Highly Selective Dopamine D3 Receptor Antagonists with Arylated Diazaspiro Alkane Cores

Article abstract of DOI:10.1021/acs.jmedchem.7b01248

A series of potent and selective D₃ receptor (D₃R) analogues with diazaspiro alkane cores were synthesized. Radioligand binding of compounds 11, 14, 15a, and 15c revealed favorable D₃R affinity (K_i = 12-25.6 nM) and were highly selective for D₃R vs D₃R (ranging from 264- to 905-fold). Variation of these novel ligand architectures can be achieved using our previously reported 10-20 min benchtop C-N cross-coupling methodology, affording a broad range of arylated diazaspiro precursors.

Full text of DOI:10.1021/acs.jmedchem.7b01248

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Brief Article
3
Highly Selective Dopamine D Receptor
Antagonists with Arylated Diazaspiro Alkane Cores
Sean W. Reilly, Suzy Griffin, Michelle Taylor, Kristoffer Sahlholm, Chi-Chang
Weng, Kuiying Xu, Daniel A. Jacome, Robert R. Luedtke, and Robert H. Mach
J. Med. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jmedchem.7b01248 • Publication Date (Web): 10 Nov 2017
Downloaded from http://pubs.acs.org on November 12, 2017
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Highly Selective Dopamine D3 Receptor Antagonists with Arylated Dia-
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Sean W. Reilly,a Suzy Griffin,c Michelle Taylor,c Kristoffer Sahlholm,a Chi-Chang Weng,a Kuiying
Xu,a Daniel A. Jacome,b Robert R. Luedtke,c Robert H. Mach*,a
aDepartment of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania
19104, USA
bDepartment of Systems Pharmacology and Translational Therapeutics, University of Pennsylvania, 421 Curie Blvd.,
Philadelphia, PA 19104, USA
cUniversity of North Texas Health Science Center, the Department of Pharmacology and Neuroscience, 3500 Camp
Bowie Boulevard, Fort Worth, TX 76107, USA
KEYWORDS (Word Style “BG_Keywords”). If you are submitting your paper to a journal that requires keywords, pro-
vide significant keywords to aid the reader in literature retrieval.
ABSTRACT: A series of potent and selective D3 receptor (D3R) analogues with diazaspiro alkane cores were synthesized.
Radioligand binding of compounds 11, 14, 15a, and 15c revealed favorable D3R affinity (Ki = 12-25.6 nM) and were highly
selective for D3R vs D3R (ranging from 264-905-fold). Variation of these novel ligand architectures can be achieved using
our previously reported 10-20 minute benchtop C–N cross-coupling methodology, affording a broad range of arylated dia-
zaspiro pre-cursors.
INTRODUCTION
Dopamine is a crucial neurotransmitter that acts by
stimulating G-protein coupled receptors responsible for
many neurological processes such as emotion, reward, and
motivation.1 These receptors are classified into two sub-
types, D1-like and D2-like, based on sequence identity and
similarity in signal transduction. The D2-like dopamine D3
receptor (D3R) is a protein of interest for various neurolog-
ical and neuropsychiatric disorders including schizophre-
nia, Parkinson’s disease, dementia, anxiety and depres-
sion.2-5 The high appeal of the D3R stems from the high
density of its expression within the mesolimbic pathway of
the CNS.6-9 This region of the brain is responsible for the
reward and motivational mechanisms associated with drug
addition, thus, making the D3R an attractive target for
pharmacological therapy in substance abuse disorders
Figure 1. Chemical structure and binding data of D3R selective
(SUD).10-13 However, the similarities shared between the
antagonists as reported in the literature.
D3R and D2R, such as an overall ~46% amino acid sequence
homology, a 78% sequence identity within the transmem-
brane-spanning segments,14 and the near-identical binding
site residues of these receptors,15 have made the develop-
ment of a FDA approved D3R selective therapeutic quite
challenging.
