Thyroid receptor ligands. Part 2: Thyromimetics with improved selectivity for the thyroid hormone receptor beta

Article abstract of DOI:10.1016/j.bmcl.2004.04.032

A set of thyromimetics having improved selectivity for TR-β1 were prepared by replacing the 3^′-isopropyl group of 2 and 3 with substituents having increased steric bulk. From this limited SAR study, the most potent and selective compounds ident

Full text of DOI:10.1016/j.bmcl.2004.04.032

Bioorganic & Medicinal Chemistry Letters 14 (2004) 3549–3553
Thyroid receptor ligands. Part 2: Thyromimetics
with improved selectivity for the thyroid hormone receptor beta
Jon J. Hangeland,a,* Arthur M. Doweyko,a Tamara Dejneka,a Todd J. Friends,a
a
Pratik Devasthale, Karin Mellstrom, Johnny Sandberg, Marlena Grynfarb,
b
b
b
€
John S. Sack,a Howard Einspahr,a, Mathias Farnegardh, Bolette Husman,
b
b
€
ꢀ
Jan Ljunggren,b Konrad Koehler,b Cheryl Sheppard,a,à Johan Malmb and Denis E. Ryonoa
aPharmaceutical Research Institute, Bristol-Myers Squibb, Princeton, NJ 08543, USA
bKaro Bio AB, Novum, Huddinge S-141 57, Sweden
Received 18 February 2004; revised 9 April 2004; accepted 10 April 2004
Abstract—A set of thyromimetics having improved selectivity for TR-b1 were prepared by replacing the 30-isopropyl group of 2 and
3 with substituents having increased steric bulk. From this limited SAR study, the most potent and selective compounds identified
were derived from 2 and contained a 30-phenyl moiety bearing small hydrophobic groups meta to the biphenyl link. X-ray crystal
data of 15c complexed with TR-b1 LBD shows methionine 442 to be displaced by the bulky R30 phenyl ethyl amide side chain.
Movement of this amino acid side chain provides an expanded pocket for the bulky side chain while the ligand–receptor complex
retains full agonist activity.
Ó 2004 Elsevier Ltd. All rights reserved.
3,5,30-Triiodo-
multiple physiological endpoints through binding to its
cognate receptor, thyroid hormone receptor (TR).1 Su-
per-physiological amounts of T3 result in weight loss,
L
-thyronine (T3; 1 in Fig. 1) influences
bone and muscle wasting, cardiac hypertrophy, and
tachycardia. Sub-physiological amounts of T3 produce
the opposite effect resulting in weight gain and brady-
cardia. The cardiac side effects associated with doses
of T3 sufficient for weight loss in obese patients prevent
its use as an anti-obesity agent.
I
3
3'
4'
I
O
I
NH2
O
OH
TR is encoded by two separate genes, TR-a and TR-b.
Each produces multiple splice variants. The predomi-
nant ones are TR-a1 and TR-b1. Tissue distribution
data1b;2 and mouse knockout studies3 have shown that
TR-a1 is the predominant receptor in cardiac tissue. In
light of the apparent central role TR-a1 plays in cardiac
response to T3 levels, it appeared possible to separate
cardiac side effects from metabolic rate increase and
cholesterol lowering by designing agents selective for
TR-b1.4;5
HO
1
5
1
R3
O
O
HO
R5
OH
2: R3 = R5 = Cl
3: R3 = R5 = Br
Figure 1. 3,5,30-Triiodo-
(3).
L-thyronine (1), KB-141 (2), and KB-131092
Initial efforts to identify TR-b1 selective thyromimetics
resulted in the discovery of KB-141 (2), which was 14-
fold selective for TR-b1.5 The SAR derived from this
study showed that substitution of the R3 and R5 iodines
of T3 by chlorine and truncation of the amino acid side
chain at R1 to acetic acid were fundamental alterations
leading to increased selectivity for TR-b1. In the present
Keywords: Thyromimetics; Thyroid hormone receptor.
