Design, synthesis, and SAR of macrocyclic tertiary carbinamine BACE-1 inhibitors

Article abstract of DOI:10.1016/j.bmcl.2007.04.072

This Letter describes the design and synthesis of tertiary carbinamine macrocyclic inhibitors of the β-secretase (BACE-1) enzyme. These macrocyclic inhibitors, some of which incorporate novel P2 substituents, display a 2- to 100-fold increase in potency r

Full text of DOI:10.1016/j.bmcl.2007.04.072

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4057–4061
Design, synthesis, and SAR of macrocyclic tertiary
carbinamine BACE-1 inhibitors
Stacey R. Lindsley,a,* Keith P. Moore,a Hemaka A. Rajapakse,a Harold G. Selnick,a
Mary Beth Young,a Hong Zhu,a Sanjeev Munshi,b Lawrence Kuo,b
Georgia B. McGaughey,c Dennis Colussi,d Ming-Chih Crouthamel,d Ming-Tain Lai,d
Beth Pietrak,d Eric A. Price,d Sethu Sankaranarayanan,d Adam J. Simon,d
Guy R. Seabrook,d Daria J. Hazuda,d Nicole T. Pudvah,e Jerome H. Hochman,e
Samuel L. Graham,a Joseph P. Vaccaa and Philippe G. Nantermeta,*
aDepartment of Medicinal Chemistry, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
bDepartment of Structural Biology, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
cDepartment of Molecular Systems, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
dDepartment of Alzheimer’s Research, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
eDepartment of Drug Metabolism, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
Received 13 March 2007; revised 20 April 2007; accepted 23 April 2007
Available online 27 April 2007
Abstract—This Letter describes the design and synthesis of tertiary carbinamine macrocyclic inhibitors of the b-secretase (BACE-1)
enzyme. These macrocyclic inhibitors, some of which incorporate novel P2 substituents, display a 2- to 100-fold increase in potency
relative to the previously described acyclic analogs while affording greater stability.
Ó 2007 Elsevier Ltd. All rights reserved.
Alzheimer’s disease (AD) is a neurodegenerative disease
of the brain that leads to a progressive decline in cogni-
tive function and ultimately incapacitation and death.1
AD is characterized by the presence of insoluble amy-
loid plaques and fibrillary tangles, key pathological fea-
tures of this devastating disease. Years of research have
established that the major component of the amyloid
plaques is Ab40–42, a neurotoxic peptide fragment of
b-amyloid precursor protein (APP), generated by the
proteolytic action of the b- and c-secretase enzymes.
Of these two enzymes, b-secretase (b-site APP Cleaving
Enzyme or BACE-1), an aspartyl protease, is considered
rate-limiting in this proteolytic cleavage process.2 Based
on the key role of BACE-1 in the b-amyloid cascade,
inhibition of BACE-1 is widely recognized as one of
the most promising therapeutic approaches for the treat-
ment and prevention of AD.3
However, the development of small molecule, brain
penetrant BACE-1 inhibitors has been challenging.
Most efforts so far have relied on the use of transition
state isosteres (hydroxyethylamines, reduced amides,
statines, and aminostatines) to interact with the cata-
lytic aspartates, often with a negative impact on phar-
macokinetics and permeation of the blood–brain
barrier (BBB).
Recently, we disclosed the discovery of non-transition
state isostere derived BACE-1 inhibitors which utilize
a single tertiary carbinamine to bind to the catalytic
aspartates.4 Inhibitors derived from this novel scaffold,
exemplified by 1a (BACE-1 IC50 = 326 nM) and 1b
(BACE-1 IC50 = 27 nM), displayed comparable potency
to classical hydroxyethylene amine (HEA) derived
inhibitors (Fig. 1). However, stability, pharmacokinet-
ics, and brain penetration remain an issue with this
new series. Examination of X-ray crystallography data
for ester 1b bound to BACE-1 revealed the close spatial
proximity between the P1 aryl group and the P3 methyl
(Fig. 2).
Keywords: BACE; AD; Aspartate protease; Macrocycle; P-gp.