used computational modeling to illustrate the extensive in-
teractions between the amino component of the ligand and
the D3R orthosteric binding site (OBS).17 Their modeling
experiments indicated ligand binding efficacy is due to a
salt bridge formation between the Asp3.32 of the receptor
and the protonated nitrogen on the amino moiety. This
binding interaction was later confirmed in the D3R crystal
structure, with later reports correlating ligand affinity to
OBS interactions, and D3R selectivity to secondary binding
Many previously reported ligand architectures designed
to selectively target the D3R employ the classic arylpipera-
zine amino template to enhance ligand affinity to the re-
ceptor (1-3, Figure 1).2, 16 In 2010, Reichert and co-workers
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Page 2 of 7
pocket interactions.18 As a result, these molecular determi- route from a previous report (Scheme 2).30 Compound 7
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3
4
5
6
7
8
nants provide a challenging path for medicinal chemists to
develop potent and selective D3R ligand scaffolds with
amino moiety alternatives to the arylpiperazine pharmaco-
phore.19
was then reacted with the appropriate arylated diazaspiro
free amine intermediate (J) to form derivatives 8-16 illus-
trated in Figure 3. Target compounds can also be devel-
oped using the alternative synthetic route depicted in
Scheme 2, by reacting the appropriate alkylated intermedi-
ate L with K in the presence of trimethylamine in ethanol.
Over the past several years, reports have emerged with
1,2,4-triazole-based scaffolds with notable D3R vs. D2R effi-
cacy and selectiviy.1, 11, 20 GlaxoSmithKline (GSK) disclosed
several 1,2,4-triazole-based compounds with excellent D3R
affinity and selectivity profiles,21-23 most notably,
GSK598,809 (4), the first D3R selective antagonist to ex-
hibit clinical evidence as a potential therapeutic for sub-
stance abuse disorders.24 More recently, Micheli and co-
workers reported a D3R antagonist with an azaspiro alkane
moiety (5), which displays sub-nanomolar affinity and ex-
cellent D3R selectivity over the D2R.25 Encouraged by these
reports, we designed a new class of thiotriazole D3R selec-
tive scaffolds using commercially available diazaspiro al-
kanes A-H (Figure 2) as modified amino cores. Spiro
synthons provide spatial modifications that are otherwise
non-accessible to saturated six-membered rings such as pi-
perazine, thus, affording unique interactions in the D3R
binding pocket. Moreover, spirocyclic compounds exhibit
lower lipophilicity compared to their monocyclic counter-
parts, resulting in increased bioavailability.26, 27 Herein we
report the synthesis and in vitro studies of diazaspiro-con-
taining 1,2,4-triazole-3-thiol ligand systems.
Scheme 2a
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aReagents and conditions: (a) 1-bromo-3-chloropropane,
K2CO3, acetone, RT, 20 h; (b) Cs2CO3, ACN, 70 °C, 12 h; (c) 1-
bromo-3-chloropropane, K2CO3, acetone, RT, 20 h; (d) TEA,
EtOH, 75 °C, 12 h.
Figure 2. Diazaspiro compounds evaluated as amino cores.
RESULTS AND DISCUSSION
Initial synthesis of the target compounds began with ar-
ylation of amino cores A-H outlined in Scheme 1. Arylated
diazaspiro synthons (I) and 6 were afforded in high yields
in just 10-20 min following our previously reported one-pot
29
Pd C–N cross-coupling methodology.28,
This catalytic
protocol can be conducted under aerobic conditions, thus
eliminating the need for an inert atmosphere or anhydrous
solvents.
Scheme 1a
Figure 3. Chemical structures of synthesized target com-
pounds.
aReagents and conditions: (a) Pd2(dba)3, RuPhos, aryl halide,
A-H, NaOt-Bu, dioxane, 100 °C, 20 min; (b) TFA, DCM, RT, 3h.
Our initial radioligand binding studies revealed dia-
zaspirocycles A-B as poor amino core candidates in the
1,2,4-triazole scaffold (Table 1, compounds 8-9). Attempts
S-Alkylation of K with 1-bromo-3-chloropropane af-
forded 7 in high yield (92%) using a modified synthetic
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to improve D3R efficacy with B were also unsuccessful with
and selectivity (91.0-fold) in comparison to lead compound
15a.
We also examined the D3R binding profile of compound
16, a piperazine congener of lead compound 15a. Although
compound 16 demonstrated a slightly higher affinity at the
D3R (Ki = 6.5 nM), a significant decrease in D3R/D2R selec-
tivity was observed (~40-fold) in contrast to 15a. This direct
comparison illustrates the potential of spiro system H to
act as a viable alternative to the classical arylpiperazine
pharmacophore in D3R ligand frameworks.
