Facile synthesis of SSR180575 and discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[18F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide, a potent pyridazinoindole ligand for PET imaging of TSPO in cancer

Article abstract of DOI:10.1016/j.bmcl.2014.07.091

A novel synthesis of the translocator protein (TSPO) ligand 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (SSR180575, 3) was achieved in four steps from commercially available starting materials. Focused structure-activity relationship development about the pyridazinoindole ring at the N3 position led to the discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (14), a novel ligand of comparable affinity. Radiolabeling with fluorine-18 (¹⁸F) yielded 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[¹⁸F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide ([¹⁸F]-14) in high radiochemical yield and specific activity. In vivo studies of [¹⁸F]-14 revealed this agent as a promising probe for molecular imaging of glioma.

Full text of DOI:10.1016/j.bmcl.2014.07.091

Bioorganic & Medicinal Chemistry Letters 24 (2014) 4466–4471
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Facile synthesis of SSR180575 and discovery of 7-chloro-N,N,5-
1
8
trimethyl-4-oxo-3(6-[ F]fluoropyridin-2-yl)-3,5-dihydro-4H-
pyridazino[4,5-b]indole-1-acetamide, a potent pyridazinoindole
ligand for PET imaging of TSPO in cancer
Yiu-Yin Cheung , Michael L. Nickels a,b, Dewei Tang a,b, Jason R. Buck a,b, H. Charles Manning a,b,c,d,e,f,
a
⇑
a
Vanderbilt University Institute of Imaging Science (VUIIS), Vanderbilt University Medical Center, Nashville, TN 37232, United States
Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, TN 37232, United States
Program in Chemical and Physical Biology, Vanderbilt University Medical Center, Nashville, TN 37232, United States
Vanderbilt-Ingram Cancer Center (VICC), Vanderbilt University Medical Center, Nashville, TN 37232, United States
Department of Biomedical Engineering, Vanderbilt University, Nashville, TN 37232, United States
Department of Neurosurgery, Vanderbilt University Medical Center, Nashville, TN 37232, United States
b
c
d
e
f
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel synthesis of the translocator protein (TSPO) ligand 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,
-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (SSR180575, 3) was achieved in four steps from
commercially available starting materials. Focused structure–activity relationship development about the
Received 28 May 2014
Revised 29 July 2014
Accepted 31 July 2014
Available online 8 August 2014
5
pyridazinoindole ring at the N3 position led to the discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-fluoro-
pyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (14),
a novel ligand of comparable
affinity. Radiolabeling with fluorine-18 (1 F) yielded 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[ F]fluoropyri-
8
18
Keywords:
TSPO
Glioma
PET
Fluorine-18
SSR180575
18
din-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide ([ F]-14) in high radiochemical yield
and specific activity. In vivo studies of [1 F]-14 revealed this agent as a promising probe for molecular
8
imaging of glioma.
Ó 2014 Elsevier Ltd. All rights reserved.
5
–7
There exists a continuing demand for the development and val-
idation of improved imaging biomarkers of cancer. Such biomark-
ers may aid in cancer diagnosis, predict clinical outcome, and
quantify response to therapeutic intervention. Imaging biomarkers
of glioma are limited and routinely include magnetic resonance
imaging (MRI), computed tomography (CT), and less frequently,
positron emission tomography (PET). Of these, PET may be the
most suitable given its exquisite sensitivity, quantitative nature,
and ability to characterize the molecular basis of the tumor. For
amino acid transport,
they can be hampered by nonspecific
6
accumulation. Thus, there remains a need to develop and validate
additional PET imaging biomarkers suitable for glioma detection
and characterization.
We have explored translocator protein (TSPO) expression as a
target for molecular imaging of cancer with PET.
