Use of carbon-11 labelled tool compounds in support of drug development

Titanium-45 (45Ti) Radiochemistry and Applications in Molecular Imaging

Molecular imaging is an important part of modern medicine which enables the non-invasive identification and characterization of diseases. With the advancement of radiochemistry and scanner technology, nuclear medicine is providing insight into efficient treatment options for individual patients. Titanium-45 (45Ti) is a lesser-explored radionuclide that is garnering increasing interest for the development of positron emission tomography (PET) radiopharmaceuticals. This review discusses aspects of this radionuclide including production, purification, radiochemistry development, and molecular imaging studies.

Light-Driven Radiochemistry with Fluorine-18, Carbon-11 and Zirconium-89

This review discusses recent advances in light-driven radiochemistry for three key isotopes: fluorine-18, carbon-11, and zirconium-89, and their applications in positron emission tomography (PET). In the case of fluorine-18, the predominant approach involves the use of cyclotron-produced [18F]fluoride or reagents derived thereof. Light serves to activate either the substrate or the fluorine-18 labeled reagent. Advancements in carbon-11 photo-mediated radiochemistry have been leveraged for the radiolabeling of small molecules, achieving various transformations, including 11C-methylation, 11C-carboxylation, 11C-carbonylation, and 11C-cyanation. Contrastingly, zirconium-89 photo-mediated radiochemistry differs from fluorine-18 and carbon-11 approaches. In these cases, light facilitates a postlabeling click reaction, which has proven valuable for the labeling of large biomolecules such as monoclonal antibodies (mAbs). New technological developments, such as the incorporation of photoreactors in commercial radiosynthesizers, illustrate the commitment the field is making in embracing photochemistry. Taken together, these advances in photo-mediated radiochemistry enable radiochemists to apply new retrosynthetic strategies in accessing novel PET radiotracers.

Preclinical Evaluation of the Reversible Monoacylglycerol Lipase PET Tracer (R)-[11C]YH132: Application in Drug Development and Neurodegenerative Diseases

Monoacylglycerol lipase (MAGL) plays a crucial role in the degradation of 2-arachidonoylglycerol (2-AG), one of the major endocannabinoids in the brain. Inhibiting MAGL could lead to increased levels of 2-AG, which showed beneficial effects on pain management, anxiety, inflammation, and neuroprotection. In the current study, we report the characterization of an enantiomerically pure (R)-[11C]YH132 as a novel MAGL PET tracer. It demonstrates an improved pharmacokinetic profile compared to its racemate. High in vitro MAGL specificity of (R)-[11C]YH132 was confirmed by autoradiography studies using mouse and rat brain sections. In vivo, (R)-[11C]YH132 displayed a high brain penetration, and high specificity and selectivity toward MAGL by dynamic PET imaging using MAGL knockout and wild-type mice. Pretreatment with a MAGL drug candidate revealed a dose-dependent reduction of (R)-[11C]YH132 accumulation in WT mouse brains. This result validates its utility as a PET probe to assist drug development. Moreover, its potential application in neurodegenerative diseases was explored by in vitro autoradiography using brain sections from animal models of Alzheimer's disease and Parkinson's disease.

Recent Developments in Carbon-11 Chemistry and Applications for First-In-Human PET Studies

Positron emission tomography (PET) is a molecular imaging technique that makes use of radiolabelled molecules for in vivo evaluation. Carbon-11 is a frequently used radionuclide for the labelling of small molecule PET tracers and can be incorporated into organic molecules without changing their physicochemical properties. While the short half-life of carbon-11 (¹¹C; t_½ = 20.4 min) offers other advantages for imaging including multiple PET scans in the same subject on the same day, its use is limited to facilities that have an on-site cyclotron, and the radiochemical transformations are consequently more restrictive. Many researchers have embraced this challenge by discovering novel carbon-11 radiolabelling methodologies to broaden the synthetic versatility of this radionuclide. This review presents new carbon-11 building blocks and radiochemical transformations as well as PET tracers that have advanced to first-in-human studies over the past five years.

Catalytic methoxylation of aryl halides using 13C- and 14C-labeled CO2

A late-stage carbon isotope strategy, which allows methoxylation from CO2, is reported. This catalytic process, that relies on the formation of BBN-OCH3, enabled 13C and 14C labeling of a series of substrates, including pharmaceuticals.

