3-[2-(Pyrrolidin-1-yl)ethyl]indoles and 3-[3-(Piperidin-1- yl)propyl]indoles: Agonists for the h5-HT(1D) receptor with high selectivity over the h5-HT(1B) subtype

Full text of DOI:10.1021/jm9704558

J . Med. Chem. 1997, 40, 3497-3500
3497
3-[2-(P yr r olid in -1-yl)eth yl]in d oles a n d
3-[3-(P ip er id in -1-yl)p r op yl]in d oles:
Agon ists for th e h 5-HT1D Recep tor w ith
High Selectivity over th e h 5-HT1B
Su btyp e
J ose´ L. Castro,*,† Leslie J . Street,*,†
Alexander R. Guiblin,† Richard A. J elley,†
Michael G. N. Russell,† Francine Sternfeld,†
Margaret S. Beer,‡ J osephine A. Stanton,‡ and
Victor G. Matassa†
Chemistry and Biochemistry Departments, Merck Sharp and
Dohme Research Laboratories, Neuroscience Research
Centre, Terlings Park, Eastwick Road,
Harlow, Essex CM20 2QR, U.K.
Received J uly 14, 1997
The discovery of Sumatriptan (1, Figure 1),1,2
a
selective serotonin 5-HT1D receptor agonist, as an ef-
fective acute treatment for migraine headache has
intensified research in this area, and over the past years
several related compounds, including Merck’s Rizatrip-
tan (MK-462, 2),3 have entered late phases of clinical
development. The mechanism of action of Sumatriptan
is still a matter of much debate,4 and both a direct
vasoconstrictor effect on intracranial, extracerebral
arteries and/or an inhibition of neuropeptide (calcitonin
gene-related peptide, substance P, neurokinin A) release
from perivascular trigeminal sensory nerves in the dura
mater and large cerebral arteries have been suggested
to play a significant role in headache relief.5,6 Since the
development of Sumatriptan and the other compounds
in the “triptan” class, it has been found that two
subtypes of the human 5-HT1D receptor exist, 5-HT1D
(previously termed 5-HT1DR) and 5-HT1B (previously
termed 5-HT1Dâ).7 The vasoconstrictor effects of Suma-
triptan are thought to be mediated by activation of
5-HT1B receptors, due to the preponderance of mRNA
coding for this subtype in human blood vessels (includ-
ing cerebral microvessels).8,9 The identity of the recep-
tor-mediating inhibition of neuropeptide release is less
clear as both 5-HT1D and 5-HT1B mRNAs have been
shown to be present in human trigeminal ganglia by
reverse transcriptase-polymerase chain reaction (RT-
PCR).9,10 Neither Sumatriptan nor the other “triptan”
agents developed so far show much selectivity for either
subtype of the 5-HT1D receptor.
Sumatriptan has been shown to produce coronary
artery vasoconstriction possibly through activation of
5-HT1B receptors,11 an effect that may or may not be
related to the chest pains experienced by some of the
patients.12 Although coronary side effects are rare, care
should be taken when the drug is given to patients with
suspected or proven coronary diseases. Although Riza-
triptan has been shown to be less effective than
Sumatriptan in causing contraction of isolated human
coronary artery segments,13 the differential expression
of 5-HT1D and 5-HT1B receptors in neural and vascular
tissue prompted us to investigate the possibility of
developing selective serotonin 5-HT1D receptor agonists
as potential second generation antimigraine agents. In
the present communication we report on the design,
F igu r e 1.
synthesis, and biological evaluation of agonists for this
receptor with up to 200-fold selectivity over the 5-HT1B
subtype.