the peripheral benzodiazepine receptor (PBR), is an 18 kDa protein
that takes part in a wide breadth of cellular processes, including
cell proliferation, apoptosis, steroid biosynthesis, and cholesterol
metabolism. Though TSPO has historically been leveraged as a
PET biomarker in neuroinflammation,
8
–13
TSPO, formerly
most oncology studies, 2-deoxy-2-[1 F]fluoroglucose ([ F]FDG) is
8
18
14
1
5–17
commonly utilized, as this tracer reflects glucose metabolism,
its elevated expression
1
18
which is usually elevated in cancer cells. However, relatively high
has also been reported in many types of cancer. This overexpres-
sion in tumors has been reported to correlate with disease progres-
sion and diminished survival and can be indicative of aggressive
glucose uptake in normal brain results in poor tumor-to-back-
ground ratios that can confound glioma detection. Moreover,
1
8
18
[
F]FDG uptake can be affected by a myriad of tangential meta-
and potentially metastatic tumors. Within this context, our labo-
bolic processes. This has led many groups to evaluate other PET
tracers of metabolism in this setting, including amino acid–based
ratory has investigated the use of TSPO PET ligands to image colon
8
9
12,13,19
cancer, breast cancer, and glioma,
as these agents could
PET probes such as [ C]MET,2
1
1
[
18
F]FET,3 and [ F]FDOPA.4 While
18
potentially serve as useful cancer imaging biomarkers. Moreover,
on a mechanistic level, we have utilized TSPO molecular imaging
probes to visualize colon cancers arising in genetically engineered
these probes can accumulate in tissues that exhibit enhanced
8
⇑
Corresponding author.
mice with aberrant Tgfb signaling, as well as tumors arising in
http://dx.doi.org/10.1016/j.bmcl.2014.07.091
960-894X/Ó 2014 Elsevier Ltd. All rights reserved.
0

Y.-Y. Cheung et al. / Bioorg. Med. Chem. Lett. 24 (2014) 4466–4471
4467
mice following loss of Apc function.20 These studies suggest the
potential importance TSPO PET agents to detect key molecular
events in oncology and to possibly serve as companion diagnostics
alongside targeted inhibitors of these pathways.
targeted four areas on the scaffold, the acetamide functionality at
C1 and the N3, N5, and C7 positions of the tricyclic pyridazinoin-
3
1,32
dole ring (Fig. 1).
The synthesis developed and reported herein
requires only four steps from a commercially available starting
material (Scheme 1). A small library of pyridazinoindoles (Table 1)
was assembled, with the point of divergence at the final condensa-
tion step with strategic aryl-hydrazines (8) (Scheme 1). The design
of the library focused primarily on incorporation of fluorine onto
an aromatic ring and potential substituents that would facilitate
We recently reported the first utilizations of two TSPO-specific
PET ligands for quantitative assessment of TSPO expression in pre-
18
clinical glioma, the aryloxyanilide N-[ F]fluoroacetyl-N-(2,5-
18
18
11
dimethoxybenzyl)-2-phenoxyaniline ([ F]PBR06, [ F]-1), and
the pyrazolopyrimidine N,N-diethyl-2-(2-(4-(2-[18F]fluoroeth-
oxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetam-
1
8
radiolabeling with F.
1
8
18
12
ide ([ F]DPA-714, [ F]-2a) (Fig. 1). Through focused library
The overall synthetic methodology is presented in Scheme 1.
Starting from a commercially available indole (4), deprotonation
with sodium hydride in DMF, followed by treatment with methyl
iodide, gave the desired N-methylated intermediate (5) (85%).
synthesis and structure–activity relationship (SAR) development
1
8
of the 5,6,7-substituted pyrazolopyrimidine scaffold of [ F]-2a,
we developed 2-(5,7-diethyl-2-(4-(2-[18F]fluoroethoxy)phenyl)-
1
8
pyrazolo[1,5-a]pyrimidin-3-yl)-N,N–diethylacetamide
[
([ F]-2b,
4
Acylation of (5) at C3 was achieved using TiCl and methyl
1
8
F]VUIIS1008), a novel and highly potent TSPO PET ligand exhib-
iting a 36-fold enhancement in affinity compared to [ F]-2a and
chloro-3-oxopropanoate in dichloroethane at 40 °C for 15 h, which
gave keto diester 6 (46%). The necessary N,N-dimethylamide moi-
ety at C1 (7) was achieved through displacement of the methoxy
group of (6) with dimethylamine in toluene and THF in a sealed
tube at 110 °C for 15 h. Final condensation with phenylhydrazine
(8, R = phenyl) gave the lead compound, SSR1805875 (3), in 32%
yield. To date, this stands as the shortest reported synthesis of this
particular TSPO ligand.
18
1
3
accessible in high radiochemical yield and specific activity.
Subsequent in vivo studies of [18F]-2b demonstrated this agent to
possess several properties for molecular imaging of TSPO-express-
1
3,21
ing cancers.
