Synthesis of carbon-11-labeled 5-HT6R antagonists as new candidate PET radioligands for imaging of Alzheimer’s disease

30 years of [11C]methyl triflate

Some scientific discoveries are well known only to a core group of researchers working on technical subjects. Nevertheless, they open new research directions, allow existing knowledge to be viewed in entirely new and useful ways, or provide a way to make something that was hard or impossible to make before. Carbon-11 methyl triflate ([¹¹C]MeOTf) is one such advance, facilitating the synthesis of many carbon-11 radio tracers and broadening the range of applications of carbon-11 radiochemistry. The year 2022 marked the 30th anniversary of the original paper in Applied Radiation and Isotopes introducing a simple synthesis of [¹¹C]MeOTf from carbon-11 methyl iodide ([¹¹C]MeI) and it also marked the end of the fruitful career and life of the researcher who developed it, Douglas Jewett. It seems fitting to say a few words on how it came to be and how it has helped advance carbon-11 radiochemistry.

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

Radiosynthesis of a carbon-11 labeled tetrahydrobenzisoxazole derivative as a new PET probe for γ-secretase imaging in Alzheimer's disease

To develop PET radiotracers for imaging of Alzheimer's disease, a new carbon-11 labeled potent and selective γ-secretase modulator (GSM) has been synthesized. The reference standard tetrahydrobenzisoxazole derivative 8 and its desmethylated precursor 9 were synthesized from cyclohex-2-en-1-one and 3-hydroxy-4-nitrobenzaldehyde in eight and nine steps with 11% and 5% overall chemical yield, respectively. The radiotracer [¹¹C]8 was prepared from its corresponding precursor 9 with [¹¹C]CH₃OTf through O-¹¹C-methylation and isolated by RP-HPLC combined with SPE in 45-50% radiochemical yield, based on [¹¹C]CO₂ and decay corrected to EOB. The radiochemical purity was >99%, and the molar activity (A_m) at EOB was 555-740 GBq/μmol.

Radiosynthesis of carbon-11 labeled PDE5 inhibitors as new potential PET radiotracers for imaging of Alzheimer's disease

To develop PET tracers for imaging of Alzheimer's disease, new carbon-11 labeled potent and selective PDE5 inhibitors have been synthesized. The reference standards (5) and (12), and their corresponding desmethylated precursors (6) and (13) were synthesized from methyl 2-amino-5-bromobenzoate and (4-methoxyphenyl)methanamine in multiple steps with 2%, 1%, 1% and 0.2% overall chemical yield, respectively. The radiotracers ([¹¹C]5) and ([¹¹C]12) were prepared from their corresponding precursors 6 and 13 with [¹¹C]CH₃OTf through O-¹¹C-methylation and isolated by HPLC combined with SPE in 40-50% radiochemical yield, based on [¹¹C]CO₂ and decay corrected to EOB. The radiochemical purity was >99%, and the molar activity (A_m) at EOB was in a range of 370-740 GBq/μmol.

Radiosynthesis of a carbon-11-labeled AMPAR allosteric modulator as a new PET radioligand candidate for imaging of Alzheimer's disease

To develop PET tracers for imaging of Alzheimer's disease, a new carbon-11-labeled AMPAR allosteric modulator 4-cyclopropyl-7-(3-[¹¹C]methoxyphenoxy)-3,4-dihydro-2H-benzo[e][1,2,4]thiadiazine 1,1-dioxide ([¹¹C]8) has been synthesized. The reference standard 4-cyclopropyl-7-(3-methoxyphenoxy)-3,4-dihydro-2H-benzo[e][1,2,4]thiadiazine 1,1-dioxide (8) and its corresponding desmethylated precursor 4-cyclopropyl-7-(3-hydroxyphenoxy)-3,4-dihydro-2H-benzo[e][1,2,4]thiadiazine 1,1-dioxide (9) were synthesized from 4-methoxyabiline and chlorosulfonyl isocyanate in eight and nine steps with 3% and 1% overall chemical yield, respectively. The target tracer [¹¹C]8 was prepared from the precursor 9 with [¹¹C]CH₃OTf through O-[¹¹C]methylation and isolated by HPLC combined with SPE in 10-15% radiochemical yield, based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The radiochemical purity was >99%, and the molar activity (A_M) at EOB was 370-740 GBq/μmol with a total synthesis time of 35-40-minutes from EOB.

Synthesis of carbon-11-labeled CK1 inhibitors as new potential PET radiotracers for imaging of Alzheimer's disease

The reference standards methyl 3-((2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)carbamoyl)benzoate (5a) and N-(2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)-3-methoxybenzamide (5c), and their corresponding desmethylated precursors 3-((2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)carbamoyl)benzoic acid (6a) and N-(2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)-3-hydroxybenzamide (6b), were synthesized from 5-amino-2,2-difluoro-1,3-benzodioxole and 3-substituted benzoic acids in 5 and 6 steps with 33% and 11%, 30% and 7% overall chemical yield, respectively. Carbon-11-labeled casein kinase 1 (CK1) inhibitors, [¹¹C]methyl 3-((2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)carbamoyl)benzoate ([¹¹C]5a) and N-(2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)-3-[¹¹C]methoxybenzamide ([¹¹C]5c), were prepared from their O-desmethylated precursor 6a or 6b with [¹¹C]CH₃OTf through O-[¹¹C]methylation and isolated by HPLC combined with SPE in 40-45% radiochemical yield, based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The radiochemical purity was >99%, and the molar activity (MA) at EOB was 370-740 GBq/μmol with a total synthesis time of ∼40-min from EOB.