Screening of the Merck collection of 5-substituted
tryptamines, synthesized during the discovery of Riza-
triptan, mainly revealed compounds with marginal
5-HT1D over 5-HT1B selectivity (3-5-fold), despite ex-
ploring different electronic and steric properties at the
indole 5-position. The pyrrolidine analogue 4a , how-
ever, showed promising levels of selectivity (9-fold; Table
1), and this was utilized as a lead for further medicinal
chemistry elaboration. The N,N-dimethylethylamine
analogue of 4a , L-741,604 (3),17 has 10-fold higher
affinity for 5-HT1D receptors but arguably lower receptor
subtype selectivity (Table 1). This, combined with the
knowledge that ketanserin (5) was a moderately selec-
tive 5-HT1D receptor ligand (1B/1D, 25), which incorpo-
rates a more elaborate amine side chain functionality,
suggested that improvements in 5-HT1D subtype selec-
tivity might be gained by substituting the pyrrolidine
ring of 4a .14 This would help to define the steric
requirements for binding to the 5-HT1D and 5-HT1B
receptors in the region of the aspartic acid binding
domain.15 In addition, a series of 4-substituted pip-
eridines (6c-h ) was explored with the aim of simplify-
ing the chemistry by removing the chiral center present
in the substituted pyrrolidines.
The 3-substituted pyrrolidines 4b-h and 4-substi-
tuted piperidines 6d -h were prepared as shown in
Schemes 1-3. Thus, hydrogenation of the enantiomeri-
cally pure benzylpyrrolidines 7a ,b and 8a ,b16 in the
presence of (BOC)2O, followed by alkylation with benzyl
bromide, gave the benzyl ethers 9 and 10 (Scheme 1).
The pyrrolidines resulting from removal of the BOC
protecting group and alkylation with 4-chlorobutanal
dimethyl acetal were subjected to Fischer indolisation
with 4-(1,2,4-triazol-4-yl)phenylhydrazine17 to give the
† Chemistry Department.
‡ Biochemistry Department.
S0022-2623(97)00455-X CCC: $14.00 © 1997 American Chemical Society

3498 J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 22
Communications to the Editor
Ta ble 1. Binding of Standard Compounds and Substituted Pyrrolidines to Cloned Human 5-HT1D Receptors
IC50 (nM)b
EC50(nM,d % 5-HTe)
h5-HT1D
compda
R
h5-HT1D
h5-HT1B
1B/1Dc
Sumatriptan (1)
Rizatriptan (2)
L-741,604 (3)
ketanserin (5)
4a
4b
4c
4d
4e
4f
5.0
11
0.30
260
3.4
6.3
23
38
2.3
16.0
41
1.4
6500
30.2
146
280
210
88
3.2
3.7
4.7
25
9.0
23
12
5.5
38
16 (100)
8.4 (90)
0.45 (117)
H
2.6 (120)
15 (81)
33 (84)
OBn
OBnf
NMeBn
CH2OBn
CH2NMeBn
CH2NHBn
CH2NHBnf
2.1 (68)
0.70
0.50
1.6
33
47
50
47
94
31
1.6 (102)
0.93 (100)
4.1 (102)
4g
4h
a
b
Where applicable, all compounds are single enantiomers with absolute stereochemistry as drawn. Displacement of [3H]-5-HT binding
to cloned 5-HT1D and 5-HT1B receptors in CHO cells. The figures are the mean of two independent determinations perfomed in triplicate.
In each case the radioligand used was at the KD for the receptor. The maximum variance from the mean of the log (IC50) values was 3.2%.
d
c Binding selectivity for 5-HT1D receptors. Measurement of agonist-induced [35S]GTPγS binding in CHO cells stably transfected with
5-HT1D receptors. e Efficacy relative to 5-HT. Values are the mean of two independent determinations. f These compounds are the
enantiomers of 4b and 4g.
Sch em e 1a
Sch em e 2a
a
Reagents: (i) MeSO2Cl, Et3N, CH2Cl2; (ii) R1R2NH, toluene,
90 °C; (iii) 90% HCO2H; (iv) 13, DME, NaI, Na2CO3, reflux.
a
Reagents: (i) Pd(OH)2, H2, (BOC)2O,MeOH/H2O; (ii) NaH,
benzyl bromide, THF; (iii) 90% HCO2H; (iv) 4-chlorobutanal
dimethyl acetal, NaI, Na2CO3, DME; (v) 4% H2SO4, 4-(1,2,4-triazol-
4-yl)phenylhydrazine, reflux.
4-[(N-benzyl-N-methylamino)methyl]piperidine by alky-
lation with 5-bromopentanal dimethyl acetal followed
by Fischer reaction as above.
indoles 4b,c,e.18 Mesylation of alcohols 11a ,b followed
by reaction with the appropriate amine and removal of
the BOC group gave the pyrrolidines 12a ,b.19 These
were N-alkylated with mesylate 1320 to give the re-
quired indoles 4d ,f-h in a single step (Scheme 2).
