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sponding guanidine 33. Interestingly, the amino-methyl
[2.2.2]bicyclooctane (41) is modestly active, although
20-fold less potent than the sterically less demanding
cyclohexylmethylamine 40. Potent inhibitors are also
obtained by appending a simple straight-chain alkyl-
amine onto the core scaold as in the asymmetric series.
For the symmetrical analogues 42±44, the 1,5-pentane-
diamine derived inhibitor is optimal.
A SAR for a related series of symmetrical amide linked
derivatives is illustrated in Table 3. In general, removal
of the external carbamyl nitrogen present in the urea
series is very well tolerated. Notably, both the phenyl-
acetic and the hydrocinnamic benzamidines 47 and 48
are remarkably potent and more active than the corre-
sponding carbamyl analogues 30 and 31. In contrast,
phenylguanidines 45 and 46 exhibit a 400-fold loss in
activity on proceeding to the hydrocinnamic case. The
amidinopiperidines 49 and 50, as well as the phenethyl-
amine 51, are also substantially more active than their
corresponding carbamyl derivatives. Although the opti-
mal piperidine in this series (52) is moderately potent,
imidazole 53 is nearly inactive.
Scheme 1. Dibasic inhibitor synthetic strategy. Reagents and condi-
tions: (a) 1, 1.0 equiv COCl2, PhCH3, MeCN, K2CO3; (b) 1.0 equiv
tert-butyl 1-piperazinecarboxylate, DIPEA, THF; (c) repeat (a); (d) 2,
tert-butyl 1-piperazinecarboxylate, DIPEA, THF; (e) TFA; (f) 3,
DIPEA, DMF; (g) aq NaOH.
Table 4 summarizes inhibitory potency for tryptase and
related proteases of the core scaold analogues 54±65.
Both alicyclic, heterocyclic and aromatic templates as
well as straight chain alkylene variations are well toler-
ated. 1,3-Adamantanedimethanol derivative 58 illus-
trates the remarkable tolerance for steric bulk in this
region. In the case of straight chain a,o-diacid derived
analogues 61±65, a clear SAR as a function of chain
length is evident with the six methylene homologue (63)
aording optimal subnanomolar activity.
serve as eective terminal nitrogen bases in the asym-
metric series, but either urea or piperidine N-methylation
(18±19) results in a substantial loss of activity. In the case
of cyclohexylamines the trans-isomer 11 demonstrated
nearly 100-fold greater potency than the cis-isomer 10.
For the terminal primary amines (12±14) a charge±
charge distance requirement for optimal activity is
apparent. In this series, a 10-fold improvement in
potency is obtained for each methylene insertion on
proceeding from the propylamine to the pentylamine
homologue.
An inter-active-site bridging mechanism has been pro-
posed for dibasic tryptase inhibitors of this general type
wherein the two terminal nitrogen bases of the inhibitor
dock into the S1 sites of adjacent A and D monomer sub-
units.6,9 The core scaold then serves to span the active-
site gap of roughly 20 A. These highly ¯exible inhibitors
have low energy conformers well suited to span this
distance with the central core of the scaold making
minimal van der Waals contact with the enzyme surface.
This binding model provides a rationale for the range of
core scaold variations tolerated in our survey.
Inhibitory potency versus tryptase and related proteases
for a series of symmetrical urea derivatives (29±44) is
illustrated in Table 2. Simultaneous replacement of both
guanidines in lead 7 with alternative nitrogen bases
aords a SAR that is more responsive in comparison to
the asymmetric series. While replacement of the guani-
dine moiety for amidine (30) is well tolerated in this
series, a 100-fold loss of potency and selectivity is
observed when a guanidine is replaced by an amidine
(30). The most active non-arylguanidine analogue in
this series is benzylamine 36, which is subnanomolar
versus tryptase and extremely selective. Benzylamine
N-methylation (37) or extension to the phenethylamine
homolog 38, results in a dramatic loss of activity. Aro-
matic ring saturation (32±33) is also tolerated, although
a reversal in the SAR as a function of methylene spacer
length is observed when compared to the corresponding
benzamidines (30±31). While the analogous ethylidene-
amino-piperidines (34±35) are inactive, piperidine 39
surprisingly is only fourfold less active than the corre-
In preclinical studies APC-2059 (60) demonstrated
optimal pharmacokinetics and safety. Additionally, in a
sheep model of allergic asthma,10 60 was eective at
blocking both the late phase and hyperresponsive phase
as determined by measuring speci®c lung resistance as a
function of time after antigen challenge with inhaled
Ascaris suum (Fig. 1).
In summary, a remarkably broad SAR exists for the
optimized series of potent, selective, competitive and
reversible inhibitors of human tryptase described herein,
with a range of core template and terminal nitrogen
base modi®cations being well tolerated. With the
exception of the rapidly metabolized primary amines,
we have found that in the symmetrical classes only
guanidine and amidine nitrogen base analogues are of
Rice, Kenneth D.