*
Corresponding authors. E-mail: philippe_nantermet@merck.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.04.072

4058
S. R. Lindsley et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4057–4061
P2
N
to inhibit BACE-1 with an IC50 of 17 nM in cell-free
medium and an IC50 of 23 nM in cells,6 thus affording
a 19-fold increase in intrinsic potency relative to 1a
(Fig. 3). Despite making a number of key contacts with
the enzyme, 2a lacks an important interaction with the
BACE-1 ‘flap’ region.4 In acyclic esters of type 1b, the
carbonyl alpha to the tertiary carbinamine accepts a
critical H-bond from the ‘flap’ residue Thr72 (Fig. 2).
This provides a significant boost in potency but at the
cost of stability, both chemically and toward plasma
esterases.4 We postulated that cyclization into a macro-
lactone might confer more stability to the ester linkage
while improving potency against BACE-1.
R
SO2Me
R2
F
O
O
R3 NH
O
NH
O
X
X
NH2
NH2
R4
P3
R1
P1
2, X = CH2, CO
1a: X = CH2, R = n-Pr
1b: X = CO, R = Me
Figure 1. Tertiary carbinamine BACE inhibitor 1 and envisioned
P1–P3 macrocyclization strategy.
Gratifyingly, macrolactone 2b exhibited improved po-
tency (ca. 10-fold boost over 2a at 2 nM) and was found
to be not only hydrolytically stable at physiological pH
Table 1. P2 sulfonamide and P3 SAR
R2
O
R3
NH
O
X
NH2
2
Compound
R2
R3
X
BACE-1
a
IC50 (nM)
2a
2b
2c
2d
2e
2f
n-Pr-N-SO2Me
n-Pr-N-SO2Me
Me-N-SO2Me
Me-N-SO2Me
Ph-N-SO2Me
Ph-N-SO2Me
Me-N-SO2Et
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CH2
CO
17
2
CH2
CO
49
2
12
CH2
CO
Figure 2. X-ray of inhibitor 1b complexed with BACE-1. RCSB ID
code: rcsb042372; PDB ID code: 2PH6.
1
56
2g
2h
2i
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
Me-N-SO2i-Pr
n-Pr-N-SO2Me
n-Pr-N-SO2Me
n-Pr-N-SO2Me
n-Pr-N-SO2Me
n-Pr-N-SO2Me
n-Pr-N-SO2Me
196
10
290b
85
Attracted by the possibility of increasing potency by
stabilizing the bioactive conformation as well as the
potential of improving the physiochemical liabilities of
the acyclic series, we embarked on the preparation of
macroethers and macrolactones of type 2.5
4-F-Ph
Me
Et
2j
2k
2l
i-Pr
CCH
cy-Pr
175
53
2m
2n
1100
In order to evaluate this strategy, we first synthesized a
macrocyclic congener of 1a. Macroether 2a was found
a IC50s are an average of at least three measurements.6
b Calculated from a mixture of four diastereoisomers.
Ser325
Asn233
Thr232
Ser325
Asn233
Thr232
O
O
O
O
O
O
S
S
N
N
Flap
O
O
Thr72
Gly230
S3
S3
NH
O
NH
Gly230
Asp228
Asp32
NH2
NH2
Asp228
Asp32
Gly230
Gly230
S1
S1
2a
2b
BACE-1 IC50 = 17 nM
sAPP_NF IC50 = 23 nM
BACE-1 IC50 = 2 nM
sAPP_NF IC50 = 5 nM
Figure 3. Macrocyclic tertiary carbinamine BACE inhibitors 2a and 2b. For both 2a and 2b, the sulfonamide imparts potency due to interactions
with Thr232, Asn233, and Ser325; the lactam NH interacts with Gly230; the two phenyl rings partially fill S1 and S3, and the tertiary carbinamine
makes critical interactions with both Asp32 and Asp228 as well as Gly230. In 2b, the ester carbonyl makes additional interactions with Thr72 of the
Flap.

S. R. Lindsley et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4057–4061
4059
but also stable in rat and human plasma and in micro-
somal preparations in the absence of NADPH (Fig. 3).
Together, these data validated our initial proposal and
warranted the synthesis of additional macrocyclic
analogs.
alkyl-sulfone portion appears somewhat more respon-
sive (2g,h), clearly favoring the smaller methyl-sulfone
substituent as in compound 2c.
