Molecular modeling, synthesis, and activity studies of novel biaryl and fused-ring BACE1 inhibitors

Article abstract of DOI:10.1016/j.bmcl.2008.10.096

A series of transition state analogues of beta-secretases 1 and 2 (BACE1, 2) inhibitors containing fused-ring or biaryl moieties were designed computationally to probe the S2 pocket, synthesized, and tested for BACE1 and BACE2 inhibitory activity. It has

Full text of DOI:10.1016/j.bmcl.2008.10.096

Bioorganic & Medicinal Chemistry Letters 19 (2009) 264–274
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Molecular modeling, synthesis, and activity studies of novel biaryl and
fused-ring BACE1 inhibitors
Srinivas Reddy Chirapu a, , Boobalan Pachaiyappan a, , Hikmet F. Nural b, Xin Cheng b, Hongbin Yuan a, ,à
,
David C. Lankin a, Samer O. Abdul-Hay a, Gregory R. J. Thatcher a, Yong Shen b, Alan P. Kozikowski a,
Pavel A. Petukhov a,
*
a Department of Medicinal Chemistry and Pharmacognosy, College of Pharmacy, University of Illinois at Chicago, 833 S Wood Street, Chicago, IL 60612, USA
b Haldeman Laboratory of Molecular and Cellular Neurobiology, Sun Health Institute, Sun City, AZ 85351, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of transition state analogues of beta-secretases 1 and 2 (BACE1, 2) inhibitors containing fused-
ring or biaryl moieties were designed computationally to probe the S2 pocket, synthesized, and tested
for BACE1 and BACE2 inhibitory activity. It has been shown that unlike the biaryl analogs, the fused-ring
moiety is successfully accommodated in the BACE1 binding site resulting in the ligands with excellent
inhibitory activity. Ligand 5b reduced 65% of Ab40 production in N2a cells stably transfected with Swed-
ish human APP.
Received 11 August 2008
Revised 15 October 2008
Accepted 17 October 2008
Available online 25 October 2008
Keywords:
Alzheimer
Ó 2008 Elsevier Ltd. All rights reserved.
BACE1
Aspartic protease
Computer-aided molecular design
Memory loss is the most common characteristic in the clinical
manifestation of Alzheimer’s disease (AD) which affects the learn-
ing of both recent as well as recalled stored information.1 It has
been hypothesized that the therapeutic targeting of amyloid pre-
cursor protein (APP) processing may mitigate the formation of
toxic fragments and prevents plaque formation. This large trans-
membrane protein can be cleaved at three distinct sites by prote-
olytic enzymes collectively referred to as ‘secretases’.2–5 The key
enzyme crucial for the release of amyloidogenic fragments during
APP processing is called b-secretase (BACE1).2 BACE1 enzyme
cleaves APP at the N-terminal side of the Ab sequence to secrete
sAPPb, producing the cell-bound, carboxyl-terminal fragment C99
(also termed C100 or CTF).3 The C99 fragment is then cleaved by
and amyloid plaques count.9 It was, therefore, suggested that low-
ering amyloid plaques in vivo may be achieved by a decrease in
production of Ab40 and Ab42 through the inhibition of BACE1, thus
opening up a new avenue for AD drug discovery.10,11 The product
of b-secretase cleavage, C99, also has toxic and pathogenic effects
in transgenic mouse cells.12,13 Another APP processing player,
a
-secretase, cleaves within the Ab sequence to generate a large
extracellular soluble fragment (sAPP ) and a smaller intracellular
fragment (C83). These fragments appear to have no pathological
significance, although sAPP may have neuroprotective character-
istics. A close-homolog of BACE1, BACE2, is thought to function as
-secretase by cleaving APP in the vicinity of the APP -secretase
a
a
a
a
cleavage site.14,15 Due to its low concentration in the brain, it ap-
pears unlikely that BACE2 plays a significant role in the generation
of Ab40 or Ab42,16 however, it may play an important role in APP
processing on the periphery affecting the muscle degeneration in
AD patients.17 Most of the known BACE1 inhibitors are relatively
polar, non-selective and target other aspartic proteases including
the highly homologous BACE2. Since BACE2 may serve as an alter-
the c-secretase enzyme in the proteolytic cascade of APP process-
ing. The resulting amyloid-beta peptides, called Ab40 and Ab42 be-
cause of the number of amino acids they are comprised of, form the
extracellular neuritic amyloid plaques—one of the key factors in
the pathogenesis of AD.3
It has been shown that BACE1 levels are significantly elevated
in vivo in rapidly autopsied brains of sporadic AD patients (<3 h)
compared with age-matched non-AD patients.6–8 There has also
been reports of a correlation (R2 = 0.54) between BACE1 activity
native a-secretase, its inhibition is highly undesirable and needs to
be addressed during the development of therapeutics that target
BACE1. Multiple studies suggest that the physiologically relevant
cleavage of APP by BACE1 occurs in endosomes where pH is opti-
mal for BACE1 enzymatic machinery18–22 or in cholesterol rich li-
pid rafts.23 Therefore, further improvement of the cell membrane
permeation properties of BACE1 inhibitors would be one of the
most important tasks for lead optimization. A proof of principal
* Corresponding author. Tel.: +1 312 996 4174; fax: +1 312 916 7107.
E-mail address: pap4@uic.edu (P.A. Petukhov).
All these authors contributed equally to this work.
Present address: Burnham Institute, San Diego, CA, USA.
à
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.10.096

S. R. Chirapu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 264–274
265
for an alternative to the passive permeation approach, a BACE1
inhibitor conjugated with a lipid fragment targeting endosomes,
was recently reported by Simons and co-authors.24
molecular modeling methods. The results of these studies are
described in this paper.
First, we established a strategy shown in Figure 3 to identify
novel scaffolds which would allow us to verify our initial hypoth-
eses. A software package for automated de novo design available
in Sybyl,26 Real-time Automated Combinatorial Heuristic Enhance-
ment of Lead compounds (RACHEL),27 was used to generate the
new scaffolds. We elected to maintain the X and Y substituents
in ligand 1, while allowing changes in portions A, B, and C (Fig.