In addition to the aryloxyanilides and pyrazolopyrimidines,
pyridazinoindoles are another series of potent TSPO ligands, as rep-
resented by 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihy-
Diversification at the N3 position of 3 was achieved through
condensation of key intermediate 7 with a series of monosubsti-
tuted aryl hydrazines (8) (31–50%). The hydrazines chosen focused
upon variation of the endogenous N3-phenyl ring, with particular
attention towards preliminary SAR and functional groups amena-
ble to PET ligand development, namely the presence of a fluorine
2
2,23
dro-4H-pyridazino[4,5-b]indole-1-acetamide (SSR180575, 3).
Developed originally by Sanofi-Aventis, the specificity and high-
2
4,25
affinity of 3 for TSPO has prompted research in cardiovascular
26
and renal pathologies, as well as neurodegenerative indica-
tions,2 inflammatory disorders,
3
22,27–29
and HIV pathogenesis.
30
The goal of this study was to determine whether optimization of
, specifically at the N3 position, would yield TSPO ligands with
atom and groups that would facilitate radiolabeling with 18
F
through an ipso-type substitution. The groups explored included:
a substituted phenyl ring (2-, 3-, 4-positions) (9–12); a 2-pyridyl
ring (13); a substituted 2-pyridyl ring (14–16).
3
comparable affinity and potential to serve as PET imaging ligands.
These experiments led to the synthesis of 3 in only four steps and
the subsequent development of 7-chloro-N,N,5-trimethyl-4-oxo-
To evaluate binding to TSPO, radiometric binding assays were
carried out with the compounds in a variety of athymic nude rat cell
lysates (C6, heart, kidney) using N-(sec-butyl)-1-(2-chlorophenyl)-
3
1
(6-fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-
-acetamide (14), a novel pyridazinoindole TSPO ligand exhibiting
3
3
11,12
binding comparable to 3 and structural features suitable for radio-
labeling with fluorine-18 (18F). Radiofluorination of either the 2-
chloro (15) or 2-bromo (16) precursor gave 7-chloro-N,N,5-tri-
N-methyl-[ H]-isoquinoline-3-carboxamide
([ H]PK11195).
For central benzodiazepine receptor (CBR) binding, radioligand dis-
3
placement was carried out against [ H]flunitrazepam in healthy, rat
1
8
methyl-4-oxo-3(6-[ F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyri-
i
brain lysate. Affinities are expressed as K ± S.E.M. (pM) in Table 1.
All experiments were performed in triplicate.
Of the eight analogs synthesized, seven proved extremely
i
potent in all three lysates, with K values comparable to those of
the parent SSR180575 (3): in C6 lysate, 0.280–2.34 pM versus
1.23 pM for 3; in heart 0.180–3.21 pM versus 0.762 pM for 3; in
kidney, 0.212–2.21 pM versus 0.596 pM for 3. Only one analog,
compound 13, bearing a 2-pyridyl ring at N3, did not appreciably
bind TSPO in any of the lysates. Interestingly, a portion of the com-
pounds synthesized showed evidence of mixed (low, high) binding
affinities. In C6 lysate, compounds 12 and 14 demonstrated high-
affinity binding at 0.671 and 1.19 pM, and low-affinity binding at
dazino[4,5-b]indole-1-acetamide ([18F]-14), which was subsequently
1
8
evaluated in vivo in a preclinical model of glioma (C6). [ F]-14
exhibited modest accumulation in normal brain, yet robust
accumulation in tumor tissue, which facilitated excellent imaging
1
8
contrast. [ F]-14 was fully displaceable by administration of non-
radioactive 14 halfway through the PET scan. Overall, these studies
1
8
illuminate [ F]-14 as a promising, novel PET ligand for evaluating
TSPO expression in gliomas and potentially other solid tumors
and diseases.
The original synthesis of SSR180575 (3) remains a patented pro-
cess at seven total steps.31 Moreover, the patented SAR studies of 3
Figure 1. Representative TSPO ligand scaffolds: Aryloxyanilide (1, PBR06); pyrazolopyrimidine (2a, DPA-714; 2b, VUIIS1008); pyridazinoindole (3, SSR180575).

4
468
Y.-Y. Cheung et al. / Bioorg. Med. Chem. Lett. 24 (2014) 4466–4471
Scheme 1. Synthetic methodology developed to generate SSR180575 (3) and a focused library of N3-substituted pyridazinoindoles (9–16).
