Synthesis of a carbon-11 radiolabeled BACE1 inhibitor

Article abstract of DOI:10.1007/s00044-019-02480-9

Beta-secretase (BACE1), a transmembrane aspartyl protease, can cleave membrane-bound β-amyloid precursor proteins (APPs) to initiate the accumulation of amyloid-β (Aβ). The inhibition of BACE-1 to limit the accumulation of neurotoxic Aβ peptides could offer a potential treatment for Alzheimer’s Disease (AD). However, little is known about the distribution and density of BACE1 in the central nervous system. As a step toward filling this gap in knowledge, we have evaluated a potential radiotracer for the imaging of BACE1 using positron emission tomography (PET). A BACE1 inhibitor, 5, is reported with blood-brain barrier (BBB) permeability and high binding affinity. To characterize the pharmacokinetics and distribution of 5 in the brain, we radiolabeled 5 with carbon-11. Using PET, we found that [¹¹C]5 shows moderate uptake in the brain when administered intravenously to rodents and further work will be performed in animal models to test its application as a PET imaging probe for the central nervous system. Our study demonstrates the effectiveness of PET at providing brain pharmacokinetic data for BACE1 inhibitors, crucial for the development of treatments for AD where CNS penetration is critical.

Full text of DOI:10.1007/s00044-019-02480-9

MEDICINAL
CHEMISTRY
RESEARCH
Medicinal Chemistry Research
https://doi.org/10.1007/s00044-019-02480-9
ORIGINAL RESEARCH
Synthesis of a carbon-11 radiolabeled BACE1 inhibitor
Yiwei Zhu1 Stephanie A. Fiedler1 Matthew L. Hibert1 Changning Wang1
●
●
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Received: 21 January 2019 / Accepted: 15 November 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Beta-secretase (BACE1), a transmembrane aspartyl protease, can cleave membrane-bound β-amyloid precursor proteins
(APPs) to initiate the accumulation of amyloid-β (Aβ). The inhibition of BACE-1 to limit the accumulation of neurotoxic Aβ
peptides could offer a potential treatment for Alzheimer’s Disease (AD). However, little is known about the distribution and
density of BACE1 in the central nervous system. As a step toward filling this gap in knowledge, we have evaluated a
potential radiotracer for the imaging of BACE1 using positron emission tomography (PET). A BACE1 inhibitor, 5, is
reported with blood-brain barrier (BBB) permeability and high binding affinity. To characterize the pharmacokinetics and
distribution of 5 in the brain, we radiolabeled 5 with carbon-11. Using PET, we found that [11C]5 shows moderate uptake in
the brain when administered intravenously to rodents and further work will be performed in animal models to test its
application as a PET imaging probe for the central nervous system. Our study demonstrates the effectiveness of PET at
providing brain pharmacokinetic data for BACE1 inhibitors, crucial for the development of treatments for AD where CNS
penetration is critical.
●
●
Keywords BACE1 PET Imaging
Introduction
2014). According to the amyloid cascade hypothesis, one of
the neuropathological characteristics of AD is the deposition
of extracellular amyloid plaques in the brain (Hardy and
Higgins 1992). The main constituent of amyloid plaques, the
39–43 amino acid amyloid-β-peptides (Aβ), initiates the
neurotoxic cascade leading to the devastating neurodegen-
erative disease. Aβ is produced in a two-step proteolysis of
the amyloid precursor protein (APP) initiated by the
sequential action of two enzymes, β-secretase (BACE1, β-site
APP cleaving enzyme 1) and γ-secretase. Numerous obser-
vations from preclinical studies show β-secretase to be a
prime therapeutic target for the treatment of AD pathogenesis
(Jonsson et al. 2012; Sathya et al. 2012; Probst and Xu 2012).
Partial reduction of BACE1 enzyme activity leads to dramatic
inhibition of AD-driven pathology, suggesting the activity of
BACE1 has a strong effect on AD (Jiang et al. 2016; Kimura
et al. 2010; McConlogue et al. 2007). Moreover, a large
number of BACE1 inhibitors have been developed which
reduce the formation of amyloid (Vassar 2014). Among them,
several inhibitors have entered clinical trials such as MK-8931
(phase II/III clinical trial) (Forman et al. 2012), AZD3293
(phase II/III clinical trial) (Eketjall et al. 2016), and E2609
(phase I trial) (Lai et al. 2012).
