H. J. M. Gijsen et al. / Bioorg. Med. Chem. Lett. 22 (2012) 797–800
799
Table 2
Effect of stereochemistry, (thio)urea, and N-alkylation on human and rat TRPA1 antagonism
Structure
Compound
A
Stereochemistry
hTRPA1 IC50
(lM)
rTRPA1 IC50 (lM)
19
40
41
42
S
S
S
RS
R
S
0.13
0.075
>10
3.9
0.020
0.012
>10
4.7
O
O
O
O
O
O
RS
NH
N
H
A
29
43
44
45
S
S
S
RS
R
S
0.050
0.013
>10
1.7
0.011
0.004
>10
0.5
O
O
O
RS
NH
N
H
A
O
38
39
S
S
RS
RS
>10
0.98
N
Me
N
H
S
O
0.085
0.003
NH
N
Me
S
compounds 16, 18, and 20 led to a drop in activity compared to the
unsubstituted 1. In contrast, meta-substitution, such as in 17, 19,
and 21–23 increased the potency relative to 1. Especially meta-
methoxy substituted 19 displayed a more than 10-fold enhanced
potency, and additional analogs around 19 were investigated.
Combination of a meta-methoxy substituent with additional
methoxy substituents as in 25–27 led to a loss in activity. Elonga-
tion of the methoxy group proved to be tolerated better, with 29
and 31 being the most potent compounds identified. Both acidic
and basic substituents were not allowed, as illustrated by 24 and
32–33, respectively.
ies to investigate the therapeutic use of TRPA1 antagonists in ani-
mal models. In general, however, this class of compounds displays
a poor druglike profile, such as very low solubility and low meta-
bolic stability. In addition, the thiopyrimidone class of compounds
is intensely colored, ranging from yellow to dark red. The potential
toxicity of thioureas is another concern.17 Further optimization
will be required to arrive at compounds which could be used as
tool compound for in vivo studies. The tolerance of polar and flex-
ible meta-phenyl substituents such as in 29 and 31, and the accep-
tance of N1 substituents as in 39, could offer initial handles for
improvement of the druglike properties.
For two of the most interesting compounds, 19 and 29, the race-
mates were separated into the enantiomers, and the corresponding
dihydropyrimidones 42 and 45 were prepared, respectively. The
results are shown in Table 2. In line with the data on the enantio-
mers of 1, only the dextrarotary enantiomers 40 and 43 proved to
be active. The absolute configuration of 40 was again determined
to be 4R via VCD. In contrast to 12, dihydropyrimidones 42 and
Acknowledgment
We thank Maarten te Molder for the synthesis of several of the
analogs.
Supplementary data
45 both had a measurable potency below 10 lM but were about
two orders of magnitudes lower in activity than their thio-analogs
19 and 29, respectively. A similar, positive influence of the meta
substitution pattern was observed, with methoxypropoxy substi-
tuted 45 being more potent than methoxy derivative 42.
Supplementary data (The details of the in vitro human and rat
TRPA1 assays and the synthesis and analytical data of 19, 29, 40,
41, 43 and 44 are described) associated with this article can be
The influence of alkylation of the thio-dihydropyrimidine nitro-
gens was briefly investigated via N-methylated compounds 38 and
39. Methylation at N3 as in 38 led to a loss in potency compared to
19, which was more pronounced for the human TRPA1 channel
than for the rat TRPA1 channel. N1-methylated 39 displayed a sim-
ilar potency as 19 in the human TRPA1 channel, and was one of the
most potent antagonists to the rat TRPA1 channel.
The blockade of the TRPA1 channel by the antagonists was con-
firmed in electrophysiological experiments on both the human and
rat TRPA1 channel for 40 and 41.3a The eutomer 40 gave a 100%
References and notes
1. (a) Hinman, A.; Chuang, H. H.; Bautista, D. M.; Julius, D. Proc. Natl. Acad. Sci.
U.S.A. 2006, 103, 19564; (b) Macpherson, L. J.; Dubin, A. E.; Evans, M. J.; Marr, F.;
Schultz, P. G.; Cravatt, B. F.; Patapoutian, A. Nature 2007, 445, 541.
2. Baraldi, P. G.; Preti, D.; Materazzi, S.; Geppetti, P. J. Med. Chem. 2010, 53, 5085.
3. (a) Brône, B.; Peeters, P. J.; Marrannes, R.; Mercken, M.; Nuydens, R.; Meert, T.;
Gijsen, H. J. M. Toxicol. Appl. Pharmacol. 2008, 231, 150; (b) Bessac, B. F.; Sivula,
M.; von Hehn, C. A.; Caceres, A. I.; Escalera, J.; Jordt, S. E. FASEB J. 2009, 23, 1102.
4. (a) Cai, X. Exper. Rev. Neurother. 2008, 8, 1675; (b) Bang, S.; Hwang, S. W. J. Gen.
Physiol. 2009, 133, 257. 5.
5. (a) Caceres, A. I.; Brackmann, M.; Elia, M. D.; Bessac, B. F.; del Camino, D.;
D’Amours, M.; Witek, J. S.; Fanger, C. M.; Chong, J. A.; Hayward, N. J.; Homer, R.
J.; Cohn, L.; Huang, X.; Moran, M. M.; Jordt, S. E. Proc. Natl. Acad. Sci. U.S.A. 2009,
106, 9099; (b) Grace, M. S.; Belvisi, M. G. Pulm. Pharmacol. Ther. 2011, 24, 286.
6. (a) Xiao, B.; Patapoutian, A. Nat. Neurosci. 2011, 14, 540; (b) Wilson, S. R.;
Gerhold, K. A.; Bifolck-Fisher, A.; Liu, Q.; Patel, K. N.; Bautista, D. M. Nat.
Neurosci. 2011, 14, 595.
blockade of ionic currents with an EC50 of 0.01
TRPA1 channel, and a 90% blockade with EC50 of 0.22
rat TRPA1 channel. The distomer 41 did not show any blockade
l
M of the human
lM of the
of the rat channel and 40–70% blockade with an EC50 of 12
the human channel.
lM of
The high potency of this class of TRPA1 antagonists for both the
human and rat TRPA1 channel makes it a potentially attractive ser-
7. Defalco, J.; Steiger, D.; Gustafson, A.; Emerling, D. E.; Kelly, M. G.; Duncton, M.
A. Bioorg. Med. Chem. Lett. 2010, 20, 276.