1-Amino-1,2,3,4-tetrahydronaphthalene-2-carboxylic acid as a Tic mimetic: Application in the synthesis of potent human melanocortin-4 receptor selective agonists

Article abstract of DOI:10.1016/j.bmcl.2005.05.012

The discovery of 1-amino-1,2,3,4-tetrahydronaphthalene-2-carboxylic acid analogs as potent human melanocortin-4 selective agonists is described.

Full text of DOI:10.1016/j.bmcl.2005.05.012

Bioorganic & Medicinal Chemistry Letters 15 (2005) 3430–3433
1-Amino-1,2,3,4-tetrahydronaphthalene-2-carboxylic acid
as a Tic mimetic: Application in the synthesis of potent
human melanocortin-4 receptor selective agonists
Raman K. Bakshi,a,* Qingmei Hong,a J. T. Olson,a Zhixiong Ye,a Iyassue K. Sebhat,a
David H. Weinberg,b Tanya MacNeil,b Rubana N. Kalyani,b Rui Tang,b
William J. Martin,b Alison Strack,b Erin McGowan,b Constantin Tamvakopoulos,a
Randy R. Miller,c Ralph A. Stearns,c Wei Tang,c D. Euan Maclntyre,b
Lex H. T. van der Ploeg,b Arthur A. Patchetta and Ravi P. Nargunda
aDepartment of Medicinal Chemistry, Merck Research Laboratories, Rahway, NJ 07065-0900, USA
bDepartment of Obesity and Metabolic Research, Merck Research Laboratories, Rahway, NJ 07065-0900, USA
cDepartment of Drug Metabolism, Merck Research Laboratories, Rahway, NJ 07065-0900, USA
Received 11 April 2005; revised 26 April 2005; accepted 3 May 2005
Abstract—The discovery of 1-amino-1,2,3,4-tetrahydronaphthalene-2-carboxylic acid analogs as potent human melanocortin-4
selective agonists is described.
Ó 2005 Elsevier Ltd. All rights reserved.
Over the last decade, the melanocortin-4 receptor
(MC4R) subtype has attracted the considerable atten-
tion of medicinal chemists and biologists because of
its potential in the treatment of obesity and sexual
dysfunction.1 Melanocortin receptors are a member
of the family of seven-transmembrane G-protein-cou-
pled receptors. Five different subtypes of melanocortin
receptors (MC1R–MC5R) have been identified and
cloned. Peptide melanocyte-stimulating hormone
(MSH) and adrenocorticotropic hormone (ACTH),
which are produced by the cleavage of a proopiomel-
anocortin (POMC), are natural ligands of these recep-
tors. These receptors have different tissue distributions
and mediate diverse physiological functions such as
hair color, adrenal function, sebaceous gland activity,
feeding, energy homeostasis, erectogenic activity, etc.
The pharmacological link between MC4R and feeding
behavior comes from the studies with both agonists
and the antagonists.2 MC4R antagonist SHU9119
and agouti protein (MC1R and MC4R antagonists)
increase food intake and body weight in rodents.3
Both non-selective peptide MC4R agonist MTII3b
and a selective small molecule MC4R agonist 14 lower
food intake and reduce body weight in rodents.
MC4R agonist 1 also stimulates erectile activity in ro-
dents.4
Compound 1 served as an important tool for studying
the MC4R pharmacology, but its limited oral bioavail-
ability precluded further development as a viable devel-
opment candidate. Herein, we disclose that the synthesis
and biological profile of 1,2,3,4-tetrahydroisoquinoline-
3-carboxylic acid (Tic) mimic 2 that incorporates the
novel Tic surrogate5 and addresses some of the core
issues associated with 1.
Keywords: MC-4 agonist; Tic; Obesity.
*
Corresponding author. Tel.: +7325946491; fax: +7325943007; e-mail:
Raman_bakshi@merck.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.05.012

