´
S. Leger et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4328–4332
4331
Table 5. Selectivity of the amide compounds versus the other
cathepsins
aminoamide 14 (Scheme 1). Subsequently, these amides
can be dehydrated using trifluoroacetic anhydride in
pyridine to give access to the nitrile analogs (compounds
1a–d). However, this approach gave low yields of nitriles
and many degradation products were observed, proba-
bly due to the presence of more than one amide bond.
An alternative route to the nitrile inhibitors involved
the preparation of substituted aminoacetonitrile precur-
sors (15a–d) and performing the peptide coupling reac-
tion at the end of the sequence as in the amide series.
Analysis of the resulting products by HPLC confirmed
the stereochemical integrity of the substituent a to the
nitrile.
Compound R
hrab Cat hCat Bb hCat
hCat
Ka IC50 IC50
Lb IC50 Sb IC50
c
c
c
c
(nM)
(nM)
(nM) (nM)
2a
2e
2g
H
238
>100,000 63,060 41,082
>100,000 55,728 76,938
>100,000 17,043 66,656
CH2CH2SMe 13
CH2Ph 10
a Humanized rabbit enzyme.
b Human enzyme.
c IC50s are an average of at least two independent titrations. See Ref.
10 for assay conditions.
In this novel amide series of cathepsin K inhibitors,
introduction of the appropriate P1 substituent
resulted in a partial recovery of the potency loss
encountered when a nitrile warhead is replaced by a
primary amide. These compounds were shown to be
competitive substrates for cathepsin K, although they
are turned over at such a slow rate that they behave
like inhibitors. This series of compounds also showed
excellent selectivity against the cathepsins B, L,
and S.
The value of the catalytic constant kcat of the amide
compound toward the enzyme appears to be related to
the size of the substitution a to the amide. At the same
time, the affinity of these compounds (estimated Km) for
the active site is improved by the P1 substitution. These
combined effects result in the compounds docking
tightly in the active site; their hydrolysis rates are very
slow when compared to the synthetic substrate, leading
these compounds to behave like enzyme inhibitors.
Introducing P1 substitution also has the potential to im-
prove selectivity over other cathepsins. Selected exam-
ples of primary amide compounds are listed in Table 5
with inhibition activity against cathepsins B, L, and
S. The unsubstituted amide 2a displays a 170-fold selec-
tivity for cathepsin K versus cathepsin S. The most po-
tent cathepsin K inhibitors 2e and 2g both display
excellent selectivity with 2e being slightly superior with
regard to its cathepsin L selectivity (4300-fold vs 1700-
fold for 2g). The substitution in P1 improves potency
against cathepsin K while having little effect in cathepsin
B, L, and S.
References and notes
1. Troen, B. R. Drug News Perspect. 2004, 17, 19.
2. Lecaille, F.; Kaleta, J.; Bro¨mme, D. Chem. Rev. 2002, 102,
4459.
3. Yasuda, Y.; Kaleta, J.; Bro¨mme, D. Adv. Drug Deliv. Rev.
2005, 57, 973.
4. Grabowska, U. B.; Chambers, T. J.; Shiroo, M. Curr.
Opin. Drug Discov. Devel. 2005, 8(5), 619.
5. Black, W. C.; Bayly, C. I.; Davis, D. E.; Desmarais, S.;
´
´
Falgueyret, J. P.; Leger, S.; Li, C. S.; Masse, F.; McKay,
D. J.; Palmer, J. T.; Percival, M. D.; Robichaud, J.; Tsou,
N.; Zamboni, R. Bioorg. Med. Chem. Lett. 2005, 15, 4741.
These amide inhibitors can be prepared in one step via a
peptide coupling of the acid precursor 1312,13 and an
ˆ
6. Li, C. S.; Deschesnes, D.; Desmarais, S.; Falgueyret, J. P.;
Gauthier, J. Y.; Kimmel, D. B.; Leger, S.; Masse, F.;
´
´
McGrath, M.; McKay, D. J.; Percival, M. D.; Riendeau,
´
D.; Rodan, S. B.; Therien, M.; Truong, V. L.; Wesolow-
ski, G.; Zamboni, R.; Black, W. C. Bioorg. Med. Chem.
Lett. 2006, 16, 1985.
CF3
H2N CONH2
CO2H
N
7. Catalano, J. J.; Deaton, D. N.; Furfine, E. S.; Hassell, A.
M.; McFadyen, R. B.; Miller, A. B.; Miller, L. R.;
Shewchuk, L. M.; Willard, D. H., Jr.; Wright, L. L.
Bioorg. Med. Chem. Lett. 2004, 14, 275.
8. Altmann, E.; Aichholz, R.; Betschart, C.; Buhl, T.; Green,
J.; Lattmann, R.; Missbach, M. Bioorg. Med. Chem. Lett.
2006, 16, 2549.
9. The bound complex coordinates shown are obtained from
molecular dynamic simulations. Starting with the PDB
entry 1MEM for the Cat-k structure, a 700 ps trajectory
was performed on the covalently bound complex in TIP3P
water with periodic boundary conditions. The image is
generated by averaging coordinate snapshots of the last
100 ps.
10. Falgueyret, J. P.; Black, W. C.; Cromlish, W.;
Desmarais, S.; Lamontagne, S.; Mellon, C.; Riendeau,
D.; Rodan, S.; Tawa, P.; Wesolowski, G.; Bass, K.
E.; Venkatraman, S.; Percival, M. D. Anal. Biochem.
2004, 335, 218.
+
H
R
13
14
MeO2S
a
CF3
H
CONH2
R
N
N
H
O
MeO2S
2a-h
H
H2N CN
BocN CONH2
b, c
a
+
1a-d
12
R
R
15a-d
Scheme 1. Reagents and conditions: (a) HATU, Et3N, DMF, 18 h, rt,
75–95% yield; (b) TFAA, pyridine, 30 min, rt, 70–75% yield; (c)
methane sulfonic acid, THF, 18 h, rt, 60–75% yield.
11. Robichaud, J.; Oballa, R.; Prasit, P.; Falgueyret, J. P.;
Percival, M. D.; Wesolowski, G.; Rodan, S. B.; Kimmel,