J. Lee et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1879–1882
1881
Mitsunobu coupling reaction between IX and a phtha-
limide, and deprotection of the resulting phthalimide X
using hydrazine provided the aminomethyl proline inter-
mediate XI. The amide coupling between 3-(6-acetyl-
amino-pyridin-3-yl)-acrylic acid and XI using EDCI
provided the desired target (S)-II.15 The corresponding
enantiomer was prepared using (R)-proline tert-butyl ester.
Result and Discussion
The two enantiomers, (S)-II and (R)-II, along with the
intermediates VIII, IX, and X were assayed in both B1
and B2 receptor binding assays.16 As shown in Table 1,
the two enantiomers, (S)-and ( R)-II, showed potent
binding affinities toward the B2 receptor, but the com-
pounds did not show any affinities toward the B1
receptor. (S)-II had a Ki of 33 nM and (R)-II had a Ki of
46 nM toward the B2 receptor. None of the inter-
mediates showed any binding affinity to either of the B1
and B2 receptor, suggesting that the right side of the
molecules was essential for the binding activity. The two
compounds [(R)-, (S)-II] were also evaluated for their
functional activity using GTPgS assay17 and shown to
be functional antagonist to the B2 receptor (data not
shown). (S)-II was further evaluated in the Graded
Abdominal Irritant Test (GrAIT).18 Upon oral admin-
istration of the compound in mouse at 160 mmol/kg, it
showed 59% inhibition.
Figure 4. FR 173675 cyan, R-isomer magenta, S-isomer green.
In summary, we designed b-turn containing surrogates
of FR 173657 based on our initial molecular modeling
studies. The pyrrolidine containing compounds II were
prepared as first targets and the compounds were potent
binders to the bradykinin B2 receptor. Furthermore, the
more potent isomer (S)-II was orally active in GrAIT,
suggesting that the novel compounds could be useful in
antinociception. Finally, it would be interesting to see if
one can apply the result we obtained to other bioactive
molecules that contain the cis-amide conformational
moiety.
As discussed before, our initial conformational analysis
suggested that the (R)-isomer would be more potent
than the (S)-isomer, but the binding data indicated that
the (S)-isomer was actually a little more potent than the
(R)-isomer. Since the experimental results were opposite
of what we expected from the conformational analysis,
we went back to the analysis. Upon further study, we
were able to find a more optimal conformation for the
(S)-isomer by inverting the nitrogen of the pyrrolidine
ring. When the ring is inverted, the (S)-isomer fits as
well to the estimated conformation of FR 173657 as the
(R)-isomer. In fact the fit for the (S)-isomer is slightly
better, as shown in Figure 4. The reason that both iso-
mers show similar potency is probably because each
isomer can adopt a different conformation with respect
to inverting the nitrogen. The (S)-isomer would bind to
the receptor in an inverted position, and the (R)-isomer
would bind to the receptor in the non-inverted way as
shown in Figure 4.
References and Notes
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Table 1. In vitro binding affinity against bradykinin B1 and B2
receptors
Compd
B1% Inhibition
@ 1 mM
B2% Inhibition
@ 1 mM
Inhibition Constant
Ki, (nM)
(R)-VIII
(R)-IX
(R)-X
1
9
1
1
6
1
5
1
11
12
8
76
19
19
8
—
—
—
46Æ3
(R)-II
(S)-VIII
(S)-IX
(S)-X
—
—
—
33Æ1
(S)-II
84