Synthesis of 11C-labeled Sulfonyl Carbamates through a Multicomponent Reaction Employing Sulfonyl Azides, Alcohols, and [11C]CO

The Hantzsch reaction for nitrogen-13 PET: preparation of [13N]nifedipine and derivatives

Nitrogen-13 is an attractive but under-used PET radionuclide for labelling molecules of biological and pharmaceutical interest, complementing other PET radionuclides. Its short half-life (t1/2 = 9.97 min) imposes synthetic challenges, but we have expanded the hitherto limited pool of 13N labelling strategies and tracers by adapting the multicomponent Hantzsch condensation reaction to prepare a library of 13N-labelled 1,4-dihydropyridines from [13N]ammonia, including the widely-used drug nifedipine. This represents a key advance in 13N PET radiochemistry, and will serve to underpin the renewed interest in clinical opportunities offered by short-lived PET tracers.

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.

[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.

Chemistry for Positron Emission Tomography: Recent Advances in 11 C-, 18 F-, 13 N-, and 15 O-Labeling Reactions

Positron emission tomography (PET) is a molecular imaging technology that provides quantitative information about function and metabolism in biological processes in vivo for disease diagnosis and therapy assessment. The broad application and rapid advances of PET has led to an increased demand for new radiochemical methods to synthesize highly specific molecules bearing positron-emitting radionuclides. This Review provides an overview of commonly used labeling reactions through examples of clinically relevant PET tracers and highlights the most recent developments and breakthroughs over the past decade, with a focus on 11 C, 18 F, 13 N, and 15 O.

A [11 C] CO dispensing system for rapid screening of carbonylation reactions

[¹¹ C] CO is a highly versatile synthon that allows for labeling at carbonyl positions of many molecules by means of transition metal-mediated carbonylation reactions. The intrinsic complexity of carbonylation reactions often requires tedious screening of reaction conditions for obtaining satisfying yields. Herein, a [¹¹ C] CO dispending system for performing multiple reactions with a single batch of cyclotron-produced [¹¹ C]CO₂ is described. This semiautomated setup allows for more rapid and efficient screening of reactions and reaction conditions compared with the traditional "one beam for one reaction" strategy.

Synthesis of 11C-Labelled Ureas by Palladium(II)-Mediated Oxidative Carbonylation

Positron emission tomography is an imaging technique with applications in clinical settings as well as in basic research for the study of biological processes. A PET tracer, a biologically active molecule where a positron-emitting radioisotope such as carbon-11 has been incorporated, is used for the studies. Development of robust methods for incorporation of the radioisotope is therefore of the utmost importance. The urea functional group is present in many biologically active compounds and is thus an attractive target for incorporation of carbon-11 in the form of [¹¹C]carbon monoxide. Starting with amines and [¹¹C]carbon monoxide, both symmetrical and unsymmetrical ¹¹C-labelled ureas were synthesised via a palladium(II)-mediated oxidative carbonylation and obtained in decay-corrected radiochemical yields up to 65%. The added advantage of using [¹¹C]carbon monoxide was shown by the molar activity obtained for an inhibitor of soluble epoxide hydrolase (247 GBq/μmol–319 GBq/μmol). DFT calculations were found to support a reaction mechanism proceeding through an ¹¹C-labelled isocyanate intermediate.

New methodologies for the preparation of carbon-11 labeled radiopharmaceuticals

PURPOSE

This short review aims to cover the more recent and promising developments of carbon-11 (11C) labeling radiochemistry and its utility in the production of novel radiopharmaceuticals, with special emphasis on methods that have the greatest potential to be translated for clinical positron emission tomography (PET) imaging.

METHODS

A survey of the literature was undertaken to identify articles focusing on methodological development in 11C chemistry and their use within novel radiopharmaceutical preparation. However, since 11C-labeling chemistry is such a narrow field of research, no systematic literature search was therefore feasible. The survey was further restricted to a specific timeframe (2000-2016) and articles in English.

RESULTS

From the literature, it is clear that the majority of 11C-labeled radiopharmaceuticals prepared for clinical PET studies have been radiolabeled using the standard heteroatom methylation reaction. However, a number of methodologies have been developed in recent years, both from a technical and chemical point of view. Amongst these, two protocols may have the greatest potential to be widely adapted for the preparation of 11C-radiopharmaceuticals in a clinical setting. First, a novel method for the direct formation of 11C-labeled carbonyl groups, where organic bases are utilized as [11C]carbon dioxide-fixation agents. The second method of clinical importance is a low-pressure 11C-carbonylation technique that utilizes solvable xenon gas to effectively transfer and react [11C]carbon monoxide in a sealed reaction vessel. Both methods appear to be general and provide simple paths to 11C-labeled products.

CONCLUSION

Radiochemistry is the foundation of PET imaging which relies on the administration of a radiopharmaceutical. The demand for new radiopharmaceuticals for clinical PET imaging is increasing, and 11C-radiopharmaceuticals are especially important within clinical research and drug development. This review gives a comprehensive overview of the most noteworthy 11C-labeling methods with clinical relevance to the field of PET radiochemistry.

Synthesis of 11C-labeled Sulfonyl Carbamates through a Multicomponent Reaction Employing Sulfonyl Azides, Alcohols, and [11C]CO

We describe the development of a new methodology focusing on 11C-labeling of sulfonyl carbamates in a multicomponent reaction comprised of a sulfonyl azide, an alkyl alcohol, and [11C]CO. A number of 11C-labeled sulfonyl carbamates were synthesized and isolated, and the developed methodology was then applied in the preparation of a biologically active molecule. The target compound was obtained in 24±10 % isolated radiochemical yield and was evaluated for binding properties in a tumor cell assay; in vivo biodistribution and imaging studies were also performed. This represents the first successful radiolabeling of a non-peptide angiotensin II receptor subtype 2 agonist, C21, currently in clinical trials for the treatment of idiopathic pulmonary fibrosis.