A series of C-terminal amino alcohol dipeptide Aβ inhibitors

Article abstract of DOI:10.1016/S0960-894X(02)00684-4

Potent, small molecule Aβ inhibitors have been prepared that incorporate an alanine core bracketed by an N-terminal arylacetyl group and various C-terminal amino alcohols. The compounds exhibit stereospecific inhibition as demonstrated in an in vitro assa

Full text of DOI:10.1016/S0960-894X(02)00684-4

Bioorganic & Medicinal Chemistry Letters 12 (2002) 3051–3053
A Series of C-Terminal Amino Alcohol Dipeptide Aꢀ Inhibitors
Albert W. Garofalo,a,* David W. G. Wone,a Angela Phuc,a James E. Audia,b
Cheryl A. Bales,a Harry F. Dovey,a Darren B. Dressen,a Beverly Folmer,a
Erich G. Goldbach,a Ashley C. Guinn,a Lee H. Latimer,a Thomas E. Mabry,b
Jeffrey S. Nissen,b Michael A. Pleiss,a Stephen Sohn,a Eugene D. Thorsett,a
Jay S. Tunga and Jing Wua
aElan, 800 Gateway Blvd, South San Francisco, CA 94080, USA
bEli Lilly and Company, Lilly Corporate Center, Indianapolis, IN 46285, USA
Received 17 June 2002; accepted 6 August 2002
Abstract—Potent, small molecule Ab inhibitors have been prepared that incorporate an alanine core bracketed by an N-terminal
arylacetyl group and various C-terminal amino alcohols. The compounds exhibit stereospecific inhibition as demonstrated in an in
vitro assay.
# 2002 Elsevier Science Ltd. All rights reserved.
b-Amyloid peptide (Ab) has been implicated in the
pathogenesis of Alzheimer’s Disease (AD). This protein
is a 39–43 amino acid peptide derived from amyloid
precursor protein (APP) by the action of b- and g-
secretases. b-Secretase has recently been identified1 and
acts on the extracellular portion of APP to form the 99
amino acid peptide, CTFb, which contains the N-ter-
minus of Ab. g-Secretase has not been definitively
characterized but acts on a portion of APP within the
lipid bilayer to create the C-terminal end of Ab. Inhibi-
tion of either of these enzymes is an approach being
pursued to reduce the production of Ab in the hope of
altering AD progression. Herein we describe a series of
amino alcohol dipeptides designed to inhibit Ab forma-
tion at the level of g-secretase.
used in this study is illustrated for compound 4b in
Scheme 1. Carbodiimide coupling of l-norleucine
methyl ester (1) with Boc-protected l-alanine afforded
dipeptide 2.4 Deprotection in neat trifluoroacetic acid
followed by a second carbodiimide coupling with (S)-
3,5-difluoromandelic acid5 gave
3
(3,5-diflouro-
phenylacetic acid was employed for compounds 4a, c, d,
f, g, i, k, m, and o). Reduction of the ester with LiBH4 in
THF then gave amino alcohol 4b.
Previously, peptidic esters and amides have been dis-
closed that inhibit Ab formation.2 The peptidic esters
and amides were shown to be effective with no observ-
able in vitro cell toxicity.3 Continued medicinal chem-
istry efforts with those compounds led to the
development of the dipeptide amino alcohols. We
looked at both primary and secondary C-terminal alco-
hols and observed a large difference in activity between
the two. The synthesis of the amino alcohol analogues
.
Scheme 1. Synthesis of compound 4b. (a) Boc-Ala-OH, EDC HCl,
HOBT, NMM, THF; (b) CF3CO2H; (c) (S)-3,5-difluoromandelic acid,
.
EDC HCl, HOBT, NMM, THF; (d) LiBH4, THF.
*Corresponding author. Tel.:+1-650-877-7647; fax:+1-650-877-7486;
e-mail: al.garofalo@elan.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00684-4

