Detailed evaluation of different 68Ge/68Ga generators: an attempt toward achieving efficient 68Ga radiopharmacy

The present study is aimed at carrying out a comparative performance evaluation of different types of ⁶⁸Ge/⁶⁸Ga generators to identify the best choice for use in ⁶⁸Ga‐radiopharmacy. Over the 1 year period of evaluation, the elution yields from the CeO₂‐based and SiO₂‐based ⁶⁸Ge/⁶⁸Ga generators remained almost consistent, in contrast to the sharp decrease observed in the elution yields from TiO₂ and SnO₂‐based generators. The level of ⁶⁸Ge impurity in ⁶⁸Ga eluates from the CeO₂ and SiO₂‐based ⁶⁸Ge/⁶⁸Ga generator was always <10^?3%, while this level increased from 10^?3% to 10^?1% in case of TiO₂ and SnO₂‐based generators. The level of chemical impurities in ⁶⁸Ga eluates from CeO₂ and SiO₂‐based ⁶⁸Ge/⁶⁸Ga generators was negligibly low (<0.1 ppm) in contrast to the significantly higher level (1–20 ppm) of such impurities in eluates from other two generators. As demonstrated by radiolabeling studies carried out using DOTA‐coupled dimeric cyclic RGD peptide derivative (DOTA‐RGD₂), CeO₂‐PAN and SiO₂‐based generators are directly amenable for radiopharmaceutical preparation, whereas the other generators can be only used after post‐elution purification of ⁶⁸Ga eluates. Clinically relevant dose of ⁶⁸Ga‐DOTA‐RGD₂ was prepared in a hospital radiopharmacy for non‐invasive visualization of tumors in breast cancer patients using positron emission tomography.     

Rhodium‐mediated [11C]carbonylation: a library of N‐phenyl‐N′‐{4‐(4‐quinolyloxy)‐phenyl}‐[11C]‐urea derivatives as potential PET angiogenic probes

As part of our ongoing investigation into the imaging of angiogenic processes, a small library of eight vascular endothelial growth factor receptor‐2 (VEGFR‐2)/platelet‐derived growth factor receptor β dual inhibitors based on the N‐phenyl‐N′‐4‐(4‐quinolyloxy)‐phenyl‐urea was labelled with ¹¹C (β⁺, t_1/2=20.4 min) in the urea carbonyl position via rhodium‐mediated carbonylative cross‐coupling of an aryl azide and different anilines. The decay‐corrected radiochemical yields of the isolated products were in the range of 38–81% calculated from [¹¹C]carbon monoxide. Starting with 10.7±0.5 GBq of [¹¹C]carbon monoxide, 1‐[4‐(6,7‐dimethoxy‐quinolin‐4‐yloxy)‐3‐fluoro‐phenyl]‐3‐(4‐fluoro‐phenyl)‐[¹¹C]‐urea (2.1 GBq) was isolated after total reaction time of 45 min with a specific activity of 92±4 GBq µmol^?1. Copyright © 2009 John Wiley & Sons, Ltd.     

Fusarinine C,a novel siderophore‐based bifunctional chelator for radiolabeling with Gallium‐68

Fusarinine C (FSC), a siderophore‐based chelator coupled with the model peptide c(RGDfK) (FSC(succ‐RGD)₃), revealed excellent targeting properties in vivo using positron emission tomography (PET). Here, we report the details of radiolabeling conditions and specific activity as well as selectivity for ⁶⁸Ga. ⁶⁸Ga labeling of FSC(succ‐RGD)₃ was optimized regarding peptide concentration, pH, temperature, reaction time, and buffer system. Specific activity (SA) of [⁶⁸Ga]FSC(succ‐RGD)₃ was compared with ⁶⁸Ga‐1,4,7‐triazacyclononane, 1‐glutaric acid‐4,7 acetic acid RGD ([⁶⁸Ga]NODAGA‐RGD). Stability was evaluated in 1000‐fold ethylenediaminetetraacetic acid (EDTA) solution (pH 7) and phosphate‐buffered saline (PBS). Metal competition tests (Fe, Cu, Zn, Al, and Ni) were carried out using [⁶⁸Ga]‐triacetylfusarinine C. High radiochemical yield was achieved within 5 min at room temperature, in particular allowing labeling with ⁶⁸Ga up to pH 8 with excellent stability in 1000‐fold EDTA solution and PBS. The 10‐fold to 20‐fold lower concentrations of FSC(succ‐RGD)₃ led to the same radiochemical yield compared with [⁶⁸Ga]NODAGA‐RGD with SA up to 1.8 TBq/µmol. Metal competition tests showed high selective binding of ⁶⁸Ga to FSC. FSC is a multivalent siderophore‐based bifunctional chelator allowing fast and highly selective labeling with ⁶⁸Ga in a wide pH range and results in stable complexes with high SA. Thus it is exceptionally well suited for the development of new ⁶⁸Ga‐tracers for in vivo molecular imaging with PET.     