R. K. Bakshi et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3430–3433
Table 1. Binding affinity and selectivity for the human MC4Ra
3431
b
Compound
IC50 (nM)
MC4R MC3R
1.2 0.11 761 31
MC1R
MC5R
326
34 1.6
1
2063 73
1400 120
940 58
220 0.47
530 86
980 95
620 16
9
11
12
13
2
1.9 0.32
0.56 0.09
1.1 0.16
83 14
48 4.2
97 46
40 6.2
54 23
64 6.3
24 2.8
31 12
0.37 0.02 120 16
0.77 0.07 32 2.7
1.1 0.25 170 57
14
15
a Values represent means standard error. All data represent at least
three determinations.
b Displacement of [125I]-NDP-a-MSH from human receptors expressed
in CHO cells.
Scheme 1. Reagents: (i) chlorosulfonyl isocyanate/ether; (ii) (Boc)2O/
DMAP/Et3N/CH2Cl2; (iii) (a) LiOH/THF/H2O, (b) HCl.
Tic mimetic amino acids 6a–c were synthesized using the
chemistry described in Scheme 1. The reaction of olefins
3a–c with chlorosulfonyl isocyanate furnished azet-2-
ones 4a–c in good yield.6 The azet-2-ones were con-
verted to the Boc intermediates 5a–c and subsequently
hydrolyzed with lithium hydroxide to give the cis amino
acids 6a–c.7
say (Table 1) and 6-fold more potent in functional assay
(Table 2) compared to 11 and 13. This result clearly sug-
gests that the ring size was important and that tetralin
ring in 12 was the preferred choice. Compound 12 was
further separated by preparative reverse-phase HPLC
to afford diastereomer 2 (cis 1R,2R)9 and 14 (cis 1S,2S).
Analog 2 was 4-fold more potent in functional assay
compared to the 1S,2S diastereomer 14, suggesting that
there is a chiral recognition of the tetralin ring at the
binding site. Furthermore, all these compounds had good
selectivity in binding against MC3 and MC5 receptors.
The amino acids 6a–c were coupled with capped peptide
10 (Scheme 2) to give MC4R compounds 11, 12, and 13
as diastereomeric mixtures. Intermediate 10 itself was
obtained by EDC coupling of N-Boc-D-(4-Cl)Phe 7 with
privileged structure 8, followed by the removal of the
Boc protecting group.
Compound 2 was further evaluated in pharmacokinetic
(PK) studies in rats and dogs (Table 3). Clearance in the
rat was rapid, resulting in a short half-life and low oral
bioavailability. The pharmacokinetics of 2 in the dog
was improved and exhibited slower clearance and better
oral bioavailability.
The diastereomeric mixtures 11, 12, and 13 were evalu-
ated for MC4 activity in competitive binding assay
(Table 1) and functional assay (Table 2).8 Compound
12 was 2- to 3-fold more potent in competitive binding as-
Scheme 2. Reagents: (i) EDC, HOBT, NMM, CH2Cl2; (ii) HCl/dioxane, CH2Cl2; (iii) (a) 6a–c, EDC, HOBT, NMM, CH2Cl2; (b) HCl/dioxane,
CH2Cl2; (iv) HPLC separation.