* Corresponding author. Tel.: +1-609-818-4958; fax: +1-609-818-3450;
e-mail: jon.hangeland@bms.com
Present address: PO Box 6395, Lawrenceville, NJ, USA.
Present address: Cardinal Health PTS, Inc., Research Triangle Park,
NC, USA.
à
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.04.032

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J. J. Hangeland et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3549–3553
study, we report our efforts to improve selectivity for
TR-b1 through modulation of the steric bulk and physio-
chemical properties of the substituents located at R30 of
KB-141. At the outset of this study, such modifications
were known to give thyromimetics with good binding
affinity and full agonist activity but with unknown iso-
form selectivity.6a Recent disclosures from investigators
at Pfizer6b and UCSF6c describe work carried out in
parallel with this study. Both show that replacing the 30-
isopropyl group with more sterically demanding side
chains is a general strategy for gaining improved selec-
tivity for TR-b. We expand on these initial disclosures
with a third chemotype, which gave improved selectivity
for TR-b through the same strategy.
30-phenoxys, and 30-amides (synthesis in Scheme 2). The
latter two series were derived from the 3,5-dibromo-
phenyl acetic acid analog of KB-141 (3) in order to
provide a higher affinity starting point albeit with
slightly lower selectivity (Table 1).
Replacement of the 30-isopropyl group of 2 with the
more sterically demanding phenyl group (9a) caused a
greater loss of affinity for TR-a1 than for TR-b1,
resulting in improved selectivity for TR-b1 (Table 1).
Placement of a trifluoromethyl group in the meta posi-
tion of the 30-phenyl ring (9c) showed that lipophilic
groups provide increased binding affinity for both TR
isoforms and maintained selectivity for TR-b1. The
corresponding ortho- and para-substituted analogs (9b
and 9d, respectively) were relatively unselective. This
initial observation was expanded upon with a series of
Three structural motifs were surveyed: 30-phenyls and
related heterocycles (synthesis in Scheme 1), and
Cl
Cl
HO
HO
O
a
OH
Cl
d-h
(i,e,f,j,h)1
Cl
CH3
BF4
Cl
Cl
O
R
O
O
O
O
c
4
5
O
HO
Cl
9a-s
OH
R4'
CH3
I
O
Cl
O
b
R4'
R4'
8a: R4' = CH3
8b: R4' = Bz
O
O
2
6a: R4' = CH3
6b: R4' = Bz
7a: R4' = CH3
7b: R4' = Bz
X
R =
N
9a: X = H
9h: X = 3-OCH3
9i : X = 3-OCHF2
9j: X = 3-OCF3
9k: X = 2-OH
9n: R =
9o: R =
9p: R =
9q: R =
9r: R =
9b: X = 2-CF3
9c: X = 3-CF3
9d: X = 4-CF3
9e: X = 3- Et
9f : X = 3-iPr
9g: X = 3-Ph
N
N
N
N
N
S
9l: X = 3-OH
9m: X = 4-OH
2
9s: R = C6H11
Scheme 1. Synthesis of the 30-phenyl series. Reagents and conditions: (a) (i)8 oxalyl chloride, cat. DMF, CH2Cl2; (ii) CH2N2, Et2O; (iii) PhCO2Ag,
triethylamine, CH3OH; (b)9 (i) I2, HNO3, Ac2O, TFA; (ii) aq NaBF4; (c)9 Cu0, triethylamine, CH2Cl2; (d) Br2, DCM; (e)10 bis(pinacolato)diborane,
PdCl2dppf, KOAc, DMSO, 85 °C; (f) aryl bromide, Na2CO3, Pd(PPh3)4, dimethoxyethane/H2O, 80 °C; (g) BBr3, CH2Cl2, ꢀ78 to 0 °C; (h) aq NaOH,
2
CH3OH; (i) Br2, HOAc, NaOAc; (j) H2 (1 atm), PtO2, ethanol.1 Alternate reaction sequence for 6h–j for which R4 ¼ Bz. Trifluoromethanesulfonic
0
acid cyclohex-1-enyl ester used in place of aryl bromide in step (f) followed sequentially by steps (j), (g), and (h).