3). The bridging group C was designed to maintain the orientation
of the polar groups (if any) in positions A and B, to decrease the
number of rotatable bonds, and to form an additional interaction
with the binding site. The A, B, C fragments were allowed to be
either ‘nothing’ (in case of the fragment C only), H, or a group se-
lected from an in-house database of small molecule fragments.
The RATOMS (number of ring atoms) descriptor of the chemical
component corresponding to A, B, and C was set to minimum 5
and maximum 10. This database was generated by RACHEL from
a set of drugs and drug-like small molecules found on the Drug-
Bank web-site.28 The RACHEL scoring function was amended to
promote selection of cyclic structures by setting the maximum
number of rotatable bonds to 4 and increasing the weight of the
function from 1 to 4. We selected Asn233 as a target site for grow-
ing substituents and the molecular weight was limited to 200. The
newly generated ligands by RACHEL were re-docked to the binding
site using FlexX docking software. Among all potential ligands gen-
erated by RACHEL bi-cycles and biaryls stood out as groups of li-
gands having the binding poses similar to those found in BACE1
X-rays, targeting the S2 site, and having fewer or no hydrogen bond
donors or acceptors. Upon visual inspection and evaluation of syn-
thetic accessibility two scaffolds were selected for further syn-
thetic elaboration: saturated bicycles with fused 6 or 7 ring
systems containing a heteroatom and biaryls with 5- and 6-mem-
bered terminal aryl (Fig. 3). It was further decided not to use the
actual FlexX docking score because (1) docking scores are typically
less reliable compared with the accuracy of the binding poses pre-
diction, and (2) we expected that the binding site residues in S2
may change their conformation to accommodate the new substit-
uents. To explore if these new chemotypes are indeed compatible
with the binding site of BACE1 two diverse series of biaryl and
5,6,7-membered bicyclic ligands shown in Table 1 were advanced
for further medicinal chemistry efforts and biological testing.
The biaryl and fused-ring ligands were prepared using a com-
mon intermediate 12 according to a published procedure.30 The
synthesis of biaryl ligands is shown in Scheme 1. Commercially
available substituted bromoisophthalic acid 7 was converted into
9 by (1) hydrolysis of 7 with 1 N NaOH which afforded the mono-
The BACE1 binding site and a common binding pose of one of
the recently developed BACE1 inhibitors is shown in Figure 1
(PDB code: 2B8L).25 The in vitro and cell-based potencies, as
reported by Stachel et al., are 15 and 29 nM, respectively.25 In
the course of our preliminary modeling efforts, we identified the
S2 site (Fig. 1) as a potential target for further improvement of
BACE1 activity and selectivity vs. BACE2. First, the BACE1 S2 site
is formed by highly polar residues Asn233, Arg235, and Ser325,
and is solvent-exposed. Many of the known ligands target S2 with
polar groups that impose strict geometric requirements for their
optimal binding. Replacing these polar substituents with either
non-polar ones or groups that require only one or two hydrogen
bonds would lead to less polar ligands in general. Second, the
BACE1 S2 site contains a highly flexible residue Arg235, which
adopts various conformations depending on the nature of P2 sub-
stituents. The extreme displacements of the Arg235 side-chains are
observed in the X-ray structures of 2B8L and 1W51 and are shown
in Figure 2. This observation suggests that the S2 site may accom-
modate novel scaffolds that would not otherwise be considered in
the structure-based drug design efforts for BACE1 because they
would poorly fit into the binding site. Third, despite the very large
homology between BACE1 and BACE2 proteins the conformational
changes that are induced by the ligands would not be identical and,
thus, an improvement in selectivity may be observed.
Collectively, these observations prompted us to hypothesize
that by lowering the number of polar groups we might gain more
freedom in the positioning of the ligand since in this case the li-
gand has to form fewer highly directional hydrogen bonds. The loss
in hydrogen bonding potential would, thus, be compensated for by
non-directional van der Waals (vdW) interactions. In addition,
compounds with fewer polar atoms are expected to have a more
favorable drug-like profile. We also hypothesized that the in-
creased flexibility of the residues in the S2 pocket makes it a per-
fect candidate for exploring different chemical scaffolds for
further improvement activity, selectivity, and drug-like profile.
To validate our hypothesis, we designed two series of compounds
using computer-aided drug design, synthesized them, tested their
activities against BACE1 and BACE2, and analyzed the results using
acid 8 and (2) subsequent coupling of 8 with (R)-a-methylbenzyl-
amine in the presence of EDCA and HOBt resulting in the formation
of 9. Intermediates 10 required for final products 2c and 3c were
prepared by a Sonogashira coupling of bromide 9 with the mono
TMS derivative of acetylene and phenylacetylene, respectively. Su-
zuki coupling of 3-thiopheneboronic acid or 4-chlorophenylbo-
ronic acid with 9 resulted in intermediates 10 required for the
final products 3a and 3b. Sultam intermediates 10 required for
the final products 4a and 4b were prepared by coupling 10 with
the corresponding ring sultams. The final compounds 2c, 3a–3d,
4a–4b were obtained by hydrolysis of 10 to form the acid interme-
diate 11 followed by subsequent coupling with the TFA salt of (2R,
3S)-N-1-2-hydroxy-4-phenylbutane-1,3-diamine.31–33 Compounds
2a and 2b were prepared using hydroxy and methoxy derivatives
of diethyl esters of isophthalic acid in four steps: hydrolysis of
one of the ester group with 1 N NaOH, coupling with the left side
Figure 1. The binding site of BACE1, ligand 1, and a crystal water molecule in S2.
