichloro-[1,2,5]thiadiazole with sodium
methoxide in methanol gave 3-chloro-4-methoxy-
[1,2,5]thiadiazole. Reaction of this thiadiazole with N-
Boc piperazine gave the corresponding compound 5
where R=OCH3. Compound 5 was deprotected by
stirring overnight at room temperature in 4 N HCl in
dioxane. Coupling of [1,2,5]thiadiazolepiperazine 5 with
N-Boc protected amino acids in methylene chloride at
room temperature using the coupling agent, DCC, in
the presence of triethylamine and1-hryodxy-
2
CompdR
R3
X
5-HT1A
Ki (nM)
c-AMP
%Emax
11a
11b
CH3
CH3
Cyclohexyl
Phenyl
CH
CH
1.5
9.9
0
IC50=72.7 (nM)
0
IC50=6.4 (nM)
NTa
11c
11d
CH3 N-Morpholino CH 46%@1 (mM)
1-Methylcyclohexyl 3.0
H
N
0
IC50=113 (nM)
aNT=not tested.
benzotriazole hydrate gave amide 7. Removal of the N-
BOC protecting group with 4 N HCl in dioxane at room
temperature gave amino amide 8, which was reduced
with 1 M BH3 in THF to give amine 9. Amine 9 was
treatedwith a carboxylic acidchloride or with a car-
boxylic acidin the presence of a coupling reagent, such
Studies using both R and S phenylalanine suggest that
ligandbinding to the 5-HT receptor is not dependent
1A
upon stereochemistry in this series. However, even
though both stereoisomers have potent affinity for the
3
receptor, when R2 is H, andR is cyclohexyl (10a) the

A. L. Sabb et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1069–1071
Table 3. a1 and dopamine affinity for selected compounds
1071
IC50 (nM)
D3
2
CompdR
R
R3
X
Stereo
a1 %inhib. @ 100 (nM)
D2
D4
10a
10b
11a
11b
11d
Cl
Cl
OCH3
OCH3
OCH3
H
Cyclohexyl
Cyclohexyl
Cyclohexyl
Phenyl
CH
CH
CH
CH
N
S
17
0
0
15
NTa
NT
11%@100nM
NT
3516
NT
NT
NT
NT
489
NT
NT
NT
NT
CH3
CH3
CH3
H
R
R
R
R
1-Me-Cyclohexyl
1102
aNT=not tested.
2
compoundis a partial agonist whereas when R
is
determined. Both R and S enantiomers of the phenyla-
lanine derivatives had affinity for the 5-HT1A receptor
andselectivity versus a1.
3
methyl andR is cyclohexyl (10b) the compoundis an
antagonist. In additon, it was observed that binding
affinity is remarkably sensitive to the nature of the
amide group as demonstrated by compounds 10c and
10d. Since it was our objective to develop potent and
selective 5-HT1A antagonists, we focusedour SAR
study on analogues of 10b in which there was steric
crowding around the amide portion of the molecule.
The methoxy derivatives were selected for study, since
both (+)-WAY-100135 andWAY-100635 contain a
methoxyarylpiperazine group. Compounds 11a,b,d
demonstrate that either a methyl group at R2 or a
1-methylcyclohexyl group at R3 give potent 5-HT1A
antagonists. Compound 11b shows that a phenyl amide
but not pyridyl amides (10c and 10d) are toleratedby
the receptor. Compound 11c, in which R3 is morpho-
line, lost affinity for the 5-HT1A receptor.
R enantiomers of the 3-pyridylalanine derivatives had
affinity for the 5-HT1A receptor andselectivity for both
a1 anddopamine receptors. ( S enantiomers were not
tested.) Steric crowding in the amide region of either
series of molecules favoredantagonism. The antagonist
with the best profile is 11a which has a Ki=1.5 nM at
the human 5-HT1A receptor, functions as a full antago-
nist (no effect in the c-AMP assay), andhas no affinity
at a1 anddopamine receptors.
References and Notes
1. Harder, J. A.; Ridley, R. M. Neuropharmacology 2000, 39,
547.
2. Boast, C.; Bartolomeo, A. C.; Morris, H.; Moyer, J. Neu-
robiol. Learn. Mem. 1999, 71, 259.
3. Carli, M.; Silva, S.; Balducci, C.; Samanin, R. Neuro-
pharmacology 1999, 38, 1165.
4. All final compounds were characterized by C, H, and N
microanalysis, NMR, IR, andmass spectrometry. For experi-
mental details see Sabb, A. L.; Vogel, R. L.; Kelly, M. G.;
Palmer, Y. L.; Childers, W. E. WO 0051999, 2000; Chem.
Abstr. Number 133:222746 AN 2000:628129.
In order to evaluate the selectivity of compounds 10 and
11, representatives of each type were testedin receptor
binding assays for affinity at a16 anddopamine (D 2, D3,
andD ) receptors.7 Results of these binding assays are
4
summarizedin Table 3. In each case examined, signif-
icant selectivity for the 5-HT1A receptor versus a1 and
dopamine receptors was observed.
5. Dunlop, J.; Zhang, Y.; Smith, D. L.; Schechter, L. J.
Pharmacol. Tox. Meth. 1998, 40, 47.
6. Morrow, A. L.; Creese, I. Mol. Pharmacol. 1986, 29, 321.
7. Mewshaw, R. E.; Nelson, J. A.; Shah, U. S.; Shi, X.;
Mazandarani, H.; Coupet, J.; Marquis, K. L.; Brennan, J. A.;
Andree, T. H. Bioorg. Med. Chem. Lett. 1999, 9, 2593.
In summary, a series of phenylalanine and3-pyr-
idylalanine derivatives of 4-substituted [1,2,5]thiadiazole-
piperazines was synthesizedandevaluatedfor affinity
andintrinsic activity at human 5-HT
receptors.
1A
Selectivity versus a1 anddopamine receptors was also