Synthesis of [11C]FEDAA1106 as a new PET imaging probe of peripheral benzodiazepine receptor expression

Targeting the Translocator Protein (18 kDa) in Cardiac Diseases: State of the Art and Future Opportunities

Mitochondria dysfunctions are typical hallmarks of cardiac disorders (CDs). The multiple tasks of this energy-producing organelle are well documented, but its pathophysiologic involvement in several manifestations of heart diseases, such as altered electromechanical coupling, excitability, and arrhythmias, is still under investigation. The human 18 kDa translocator protein (TSPO) is a protein located on the outer mitochondrial membrane whose expression is altered in different pathological conditions, including CDs, making it an attractive therapeutic and diagnostic target. Currently, only a few TSPO ligands are employed in CDs and cardiac imaging. In this Perspective, we report an overview of the emerging role of TSPO at the heart level, focusing on the recent literature concerning the development of TSPO ligands used for fighting and imaging heart-related disease conditions. Accordingly, targeting TSPO might represent a successful strategy to achieve novel therapeutic and diagnostic strategies to unravel the fundamental mechanisms and to provide solutions to still unanswered questions in CDs.

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.

The 18-kDa Translocator Protein PET Tracers as a Diagnostic Marker for Neuroinflammation: Development and Current Standing

Translocator protein (TSPO, 18 kDa) is an evolutionary, well-preserved, and tryptophan-rich 169-amino-acid protein which localizes on the contact sites between the outer and inner mitochondrial membranes of steroid-synthesizing cells. This mitochondrial protein is implicated in an extensive range of cellular activities, including steroid synthesis, cholesterol transport, apoptosis, mitochondrial respiration, and cell proliferation. The upregulation of TSPO is well documented in diverse disease conditions including neuroinflammation, cancer, brain injury, and inflammation in peripheral organs. On the basis of these outcomes, TSPO has been assumed to be a fascinating subcellular target for early stage imaging of the diseased state and for therapeutic purposes. The main outline of this Review is to give an update on dealing with the advances made in TSPO PET tracers for neuroinflammation, synchronously emphasizing the approaches applied for the design and advancement of new tracers with reference to their structure-activity relationship (SAR).

Synthesis of 2-[11C]methoxy-3,17β-O,O-bis(sulfamoyl)estradiol as a new potential PET agent for imaging of steroid sulfatase (STS) in cancers

Steroid sulfatase (STS) catalyzes the hydrolysis of steroid sulfates to estrones, the main source of estrogens in tumors. Carbonic anhydrase II (CAII) is highly expressed in red blood cells through a coordination of the monoanionic form of the sulfamate moiety to the zinc atom in the enzyme active site, and CAII is highly expressed in several tumors. 2-Methoxy-3,17β-O,O-bis(sulfamoyl)estradiol (5) is a dual-function STS-CAII inhibitor inhibited STS with 39 nM IC(50) value selectively over CAII with 379 nM IC(50) value. This compound exhibited potent antiproferative activity with mean graph midpoint value of 87 nM in the NCI 60-cell-line panel, and antiangiogenic in vitro and in vivo activity in an early-stage Lewis lung model as well. The compound has been recently developed as a multitargeted anticancer agent. Both STS and CAII are over-expressed in cancers and have become attractive targets for cancer treatment and molecular imaging of cancer. Here we report the first design and synthesis of 2-[(11)C]methoxy-3,17β-O,O-bis(sulfamoyl)estradiol ([(11)C]5) as a new potential imaging agent for biomedical imaging technique positron emission tomography (PET) to image STS in cancers. The authentic standard 5 was synthesized from 17β-estradiol by published procedures in 5 steps with 40% overall chemical yield. The precursor 2-hydroxy-3,17β-O,O-bis(sulfamoyl)estradiol (14a) for radiolabeling was synthesized from 17β-estradiol in 10 steps with 5% overall chemical yield. The target tracer [(11)C]5 was prepared from the precursor 14a with [(11)C]CH(3)OTf through O-[(11)C]methylation and isolated by HPLC combined with solid-phase extraction (SPE) purification in 40-50% radiochemical yields based on [(11)C]CO(2) and decay corrected to end of bombardment (EOB), with 370-740 GBq/μmol specific activity at EOB.

In vivo imaging of neuroinflammation: a comparative study between [(18)F]PBR111, [ (11)C]CLINME and [ (11)C]PK11195 in an acute rodent model

PURPOSE

The key role of neuroinflammation in acute and chronic neurological disorders has stimulated the search for specific radiotracers targeting the peripheral benzodiazepine receptor (PBR)/18 kDa translocator protein (TSPO), a hallmark of neuroinflammation. Here we evaluate the new radiotracer for positron emission tomography (PET) [(18)F]PBR111 in a rodent model of acute inflammation and compare it with [(11)C]CLINME, an (11)C-labelled tracer of the same chemical family, and with the isoquinolinic carboxamide [(11)C]PK11195.