Alzheimer’s Disease (AD) is a neurodegenerative disease
which results in the impairment of cognitive capabilities
including progressive memory loss, impaired judgment, a
decline in language ability, and cognitive and behavioral dis-
order, which ultimately leads to death. In 2013, official death
certificates recorded 84,767 deaths from AD, making AD the
sixth leading cause of death in the United States and the fifth
leading cause of death in Americans age 65 and above. Today
more than 5 million Americans live with AD and a new case is
diagnosed every 66 s (http://www.alz.org/facts/. 2016).
To date, much attention has been paid to strategies to
identify underlying pathologic mechanisms and decrease the
progression of AD (Oehlrich et al. 2014), and there are many
potential targets for therapeutics, such as AD autophagy drugs
(Friedman et al. 2015) and epigenetic drugs (Heerboth et al.
* Changning Wang
cwang15@mgh.harvard.edu
1
Athinoula A. Martinos Center for Biomedical Imaging,
BACE1 protein and activity are elevated in several neuro-
logical disorders, such as AD (Yang et al. 2003) and Down’s
Department of Radiology, Massachusetts General Hospital,
Harvard Medical School, Charlestown, MA 02129, USA

Medicinal Chemistry Research
Syndrome (Sun et al. 2006). In the AD brain, BACE1 protein
levels and activity were increased significantly in high-order
association brain regions affected by Aβ deposition compared
to non-demented control brains as measured by ELISA.
However, the mechanisms by which BACE1 enzymes change
in AD patients and BACE1 enzymes are modified in human
CNS diseases have yet to be determined in vivo. A better
understanding of the role of BACE1 in AD may facilitate the
development of novel treatments for AD. Therefore, our goal
was to develop a positron emission tomography (PET) imaging
probe and evaluate the expression and distribution of BACE1
in vivo in order to improve our understanding of the relation-
ship between BACE1 and AD, potentially accelerating the
diagnosis of AD and leading to earlier intervention in the
treatment of AD.
PET is a powerful non-invasive imaging tool that could be
used to visualize BACE1 in animals and humans. It can
provide a method for quantification of the distribution and
alterations of BACE1. However, until now no imaging
radiotracer has been developed to target BACE1, although a
few BACE1 inhibitors have been radiolabeled with limited
brain uptake (Nordeman et al. 2014). Kawai et al. has
reported a radiolabeled naphthalene- based BACE-1 inhibitor
(Me-NCFB, IC50 = 2.3 0.80 μM), with a brain/plasma ratio
of 1–1.5, which is not high for a PET radiotracer. They also
do not report any information about specific binding or
in vivo PET imaging (Kawai et al. 2013). Another
hydroxyethylamine-based BACE-1 inhibitor, BSI-IV, has
been labeled, but shows very low brain uptake (Nordeman
et al. 2014). We chose a BACE-1 inhibitor, compound 5, to
be labeled with C-11 in this manuscript. Compound 5 is
reported as a BACE-1 inhibitor (Ginman et al. 2013), it has a
brain/plasma ratio of 8.9 and Log D of 2.5, which are ideal
properties for a PET imaging probe. We expect compound 5
would be a good candidate for further development. There-
fore, we aim to present a potential probe with not only good
affinity to BACE1, but also with high brain uptake.
were recorded on a Varian 500 MHz magnet and were
reported in ppm units downfield from trimethylsilane.
Analytical separation was conducted on an Agilent
1100 series high-pressure liquid chromatography (HPLC)
system fitted with a diode-array detector, quaternary pump,
vacuum degasser, and autosampler. Mass spectrometry data
were recorded on an Agilent 6310 ion trap mass spectro-
meter (ESI source) connected to an Agilent 1200 series
HPLC with quaternary pump, vacuum degasser, diodearray
detector, and autosampler. Compounds 5 and 6 were syn-
thesized (from Sai Life Sciences Ltd.) with >98% purity, as
determined by a HPLC system (Agilent XDB C-18
(150 mm × 4.6 mm × 3 μ) mobile phase: A: 0.1% TFA in
water/B: 0.1% TFA in acetonitrile. Flow rate: 5.0 mL/min).
