SB-656104-A: A novel 5-HT7 receptor antagonist with improved in vivo properties

Article abstract of DOI:10.1016/S0960-894X(02)00690-X

A focused SAR study around the previously reported selective 5-HT₇ receptor antagonist, SB-269970-A has resulted in the identification of a structurally related analogue having an improved pharmacokinetic profile. Replacement of the phenolic gr

Full text of DOI:10.1016/S0960-894X(02)00690-X

Bioorganic & Medicinal Chemistry Letters 12 (2002) 3341–3344
SB-656104-A: A Novel 5-HT7 Receptor Antagonist with
Improved In Vivo Properties
Ian T. Forbes,* Sara Douglas, Andrew D. Gribble, Robert J. Ife, Andrew P. Lightfoot,
Ashley E. Garner, Graham J. Riley, Phillip Jeffrey, Alexander J. Stevens, Tania O. Stean
and David R. Thomas
GlaxoSmithKline, New Frontiers Science Park, Harlow, Essex CM19 5AW, UK
Received 17 June 2002; accepted 6 August 2002
Abstract—A focused SAR study around the previously reported selective 5-HT7 receptor antagonist, SB-269970-A has resulted in
the identification of a structurally related analogue having an improved pharmacokinetic profile. Replacement of the phenolic
group in SB-269970-A with an indole moiety, and replacement of the piperidinyl 4-methyl group with a heterocyclic ring system
proved to be the key changes leading to the identification of SB-656104-A.
# 2002 Elsevier Science Ltd. All rights reserved.
The 5-HT7 receptor is the most recent addition to the
serotonin sub-family of G-protein coupled receptors.1
Receptor localisation studies in various species indicate
the presence of 5-HT7 receptors both centrally and per-
ipherally, with the highest receptor densities being loca-
ted in the thalamus, hypothalamus, limbic and cortical
regions of the brain.2 Although the biological func-
tion(s) of this receptor are poorly understood, recent
reports suggest that 5-HT7 receptors are involved in the
control of circadian rhythms,3 or in the aetiology of
depression.4 Further clarification of these hypotheses
awaits the discovery and evaluation of selective tool com-
pounds. We have previously reported that the selective 5-
HT7 receptor antagonist SB-269970-A inhibits rapid eye
movement (REM) sleep in rats, suggesting involvement of
5-HT7 receptors in the physiology of sleep.5
tainly due to the presence of the phenolic hydroxyl
group and Phase II metabolism. Thus we embarked
upon a chemical programme aimed at replacing the
phenolic moiety in SB-269970-A with metabolically
more stable bioisosteres.
Initially, we investigated the synthesis of heterocyclic
analogues of SB-269970-A, since this strategy has pro-
ven successful in many related cases.6 Compounds in
Table 1 were synthesised via two main strategies. Route
A (Scheme 1) involved the preparation of the key
4-methyl-3-nitrosulfonamide intermediate (1a) using the
previously reported methodology.7 Leimgruber–Batcho
transformation8 afforded the indole (2), which was
reduced with sodium cyanoborohydride to the indoline
(3). Direct reduction of the nitro group in (1a) followed
by diazotisation afforded the indazole (4).
The alternative strategy (Route B) involved the key
intermediate (1b). Reaction with ammonia or sodium
hydroxide afforded the corresponding 4-amino and
4-hydroxy analogues respectively, which were converted
using standard methodologies to the benzimidazole (5),
benzotriazole (6), benzimidazolone (7) and the
benzoxazolidinone (8) (Scheme 2).
Unfortunately, further studies with SB-269970-A have
been hampered by its extremely high in vivo blood
clearance in the rat (CLb 140 mL/min/kg),5 almost cer-
The 5-HT7 binding affinities9 for compounds (2)–(8) are
shown in Table 1. It can be seen that in all cases, repla-
cement of the phenol with a heterocyclic ring caused a
reduction in activity of varying degrees. Thus the indole
(2), indazole (4) and benzimidazole (5) retained significant
*Corresponding author. Tel.: +44-1279-622126; fax: +44-1279-
622790; e-mail: ian_t_forbes@gsk.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00690-X