Synthesis and biological evaluation of 68Ga‐labeled Pteroyl‐Lys conjugates for folate receptor‐targeted tumor imaging

In order to develop novel ⁶⁸Ga‐labeled PET tracers for folate receptor imaging, two DOTA‐conjugated Pteroyl‐Lys derivatives, Pteroyl‐Lys‐DOTA and Pteroyl‐Lys‐DAV‐DOTA, were designed, synthesized and radiolabeled with ⁶⁸Ga. Biological evaluations of the two radiotracers were performed with FR‐positive KB cell line and athymic nude mice bearing KB tumors. Both ⁶⁸Ga‐DOTA‐Lys‐Pteroyl and ⁶⁸Ga‐DOTA‐DAV‐Lys‐Pteroyl exhibited receptor specific binding in KB cells in vitro. The tumor uptake values of ⁶⁸Ga‐DOTA‐Lys‐Pteroyl and ⁶⁸Ga‐DOTA‐DAV‐Lys‐Pteroy were 10.06 ± 0.59%ID/g and 11.05 ± 0.60%ID/g at 2 h post‐injection, respectively. Flank KB tumor was clearly visualized with ⁶⁸Ga‐DOTA‐DAV‐Lys‐Pteroyl by Micro‐PET imaging at 2 h post‐injection, suggesting the feasibility of using ⁶⁸Ga‐labeled Pteroyl‐Lys conjugates as a novel class of FR targeted probes.     

Novel developed HPLC analyses of [68Ga]Ga/[177Lu]Lu-EDTMP and [68Ga]Ga/[177Lu]Lu-DOTA-Zoledronate

Radiolabeled phosphonates have found widespread applications in nuclear medicine for the treatment of bone metastases. Even though considerable attention has been devoted to the administration of radiolabeled phosphonates to the patients, quality control (QC) studies belong to detection of them are still speculative and need to be improved. Herein, practical, efficient, and more reliable high-performance liquid chromatograpy (HPLC) methods were described for the first time. Radiochemical purity was easily determined by our developed analytical methods. N,N-Dimethylhexylamine and N,N-dimethyltetradecylamine were furnished as suitable ion pair reagents for the identification of radiopharmaceutical products. Effects of column type, pH of the mobile phase, and salt concentration were also examined.     

An improved assay for 68Ga‐hydroxide in 68Ga‐DOTATATE formulations intended for neuroendocrine tumour imaging

The objective of this study was to identify a more rapid assay for ⁶⁸Ga(OH)₃ impurity in ⁶⁸Ga‐DOTATATE formulations. Three methods were used to prepare ⁶⁸Ga(OH)₃ reference material (pharmacopoeial, bench titration and automated radiosynthesis), and four quality control methods for its assessment (thin layer chromatography, membrane filtration, HPLC and solid phase extraction). The optimal method of preparing ⁶⁸Ga(OH)₃ was by titrating ⁶⁸Ga³⁺ with buffered sodium hydroxide solutions to pH 5.6 ± 0.2. The precipitate was quantitatively isolated by membrane filtration (0.02 µm)/hydrochloric acid (HCl; pH 5.6) solvent, and also it remained 100% at the origin on instant thin layer chromatography with silica gel paper/HCl (pH 5.6) solvent. For ⁶⁸Ga‐DOTATATE samples, the thin layer chromatography technique was used with a single paper strip developed separately on two occasions, once in HCl (pH 5.6) and next in methanol solvent. This so‐called double‐developed (DD) method separated ⁶⁸Ga(OH)₃ impurity located at the origin, from ⁶⁸Ga‐DOTATATE plus ⁶⁸Ga³⁺ at ~R_f 0.4, and it was superior to the other methods. It assayed for the impurity similarly to the pharmacopoeial method. The advantages of the DD method were that it required inexpensive test materials and it reproducibly determined % ⁶⁸Ga(OH)₃ in ⁶⁸Ga‐DOTATATE in 12 min, 13 min earlier than the pharmacopoeial method. This time efficiency resulted in a surplus of 12% ⁶⁸Ga‐DOTATATE counts in the product vial, and this provided a contingency of radioactivity or time for the injection/imaging processes in the Nuclear Medicine Department.     