[11C]carbon dioxide (1.2 Ci) was obtained via the 14N (p,α)
11C reaction on nitrogen with 2.5% oxygen, with 11 MeV
protons (Siemens Eclipse cyclotron), and trapped on
molecular sieves in a TRACERlab FX-MeI synthesizer
(General Electric). [11C]Methane was obtained by the
reduction of [11C] carbon dioxide in the presence of Ni/
hydrogen at 350 °C and recirculated through an oven con-
taining I2 to produce [11C]methyl iodide via a radical
reaction (Wang et al. 2014).
The animal studies were carried out at Massachusetts
General Hospital (PHS Assurance of Compliance no.
A3596-01). The Subcommittee on Research Animal Care
(SRAC) serves as the Institutional Animal Care and Use
Committee (IACUC) for the Massachusetts General Hos-
pital (MGH). SRAC reviewed and approved all procedures
detailed in this paper.
PET-CT imaging was performed in anesthetized (iso-
flurane) rats (Sprague−Dawley) to minimize discomfort.
Highly trained animal technicians monitored animal safety
throughout all procedures, and veterinary staff were
responsible for daily care. All animals were socially housed
in cages appropriate for the physical and behavioral health
of the individual animal. Animals were given unlimited
access to food and water, with additional nutritional sup-
plements provided as prescribed by the attending veterinary
staff. Animals were euthanized at the end of the study using
sodium pentobarbital (200 mg/kg, ip).
Our goal is to develop a PET brain imaging radiotracer
for BACE1. In this paper, we describe the design and
synthesis of [11C]5, and characterize the BBB penetration of
[11C]5 in vivo in a rodent model with PET. Results from our
imaging studies indicate that [11C]5 has moderate BBB
penetration in rats, although it may not ideal for further
imaging study, it will be a good candidate compound for
further radiotracer development and scaffold optimization.
Chemical synthesis
Intermediate 1 (4.0 g, 20 mmol), 3-Methoxy phenyl boronic
acid (3.3 g, 22 mmol), [1,1′-Bis(diphenylphosphino)ferro-
cene]palladium(II) dichloride (0.86 mg, 1 mmol), and
sodium carbonate (5.5 g, 52 mmol) were added to a flask.
The mixture was stirred at 80 °C for 2 h in 30 mL dime-
thoxyethane/water (2:1). The reaction mixture was then
diluted with water and extracted with ethyl acetate, and
purified by flash column chromatography to obtain inter-
mediate 2 (4 g, 90%). Intermediate 2 (2 g, 8.8 mmol) was
Materials and methods
General materials and methods
All reagents and solvents were of ACS-grade purity or
higher and used without further purification. NMR data

Medicinal Chemistry Research
dissolved in 2-methyltetrahydrofuran at −78 °C, vinyl-
magnesium bromide (2.6 g, 20 mmol) was added, stirred for
1 h at −78 °C, and warmed to room temperature (rt) for 2 h.
It was then quenched with saturated ammonium chloride
solution, extracted with ethyl acetate, and purified by flash
column chromatography to obtain intermediate 3 (1.3 g,
58%). Thiourea (0.39 g, 5.5 mmol) and 1 M hydrochloric
acid (5.5 mL, 5.5 mmol) in acetic acid was added to a
solution of intermediate 3 (1.3 g, 5.1 mmol) in acetic acid at
rt and stirred at 45 °C for 16 h. The reaction mixture was
concentrated and re-crystallized from a mixture of methanol
and diethyl ether to obtain intermediate 4 (0.8 g, 50%).
