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 Evaluation of Clinical Candidate AZD3759, a Potent, Oral Active, Central Nervous System-Penetrant, Epidermal Growth Factor Receptor Tyrosine Kinase Inhibitor

Recent reports suggest that an increasing number of patients with lung cancer, especially those with activating mutations of the epidermal growth factor receptor (EGFR), also present with brain metastases and leptomeningeal metastases. These patients have poor prognosis as there are no approved drugs for these indications. Available agents have poor efficacy for these patients even at well above their standard dose. Herein, we report the discovery of (4-[(3-chloro-2-fluorophenyl)­amino]-7-methoxyquinazolin-6-yl (<i>2R</i>)-2,4-dimethylpiperazine-1-carboxylate <b>1m</b> (AZD3759), an investigational drug currently in Phase 1 clinical trial, which has excellent central nervous system penetration and which induces profound regression of brain metastases in a mouse model

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>