2
D. A. Rudolph et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
As is shown in the Table, the rac-8-methyl-5,6-dihydro-[1,2,4]-
At least two peripherally restricted P2X7 compounds have pro-
triazolo[4,3-a]pyrazin-7(8H)-yl)methanones 7a–c are very active
P2X7 antagonists at the human ion channel. However, only the
pyridyl analog 7b had moderate activity at the rat receptor.
Similarly, the rac-6-methyl-5,6-dihydro-[1,2,4]triazolo[4,3-a]pyra-
zin-7(8H)-yl)methanones 7d and 7e were also active at the human
P2X7 and less so at the rat ion channel.
Interestingly, the rac-5-methyl-5,6-dihydro-[1,2,4]triazolo[4,3-
a]pyrazin-7(8H)-yl)methanones 7f and 7g have far weaker func-
tional activity as measured by the human FLIPR assay. Also of inter-
est in this data set are compounds 7i and 7k.
gressed into clinical trials, primarily for the treatment of arthritis.
However, to our knowledge, none have shown efficacy to date.
Most notable is the Pfizer compound, CE-224,535 (3), a compound
that effectively inhibited interleukin 1b (IL-1b) in human for
3 months, but this compound did not show efficacy in a rheuma-
toid arthritis trial.10 Structure 4 was also of interest, as this com-
pound was reported by Glaxo SmithKline to be a CNS penetrant
P2X7 compound11 with moderate rodent affinity (rat pIC50 = 6.5),
something which is lacking in many published P2X7 compounds,
including compound 3.
Both compounds have very good rat activity (<20 nM) and these
2-chloro-3-trifluoromethylbenzamides provide useful leads for
subsequent compound elaboration.
Of relevance to our research, recently the synthesis of 11C-
GSK1482160 (4) was also reported.12
O
Cl
O
Cl
O
Cl
CF3
CF3
Cl
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
5e
6
5a
F
hP2X7 Ki=4.2 nM
Another set of interesting compounds in the literature are rep-
resented by compounds 5a and 5e. These compounds are reported
to have P2X7 pIC50s of >8.0.13 Our team at Janssen also recently
described the pharmacological characterization of a series of
1,2,3 triazoles as represented by compound 6.14
We profiled several compounds from this series of 1,2,4-tria-
zoles as comparator data, and the results of our in vitro testing
are shown in Table 1.15 Of the compounds tested, all but one have
human P2X7 IC50s of less than 10 nanomolar, and interestingly all
the compounds are weak inhibitors of the rat P2X7 receptor (with
the exception of 5b which has moderate rat activity).
Following up on these data, our goals for this part of the P2X7
project were to prepare novel chemotypes with equivalent or
improved human IC50s, greatly improved rat IC50s and the appro-
priate physical properties that would favor penetration into the
CNS and target engagement at the P2X7 receptor in rat. A sec-
ondary goal of this research was to prepare novel P2X7 antagonists
that could be used as potential PET tracers in order to demonstrate
target engagement in vivo.
One aspect of this work led us to investigate how substitution
on the triazolopiperidine ring system would affect the conforma-
tion of the amide functionality and P2X7 activity. Some initial
efforts in this area included the preparation of the compounds
depicted in Table 2.
After an initial compound screen (as racemates, Table 2) many
of the more promising compounds were either purified to separate
and characterize each enantiomer, or synthesized as single enan-
tiomers, and the data for these compounds along with data for
some additional analogs are shown in Table 3. All of the P2X7
activity for the 8-methyl-5,6-dihydro-[1,2,4]triazolo[4,3-a]pyra-
zin-7(8H)-yl)methanones resides in a single enantiomer (compare
7l and 7m).17 However the rat activity for the active enantiomer 7l
is not ideal.
The rat activity for compound 7k in Table 2 led to further inves-
tigation of the 6-methyl-5,6-dihydro-[1,2,4]triazolo[4,3-a]pyrazin-
7(8H)-yl)methanone series and resulted in the synthesis of 7n–x.
As shown in Table 3, many of these compounds are potent P2X7
antagonists and again, most of the activity is found in one enan-
tiomer, and in this case the activity is due to the S-methyl com-
pounds. For this reason additional analogs were prepared and
some of those compounds are shown in the Table. Several of the
compounds are potent at the human and the rat P2X7 channel,
including the 4-fluorophenyl compounds 7n and 7p, the pyrazines
7q and 7r and the substituted pyridines 7u and 7x, which were
prepared in order to evaluate their potential utility as PET tracers.
Compounds 5a–e were prepared using literature procedures12
and compounds 7a–x were prepared by the procedures outlined
in Schemes 1 and 2.
As such, the rac-8-methyl-5,6-dihydro-[1,2,4]triazolo[4,3-
a]pyrazin-7(8H)-yl)methanones (7a–c and 7h–i) were prepared
from 3-methylpiperazin-2-one (8, Scheme 1) via BOC protection
of the amine followed by activation of the amide and conden-
sation with a hydrazide to give 9. Subsequent removal of the
BOC protecting group and an amino acid coupling provides
the products 7a–c and 7h–i. Compounds 7l–m were isolated
after chiral chromatography. The rac-5-methyl-5,6-dihydro-
[1,2,4]triazolo[4,3-a]pyrazin-7(8H)-yl)methanones 7f–g were
synthesized in the same fashion, starting with 6-methylpiper-
azin-2-one.
The 6-methyl-5,6-dihydro-[1,2,4]triazolo[4,3-a]pyrazin-7(8H)-
yl)methanones 7d–e, 7j–k and 7n–x were prepared using the
routes outlined in Scheme 2.
In this route, (S)-tert-butyl (1-hydroxypropan-2-yl)carbamate
13 was converted the azide 14, reduced to the corresponding
amine and condensed with chloroacetyl chloride to give 15.
Table 1
IC50 data16 for comparator compounds 5
O
Cl
N
N
R2
N
N
R1
Compd
R1
R2
hP2X7 IC50 (nM)
rP2X7 IC50 (nM)
3320 2010
5a
5b
5c
5d
5e
2-Pyrazinyl
2-Pyrazinyl
2-Pyridyl
3-Cl
3-CF3
3-Cl
1.9 0.3
2.1 0.3
7.9 3.2
9.7 0.4
13.5 2.3
92
7
4430 510
1310 40
6390 4400
2-Pyridyl
4-F-phenyl
3-CF3
3-CF3
Rudolph, Dale A.
