Synthesis of an acromelic acid A analog-based 11C-labeled PET tracer for exploration of the site of action of acromelic acid A in allodynia induction

A comparative study on Suzuki-type 11 C-methylation of aromatic organoboranes performed in two reaction media

The Suzuki-type cross coupling reaction is a palladium-mediated multistep reaction that has been used to synthesize several 11 C-labeled tracers for PET. However, the impact of the selected organoborane reagent and reaction medium on the radiochemical yield (RCY) has not been thoroughly investigated. To bridge this gap, we studied the synthesis of 1-[11 C]methylnaphthalene using four different organoborane precursors in reactions performed in DMF/water and THF/water. In the synthesis of 1-[11 C]methylnaphthalene, the best radiochemical yields (RCYs), approximately 50%, were obtained with boronic acid and pinacol ester precursors, whereas less than 4% RCY was obtained when performing the reaction with the N-methylimidodiacetic acid boronic ester (MIDA ester) precursor. 1-[11 C]methylnaphthalene was obtained in higher yields in almost all syntheses performed in THF/water as compared to DMF/water. This observation was in line with previously reported results for [11 C]UCB-J, a tracer for the synaptic vesicle glycoprotein 2A (SV2A) receptor, that also was obtained in higher RCY when synthesized in THF/water. The same trend was observed with [11 C]cetrozole, where the RCY was more than doubled in THF/water compared to the previously published synthesis performed in DMF. These results suggest that THF/water could be the preferred reaction medium when producing PET tracers via the Suzuki-type coupling reaction.

Positron Emission Tomography (PET) Ligand Development for Ionotropic Glutamate Receptors: Challenges and Opportunities for Radiotracer Targeting N-Methyl-d-aspartate (NMDA), α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA), and Kainate Receptors

Ionotropic glutamate receptors (iGluRs) mediate excitatory neurotransmission within the mammalian central nervous system. iGluRs exist as three main groups: N-methyl-d-aspartate receptors (NMDARs), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), and kainate receptors. The past decades have witnessed a remarkable development of PET tracers targeting different iGluRs including NMDARs and AMPARs, and several of the tracers have advanced to clinical imaging studies. Here, we assess the recent development of iGluR PET probes, focusing on tracer design, brain kinetics, and performance in PET imaging studies. Furthermore, this review will not only present challenges in the tracer development but also provide novel approaches in conjunction with most recent drug discovery efforts on these iGluRs, including subtype-selective NMDAR and transmembrane AMPAR regulatory protein modulators and positive allosteric modulators (PAMs) of AMPARs. These approaches, if successful as PET tracers, may provide fundamental knowledge to understand the roles of iGluR receptors under physiological and pathological conditions.

Boron reagents for divergent radiochemistry

This review discusses boron reagents as precursors for divergent radiolabelling with a focus on carbon-11, fluorine-18 and iodine-123, -125, -131.

Synthesis of PET probe O6-[(3-[11C]methyl)benzyl]guanine by Pd0-mediated rapid C-[11C]methylation toward imaging DNA repair protein O6-methylguanine-DNA methyltransferase in glioblastoma

O⁶-Benzylguanine (O⁶-BG) is a substrate of O⁶-methylguanine-DNA methyltransferase (MGMT), which is involved in drug resistance of chemotherapy in the majority of glioblastoma multiform. For clinical diagnosis, it is hoped that the MGMT expression level could be determined by a noninvasive method to understand the detailed biological properties of MGMT-specific tumors. We synthesized ¹¹C-labeled O⁶-[(3-methyl)benzyl]guanine ([¹¹C]mMeBG) as a positron emission tomography probe. Thus, a mixed amine-protected stannyl precursor, N⁹-(tert-butoxycarbonyl)-O⁶-[3-(tributylstannyl)benzyl]-N²-(trifluoroacetyl)guanine, was subjected to rapid C-[¹¹C]methylation under [¹¹C]CH₃I/[Pd₂(dba)₃]/P(o-CH₃C₆H₄)₃/CuCl/K₂CO₃ in NMP, followed by quick deprotection with LiOH/H₂O, giving [¹¹C]mMeBG with total radioactivity of 1.34GBq and ≥99% radiochemical and chemical purities.

Small Molecule Radiopharmaceuticals - A Review of Current Approaches

Radiopharmaceuticals are an integral component of nuclear medicine and are widely applied in diagnostics and therapy. Though widely applied, the development of an “ideal” radiopharmaceutical can be challenging. Issues such as specificity, selectivity, sensitivity, and feasible chemistry challenge the design and synthesis of radiopharmaceuticals. Over time, strategies to address the issues have evolved by making use of new technological advances in the fields of biology and chemistry. This review presents the application of few advances in design and synthesis of radiopharmaceuticals. The topics covered are bivalent ligand approach and lipidization as part of design modifications for enhanced selectivity and sensitivity and novel synthetic strategies for optimized chemistry and radiolabeling of radiopharmaceuticals.

An improved synthesis of [11C]MENET via Suzuki coupling with [11C]methyl iodide

[(11) C]MENET, a promising norepinephrine transporter imaging agent, was prepared by Suzuki cross coupling of 1 mg N-t-Boc pinacolborate precursor with [(11) C]CH3 I in DMF using palladium complex generated in situ from Pd2 (dba)3 and (o-CH3 C6 H4 )3 P together with K2 CO3 as the co-catalyst, followed by deprotection with trifluoroacetic acid. This improved radiolabeling method provided [(11) C]MENET in high radiochemical yield at end of synthesis (EOS, 51 ± 3%, decay-corrected from end of (11) CH3 I synthesis, n = 6), moderate specific activity (1.5-1.9 Ci/µmol at EOS), and high radiochemical (>98%) and chemical purity (>98%) in a synthesis time of 60 ± 5 min from the end of bombardment.

PET and SPECT tracers for glutamate receptors

Radioligands for PET imaging of glutamate receptors will have the potential for studying neurological and neuropsychiatric disorders and their diagnosis and therapeutic intervention. Glutamate is the major excitatory neurotransmitter in the brain and is implicated in the pathophysiology of many neurodegenerative and neuropsychiatric disorders. Glutamate and its receptors are potential targets in the treatment of these disorders. Glutamate signaling is mediated through ionotropic and metabotropic receptors. The abundant concentration of these receptors can facilitate their in vivo quantification using positron emission tomography (PET). Glutamate receptors are a potentially important set of targets for monitoring disease progression, for evaluating the effect of therapy and for new treatment development based on the quantification of receptor occupancy. Here, we review the PET and single-photon emission computed tomography (SPECT) radioligands that have been developed for imaging glutamate receptors in living brain.