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3
4
5
6
7
8
ortho-substituted fluorobenzene, resulting in diminished
D3R affinity of 9b. Binding activity and selectivity improved
with 10, containing slightly bulkier core C, when compared
to A and B. When investigating D in compound 11, we ob-
served moderate affinity (Ki = 24.2 nM) and good selectivity
(D3R/D3R = 264) at the D3R. Replacing D with amino cores
E and F resulted in lower D3R potency and selectivity com-
pared to 11. Compounds 13 and 14, however, displayed more
promising binding profiles than 8-9, scaffolds also with
azetidine containing diazaspiro cores (A-C).
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Compounds 14, 15a, 15c, and 16 were then evaluated for
human serotonin 5-HT1A receptor (5-HT1AR) binding affin-
ity (Figure 4), a common off-target binding receptor with
piperazine containing D3R scaffolds.31-35 As suspected, we
observed low binding profiles at the 5-HT1A receptor with
selected spiro compounds 14 (Ki = 724 nM), 15a (Ki = 931
nM), and 15c (Ki = 587 nM). It should also be noted, we
briefly furthered the profiles for compounds 14, 15a, and
15c at the 5-HT2A and 5-HT2c receptors as well, and ob-
served low binding affinities comparable to those obtained
at the 5-HT1AR (data not shown). Piperazine analogue 16,
however, expressed a 0.22 nM binding affinity to the 5-
HT1AR, in contrast to the receptor affinity observed with
lead compound 15a. In regards to our current research fo-
cus in PET probes development, eliminating off-target af-
finities of D2-like radioligands, such as for serotonin recep-
tors, is pivotal in developing a radiotracer that can image
the D3R in clinical settings.
When investigating bulkier diazaspiro cores G-H as po-
tential amino surrogates with the selected 1,2,4-triazole
system, a noticeable increase in D3R potency and selectiv-
ity was observed. For example, compounds 14 and 15a ex-
hibited good binding D3R affinity (Ki = 19.6 nM and 12.0
nM, respectively), with 15a demonstrating over 900-fold
selectivity. Introduction of the -OCH3 functional group in
para position (15b) led to a reduction in receptor affinity
(Ki = 97.7 nM), and a slight increase in D3R selectivity. Re-
placing the aryl group with a para-substituted fluoroben-
zene on the H amino moiety (compound 15c) resulted in
moderate D3R binding affinity (Ki = 25.6 nM) and selectiv-
ity (383-fold selectivity) in contrast to 15a. The insertion of
a benzonitrile and pyridine moiety (15d and 15e, respec-
tively) led to significant regression in D3R affinity and se-
lectivity. Diazaspiro synthon 6 was evaluated as a potential
bioisostere for the classical dichlorphenylpiperazine dis-
played in many antipsychotics. However, the binding pro-
file for 15f revealed decreased D3R affinity (Ki = 82.4 nM)
Table 1. D3 and D2 Receptor Binding Affinities of Diazaspiro Analoguesa
Kib ± SEM (nM)
Compound
Ar
D3R/D2R Ratioe
cLog Pf
D3Rc
D2Rd
8
9a
2-OCH3
2-OCH3
4-F
833 ± 154
21,216 ± 4,636
114,945 ± 31,960
>109,574 ± NA
60,152 ± 13,327
6,370 ± 1,145
37,579 ± 1,806
29,300 ± 3,139
6,168 ± 939
25.5
24.0
>6.0
313
3.95
3.85
4.23
3.90
4.16
3.76
3.76
4.32
4.88
4.88
5.32
4.05
4.83
6.54
4.09
4,790 ± 280
18,1888 ± 2,284
192 ± 2.3
9b
10
11(rac)g
2-OCH3
2-OCH3
2-OCH3
2-OCH3
2-OCH3
2-OCH3
4-OCH3
4-F
24.2 ± 5.6
235 ± 41.1
264
160
12
13
169 ± 20.2
19.6 ± 4.7
12.0 ± 2.8
173
14
315
15a
15b
15c
15d
15e
15f
10,895 ± 2,069
104,847 ± 29,076
9,792 ± 1,790
83,671 ± 62,789
57,101 ± 29,507
7,501 ± 12.9
905
1,073
383
96.1
130
97.7 ± 17.4
25.6 ± 5.6
871 ± 66.9
438 ± 33.6
82.4 ± 1,053
6.5 ± 0.88
pyridine-2-yl
3-CN
2,3-Cl
91.0
40.2
16
2-OCH3
260 ± 44.2
aSpiperone assayed under the same conditions as a reference blocker D2R 0.06 (nM) ± 0.001; D3R 0.33 (nM) ± 0.02; D4R 0.45 (nM)
b
c
± 0.01. Mean ± SEM, Ki values were determined by at least three experiments. Ki values for D3 receptors were measured using
d
human D3 expressed in HEK cells with [125I]ABN as the radioligand. Ki values for D2 receptors were measured using human D3
e
f
expressed in HEK cells with [125I]ABN as the radioligand. Ki for D3 receptors/Ki for D2 receptors. Calculated using ChemDraw
Professional 15.1. grac = racemate.