Methanesulfonic acid (0.54 g, 5.6 mmol) was added drop-
wise to a solution of intermediate 4 (0.8 g, 2.6 mmol) in
trifluoroacetic acid at 0 °C and then stirred at rt for 1 h. The
reaction mixture was concentrated and dissolved in
dichloromethane, washed with aq. sodium bicarbonate
solution and purified by column chromatography to obtain
compound 5 as yellow solid (0.35 g, 44%). 1H-NMR
(400 MHz, DMSO-d6): δ 7.59 (s, 1H), 7.34–7.46 (m, 4H),
7.17 (d, J = 8.0 Hz, 1H), 7.12 (s, 1H), 6.92 (d, J = 8.0 Hz,
1H), 5.82(s, 2H), 3.8 (s, 3H), 2.88–2.90 (m, 1H), 2.54–2.56
(m, 1H), 2.05–2.08 (m, 1H), 1.71–1.71 (m, 1H), 1.44 (s,
3H). LC-MS calculated for C18H20N2OS [M]: 312.1; found
[M + H]+: 313.1. Boron tribromide (0.6 g, 2.4 mmol) was
added dropwise to a solution of compound 5 (0.25 g,
0.8 mmol) in dichloromethane at −78 °C, then stirred at
0 °C for 1 h. The reaction was quenched with water, pH
adjusted to 7 and the mixture was extracted with 5%
methanol in ethyl acetate. The organic layer was con-
centrated and purified by flash column chromatography to
cartridge, rinsing with water (5 mL), eluting with ethanol
(1 mL), and diluting with 0.9% saline (9 mL).
Rodent PET/CT acquisition and post processing
Male Sprague-Dawley rats (350 g, Charles River Labora-
tories, Inc, Wilmington, MA) were utilized in pairs, anes-
thetized with gaseous isoflurane at 3% in a carrier of 1.5 L/
min medical oxygen and maintained at 2% isoflurane for the
duration of the scan. The rats were arranged side-by-side in
a Triumph Trimodality PET/CT/SPECT scanner (Gamma
Medica, Northridge, CA). Rats were injected with [11C]5
(50–100 µCi per animal) after 5 min pretreatment of unla-
beled 5 (1 mg/kg) or vehicle via a lateral tail vein cathe-
terized at the start of PET acquisition. Dynamic PET
acquisition lasted for 60 min, and was followed by com-
puted tomography (CT) for anatomic co-registration and
PET attenuation correction. PET data were reconstructed
using a 3D-MLEM method resulting in a full width at half-
maximum resolution of 1 mm. Reconstructed images were
exported from the scanner in DICOM format along with an
anatomic CT for rodent studies. These files were imported
to PMOD (PMOD Technologies, Ltd.) and manually co-
registered using six degrees of freedom.
Rodent PET/CT image analysis
Volumes of interest (VOIs) were drawn manually as spheres
in brain regions guided by high-resolution CT structural
images and summed PET data, with a radius of no <1 mm to
minimize partial volume effects. Time-activity curves
(TACs) were exported as decay-corrected activity per unit
volume. The TACs were expressed as percent injected dose
per unit volume for analysis.
1
obtain compound 6 as white solid (45 mg, 19%). H-NMR
(400 MHz, DMSO-d6): δ9.46 (s, 1H), 7.59 (s, 1H),
7.42–7.46 (m, 2H), 7.32 (d, J = 7.2 Hz, 1H), 7.23–7.27 (m,
1H), 7.05 (d, J = 7.2 Hz, 1H), 7.0 (s, 1H), 6.76–6.78 (m,
1H), 3.0 (s, 1H), 2.61–2.66 (m, 1H), 2.50 (s, 1H), 1.90–1.93
(m, 1H), 1.57 (s, 3H). LC-MS calculated for C17H18N2OS
[M]: 298.1; found [M + H]+: 299.0.
Results and discussion
BACE1, a transmembrane aspartyl protease, has been
considered as a potentially powerful therapeutic target for
the diagnosis and treatment of AD for many years. One of
the major factors associated with AD is the accumulation of
amyloid plaques, the primary component of which is Aβ
which is excised from APP by the sequential action of
BACE1. As a result, inhibition of BACE1 is an exciting
therapeutic target for the treatment of AD.
However, information about the density and distribution
of BACE1 in the brain is limited due to the lack of effective
tools to quantify the expression of cerebral BACE1 in vivo.
