Pyrrolidino-tetrahydroisoquinolines as potent dual H3 antagonist and serotonin transporter inhibitors

Article abstract of DOI:10.1016/j.bmcl.2007.01.106

A series of novel and potent pyrrolidino-tetrahydroisoquinolines with dual histamine H₃ antagonist/serotonin transporter inhibitor activity is described. A highly regio- and diastereoselective synthesis of the pyrrolidino-tetrahydroisoquinoline core involving acid mediated ring-closure of an acetophenone intermediate followed by reduction with NaCNBH₃ was developed. In vitro and in vivo data are discussed.

Full text of DOI:10.1016/j.bmcl.2007.01.106

Bioorganic & Medicinal Chemistry Letters 17 (2007) 2603–2607
Pyrrolidino-tetrahydroisoquinolines as potent dual
H3 antagonist and serotonin transporter inhibitors
John M. Keith,* Leslie A. Gomez, Ronald L. Wolin, Ann J. Barbier,ꢀ
Sandy J. Wilson, Jamin D. Boggs, Curt Mazur, Ian C. Fraser, Brian Lord,
Leah Aluisio, Timothy W. Lovenberg and Nicholas I. Carruthers
Johnson & Johnson Pharmaceutical Research and Development L.L.C., 3210 Merryfield Row, La Jolla, CA 92121, USA
Received 5 January 2007; revised 30 January 2007; accepted 31 January 2007
Available online 4 February 2007
Abstract—A series of novel and potent pyrrolidino-tetrahydroisoquinolines with dual histamine H3 antagonist/serotonin transporter
inhibitor activity is described. A highly regio- and diastereoselective synthesis of the pyrrolidino-tetrahydroisoquinoline core involv-
ing acid mediated ring-closure of an acetophenone intermediate followed by reduction with NaCNBH3 was developed. In vitro and
in vivo data are discussed.
Ó 2007 Elsevier Ltd. All rights reserved.
Depression is a major health issue, with more than 340
million people affected worldwide.1 There is a growing
awareness that patients with depressive disorders often
also suffer cognitive impairment,2 and some studies indi-
cate that these deficits may persist even after remission.3
Fatigue is another prominent symptom of depression.4
Selective serotonin reuptake inhibitors (SSRIs) are the
most frequently prescribed antidepressant drugs. How-
ever, these drugs often fail to improve these symptoms
even as mood improves.5,6 Some SSRIs even induce fa-
tigue and excessive sleepiness.7,8
Histamine H3 receptor antagonists also improve cogni-
tion in a variety of animal models16 and increase wake-
fulness17 without showing nonspecific stimulant effects
such as increased locomotor activity.18 H3 receptor
antagonists have also been shown to decrease REM
sleep,17 which has been observed to improve the mood
of depressed patients in sleep deprivation studies.20 Thus
the case can be made that H3 antagonists would be use-
ful adjuncts to antidepressant therapy for partial
responders to SSRI therapy with persistent fatigue and
sleepiness.21 In the current paper, we describe some as-
pects of our medicinal chemistry efforts to synthesize
molecules that are both H3 receptor antagonists and
serotonin reuptake inhibitors.
Attempts to address these therapeutic needs have largely
relied on the use of modafinil in conjunction with an
SSRI. This drug, a molecule of undetermined mecha-
nism of action,9 improves cognition and increases wake-
fulness.10,11 In recent years, several small trials
investigating the usefulness of modafinil as antidepres-
sant monotherapy or as co-therapy with SSRIs have
been published, bearing witness to the medical interest
in the addition of a mild wakefulness promoting agent
to the current antidepressant regimes.12–15
One of our strategies for the preparation of a dual activ-
ity H3 antagonist/serotonin transporter (SERT) inhibi-
tor was to introduce an H3 pharmacophore22,23 to a
known SERT inhibitor.24,25 As one part of our efforts
to do this, we elected to use a pyrrolidino-isoquinoline
scaffold, developed as part of an earlier inhouse anti-de-
pressant program,26 as a template to which H3 activity
could be added (Fig. 1A). Pharmacophore models for
the H3 receptor suggest a near linear disposition (m-
or p-) of two tertiary amines separated by a 3- or 4-
substituted phenylene and a hydrophobic chain of at
least four atoms should lead to potent antagonists.22
We envisioned attachment of a piperidinylpropyloxy
side chain to the pyrrolidino-isoquinoline scaffold in
Keywords: Histamine H3; Serotonin transporter; Microdialysis;
Reduction.
*
Corresponding author. Tel.: +1 858 784 3275; e-mail:
jkeith@prdus.jnj.com
ꢀ
Present address: EnVivo Pharmaceuticals, Inc., 480 Arsenal Street,
Building 1, Watertown, MA 02472, USA.
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.01.106

