Tricyclic azepine derivatives as selective brain penetrant 5-HT6 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2008.08.010

Starting from a benzazepine sulfonamide 5-HT₆ receptor antagonist lead with limited brain penetration, application of a strategy of conformational constraint and reduction of hydrogen bond donor count led to a novel series of tricyclic derivati

Full text of DOI:10.1016/j.bmcl.2008.08.010

Bioorganic & Medicinal Chemistry Letters 18 (2008) 5698–5700
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Tricyclic azepine derivatives as selective brain penetrant 5-HT6
receptor antagonists
b
a
a
a
Giancarlo Trani a, , Stuart M. Baddeley , Michael A. Briggs , Tsu T. Chuang , Nigel J. Deeks ,
Christopher N. Johnson a, Abir A. Khazragi b, Tania L. Mead a, Andrew D. Medhurst a, Peter H. Milner c,
Leann P. Quinn a, Alison M. Ray a, Dean A. Rivers a, Tania O. Stean a, Geoffrey Stemp c, Brenda K. Trail a,
David R. Witty a
*
a Neurosciences Centre of Excellence for Drug Discovery, GlaxoSmithKline, New Frontiers Science Park, Third Avenue, Harlow, Essex CM19 5AW, UK
b Molecular Discovery Research, GlaxoSmithKline, New Frontiers Science Park, Third Avenue, Harlow, Essex CM19 5AW, UK
c Immunoinflammation Centre of Excellence for Drug Discovery, GlaxoSmithKline, Gunnels Wood Road, Stevenage, Hertfordshire SG1 2NY, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Starting from a benzazepine sulfonamide 5-HT6 receptor antagonist lead with limited brain penetration,
application of a strategy of conformational constraint and reduction of hydrogen bond donor count led to
a novel series of tricyclic derivatives with high 5-HT6 receptor affinity and excellent brain:blood ratios.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 18 June 2008
Revised 1 August 2008
Accepted 3 August 2008
Available online 7 August 2008
Keywords:
5-HT6 receptor antagonists
Conformational constraint
CNS penetration
5-HT6 receptor mRNA is almost exclusively expressed within
the brain, and many CNS drugs have high affinity for the 5-HT6
receptor. Hence, there has been interest in elucidating the role of
the 5-HT6 receptor in CNS disorders through the development of
selective agents. Several classes of ligands have been disclosed in
recent years,1 including the highly potent and selective 5-HT6
receptor antagonist SB-271046 1,2 and a number of 5-HT6 receptor
antagonists are currently in clinical trials for treatment of cognitive
disorders, for example, Alzheimer’s disease. The discovery that 5-
HT6 receptor antagonists, such as 1, have a beneficial effect on
memory consolidation in animal models of cognitive enhance-
ment, such as the Morris water maze and Object Recognition par-
adigms,3,4 suggested a possible role for 5-HT6 receptor antagonists
in the treatment of learning and memory disorders.5 The proposed
involvement of 5-HT6 receptor antagonism in memory consolida-
tion is further supported by the effect of 1 on neuronal cell adhe-
sion molecule (NCAM) polysialylation in rat brain.6 NCAM
polysialylation is a process which contributes to learning-associ-
ated neuronal remodelling in the adult central nervous system.
Acute and chronic administration of 1 has been shown to increase
the frequency of NCAM polysialylated neurons, activated in the
entorhinal and perirhinal cortex, as well as the dentate gyrus in re-
sponse to water maze spatial learning.7 These data imply that 5-
HT6 receptor antagonists may have a beneficial effect on synaptic
plasticity in brain regions that are critical to information process-
ing, a property which may underpin their broad spectrum of activ-
ity in preclinical models of cognition enhancement.
Although 1 had activity in a range of centrally mediated cogni-
tion models, its brain:blood ratio in rat was low (0.05:1) and the
compound was shown to be a substrate for the P-glycoprotein
(P-gp) efflux pump with low passive permeability. A successful
strategy for improving the brain penetration of 1 has been de-
scribed,8 in which conformational constraint and concomitant re-
moval of an acidic NH group led to the highly brain penetrant
and orally bioavailable indolylpiperazine 699929 2.