Our goal was to develop a radiotracer targeting BACE1 for
PET imaging. The development of specific PET radiotracers
to measure BACE1 in the brain is challenging. A radiotracer
to be used for brain imaging must have high BBB
Radiosynthesis of [11C]5
[11C]methyl iodide was trapped in a TRACERlab FX-M
synthesizer reactor (General Electric) preloaded with a
solution of precursor (6) (1.0 mg) in dry dimethylformamide
(300 μl) with potassium hydroxide (15 mg). The solution
was stirred at 100 °C for 3 min, and water (0.7 ml) was
added. The reaction mixture was purified by reverse phase
semi-preparative HPLC (Agilent Eclipse XDB-18 column,
(250 mm × 9.4 mm × 5 μ), mobile phase: A: 0.1% TFA in
water/B: 0.1% TFA in acetonitrile, A:B = 70: 30; Flow rate:
5.0 mL/min), and the desired fraction was collected. The
final product was diluted with 20 mL water and reformu-
lated by loading onto a solid-phase exchange (SPE) C-18

Medicinal Chemistry Research
penetration, high brain uptake, great binding affinity, and
high specific binding (Wang et al. 2014). Some of these
factors, such as specific binding, are difficult to predict and
can only be measured in vivo by pre-saturating the target
with an unlabeled form of the radiotracer. Compound 5
(IC50 = 2 µM, LogD = 2.5, MW = 312.43) is one of the
most optimal BACE1 inhibitors with high target activity
and high brain permeability (Ginman et al. 2013). To fur-
ther quantify these properties in mice and investigate the
ability of in vitro measurements of permeability to predict
in vivo results, 5 was administered orally in mice. We found
that 5 penetrated the blood-brain barrier well with a high
total brain/plasma ratio (8.9) (Ginman et al. 2013). There-
fore, we choose compound 5 for radiolabeling and PET
imaging study in this paper. Compound 5 has been screened
by the central nervous system PET multiparameter optimi-
zation (CNS PET MPO) design tool, which was reported by
Pfizer (Zhang et al. 2013). The CNS PET MPO value is 3.3,
which is in the ideal range for radiotracer development
(CNS PET MPO > 3).
Synthesis of nonradioactive standards compound 5 and
its O-desmethyl precursor 6 were achieved in good yield as
shown in Scheme 1. Briefly, 1-(3-bromophenyl)ethan-1-one
(1) was coupled with (3-methoxyphenyl)boronic acid to
form intermediate 2. The intermediate 2 was then reacted
with vinylmagnesium bromide to form 3. Reference stan-
dard 5 was obtained by the reaction of 3 with thiourea and
methanesulfonic acid. Radiolabeling precursor 6 was
accomplished through O-demethylation in the presence of
boron tribromide.
was stirred at 100 °C for 3 min and water (0.7 mL) was
added.
The reaction mixture was purified by reverse phase semi-
preparative HPLC and the desired fraction was collected.
The final product was reformulated by loading onto a solid-
phase exchange (SPE) C-18 cartridge rinsing with water
(5 mL), eluting with ethanol (1 mL), and diluting with saline
(0.9%, 9 mL) to formulate the product as isotonic solution.
The chemical and radiochemical purity of the final product
was tested by analytical HPLC. The time required for the
synthesis from end of cyclotron bombardment to end of
synthesis was 35–40 min. The radiochemical yield (RCY)
was 0.41% (n = 3, decay-uncorrected to trapped [11C]
methyl iodide) with specific activity 0.4–0.7 mCi/nmol (at
time of injection). Chemical and radiochemical purities
were ≥95% at time of injection.