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D, G, and H alleviates serotonin binding, a common off-
1
2
3
4
5
6
7
8
target effect of piperazine containing D3R ligands, illus-
trated by 16. Access to arylated diazaspiro synthons can be
readily achieved in just 10-20 minutes using our previously
reported C–N cross-coupling conditions, affording a con-
venient synthetic route to these novel D3R scaffolds. Inves-
tigation is currently ongoing to further improve D3R bind-
ing profiles of these compounds and will be reported in due
course.
C o m p o u n d [M ]
14 [7.24 x 10 -7
]
15a [9.31 x 10 -7
15c [5.87 x 10 -7
]
]
16 [2.22 x 10 -1 0
]
1 .0
0 .5
0 .0
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EXPERIMENTAL SECTION
General. Chemical compounds A-H and K were purchased
and used without further purification. NMR spectra were
taken on a Bruker DMX 500 MHz. Compound structures and
1
identity were confirmed by H and 13C NMR, and mass spec-
-1 2 -1 1 -1 0
-9
-8
-7
-6
-5
-4
L o g [C o m p o u n d ] (M )
troscopy. Compound purity greater than 95% was determined
by LCMS analysis using a 2695 Alliance LCMS. All other com-
mercial reagents were purchased and used without further pu-
rification. Purification of organic compounds were carried out
on a Biotage Isolera One with a dual-wavelength UV-VIS de-
tector. Chemical shifts (δ) in the NMR spectra (1H and 13C)
were referenced by assigning the residual solvent peaks.
Synthesis for Compound 7. 3-((3-chloropropyl)thio)-4-me-
thyl-5-phenyl-4H-1,2,4-triazole (7). A mixture of K (2.00 g,
10.46 mmol), 1-bromo-3-chloropropane (10.34 mL, 104.60
mmol), and K2CO3 (2.17 g, 15.00 mmol) were stirred in acetone
(30.00 mL) at room temperature for 20 h. The crude reaction
mixture was then filtered and solvent was removed under re-
duced pressure. The residue was purified by flash chromatog-
raphy on silica gel eluding with hexane/EtOAc (5:2) affording
Figure 4. Competition curve of 14, 15a, 15c, and 16 at the 5-
HT1AR. [3H]8-OH-DPAT used as competitive substrate in
binding assay. Data shown are determined by at least three
experiments.
Finally, we evaluated compounds 11, 14, 15a, and 15c for
functional activity using adenylyl cyclase and β-arrestin re-
cruitment assays (Figure 5). With the exception of 15a act-
ing as a partial agonist for adenylyl cyclase, compounds 11,
14, and 15c had low activity in these assays, thus, consistent
with their being antagonists at D3R.
A d e n ylyl C ycla se
1
2.59 g, 92% yield (white solid). H NMR (500 MHz, CDCl3) δ
 -A rre stin
7.55-7.53 (m, 2H), 7.41-7.39 (m, 3H), 3.63 (t, J = 6.1 Hz, 2H), 3.52
(s, 3H), 3.32 (t, J = 6.8 Hz, 2H), (quint, J = 6.4, 2H); 13C NMR
(125 MHz, CDCl3) δ 155.9, 151.2, 130.0, 128.8, 128.5, 127.0, 53.4,
43.1, 31.9, 31.5, 29.9; LC-MS (ESI) m/z: 268.05 [M+H].