H
Me
Cl
N
NH
H
N
N
N
S
S
O O
N
S
Cl
OMe
Cl
O O
SB-271046, 1
699929, 2
A cross screen against the 5-HT6 receptor led to the identification of
benzazepine 3 as a potent and selective 5-HT6 receptor antagonist
lead (pKi = 8.7).9 Although compound 3 had brain:blood ratio
0.5:1, we reasoned that by applying the same strategy of conforma-
tional constraint to 3, to give compounds such as 4, we could fur-
ther improve the brain:blood ratio.
* Corresponding author. Tel.: +44 01279 627819; fax: +44 01279 627685.
E-mail address: Giancarlo.Z.Trani@gsk.com (G. Trani).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.08.010

G. Trani et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5698–5700
5699
was alkylated with methyl iodide, then the Boc group was removed
using HCl to afford the desired product 23 in 38% overall yield.
In order to investigate the effect of ring size and aromaticity,
compound 24 (Scheme 3) was synthesised starting from interme-
diate 7. A Sonogashira coupling with 3,3-bis(ethyloxy)-1-propyne
followed by catalytic hydrogenation afforded 13 in 74% yield. Ring
closure was performed in 66% yield by hydrogenation under aque-
ous acidic conditions, the resulting tricycle was sulfonylated with
benzenesulfonyl chloride and the Boc group was removed using
HCl to afford the desired product.
Conformational
constraint
NH
NH
HN
SO2Ph
N
Removal of NH
PhO2S
4
3
This Letter describes the successful application of the same strategy
to a benzazepine sulfonamide starting point, leading to a novel ser-
ies of selective tricyclic 5-HT6 receptor antagonists with examples
demonstrating excellent brain:blood ratio.
All final compounds were purified by flash chromatography and
converted to hydrochloride salts and were evaluated for functional
potency in a cAMP accumulation assay,11 and data are shown in
Table 1.
Tricyclic compound 4 encouragingly showed high potency as a
functional antagonist at the 5-HT6 receptor (fpKi = 8.8). Unfortu-
nately, it had low in vitro metabolic stability in rat microsomes
(Table 1). We therefore investigated variations at the arylsulfone
moiety and indole portion of the tricyclic core. 3-Substituted phe-
nylsulfones (15, 17) were favoured for optimal potency and im-
proved metabolic stability within the series. However, more
lipophilic substituents (14, 15) gave rise to increased P450 inhibi-
Novel compounds 4 and 14–24 (Table 1) were prepared as
shown in Schemes 1–3. The benzazepine nitrate salt 5 was selec-
tively nitrated at C-7 with nitric acid, then the product was pro-
tected as a Boc derivative 6. Catalytic hydrogenation followed by
reaction with N-iodosuccinimide led to the formation of compound
7 which underwent a Sonogashira coupling with trimethylsilyl-
acetylene to afford 8 in excellent yield. Treatment of 8 with an aryl-
sulfonyl chloride in the presence of pyridine, in some cases, led to
the formation of the bis-sulfonylated material. This problem was
overcome by treating 8 with bis-TMS-trifluoroacetamide, which
enabled a transient TMS-protection of the nitrogen; the subse-
quent in situ addition of the phenylsulfonyl chloride, followed by
aqueous work up, afforded the desired monosulfonylated product
9 in 98% yield.10 Ring closure to form 10 was effected in 86% yield
using copper (I) iodide, and removal of the phenylsulfone and the
trimethylsilyl protecting group was achieved with sodium meth-
oxide in methanol to form 11 in 99% yield (Scheme 1).
Sulfonylation of 11 with a range of arylsulfonyl chlorides and
removal of the Boc group using HCl afforded final compounds 4
and 14–19 in good yields. Compounds 20 and 21 were obtained
by chlorination of 11 with N-chlorosuccinimide followed by sulf-
onylation and Boc removal. Compound 22 was obtained by reduc-
tive alkylation of 15 with formaldehyde in the presence of sodium
triacetoxyborohydride.
tion against the 3A4 isoform with IC50 values < 10 lM (Table 1).
