Synthesis and evaluation of two 18F-labeled imidazo[1,2-a]pyridine analogues as potential agents for imaging β-amyloid in Alzheimer's disease

Article abstract of DOI:10.1016/j.bmcl.2006.02.055

Two new fluorinated imidazo[1,2-a]pyridine derivatives, 6-(2′-fluoroethyl)-2-(4′-dimethylamino)phenylimidazo[1,2-a]pyridine (FEPIP) and 6-(3′-fluoropropyl)-2-(4′-dimethylamino)phenylimidazo[1,2-a]pyridine (FPPIP), were synthesized. The binding affinity fo

Full text of DOI:10.1016/j.bmcl.2006.02.055

Bioorganic & Medicinal Chemistry Letters 16 (2006) 3015–3018
18
Synthesis and evaluation of two F-labeled
imidazo[1,2-a]pyridine analogues as potential agents
for imaging b-amyloid in Alzheimer’s disease
Fanxing Zeng,a Jeanine A. Southerland,b Ronald J. Voll,a John R. Votaw,a
Larry Williams,a Brian J. Ciliax,b Allan I. Leveyb and Mark M. Goodmana,*
aDepartment of Radiology, Division of Radiological Sciences, Emory University, Atlanta, GA 30322, USA
bCenter for Neurodegenerative Disease and Department of Neurology, Emory University, Atlanta, GA 30322, USA
Received 12 December 2005; revised 21 February 2006; accepted 21 February 2006
Available online 29 March 2006
Abstract—Two new fluorinated imidazo[1,2-a]pyridine derivatives, 6-(20-fluoroethyl)-2-(40-dimethylamino)phenylimidazo[1,2-a]pyr-
idine (FEPIP) and 6-(30-fluoropropyl)-2-(40-dimethylamino)phenylimidazo[1,2-a]pyridine (FPPIP), were synthesized. The binding
affinity for FEPIP and FPPIP to amyloid plaques in human AD cortical tissues was determined. Radiolabeling, in vitro film auto-
radiography, and micro-PET study were performed with [18F]FPPIP to determine its utility as a radioligand for amyloid plaque
imaging in the brain of AD patients.
Ó 2006 Elsevier Ltd. All rights reserved.
Alzheimer’s disease (AD) is the most prevalent neurode-
generative disorder among the elderly. Neuritic plaques
containing b-amyloid (Ab) peptides and neurofibrillary
tangles in postmortem brain are the two pathological
hallmarks characteristic of AD and provide the basis
for the definitive, albeit postmortem, diagnosis of
AD.1,2 While there are no definitive treatments available
to affect a cure of AD, much recent interest has been giv-
en to the development of anti-amyloid therapies aimed
at halting and reversing amyloid formation and deposi-
tion. Therefore, development of amyloid-specific imag-
ing agents could be potentially useful for early
diagnosis and further neuropathogenesis studies of
AD. Amyloid-imaging tracers could also facilitate the
evaluation of the efficacy of anti-amyloid therapies
currently under intense development.
and
compounds have been reported for targeting Ab pla-
ques.12–14 123I]IMPY ([123I]6-iodo-2-(40-N,N0-dimeth-
ylamino)phenylimidazo[1,2-a]pyridine) has recently
been reported as a potential SPECT agent for imaging
Ab plaques with high binding affinity for preformed syn-
thetic Ab 40 aggregates (Ki = 15 nM) and human AD
cortical homogenates (Kd = 5.3 nM) and desirable phar-
macokinetics in normal mouse brain (2.9% initial dose/
brain at 2 min and 0.2% initial dose/brain at 60 min).
[
123I]IMPY.10,11 Other structurally similar
[
As part of an effort to develop 18F-labeled tracers for
PET imaging of Ab plaques, we developed two fluori-
nated analogues of IMPY, in which its iodo group is
replaced with fluoroethyl (FEPIP) or fluoropropyl
(FPPIP). We report here the synthesis and in vitro affin-
ities of FEPIP and FPPIP, and the radiolabeling and
evaluation of [18F]FPPIP as in vivo PET imaging agent
for Ab plaques. (Fig. 1).
Efforts to develop radiolabeled Ab imaging agents for
both positron emission tomography (PET) and single-
photon emission computed tomography (SPECT)
have generated a few lead compounds, including
The syntheses of unlabeled FEPIP and FPPIP are
shown in Scheme 1. IMPY (1) was prepared from
condensation reaction between commercially available
2-amino-5-iodopyridine and 2-bromo-40-dimethylami-
noacetophenone15 in the presence of a mild base such
as sodium bicarbonate.11 Palladium catalyzed coupling
of 1 with tributyl(vinyl)tin produced the alkene 2 in
[11C]PIB,3,4 [11C]SB-13,5,6 [18F]FDDNP,7,8 123I]IBOX,9
[
Keywords: Fluorine-18; b amyloid; Alzheimer’s disease; PET imaging.
*
Corresponding author. Tel.: +1 404 727 9366; fax: +1 404 727
3488; e-mail: mgoodma@emory.edu
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.02.055