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I. T. Forbes et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3341–3344
Scheme 1. Reagents and conditions for route A: (a) dimethylformamide dimethylacetal, 100 ꢀC, 6 h; (b) Raney Nickel, hydrazine, MeOH, THF,
45 ꢀC; (c) sodium cyanoborohydride, acetic acid, rt, 4 h; (d) Raney Nickel, hydrazine, MeOH, THF, 45 ꢀC; (e) NaNO2, acetic acid, rt, 24 h.
Scheme 2. Reagents and conditions for route B: (a) aqueous ammonia, dioxan, 90 ꢀC, sealed tube; (b) 50 psi H2, 10% palladium–charcoal, EtOH;
(c) formic acid, NaHCO3, H2O, 100 ꢀC; (d) 50 psi H2, 10% palladium–charcoal, EtOH; (e) NaNO2, acetic acid, HCl, rt; (f) 50 psi H2, 10% palla-
dium–charcoal, EtOH; (g) phosgene, CH2Cl2, Hunig’s base; (h) NaOH, dioxan, reflux; (i) 50 psi H2, 10% palladium–charcoal, EtOH; (j) phosgene,
CH2Cl2, Hunig’s base.
levels of 5-HT7 binding affinity, whereas the benzimida-
zolidinone (7) and the oxazolidinone (8) showed the
greatest reduction in affinity. Attempts to correlate the
pKa of the heterocyclic NH with the 5-HT7 receptor affi-
nity failed to reveal any trend. In the case of (7) and (8) it
appears that a carbonyl group at C-2 is not well tolerated,
suggesting that polarity and/or steric interactions may be
influencing factors. Overall, the most interesting analo-
gue was the indole (2), which was one of the few analogues
to possess a similar 5-HT7 receptor affinity to the phenol
SB-269970-A. Unfortunately, (2) showed high blood
clearance and zero bioavailability in the rat.10 In vitro
metabolism studies on closely related structures indi-
cated that hydroxylation may be occurring on the 4-
methylpiperidine ring system,10 so we followed up these
leads with modifications to the piperidine ring.
receptor modelling studies indicated the presence of a
large lipophilic pocket around this region.
The preparation of analogues (9)–(16) involved similar
methodologies to those for (2), except that the indole
ring system was introduced prior to introduction of the
4-substituted piperidine moiety, allowing parallel
synthesis of target compounds (Scheme 3).
Initially, we investigated the preparation of alternative
R groups at the C-4 position in (2). We were particu-
larly interested in aromatic C-4 substituents, since
It was most gratifying to see that replacement of the
4-methyl group in (2) with a variety of aromatic groups