Encouragingly, analysis of brain and blood samples following oral
dosing of 15 in the rat showed that the brain:blood ratio was
5:1, demonstrating that the strategy for improving CNS penetra-
tion was successful. However, absolute concentrations in blood
were low (38 nM at 1 mg/kg oral dose), suggesting that further
improvement in metabolic stability was required. Replacement of
the phenyl ring with a heterocycle led to either a decrease in po-
tency (pyridine 18) or a greater in vitro metabolic instability (thi-
ophene 19). Introduction of a chlorine substituent at the 3-position
of the indolyl core (20, 21) was targeted in order to block a poten-
tial metabolically labile centre. Both compounds 20 and 21 showed
improved in vitro metabolic stability in rat microsomes relative to
Variation of the point of attachment of the arylsulfonyl group
was also investigated (Scheme 2). Anion formation on 11 was fol-
lowed by reaction with diphenyldisulfide and oxidation with mag-
nesium monoperoxyphthalate (MMPP) to form sulfone 12. This
both 4 and 15, and an IC50 against the 3A4 isoforms of >10 lM.
Methylation of the azepine nitrogen (22) gave excellent 5-HT6
antagonist potency but led to high in vitro metabolic instability,
Table 1
5-HT6 receptor functional activity (fpKi),a rat microsomal clearance and cytochrome P450 inhibition at the 3A4 isoform
X
PhO2S
NH
N
R
NH
N
N
N
ArO2S
SO2Ph
Me
24
23
Compoundb
Ar
X
R
fpKi
Rat CLi12 (mL/min/g liver)
P450 IC50
3A4 DEF
(l
M)
13
3A4 PPR
4
Ph
H
H
H
H
H
H
H
Cl
Cl
H
—
—
H
H
H
H
H
H
H
H
H
Me
—
—
8.8
8.1
8.8
8.1
8.7
7.0d
8.1
8.6
8.4
8.9
8.7
7.4d
7.1
4.5
2.5
ndc
2.7
3.6
21
1.7
<0.5
22
12
2.5
3
20
3.2
7
14
15
16
17
18
19
20
21
22
23
24
C6H4(2-Cl)
C6H4(3-Cl)
C6H4(4-Cl)
C6H4(3-OMe)
2-Pyridyl
3-Thienyl
Ph
C6H4(3-Cl)
C6H4(3-Cl)
—
ndc
17
39
9.7
12
12
15
7.8
ndc
ndc
27
77
22
19
20
71
13
ndc
4.6
—
ndc
a
Unless otherwise stated all fpKi values represent the mean of at least three experiments, each within 0.3 of the mean.
b
c
All new compounds gave satisfactory analytical and/or mass spectral data.14
Not determined.
N = 2.
d

5700
G. Trani et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5698–5700
I
OEt
a, b
c, d
c, d, e
NBoc
a, b
NBoc
NH
NH
EtO
7
O2N
H2N
NBoc
N
.HNO3
H2N
SO2Ph
6
7
5
13
24
e
TMS
Scheme 3. Reagents and conditions: (a) 3,3-bis(ethyloxy)-1-propyne, CuI,
PdCl2(PPh3)2, Et2NH, 45 °C, 20 h, 80%; (b) H2, Pd/C, EtOH, RT, 2 h, 92%; (c) H2
(50 psi), Pd/C, THF/AcOH/H2O, 50 °C, 3 days, 66%; (d) PhSO2Cl, Et3N, DCM, RT, 2
days, 15%; (e) HCl (1 M in Et2O), MeOH, 40 °C, 15 h, 53%.
TMS
f
NBoc
NBoc
HN
SO2Ph
H2N
8
9
tency (fpKi = 8.6), good selectivity against a number of receptors
(for example, fpKi at 5-HT2A = 7.6, 5-HT2B = 6.4, 5-HT2C = 6.8), a
good P450 and in vitro metabolic stability profile, and was there-
fore progressed to a rat ex vivo binding assay,15 showing an
ED50 = 4 mg/kg following oral dosing.
g
3
2
h
TMS
NBoc
NBoc
N
N
References and notes
H
PhO2S
1. Glennon, R. A. J. Med. Chem. 2003, 46, 2795.
10
11
2. Bromidge, S. M.; Brown, A. M.; Clarke, S. E.; Dodgson, K.; Gager, T.; Grassam, H.
L.; Jeffrey, P. M.; Joiner, G. F.; King, F. D.; Middlemiss, D. N.; Moss, S. F.;
Newman, H.; Riley, G. J.; Routledge, C.; Wyman, P. J. Med. Chem. 1999, 42, 202.