METHODS

We studied radiometabolites by HPLC, in vitro binding by autoradiography and in vivo brain kinetics as well as in vivo specificity of binding using PET imaging.

RESULTS

We show that this radiotracer has a high in vitro specificity for PBR/TSPO versus central benzodiazepine receptors, as reflected by the drastic reduction of its binding to target tissue by addition of PK11195 or PBR111, while addition of flumazenil does not affect binding. Only intact [(18)F]PBR111 is detected in brain up to 60 min after i.v. injection, and PET imaging shows an increased uptake in the lesion as compared to the contralateral side as early as 6 min after injection. Administration of an excess of PK11195 and PBR111, 20 min after [(18)F]PBR111 administration, induces a rapid and complete displacement of [(18)F]PBR111 binding from the lesion. Modelling of the PET data using the simplified reference tissue model showed increased binding potential (BP) in comparison to [(11)C]PK11195.

CONCLUSION

[(18)F]PBR111 is a metabolically stable tracer with a high specific in vitro and in vivo binding to TSPO. In addition, considering the longer half-life of (18)F over (11)C, these results support [(18)F]PBR111 as a promising PET tracer of the PBR/TSPO for neuroinflammation imaging.

New iodinated quinoline-2-carboxamides for SPECT imaging of the translocator protein

With the aim of developing new SPECT imaging agents for the translocator protein (TSPO), a small library of iodinated quinoline-2-carboxamides have been prepared and tested for binding affinity with TSPO. N,N-Diethyl-3-iodomethyl-4-phenylquinoline-2-carboxamide was found to have excellent affinity (K(i) 12.0 nM), comparable to that of the widely used TSPO imaging agent PK11195.

Biodistribution and radiation dosimetry in humans of a new PET ligand, (18)F-PBR06, to image translocator protein (18 kDa)

As a PET biomarker for inflammation, translocator protein (18 kDa) (TSPO) can be measured with an (18)F-labeled aryloxyanilide, (18)F-N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline ((18)F-PBR06), in the human brain. The objective of this study was to estimate the radiation absorbed doses of (18)F-PBR06 based on biodistribution data in humans.

METHODS

After the injection of (18)F-PBR06, images were acquired from head to thigh in 7 healthy humans. Urine was collected at various time points. Radiation absorbed doses were estimated by the MIRD scheme.

RESULTS

Moderate to high levels of radioactivity were observed in organs with high densities of TSPO and in organs of metabolism and excretion. Bone had low levels of radioactivity. The effective dose was 18.5 muSv/MBq.

CONCLUSION

The effective dose of (18)F-PBR06, compared with other (18)F radioligands, was moderate. This radioligand had negligible defluorination, as indirectly assessed by bone radioactivity. Doses to the gallbladder wall and spleen may limit the amount of permissible injected radioactivity.

Design and synthesis of carbon-11-labeled dual aromatase-steroid sulfatase inhibitors as new potential PET agents for imaging of aromatase and steroid sulfatase expression in breast cancer

Aromatase and steroid sulfatase (STS) are particularly attractive targets in the treatment of estrogen-receptor-positive breast cancer and the development of enzyme-based cancer imaging agents for the biomedical imaging technique positron emission tomography (PET). New carbon-11-labeled sulfamate derivatives were first designed and synthesized as potential PET dual aromatase-steroid sulfatase inhibitor (DASSI) radiotracers for imaging of aromatase and STS expression in breast cancer. The target tracers 5-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-[(11)C]methoxyphenyl sulfamate ([(11)C]8a) and 4-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-[(11)C]methoxyphenyl sulfamate ([(11)C]8b) were prepared from their corresponding precursors 5-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-hydroxyphenyl sulfamate (16) and 4-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-hydroxyphenyl sulfamate (21) with [(11)C]CH(3)OTf under basic conditions through the O-[(11)C]methylation and isolated by the reversed-phase high pressure liquid chromatography (HPLC) method in 30-45% radiochemical yields based on [(11)C]CO(2) and decay corrected to end of bombardment (EOB). The specific activity at end of synthesis (EOS) was 111-185GBq/micromol.

Fully automated synthesis and initial PET evaluation of [11C]PBR28

Fully automated synthesis and initial PET evaluation of a TSPO radioligand, [11C]PBR28 (N-(2-[11C]methoxybenzyl)-N-(4-phenoxypyridin-3-yl)acetamide), are reported. These results facilitate the potential preclinical and clinical PET studies of [11C]PBR28 in animals and humans.