Rapid and efficient radiosynthesis of [123I]I-PK11195, a single photon emission computed tomography tracer for peripheral benzodiazepine receptors

A one-pot radioiodination of aryl amines via stable diazonium salts: preparation of 125I-imaging agents

An operationally simple, one-pot, two-step tandem procedure that allows the incorporation of radioactive iodine into aryl amines via stable diazonium salts is described. The mild conditions are tolerant of various functional groups and substitution patterns, allowing late-stage, rapid access to a wide range of ¹²⁵I-labelled aryl compounds and SPECT radiotracers.

Synthesis and Evaluation of a Radioiodinated Tracer with Specificity for Poly(ADP-ribose) Polymerase-1 (PARP-1) in Vivo

Interest in nuclear imaging of poly(ADP-ribose) polymerase-1 (PARP-1) has grown in recent years due to the ability of PARP-1 to act as a biomarker for glioblastoma and increased clinical use of PARP-1 inhibitors. This study reports the identification of a lead iodinated analog 5 of the clinical PARP-1 inhibitor olaparib as a potential single-photon emission computed tomography (SPECT) imaging agent. Compound 5 was shown to be a potent PARP-1 inhibitor in cell-free and cellular assays, and it exhibited mouse plasma stability but approximately 3-fold greater intrinsic clearance when compared to olaparib. An (123)I-labeled version of 5 was generated using solid state halogen exchange methodology. Ex vivo biodistribution studies of [(123)I]5 in mice bearing subcutaneous glioblastoma xenografts revealed that the tracer had the ability to be retained in tumor tissue and bind to PARP-1 with specificity. These findings support further investigations of [(123)I]5 as a noninvasive PARP-1 SPECT imaging agent.

Radiosynthesis, In Vivo Biological Evaluation, and Imaging of Brain Lesions with [123I]-CLINME, a New SPECT Tracer for the Translocator Protein

The high affinity translocator protein (TSPO) ligand 6-chloro-2-(4'-iodophenyl)-3-(N,N-methylethyl)imidazo[1,2-a]pyridine-3-acetamide (CLINME) was radiolabelled with iodine-123 and assessed for its sensitivity for the TSPO in rodents. Moreover neuroinflammatory changes on a unilateral excitotoxic lesion rat model were detected using SPECT imaging. [(123)I]-CLINME was prepared in 70-80% radiochemical yield. The uptake of [(123)I]-CLINME was evaluated in rats by biodistribution, competition, and metabolite studies. The unilateral excitotoxic lesion was performed by injection of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid unilaterally into the striatum. The striatum lesion was confirmed and correlated with TSPO expression in astrocytes and activated microglia by immunohistochemistry and autoradiography. In vivo studies with [(123)I]-CLINME indicated a biodistribution pattern consistent with TPSO distribution and the competition studies with PK11195 and Ro 5-4864 showed that [(123)I]-CLINME is selective for this site. The metabolite study showed that the extractable radioactivity was unchanged [(123)I]-CLINME in organs which expresses TSPO. SPECT/CT imaging on the unilateral excitotoxic lesion indicated that the mean ratio uptake in striatum (lesion:nonlesion) was 2.2. Moreover, TSPO changes observed by SPECT imaging were confirmed by immunofluorescence, immunochemistry, and autoradiography. These results indicated that [(123)I]-CLINME is a promising candidate for the quantification and visualization of TPSO expression in activated astroglia using SPECT.

Development of the radiosynthesis of high-specific-activity 123I-NKJ64

INTRODUCTION

(123)I-NKJ64, a reboxetine analogue, is currently under development as a potential novel single photon emission computed tomography radiotracer for imaging the noradrenaline transporter in brain. This study describes the development of the radiosynthesis of (123)I-NKJ64, highlighting the advantages and disadvantages, pitfalls and solutions encountered while developing the final radiolabelling methodology.

METHODS

The synthesis of (123)I-NKJ64 was evaluated using an electrophilic iododestannylation method, where a Boc-protected trimethylstannyl precursor was radioiodinated using peracetic acid as an oxidant and deprotection was investigated using either trifluoroacetic acid (TFA) or 2 M hydrochloric acid (HCl).

RESULTS

Radioiodination of the Boc-protected trimethylstannyl precursor was achieved with an incorporation yield of 92±6%. Deprotection with 2 M HCl produced (123)I-NKJ64 with the highest radiochemical yield of 98.05±1.63% compared with 83.95±13.24% with TFA. However, the specific activity of the obtained (123)I-NKJ64 was lower when measured after using 2 M HCl (0.15±0.23 Ci/μmol) as the deprotecting agent in comparison to TFA (1.76±0.60 Ci/μmol). Further investigation of the 2 M HCl methodology found a by-product, identified as the deprotected proto-destannylated precursor, which co-eluted with (123)I-NKJ64 during the high-performance liquid chromatography (HPLC) purification.

CONCLUSIONS

The radiosynthesis of (123)I-NKJ64 was achieved with good isolated radiochemical yield of 68% and a high specific activity of 1.8 Ci/μmol. TFA was found to be the most suitable deprotecting agent, since 2 M HCl generated a by-product that could not be fully separated from (123)I-NKJ64 using the HPLC methodology investigated. This study highlights the importance of HPLC purification and accurate measurement of specific activity while developing new radiosynthesis methodologies.

Molecular tracers for the PET and SPECT imaging of disease

The development of positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging continues to grow due to the ability of these techniques to allow the non-invasive in vivo visualisation of biological processes at the molecular and cellular levels. As well as finding application for the diagnosis of disease, these techniques have also been used in the drug discovery process. Crucial to the growth of these techniques is the continued development of molecular probes that can bind to the target biological receptor with high selectivity. This tutorial review describes the use of PET and SPECT for molecular imaging and highlights key strategies for the development of molecular probes for the imaging of both cancer and neurological diseases.

Synthesis and characterization of iodinated tetrahydroquinolines targeting the G protein-coupled estrogen receptor GPR30

A series of iodo-substituted tetrahydro-3H-cyclopenta[c]quinolines was synthesized as potential targeted imaging agents for the G protein-coupled estrogen receptor GPR30. The affinity and specificity of binding to GPR30 versus the classical estrogen receptors ER alpha/beta and functional responses associated with ligand-binding were determined. Selected iodo-substituted tetrahydro-3H-cyclopenta[c]quinolines exhibited IC(50) values lower than 20 nM in competitive binding studies with GPR30-expressing human endometrial cancer cells. These compounds functioned as antagonists of GPR30 and blocked estrogen-induced PI3K activation and calcium mobilization. The tributylstannyl precursors of selected compounds were radiolabeled with (125)I using the iodogen method. In vivo biodistribution studies in female ovariectomized athymic (NCr) nu/nu mice bearing GPR30-expressing human endometrial tumors revealed GPR30-mediated uptake of the radiotracer ligands in tumor, adrenal, and reproductive organs. Biodistribution and quantitative SPECT/CT studies revealed structurally related differences in the pharmacokinetic profiles, target tissue uptake, and metabolism of the radiolabeled compounds as well as differences in susceptibility to deiodination. The high lipophilicity of the compounds adversely affects the in vivo biodistribution and clearance of these radioligands and suggests that further optimization of this parameter may lead to improved targeting characteristics.