Table 1
Affinity of pyridazinoindoles
Compd
R
C6 Glioma K b
i
(pM)
Heart K b
i
(pM)
Kidney K b
(pM)
i
3a
9
Ph
1.23 ± 0.08
0.422 ± 0.06
0.280 ± 0.07
0.889 ± 0.2
0.762 ± 0.15
0.591 ± 0.10
0.180 ± 0.02
1.18 ± 0.09
0.877 ± 0.03
4936 ± 662.3
3.21 ± 0.4
0.596 ± 0.04
1.18 ± 0.04
0.212 ± 0.02
0.678 ± 0.2
0.459 ± 0.03; 969 ± 752
6796 ± 830.9
2.21 ± 0.4
0.495 ± 0.03
0.965 ± 0.02
2-Fluorophenyl
3-Fluorophenyl
3-Nitrophenyl
4-Fluorophenyl
2-Pyridyl
3-Fluoro-2-pyridyl
3-Chloro-2-pyridyl
3-Bromo-2-pyridyl
10
11
12
13
14
15
16
0.671 ± 0.2; 3020 ± 1007d
2409 ± 795.4
d
c
1.19 ± 0.05; 1770 ± 232.6d
2.34 ± 0.4
1.48 ± 0.2
0.676 ± 0.08
0.281 ± 0.01
a
b
c
SSR18075.
S.E.M versus [ H]PK11195.
3
3
Ki versus [ H]flunitrazepam in rat brain lysate >10
Mixed affinity binding. All lysates procured from athymic nude rats. All experiments performed in triplicate.
lM.
d
1
770 and 3020 pM, respectively. Compound 12 demonstrated sim-
TSPO in C6 glioma (1 K
i
= 6170 pM;11 2a K
= 9730 pM;12 2b
i
1
3
ilar mixed binding in kidney lysate (0.459 and 969 pM), but not the
heart. While multiple PK11195 TSPO binding sites have been
reported and merit consideration,33 recent research has also impli-
cated a genetic polymorphism for such binding variations in the
K = 270 pM ). Moreover, in our lab, compound 14 represents a
i
1000-fold increase in potency over 2b.
Of the synthesized series, the 3-fluoro analog 10 was initially
considered the candidate PET tracer, particularly given its activity
and ready availability of its 3-nitro precursor (11). However, effec-
3
4,35
human brain.
However, genetic sequencing of the C6 cell line
1
8 À
from the specific clonal population used for this study confirmed
its wild-type status. The mixed binding we observed could poten-
tially be a result of overlapping points of interaction that the ana-
logs and PK11195 possess for TSPO.
tive substitution of a nitro group with
F
on a benzene ring
requires a highly deactivated ring system, preferably with elec-
tron-withdrawing groups either ortho or para to the nitro moiety.
Neither was the case for 11 as a precursor. Similar arguments could
also be made for consideration of 9 or 12 as PET tracer candidates.
Compound 14 was ultimately chosen as the candidate PET tracer,
with 15 and 16 as possible precursors. The selectivity of 14 for
TSPO over CBR was evaluated through displacement of [ H]fluni-
trazepam in rat cerebral cortex membranes (Table 1). The selectiv-
ity of 14 for TSPO over CBR was verified with [ H]flunitrazepam,
Overall, the modifications made to lead compound 3, with the
exception of 13, were well tolerated. While substitution of the
N3-phenyl ring with 2-pyridine did adversely affect binding across
all three lysates, comparable biological activity was regained with
halogen substitution at the 3-position of the ring (14–16). This
influence on potency could be attributed to an electronic effect of
the halogen atom and merits further consideration in future stud-
ies. Similarly, 2-, 3-, and 4-fluorosubstitution on the endogenous
phenyl ring with fluorine (9, 10, 12, respectively) proved to mini-
mally affect TSPO affinity, as did 4-nitro-substitution (11). This ser-
ies of compounds are among the most potent ligands reported for
3
3
which gave a K
i
>10 lM.