The key residues in S2 site are rendered as ‘ball and stick’, whereas the ligand as
‘capped sticks’. Water 43 is shown as blue sphere. The solvent accessible area of the
binding site is rendered by red surface.
fragment (R)-a-methylbenzylamine, hydrolysis of the remaining
ester group using 1 N NaOH, and finally coupling with the TFA salt
of (2R, 3S)-N-1-2-hydroxy-4-phenylbutane-1,3-diamine.

266
S. R. Chirapu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 264–274
Figure 2. Two different conformations of Arg235 in BACE1 PDB structures 2B8L (left) and 1W51 (right) are rendered as capped sticks and are color coded (green, carbon; blue,
nitrogen; red, oxygen; yellow, sulfur).
C
B
RACHEL
A
X
X
Y
Y
X
Y
input from
medicinal
chemists
1
O
O
O
O
O
O
bi-cycles
biaryls
X, Y - the same as in ligand
A - allowed to be changed by RACHEL
using an in-house database of druglike
fragments
docking using FlexX
5,6-membered
aryls
6,7 - saturated rings
visual analysis and
evaluation of
synthesizability
X
X
Y
Y
O
O
O
O
biaryls
bi-cycles
Figure 3. De novo design strategy utilized to find P2 fragments.
The bis-esters 15, 17, and 19 were prepared according to the re-
ported procedures starting from 14 (Scheme 2).34,35 Subsequent
selective hydrolysis of 17 yielded monoester 18a as a major prod-
uct whereas 19 resulted in monoester 20a as a major product.
(2) the careful examination of the patterns of the 3JC,H correlations
present in the gHMBC 2D spectrum of each of the hydrolysis prod-
ucts, specifically the correlations pertaining to the respective car-
bonyl carbons. In the generalized structure (n = 1 or 2; Fig. 4a),
the aromatic ring proton located between both carboxylate car-
Further treatment of 16a, 18a, and 20b with (R)-a-methylbenzyl-
3
amine led to the corresponding intermediates 21–23 that upon
hydrolysis and subsequent reaction with 12 resulted in the final
products 5a, 5b, and 6c (Scheme 3). Reaction of 16b, 18b, and
20a with 12 resulted in intermediates 24–26 which upon hydroly-
bons will exhibit JC,H correlations in the gHMBC to both carbonyl
carbon signals (red and blue). The remaining aromatic ring proton
(right side) will show the indicated 3JC,H correlation only to the car-
bonyl carbon on the right side (in red). If a carbonyl exists as an es-
ter moiety, that is, with a methoxy group bonded to the carbonyl
carbon, there will be an additional correlation cross peak in the
sis and subsequent coupling with (R)-a-methylbenzylamine led to
the final ligands 5c, 6a, and 6b (Scheme 4). The chemical character-
ization of compounds 5b36 and 5c37 is attached as a footnote.
Theoretically, two regioisomeric monoesters could have been
produced from each diester 15, 17, and 19 by hydrolysis of the
right side ester or the left side ester. The structure of the hydrolysis
products produced from 17 (Fig. 4b and c) and 19 were deduced in
a self-consistent manner from the interpreted results of extensive
NMR studies (1D 1H and 13C, 2D gCOSY, gHSQC, and gHMBC). The
assignment strategy employed for the determination of the regio-
isomeric structure of the monohydrolysis products represents a
straight forward approach involving: (1) unambiguous assignment
the 1H and 13C NMR spectra for each of the hydrolysis products and
3
gHMBC 2D spectrum arising from a JC,H coupling to the protons
of the methoxy group to that carbonyl carbon. In this way, unam-
biguous assignment of the structure of the hydrolysis products can
be made with confidence. This concept is illustrated for the starting
dimethyl ester 17 (n = 1, R = R0 = Me), for which all of the 1H and 13C
resonances were assigned.
In the case of the hydrolysis product 18a derived from 17, the
1D proton NMR spectrum (400 MHz, DMSO-d6) confirmed the
presence of three chemically distinct methylene groups with reso-
nances centered at d 1.921 (m), 3.027 (triplet, J = 5.2), and 4.175
(triplet, J = 6.6), each integrating for two (2) protons and all associ-

S. R. Chirapu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 264–274
267
Table 1
BACE1 and BACE2 inhibition profile of P2 substituents.
R1
OH
H
N
H
N
H
N
O
O
Ligands
R1
BACE1 IC50 (nM)
281 42
BACE2 IC50 (nM)
132 17
logBBa
O
O
N
1
ꢀ0.9
S
2a
2b
OH
OMe
1581 269
2506 576
>100,000
>100,000
ꢀ0.8
ꢀ0.5
2c
889 108
>100,000
ꢀ0.3
S
3a
2233 384
>100,000
ꢀ0.6
Cl
3b
811 102
>100,000
ꢀ0.1
Ph
3c
1119 173
1256 140
>100,000
>100,000
ꢀ0.1
ꢀ0.5
N N
N
3d
O
S
4a
4b
1774 218
158 19
141 16
251 33
ꢀ0.9
ꢀ0.8
N
N
O
O
O
S
R2
OH
H
H
N
H
N
R1
N
O
O
Ligands
-R1-
R2
BACE1 IC50 (nM)
524 150
BACE2 IC50 (nM)
>100,000
logBBa
O
O
5a
H
ꢀ0.3
5b
H
63
8
>100,000
ꢀ0.3
(continued on next page)

268
S. R. Chirapu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 264–274
Table 1 (continued)
Ligands
-R1-
R2
BACE1 IC50 (nM)
BACE2 IC50 (nM)
>100,000
logBBa
O
5c
5d
5e
H
56
7
ꢀ0.3
O
F
F
740 95
2089 311
1949 207
ꢀ0.3
ꢀ0.2
O
O
262 36
6a
H
998 78
>100,000
ꢀ0.3
O
6b
6c
H
H
112 12
>100,000
>100,000
ꢀ0.4
ꢀ0.2
O
89 13
a
Calculated log BB.
ated with the 6-membered ring present in the monohydrolyzed
product. The gCOSY spectrum confirmed that the methylene
groups constituted an isolated mutually coupled proton spin sys-
tem. A singlet (3H) appearing at d 3.837 was assigned to the methyl
group of the monoester moiety. Two doublets were observed cen-
tered at d 7.979 and d 7.428 (J = 1.5 Hz), which were shown to be
spin coupled (gCOSY) and were assigned to the two aromatic pro-
tons present on the aromatic ring. The magnitude of the J-coupling
between the two aromatic protons confirmed the indicated meta-
relationship. The protonated carbons could be assigned from the
gCOSY spectrum confirmed that these resonances also constituted
an isolated mutually coupled proton spin system. A singlet (3H) at
d 3.854 was assigned to the methyl group of the monoester moiety.