In contrast, the R3 position is highly sensitive to modi-
fication. While fluoro substitution at the para position
of the P3 phenyl provides a modest improvement in po-
tency (2i), any attempt to reduce the size of the P3 group
(2j–n) results in diminished potency.
Table 1 lists macroethers and macrolactones of type 2
bearing a variety of P2 sulfonamides and P3 hydropho-
bic groups. As observed with the prototypical com-
pounds, macrolactones 2b, 2d, and 2f are >8-fold more
potent than the corresponding macroethers 2a, 2c, and
2e. SAR of the N-alkyl portion of the P2 group is rather
flat, especially in the macrolactone mode (2a–f). The
Macrolactone 2d represents the best combination of po-
tency and molecular weight, with a 2 nM IC50 toward
BACE-1 and a MW of 535. Unfortunately, like many
other analogs described in Table 1, inhibitor 2d suffers
from poor brain penetration ([2d]brain/[2d]plasma < 5%).7
This could be the result of low apparent permeability
(PApp = 9) and high P-gp efflux (B–A/A–B = 16).8 Mac-
rolactone 2d also displayed poor pharmacokinetics (iv
clearances approaching hepatic blood flow in rats, dogs,
and rhesus) likely due to extensive oxidative metabolism
at various positions of the P1–3 aromatic groups, as ob-
served from incubation with microsomes.
SO2Me
N
O
O
NH
O
NH2
F
3
BACE-1 IC50 = 23 nM
hu P-gp: PApp = 22, B-A/A-B = 56
In order to increase the likelihood of brain penetration,
we proposed to modulate the Lewis acidity/basicity of
the polar functional groups. Initially, we evaluated the
effect of reducing the basicity of the tertiary carbinamine
Figure 4. Attenuation of the tertiary carbinamine basicity by
b-fluorination.
Table 2. Macrolactone optimization
R2
R3
O
3
6
O
NH
4
O
1
NH2
R1
R1
R2
R3
hu P-gp
a
BACE-1 IC50 (nM)
Compound
B–A/A–Bc
b
PApp
2d
4a
H
H
Me-N-SO2Me
H
H
2
9
16
O
O
S
S
S
CF3
16
12
13
15
19
13
32
N
O
N
O
4b
4c
H
H
H
H
38
4
CF3
O
N
O
N
5
H
H
Br
H
H
270
780
9
5
1
O
N
6a
19
6b
6c
6d
6e
6f
H
2-CN-Ph
2-CN-Ph
H
F
F
F
F
F
27
130
46
78
22
46
9
1
3-F
4-F
6-F
H
ND
9
ND
3
2-CN-Ph
2-CN-Ph
6
2
2-CN-3-F-Ph
2-CN-3-F-Ph
7
1
6g
4-F
6
5
a IC50s are an average of at least three measurements.6
b Apparent permeability (10À6 cm/s).
c Efflux ratio.

4060
S. R. Lindsley et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4057–4061
by b-fluorination. The resulting methyl-fluoro macrolac-
tone 3 (Fig. 4) displayed a 10-fold loss in potency versus
2d while improving apparent permeability (PApp = 22)
and increasing P-gp efflux (B–A/A–B = 56). Similarly,
attempts at modulating the electronics of the P2 sulfon-
amide via inductive or resonance effects (4a–c, Table 2)
resulted in improved PApp but did not affect P-gp efflux.
Despite the critical BACE-1 binding contributions made
by the P2 sulfonamide (Fig. 3), it was also perceived as a
major P-gp liability. Following unsuccessful attempts to
attenuate P-gp efflux via modulation of the sulfonamide
moiety (4a–c, Table 2), efforts centered on finding alter-
native P2 groups. Replacement of the P2 sulfonamide
moiety with a bromo substituent (5) resulted in a
135-fold loss in potency; however, P-gp efflux9 was
improved. Guided by earlier findings10 and encouraged
by this result, we focused on P2 hydrophobic replace-
ments. Although it displayed reduced potency, P2 oxa-
zole11 derivative 6a was the first macrolactone to
display high intrinsic permeability (PApp = 19) without
P-gp efflux susceptibility (B–A/A–B = 1). Installation
of a 2-cyano-phenyl P2 afforded biaryl inhibitor
6b (BACE-1 IC50 = 27 nM, sAPP_NF IC50 = 68 nM,
10-fold loss in intrinsic potency relative to 2d)6 which
was also devoid of P-gp efflux. Unfortunately, this came
at the cost of permeability. A modest 16% brain penetra-
tion was achieved by 6b when administered ip at
30 mpk.7 In an attempt to increase PApp while maintain-
ing low P-gp efflux, fluorine atoms12 were sequentially
installed around the P1–3 phenyl groups (6c–g); unfor-
tunately, this strategy did not improve permeability
(PApp = 6–9).