A new 68Ga anionic concentration and purification method for automated synthesis of [68Ga]‐DOTA or NODAGA conjugated peptides in high radiochemical purity

The ⁶⁸Ge/⁶⁸Ga generator is of increasing interest for clinical PET. For successful labelling, the eluate has to be purified. The aim of our approach is to improve the existing anionic methods which have a number of advantages compared to other methods but which use high concentrated HCl, and require an additional anionizing step. A new ⁶⁸Ga‐eluate anionic purification method that enables rapid and high efficiency labelling of DOTA and NODAGA conjugated peptides in high radiochemical purity is described. The new method uses NaCl as an alternative Cl^? source to the corrosive HCl and combines the three standard steps in a single step. The recovery yield was ≥90%, and the ⁶⁸Ge breakthrough was in conformity with the European Pharmacopeia limit. An automated labelling of DOTA and NODAGA‐conjugated peptides was performed with the new method, using acetate sodium buffer, with a total duration of 13 min and a radiochemical yield >85%. The labelled peptides have a radiochemical purity exceeding 99% and can be used directly without any further purification step and without the quality control by gas chromatography. Furthermore, the new method has an economic advantage: it offers the possibility to use generator until 20 months after the calibration date.     

Radiosynthesis of O‐[11C]methyl‐L‐tyrosine and O‐[18F]Fluoromethyl‐L‐tyrosine as potential PET tracers for imaging amino acid transport

Two positron‐emitting analogues of tyrosine, O‐[¹¹C]methyl‐L ‐tyrosine and O‐[¹⁸F]fluoromethyl‐L ‐tyrosine were prepared as new tumor imaging agents. The alkylating agent, [¹¹C]methyl triflate or [¹⁸F]fluoromethyl triflate, was simply bubbled through a dimethylsulfoxide solution of L ‐tyrosine disodium salt at room temperature. After subsequent HPLC purification the labeled L ‐tyrosine analogues were obtained in decay‐corrected radiochemical yields of over 50%, based on their corresponding labeling agent, with radiochemical purities always higher than 98%. The quite straightforward preparation, together with the high radiochemical yields achieved, make both these syntheses suitable for routine production. Copyright © 2003 John Wiley & Sons, Ltd.     

Trifluoromethanesulfonic acid,an alternative solvent medium for the direct electrophilic fluorination of DOPA: new syntheses of 6‐[18F]fluoro‐L‐DOPA and 6‐[18F]fluoro‐D‐DOPA

Previous work from this laboratory has shown that the direct fluorination of 3, 4‐dihydroxy‐phenyl‐L ‐alanine (L ‐DOPA) in anhydrous HF (aHF) or BF₃/HF with F₂ is an efficient method for the synthesis of 6‐fluoro‐L ‐DOPA. Since then, ¹⁸F‐labeled 6‐fluoro‐L ‐DOPA ([¹⁸F]6‐fluoro‐L ‐DOPA) has been used to study presynaptic dopaminergic function in the human brain and to monitor gastrointestinal carcinoid tumors. This work demonstrates that the reactivity and selectivity of F₂ toward L ‐DOPA in CF₃SO₃H is comparable with that in aHF. This new synthetic procedure has led to the production of [¹⁸F]fluoro‐L ‐DOPA and [¹⁸F]fluoro‐D‐DOPA isomers in 17±2% radiochemical yields (decay corrected with respect to [¹⁸F]F₂). The 2‐ and 6‐FDOPA isomers were separated by HPLC and subsequently characterized by ¹⁹F NMR spectroscopy. The corresponding [¹⁸F]‐FDOPA enantiomers have been obtained in clinically useful quantities by a synthetic approach that avoids the use of aHF. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthesis of [18F] 4‐amino‐N‐(3‐chloro‐4‐fluorophenyl)‐N′‐hydroxy‐1,2,5‐oxadiazole‐3‐carboximidamide (IDO5L): a novel potential PET probe for imaging of IDO1 expression