3016
F. Zeng et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3015–3018
Table 1. The logP and Ki values of IMPY derivatives for binding
N
N
N
human AD cortical tissues
N
N
N
F
I
Ligand
LogP
Ki (nM)
IMPY
FEPIP
IMPY
PIB
3.58
1.303
2.42
2.84
10.3
7.8
N
N
F
N
FEPIP
FPPIP
177
48.3
FPPIP
Figure 1. Structures of IMPY and its two fluorinated analogues.
affinity (Ki = 48.3 nM). Decreasing length of the side
chain by a –CH2 group reduced the binding affinity
significantly (Ki for FEPIP was 177 nM). Distribution
coefficients of FEPIP and FPPIP were measured
between 1-octanol and phosphate buffer at pH 7.4,17
and logP values are given in Table 1. FEPIP and FPPIP
display moderate lipophilicity and the logP values are in
the optimal range (1–3) for compounds expected to
enter the brain readily.18
90% yield. The following hydroboration–oxidation reac-
tion of 2 gave the hydroxyethyl compound 3 in 86%
yield, which was then converted to FEPIP by reaction
with DAST. FPPIP was prepared in good yield by a
similar method.
The radiolabeling of [18F]FPPIP was accomplished by
treatment of tosylate precursor with anhydrous
[18F]KF and K222 in acetonitrile for 10 min at 90 °C
(Scheme 2). The entire procedure required approximate-
ly 105 min from the end of bombardment. [18F]FPPIP
was prepared in an average 51% decay-corrected yield.
Analytical HPLC demonstrated that the radiolabeled
product was over 98% radiochemically pure, and the
specific activity of the product was 1.7–2.3 Ci/lmol at
time of injection.
Upon the basis of its higher binding affinity to Ab
plaques, [18F]FPPIP plaque labeling was evaluated by
in vitro film autoradiography as shown in Figures 2
and 3. Specific binding of [18F]FPPIP to amyloid
plaques in sections from postmortem AD brains was
A
B
In vitro binding affinities of FEPIP and FPPIP to
b-amyloid were determined via the binding competition
with [125I]IMPY using human AD cortical tissues by
quantitative autoradiography.16 The results shown in
Table 1 demonstrate that IMPY and PIB displayed high
binding affinity with Ki = 10.3 and 7.8 nM, respectively,
which is comparable to the previously reported inhibi-
tion constant (Ki = 15 and 4.7 nM) obtained in a pre-
formed Ab 40 aggregate model system. FPPIP
competed well with [125I]IMPY, showing a moderate
Figure 2. In vitro autoradiographic detection of Ab amyloid deposits
with [18F]FPPIP in postmortem brain tissue sections of frontal
lobe from an AD patient and an age-matched control. (A) AD
tissue + [18F]FPPIP. (B) Control tissue + [18F]FPPIP.
I
N
O
a
N
BrCH2
C
N
+
N
I
N
NH2
N
IMPY, 1
N
N
d
b
c
1
1
N
FEPIP
N
N
HO
3
2
N
N
g
e
f
N
FPPIP
N
N
HO
N
5
4
Scheme 1. Syntheses of FEPIP and FPPIP. Reagents and conditions: (a) EtOH, reflux, 2 h; NaHCO3, reflux, 5 h, 51%; (b) Bu3SnCH@CH2,
1,4-dioxane, Et3N, (PPh3)4Pd, 100 °C, 20 h, 90%; (c) 9-BBN, THF, rt, 24 h; 3 N NaOH, 30% H2O2, rt, 5 h, 86%; (d) DAST, CH2Cl2, ꢀ78 °C
to 23 °C, 2 h, 32%; (e) Bu3SnCH2CH@CH2, toluene, (PPh3)4Pd, 100 °C, 20 h, 58%; (f) 9-BBN, THF, rt, 24 h; 3 N NaOH, 30% H2O2, rt, 5 h, 88%;
(g) DAST, CH2Cl2, ꢀ78 °C to 23 °C, 2 h, 44%.
N
N
TsCl, Et3N
[
18F]KF-K222
MeCN, 90oC
10 min, 51%
5
N
N
18F
N
CH2Cl2, rt, 15 h
52% yield
TsO
N
6
[
18F] FPPIP
Scheme 2. Radiosynthesis of [18F]FPPIP.