MeO2C
Ph
CO2Me
NHBoc
a-c
N
Ph
Br
R1
i
9
8
R3
NHBoc
R3
NHBoc
d-g
h
R3 CHO
B
10
11
12
HO
X
CO2Me
NHBoc
BocHN
BocHN
NHBoc
j or k
R1
R1
13
R3
R3
14a: X = CH2
14b: X =CO
R2
R2
p or q
l-o
CO2Me
t
BuO2C
MeO2C
Br
16
15
R2
R2
O
tBuO2C
r-t
O
R3
NH
O
X
X
NH2
BocHN
NH2
R1
R3
R1
18
17
Scheme 1. Reagents and conditions: (a) base, ArCH2Br; (b) HCl,
MeOH; (c) Boc2O, DIEA; (d) Ellman sulfinimine; (e) vinyl Grignard,
THF; (f) HCl, MeOH; (g) Boc2O, Hunig’s base; (h) 9-BBN, THF; (i)
Pd(PPh3)4, 3 N NaOH, toluene, 85 °C; (j) for 14a: LiBH4,THF; (k) for
14b: 1 N LiOH, THF; (l) 1 equiv NaOH, MeOH; (m) CDI, t-BuOH;
(n) LiBH4,THF; (o) CBr4, PPh3, DCM; (p) for 14a: AgOTf, 2,6-di-t-
Bu-pyr resin, DCE; (q) for 14b: Cs2CO3, DMF; (r) HCl, DCM; (s)
BOP, DIEA, DMF; (t) chiral preparative HPLC; for R2 = Br: Boc2O;
flash chromatography; R2–M, Pd(0); HCl, DCM.
Additionally, pharmacokinetic properties remained poor
across this series despite blocking multiple oxidative
metabolism locations by fluorination. The only molecu-
lar subunit not yet addressed in these macrocyclic
tertiary carbinamine BACE inhibitors was the P2–P3
amide. Although the amide appeared critical for potency
(H-bond to Gly230),4 it also represented a classical
recognition element for the P-gp efflux pump. Indeed,
P3-a-methylation (7, Fig. 5) as a means to increase steric
hindrance around the P2 to P3 amide did improve P-gp
efflux susceptibility when compared to non-methylated
analog 4b, but it also resulted in a 10-fold loss in potency.
block is elaborated from aldehydes of type 10 via
Ellman’s methodology14 to provide vinylic intermedi-
ate 11. Hydroboration, followed by Pd-mediated cou-
pling of aryl bromide 9, affords the P1/P3 subunit
13. Reduction or hydrolysis leads to alcohol 14a or
acid 14b. Alkylation with benzylic bromide 16 leads
to the acyclic P1–3 full assembly 17 which is deprotec-
ted and macrocyclized to 18. If the R2 group is carried
intact from the original diester 15, separation of the
diastereoisomers (at the tertiary carbinamine center)
is performed by preparative chiral HPLC. Alterna-
tively, the R2 group can be installed postmacrocycliza-
tion via Pd-mediated coupling, starting originally from
dimethyl 3-bromoisophthalate.
A general synthetic route to macrocyclic tertiary car-
binamine BACE inhibitors is illustrated in Scheme
1.13 Installation of the P1 group is achieved via alkyl-
ation of methyl alanine benzophenone imine 8, which
after hydrolysis and Boc protection of the tertiary car-
binamine provides intermediate 9. The P3 building
In conclusion, macrocyclization of tertiary carbin-
amine BACE-1 inhibitors provided 2- to 100-fold
improvement in potency and maintained greater than
1000-fold levels of selectivity against Renin and
Cathepsin D.15 Modification of the P2 group did af-
ford minor improvements in brain penetration but of-
ten at the cost of potency. Initial results also indicated
that modifications of the P2–P3 amide moiety could be
more fruitful; this will be the subject of future work,
to be described in due time.
