Shining dead bone—cause for cautious interpretation of [¹⁸F]NaF PET scans

<p><b>Background and purpose — [¹⁸F]Fluoride ([¹⁸F]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 [¹⁸F]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. [¹⁸F]NaF was injected intravenously after 7 days. 60 minutes later, the rats were killed and [¹⁸F]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 [¹⁸F]NaF uptake, on a par with the most actively growing and remodeling sites around the knee joint.</b></p> <p><b>Interpretation — [¹⁸F]NaF binds with high affinity to dead bone and calcium phosphate materials. Hence, an [¹⁸F]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>-[1-¹¹C]­propyl-3-piperidyl benzilate (PPB). In an effort to overcome the shortcomings with the technically cumbersome synthesis of [¹¹C]­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>-¹¹C-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 [¹¹C]­PBB. (<i>S</i>,<i>R</i>)-[¹¹C]<b>1</b> is a promising candidate for measuring changes in endogenous acetylcholine concentrations

Development of [<i>Carbonyl</i>-¹¹C]AZ13198083, a Novel Histamine Type‑3 Receptor Radioligand with Favorable Kinetics

The histamine subtype-3 receptor (H₃R) is implicated in a range of central nervous system disorders, and several radioligands have been developed for H₃R positron emission tomography imaging. However, a limitation of currently used PET radioligands for H₃R is the slow binding kinetics in high density brain regions. To address this, we herein report the development of three novel candidate H₃R radioligands, namely, [<i>carbonyl</i>-¹¹C]­AZ13153556 ([<i>carbonyl</i>-¹¹C]<b>4</b>), [<i>carbonyl</i>-¹¹C]­AZD5213­([<i>carbonyl</i>-¹¹C]<b>5</b>), and [<i>carbonyl</i>-¹¹C]­AZ13198083 ([<i>carbonyl</i>-¹¹C]<b>6</b>), and their subsequent preclinical evaluation in nonhuman primates (NHP). Radioligands [<i>carbonyl</i>-¹¹C]<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 ¹¹C-aminocarbonylation protocol. All three radioligands showed high brain permeability as well as a regional brain radioactivity distribution in accordance with H₃R expression (striatum > cortex > cerebellum). [<i>Carbonyl</i>-¹¹C]<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>-¹¹C]<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>-¹¹C]<b>6</b> is a promising candidate for H₃R imaging in human subjects

Development of [<i>Carbonyl</i>-¹¹C]AZ13198083, a Novel Histamine Type‑3 Receptor Radioligand with Favorable Kinetics

The histamine subtype-3 receptor (H₃R) is implicated in a range of central nervous system disorders, and several radioligands have been developed for H₃R positron emission tomography imaging. However, a limitation of currently used PET radioligands for H₃R is the slow binding kinetics in high density brain regions. To address this, we herein report the development of three novel candidate H₃R radioligands, namely, [<i>carbonyl</i>-¹¹C]­AZ13153556 ([<i>carbonyl</i>-¹¹C]<b>4</b>), [<i>carbonyl</i>-¹¹C]­AZD5213­([<i>carbonyl</i>-¹¹C]<b>5</b>), and [<i>carbonyl</i>-¹¹C]­AZ13198083 ([<i>carbonyl</i>-¹¹C]<b>6</b>), and their subsequent preclinical evaluation in nonhuman primates (NHP). Radioligands [<i>carbonyl</i>-¹¹C]<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 ¹¹C-aminocarbonylation protocol. All three radioligands showed high brain permeability as well as a regional brain radioactivity distribution in accordance with H₃R expression (striatum > cortex > cerebellum). [<i>Carbonyl</i>-¹¹C]<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>-¹¹C]<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>-¹¹C]<b>6</b> is a promising candidate for H₃R imaging in human subjects

Discovery and Preclinical Validation of [¹¹C]AZ13153556, a Novel Probe for the Histamine Type 3 Receptor

The histamine type 3 receptor (H₃) 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 H₃ 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 H₃ 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 [³H]<b>1</b> and [³H]<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 [¹¹C]<b>1</b> and [¹¹C]<b>5</b> was examined by PET imaging in mice and nonhuman primates. <i>B</i>_max values obtained from Scatchard analysis of [³H]<b>1</b> and [³H]<b>5</b> binding were in good agreement. Autoradiography with [³H]<b>5</b> on rat, guinea pig, and human brain slices showed specific binding in regions known to be enhanced in H₃ receptors, a high degree of colocalization with [³H]<b>1</b>, and virtually negligible nonspecific binding in tissue. PET measurements in mice and nonhuman primates demonstrated that [¹¹C]<b>5</b> binds specifically and reversibly to H₃ receptors in vivo with low nonspecific binding in brain tissue. Whereas [¹¹C]<b>1</b> showed similar binding characteristics in vivo, the binding kinetics appeared faster for [¹¹C]<b>5</b> than for [¹¹C]<b>1</b>. Conclusions: [¹¹C]<b>5</b> has suitable properties for quantification of H₃ receptors in nonhuman primate brain and has the potential to offer improved binding kinetics in man compared to [¹¹C]<b>1</b>