To synthesize ¹⁸F‐labeled positron emission tomography (PET) ligands, reliable labeling techniques inserting ¹⁸F into a target molecule are necessary. The ¹⁸F‐fluorobenzene moiety has been widely utilized in the synthesis of ¹⁸F‐labeled compounds. The present study utilized [¹⁸F]‐labeled aniline as intermediate in [¹⁸F]‐radiolabeling chemistry for the facile radiosynthesis of 4‐amino‐N‐(3‐chloro‐4‐fluorophenyl)‐N′‐hydroxy‐1,2,5‐oxadiazole‐3‐carboximidamide ([¹⁸F]IDO5L) as indoleamine 2,3‐dioxygenase 1 (IDO1) targeted tracer. IDO5L is a highly potent inhibitor of IDO1 with low nanomolar IC₅₀. [¹⁸F]IDO5L was synthesized via coupling [¹⁸F]3‐chloro‐4‐fluoroaniline with carboximidamidoyl chloride as a potential PET probe for imaging IDO1 expression. Under the optimized labeling conditions, chemically and radiochemically pure (>98%) [¹⁸F]IDO5L was obtained with specific radioactivity ranging from 11 to 15 GBq/µmol at the end of synthesis within ~90 min, and the decay‐corrected radiochemical yield was 18.2 ± 2.1% (n = 4).     

Temperature effects on the stereospecificity of nucleophilic fluorination: formation of trans‐[18F]4‐fluoro‐l‐proline during the synthesis of cis‐[18F]4‐fluoro‐l‐proline

Fluorine‐18 labeled (2S,4S)‐4‐fluoro‐l ‐proline (cis‐[¹⁸F]4‐FPro) has been reported to be a potential positron emission tomography tracer to study abnormal collagen synthesis occurring in pulmonary fibrosis, osteosarcomas, mammary and colon carcinomas. In this paper, we report the stereospecific radiofluorination of (2S,4R)‐N‐tert‐butoxycarbonyl‐4‐(p‐toluenesulfonyloxy) proline methyl ester (at 110°C) to produce diastereomerically pure cis‐[¹⁸F]4‐FPro in 38% radiochemical yield at the end of a 90‐min synthesis. Investigation of the effect of temperature on the stereospecificity of nucleophilic fluorination showed that diasteriomerically pure cis‐[¹⁸F]4‐FPro or trans‐[¹⁸F]4‐FPro was produced at lower temperatures (85°C–110°C) during the fluorination of (2S,4R) or (2S,4S) precursors, respectively. However, at higher temperatures (130°C–145°C), fluorination of (2S,4R) precursor produced a mixture of cis‐[¹⁸F]4‐FPro and trans‐[¹⁸F]4‐FPro diastereomers with cis‐[¹⁸F]4‐FPro as the predominant isomer. Hydrolysis of the purified fluorinated intermediate was carried out either in one step, using 2 m triflic acid at 145°C for 10 min, or in two steps where the intermediate was heated in 1 m HCl at 110°C for 10 min followed by stirring at room temperature in 1 N NaOH for 5 min. The aqueous hydrolysis mixture was loaded onto an anion exchange column (acetate form for one‐step hydrolysis) or an ion retardation column (two‐step hydrolysis) followed by a C₁₈ Sep‐Pak® (Waters Corporation, Milford, MA, USA). Pure cis‐[¹⁸F]4‐FPro was then eluted with sterile water. We also report that epimerization of cis‐[¹⁸F]4‐FPro occurs during the two‐step hydrolysis (H⁺ followed by OH^?) of the intermediate, resulting in 5 ± 3% trans‐[¹⁸F]4‐FPro, whereas the one‐step acid hydrolysis yielded pure cis‐[¹⁸F]4‐FPro in the final product. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis and in vivo evaluation of gallium‐68‐labeled glycine and hippurate conjugates for positron emission tomography renography

