S. Ahmad et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1128–1133
1131
tion of the resulting aldehyde (with concomitant removal of the
protecting group when PG = boc) afforded 17a–c and 3b. Removal
of the cbz protecting group accompanied by reduction of the iodo
group of 17a via palladium catalyzed hydrogenolysis afforded 3.
Compound 3a was prepared from 17b via Suzuki coupling using
trimethylboroxine in the presence of catalytic amounts of tetra-
kis(triphenylphosphine)palladium(0) and potassium carbonate
prior to removal of the cbz protecting group. Compound 3c was
similarly prepared from the dibromo compound 17c using a Negi-
shi coupling reaction. The isomeric 6,6,7-tricyclic analog of 3b,
compound 4a, could be prepared from 16d in low overall yield
via a sequence that involved hydroboration followed by Dess–Mar-
tin periodinane oxidation of the resulting primary alcohol to the
corresponding aldehyde and subsequent intramolecular reductive
amination (Scheme 5).
chloro analog 3b (5-HT2C Ki = 9 nM) displayed 25-fold improve-
ment in the binding affinity for 5-HT2C over the corresponding ana-
log in the 6,6,6-series (2e, Ki = 223 nM). In addition, compounds in
this series showed a marked improvement in 5-HT2C functional
activity as demonstrated by a 22-fold increase in potency for com-
pound 3a (EC50 = 2.0 nM, IA = 1.0) over compound (+)-2a. However,
in contrast to most compounds in the 6,6,6-series, compounds 3a
and the chloro analog 3b (as well as the baseline compound 3)
are partial agonists (IA = 0.4–0.6) of the 5-HT2B as well as full ago-
nists (IA = 0.8–1.0) at the 5-HT2A receptors. Extensive SAR studies
were carried out in this series in order to increase functional selec-
tivity for the 5-HT2C receptor over 5-HT2A and 5-HT2B receptors.
These efforts resulted in the generation of the dimethyl analog
(+)-3c with single digit nM 5-HT2C binding and function
(Ki = 6.5 nM, EC50 = 6.6 nM, IA = 1.0), no measurable activation of
Table 1 displays in vitro data for select analogs in the 6,6,6- and
two isomeric 6,6,7-tricyclic amine derived series. All compounds
were initially screened in binding assays for the target 5-HT2C
receptor as well as for 5-HT2B and 5-HT2A selectivity.14,15 Com-
pounds with significant binding affinities were also evaluated for
activity in corresponding functional assays. The baseline com-
pound 2 in the 6,6,6-tricyclic series displayed a modest binding
the 5-HT2B receptor (IA = 0 at 10 lM) and only weak activation
(EC50 = 952 nM, IA = 0.28) of the 5-HT2A receptor. Consistent with
the cell based 5-HT2B functional data, compound (+)-3c did not
show any significant activity in a tissue based (rat fundus) assay
at concentrations up to 10
nist 3a (5-HT2B EC50 = 61 nM, IA = 0.44) produced a weak (IA = 0.2)
response in the rat fundus at 10 M.
lM. In contrast, the 5-HT2B partial ago-
l
affinity for the 5-HT2C receptor (1.9
l
M). However, it displayed full
Based on these data, two compounds were selected for detailed
in vivo testing and characterization. Compound (+)-2a displayed
favorable PK properties in rat with 49% oral bioavailability when
dosed at 1 mpk (Table 2). In addition, when orally dosed at
10 mpk (Table 3), compound (+)-2a exhibited plasma and total
brain exposures of 2122 nM and 3010 nmol/kg, respectively, 4 h
post administration. Demonstrating reasonable exposure, the ago-
nist was advanced into rat efficacy models. In an acute (20 h) feed-
ing assay (Fig. 1), compound (+)-2a produced a dose dependent
reduction in food intake with a robust 22% reduction in feeding
at 10 mpk. Further evaluation of (+)-2a was carried out in a sub-
chronic 4 day weight loss model in the rat (Fig. 2). After 30 mpk
daily oral dosing, compound (+)-2a produced a statistically signif-
icant 4.1% reduction in body weight (as well as a 16% reduction in
cumulative caloric intake) as compared to vehicle-treated animals.