2604
J. M. Keith et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2603–2607
N
O
N
N
(a,b,c)
(d)
OEt
OEt
N
N
N
HO
O
O
O
O
A
B
JNJ-5207852
hH3 Ki = 0.46 nM
Figure 1. (A) SERT inhibitor template. (B) Dual SERT inhibitor and
H3 antagonist template with the H3 pharmacophore in bold.
N
N
H
N
(e)
1
N
O
O
such a way as to give a complete H3 pharmacophore
(Fig. 1B).
Scheme 2. Reagents and conditions: (a) 1-bromo-3-chloropropane,
3 equiv K2CO3, acetone, reflux, 2 d, 92%; (b) 3.5 equiv NaI, acetone,
reflux, 2 d, 98%; (c) 1.3 equiv piperidine, 3 equiv K2CO3, EtOH, 60 °C,
2 d, 87%; (d) i—2 equiv N-vinylpyrrolidinone, 1.4 equiv NaH, THF,
rt–reflux, 2 h; ii—6 N HCl reflux, 24 h, 72%; (e) LiAlH4, THF, 24 h, rt,
88%.
Our early synthetic approach to this class of molecules
was to couple a suitably functionalized aryl pyrrolidine
with a mandelic acid derivative and then perform an
acid mediated ring closure on the a-hydroxyamide
(Scheme 1). Reduction of the resultant lactam and
installation of the basic side chain would follow.
With intermediate 1 in hand we examined the use of sty-
rene oxides and bromo-acetophenones as mandelic acid
surrogates.
Unfortunately, preparation of the amide (either with
HATU or thermally) was low yielding and produced a
number of side products. Subsequent ring closure with
polyphosphoric acid (PPA) or methanesulfonic acid
(MSA), then gave a mixture of regio- and diastereomeric
products, of which, the desired 9-cis isomer was a minor
component.27 Further difficulty in installing the H3
pharmacophore led us to examine alternate synthetic
approaches to these molecules.
Alkylation of 1 with substituted styrene oxides gave suit-
able precursors for acid mediated ring closure (Scheme
3). The regioselectivity of epoxide opening was highly
dependent on the aryl ring substituent. Strongly elec-
tron-withdrawing substituents preferentially gave prod-
ucts of attack at the terminal carbon. With all other
substituents, significant (>50% with 4-methyl) opening
occurred at the benzylic position. The regioisomeric
alcohols were difficult to separate from 2 and, if carried
forward, complicated isolation of products 3 and 4.
As it was desirable to have a common intermediate from
which to develop SAR, we elected to construct an aryl
pyrrolidine28 bearing the desired basic side chain
(Scheme 2). The route in Scheme 2 can be performed
on a 100g scale without the use of chromatography by
subjecting the crude products to short-path or Kugel-
rohr distillation.
However, formation of the regioisomeric alcohol could
be avoided by alkylation of 1 with suitably functional-
ized 2-bromo acetophenones followed by reduction of
the amino ketone 5 with NaBH4 to give aminoalcohol
2 (Scheme 4). This approach allowed for the introduc-
tion of strongly electron-donating substituents on the
phenyl ring that were otherwise inaccessible via the
epoxide route. Despite improvements to reaction clean-
liness, the reaction still suffered from poor regio- and
diastereoselectivity. Of four possible isomers, the desired
9-cis product remained a minor component.
O
R
R
HO
OH
(a)
(b,c)
+
O
OH
H
N
N
In contrast, acid mediated ring closure of ketone 5 was
regiospecific, giving only the 9-substituted product as a
mixture of dihydroisoquinoline and isoquinolinium spe-
cies 6 (Scheme 5). Reduction of products 6 to give 7 was
then achieved using NaCNBH3 and HCl.29 Both prod-
ucts are reduced under these conditions and with com-
plete diastereoselectivity for the desired 9-cis isomer 7.
MeO
MeO
R
R
OMe
7
6
+
Binding affinities for compounds 7a–u are shown in
Table 1.30 It is apparent from Table 1 that H3 receptor
affinity is minimally influenced by the substituents on
the pendant aryl ring. All compounds in this series were
shown to be potent antagonists of the H3 receptor in our
functional assay with pA2 values ranging from 8.5 to 10.
N
N
MeO
9
1
Scheme 1. Reagents and conditions: (a) refluxing in xylenes for 2 d
with a Dean–Stark trap or HATU 30–50%; (b) MSA, rt, 40–85%; (c)
BH3ÆTHF, 50–90%.

