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

Reactive Palladium-Ligand Complexes for 11C-Carbonylation at Ambient Pressure: A Breakthrough in Carbon-11 Chemistry

The Pd-Xantphos-mediated ¹¹C-carbonylation protocol (also known as the "Xantphos- method"), due to its simplistic and convenient nature, has facilitated researchers in meeting a longstanding need for preparing ¹¹C-carbonyl-labeled radiopharmaceuticals at ambient pressure for positron emission tomography (PET) imaging and drug discovery. This development could be viewed as a breakthrough in carbon-11 chemistry, as evidenced by the rapid global adoption of the method by the pharmaceutical industry and academic laboratories worldwide. The method has been fully automated for the good manufacturing practice (GMP)-compliant production of novel radiopharmaceuticals for human use, and it has been adapted for "in-loop" reactions and microwave technology; an impressive number of ¹¹C-labeled compounds (>100) have been synthesized. Given the simplicity and efficiency of the method, as well as the abundance of carbonyl groups in bioactive drug molecules, we expect that this methodology will be even more widely adopted in future PET radiopharmaceutical research and drug development.

Imaging Histamine H3 Receptors with Positron Emission Tomography

Positron emission tomography (PET) provides a unique tool to study the biochemistry of the human brain in vivo. By using PET probes that are binding selectively to certain receptor subtypes, brain PET allows the quantification of receptor levels in various brain areas of human subjects. This approach has the potential to reveal abnormal receptor expressions that may contribute to the physiopathology of some psychiatric and neurological disorders. This approach also has the potential to assist in the drug development process by determining receptor occupancy in vivo allowing selection of proper drug dosage to produce therapeutic effects. Several PET tracers have been developed for histamine H3 receptors (H3R). However, despite the potential of PET to elucidate the role of H3R in vivo, only limited work has been conducted so far. This article reviews the work that has been done in this area. Notably, we will cover the limitations of the first-generation PET radioligand for H3R and present the advantages of novel radioligands that promise an explosion of clinical PET research on the role of H3R in vivo.

One-Pot Synthesis of 11 C-Labelled Primary Benzamides via Intermediate [11 C]Aroyl Dimethylaminopyridinium Salts

Electrophilic ¹¹C‐labelled aroyl dimethylaminopyridinium salts, obtained by carbonylative cross‐coupling of aryl halides with [¹¹C]carbon monoxide, were prepared for the first time and shown to be valuable intermediates in the synthesis of primary [¹¹C]benzamides. The methodology furnished a set of benzamide model compounds, including the two poly (ADP‐ribose) polymerase (PARP) inhibitors niraparib and veliparib, in moderate to excellent radiochemical yields. In addition to providing a convenient and practical route to primary [¹¹C]benzamides, the current method paves the way for future application of [¹¹C]aroyl dimethylaminopyridinium halide salts in positron emission tomography (PET) tracer synthesis.

Development of a fully automated low-pressure [11 C]CO carbonylation apparatus

[¹¹C]carbon monoxide ([¹¹C]CO) is a versatile synthon for radiolabeling of drug‐like molecules for imaging studies with positron emission tomography (PET). We here report the development of a novel, user‐friendly, fully automated, and good manufacturing practice (GMP) compliant low‐pressure synthesis module for ¹¹C‐carbonylation reactions using [¹¹C]CO. In this synthesis module, [¹¹C]CO was reliably prepared from cyclotron‐produced [¹¹C]carbon dioxide ([¹¹C]CO₂) by reduction over heated molybdenum and delivered to the reaction vessel within 7 min after end of bombardment, with an overall radiochemical yield (RCY) of 71%. [¹¹C]AZ13198083, a histamine type‐3 receptor ligand, was used as a model compound to assess the functionality of the radiochemistry module. At full batch production conditions (55 μA, 30 min), our newly developed low‐pressure ¹¹C‐carbonylation apparatus enabled us to prepare [¹¹C]AZ13198083 in an isolated radioactivity of 8540 ± 1400 MBq (n = 3). The radiochemical purity of each of the final formulated batches exceeded 99%, and all other quality control tests results conformed with specifications typically set for carbon‐11 labeled radiopharmaceuticals. In conclusion, this novel radiochemistry system offers a convenient GMP compliant production drugs and radioligands for imaging studies in human subjects.

