Bioorganic & Medicinal Chemistry Letters 13 (2003) 2137–2140
Nicotinyl Aspartyl Ketones as Inhibitors of Caspase-3
Elise Isabel,a,* W. Cameron Black,a Christopher I. Bayly,a Erich L. Grimm,a
Marc K. Janes,a Daniel J. McKay,a Donald W. Nicholson,a Dita M. Rasper,a
Johanne Renaud,a Sophie Roy,a John Tam,a Nancy A. Thornberry,b
John P. Vaillancourt,a Steven Xanthoudakisa and Robert Zambonia
aMerck Frosst Centre for Therapeutic Research, Merck Frosst Canada & Co., PO Box 1005, Pointe-Claire-Dorval,
Quebec, Canada H9R 4P8
bMerck Research Laboratories, Rahway, NJ 07065, USA
Received 15 May 2002; accepted 8 April 2003
Abstract—Caspase-3 is a cysteinyl protease that mediates apoptotic cell death. Its inhibition may have an important impact in the
treatment of several degenerative diseases. Since P1 aspartic acid is a required element of recognition for this enzyme, a library of
capped aspartyl aldehydes was synthesized using solid-phase chemistry. The 5-bromonicotinamide derivative of the aspartic acid
aldehyde was identified to be an inhibitor of caspase-3. Substitution at the 5-position of the pyridine ring and conversion of the
aldehyde to ketones led to a series of potent inhibitors of caspase-3.
# 2003 Elsevier Science Ltd. All rights reserved.
The human caspases are a family of at least 11 cysteinyl-
aspartate-specific proteinases that are central compo-
nents in the molecular pathways that result in the
apoptosis of cells.1 These enzymes are divided into three
groups. Group I caspases (1, 4, and 5) mediate cytokine
maturation and are implicated in the inflammatory
response. Group II caspases (2, 3 and 7) are the major
effectors of cell death. Group III caspases (6, 8, 9, and
10) are upstream activator enzymes of the group II cas-
pases. Caspase-14 is a heratinocyte specific caspase that
has not yet been classified. Caspase-3 appears to be a
critical participant in apoptosis in neurons. Prototype
peptidyl inhibitors of caspase-3 have shown efficacy in
models such as stroke, traumatic brain/spinal cord
injury, hypoxic brain damage, and cardiac ischemia/
reperfusion injury.2 DEVD-CHO is a tetrapeptide inhi-
bitor based on the preferred amino acid sequence
recognized by caspase-3. The aspartic acid is an essen-
tial element of recognition of the enzyme and was used
as the basis to develop new inhibitors. Here we report
nicotinyl aspartyl ketones as potent and selective
inhibitors of caspase-3.
In order to generate libraries of aspartyl aldehydes,
rapid analogues synthesis was performed using solid-
phase chemistry (Scheme 1). Fmoc-aspartyl aldehyde 1
was reacted with a semi-carbazone linked resin 33.3,4
The Fmoc protecting group was then cleaved under
standard conditions to afford the polymer 2. The free
amine was submitted to peptidic couplings with a wide
variety of carboxylic acids. Reactions were routinely
conducted on a 100-mg scale of resin. Subsequent
treatment with 9:1 TFA/H2O led to cleavage from the
resin with concomitant t-butyl ester removal to give
final compounds with a good level of purity. Two alde-
hyde replacements were considered: alkyl and thioether
ketones. Alkyl ketones were prepared from Fmoc
aspartate alcohol 9 (Scheme 2), which were submitted to
Swern oxidation followed by an in situ addition of
Grignard reagents. Resulting alcohols were oxidized to
ketones using the Dess–Martin periodinane. These
ketones were then attached to resin 33. The Fmoc pro-
tecting group was cleaved and the free amine was cou-
pled to the 5-bromonicotinic acid. Cleavage from the
resin with wet TFA afforded ketones such as 10–12.
Thioether ketones were synthesized using the Fmoc-
aspartic acid b-t-butyl ester 13 (Scheme 3). After for-
mation of the mixed anhydride, treatment with CH2N2
and addition of a 1:1 mixture of aqueous 48% HBr/
CH3COOH, the a-bromoketone 14 was obtained. As in
*Corresponding author. Tel.: +1-514-428-3655; fax: +1-514-428-
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00390-1