J. M. Keith et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2603–2607
2605
R
R
O
(a)
(b)
+
N
OH
1
N
O
2
R
R
N
N
O
N
+
N
O
3
4
Scheme 3. Reagents and conditions: (a) EtOH, D, 4 h, quantitative mix of regioisomers; (b) MSA, rt–140 °C, 1–6 h (temperature and time depend on
R).
assays. Consistent with earlier work on the pyrrolidi-
R
O
R
no-isoquinoline scaffold,31 most of the rSERT and
essentially all of the hSERT affinity was due to the (-)-
enantiomer.32 Affinity for the H3 receptor was slightly
higher for the (+)-isomer.
Br
O
(a)
(b)
+
2
N
1
N
The 4-methoxyphenyl derivative (Table 1, 7o) was exam-
ined in the 5-hydroxytryptophan potentiated (5-HTP)
head twitch model for SERT inhibition.33 Mice dosed
ip with this compound exhibited significant and dose-de-
pendent increases in head twitches 24 h after administra-
tion. At 3 mg/kg the head twitch response (HTR)
averaged 384% (n = 3) above vehicle treated mice
(n = 3). At 10 mg/kg the HTR increased to 661%
(n = 3) above vehicle. The HTR did not differ signifi-
cantly from the vehicle treated animals 1 h after dosing
suggesting slow uptake into the brain.
O
5
Scheme 4. Reagents and conditions:(a) Et3N, THF, 30 min, 80–99%;
(b) NaBH4, quant.
R
R
N
N
N
(a)
(b)
5
N
Evaluation of 7o in a rat PK study34 revealed the com-
pound reached a maximal plasma concentration rela-
tively quickly (0.67 h for the oral dose), but had a long
plasma half-life (15.7 h). Bioavailability was modest at
16%. A mouse blood–brain barrier (BBB) penetration
study (30mg/kg ip) gave results consistent with the phar-
macology observed in the rat 5-HTP study in that 7o
was demonstrated to slowly absorb into the brain
(Cmax = 1.28 lM at 24 h). The BBB coefficient was
0.96.35
O
O
6
( ) 7
Scheme 5. Reagents and conditions: (a) MSA, rt–80 °C, 1–6 h
(temperature and time depend on R); (b) NaCNBH3, HCl, MeOH,
bromocresol green, pH 4.5–5.5, 30 min–1 h, 15–70%.
In contrast, rSERT and hSERT Ki values were strongly
influenced by aryl ring substitution. Generally, substitu-
ents in the 3- or 4-positions gave more potent analogs
than their 2-substituted counterparts. Both electron-do-
nating and electron-withdrawing groups were tolerated.
Exceptions include 4-CN (7i), 3- and 4-CF3 (7k and 7l),
and 4-OH (7r). Compounds 7i and 7k are especially
interesting as these moieties are found in the marketed
anti-depressants citalopram (4-CN) and fluoxetine
(4-CF3). With one exception (7m), these compounds
demonstrated somewhat weaker binding affinities for
the human serotonin transporter than the rat serotonin
transporter.
In vivo microdialysis coupled with HPLC-electrochemi-
cal detection was used to assess the effects of 7o on the
extracellular concentration of serotonin (5-HT) and
dopamine in the frontal cortex of the rat brain.36 Com-
pound 7o increased extracellular levels of 5-HT 500%
and dopamine (300%) over basal values 8 h after ip
administration (1 mg/kg) and maintained those levels
of elevation for the duration of the experiment (24 h).
The above in vivo experiments describe a compound
with a delayed but prolonged pharmacological response
due to slow absorption into, and subsequent very slow
Racemate 7o was resolved via chiral HPLC and the indi-
vidual enantiomers examined in our in vitro binding