R
O
Br
O
O
Br
Br
HO
d-g
b,c
O
O
a
HO
Br
O
O
O
H
O
O
HO
Br
R = X
OH
O
Br
O
10
11
12
f,h
Br
13a: X = H
13f: X = 2-OH
13b: X = 2-CF3 13g: X = 3-OH
O
R3
O
13c: X = 3-CF3
13d: X = 4-CF3
13e: X = 3-Ph
13h: X = 4-OH
i,g
R
O
O
N
O
HO
O
H
HO
R5
OH
HO
Br
O
15a-j: R3 = R5 = Br
16: R = (CH2)2Ph
14
R
3 = R5 = Cl
O
15a: R = Ph
15f: R =
15g: R =
15j: R =
15h: R =
15b: R = Bz
N
O
15c: R = (CH2)2Ph
15d: R = (CH2)3Ph
15e: R = (CH2)4Ph
15i : R = CH2CHPh2
O
Scheme 2. Synthesis of the 30-phenoxy and 30-amide series. Reagents and conditions: (a) Br2, H2O, rt; (b)9 7a, Cu0, triethylamine, CH2Cl2; (c)
Cl2CHOCH3, SnCl4, CH2Cl2, 0 °C; (d) H2O2, H2SO4, CH3OH; (e) aryl boronic acid, Cu(OAc)2, triethylamine, CH2Cl2; (f) BBr3, CH2Cl2, ꢀ78 to
0 °C; (g) aq NaOH, CH3OH; (h) NaClO, NaHSO3, THF/H2O; (i) diverse amines, EDCI, HOAt, triethylamine, CH2Cl2/DMF.

J. J. Hangeland et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3549–3553
Table 1. IC50 and selectivity data for compounds 1–3, 9a–s, 13a–h, 15a–k, and 16
3551
Compounds
IC50 (nM)a;b
TR-b
Selc
Compounds
IC50 (nM)a;b
TR-b
Selc
TR-a
0.24
25
1.4
TR-a
1
0.26
1.1
0.9
14
9
13a
13b
13c
13d
13e
13f
13g
13h
123
244
50
5.97
7.63
1.1
12
19
27
20
5
2
3
0.1
2.9
9a
9b
9c
9d
9e
9f
9g
9h
9i
152
18
31
3
326
499
1475
549
237
9.5
3.4
0.94
60
40
25
4
83
47
10
7
515
13
67
0.20
4.0
38
18
13
16
23
17
13
9
6.0
23
124
454
297
30
21
11
0.77
3.4
50
15a
15b
15c
15d
15e
15f
15g
15h
15i
424
84
35
4.2
7
12
14
18
9
15
0.62
6.8
9j
100
1052
1160
847
1525
819
145
269
558
258
203
281
228
93
9k
9l
18
37
4.8
75
87
4
9m
9n
9o
9p
9q
9r
9s
6
11
7
139
41
6
96
18
8.5
0.47
12
1
23
10
124
67
15j
1133
67
3.5
2
28
12
20
16
127
21
7.7
a Competitive binding affinities versus radioiodinated T3 using full length cloned hTR-a1 and hTR-b1.5;11
b IC50 values determined from a minimum of duplicate data. Values have an average variability of ꢁ25%.
c Selectivity ¼ (IC50 hTR-a1)/(IC50 hTR-b1 ꢂ 1:7).5;11
analogs (9e–g) designed to probe the effects of more
sterically demanding substituents in the meta position.
The rank order of binding affinity for this series was
found to be Et > CF3 > i-Pr > Ph, with selectivity
decreasing for TR-b1 as the meta substituents increased
in size. Isosteric replacement of ethyl with methoxy (9h),
difluoromethoxy (9i), and trifluoromethoxy (9j) lowered
both binding affinity and selectivity for TR-b relative to
9e. Introduction of hydrophilic groups onto the 30-phen-
yl ring, such as hydroxy (9k–m), resulted in concomi-
tant loss of binding affinity and selectivity for TR-b1
regardless of its position on the 30-phenyl ring. In an
attempt to decrease the overall hydrophobicity of the 30-
phenyl series, several analogs containing heterocyclic
isosteres of the 30-phenyl ring (9n–r) were synthesized.
