5 research outputs found
Shining dead bone—cause for cautious interpretation of [<sup>18</sup>F]NaF PET scans
<p><b>Background and purpose — [<sup>18</sup>F]Fluoride ([<sup>18</sup>F]NaF) PET scan is frequently used for estimation of bone healing rate and extent in cases of bone allografting and fracture healing. Some authors claim that [<sup>18</sup>F]NaF uptake is a measure of osteoblastic activity, calcium metabolism, or bone turnover. Based on the known affinity of fluoride to hydroxyapatite, we challenged this view.</b></p> <p><b>Methods — 10 male rats received crushed, frozen allogeneic cortical bone fragments in a pouch in the abdominal wall on the right side, and hydroxyapatite granules on left side. [<sup>18</sup>F]NaF was injected intravenously after 7 days. 60 minutes later, the rats were killed and [<sup>18</sup>F]NaF uptake was visualized in a PET/CT scanner. Specimens were retrieved for micro CT and histology.</b></p> <p><b>Results — MicroCT and histology showed no signs of new bone at the implant sites. Still, the implants showed a very high [<sup>18</sup>F]NaF uptake, on a par with the most actively growing and remodeling sites around the knee joint.</b></p> <p><b>Interpretation — [<sup>18</sup>F]NaF binds with high affinity to dead bone and calcium phosphate materials. Hence, an [<sup>18</sup>F]NaF PET/CT scan does not allow for sound conclusions about new bone ingrowth into bone allograft, healing activity in long bone shaft fractures with necrotic fragments, or remodeling around calcium phosphate coated prostheses</b></p
Discovery of a Novel Muscarinic Receptor PET Radioligand with Rapid Kinetics in the Monkey Brain
Positron emission
tomography (PET), together with a suitable radioligand,
is one of the more prominent methods for measuring changes in synaptic
neurotransmitter concentrations in vivo. The radioligand of choice
for such measurements on the cholinergic system is the muscarinic
receptor antagonist <i>N</i>-[1-<sup>11</sup>C]propyl-3-piperidyl
benzilate (PPB). In an effort to overcome the shortcomings with the
technically cumbersome synthesis of [<sup>11</sup>C]PPB, we designed
and synthesized four structurally related analogues of PPB, of which
(<i>S</i>,<i>R</i>)-1-methylpiperidin-3-yl)2-cyclopentyl-2-hydroxy-2-phenylacetate
(<b>1</b>) was found to bind muscarinic receptors with similar
affinity as PPB (3.5 vs 7.9 nM, respectively). (<i>S</i>,<i>R</i>)<b>-1</b> was radiolabeled via <i>N</i>-<sup>11</sup>C-methylation at high radiochemical purity
(>99%) and high specific radioactivity (>130 GBq/μmol).
In vitro
studies by autoradiography on human brain tissue and in vivo studies
by PET in nonhuman primates demonstrated excellent signal-to-noise
ratios and a kinetic profile in brain comparable to that of [<sup>11</sup>C]PBB. (<i>S</i>,<i>R</i>)-[<sup>11</sup>C]<b>1</b> is a promising candidate for measuring changes in
endogenous acetylcholine concentrations
Development of [<i>Carbonyl</i>-<sup>11</sup>C]AZ13198083, a Novel Histamine Type‑3 Receptor Radioligand with Favorable Kinetics
The
histamine subtype-3 receptor (H<sub>3</sub>R) is implicated
in a range of central nervous system disorders, and several radioligands
have been developed for H<sub>3</sub>R positron emission tomography
imaging. However, a limitation of currently used PET radioligands
for H<sub>3</sub>R is the slow binding kinetics in high density brain
regions. To address this, we herein report the development of three
novel candidate H<sub>3</sub>R radioligands, namely, [<i>carbonyl</i>-<sup>11</sup>C]AZ13153556 ([<i>carbonyl</i>-<sup>11</sup>C]<b>4</b>), [<i>carbonyl</i>-<sup>11</sup>C]AZD5213([<i>carbonyl</i>-<sup>11</sup>C]<b>5</b>), and [<i>carbonyl</i>-<sup>11</sup>C]AZ13198083 ([<i>carbonyl</i>-<sup>11</sup>C]<b>6</b>), and their subsequent preclinical evaluation in
nonhuman primates (NHP). Radioligands [<i>carbonyl</i>-<sup>11</sup>C]<b>4</b>–<b>6</b> were produced and
isolated in high radioactivity (>1000 MBq), radiochemical purity
(>99%),
and moderate molar activity (19–28 GBq/μmol at time of
injection) using a palladium-mediated <sup>11</sup>C-aminocarbonylation
protocol. All three radioligands showed high brain permeability as
well as a regional brain radioactivity distribution in accordance
with H<sub>3</sub>R expression (striatum > cortex > cerebellum).