2606
J. M. Keith et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2603–2607
Table 1. Binding data for rat and human serotonin reuptake transporters and for the human histamine H3 receptor for compounds 7a–u
Compound 7
R
rSERT Ki (nM)
hSERT Ki (nM)
hH3 Ki (nM)
a
H
4-Me
2 ( 0.7)
3 ( 0)
7 ( 1.2)
4 ( 0)
4.7 ( 0.8)
15.7 ( 4)
14.3 ( 4.1)
7.3 ( 1.6)
4.7 ( 2.8)
10.7 ( 1.8)
4.3 ( 1.1)
59 ( 5)
0.9 ( 0.1)
2 ( 0)
4 ( 1.2)
b
c
4-C„CH
3-C„CH
4-Cl
d
8.7 ( 1.8)
6.3 ( 1.5)
4.0 ( 1.9)
8.7 ( 1.8)
3.7 ( 1.1)
24.7 ( 2.3)
5.3 ( 2.3)
26.7 ( 5.8)
21.7 ( 4.0)
11 ( 0.7)
3.3 ( 0.4)
2.3 ( 1.0)
50.5 ( 2.9)
2.2 ( 1.0)
4.7 ( 1.5)
14.0 ( 1.9)
9.3 ( 2.3)
1.3 ( 0.4)
20.7 ( 4.7)
3.3 ( 1.1)
2 ( 0)
e
2.2 ( 0.5)
1.2 ( 0.5)
2.3 ( 0.4)
1.7 ( 0.4)
1.2 ( 0.5)
1.3 ( 0.4)
4.7 ( 1.5)
2.0 ( 0.7)
4 ( 0)
f
3-Cl
2-Cl
g
h
4-F
4-CN
i
j
4-NO2
4-CF3
3-CF3
4-OCF3
4-SMe
4-MeO
4-MeO
4-MeO
3-MeO
2-MeO
4-OH
3-OH
2-OH
4-Me2N
6.8 ( 0.7)
40.3 ( 17.8)
24 ( 7.4)
8.7 ( 1.5)
5.2 ( 1.0)
3.3 ( 0.8)
2000 ( 0)
2.3 ( 1.6)
7.5 ( 3.0)
33.5 ( 7.40)
68.7 ( 12.5)
5.3 ( 2.3)
150 ( 32)
5.3 ( 2.0)
k
l
m
n
2.5 ( 0.9)
0.7 ( 0.2)
0.8 ( 0.2)
2.5 ( 0.4)
1.7 ( 0.4)
1.3 ( 0.4)
1.7 ( 0.4)
0.7 ( 0.1)
3.7 ( 1.1)
1.6 ( 0.5)
o
(+) o
(ꢀ) o
p
q
r
s
t
u
rSERT = rat serotonin transporter, hSERT = human serotonin transporter, hH3 = human histamine H3 receptor: numbers in parentheses are the
standard error of the mean (SEM) for each data set: n P 3 for all in vitro data.
6. Fava, G. A.; Fabbri, S.; Sonino, N. Prog. Neuro-Psycho-
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elimination from, the brain. Despite the modest bio-
availability of the compound, the relatively high brain
concentrations obtained coupled with the high affinity
of 7o for the SERT, robust pharmacology was observed
in both the 5-HTP and microdialysis experiments.
In conclusion, we have developed a regio- and diastereo-
specific synthesis of pyrrolidino-tetrahydroisoquinolines
possessing potent dual H3 antagonist/SERT inhibitor
activity. A profiled member of this class gave robust
responses in both 5-HTP and microdialysis experiments.
Such dual activity compounds could be useful in the
treatment of depression.
14. DeBattista, C.; Doghramji, K.; Menza, M. A.; Rosenthal,
M. H.; Fieve, R. R.; Bielski, R.; DuBoff, E.; Fanelli, J.;
Hartford, J. T.; Kirby, L.; Udelman, H. D.; Zajecka, J.
J. Clin. Psychiatry 2003, 64, 1057.
15. Ninan, P. T.; Hassman, H. A.; Glass, S. J.; McManus, F.
C. J. Clin. Psychiatry 2004, 65, 414.
Acknowledgments
We warmly thank Bruce E. Maryanoff for helpful
discussions regarding the described chemical series
and Heather McAllister for separating the enantiomers
of 7o.
16. Witkin, J. M.; Nelson, D. L. Pharmacol. Ther. 2004, 103,
1.
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31. See Ref. 26.
20
20
32. Compound (ꢀ)7o ½aꢁD ꢀ66° (c 0.006, CH2Cl2); (+)7o ½aꢁD
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34. iv dose = 1mg/kg, oral dose = 10mg/kg; n = 3 for each.
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ed.; Academic Press: San Diego, CA, 1997.
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27. The 7-cis, 7-trans, and 9-trans isomers were all significant-
ly less potent then the 9-cis isomers: data not shown.