The objective of this study was to evaluate four new ⁶⁸Ga‐labeled 1,4,7,10‐cyclododeca‐1,4,7,10‐tetraacetic acid (DOTA)/1,4,7‐triazacyclononane‐1,4,7‐triacetic acid derived (NODAGA)‐glycine/hippurate conjugates and select a lead candidate for potential application in positron emission tomography (PET) renography. The non‐metallated conjugates were synthesized by a solid phase peptide synthesis method. The ⁶⁸Ga labeling was achieved by reacting an excess of the non‐metallated conjugate with ⁶⁸GaCl₄^? at pH ?4.5 and 10‐min incubation either at room temperature for NODAGA or 90 °C for DOTA. Radiochemical purity of all ⁶⁸Ga conjugates was found to be >98%. ⁶⁸Ga‐NODAGA‐glycine displayed the lowest serum protein binding (0.4%) in vitro among the four ⁶⁸Ga conjugates. Biodistribution of ⁶⁸Ga conjugates in healthy Sprague Dawley rats at 1‐h post‐injection revealed an efficient clearance from circulation primarily through the renal–urinary pathway with <0.2% of injected dose per gram remaining in the blood. The kidney/blood and kidney/muscle ratios of ⁶⁸Ga‐NODAGA‐glycine were significantly higher than other ⁶⁸Ga conjugates. On the basis of these results, ⁶⁸Ga‐NODAGA‐glycine was selected as the lead candidate. ⁶⁸Ga‐NODAGA‐glycine PET renograms obtained in healthy rats suggest ⁶⁸Ga‐NODAGA‐glycine as a PET alternate of ^99mTc‐Diethylenetriaminepentaacetic acid (DTPA).     

Bis(Methylpyridine)‐EDTA Derivative as a Potential Ligand for PET Imaging: Synthesis,Complexation, and Biological Evaluation

A novel transitional metal ligand derivatized from EDTA‐conjugated 2‐amino‐4‐methyl pyridine, an acyclic vehicle (EDTA‐Mepy₂) was designed, synthesized, and characterized for PET imaging with ⁶⁸Ga. The drug likeliness and appropriate lipophilicity were first analyzed by molecular docking studies which shows interactive property of ligand with serum albumin protein (HSA: PDB 1E78), at Lys199, Arg257, and His242 residues, which make it more appropriate in transportation as a specific ligand for PET imaging. As a confirmation, binding constant of the ligand with human serum albumin was calculated at λ_ex = 350 nm which was found to be 4.9 × 10³ m ^?1. The pharmacokinetics of ⁶⁸Ga‐EDTA‐Mepy₂ was analyzed by blood kinetics (t_1/2 slow: 3 h 56 min and t_1/2 fast: 32 min) and biodistribution (maximum%ID/g was found in kidney at 1 h). Further the capability of this ligand was analyzed as optical marker also, by recording λ_ex = 380 nm, RFU = 8000; 710 nm, RFU = 1000 units at fixed λ_em = 280 nm. Additionally, in physiological conditions where its stability was calculated, suggests 15–20 times selectivity over the endogenously present metal ions (K_GaL/K_ZnL = 14.3, K_GaL/K_CuL = 18.1).     