Two signals were observed at d 7.937 and d 7.582, which were
shown to be spin coupled (gCOSY) (J = 1.7 Hz) and which were as-
signed to the two aromatic protons present on the aromatic ring.
The magnitude of the J-coupling between the two aromatic protons
is also consistent with the indicated meta-relationship. Similar to
the NMR of 18a, the hydrolysis product of 17, a broad resonance
(1H) at d ꢁ13 ppm was also observed and was assigned to the pro-
ton of the expected free acid functionality. The protonated carbons
could be easily assigned from the gHSQC spectrum (1JC,H). The
gHMBC of hydrolysis product 20a from 19 showed correlations
from the proton at d 7.937 to the both of the carbonyl carbons
appearing at d 166.0 and 167.2 ppm. There was a correlation ob-
served from the proton signal at d 7.582 to the carbonyl carbon
at d 166.0. The carbonyl at d 167.2 also showed a correlation in
the gHMBC spectrum to a methyl singlet (d 3.854) indicating that
the hydrolysis of 19 proceeded at the right side carbonyl (red, in
the generalized structure). Analysis of the 2D gCOSY, gHSQC, and
gHMBC data together with the 1D 1H and 13CNMR data obtained
for both hydrolysis products permitted a self-consistent assign-
ment of the 1H chemical shifts and of the 13C chemical shifts for
both regioisomers. The 1H and 13C NMR assignments of intermedi-
ates 17 and 19 and their hydrolysis products 18a and 20a are
shown in Figure 4d and e.
3
gHSQC spectrum (1JC,H) and gHMBC spectrum (2JC,H and JC,H). A
broad resonance (1H) appeared at d ꢁ13 ppm and was assigned
to the proton of the free acid. The gHMBC of hydrolysis product
18a from 17 showed correlations from the proton at d 7.979 to both
of the carbonyl carbons at d 166.2 and 166.3 ppm. There was a cor-
relation observed from both the proton signal at d 7.428 to the car-
bonyl carbon appearing at d 166.3 as well as a correlation cross
peak to the ester methyl group at d 3.837 indicating that the hydro-
lysis of 17 proceeded at the left side carbonyl in the generalized
structure. In contrast, the 1D proton NMR spectrum (400 MHz,
DMSO-d6) of the hydrolysis product 20a from 19 confirmed the
presence of four chemically distinct methylene groups with
slightly broadened resonances centered at (d 1.673, 1.899, 2.992–
3.018 m, and 4.012 t (J = 6.6 Hz) each integrating for two (2) pro-
tons and all associated with the 7-membered ring present in the
mono-hydrolyzed product from 19. As in the case of 20a, the

S. R. Chirapu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 264–274
269
Br
Br
(b)
see text
H
N
X2
X1
X2
O
O
O
O
9
7
8
X1 = X2 = OMe
X1 = OH
(a)
R1
R1
OH
(c)
H
N
H
N
H
N
H
N
X2
OH
H
N
H2N
O
O
O
O
10 X2 = OMe
12
(a)
13
X2 = OH
11
Cl
Ph
S
N N
N
O
O
Ph
S
S
N
O
O
R1 =
2c
3a
3b
3d
4a
4b
3c
Scheme 1. Reagents and conditions: (a) 1 N NaOH; THF/MeOH (50:50) (90%); (b) (R)-
a-methylbenzylamine, EDCA/HOBt, 0 °C to rt (92%); (c) 8, EDCA/HOBt, 0 °C to rt (90%);
(c) BOP, DIPEA (90%). In this and further schemes the typical yields of the reactions are provided in parentheses.
For IC50 measurements, 293T cells stably transfected with
pcDNA-BACE1 were maintained in 200 g zeocin/ml DMEM, 10%
FBS. Cells were lysed by lysis buffer (PBS with 1% Triton X100
and 0.1% SDS), and the lysate was adjusted to 4 g/ l. The BACE1
inhibitors were diluted to the desired concentration with reaction
buffer (100 mM Tris–HCl, 100 mM NaCl, pH 4.5). The consistent
expression levels of BACE1 protein and enzyme activity were con-
l
firmed by BACE1 ELISA and BACE1 activity assay. We notice that
pH 4.5 is not ideal for Tris buffer, however, we found that Tris buf-
fer does not interfere with the activity and stability of the BACE1
(and BACE2) lysates at pH 4.5 necessary for their enzymatic
activity.2,9,38
l
l
O
O
O
(b)
+
MeO
OH
MeO
OMe
HO
OMe
O
O
O
O
O
O
(a)
15
17
16a Major (predicted)
16b Minor (predicted)
O
O
O
OH
(b)
(b)
(c,d,e)
+
HO
MeO
OH
OMe
MeO
OMe
MeOOC
COOMe
O
O
O
O
O
O
14
18a Major
18b Minor
(f,d,e)
O
O
O
+
MeO
OMe
MeO
HO
OH
OMe
O
O
O
O
O
O
20a Major
19
20b Minor
Scheme 2. Reagents and conditions: (a) PdCl2, Cu(OAc)2, LiCl, MeOH/H2O (2 mL:0.1 mL) (85%); (b) 1 N NaOH; THF/MeOH (50:50), 0 °C to rt (80%); (c) tetravinyltin, Cu(OAc)2,
O2, CH3CN, 2 days (82%); (d) 2nd generation Grubbs catalyst, DCM, rt (80%)29; (e) H2, Pd/C, MeOH (90%); (f) allyl bromide, K2CO3, acetone, reflux (96%).