4-Substituted piperidines 6d -h were prepared starting
from 4-hydroxypiperidine, which was alkylated with
5-bromopentanal dimethyl acetal to afford alcohol 14
(Scheme 3). Fischer indolization, followed by Parick
oxidation of the intermediate 4-piperidinol, gave the
versatile ketone intermediate 15, easily converted into
piperidines 6d -h by reductive amination with the
appropriate amine. Compound 6c was prepared from
The compounds in Tables 1 and 2 were evaluated for
their affinity to cloned human 5-HT1D and 5-HT1B
receptors stably expressed in CHO cells.21 Their in-
trinsic efficacy, expressed as percent of the maximal
5-HT response, was measured in the same cell lines
using agonist-induced [35S]GTPγS binding.22,23 It can
be seen from the data in Table 1 that substitution at
C-3 of the pyrrolidine ring of 4a with a benzyloxy group
improved 5-HT1D over 5-HT1B selectivity (4b: 1B/1D,
23). The same effect was observed when this group was
attached through a methylene spacer (4e: 1B/1D, 38);
in this case, however, 5-HT1D affinity remained un-
changed compared to 4a . Both, 4b and 4e behaved as

Communications to the Editor
J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 22 3499
Ta ble 2. Binding of Substituted Piperidines to Cloned Human 5-HT1D Receptors
IC50 (nM)b
EC50 (nM,d % 5-HTe)
h5-HT1D
compda
R
h5-HT1D
h5-HT1B
1B/1Dc
6a
6b
6c
6d
6e
6f
200
24
0.95
2.2
4.8
0.35
568
15
28
145
125
35
3
0.6
29
66
26
H
CH2NMeBn
NMeBn
NHBn
0.90 (93)
1.6 (94)
8.0 (76)
1.5 (96)
100
Me
N
H
Ph
6g
6h
1.3
0.9
114
185
88
1.8 (95)
OH
N
H
Ph
206
1.1 (104)
OH
N
H
Ph-4F
a-e See corresponding footnotes for Table 1.
Sch em e 3a
groups without detrimental effect on affinity, it usually
resulted in lower 1B/1D selectivity. By contrast with
the pyrrolidine-derived 5-HT1D receptor agonists 4,
which appeared to prefer a two-carbon linker between
the indole nucleus and the pyrrolidine nitrogen, a
trimethylene chain was optimal for the piperidine-based
agonists 6 (e.g. 6a vs 6b, Table 2). In addition, direct
attachment of the amino functionality to C-4 of the
piperidine was preferred rather than through a meth-
ylene group (cf. 6c and 6d ), and 6d was a high-affinity,
full agonist with good 1B/1D selectivity. Removal of the
N-Me group of 6d to give 6e was slightly detrimental
for 1B/1D selectivity, due to a 2-fold loss in 5-HT1D
affinity. Gratifyingly, this could be restored and im-
proved by substitution at the benzylic position with
methyl (6f) or hydroxymethyl (6g) groups, with reten-
tion of full agonist properties. Moreover, introduction
of fluorine at the 4-position of the pendent phenyl ring
of 6g afforded a compound, 6h , with excellent affinity
and selectivity (1B/1D, 206) for 5-HT1D receptors. Thus,
piperidine 6h (L-772,405) represents the most selective
5-HT1D receptor full agonist reported to date. The
pyrrolidines and piperidines also showed selectivity in
the functional assay for the 5-HT1D receptor. Thus both
pyrrolidine 4g (EC50: 1D 0.93 nM; 1B 456 nM) and
piperidine 6h (EC50: 1D 1.1 nM; 1B 1100 nM) have
relatively low potency for the 5-HT1B receptor in the
GTPγS binding assay.
a
Reagents: (i) (MeO)2CH(CH2)4Br, K2CO3, DMF, 90 °C; (ii) 4%
H2SO4, 4-(1,2,4-triazol-4-yl)phenylhydrazine, reflux; (iii) pyridine‚
SO3, DMSO, 25 °C; (iv) R1R2NH, NaCNBH3, AcOH, MeOH.