Without exception, binding affinity and selectivity for
TR-b1 were decreased. The fully saturated analog of 9a,
compound 9s (30-cyclohexyl) had similar binding affinity
and somewhat less selectivity for TR-b1. In all of the
aforementioned analogs, full agonist activity was
maintained (>80% induction gene transcription nor-
malized to T3)7 with one exception; the pyridin-3-yl
analog (9o) had only partial agonist activity (60%
induction)7 for TR-a1 but was a full agonist with TR-
b1.
were probed using the meta phenyl analog (13e). Con-
sistent with the result obtained in the 30-phenyl series,
this analog lost both binding affinity and TR-b1 selec-
tivity. However, in contrast to the 30-phenyl series where
hydroxy groups universally caused the loss of both
affinity and selectivity, a hydroxy group positioned para
to the biphenyl ether linkage yielded a compound (13h)
that was approximately equipotent with 13a and slightly
more selective. The other two positional isomers, 13f
and 13g, were significantly less potent and less selective.
Amide formation at C-30 provided ready entry into more
diverse structures through simple amide bond coupling
with carboxylic acid 14 (Scheme 2). Binding affinity for a
homologous series of phenyl alkyl amides (15a–e)
peaked with the 2-phenyl ethyl amide 15c, while selec-
tivity for TR-b1 was approximately the same for 15c
and its homolog 15d. However, the increase in selectivity
over that of 3 was modest (1.5-fold). Another trend
clearly established by this series was the gradual
decrease in agonist activity for both isoforms as the
aliphatic chain length increased reaching a low of 37%
and 57% induction of gene transcription7 for TR-a1 and
TR-b1, respectively, for compound 15e. Among the
more interesting analogs examined in this series was the
2,2-diphenyl ethyl amide 15i, which was equipotent with
15c and as selective, despite challenging the receptor
binding pocket with a second terminal phenyl ring. As
with the 30-phenyl series, introduction of a basic pyridine
onto the amide side chain (15j) caused a loss of binding
affinity and selectivity for TR-b1.
The SAR of the 30-phenoxy series diverged somewhat
from that of the 30-phenyl series. The parent compound
in the 30-phenoxy series (13a) was approximately equi-
potent with 9a and less selective for TR-b1. Systematic
placement of trifluoromethyl groups around the
30-phenoxy ring (13b–d) showed a preference for
hydrophobic groups in the meta position, consistent
with the trend observed in the 30-phenyl series. The steric
limitations for the meta position in the 30-phenoxy series
Insight into the mechanism by which the receptor
accommodates sterically demanding 30-side chains was
obtained by comparison of X-ray crystal data of the

3552
J. J. Hangeland et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3549–3553
the 30-phenyl series, however, improved TR-b selectivity
was not universally observed. For example, the SAR
revealed by compounds 9a–j shows that both good
affinity and improved selectivity is obtained when the 30-
phenyl ring is either unsubstituted or substituted by a
small, hydrophobic group in the meta position. In con-
trast, compound 9b, which bears a small hydrophobic
group in the ortho position of the 30-phenyl ring, pref-
erentially gains affinity for TR-a and hence has lower
TR-b selectivity relative to compound 9a. These SAR
data point to the likelihood that specific contacts be-
tween the 30-group of the ligand and the 30-binding
pocket of the LBD provide additional determinants for
influencing selectivity for TR-b or for TR-a. The details
of such interactions and their potential impact on the
design of selective thyromimetics awaits further X-ray
crystal analysis.
Figure 2. X-ray crystal structure of 15c/TR-b1 LBD complex12 illus-
trating the movement of M442 (shown in yellow for the X-ray struc-
ture of 2/TR-b1 LBD5).