Modified PEG‐anilinoquinazoline derivatives as potential EGFR PET agents

Inhibition of epidermal growth factor receptor tyrosine kinase (EGFR‐TK) has emerged as a major approach for cancer‐targeted therapy. Consequently, there has been a great interest in the use of labeled EGFR‐TK inhibitors as positron emission tomography (PET) imaging agents. Currently, the developed agents did not yield adequate PET imaging of animal models probably due to poor solubility, rapid washout from blood, and low stability in vivo. In order to overcome these hurdles, new derivatives of previously reported inhibitors (ML04, 2) with decreased log P and increased solubility were designed and synthesized. These compounds (3–5) exhibited high autophosphorylation inhibitory potency with an IC₅₀ of 5–35 nM, decreased log P's (3.1, 3.34, and 3.45, respectively), and significantly increased solubility (630, 300, and 120 µg/mL, respectively) relative to the previously reported parent compound 2 (log P=3.7, solubility=3.5 µg/mL). The labeling of compound 5 with [¹⁸F] and compounds 3 and 4 with [¹¹C] and [¹²⁴I], respectively, involved a one‐step radiosynthesis. Compounds 3–5 were obtained with a total decay‐corrected radiochemical yields of 13, 31, and 5%, respectively, and were found to be stable in blood. The positive outcome achieved with compounds 3–5 merits further in vivo evaluation as PET bioprobes. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis of (R)‐ and (S)‐[O‐methyl‐11C]N‐[2‐[3‐(2‐cyano‐phenoxy)‐2‐hydroxy‐propylamino]‐ethyl]‐N′‐(4‐methoxy‐phenyl)‐urea as candidate high affinity β1‐adrenoceptor PET radioligands

Molecular imaging and quantification of myocardial β₁‐adrenoceptor (AR) rather than total β‐AR density is of great clinical interest since cardiac biopsy studies suggest that myocardial β₁‐AR density is reduced in patients with chronic heart failure whereas cardiac β₂‐AR density may vary. Positron emission tomography (PET), with appropriate radioligands, offers the possibility to assess β‐AR density non‐invasively in humans. However, no PET radioligand for the selective imaging of cardiac β₁‐ARs is clinically available. Here some derivatives of the well characterized β₁‐AR selective antagonist, ICI 89,406, namely the enantiomers of N‐[2‐[3‐(2‐cyano‐phenoxy)‐2‐hydroxy‐propylamino]‐ethyl]‐N′‐(4‐hydroxy‐phenyl)‐urea ( 5a and 5b ) were synthesized and evaluated in vitro. The (R)‐isomer 5a was more β₁‐selective but has lower affinity than its (S)‐enantiomer 5b (β₁‐AR selectivity: 6100 vs 1240; β₁‐affinity: K₁ = 0.288 nM vs K₁ = 0.067 nM). Etherification of the analogous desmethyl precursors, 5e and 5f , respectively, with [¹¹C]iodomethane gave ¹¹C‐labelled versions of 5a and 5b , namely 5g and 5h , in 44 ± 5% radiochemical yield (decay‐corrected) and 97.4 ± 1.3% radiochemical purity with specific radioactivities of 26.4 ± 9.4 GBq/µmol within 41.2 ± 3.4 min from the end of bombardment (n = 14). 5g and 5h are now being evaluated as candidate radioligands for myocardial β₁‐ARs. Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis of [18F]‐labeled adenosine analogues as potential PET imaging agents

The syntheses of adenosine analogues, 2′‐deoxy‐2′‐[¹⁸F]fluoro‐9‐β‐D ‐arabinofuranosyladenine ([¹⁸F]‐FAA) and 3′‐deoxy‐3′‐[¹⁸F]fluoro‐9‐β‐D ‐xylofuranosyladenine ([¹⁸F]‐FXA) are reported. Adenosine ( 1 ) was converted to its methoxytrityl derivatives 2 and 3 as a mixture. After separation, these derivatives were converted to their respective triflates 4 and 5 . Each triflate was reacted with tetrabutylammonium[¹⁸F]fluoride to produce 6b or 7b , which by acidic hydrolysis yielded compounds 8b and 9b . Crude preparations were purified by HPLC to obtain the desired pure products. The radiochemical yields were 10‐18% decay corrected (d. c.) for 8b and 30‐40% (d. c.) for 9b in 4 and 3 runs, respectively. Radiochemical purity was >99% and specific activity was >74 GBq/μmol at the end of synthesis (EOS). The synthesis time was 90‐95 min from the end of bombardment (EOB). Copyright © 2003 John Wiley & Sons, Ltd.     