270
S. R. Chirapu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 264–274
Fused-ring
Fused-ring
b, c
a
H
N
HO
OMe
OMe
OH
H
N
H2N
O
O
O
O
12
16a
18a
20b
21
22
23
Fused-ring
OH
H
N
H
N
H
N
O
O
5a
5b
6c
Scheme 3. Reagents and conditions: (a) (R)-a-methylbenzylamine, EDCA/HOBt, 0 °C to rt (92%); (b) 1 N NaOH; THF/MeOH (50:50), 0 °C to rt (90%); (c) EDCA/HOBt, 0 °C to rt
(85%).
The BACE1 substrate (EDANS-SEVNLDAEFR-DABCYL) was dis-
solved in DMSO as stock and then diluted to 10 M working solu-
tion. We then mixed 20 g of 293T/pcDNA-BACE1 of cell lysate
with BACE1 inhibitor and the substrate, and the final substrate
concentration was 5 M. Fluorescence was detected in microplate
reader (Bio-tek) every 5 min at emission length 500 nm as well as
excitation at about 430 nm. Again, the consistent expression levels
of BACE1 protein and enzyme activity were confirmed by BACE1
ELISA and BACE1 activity assay. Maximal velocities were calculated
by the time point within 20 min. Similarly, 293T cells stably trans-
BACE2 inhibition was then calculated and the results are shown
in Table 1.
l
l
The following procedure was implemented to determine the
efficacy of our best BACE1 inhibitors in reducing the Ab40 produc-
tion. Mouse neuroblastoma cells (N2a) stably transfected with
Myc-epitope tagged Swedish mutant APP 695 cDNA (a kind gift
from Dr. Gopal Thinakaran) were cultured in 1:1 Opti-MEM/Dul-
becco’s modified Eagle’s medium (high glucose) containing 5% fetal
bovine serum, 1% penicillin/streptomycin and 0.2 mg/ml G418 in a
humified air incubator at 37 °C (5% CO2–95% O2). Cells were plated
in 24 well plates for 24 h at a concentration of 15 ꢂ 104 cell/well.
After 24 h, the growth medium was discarded and replaced with
0.5 ml of serum reduced medium consisting of Dulbecco’s modified
Eagle’s medium (high glucose) and 0.2% fetal bovine serum. Fifteen
minutes after medium replacement, the cells were treated with
drugs dissolved in DMSO and incubated for 24 h. At the end of
l
fected with pcDNA-BACE2 were maintained in 200
(G148)/ml DMEM, 10% FBS. Cells were lysed by lysis buffer (PBS
with 1% Triton X100 and 0.1% SDS), 5 g protein were used in
lg Geneticin
l
the assay. The BACE2 inhibitors were diluted to desired concentra-
tion with a reaction buffer (100 mM Tris–HCl, 100 mM NaCl, pH
4.5). The BACE2 substrate (MCA-ERHADGLALALEPA(K-Dnp) was
dissolved in DMSO as stock. The cell lysate was mixed with BACE2
the incubation period, 100 ll of conditioned media were collected
inhibitors and the substrate (final concentration 5
l
M). Fluores-
and the protease inhibitor AEBSF (4-(2-aminoethyl) benzene sulfo-
nyl fluoride hydrochloride) was immediately added at a final con-
centration of 1 mM. The concentration of Ab40 was determined
using human beta amyloid [1–40] colorimetric ELISA kit from
cence was detected every 5 min at an emission length of 430 nm
as well as excitation at about 340 nm. Maximal velocities were cal-
culated by the time point within 20 min. The IC50 of BACE1 and
Fused-ring
Fused-ring
OH
b, c
a
H
N
H
N
MeO
MeO
OH
OH
H
N
H2N
O
O
O
O
Fused-ring
16b
18b
20a
24
25
26
OH
H
N
H
N
H
N
12
O
O
6a
6b
5c
Scheme 4. Reagents and conditions: (a) EDCA/HOBt, 0 °C to rt (90%); (b) 1 N NaOH; THF/MeOH (50:50), 0 °C to rt (95%); (c) (R)-a-methylbenzylamine, EDCA/HOBt, 0 °C to rt
(90%).

S. R. Chirapu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 264–274
271
[CH2]n
O
H
O
RO
O
OR'
(a)
H
17 ( n = 1 R = R' = Me )
19 ( n = 2 R = R' = Me )
1H and 13C NMR Data
δ4.214 t
66.1
δ2.021 m
δ3.159 t
21.7
24.2
155.6
128.7
52.2
O
O
H δ7.621 d
δ3.922 s
121.7
OMe
166.1
129.2
δ3.908 s
52.0
MeO
MeO
OMe
123.6
( CDCl3 )
167.0
O
O
H
O
O
δ8.129 d
129.4
(c)
(b)
17
13C NMR Data
65.7
O
30.9
24.3
28.6
73.4
21.1
23.5
155.2
120.7
160.9
124.5
O
129.4
52.1
128.7
166.3
129.7
OH
140.8
52.4
HO
OMe
MeO
122.5
125.2
O
O
166.2
(DMSO-d6)
O
O
166.0
(DMSO-d6)
167.2
130.7
130.3
18a
20a
(e)
(d)
Figure 4. (a) General structure of ligands 17 and 19; (b) 1H NMR assignments for 17; (c) 13C NMR assignments for 17; (d) 13C NMR assignments for 18a; (e) 13C NMR
assignments for 20a.