Isotopic labeling of 3 with both C and 18F has been previously
1
1
3
6
reported in the patent literature. In the research literature,
Thominiaux et al. reported
pyridazino[4,5-b]indole ([indole-N-methyl- C]SSR180575) and N,N-
1
1
C labeling at the 5-methyl-
1
1

Y.-Y. Cheung et al. / Bioorg. Med. Chem. Lett. 24 (2014) 4466–4471
4469
dimethylacetamide ([acetamide-N-methyl-11C]SSR180575)37 moie-
Adaptation of these reaction conditions to large-scale box pro-
1
8
ties. Within the context of radiochemical yield and purity, and
ductions enabled preclinical production of [ F]-14 in the GE
TRACERlab™ FXFÀN module with labeling conditions of 165 °C for
20 min. A lowering of the reaction temperature from 180 to
165 °C was deemed necessary due to the limitations of heating
reliability at such an extreme temperature using the TRACERlab™
1
1
in vivo pharmacological properties, [indole-N-methyl- C]-
SSR180575 was advanced as a candidate for imaging neuroinflam-
3
7
mation and afforded higher image contrast when compared to
11
the traditional isoquinoline [ C]PK11195 in a model of acute
neuroinflammation (rat).38 Moreover, competition studies
platform. Purification of [ F]-14 was carried out with preparative
18
11
38
18
demonstrated a high specific binding of [ C]SSR180575 for TSPO.
HPLC in 45% ethanol and 55% water. The retention time of [ F]-14
However, as far as the authors are aware, no studies to date have
been reported for the use of isotopically labeled SSR180575 for
PET imaging in oncology. Since our goal was to explore 3 as a poten-
tial radiopharmaceutical lead for translation into preclinical cancer
imaging studies, radiofluorination was deemed to be the most
effective means to achieve this goal. Though useful in the research set-
ting, the 20.4 min half-life of 11C limits broader utility of the tracer
was 23–28 min according to gamma detection and corresponded
to the UV retention time of nonradioactive 14. Radiochemical pur-
ity was consistently greater than 99% (n = 9), with decay-corrected
yields ranging from 9.3–19.3% and specific activities as high as
5559 Ci/mmol (206 TBq/mmol) (n = 9). However, the apparent spe-
cific activity was diminished using 42.5% ethanol and 57.5% water,
despite the longer retention time.
(
shipment to satellite locations, dynamic PET studies), thus making
The in vivo performance of [18F]-14 was evaluated in glioma-
bearing (C6), male Wistar rats using microPET imaging, with a
1
8
F (109.4 min half-life) an attractive alternative.
We initially applied microfluidic-radiolabeling approaches to
typical study shown in Figure 2. MRI (T
2
-weighted) was used to
elucidate stability and ascertain labeling feasibility of precursors
localize tumors and for registration of anatomical features with
Ò
11,12,18
18
1
5 and 16. Using a commercial microfluidic module (NanoTek )
PET (Fig. 2A).
Dynamic PET imaging with [ F]-14 illustrated
1
8
À
that enabled carefully controlled stoichiometry between [ F]
and precursor at set temperatures (°C) with controllable reaction
times ( L/min flow rates), we carried out multiple, small-scale,
that the majority of tracer accumulation in the brain was localized
to the tumor, with modest accumulation that exceeded plasma in
contralateral, non-tumor brain (Fig. 2B and C). The tumor-selective
l
1
8
sequential radiolabelings in a relatively short amount of time,
allowing rapid optimization of labeling conditions (Table 2).
The information gathered with the NanoTekÒ module was used
to better inform the transition to the GE TRACERlab™ FXFÀN
module for larger, preclinical-scale productions. Using cyclo-
characteristics of [ F]-14 afforded excellent imaging contrast
between tumor and contralateral tissue. Figure 2G illustrates
timeÀactivity curves (TACs) typical of representative studies for
tumor (blue), normal brain (green), and plasma (red) over a
90-min dynamic acquisition. After an initial spike in radioactivity
1
8
À
+
18
tron-generated
series of reaction temperatures (80–180 °C) at a transfer rate of
L/min was investigated, with product formation monitored
F
and K -K2.2.2/K
2
CO
3
in anhydrous DMSO, a
consistent with tracer injection and rapid distribution, [ F]-14
1
8
quickly cleared the plasma (red). We found that [ F]-14 accumu-
lation in the tumor (blue), relative to normal brain (green), reached
a tumor-to-normal brain ratio greater than 10:1. Preliminary
radio-TLC analysis of plasma samples (2, 12, 30, 60 min) taken
during the scan (n = 2) suggested minimal tracer metabolism over
the 60 min (% parent at 60 min >90%).
40 l
1
8
by radio-TLC for % F-incorporation (Table 2). For the chloro
precursor (15), product formation was observed at 180 °C
(
1.41%) at a transfer rate of 40
temperature of 180 °C and decreasing the transfer rate to
L/min gave modest incorporation (5.2%, Table 2, entry 3).