I. T. Forbes et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3341–3344
3343
Scheme 3. Reagents and conditions: (a) carbon tetrabromide, triphenylphosphine, CH2Cl2, 0ꢀC, 1 h; (b) 4-substituted piperidine, MeCN, NaHCO3,
80ꢀC, 12 h.
Table 1. Heterocyclic phenol isosteres
No.
pKi9 5-HT7
No.
pKi9 5-HT7
SB-269970-A
8.90Æ0.10
8.62Æ0.10
5
6
8.19Æ0.11
7.59Æ0.04
2
3
4
7.64Æ0.10
8.19Æ0.03
7
8
7.24Æ0.04
6.78Æ0.10
significantly enhanced the 5-HT7 receptor affinity (Table
2). The greatest increases were seen with the indole (9),
the benzimidazolone (11) and the 4-fluorophenoxy ana-
logue (14). Unfortunately, it was found that these
modifications also markedly increased unwanted affinity
for the adrenergic a1B receptor, with the greatest poten-
tiation being observed with compounds (9) and (16).
Only compounds (12) and (13) retained relatively low
affinity for this receptor. Overall, the 4-chlorophenoxy
derivative (13; free base SB-656104; hydrochloride salt
SB-656104-A) was selected as the most promising ana-
logue for further studies.
Table 2. 4-Substituted piperidines
SB-656104 was evaluated in a previously described
functional model of 5-HT7 receptor activation,5 block-
ing 5-carboxamidotryptamine (5-CT) stimulated adeny-
Table 3. Receptor binding selectivity profile of SB-656104
Receptor
Affinity (pKi)
5-HT1A
5-HT1B
5-HT1D
5-HT1E
5-HT1F
5-HT2A
5-HT2B
5-HT2C
5-HT4
6.25Æ0.06
6.20Æ0.05
7.60Æ0.02
<5.3
No.
R
pKi9 5-HT7
pKi9 a1B
2
9
Me
Indol-3-yl
Indol-2-yl
8.62Æ0.10
9.17Æ0.03
8.20Æ0.04
9.15Æ0.08
8.74Æ0.02
8.70Æ0.10
9.30Æ0.06
9.10Æ0.12
9.08Æ0.02
<5
<5.7
8.89Æ0.12
7.05Æ0.05
7.36Æ0.03
6.51Æ0.11
6.66Æ0.01
7.52Æ0.01
7.03Æ0.07
7.72Æ0.06
7.20Æ0.02
7.04Æ0.08
6.57Æ0.08
5.72Æ0.10
6.74Æ0.01
6.07Æ0.06
8.70Æ0.10
10
11
12
13
14
15
16
Benzimidazol-2-on-3-yl
Benzoxazol-2-on-3-yl
4-Chlorophenoxy
4-Fluorophenoxy
4-Chlorobenzoyl
4-Fluorobenzoyl
5-HT5A
5-HT6
5-HT7