Figure 5. (a) Gold docked conformations of all ligands in BACE1 binding site. The P2 moieties are circled yellow; (b) biaryl compounds 3a–3d in BACE1 binding site; (c) cyclic
ring sultam compounds 4a–4b in BACE1 binding site rendered as ribbon and tube representation. Residues Asn233 and Arg235 are rendered as ball-and-stick
representations; (d) overlay of docked poses of ligands 5a–5c when side-chain flexibility of Arg235 is considered; (e) overlay of docked poses of ligands 6a–6c when side-
chain flexibility of Arg235 is considered; (f) ligand 5e in BACE1 binding site. Different orientations of methyl group in Ala335 observed in X-ray crystal structure is rendered as
a ball-and-stick representation and circled in red for clarity.

272
S. R. Chirapu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 264–274
Invitrogen. The procedure was followed as instructed in the sup-
plied protocol.
are shown in Figure 5b. The 3-thiophene moiety in 3a did not sig-
nificantly influence the activity compared to 2b. A replacement of
R1 with the bulkier p-chlorophenyl moiety in ligand 3b led to a 3-
fold increase in potency compared to 3a and an almost equal activ-
ity to 2c, a ligand with a smaller R1 acetylene substituent. An anal-
ysis of the docking pose of 3b shows that binding of its p-
chlorophenyl substituent requires a displacement of the conserved
water molecule WAT43 (in 2B8L) typically mediating hydrogen
bonding between the side-chains of Ser325, Arg235, and Gln326
(discussed below). It is likely that an induced fit disorder caused
by the removal of the conserved water molecule may contribute
to improved binding energy of 3b.44 When the GOLD program
was allowed to determine whether WAT43 should be bound or dis-
placed upon binding of 3b, the resulting ligand–protein complex
did not contain WAT43. Probing of the binding site with larger, ex-
tended substituents as in the phenylacetylene- and N-benzyltriaz-
ole-containing ligands 3c and 3d resulted in a 1.4- and 1.6-fold loss
of activity compared to 3b, respectively. The loss was more pro-
nounced in 3d where the benzyl appendage appears to be com-
pletely solvent-exposed. Unlike 3c and 3d, ligands 3a and 3b fit
well within the cleft of the S2 pocket. The predicted logBB values
for the biaryl series range from modest ꢀ0.6 and ꢀ0.5 for com-
pounds 3a and 3d, respectively, to ꢀ0.1 for both 3b and 3c. Simi-
larly to ligands 2a–c ligands 3a–d are not active against BACE2 at
The subsequent molecular modeling analysis was performed in
the following way. The X-ray coordinates of BACE1 (PDB code:
2B8 L) were downloaded from the Protein Databank. The catalytic
residue Asp32 was protonated according to the published theoret-
ical studies.39,40 The active site was defined as a sphere enclosing
residues within 10 Å of the bound ligand. The 3D structures of li-
gands shown in Table 1 were built using Sybyl 7.326 and were en-
ergy minimized using BFGS algorithm, 5000 iterations, and
gradient of 0.005 kcal/mol/Å as the termination criterion. The
resulting minimized molecules were docked to the binding site
of BACE1 using the GOLD software.41 All poses outputted by the
docking program were visualized; however, only the pose with
the best fitness score was used for the following SAR analysis.
Wherever appropriate, docking under induced-fit mode was ap-
plied to accommodate the side-chain flexibility of Arg235. Prior
to the SAR analysis, the ability of the docking program to success-
fully reproduce the binding mode of the co-crystallized ligand 1
was evaluated. It was found that GOLD was able to reproduce
the X-ray binding mode of co-crystallized 1 with a RMSD of
0.36 Å. We also calculated the logBB values to confirm that the
new ligands exhibit improved blood–brain permeation properties.
The logBB values of all inhibitors 1, 2a–2c, 3a–3d, 4a–4b, 5a–5e,
6a–6c were calculated using Clark’s equation.42 The actual coeffi-
cients in the Clark equation were refitted using the original data
in the Clark paper to account for the difference between the polar
surface area and ClogP used by Clark and those generated by the
Tripos software. The contact van der Waals surface area (cvdWSA)
was calculated using MOLCAD separated surfaces between R1 and
the residues Thr231, Thr232, Asn233, Ser325, Arg235, Gln73, and
Thr72 proximal to R1. The figures were generated either with Vida
3.0 or with Sybyl8.0.
concentrations up to 100 lM.
To explore the impact of rigidification/cyclization of the sulfon-
amide group of ligand 1 on BACE1 and BACE2, compare the result-
ing ligands with biaryls without sulfonamide group, and to find a
future pharmacophore for the hot spot Asn233 (Leu246 in BACE2),
we have tested two ring sultams 4a and 4b. The 6-membered sul-
tam 4b is found to be about 11-fold more active than 5-membered
sultam 4, 1.8-fold more active than ligand 1 at BACE1, and 1.9-fold
less active at BACE2 than ligand 1. The 5-membered sultam 4a, on
the other hand, was 12.6-fold more active at BACE2 than at BACE1.
Sultam 4a is exclusively BACE2 selective among all the ligands that
were tested. Although it is not sufficient to talk about a general
trend, it appears that a sulfonamide group has to be present in
S2 to maintain the activity of the ligands at BACE2. An analysis
of the binding modes of ligands shows that the SO2 group in sul-
tams points toward the S2 site whereas the alicyclic ring is sol-
vent-exposed ( Fig. 5c). This predicted mode of binding is very
similar to the one that is observed in the X-ray crystal structure
of the 6-membered sultam published during the preparation of
this manuscript (PDB code: 2VNM). In 4b, one of the SO2 oxygen
atoms forms a hydrogen-bond with backbone NH of Asn233 (N–
O distance = 3.3 Å, N. . .H. . .O angle = 170°), whereas the other oxy-
gen atom forms ion–dipole interactions with epsilon nitrogen (NE)
of Arg235 (distance = 2.8 Å). Ligand 4a, owing to its constrained 5-
membered ring, forms a weak hydrogen bond with Asn233 (N–O
distance = 3.7 Å, N. . .H. . .O angle = 142°) and weak ion–dipole
interactions with NE of Arg235 (distance = 3.7 Å) and hence less ac-
tive compared to 4b. Further, the cvdWSA in Å2 of 4a and 4b are
278 and 292, respectively. The calculated logBB values of com-
pounds 4a and 4b are similar to that of ligand 1.