1 0 0
5 0
0
General Method for Preparing Diazaspiro Analogues 8-
16. The appropriate arylated diazaspiro and piperazine com-
pounds (I), obtained following our previous report,28, 29 were
dissolved in CH2Cl2 (2 mL), followed by dropwise addition of
CF3COOH (2 mL), and stirred at room temperature for 3 h.
Volatiles were then removed under reduced pressure and the
crude product was neutralized with a saturated NaHCo3 (aq) so-
lution (10 mL). The reaction mixture was extracted with
CH2Cl2 (3 x 20 mL), and the organic layers were combined,
dried, and concentrated to afford the free-amine intermedi-
ates (J) that were used as such in the following steps.
An equimolar mixture of the appropriate intermediate J
(0.50 mmol), 7 (0.50 mmol), and Cs2CO3 (1.0 mmol) was
stirred in acetonitrile (5 mL) at 70 °C for 12 h. The crude reac-
tion mixture was then filtered and solvent was removed under
reduced pressure. The residue was loaded onto a Biotage
SNAP flash purification cartridge and eluded with 10% 7N NH3
in MeOH solution/CH2Cl2 to give the target compounds 8-16.
Alternative Method for Preparing Diazaspiro Ana-
logues 8-16. A mixture of J (1 mmol), 1-bromo-3-chloropro-
pane (2 mmol), and K2CO3 (1.5 mmol) was stirred in acetone
(5 mL) at room temperature for 20 h. The crude reaction mix-
ture was then filtered and solvent was removed under reduced
pressure. The residue was loaded onto a Biotage SNAP flash
purification cartridge and eluded with 5% MeOH in CH2Cl2
affording intermediates L.
-5 0
l
e
1
4
a
c
l
o
1
1
5
5
o
d
1
1
r
i
i
r
p
e
n
p
i
o
u
l
a
Q
H
C o m p o u n d
Figure 5. The efficacy of compounds to inhibit forskolin-de-
pendent stimulation of adenylyl cyclase and β-arrestin bind-
ing is shown. Each test compound was used at a concentration
equal to 10x the Ki value. The bar graph represents the mean
percent efficacy ± SEM relative to the full agonist Quinpirole.
Mean ± SEM values were determined by at least three experi-
ments. Haloperidol was included as a prototypical antagonist.
CONCLUSION
We have identified several ligand architectures contain-
ing modified amino cores that exhibit excellent selectivity
and good affinity at the D3R. In addition, diazaspiro cores
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A mixture of K (1 mmol), TEA (1.5 mmol), and ethanol (10
capable of hydrolyzing substrate and generating a chemilumi-
nescent signal. Following the manufacturer’s protocol, the D3
dopamine receptor expressing U2OS cells are seeded at a con-
centration of 10,000 cells per well, in white, 96-well, clear-bot-
tomed plates that are provided with the kit. After a 48 hour
incubation (37°C, 5% CO2), test compound or control com-
pounds (quinpirole included as a prototypical full agonist and
haloperidol as a prototypical antagonist) are added (at a dose
of 10x the Ki value) to the appropriate wells and incubated for
90 minutes (37°C, 5% CO2). Kit substrate buffer is added
(room temperature, 60 min in the dark) to each well and the
luminescence is determined using an EnSpire Alpha 2390 mul-
tilabel plate reader (Perkin Elmer).
1
2
3
4
5
6
7
8
mL) was stirred at 75 °C for 15 min. The appropriate interme-
diate L (1 mmol) was then added, and the solution was stirred
at 75 °C for 12 h. Solvent from the crude reaction mixture was
then removed under reduced pressure. The residue was
loaded onto a Biotage SNAP flash purification cartridge and
eluded with 10% 7N NH3 in MeOH solution/CH2Cl2 to give the
target compounds 8-16.
Compounds 8-16 were taken up with CH2Cl2 followed by
dropwise addition of a 2.0M HCl solution in diethyl ether. Af-
ter stirring at rt for 1 h, the solvent was removed under re-
duced pressure to afford the desire compound as a hydrochlo-
ride salt for in vitro studies.