Dimethyl analog (+)-3c was similarly evaluated in vivo. In a rat
pharmacokinetic study (10 mpk, po only), (+)-3c displayed an AUC
functional agonism (IA = 0.96) with an EC50 value of 137 nM. Intro-
duction of small hydrophobic substituents such as chloro, methyl
and trifluoromethyl at the ‘A’ ring resulted in the generation of a
variety of compounds that displayed, with a few exceptions, signif-
icantly improved 5-HT2C binding and functional activities (e.g.,
compounds (+)-2a and 2e). Most compounds in the 6,6,6-tricyclic
series displayed relatively higher binding affinity for the 5-HT2B
over the 5-HT2C receptor. However, compounds in this series were
completely devoid of functional activity at the 5-HT2B receptor.
Thus, while (+)-2a (5-HT2C EC50 = 45 nM, IA = 1) is a potent binder
at the 5-HT2B receptor (Ki = 8.6 nM), the lack of functional agonism
of the 5-HT2B receptor at concentrations up to 10 lM (IA = 0) rep-
resented a more desirable profile. Furthermore, compound (+)-2a,
as well as most compounds in the 6,6,6-series, displayed no signif-
icant functional agonism at the 5-HT2A receptor (EC50 >10 lM).
Expansion of the ‘C’ ring afforded two isomeric series of 6,6,7-
tricyclic amines with significantly different biological profiles.
Chloro compound 4a showed a significantly diminished binding
affinity for the 5-HT2C receptor (Ki = 1547 nM) as compared to 2e.
In contrast, compounds in the isomeric series (represented by com-
pound 3) were dramatically more potent at the 5-HT2C receptor. As
an example, compound 3a with a Ki value of 2.9 nM showed a 30-
fold increased affinity for the 5-HT2C receptor over the correspond-
ing analog in the 6,6,6-series ((+)-2a, Ki = 89 nM). Similarly, the
(0–8 h) of 9.2 lM h in addition to plasma and total brain exposures
of 714 nM and 108 nmol/kg, respectively, at 8 h post dose (Table 3).
In the 20 h feeding model in rat (Fig. 1), compound (+)-3c produced
statistically significant 18% and 25% reductions in food intake at 10
and 30 mpk doses, respectively. As observed with (+)-2a, there
were minimal effects on locomotor activity and no signs of overt
clinical toxicity or malaise with dosing of these compounds. In a
separate acute feeding study, the effect on feeding by (+)-3c was
Table 2
Pharmacokinetic data for compound (+)-2a in male Sprague–Dawley ratsa
Route
Dose (mpk)
T1/2 (h)
Cl (mL/min/kg)
Vdss (L/kg)
Tmax (h)
Cmax (nM)
AUC0-1 (nM h)
%F
Intravenous
Oral
1
1
0.9 0.1
2.5 1.6
27 7.4
ND
1.3 0.1
ND
ND
1.3 0.6
ND
293 95
2344 690
1154 161
—
49
a
Values are reported as means of 3 animals each iv and po. ND = value not determined.
Table 3
Oral pharmacokinetic data for compounds (+)-2a and (+)-3c in male Sprague–Dawley ratsa
Compound
Dose (mpk)
Tmax (h)
Cmax (nM)
AUC (nM h)b
Plasma level (nM)c
Brain level (nmol/kg)c
(+)-2a
(+)-2a
(+)-3c
1
10
10
1
0.5
1.5
398
5376
1749
1040
12637
9242
219
2122
714
326
3010
108
a
b
c
Compounds were dosed only po; values are reported as means of 2 animals.
The reported data was collected over 4 h for compound (+)-2a and 8 h for compound (+)-3c.
Exposure levels for (+)-2a and (+)-3c at 4 and 8 h, respectively.
Ahmad, Saleem