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I. T. Forbes et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3341–3344
Table 4. In vivo comparison of SB-269970-A and SB-65610411
In vivo data
SB-269970-A
SB-656104a
CLb (rat)
t1/2 (rat)
Steady state brain:blood ratio
Fpo (rat)
ED50 to antagonise 5-CT induced
hypothermia in guinea pig5
140 mL/min/kg
<0.5 h
0.8
57Æ4 mL/min/kg
2.0Æ0.2 h
0.9
16Æ8%
Not measured
3.0Æ0.2 mg/kg ip
(Maximal inhibition at 1 h)
2.0Æ0.01 mg/kg po
(Maximal inhibition at 2 h)
aFree base used for DMPK studies; hydrochloride salt used for guinea pig hypothermia model.
2. (a) Bard, J. A.; Zgombick, J.; Adham, N.; Vaysee, P.;
Branchek, T. A.; Weinshank, R. L. J. Biol. Chem. 1993, 268,
23422. (b) Ruat, M.; Traiffort, E.; Leurs, R.; Tardivel-
Lacombe, J.; Diaz, J.; Arrang, J.-M.; Schwarz, J. C. Proc.
Natl. Acad. Sci. 1993, 90, 8547. (c) To, Z. P.; Bonhaus, D. W.;
Eglen, R. M.; Jakeman, L. B. Br. J. Pharm. 1995, 115, 107. (d)
Shen, Y.; Monsma, F. J.; Metcalf, M. A.; Jose, P. A.; Ham-
blin, M. W.; Sibley, D. R. J. Biol. Chem. 1993, 268, 18200.
3. Lovenberg, T. W.; Baron, B. M.; Lecea, L. de.; Miller, J. O.;
Prosser, R. A.; Rea, M. A.; Foye, P. E.; Rucke, M.; Slone,
A. L.; Siegel, B. W.; Danielson, P. E.; Sutcliffe, J. G.; Erlan-
der, M. G. Neuron 1993, 11, 449.
4. Guscott, M. Poster Presentation, Society for Neurosciences
Satellite Meeting on Serotonin, New Orleans, November, 2000.
5. Hagan, J. J.; Price, G. W.; Jeffrey, P.; Deeks, N. J.; Stean,
T.; Piper, D.; Smith, M. I.; Upton, N.; Medhurst, A. D.;
Middlemiss, D. N.; Riley, G. J.; Lovell, P. J.; Bromidge, S. M.;
Thomas, D. R. Br. J. Pharm. 2000, 130, 539.
6. For a recent example see: Wright, J. L.; Gregory, T. F.;
Kesten, S. R.; Boxer, P. A.; Serpa, K. A.; Meltzer, L. T.; Wise,
L. D.; Espitia, S. A.; Konkoy, C. S.; Whittemore, E. R.;
Woodward, R. M. J. Med. Chem. 2000, 43, 3408.
7. Lovell, P. J.; Bromidge, S. M.; Dabbs, S.; Duckworth,
D. M.; Forbes, I. T.; Jennings, A. J.; King, F. D.; Middlemiss,
D. N.; Rahman, S. K.; Saunders, D. V.; Collin, L. L.; Hagan,
J. J.; Riley, G. J.; Thomas, D. R. J. Med. Chem. 2000, 43, 342.
8. Batcho, A. D.; Leimgruber, W. Org. Synth. 1985, 63, 214.
9. pKi values represent the mean of at least three determina-
tions using cell lines stably expressing the cloned human
receptor. For details of the radioligand binding assays, see ref
7.
lyl cyclase activity with a calculated pA2 of 8.1Æ0.1,
confirming its antagonist profile. The selectivity profile
for SB-656104 across the range of serotonin receptor
sub-types is shown in Table 3. Encouragingly, SB-
656104 shows ꢁ100-fold binding selectivity over most
other 5-HT receptor subtypes, with the exception of 5-
HT2A (30-fold), 5-HT2B (50-fold) and 5-HT1D (12-fold).
Of crucial importance was the in vivo profile of SB-
656104, which is summarised in Table 4, in comparison
with the former tool compound SB-269970-A. Impor-
tantly, SB-656104 shows a greatly improved pharmaco-
kinetic profile in the rat compared to SB-269970-A,
possessing a lower blood clearance and a significantly
improved half life. Brain penetration with SB-656104 is
similar to that measured for SB-269970-A (Table 4).
Significantly, the presence of the 4-chlorophenoxy
piperidine ring in SB-656104 confers oral bioavailability
(16%) which was notably lacking in the earlier lead (2).
Evaluation of SB-656104-A (hydrochloride salt) in an in
vivo functional assay (5-CT induced hypothermia in gui-
nea pigs)5 resulted in a similar ED50 profile to SB-269970-
A, but with administration being via the oral route.
In summary, through a rational SAR study based
around SB-269970-A we have identifed SB-656104-A as
a novel, potent and selective 5-HT7 receptor antagonist
with a greatly improved pharmackinetic profile, which
represents an improved tool compound for character-
isation of 5-HT7 receptors in the CNS. Further
studies with SB-656104-A will be the subject of a future
publication.12
10. Unpublished DMPK data from GlaxoSmithKline
11. DMPK protocol for SB-269970-A as previously pub-
lished.5 For SB-656104 (free base), iv dosing in male Sprague–
Dawley rats (n=3): constant infusion over 1 h to achieve tar-
get dose of 1 mg free base/kg; oral dosing: an oral suspension
of SB-656104 was administered to male Sprague–Dawley rats
(n=3) at a target dose of 3 mg free base/kg. Blood and brain
homogenate samples were analysed by HPLC/MS/MS.
12. Forbes, I. I.; Douglas, S.; Gribble, A. D.; Ife, R. J.;
Lightfoot, A. P.; Garner, A. E.; Riley, G. J.; Jeffrey, P.; Mas-
sagrande, M.; Melotto, S.; Stevens, A. J.; Stean, T. O.;
Thomas, D. R. manuscript in preparation.
References and Notes
1. For a recent review of serotonin receptors see: Hoyer, D.;
Hannon, J. P.; Martin, G. R. Pharmacol. Biochem. Behav.
2002, 71, 533.