F. Zeng et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3015–3018
3017
3
SCWM
Frontal CX
Cere
2.5
2
1.5
1
0.5
0
0
20
40
60
80
100
120
Time (min)
Figure 4. Time–activity curves of brain regions for [18F]FPPIP in a
rhesus monkey.
imaging agent for Ab, based on its favorable pharma-
cokinetics in a rhesus monkey and its high specific
labeling of Ab plaques in vitro. However, more work
is required to refine this structure in order to increase
binding affinity.
Figure 3. Comparison between plaques visualized by [18F]FPPIP
autoradiography and Ab immunohistochemistry. (A) Film autoradio-
gram of [18F]FPPIP displays selective in vitro binding to Ab plaques in
AD brain sections. (B) The same labeled plaques are confirmed by
immunohistochemistry with 4G8. (C) and (D) Images of those regions
demarcated by the rectangles in (A) and (B) were magnified,
respectively, and arrows demarcate the same distribution of plaques.
References and notes
clearly observed in cortical gray matter, but not in the
white matter, and the specific binding was eliminated
in the AD specimen with the pretreatment with nonra-
dioactive FPPIP (data not shown). The control brain
sections showed no specific binding, correlating well
with the absence of amyloid plaques in these brains.
To confirm the [18F]FPPIP-labeled plaques at a micro-
scopic level and to determine the specificity of [18F]FPPIP
binding for Ab plaques, AD brain sections labeled with
[18F]FPPIP were subsequently immunostained with a
monoclonal antibody (4G8, Signet, MA) specific for
b-amyloid proteins.19 Clearly, [18F]FPPIP labeled all of
the plaques detected by 4G8 on AD brain sections and a
good correlation with Ab plaques’ labeling was demon-
strated (marked with arrowheads).
1. Klunk, W. E.; Abraham, D. J. Psychol. Dev. 1988, 6, 121.
2. Hardy, J.; Selkoe, D. J. Science 2002, 297, 353.
3. Mathis, C. A.; Wang, Y.; Holt, D. P.; Huang, G.; Debnath,
M. L.; Klunk, W. E. J. Med. Chem. 2003, 46, 2740.
4. Wang, Y.; Klunk, W. E.; Debnath, M. L.; Huang, G. F.;
Holt, D. P.; Shao, L.; Mathis, C. A. J. Mol. Neurosci.
2004, 24, 55.
5. Verhoeff, N. P.; Wilson, A. A.; Takeshita, S.; Trop, L.;
Hussey, D.; Singh, K.; Kung, H. F.; Kung, M.-P.; Houle,
S. Am. J. Geriatr. Psychiatr. 2004, 12, 584.
6. Ono, M.; Wilson, A.; Nobrega, J.; Westaway, D.;
Verhoeff, P.; Zhuang, Z.-P.; Kung, M.-P.; Kung, H. F.
Nucl. Med. Biol. 2003, 30, 565.
7. Agdeppa, E. D.; Kepe, V.; Liu, J.; Flores-Torres, S.;
Satyamurthy, N.; Petric, A.; Cole, G. M.; Small, G. W.;
Huang, S. C.; Barrio, J. R. J. Neurosci. 2001, 21, RC189.
8. Nordberg, A. Lancet Neurol. 2004, 3, 519.
The micro-PET imaging study of [18F]FPPIP to assess