[<i>Carbonyl</i>-<sup>11</sup>C]<b>6</b> displayed
the most
favorable in vivo kinetics and brain uptake, with an early peak in
the striatal time–activity curve followed by a progressive
washout from the brain. The specificity and on-target kinetics of
[<i>carbonyl</i>-<sup>11</sup>C]<b>6</b> were next
investigated in pretreatment and displacement studies. After pretreatment
or displacement with <b>5</b> (0.1 mg/kg), a uniformly low distribution
of radioactivity across the NHP brain was observed. Collectively,
this work demonstrates that [<i>carbonyl</i>-<sup>11</sup>C]<b>6</b> is a promising candidate for H<sub>3</sub>R imaging
in human subjects
Development of [<i>Carbonyl</i>-<sup>11</sup>C]AZ13198083, a Novel Histamine Type‑3 Receptor Radioligand with Favorable Kinetics
The
histamine subtype-3 receptor (H<sub>3</sub>R) is implicated
in a range of central nervous system disorders, and several radioligands
have been developed for H<sub>3</sub>R positron emission tomography
imaging. However, a limitation of currently used PET radioligands
for H<sub>3</sub>R is the slow binding kinetics in high density brain
regions. To address this, we herein report the development of three
novel candidate H<sub>3</sub>R radioligands, namely, [<i>carbonyl</i>-<sup>11</sup>C]AZ13153556 ([<i>carbonyl</i>-<sup>11</sup>C]<b>4</b>), [<i>carbonyl</i>-<sup>11</sup>C]AZD5213([<i>carbonyl</i>-<sup>11</sup>C]<b>5</b>), and [<i>carbonyl</i>-<sup>11</sup>C]AZ13198083 ([<i>carbonyl</i>-<sup>11</sup>C]<b>6</b>), and their subsequent preclinical evaluation in
nonhuman primates (NHP). Radioligands [<i>carbonyl</i>-<sup>11</sup>C]<b>4</b>–<b>6</b> were produced and
isolated in high radioactivity (>1000 MBq), radiochemical purity
(>99%),
and moderate molar activity (19–28 GBq/μmol at time of
injection) using a palladium-mediated <sup>11</sup>C-aminocarbonylation
protocol. All three radioligands showed high brain permeability as
well as a regional brain radioactivity distribution in accordance
with H<sub>3</sub>R expression (striatum > cortex > cerebellum).
[<i>Carbonyl</i>-<sup>11</sup>C]<b>6</b> displayed
the most
favorable in vivo kinetics and brain uptake, with an early peak in
the striatal time–activity curve followed by a progressive
washout from the brain. The specificity and on-target kinetics of
[<i>carbonyl</i>-<sup>11</sup>C]<b>6</b> were next
investigated in pretreatment and displacement studies. After pretreatment
or displacement with <b>5</b> (0.1 mg/kg), a uniformly low distribution
of radioactivity across the NHP brain was observed. Collectively,
this work demonstrates that [<i>carbonyl</i>-<sup>11</sup>C]<b>6</b> is a promising candidate for H<sub>3</sub>R imaging
in human subjects
Discovery and Preclinical Validation of [<sup>11</sup>C]AZ13153556, a Novel Probe for the Histamine Type 3 Receptor
The histamine type 3 receptor (H<sub>3</sub>) is a G protein-coupled
receptor implicated in several disorders of the central nervous system.
Herein, we describe the radiolabeling and preclinical evaluation of
a candidate radioligand for the H<sub>3</sub> receptor, 4-(1<i>S</i>,2<i>S</i>)-2-(4-cyclobutylpiperazine-1-carbonyl)cyclopropyl]-<i>N</i>-methyl-benzamide (<b>5</b>), and its comparison
with one of the frontrunner radioligands for H<sub>3</sub> imaging,
namely, GSK189254 (<b>1</b>). Compounds <b>1</b> and <b>5</b> were radiolabeled with tritium and carbon-11 for in vitro
and in vivo imaging experiments. The in vitro binding of [<sup>3</sup>H]<b>1</b> and [<sup>3</sup>H]<b>5</b> was examined by
(i) saturation binding to rat and nonhuman primate brain tissue homogenate
and (ii) in vitro autoradiography on tissue sections from rat, guinea
pig, and human brain. The in vivo binding of [<sup>11</sup>C]<b>1</b> and [<sup>11</sup>C]<b>5</b> was examined by PET imaging
in mice and nonhuman primates. <i>B</i><sub>max</sub> values
obtained from Scatchard analysis of [<sup>3</sup>H]<b>1</b> and
[<sup>3</sup>H]<b>5</b> binding were in good agreement. Autoradiography
with [<sup>3</sup>H]<b>5</b> on rat, guinea pig, and human brain
slices showed specific binding in regions known to be enhanced in
H<sub>3</sub> receptors, a high degree of colocalization with [<sup>3</sup>H]<b>1</b>, and virtually negligible nonspecific binding
in tissue. PET measurements in mice and nonhuman primates demonstrated
that [<sup>11</sup>C]<b>5</b> binds specifically and reversibly
to H<sub>3</sub> receptors in vivo with low nonspecific binding in
brain tissue. Whereas [<sup>11</sup>C]<b>1</b> showed similar
binding characteristics in vivo, the binding kinetics appeared faster
for [<sup>11</sup>C]<b>5</b> than for [<sup>11</sup>C]<b>1</b>. Conclusions: [<sup>11</sup>C]<b>5</b> has suitable
properties for quantification of H<sub>3</sub> receptors in nonhuman
primate brain and has the potential to offer improved binding kinetics
in man compared to [<sup>11</sup>C]<b>1</b>