3052
A. W. Garofalo et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3051–3053
Results and Discussion
We examined the ability of these compounds to inhibit
Ab formation in a human kidney cell line (HEK 293)
stably transfected with the gene for APP751 harboring
the Swedish double mutation.6 This cell line secretes
both the 40 and 42 residue forms of Ab. Inhibition
relating to the amount of Ab 40 and 42 was measured in
the cell culture medium.3 Previously observed SAR with
dipeptide esters established the importance of a central
l-amino acid in those inhibitors. To illustrate this point,
diastereomeric dipeptide esters 5 and 6 with both nat-
ural and unnatural configuration at the alanine center
are shown in which a clear advantage is observed for the
(S,S)-diastereomer, 6, over the (R,S)-diastereomer, 5.
With the exception of compounds 4a and 4o, N-terminal
(S)-3,5-difluoromandelic acid derivatives generally exhib-
ited higher potency than the 3,5-difluorophenylacetic acid
derivatives. The most active compound in the series was
the aminophenylpentanol derivative, 4n. It was observed
that within the mandelic acid series of compounds (4b, h, j,
l, n, and p), increasing cellular activity corresponded
roughly with increasing cLog P (Table 2).7
The effect of substitution at C1 of the amino alcohol was
examined with the compounds listed in Table 3. These
secondary alcohols have greatly reduced potency com-
pared to the primary alcohols. Three sets of diastereomeric
pairs were prepared. The ED50 values obtained show a
slight preference for the (2S,1R) analogues over the (2R,1S)
analogues (cf. 8e with 8f). Alkylation to form the N-methyl
amides, 8c and 8d, offered no improvement in activity.
We also prepared a diastereomeric pair of amino indanol
based derivatives, 9a and 9b. As with other secondary
alcohols, the (2S,1R) configuration was preferred. The
ED50 of 9b is comparable to that of the primary amino
alcohols that were examined. The enhanced activity of this
compound compared to other secondary alcohols may be
due to the restricted conformation imposed by the indanol
ring that places the hydroxyl group in a favorable position.
We continued our studies with dipeptide alcohols using
l-alanine as the core amino acid. As shown by compar-
ison of primary (S)-amino alcohols 4a–c with (R)-amino
alcohols 4d–f, the stereochemistry of the amino alcohol
has a large influence on activity with the S-configuration
being preferred (Table 1). The importance of an alipha-
tic group of correct configuration at C2 of the amino
alcohol was also shown by the synthesis of compound 7.
This glycinol derivative, lacking substitution at C2, has
an ED50 of >10 mM.
Table 1. ED50 values for primary amino alcohol dipeptides
Compd
R1
R2
ED50 (mM)
Table 2. Comparison of ED50 and cLog P values
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
H
OH(
H
H
OH(
H
H
OH(
H
OH(
H
OH(
H
(S)-n-Bu
S)-n-Bu
(S)-C6H5
(R)-n-Bu
R)-n-Bu
(R)-C6H5
0.46
0.60
0.18
>29
24.5
>10
0.80
0.20
(S)-CH2C6H11
S)-CH2C6H11
(S)-CH2CꢀCH10.0
S)-CH2CꢀCH2.30
(S)-CH2Ph
S)-CH2CH2Ph
(S)-CH2CH2CH2Ph
S)-CH2CH2CH2Ph
(S)-CH2CH2SCH3
S)-CH2CH2SCH3
Compd
R
ED50 (mM)
cLog P
4b
4h
4j
4l
4n
4p
(S)-n-Bu
(S)-CH2C6H11
0.60
0.20
0.08
0.27
0.13
0.96
1.25
0.91
2.70
0.27
0.87
0.13
0.49
0.96
(S)-CH2CꢀCH2.30
(S)-CH2CH2Ph
(S)-CH2CH2CH2Ph
(S)-CH2CH2SCH3
2.02
2.44
0.26
OH(
H
OH(

A. W. Garofalo et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3051–3053
Table 3. ED50 values for secondary amino alcohol dipeptides
3053
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Compd
R1
R2
R3
ED50 (mM)
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8a
8b
8c
8d
8e
8f
H
H
Me
Me
H
(R)-Me
(S)-Me
(R)-Me
(S)-Me
(R)-Ph
(S)-Ph
(R)-Ph
(R)-Ph
(S)-Ph
(S)-Ph
>10
>10
>10
>10
>10
3.5
3. Dovey, H. F.; John, V.; Anderson, J. P.; Chen, L. Z.; de
Saint Andrieu, P.; Fang, L. Y.; Freedman, S. B.; Folmer, B.;
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H
H
(S)-Ph
(S)-CH2Ph
(R)-Ph
(R)-CH2CO2CH3
8g
>10
In conclusion, we have prepared a novel series of potent
compounds for inhibiting Ab. The majority of the pri-
mary amino alcohols showed sub-micromolar activity in
vitro with compound 4n being the most active. Second-
ary acyclic amino alcohols were prepared which exhib-
ited micromolar activity. This activity could be
improved with the formation of cyclic amino alcohol
derivatives of specified configuration.
4. Compounds 4e and 4f were similarly prepared from their
commercially available amino alcohols.
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