partial agonists in the GTPγS binding assay when
compared to serotonin. Replacement of the oxygen atom
in 4b and 4e by a methylated nitrogen, to give the
benzylamines 4d and 4f, respectively, appeared to be
tolerated only in the latter case. This change, however,
was found to be beneficial for improving the efficacy of
the compounds, and benzylamine 4f was as efficacious
as 5-HT in the in vitro functional assay. A further 2-fold
increase in 1B/1D binding selectivity was achieved by
removal of the N-Me group on 4f while retaining sub-
nanomolar afffinity and high efficacy (4g, L-760,790;
IC50, 0.5 nM; 1B/1D, 94). The absolute stereochemistry
of the pyrrolidine C-3 chiral center has an effect on both
affinity and selectivity, which is illustrated in Table 1
by comparison of the benzyl ethers 4b and 4c and the
benzylamines 4g and 4h . Although substitution at C-2
of the pyrrolidine was shown to tolerate a variety of
The 5-HT1D and 5-HT1B receptors can therefore be
differentiated by appropriate substitution of the ligand
in the region which binds to the aspartic acid. Whereas
relatively bulky substituents are tolerated by the
5-HT1D receptor and indeed, in many cases, affinity is
improved by this substitution, the 5-HT1B receptor only
tolerates dimethylamino substitution of the tryptamine
and suggests that the binding pocket of the 5-HT1D
receptor which accommodates benzylamine substitution
is not present for the 5-HT1B receptor.15 The selectivity

3500 J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 22
Communications to the Editor
(10) Rebeck, G. W.; Maynard, K. I.; Hyman, B. T.; Moskowitz, M. A.
Selective 5-HT1DR Serotonin Receptor Gene Expression in Trigem-
inal Ganglia: Implications for Antimigraine Drug Development.
Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 3666-3669.
(11) (a) Bax, W. A.; Renzenbrink, G. J .; Van Heuven-Nolsen, D.;
Thijssen, E. J . M.; Bos, E.; Saxena, P. R. 5-HT Receptors
Mediating Contractions of Isolated Human Coronary Artery.
Eur. J . Pharmacol. 1993, 239, 203-210. (b) Kaumann, A. J .;
Frenken, M.; Posival, H.; Brown, A. M. Variable Participation
of 5-HT1-Like Receptors and 5-HT2 Receptors in Serotonin-
Induced Contraction of Human Isolated Coronary Arteries.
5-HT1-Like Receptors Resemble Cloned 5-HT1Dâ Receptors.
Circulation 1994, 90, 1141-1153.
of pyrrolidine 4g and piperidine 6h vs other cloned
serotonin receptors was also explored using radioligand
binding techniques. Thus, 4g and 6h showed the
following affinities (IC50, nM) for h5-HT1A (6.4 and 105,
respectively), h5-HT1E (>10 000), h5-HT1F (>10 000),
r5-HT2A (>4000), r5-HT5A (>1500), r5-HT6 (>5000), and
r5-HT7 (3800 and 318, respectively). The selectivity
observed for 6h over 5-HT1A receptors (115-fold) is
noteworthy as this has been difficult to achieve with
other 5-HT1D receptor agonists. Additionally, both 4g
and 6h had >1 µM affinity at over 100 other GPCRs,
ion channels, and proteins.
In summary, two series of high-affinity 5-HT1D recep-
tor full agonists, with up to 200-fold selectivity over the
5-HT1B subtype, have been identified. The pyrrolidine
4g and piperidine 6h show very good selectivities over
a range of other serotonin and non-serotonin receptors
and, therefore, constitute new useful tools to delineate
the role of 5-HT1D receptors in migraine and other
diseases.
(12) Ottervanger, J . P.; Stricker, B. H. Ch. Cardiovascular Adverse
Reactions to Sumatriptan. Cause for Concern? CNS Drugs 1995,
3 (2), 90-98.
(13) (a) Ferro, A.; Longmore, J .; Hill, R. G.; Brown, M. J .
A
Comparison of the Contractile Effects of 5-Hydroxytryptamine,
Sumatriptan and MK-462 on Human Coronary Artery in vitro.