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R. K. Bakshi et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3430–3433
Table 2. Functional activity of compounds at human melanocortin receptorsa
EC50 (nM) [%max]c
b
Compound
MC1R
MC4R
MC3R
MC5R
1
2850 450[95]
1000 180[26]
430 93[21]
280 26[32]
780 43[23]
280 40[22]
960 45[19]
2.1 0.2[97]
21 2.6[34]
2487 43[32]
110 8.4[26]
150 12[19]
[12]
737 65[61]
180 33[75]
200 32[87]
800 75[60]
250 18[68]
130 19[94]
140 10[100]
11
12
13
2
3.3 0.55[52]
17 5.1[51]
1.9 0.13[81]
7.1 0.64[48]
1.5[108]
[4]
99 31[24]
390 41[26]
14
15
a Values represent means standard error. All data represent at least three determinations.
b Concentration of compound at 50% maximum cAMP accumulation.
c Percentage of cAMP accumulation at 10 lM compound relative to a-MSH.
Table 3. Pharmacokinetic data for 2
Studies of the in vitro metabolism of compound 2 in
human liver microsomes indicated that it was both a
reversible and time-dependent inhibitor of CYP3A4.12
PK parameter
Rata
Dogb
F (%)
11
35
42
11
2.9
3.4
2
Cl (mL/min/kg)
Vdss (L/kg)
t1/2 (h)
Since CYP3A4 is an enzyme that is involved in the
metabolism of many drugs, time-dependent inhibition
of this enzyme by compound 2 was of great concern to
us because of potential drug–drug interactions. We rea-
soned that an understanding of the metabolism of com-
pound 2 would help us design analogs that did not have
this potential liability. Those studies indicated that oxi-
dation of the amino group of the tetralin was involved in
the time-dependent inhibition of CYP3A4.12
3.9
2
tmax (h)
1.13
a Compound dosed in Sprague–Dawley rats as a EtOH/PEG/water
(1:4:5) solution at 1 mg/kg iv and 4 mg/kg po.
b Compound dosed in beagles as a solution in EtOH/PEG/water (1:4:5)
at 0.5 mg iv and 2.0 mg/kg po.
Compound 2 was evaluated in an overnight food intake
study at 10 mg/kg po in diet induced obese (DIO) Spra-
gue–Dawley rats for its effect on food intake and body
weight. The compound was administered orally, by
gavage, 1 h before lights off and food intake, and body
weight was measured 18 h post-dosing. Compound 2
was efficacious both in food intake reduction (20%)
and body weight reduction (7 g).10
It was envisioned that introducing the methyl group a to
the basic amine, as shown in compound 15, might atten-
uate the oxidation of amine by CYP3A4. Route to 15
began with amino acid 20 whose synthesis is described
in Scheme 3 and in ways was analogous to 6a–c. The
transformation of the desired amino acid 20 to 15 was
accomplished by using chemistry described in Scheme 1.
Erectogenic activity of compound 2 was evaluated in a
rat ex copula model. In this model, each rat served as
its own control where the effect of compound was com-
pared to that produced by the vehicle. The mean
number of erections elicited over a 15-min period was
determined by a visual count of video-taped events. As
shown below 2 (Fig. 1), increasing the number of erectile
events in rat ex copula erectogenic assay11 at 2 and 5 mg/
kg, iv (57% and 94%), failed to elicit significant pro-erec-
tile activity at oral doses up to 20 mg/kg (35%: P = 0.05).
The human MC4R receptor binding (Table 1) and func-
tional activity (Table 2) of compound 15 were similar to
those of compound 2. Unlike compound 2, however,
compound 15 was not a potent reversible inhibitor of
Scheme 3. Reagents: (i) chlorosulfonyl isocyanate/ether; (ii) (Boc)2O/
DMAP/Et3N/CH2Cl2; (iii) (a) Separation of enantiomers by chiral
HPLC, (b) LiOH/THF/H2O, (c) HCl.
Figure 1. Ex copula erectogenic effect of compound 2 monitored 60–
75 min post-dose. *P < 0.05 paired t test.

R. K. Bakshi et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3430–3433
3433
Strack, A. M.; McGowan, E.; Cashen, D. E.; Drisko, J.
E.; Hom, G. J.; Howard, A. D.; MacIntyre, D. E.; Ploeg,
L. H. T.; Patchett, A. A.; Nargund, R. P. J. Med. Chem.
2002, 45, 4589.
human CYP3A4 and it was also not a time-dependent
inhibitor of that enzyme. Thus, as we had hoped, intro-
duction of the methyl group a to the basic amine pre-
vented the time-dependent inhibition of CYP3A4.
5. Tic mimic 2 design is based on disconnection shown below
In summary, we report the design, synthesis, and biolog-
ical profile of MC4R agonists based on tetralin Tic mi-
metic 2. This compound was a potent MC4 agonist and
was efficacious in animal models for food intake and
erectile dysfunction. Compound 15 was designed as an
MC4R agonist that had a lower potential for drug–drug
interactions based on the results from in vitro metabo-
lism studies.
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