Synthesis of 3′‐deoxy‐3′‐[18F]fluoro‐1‐β‐D‐xylofuranosyluracil ([18F]‐FMXU) for PET

The synthesis of a pyrimidine analog, 3′‐deoxy‐3′‐[¹⁸F]‐fluoro‐1‐β‐D ‐xylofuranosyluracil ([¹⁸F]‐FMXU) is reported. 5‐Methyluridine 1 was converted to its di‐methoxytrityl derivatives 2 and 3 as a mixture. After separation the 2′,5′‐di‐methoxytrityluridine 2 was converted to its 3′‐triflate 4 followed by derivatization to the respective N³‐t‐Boc product 5 . The triflate 5 was reacted with tetrabutylammonium[¹⁸F]fluoride to produce 6 , which by acid hydrolysis yielded compound 7 . The crude preparation was purified by HPLC to obtain the desired product [¹⁸F]‐FMXU. The radiochemical yields were 25–40% decay corrected (d. c.) with an average of 33% in four runs. Radiochemical purity was >99% and specific activity was >74 GBq/µmol at the end of synthesis (EOS). The synthesis time was 67–75 min from the end of bombardment (EOB). Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis and PET evaluation of (R)‐[S‐methyl‐11C]thionisoxetine,a candidate radioligand for imaging brain norepinephrine transporters

Introduction: (R)‐3‐(2‐(methylthio)phenoxy)‐N‐methyl‐3‐phenylpropan‐1‐amine [(R)–thionisoxetine; 1 ] is a potent inhibitor of the norepinephrine transporter (NET). We aimed to label 1 with carbon‐11 (t_1/2 = 20.4 min) for evaluation as a radioligand for imaging NET in living brain with positron emission tomography (PET). Methods: Methyl 3‐(2‐((R)‐3‐(methylamino)‐1‐phenylpropoxy)phenylthio)‐propanoate (MPPP) and 1 were each prepared from o‐hydroxythiophenol in three steps. Treatment of MPPP with potassium t‐butoxide and [¹¹C]methyl iodide in tetrahydrofuran gave [S‐methyl‐¹¹C]thionisoxetine ([¹¹C] 1 ), which was purified with HPLC. The distribution of radioactivity in brain after intravenous injection of [¹¹C] 1 into cynomolgus monkey was followed with PET and the appearance of radiometabolites in plasma monitored with radio‐HPLC. Results: [¹¹C] 1 was obtained in high yield from [¹¹C]methyl iodide. Of the radioactivity injected into monkey, 2.4% entered brain. Ratios of radioactivity in thalamus, mesencephalon, occipital cortex and caudate to that in cerebellum at 93 min were 1.3, 1.2, 1.2 and 1.1, respectively. The radioactivity in plasma corresponding to unchanged radioligand decreased to 53% at 45 min, with the remainder represented by hydrophilic radiometabolites. Conclusions: MPPP is an effective precursor for ¹¹C‐methylation to [¹¹C] 1 , suggesting that the S‐γ‐propionic acid methyl ester protecting group may have wider value in the ¹¹C‐labeling of aryl methyl sulfides. However, the relatively low ratios of radioactivity to the cerebellum together with an unexpected accumulation of radioactivity in the caudate, makes [¹¹C] 1 an unpromising NET radioligand. Copyright © 2006 John Wiley & Sons, Ltd.     

Microwave accelerated 68Ga‐labelling of oligonucleotides

Oligonucleotides are extensively used for characterization of gene expression in vitro and have now been studied as inhibitors of gene expression in vivo in various diseases. Labelled antisense oligonucleotides are therefore of potential interest for possible in vivo imaging of gene expression, considering the biology of tumors and applications in designing novel molecule‐targeted therapies. In the present work a method of microwave accelerated ⁶⁸Ga‐labelling of oligonucleotides and analysis of the resulting tracers are described. Four modified and functionalized 17‐mer oligonucleotides with a hexylamine group in the 3′‐ or 5′‐position were studied. The oligonucleotides were conjugated to the bifunctional chelator, 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA), and then labelled with ⁶⁸Ga(T_1/2=68 min) using microwave activation. The isolated decay‐corrected radiochemical yields ranged from 30 to 52%. Labelled products were stable in water and ethanol for more than 4 h. The impact of the labelling procedure on the oligonucleotide probes was investigated using hybridization to a complementary 17‐mer sense oligonucleotide in solution. Chemical modification did not influence either the labelling or hybridization ability of the oligonucleotides. The radiolabelled oligonucleotides will be used for the further in vitro and in vivo biology studies. Copyright © 2003 John Wiley & Sons, Ltd.