3. Rogers, D. C.; Hagan, J. J. Psychopharmacology 2001, 158, 114.
4. King, M. V.; Sleight, A. J.; Woolley, M. L.; Topham, I. A.; Marsden, C. A.; Fone, K.
C. F. Neuropharmacology 2004, 47, 195.
i, j, k
j, k
Cl
5. Reavill, C.; Rogers, D. C. Curr. Opin. Invest. Drugs 2001, 2, 104.
6. Regan, C. M.; Foley, A. G.; Murphy, K. J.; Hirst, W. D.; Gallagher, H. C.; Hagan, J.
J.; Upton, N.; Walsh, F. S. Abstracts of Papers, Society for Neuroscience 33rd
Annual Meeting, New Orleans, USA, Nov 8–12, 2003; 835.21.
7. Johnson, C. N.; Roland, A.; Upton, N. Drug Discov. Today: Therapeut. Strategies
2004, 1, 13.
8. Ahmed, M.; Briggs, M. A.; Bromidge, S. M.; Buck, T.; Campbell, L.; Deeks, N. J.;
Garner, A.; Gordon, L.; Hamprecht, D. W.; Holland, V.; Johnson, C. N.; Medhurst,
A. D.; Mitchell, D. J.; Moss, S. F.; Powles, J.; Seal, J. T.; Stean, T. O.; Stemp, G.;
Thompson, M.; Trail, B.; Upton, N.; Winborn, K.; Witty, D. R. Bioorg. Med. Chem.
Lett. 2005, 15, 4867.
NH
N R
N
N
ArO2S
ArO2S
R = H 4, 14-19
22 (R = Me, Ar = 3-Cl-Ph)
20, 21
l
For 15
Scheme 1. Reagents and conditions: (a) conc. HNO3, RT, 15 h, 66%; (b) Boc2O, Et3N,
CH2Cl2, RT, 3 h, 83%; (c) H2 (50 psi), Pd/C, EtOH/dioxane, RT, 2 h, quantitative; (d) N-
iodosuccinimide, MeCN, 0 °C–RT, 20 h, 68%; (e) TMS-acetylene, CuI, PdCl2(PPh3)2,
Et2NH, RT, 15 h, 98%; (f) bis-TMS-trifluoroacetamide, pyridine, PhSO2Cl, CH2Cl2,
0 °C, 3 h, 98%; (g) CuI, DMF, Et3N, 80 °C, 20 h, 86%; (h) NaOMe, MeOH, 45 °C, 3 h,
99%; (i) N-chlorosuccinimide, CH2Cl2, RT, 1 h, 69%; (j) ArSO2Cl, 2-tert-butylimino-2-
diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine (BEMP), CH2Cl2 or
THF, RT, up to 20 h, 44–99%; (k) HCl (4 M in 1,4-dioxane) 50 °C, up to 3 h or MeOH,
HCl (1 M in Et2O), 40 °C, up to 15 h, 8–93%; (l) H2CO, NaBH(OAc)3, AcOH, NaOAc,
MeOH, RT, 10 min, 55%.
9. Bromidge, S. M.; Johnson, C. N.; Moss, S. F.; Rahman, S. S.; Witty, D. R.
WO2003068751, Chem. Abstr. 2003, 139, 197390.
10. Preparation of 9. To
a
solution of 1,1-dimethylethyl 7-amino-8-
[(trimethylsilyl)ethynyl]-1,2,4,5-tetrahydro-3H-3-benzazepine-3-carboxylate
(4.1 g, 11.45 mmol) and pyridine (3.7 ml, 45.8 mmol) in CH2Cl2 (45 ml) cooled
to 0 °C in an ice bath was added N, O-bis(trimethylsilyl) trifluoroacetamide
(6.1 ml,
23 mmol)
dropwise
under
argon
atmosphere.