Using PET-CT, we determined that [11C]5 exhibited
moderate BBB penetration and low brain uptake over the
scanning time (60 min) when administered intravenously to
rats as shown in Figs. 1 and 2. A concentration of 0.2–0.4
SUV (standardized uptake value) was distributed in the
brain tissue. Coregistration of the PET image with a CT of
the rat indicated that the areas appearing to have a high level
of radioactivity are likely outside of the brain. In order to
determine the property of specific binding, we also con-
ducted a PET imaging study with 5-min pretreatment of
reference standards (5, 1.0 mg/kg). However, no change in
permeability or retention was observed after pretreatment
which indicated 5 has high non-specific binding. The
observed moderate brain uptake may be due to the differ-
ence in the route of administration (p.o. vs i.v.). Addition-
ally, 5 may interact with P-glycoprotein (P-gp) which limits
its brain penetrance. P-gp inhibitors could be used to block
P-gp efflux and possibly increase BBB penetration of [11C]
5. There are many other reasons to affect the brain uptake of
radiotracers, including tracer stability and specific activity
The high polar radiometabolites from unstable tracers will
The synthesis of the target carbon-11 labeled compound
[11C]5 was accomplished using O-desmethyl precursors in
dimethylformamide with [11C]methyl iodide as outlined in
Scheme 2. [11C]methyl iodide was trapped in a TRACERlab
FX-M synthesizer reactor (General Electric) preloaded with
a solution of precursor 6 (1.0 mg) and potassium hydroxide
(6.0 mg) in dry dimethylformamide (300 μL). The solution
Scheme 1 Reagents and conditions: a [1,1′-Bis(diphenylphosphino)
ferrocene]palladium(II) dichloride, sodium carbonate, dimethox-
yethane/water, 80 °C, 2 h, 90%; b vinylmagnesium bromide, 2-
methyltetrahydrofuran, −78 °C to rt, 58%; c thiourea, hydrochloric
acid, acetic acid, 45 °C, 50%; d Methanesulfonic acid, trifluoroacetic
acid, rt, 44%; e Boron tribromide, dichloromethane, −78 to 0 °C, 19%

Medicinal Chemistry Research
Scheme 2 Radiosynthesis of
[11C]5. Reagents and conditions:
a [11C]methyl iodide, Potassium
hydroxide, Dimethylformamide,
100 °C for 3 min
Conclusion
In summary, we successfully labeled 5, a BACE1 inhibitor,
with [11C]methyl iodide by the methylation of precursor. Using
PET, we determined that [11C] 5 exhibits moderate BBB
penetration with non-specific binding, but this does not
necessarily exclude it as a candidate for AD treatment. To this
end, ongoing work is aimed at clarifying the BBB penetration
mechanism of compound 5 and imaging animal models of AD.
Acknowledgements We are grateful to the Martinos Center radio-
pharmacy staff (Judit Sore, Kari Phan, Garima Gautam, and Samantha
To) for help with radiotracer production. We are grateful to Brendan
Taillon for assisting with rodent PET-CT studies. This research was
supported by the Harvard/MGH Nuclear Medicine Training Program
from the Department of Energy under Grant DE-SC0008430 (C.W.).
This research was carried out at the Athinoula A. Martinos Center for
Biomedical Imaging at the Massachusetts General Hospital, using
resources provided by the Center for Functional Neuroimaging
Technologies, P41EB015896, a P41 Regional Resource supported by
the National Institute of Biomedical Imaging and Bioengineering
(NIBIB), and the National Institutes of Health. This work also
involved the use of instrumentation supported by the NIH Shared
Instrumentation Grant Program and/or High-End Instrumentation
Grant Program; specifically, Grant Number: S10RR015728.
Fig. 1 Rodent in vivo PET imaging with [11C]5 reveals limited uptake
in brain. Summed PET images (0–60 min) following injection with
[11C]5 at baseline or after 5-min pretreatment with 1.0 mg/kg unla-
beled 5 (self-blocking)
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of
interest.
Publisher’s note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional affiliations.
Fig. 2 Whole brain time–activity curves for brain regions of interest in
baseline [11C]5 study and 5-min pre-administration study with 1 mg/kg
5. There are reasons for the blocking study has an activity curve above
the baseline study, for example, the blocking agent binds to BET
peripherally, and increase the activity in the blood pool or the blocking
agents have efflux transporter interaction which potentiate brain entry
References
Eketjall S et al. (2016) AZD3293: a novel, orally active BACE1
inhibitor with high potency and permeability and markedly slow
off-rate kinetics. J Alzheimers Dis 50(4):1109–1123
Forman M, Palcza J, Tseng J, Leempoels J, Ramael S, Han D, Jhee S,
Tanen M, Ereshefsky L, Laterza O, Dockendorf M, Krishna G,
Ma L, Wagner J, and Troyer M (2012) The novel BACE inhibitor
MK-8931 dramatically lowers cerebrospinal fluid Aβ peptides in
health subjects following single- and multiple-dose administra-
tion. Alzheimers Dement 8:704
Friedman LG, Qureshi YH, Yu WH (2015) Promoting autophagic
clearance: viable therapeutic targets in Alzheimer’s disease.