9
10
11
12
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14
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Receptor Binding Assays. The binding properties of
membrane-associated receptors were characterized by a filtra-
tion binding assay.36 For human D2R (long isoform) and D3R
expressed in HEK 293 cells, membrane homogenates were sus-
pended in 50 mM Tris-HCl/150 mM NaCl/ 10 mM EDTA
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications. Molecular formula strings (CSV), along
°
buffer, pH 7.5 and incubated with [125I]IABN36 at 37 C for 60
1
with H and 13C NMR spectra, and mass spectral data of iso-
min, using 20 μM (+)-butaclamol to define the nonspecific
binding. Human 5-HT1AR binding was assessed using mem-
branes from heterologously expressing CHO-K1 cells (Perki-
nElmer, Waltham, MA), suspended in buffer containing 50
mM Tris-HCl, 10 mM MgSO4, 0.5 mM EDTA, and 0.1% (w/v)
ascorbic acid, pH 7.4 and incubated with [3H]-8-OH-DPAT
lated compounds 6, and 8-16 (PDF).
AUTHOR INFORMATION
Corresponding Author
*Phone (215) 746-8233; E-mail: rmach@pennmedicine.up-
enn.edu.
°
(PerkinElmer) at 27 C for 60 min. Nonspecific binding was
determined in the presence of 10 µM metergoline (Tocris Bio-
science, Bristol, UK).
Author Contributions
The radioligand concentration was equal to approximately
0.5 (D3/2R) or 1.5-2 (5-HT1AR) times the Kd value, and the con-
centration of the competitive inhibitor ranged over 5 orders of
magnitude for competition experiments. For each competi-
tion curve, two concentrations of inhibitor per decade were
used, and triplicates were performed. Binding was terminated
by the addition of ice cold wash buffer (D2/3R; 10 mM Tris-HCl,
150 mM NaCl, pH 7.5, 5-HT1AR; 10 mM TrisHCl, pH 7.4) and
filtration over a glass-fiber filter (D3/2R; Schleicher and Schuell
No. 32, 5-HT1AR; Whatman grade 934-AH, GE Healthcare Bio-
Sciences, Pittsburgh, PA). Packard Cobra (D3/2R) or Perki-
nElmer MicroBeta2 (5-HT1AR) scintillation counters were used
to measure the radioactivity. The equilibrium dissociation
constant and maximum number of binding sites were gener-
ated using unweighted nonlinear regression analysis of data
modeled according to the equation describing mass R-bind-
ing. The concentration of inhibitor that inhibits 50% of the
specific binding of the radioligand (IC50 value) was deter-
mined by using nonlinear regression analysis to analyze the
data of competitive inhibition experiments. Competition
curves were modeled for a single site, and the IC50 values were
converted to equilibrium dissociation constants (Ki values) us-
ing the Cheng and Prusoff37 correction. Mean Ki values ± SEM
are reported for at least three independent experiments.
β-Arrestin Assay. The PathHunter eXpress human D3 do-
pamine receptor-expressing human bone osteosarcoma epi-
thelial cell line-based (U2OS cell line) β-Arrestin GPCR Assay
kit (DiscoverX) was used to determine the efficacy of test com-
pounds for β-arrestin-2 binding. The PathHunter® β-Arrestin
D3 receptor cell line was genetically engineered to co-express
a ProLink™ (PK) tagged receptor and the Enzyme Acceptor
(EA) tagged β-Arrestin. Activation of the Dopamine D3 recep-
tor-PK chimeric protein induces β-Arrestin-EA binding, lead-
ing to complementation of two β-galactosidase enzyme frag-
ments (EA and PK), resulting in a functional enzyme that is
The manuscript was written through contributions of all au-
thors. / All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
National Institute on Drug Abuse [(R01 DA29840-07 to
R.H.M.), (R01 DA23957-06 to R. R. Luedtke, University of
North Texas Health Science Center-Fort Worth)] is gratefully
acknowledged for financial support. SWR is supported by
training grant 5T32DA028874-07.
ABBREVIATIONS
ACN, acetonitrile; AMP, adenosine monophosphate; CNS,
central nervous system; DCM, dichloromethane; D2R, dopa-
mine D2 receptor; D3R, dopamine D3 receptor; HEK cells, hu-
man embryonic kidney 293 cells; [125I]IABN, [125I]-N-benzyl-5-
iodo-2,3-dimethoxy[3.3.1]azabicyclononan-3-β-ylbenzamide;
Pd, palladium; PET, positron emission tomography; TFA, tri-
fluoroacetic acid.
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