Then,
phenylsulfonylchloride (1.53 ml, 12 mmol) was added dropwise over
a
period of 15 min at 0 °C. After 3 h, the reaction mixture was poured into
water, and the aqueous solution extracted with CH2Cl2. The combined organics
were dried over MgSO4 and concentrated in vacuo. The resulting crude
material was purified by flash chromatography with a gradient of acetone in
toluene to afford the desired compound 9 (5.6 g, 98%); dH (CDCl3) 0.24 (9H, s),
1.47 (9H, s), 2.76 (2H, br s), 2.86–2.88 (2H, m), 3.47–3.51 (4H, m), 7.06 (2H, d),
7.39–7.44 (3H, m), 7.52 (1H, t), 7.75 (2H, d).
possibly due to N-dealkylation. Sulfone 23 was prepared as a reg-
ioisomer of 4 with the view to blocking a potential metabolic site
on the pyrrole ring. Whilst the change was tolerated potency-wise,
compound 23 also had low in vitro metabolic stability. Expanding
the conformational constraint from a five- to a six-membered ring
(24) led to a loss of potency at the 5-HT6 receptor.
In summary, the conformational constraint of benzazepine 3
and concomitant removal of an acidic NH proved to be a successful
strategy for obtaining compounds with the potential for improved
brain penetration, as exemplified by compound 15, which proved
to be approximately 10-fold more brain penetrant than the starting
benzazepine 3. Compound 20 combined high 5-HT6 antagonist po-
11. Membranes of HeLa cells expressing the 5-HT6 receptor were treated with the
test compound as a solution in DMSO, the agonist 5-HT and ATP. The resulting
mixture was incubated to allow the production of cAMP which was then
measured using a DiscoveRx HitHunter chemiluminescence cAMP assay kit.
12. Rat microsomal intrinsic clearance (CLi) determination: A 2 mM stock solution
of the 5-HT6 receptor antagonist in dimethylsulfoxide (DMSO) was prepared
and used to generate a 0.1 mM final working solution in DMSO. This was
spiked into 50 mM phosphate buffer (pH 7.4) containing 0.5 mg/mL
microsomal protein to give
a final incubation concentration of 0.5 lM
substrate after the addition of NADP co-factor solution. The solutions were
mixed well and pre-incubated for 5 min at ca. 37 °C before the reaction was
started with the addition of co-factor. Samples (50
lL) were then taken at 0, 3,
6, 9, 12, 15, 18, 24 and 30 min and at 30 min for the controls into 200
lL
acetonitrile containing internal standard. This method was carried out in
triplicate. Samples were analysed for parent compound using a specific HPLC/
MS/MS method.
PhO2S
PhO2S
13. Crespi, C. L.; Miller, V. P.; Penman, B. W. Anal. Biochem. 1997, 248, 188.
14. 1H NMR spectra were recorded at 250 or 400 MHz in CDCl3 or DMSO-d6 as
solvent. Compound 20, (HCl salt); dH (DMSO-d6) 3.15–3.28 (8H, m), 7.42 (1H,
s), 7.62 (2H, t), 7.73 (1H, t), 7.89 (1H, s), 8.06 (2H, d), 8.10 (1H, s), 9.11 (2H, br s).
Mass Spectrum: C18H1735ClN2O2S requires 360; found 361 (MH+).
c, d
a, b
11
NH
NBoc
N
Me
N
H
15. Rats received vehicle or the selective 5-HT6 receptor antagonist, dosed orally,
4 h pre-treatment. Animals were then sacrificed, and striatum was removed.
Tissue was homogenised, and a binding assay performed with the selective 5-
12
23
Scheme 2. Reagents and conditions: (a) NaH, PhSSPh, DMF, RT, 15 h, 64%; (b)
MMPP, MeOH/H2O, RT, 22 h, 77%; (c) K2CO3, MeOH, RT, 0.5 h, then MeI, DMF, RT,
20 h, 93%; (d) HCl, 1,4-dioxane, RT, 1 h, 83%.
HT6 receptor antagonist radioligand
determined from the dose–response curve. Drug analysis of blood and brain
samples using LC/MS/MS allowed direct measurement of brain:blood ratio.
[
125I]-258585. An ED50 value was