brain penetration of the radiolabeled derivatives was
performed in a rhesus monkey that presumably had no
amyloid deposits in its brain. Thus, this experiment re-
flects brain entry and clearance from normal brain tis-
sue. The time–activity curves shown in Figure 4
indicate that [18F]FPPIP easily penetrates the blood–
brain barrier after intravenous injection, with peak value
(SUV) of 1.6–2.7 at 9 min. Relatively fast nonspecific
binding clearance was observed with the radioactivity
ratios of peak—to -105 min 2.6, 1.9, and 2.0 in cerebel-
lum, frontal cortex, and subcortical white matter devoid
of specific binding sites. No bone uptake of radioactivity
was observed in the skull after the intravenous adminis-
tration of [18F]FPPIP to the monkey, indicative of low
in vivo defluorination.
9. Zhuang, Z.-P.; Kung, M. P.; Hou, C.; Plossl, K.;
Skovronsky, D. M.; Gur, T.; Trojanowski, J. Q.; Lee, V.
M.; Kung, H. F. Nucl. Med. Biol. 2001, 28, 887.
10. Kung, M. P.; Hou, C.; Zhuang, Z.-P.; Zhang, B.;
Skovronsky, D. M.; Gur, T.; Lee, V. M.-Y.; Trojanowski,
J. Q.; Kung, H. F. Brain Res. 2002, 956, 202.
11. Zhuang, Z. P.; Kung, M-P.; Wilson, A.; lee, C. W.; Plossl,
K.; Hou, C.; Holtzman, D. M.; Kung, H. F. J. Med.
Chem. 2003, 46, 237.
12. Cai, L.; Chin, F. T.; Pike, V. W.; Toyama, H.; Liow, J.-S.;
Zoghbi, S. S.; Modell, K.; Briard, E.; Shetty, U. H., et al.
J. Med. Chem. 2004, 47, 2208.
13. Lee, C.-W.; Kung, M.-P.; Hou, C.; Kung, H. F. Nucl.
Med. Biol. 2003, 30, 573.
14. Zhang, W.; Oya, S.; Kung, M.-P.; Hou, C.; Maier, D. L.;
Kung, H. F. J. Med. Chem. 2005, 48, 5980.
15. Diwu, Z.; Beachdel, C.; Klaubert, D. H. Tetrahedron Lett.
1998, 39, 4987.
16. For the quantitative autoradiography, we modified the
binding protocol used by Kung et al. See: (a) Kung, M. P.;
Hou, C.; Zhuang, Z.-P.; Skovronsky, D.; Kung, H. F.
Brain Res. 2004, 1025, 98; and performed calibrated
In summary, two new Ab plaque ligands, FEPIP and
FPPIP, have been synthesized and biologically evaluat-
ed. [18F]FPPIP is a promising candidate as PET

3018
F. Zeng et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3015–3018
densitometry as described by Penney et al; (b) Penney, J. 17. Wilson, A. A.; Houle, S. J. Labelled Compd. Radiopharm.
B.; Pan, H. S. Jr.; Young, A. B.; Frey, K. A.; Dauth, G.
W. Science 1981, 214, 1036; using [I-125] calibrated
[C-14]standards. Miller, J. A.; Zahniser, N. R. Neurosci.
Lett. 1987, 81, 345; and Baskin, D. G.; Wimpy, T. H.
Neurosci. Lett. 1989, 104, 171.
1999, 42, 1277.
18. Dishino, D. D.; Welch, M. J.; Kilbourn, M. R.; Raichle,
M. E. J. Nucl. Med. 1983, 24, 1030.
19. Arai, T.; Akiyama, H.; Ikeda, K.; Kondo, H.; Mori, H.
Brain Res. 1999, 823, 202.