The development of 11C-carbonylation chemistry: A systematic view

The prospects for using carbon-11 labelled compounds in molecular imaging has improved with the development of diverse synthesis methods, including ¹¹C-carbonylations and refined techniques to handle [¹¹C]carbon monoxide at a nanomole scale. Facilitating biological research and molecular imaging was the driving force when [¹¹C]carbon monoxide was used in the first in vivo application with carbon-11 in human (1945) and when [¹¹C]carbon monoxide was used for the first time as a chemical reagent in the synthesis of [¹¹C]phosgene (1978). This review examines a rich plethora of labelled compounds synthesized from [¹¹C]carbon monoxide, their chemistry and use in molecular imaging. While the strong development of the ¹¹C-carbonylation chemistry has expanded the carbon-11 domain considerably, it could be argued that the number of ¹¹C-carbonyl compounds entering biological investigations should be higher. The reason for this may partly be the lack of commercially available synthesis instruments designed for ¹¹C-carbonylations. But as this review shows, novel and greatly simplified methods to handle [¹¹C]carbon monoxide have been developed. The next important challenge is to make full use of these technologies and synthesis methods in PET research. When there is a PET-tracer that meets a more general need, the incentive to implement ¹¹C-carbonylation protocols will increase.

"In-loop" carbonylation-A simplified method for carbon-11 labelling of drugs and radioligands

Transition‐metal mediated carbonylation with ¹¹C‐labelled carbon monoxide ([¹¹C]CO) is a versatile method for introducing ¹¹C (t_1/2 = 20.3 min) into drugs and radioligands for subsequent use in positron emission tomography (PET). The aim of the current study was to perform the ¹¹C‐carbonylation reaction on the interior surface of a stainless‐steel loop used for high performance liquid chromatography (HPLC). In the experimental setup, cyclotron produced ¹¹C‐labelled carbon dioxide ([¹¹C]CO₂) was converted to [¹¹C]CO by reduction over heated Molybdenum and swept into an HPLC loop pre‐charged with the appropriate reaction mixture. Following a 5 min reaction, the radiochemical purity (RCP) and the trapping efficiency (TE) of the reaction mixture was determined. After optimization, [¹¹C]N‐Benzylbenzamide was obtained in quantitative radiochemical yield (RCY) following a 5 min reaction at room temperature. The methodology was further applied to label [¹¹C]benzoic acid (RCP≥99%, TE>91%), [¹¹C]methyl benzoate (RCP≥99%, TE>93%) and [¹¹C]phthalide (RCP≥99%, TE>88%). A set of pharmaceuticals was finally radiolabelled using non‐optimized conditions. Excellent yields were obtained for the histamine‐3 receptor radioligand [¹¹C]AZ13198083, the oncology drug [¹¹C]olaparib and the dopamine D2 receptor radioligand [¹¹C]raclopride, whereas a moderate yield was observed for the high‐affinity dopamine D2 receptor radioligand [¹¹C]FLB457. The presented “in‐loop” process proved efficient for diverse ¹¹C‐carbonylations, providing [¹¹C]amides, [¹¹C]esters and [¹¹C]carboxylic acids in moderate to excellent RCYs. Based on the advantages associated with performing the radiolabelling step as an integrated part of the purification system, this methodology may become a valuable addition to the toolbox of methodologies used for ¹¹C‐carbonylation of drugs and radioligands for PET.

[11C]Carbon monoxide: advances in production and application to PET radiotracer development over the past 15 years

[¹¹C]Carbon monoxide is an appealing synthon for introducing carbon-11 at a carbonyl position (C=O) in a wide variety of chemotypes (e.g., amides, ketones, acids, esters, and ureas). The prevalence of the carbonyl group in drug molecules and the present-day broad versatility of carbonylation reactions have led to an upsurge in the production of this synthon and in its application to PET radiotracer development. This review focuses on the major advances of the past 15 years.

Histamine H3 receptor antagonists/inverse agonists: Where do they go?

Since the discovery of the histamine H₃ receptor in 1983, tremendous advances in the pharmacological aspects of H₃ receptor antagonists/inverse agonists have been accomplished in preclinical studies. At present, there are several drug candidates that reached clinical trial studies for various indications. However, entrance of these candidates to the pharmaceutical market is not free from challenges, and a variety of difficulties is engaged with their developmental process. In this review, the potential role of H₃ receptors in the pathophysiology of various central nervous system, metabolic and allergic diseases is discussed. Thereafter, the current status for H₃ receptor antagonists/inverse agonists in ongoing clinical trial studies is reviewed and obstacles in developing these agents are emphasized.