Similar results were obtained using precursor 16 (Table 2, entry
). Despite modest yields, the data generated from these exper-
lL/min. Maintaining a reaction
1
4
l
To validate the PET, imaging-matched brains were processed for
post-mortem staining and immunohistochemistry for TSPO. Ex vivo
histological analysis correlated well with PET imaging data, with
close agreement between tumor tissue (H&E, Fig. 2E), elevated
TSPO expression (immunohistochemistry, Fig. 2F), and tumor accu-
6
iments also highlighted the thermal stability (up to 180 °C) of
both 15 and 16, verified by HPLC of crude reaction mixtures.
Bromo-precursor 16 was ultimately chosen going forward given
its optimal chromatographic resolution from the final product
1
8
mulation of [ F]-14 (Fig. 2D).
To evaluate the in vivo TSPO specificity of [18F]-14, we per-
formed displacement studies in C6-bearing rats using the non-
radioactive analog (14). As shown in the TAC in Figure 2H, during
the dynamic PET study, a bolus (IV) infusion of nonradioactive 14
1
8
(
[ F]-14). These observations provided a confident basis to pur-
sue the radiolabeling reaction in a sealed system at 180 °C for a
longer period of time.
Table 2
Initial radiofluorination of precursors 15 and 16 using microfluidics
Run
Temp (°C)
Transfer rate (
lL)
RCY (%)
1
2
3
4
5
6
Cl
Cl
Cl
Br
Br
Br
140
160
180
140
160
180
40
40
14
40
40
60
0
0
5.2
0
1
7
⁄
Precursors 15 and 16 stable up to 180 °C.

4
470
Y.-Y. Cheung et al. / Bioorg. Med. Chem. Lett. 24 (2014) 4466–4471
Figure 2. Imaging of preclinical glioma using [18F]-14. (A) T
histology. (F) TSPO immunohistochemistry. (G) Typical dynamic PET TAC. (H) Displacement study PET TAC.
2
-weighted MRI (coronal). (B) Fused MRI-PET (coronal). (C) Dynamic PET (coronal). (D) Dynamic PET (axial). (E) H&E
(
10 mg/kg) was administered approximately 30 min after injection
the brain and neuroinflammation. Future head-to-head compari-
sons between this tracer and emerging, potent TSPO ligands
are underway, as well as exploration of other promising applica-
tions in oncology, neuroinflammation, and other diseases.
1
8
13,21
of [ F]-14. Summation of the first 30 min of the PET scan prior to
injection of cold 14 (0–30 min) demonstrated typical accumulation
characteristics of [ F]-14. However, summation of the final 30 min
1
8
of the PET scan (30–60 min) demonstrated significant displace-
1
8
Acknowledgment
ment of [ F]-14 from tumor tissue (blue). TAC analysis demon-
strated that after injection of 14, tumor binding was reduced to
approximately 40% of the peak tumor uptake. During tumor dis-
placement, we observed a minor influx of tracer into normal brain
We acknowledge funding from the National Institutes of Health
(
1
K25 CA127349, P50 CA128323, S10 RR17858, U24 CA126588,
R01 CA163806, P50 CA095103, P30 DK058404), The Kleberg
1
8
(
green). The displaced [ F]-14 then rapidly cleared the normal
brain and entered the plasma, subsequently elevating plasma
red) radioactivity. These studies suggest a level of displaceable
binding indicative of a high degree of specific binding and revers-
Foundation, and The Lustgarten Foundation. Dr. M. Noor Tantawy,
George H. Wilson, and Dr. Daniel Colvin are acknowledged for sup-
port of the imaging studies.
(
1
8
ibility of [ F]-14 to TSPO in tumor tissue.
Supplementary data
In this work, we report the shortest published synthesis of the
pyridazinoindole TSPO ligand SSR180575 (3) to date. Initial SAR
exploration at the N3 position of 3 provided a series of analogs of
comparable potency, one of which (14) could be adapted into a
Supplementary data (analytical results for all compounds;
methods; supplemental figures) associated with this article can
be found, in the online version, at http://dx.doi.org/10.1016/
j.bmcl.2014.07.091.
18
potential PET imaging tracer ([ F]-14). Subsequent preclinical
1
8
studies illuminated [ F]-14 as a promising, novel TSPO PET ligand
for imaging glioma. Compared to previous TSPO PET ligands in
11
39
References and notes
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