Next, we synthesized a series of fused-ring compounds which
were subsequently tested for the inhibition of BACE1 and BACE2
(Table 1). We synthesized and tested three sets of regioisomers
that differ by the size and position of the ring fused to the central
phenyl moiety. We found that the degree of lipophilicity and flex-
ibility incorporated at the P2 position in 5a–5c and their regioi-
somers 6a–6c correlates with the BACE1 inhibitory activity of the
ligands: (less lipophilic, less active) 5a < 5b < 5c (more lipophilic,
more active) and similarly 6a < 6b < 6c. Upon docking using GOLD
under induced-fit mode, it was found that ligands 5c and 6c, con-
taining the more flexible 7-membered ring, form better vdW con-
tact with the enzyme as compared with the ligands 5b and 6b,
The best docking poses were visualized in the binding site (Fig-
ure 5a), and the SAR of the synthesized compounds is discussed be-
low. Hydroxyethylamine (HE) motif is well known to mimic the
transition state of the peptides cleaved by aspartic proteases, and
it has been successfully employed in the BACE1 inhibitor design.43
Multiple transition state analogs containing HE motif were crystal-
lized. Our docking confirms that the binding modes of our ligands
are similar to those found for the transition state analogs.
Substitution of R1 by a simple hydroxyl or a methoxy group in
2a and 2b (Table 1) resulted in no activity at BACE2 and approxi-
mately 6- and 9-fold loss in BACE1 activity compared to ligand 1,
respectively, a relatively small trade-off in exchange of improve-
ment of the drug-like profile and BACE1/BACE2 selectivity. The
docking shows that the solvent-exposed hydroxyl or methyl group
in 2a and 2b do not form favorable contacts with the binding site.
In the induced-fit mode docking, the side-chain guanidine group of
Arg235 is able to interact with the hydroxyl or methoxy groups in
2a and 2b, respectively. Substitution of R1 in 1 with an acetylene
moiety in 2c led to a mere 3-fold decrease in BACE1 activity. As
well as in the case of ligands 2a and 2b this is a small trade-off
for replacing methyl sulfonamide group of 1 with a small acetylene
group in 2c. This observation supports our initial hypothesis that
the loss of hydrogen-bonding contribution may be compensated
by non-directional vdW interactions of the P2 substituents as a re-
sult of possible induced fit changes. Indeed, the docking shows that
the P2 acetylene moiety forms extensive vdW contact with the al-
kyl side-chains of Arg235 and Thr231 located in S2. None of 2a–c
ligands showed BACE2 inhibition below 100 lM concentration.
As expected, the relatively less polar 2a–c are predicted to have
better logBB of ꢀ0.3 compared to ꢀ0.9 for 1.
Encouraged by these initial findings, we synthesized and tested
ligands 3a–3d to explore whether the S2 pocket can accommodate
larger biaryl substituents. The docking poses of these compounds

S. R. Chirapu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 264–274
273
which contain only a moderately flexible 6-membered ring. Li-
gands 5b and 6b possess more efficient vdW interactions with
the binding site than their 5-membered ring analogs 5a and 6a
(Fig. 5d and e). The cvdWSA in Å2 calculated under the induced-
fit mode for 5a–5c are 183, 258, 266, whereas for compounds 6a–
6c it is 209, 197, and 227, respectively. The cvdWSA correlates with
activity of compounds 5a–5c but not 6a–6c suggesting that this
parameter alone is not sufficient to explain the trend in activity.
To find a plausible explanation for the difference in activity pro-
files of 5a–5c and their regioisomers 6a–6c, we analyzed the X-ray
co-crystal structures of BACE1 protein available in Protein Data
Bank. Two interesting observations resulted from this analysis.
First, the group at the P2 position appears to control the position
of Arg235 through an induced-fit effect. Depending on the steric
bulk and electronic properties of the P2 substituent, Arg235 is
forced to adopt different conformations. The maximum change in
the location of Arg235 was observed between 1W51 and 2B8L with
a deviation of 4.4 Å between the two guanidine carbon atoms in the
Arg235 side-chain. Second, in those cases where the P2 has N-meth-
ylsulfonamide moiety (PDB codes: 1TQF, 2IRZ, 2B8L, 2IS0, 2NTR,
2OAH, 2P4J, 2P8H, and 2PH6), a conserved water molecule medi-
ates hydrogen bond between the side-chains of Arg235, Ser325,
and Gln326. On the other hand, if Arg235 changes its position as
in 1W51 the water molecule is not observed in the crystal struc-
tures. It appears that both observations are interconnected—those
ligands responsible for the movement of Arg235 also contribute
to the displacement of the water molecule and loss of the hydrogen
bonds between this water molecule and residues Ser325, Arg235,
and Gln326. To account for the induced-fit effect Arg235 was al-
lowed to change conformation during docking with Gold. Fused-
ring compounds 5a–5c has an oxygen atom locked in a position
amenable for hydrogen bonding with side-chain NH2 of Arg235.
Our docking experiments suggest that in order to facilitate this
hydrogen bond, Arg235 must lose its contact with water (WAT43
in 2B8L). Similar hydrogen-bonding interaction is not possible in
regioisomers 6a–6c because the ring precludes their contact with
Arg235 and hence a near 2-fold drop in potency compared to 5a–
5c is observed. The calculated logBB is ꢀ0.3 for ligands 5a–5c and
ꢀ0.3, ꢀ0.4 and ꢀ0.2 for 6a–6c, respectively. This shows improve-
ment when compared to the ligand with the sulfonamide moiety
bound to S2 pocket. As with the other ligands in the biaryl and
fused-ring series that according to the docking experiments require
an induced fit, ligands 5a–c and 6a–c did not show inhibition of
BACE2.