Br. J . Clin. Pharmacol. 1995, 40 (3) 245-251. (b) Longmore, J .;
Boulanger, C. M.; Desta, B.; Hill, R. G.; Schofield, W. N.; Taylor,
A. A. 5-HT1D Receptor Agonists and Human Coronary Artery
Reactivity in vitro: Crossover Comparisons of 5-HT and Sumatrip-
tan with Rizatriptan and L-741, 519. Br. J . Clin. Pharmacol.
1996, 42, 431-441.
(14) Although the binding mode of ketanserin might be totally
different to that of 4a , we hypothesized that perhaps the
benzoyleneurea or the 4-fluorobenzoyl moieties of the former
might mimic the indole portion of the latter. If this was to be
the case, the 5-HT1D selectivity could arise from differential
interaction of both receptors with the other appended piperidine
substituent, and therefore exploration of space around the
pyrrolidine ring of 4a was investigated.
(15) Bremner, D. H.; Ringan, N. S.; Wishart, G. Modeling of the
agonist binding site of serotonin human 5-HT1A, 5-HT1DR and
5-HT1Dâ receptors. Eur. J . Med. Chem. 1997, 32, 59-69.
(16) Nielsen, L.; Brehm, L.; Krogsgaard-Larsen, P. GABA Agonists
and Uptake Inhibitors. Synthesis, Absolute Stereochemistry, and
Enantioselectivity of (R)-(-)- and (S)-(+)-Homo-â-proline. J . Med.
Chem. 1990, 33, 71-77.
(17) Sternfeld, F.; Baker, R.; Broughton, H. B.; Guiblin, A. R.; J elley,
R. A.; Matassa, V. G.; Reeve, A. J .; Beer, M. S.; Stanton, J . A.;
Hargreaves, R. J .; Shepheard, S. L.; Longmore, J .; Razzaque,
Z.; Graham, M. I.; Sohal, B.; Street, L. J . The Chemical Evolution
of N,N-Dimethyl-2-[5-(1,2,4-triazol-4-yl)-1H-indol-3-yl]ethylamine
(L-741,604) and Analogues: Potent and Selective Agonists for
5-HT1D Receptors. Bioorg. Med. Chem. Lett. 1996, 6, 1825-1830.
(18) Castro, J . L.; Street, L. J .; et al. Azetidine, Pyrrolidine and
Piperidine Derivatives. World Patent Appl. WO 96/04274, 1996.
(19) Note that in the case of 11a , this operation results in inversion
of the pyrrolidine C3-chiral center.
(20) Mesylate 13 was synthesised from 4-(1,2,4-triazol-4-yl)aniline
following a method similar to that developed for the preparation
of MK-462: Chen, C.; Lieberman, D. R.; Larsen, R. D.; Reamer,
R. A.; Verhoeven, T. R.; Reider, P. J .; Cottrell, I. F.; Houghton,
P. G. Synthesis of the 5-HT1D Receptor Agonist MK-462 via a
Pd-Catalyzed Coupling Reaction. Tetrahedron Lett. 1994, 35,
6981-6984.
(21) Veldman, S. A.; Bienkowski, M. J . Cloning and Pharmacological
Characterization of a Novel Human 5-Hydroxytryptamine1D
Receptor Subtype. Mol. Pharmacol. 1992, 42, 439-444.
(22) In this assay, interaction of the agonist-occupied receptor with
the G-protein results in dissociation of GDP from the G-protein
R-subunit and the binding of a molecule of GTP. Under normal
conditions the R-subunit dissociates from the âγ-subunits and,
following modulation of its effector, intrinsic R-subunit GTPase
hydrolyzes the GTP to GDP. Because [35S]GTPγS is resistant
to this GTPase, it accumulates in the membrane and it can be
measured by virtue of its radiolabel (see for example ref 22).
(23) Lazareno, S.; Birdsall, N. J . M. Pharmacological Characterization
of Acetylcholine-stimulated [35S]GTPγS Binding Mediated by
Human Muscarinic m1-m4 Receptors: Antagonist Studies. Br.
J . Pharmacol. 1993, 109, 1120-1127.
Ack n ow led gm en t. We thank Mr. Roy Pengilley for
technical assistance.
Su p p or tin g In for m a tion Ava ila ble: Synthetic proce-
dures for pyrrolidine 4g and piperidine 6h (8 pages). Ordering
information is given on any current masthead page.
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J M9704558