O
O
S
N
CF3
7
O
BACE-1 IC50 = 380 nM
hu P-gp: PApp = 18, B-A/A-B = 1
O
NH
O
NH2
Figure 5. P3 amide modulation.

S. R. Lindsley et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4057–4061
4061
Verlinde, C. L. M. J.; Hol, W. G. J.; Pei, D. J. Med. Chem.
2004, 47, 4941.
Acknowledgments
6. Amyloid precursor protein (APP) modified to the NFEV
sequence at the site of proteolysis was used in our assays.
Intrinsic potency refers to the inhibition of the BACE-1
enzyme in a cell-free medium. Functional potency refers to
the inhibition of APP processing by BACE-1 transfected
to a HEK293 cell line. See: Shi X.-P.; Tugusheva K.; Bruce
J. E.; Lucka A.; Chen-Dodson E.; Hu B.; Wu G.-X.; Price
E.; Register R. B.; Lineberger J.; Miller R.; Tang M.-J.;
Espeseth A.; Kahana J.; Wolfe A.; Crouthamel M.-C.;
Sankaranarayanan S.; Simon A.; Chen L.; Lai M.-T.;
Pietrak B.; DiMuzio J.; Li Y.; Xu M.; Huang Q.; Garsky
V.; Sardana M. K.; Hazuda D. J. J. Alzheimer’s Dis. 2005,
7, 139.
7. Brain penetration was assessed from mice brain homog-
enates, 2 h post-ip dosing in 25% DMSO/75% PEG300 at
30 mpk. Total brain concentration versus total plasma
concentration is reported, at 2 h.
8. (a) Hochman, J. H.; Yamazaki, M.; Ohe, T.; Lin, J. H.
Curr. Drug Met. 2002, 3, 257; (b) Yamazaki, M.; Neway,
W. E.; Ohe, T.; Chen, I-W.; Rowe, J. F.; Hochman, J. H.;
Chiba, M.; Lin, J. H. J. Pharmacol. Exp. Ther. 2001, 296,
723.
9. The accuracy of B–A/A–B ratio measurements might be
compromised for compounds displaying PApp < 10.
10. Eickmeier, C.; Fuchs, K.; Peters, S.; Dorner-Ciossek, C.
PCT Int. Appl. 2006, WO2006103038. Barrow, J. C.;
Coburn, C. A.; Nantermet, P. G.; Selnick, H. G.; Stachel,
S. J.; Stanton, M. G.; Stauffer, S. R.; Zhuang, L.; Davis, J.
R. PCT Int. Appl. 2005, WO2005065195. Coburn, C. A.;
Stachel, S. J.; Vacca, J. P. PCT Int. Appl. 2005,
WO2005004802.
The authors thank the Laboratory Animal Resource
and Drug Metabolism departments for animal dosing
and sample analysis, the analytical chemistry, mass spec-
trometry, and NMR groups for analytical and spectro-
scopic data, Carl Homnick for chiral separations, and
Dr. Wes Trotter for proofreading this manuscript.
References and notes
1. (a) Nussbaum, R. L.; Ellis, C. E. N. Eng. J. Med. 2003,
348, 1356; (b) Stachel, S. J.; Vacca, J. P. Curr. Top. Med.
Chem. 2006, 6, 551, and following articles.
2. Cummings, J. L. N. Eng. J. Med. 2004, 351, 56.
3. (a) Roberds, S. L.; Anderson, J.; Basi, G.; Bienkowski, M.
J.; Branstetter, D. G.; Chen, K. S.; Freedman, S.; Frigon,
N. L.; Games, D.; Hu, K.; Johnson-Wood, K.; Kappen-
man, K. E.; Kawabe, T.; Kola, I.; Kuehn, R.; Lee, M.;
Liu, W.; Motter, R.; Nichols, N. F.; Power, M.; Robert-
son, D. W.; Schenk, D.; Schoor, M.; Shopp, G. M.; Shuck,
M. E.; Sinha, S.; Svensson, K. A.; Tatsuno, G.; Tintrup,
H.; Wijsman, J.; Wright, S.; McConlogue, L. Hum. Mol.
Genet. 2001, 10, 1317; (b) Durham, T.; Shepherd, T. Curr.