5e are found to be less active than non-fluorinated analogs 5b and
5c by ca. 5- and 12-fold, respectively. Ligand 5e exhibits 2.8-fold
better activity than 5d perhaps for the same reasons 5c is more ac-
tive than 5a and 5b—larger area of contact with the binding site
due to flexibility of the 7-membered ring. To understand what
could be the reason for overall lower activity of 5d and 5e com-
pared to their non-fluorinated analogs we visually inspected the
crystal structures of BACE1 with different P2 substituents and
found that the induced-fit effect of P2 affects the orientation of
the methyl group in Ala335 (Fig. 5f). Of particular interest, the
RMSD between CB atoms for crystal structures 2P8H (P3 = p-flu-
oro-a-methylbenzylamino, P2 = N-methylsulfonamide) and 2IQG
(P3 = N,N-dipropanoyl and P2 = Me) is 1.2 Å. Because of the prox-
imity of the methyl group in Ala335 and the fluorine atom in the
BACE1 inhibitors, any conformational changes in S2 may result in
mutual steric clashing. If an induced fit caused by the fused rings
in series 5 and 6 takes place it may, indeed, affect the position of
Ala 335 and explain the overall lower activity of the fluorinated
compounds 5d and 5e compared to their non-fluorinated analogs.
The predicted logBB values of 5d and 5e are almost the same as
those of the non-fluorinated compounds. Unlike non-fluorinated
fused-ring ligands in series 5 and 6, both fluorinated ligands 5d
and 5e showed a reasonable, ca. 2 lM, activity against BACE2.
To find a possible explanation for BACE1 selectivity of the li-
gands we tried to compare qualitatively the flexibility of the BACE1
and BACE2 proteins using their b-factors. A quantitative compari-
son of the b-factors is not possible since there is large number of
the same residues in different X-ray structures of BACE1 proteins
that differ in their b-factors depending on the co-crystallized li-
gand. It is clear, however, that the flexibility patterns for the resi-
dues in the S2 pocket and adjacent to it areas of the BACE1 and
BACE2 proteins are different, suggesting that induced fit may play
a role in an improved selectivity toward BACE1.
To investigate the effect of BACE1 inhibitors on APP processing
the most active ligands 5b and 5c were tested for reduction in
Ab40 production in mouse neuroblastoma cells (N2a) transfected
with Swedish mutant APP695 (Fig. 6). Compounds 5b and 5c reduce
ca. 65% and 35% of Ab production, respectively, compared to the
control. At the moment it is unclear why almost equally active 5b
and 5c exhibit 1.9-fold difference in reduction of Ab40. It may be re-
lated to their metabolic stability or delivery to the site of action.
This observation is being actively investigated at this time.
In summary, by using computer-aided drug design methods, we
have designed, synthesized, tested the activity of the fused-ring and
biaryl compounds against BACE1 and BACE2, and analyzed the
resulting SAR using docking protocols. The fused-ring compounds
are in general more active than the biaryl-based ligands with an
activity range from 56 to 998 nM. This is comparable to the activity
of the ligands with polar substituents occupying the S2 binding
pocket, which lends support for our initial hypothesis that the S2
site may be targeted with less polar substituents. Most of the li-
gands displayed more favorable calculated logBB. The side-chain
flexibility of Arg235 and, perhaps, adjacent residues, and possibly
the presence of a water molecule mediating hydrogen bond interac-
tions between Arg235, Ser325, and Gln326 in S2 appears to play an
important role in accommodating the fused-ring and biaryl-based
ligands in the binding site. Most of the fused-ring and biaryl-based
ligands are BACE1 selective, and thus support our initial hypothesis
that differences in the flexibility of BACE1 and BACE2 proteins and
the ability to accommodate the ligands through induced fit can be
utilized to design BACE1/2 selective ligands. In addition, it appears
that the sulfonamide group is required to maintain BACE2 activity.
The fused-ring ligands 5b and 5c combine the best attributes of
acceptable logBB, BACE1 inhibitory activity, BACE2 selectivity,
and ability to reduce the Ab production. A similar approach may
be used to explore other binding pockets of BACE1 (and BACE2) to
Several publications have reported that fluorinated BACE1
inhibitors exhibited better activity than the non-fluorinated inhib-
itors due to increased lipophilicity.30,45 Unlike the compounds de-
scribed in these publications, both fluorinated compounds 5d and
Figure 6. Reduction of Ab40 levels by BACE1 inhibitors detected in N2a cells stably
transfected with Swedish human APP.

274
S. R. Chirapu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 264–274
24. Rajendran, L.; Schneider, A.; Schlechtingen, G.; Weidlich, S.; Ries, J.; Braxmeier,
T.; Schwille, P.; Schulz, J. B.; Schroeder, C.; Simons, M.; Jennings, G.; Knolker, H.
J.; Simons, K. Science 2008, 320, 520.
create new chemotypes for further development of therapeutics to
treat AD and to serve as general probes for drug discovery.
25. Stachel, S. J.; Coburn, C. A.; Steele, T. G.; Crouthamel, M. C.; Pietrak, B. L.; Lai, M.
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Acknowledgments
26. Sybyl7.3. Tripos Inc.: 1699 South Hanley Rd., St. Louis, Missouri, 63144, USA,
2006.
27. Chris M. W. Ho at Drug Design Methodologies, L. http://www.newdrugdesign.
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This research is supported by NIH R21 AG027454 (P.A.P.) and
NIH AG025888 (Y.S.), Arizona Alzheimer Consortium (Y.S.), and
Hans Vahlteich endowment program grant of College of Pharmacy
(P.A.P.) at University of Illinois at Chicago. G.T and S.O.A are grate-
ful for the kind gift of N2a cells from Dr. Gopal Thinakaran (Univer-
sity of Chicago, Chicago, IL). We thank Subash Velaparthi, Gilles
Pieffet, and Reaz Uddin for useful comments and suggestions.
The authors thank OpenEye Scientific Software for providing aca-
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& Biological Sciences,
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