Neurotherapeutics 12(1):94–108
Ginman T et al. (2013) Core refinement toward permeable beta-
secretase (BACE-1) inhibitors with low hERG activity. J Med
Chem 56(11):4181–4205
lead to poor brain penetration, and not suitable for PET
imaging study. So we will choose the candidates with
relatively good stability for radiolabeling to minimize the
effects from radiometabolites. The radiotracer with low
specific activity has more “unlabeled” version of com-
pounds, that will potentially compete with the radiotracer
for target binding in the brain. Therefore, the radiosynthesis
conditions need to be optimized to achieve moderate to high
specific activity. Additionally, we plan to conduct [11C]5
imaging in non-human primates to study if there is species-
dependent brain permeability.

Medicinal Chemistry Research
Hardy JA, Higgins GA (1992) Alzheimer’s disease: the amyloid cas-
cade hypothesis. Science 256(5054):184–185
Heerboth S et al. (2014) Use of epigenetic drugs in disease: an over-
view. Genet Epigenet 6:9–19
Jiang Y et al. (2016) Partial BACE1 reduction in a Down syndrome
mouse model blocks Alzheimer-related endosomal anomalies and
cholinergic neurodegeneration: role of APP-CTF. Neurobiol
Aging 39:90–98
Jonsson T et al. (2012) A mutation in APP protects against Alzhei-
mer’s disease and age-related cognitive decline. Nature 488
(7409):96–99
Nordeman P et al. (2014) (11)C-Labeling of a potent hydro-
xyethylamine BACE-1 inhibitor and evaluation in vitro and
in vivo. Nucl Med Biol 41(6):536–543
Oehlrich D, Prokopcova H, Gijsen HJ (2014) The evolution of
amidine-based brain penetrant BACE1 inhibitors. Bioorg Med
Chem Lett 24(9):2033–2045
Probst G, Xu YZ (2012) Small-molecule BACE1 inhibitors: a patent
literature review (2006 - 2011). Expert Opin Ther Pat 22
(5):511–540
Sathya M et al. (2012) BACE1 in Alzheimer’s disease. Clin Chim
Acta 414:171–178
Kawai T et al. (2013) Synthesis and evaluation of 11C-labeled
naphthalene derivative as a novel non-peptidergic probe for the
beta-secretase (BACE1) imaging in Alzheimer’s disease brain.
Nucl Med Biol 40(5):705–709
Kimura R, Devi L, Ohno M (2010) Partial reduction of BACE1
improves synaptic plasticity, recent and remote memories in
Sun X et al. (2006) Increased BACE1 maturation contributes to the
pathogenesis of Alzheimer’s disease in Down syndrome. FASEB
J 20(9):1361–1368
Vassar R (2014) BACE1 inhibitor drugs in clinical trials for Alzhei-
mer’s disease. Alzheimers Res Ther 6(9):89
Wang C et al. (2014) In vivo imaging of histone deacetylases
(HDACs) in the central nervous system and major peripheral
organs. J Med Chem 57(19):7999–8009
Alzheimer’s disease transgenic mice.
(1):248–261
J Neurochem 113
Lai R, Albala B, Kaplow JM, Aluri J, Yen M, Satlin A (2012) First-in-
human study of E2609, a novel BACE1 inhibitor, demonstrates
prolonged reductions in plasma beta-amyloid levels after single
dosing Alzheimers Dement 8:96
McConlogue L et al. (2007) Partial reduction of BACE1 has dramatic
effects on Alzheimer plaque and synaptic pathology in APP
Transgenic Mice. J Biol Chem 282(36):26326–26334
Yang LB et al. (2003) Elevated beta-secretase expression and enzymatic
activity detected in sporadic Alzheimer disease. Nat Med 9(1):3–4
Zhang L et al. (2013) Design and selection parameters to accelerate the
discovery of novel central nervous system positron emission
tomography (PET) ligands and their application in the develop-
ment of a novel phosphodiesterase 2A PET ligand. J Med Chem
56(11):4568–4579