Opin. Drug Discov. Dev. 2006, 9, 776; (c) Guo, T.; Hobbs,
D. W. Curr. Med. Chem. 2006, 13, 1811; (d) Ziora, Z.;
Kimura, T.; Kiso, Y. Drugs Future 2006, 31, 53; (e) Beher,
D.; Graham, S. L. Expert Opin. Investig. Drugs 2005, 14,
1385; (f) Gosh, A. K.; Kumaragurubaran, N.; Tang, J.
Curr. Top. Med. Chem. 2005, 5, 1609; (g) Thompson, L.
A.; Bronson, J. J.; Zusi, C. Curr. Pharm. Design 2005, 11,
3383.
11. Reeder, M. R.; Imbordino, R. J. PCT Int. Appl. 2004,
WO2004000821.
4. Rajapakse, H. R.; Nantermet, P. G.; Selnick, H. G.;
Munshi, S.; McGaughey, G. B.; Lindlsey, S. R.;
Young, M. B.; Lai, M. T.; Espeseth, A. S.; Shi, X.
P.; Colussi, D.; Pietrak, B.; Crouthamel, M. C.;
Tugusheva, K.; Huang, Q.; Xu, M.; Simon, A. J.;
Kuo, L.; Hazuda, D. J.; Graham, S.; Vacca, J. P.
J. Med. Chem. 2006, 49, 7270.
12. Leach, A. G.; Jones, H. W.; Cosgrove, D. A.; Kenny, P.
W.; Ruston, L.; MacFaul, P.; Wood, J. M.; Colclough, N.;
Law, B. J. Med. Chem. 2006, 49, 6672.
13. See Nantermet, P. G.; Rajapakse, H. A.; Selnick, H. G.;
Lindsley, S. R.; Moore, K. P.; Stachel, S. J.
WO2006055434, for detailed preparations of representa-
tive examples.
5. (a) Tyndall, J. D. A.; Fairlie, D. P. Curr. Med. Chem.
2001, 8, 893; (b) Bell, I. M.; Gallicchio, S. N.; Abrams, M.;
Beese, L. S.; Beshore, D. C.; Bhimnathwala, H.; Bogusky,
M. J.; Buser, C. A.; Culberson, J. C.; Davide, J.; Ellis-
Hutchings, M. E.; Fernandes, C.; Gibbs, J. B.; Graham, S.
L.; Hamilton, K. A.; Hartman, G. D.; Heimbrook, D. C.;
Homnick, C. F.; Huber, H. E.; Huff, J. R.; Kassahun, K.;
Koblan, K. S.; Kohl, N. E.; Lobell, R. B.; Lynch, J. J.;
Robinson, R.; Rodrigues, A. D.; Taylor, J. S.; Walsh, E.
S.; Williams, T. M.; Zartman, B. C. J. Med. Chem. 2002,
45, 2388; (c) Tsantrizos, Y. S.; Bolger, G.; Bonneau, P.;
Cameron, D. R.; Goudreau, N.; Kukolj, G.; LaPlante, S.
R.; Llinas-Brunet, M.; Nar, H.; Lamarre, D. Angew.
Chem. Int. Ed. 2003, 42, 1356; (d) Hu, X.; Nguyen, K. T.;
14. Cogan, D. A.; Liu, G.; Ellman, J. Tetrahedron 1999, 55,
8883.
15. IC50 against Renin for 2a, 2b, 6b, respectively: 144, 169,
275 lM. IC50 against Cathepsin D for 2a, 2b, 6b,
respectively: 275, 97, 48 lM. IC50 against BACE-2 for
2a, 2b, 6b, respectively: 1.6 lM, 54 nM, 41 nM. It is
rationalized that this selectivity is built into the isophthal-
imide scaffold, resulting from the hydrophilic nature of the
BACE-1 S2 pocket as compared to BACE-2, Renin, and
Cathepsin D.4 For additional reference, see: Brady, S. F.;
Singh, S.; Crouthamel, M.-C.; Holloway, M. K.; Coburn,
C. A.; Garsky, V. M.; Bogusky, M.; Pennington, M. W.;
Vacca, J. P.; Hazuda, D.; Lai, M.-T. Bioorg. Med. Chem.
Lett. 2004, 14, 601.