Metallaphotoredox aryl and alkyl radiomethylation for PET ligand discovery

Positron emission tomography (PET) radioligands (radioactively labelled tracer compounds) are extremely useful for in vivo characterization of central nervous system drug candidates, neurodegenerative diseases and numerous oncology targets¹. Both tritium and carbon-11 radioisotopologues are generally necessary for in vitro and in vivo characterization of radioligands², yet there exist few radiolabelling protocols for the synthesis of either, inhibiting the development of PET radioligands. The synthesis of such radioligands also needs to be very rapid owing to the short half-life of carbon-11. Here we report a versatile and rapid metallaphotoredox-catalysed method for late-stage installation of both tritium and carbon-11 into the desired compounds via methylation of pharmaceutical precursors bearing aryl and alkyl bromides. Methyl groups are among the most prevalent structural elements found in bioactive molecules, and so this synthetic approach simplifies the discovery of radioligands. To demonstrate the breadth of applicability of this technique, we perform rapid synthesis of 20 tritiated and 10 carbon-11-labelled complex pharmaceuticals and PET radioligands, including a one-step radiosynthesis of the clinically used compounds [¹¹C]UCB-J and [¹¹C]PHNO. We further outline the direct utility of this protocol for preclinical PET imaging and its translation to automated radiosynthesis for routine radiotracer production in human clinical imaging. We also demonstrate this protocol for the installation of other diverse and pharmaceutically useful isotopes, including carbon-14, carbon-13 and deuterium.

Fully Automated Radiosynthesis of [11C]Guanidines for Cardiac PET Imaging

Radiolabeled guanidines such as meta-iodobenzylguanidine (MIBG) find utility in nuclear medicine as both diagnostic imaging agents and radiotherapeutics and, over the years, numerous methods for incorporating radionuclides into guanidines have been developed. In connection with a project developing new positron emission tomography (PET) radiotracers for cardiac sympathetic nerve density, we had cause to prepare [¹¹C]3F-PHPOG. However, it quickly became apparent that radiolabeling of guanidine scaffolds with carbon-11 has remained challenging, and historical methods lack compatibility with modern automated radiochemistry synthesis platforms and current Good Manufacturing Practice (cGMP) requirements. To address this challenge, we report a new automated method for radiolabeling guanidines with carbon-11. The method was used to prepare a series of [¹¹C]guanidines in good radiochemical yield (8-76% by radio-HPLC) and was found to have broad substrate scope and tolerance of unprotected OH and NH functional groups. The method was used to synthesize [¹¹C]3F-PHPOG for preclinical imaging, and suitability of the radiotracer for preclinical use was demonstrated through preliminary cardiac PET in New Zealand white rabbits which revealed good cardiac uptake and expected retention in the heart.

Carbon-11 carboxylation of trialkoxysilane and trimethylsilane derivatives using [11C]CO2

A simple and rapid carbon-11 carboxylation radiosynthesis method.

A novel carboxylation radiosynthesis methodology is described starting from cyclotron-produced [¹¹C]CO₂ and fluoride-activated silane derivatives. Six carbon-11 labelled carboxylic acids were obtained from their corresponding trimethylsilyl and trialkoxysilyl precursors in a one-pot labelling methodology. The radiochemical yields ranged from 19% to 93% within 12 minutes post [¹¹C]CO₂ delivery with a trapping efficiency of 21–89%.

Synthesis and preliminary radiopharmacological characterisation of an 11 C-labelled azadipeptide nitrile as potential PET tracer for imaging of cysteine cathepsins

An O-methyltyrosine-containing azadipeptide nitrile was synthesised and investigated for its inhibitory activity towards cathepsins L, S, K, and B. Labelling with carbon-11 was accomplished by reaction of the corresponding phenolic precursor with [¹¹ C]methyl iodide starting from cyclotron-produced [¹¹ C]methane. Radiopharmacological evaluation of the resulting radiotracer in a mouse xenograft model derived from a mammary tumour cell line by small animal PET imaging indicates tumour targeting with complex pharmacokinetics. Radiotracer uptake in the tumour region was considerably lower under treatment with the nonradioactive reference compound and the epoxide-based irreversible cysteine cathepsin inhibitor E64. The in vivo behaviour observed for this radiotracer largely confirms that of the corresponding ¹⁸ F-fluoroethylated analogue and suggests the limited suitability of azadipeptide nitriles for the imaging of tumour-associated cysteine cathepsins despite target-mediated uptake is evidenced.