Improvements to the TR-b1 selectivity and potency of
KB-141 have been achieved through SAR studies that
vary the steric bulk of R30 hydrophobic groups. This
study, in agreement with others reported previously,6b;c
showed that increasing the steric bulk of the 30-group of
thyromimetics is a general approach for improving
selectivity for TR-b. These results further suggest that
partial agonists and, perhaps, antagonists18 can be de-
signed by varying the trajectories and steric bulk of
groups located at the 30-position of thyromimetics in a
way distinct from those needed to create selective
agonists.
TR-b1 LBD complexed separately to 25 and 15c12 (Fig.
2). Comparison of key hydrogen bonding interactions
between the LBD and the two ligands (e.g., arg282 and
arg320 and the R1 acetic acid side chain; his435 and the
R40 phenol) showed the location and orientation of the
R1 side chain and biaryl ether core of 15c and 2 to be
substantially the same. The key structural difference
between the two complexes was movement of met442
(shown in yellow for the 2/TR-b1 complex), resulting in
significant enlargement of the 30-binding pocket imme-
diately adjacent to the 30-phenyl ethyl amide of 15c in a
manner similar to that previously disclosed.6c The phen-
yl ethyl side chain of the ligand adopted a gauche
conformation, complementing the shape of the enlarged
binding pocket.
Acknowledgements
The authors would like to than thank Dr. Minsheng
Zhang and Ms. Yolanda Caringal for insightful com-
ments during the course of this study.
Investigation into the significance of these findings was
carried out by docking compounds 15a–e into a binding
site model based on the 15c/TR-b1 structure.17 The
computed energies (Eass kJ/mol) for this series correlated
well with their observed TR-b1 IC50 values (r2 ¼
0:93; n ¼ 5) suggesting that once the bulky 30-substi-
tuents displaced met442, the differences in relative
binding affinities were largely accounted for by steric
interactions in the newly formed pocket. Movement of
met442 may be involved in the binding of selective
analogs from the 30-phenyl and 30-phenoxy series as well.
The influence that this conformational change has on
isoform selectivity is difficult to pinpoint owing to the
lack of a complementary X-ray crystal structure with the
TR-a1 LBD. In a separate study, a detailed analysis of
binding data and crystallographic data for a novel TR-b
selective thyromimetic, GC-25, and its progenitor GC-1,
provided evidence that the 30-binding pocket of TR-b is
more flexible compared to that of TR-a and that this
difference plays a dominant role in the improved selec-
tivity of GC-25 for TR-b.6c It is reasonable to speculate,
therefore, that a similar mechanism may be partly
responsible for the increased TR-b selectivity observed
for each of the three series describe in this work and for
compounds disclosed by the group at Pfizer.6b Within
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T3, which is normalized to 100%. Percent gene induction is
measured using CHO cells transfected separately with
human TR-a1 or human TR-b1 gene constructs and a
reporter gene coding for soluble alkaline phosphatase
under control of a thyroid hormone receptor response
element.5;11
a reservoir composed of 1.5 M sodium acetate and 0.1 M
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ꢀ
3.0 A data was collected at beam line BW7B, DESY,
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ꢀ
length ¼ 0.8337 A). The data was indexed and reduced by
Denzo and Scalepack13 and belongs to space group
ꢀ
ꢀ
P3ð1Þ21 (a ¼ b68:6 A, c ¼ 129:2 A) with one molecule in
the asymmetric unit. The structure was determined by
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ꢀ
was used in the refinement, but according to obsolete
rejection criteria, the data is only complete and acceptable
ꢀ
to 3.2 A resolution. The structure was refined with Refmac
using TLS15 and the model was built using O.16 The final
structure has an Rfree of 34.7 and Rwork of 26.1. The
ꢀ
final geometry is good (rmsd for distance is 0.015 A and
for angle is 1.5°). The coordinates have been deposited at
www.rcsb.org (1r6g.pdb).
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