Fluorine-18 radiolabeling of a nitrophenyl sulfoxide and its evaluation in an SK-RC-52 model of tumor hypoxia.

The significance of imaging hypoxia with the positron emission tomography ligand [(18) F]FMISO has been demonstrated in a variety of cancers. However, the slow kinetics of [(18) F]FMISO require a 2-h delay between tracer administration and patient scanning. Labeled chloroethyl sulfoxides have shown faster kinetics and higher contrast than [(18) F]FMISO in a rat model of ischemic stroke. However, these nitrogen mustard analogues are unsuitable for routine production and use in humans. Here, we report on the synthesis and in vitro and in vivo evaluation of a novel sulfoxide, which contains an ester moiety for hydrolysis and subsequent trapping in hypoxic cells. Non-decay corrected yields of radioactivity were 1.18 ± 0.24% (n = 27, 2.5 ± 0.5% decay corrected radiochemical yield) based on K[(18) F]F. The radiotracer did not show any defluorination and did not undergo metabolism in an in vitro assay using S9 liver fractions. Imaging studies using an SK-RC-52 tumor model in BALB/c nude mice have revealed that [(18) F]1 is retained in hypoxic tumors and has similar hypoxia selectivity to [(18) F]FMISO. Because of a three times faster clearance rate than [(18) F]FMISO from normoxic tissue, [(18) F]1 has emerged as a promising new radiotracer for hypoxia imaging.

Fully automated synthesis and coupling of [(18) F]FBEM to glutathione using the iPHASE FlexLab module.

Site-specific radiolabelling of peptides or antibodies using [(18) F]FBEM is often preferred over non-site-specific radiolabelling with [(18) F]SFB because it does not affect the affinity of the antibody to its target. Unfortunately, the synthesis of [(18) F]FBEM and its conjugation to thiol containing macromolecules requires some manual intervention, which leads to radiation exposure of the radiochemist. In this publication, we report on the complete automation of [(18) F]FBEM production and its subsequent conjugation to glutathione using a slightly modified iPHASE FlexLab module. [(18) F]FBEM was produced in 1.185 ± 0.168 GBq (15-20%; n = 10; 0.75 ± 0.106 GBq non-decay corrected) with a specific activity of 57 ± 10 GBq/µmol. Radiochemical purity was 97 ± 1% and the synthesis time including HPLC purification and reformulation was 70 min. After evaporation to dryness, [(18) F]FBEM was conjugated to glutathione in PBS buffer pH 7.4 in quantitative yields. This fully automated method does not require any manual intervention and therefore reduces the radiation exposure to the operator.

Hyperconjugation involving strained carbon-carbon bonds. structural analysis of ester and ether derivatives and one-bond 13C-13C coupling constants of α- and ß-nopinol.

σ(C-C)-σ*(C-O) interactions involving the strained carbon-carbon bonds of α- and ß-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C-OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C-C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the (13)C-(13)C one-bond coupling constants. For the donor C-C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C-C bond coupling constants increase, consistent with bond strengthening, as the electron demand of OR increases.

Improved synthesis of [(18)F]FLETT via a fully automated vacuum distillation method for [(18)F]2-fluoroethyl azide purification.

The synthesis of [(18)F]2-fluoroethyl azide and its subsequent click reaction with 5-ethynyl-2'-deoxyuridine (EDU) to form [(18)F]FLETT was performed using an iPhase FlexLab module. The implementation of a vacuum distillation method afforded [(18)F]2-fluoroethyl azide in 87±5.3% radiochemical yield. The use of Cu(CH3CN)4PF6 and TBTA as catalyst enabled us to fully automate the [(18)F]FLETT synthesis without the need for the operator to enter the radiation field. [(18)F]FLETT was produced in higher overall yield (41.3±6.5%) and shorter synthesis time (67min) than with our previously reported manual method (32.5±2.5% in 130min).

Detection of activated platelets in a mouse model of carotid artery thrombosis with 18 F-labeled single-chain antibodies.

INTRODUCTION: Activated platelets are key players in thrombosis and inflammation. We previously generated single-chain antibodies (scFv) against ligand-induced binding sites (LIBS) on the highly abundant platelet glycoprotein integrin receptor IIb/IIIa. The aim of this study was the construction and characterisation of a novel (18)F PET radiotracer based on this antibody. METHODS: ScFv(anti-LIBS) and control antibody mut-scFv were reacted with N-succinimidyl-4-[(18)F]fluorobenzoate (S[(18)F]FB). Radiolabeled scFv was incubated with in vitro formed platelet clots and injected into mice with FeCl(3) induced thrombus in the left carotid artery. Clots were imaged in the PET scanner and amount of radioactivity measured using an ionization chamber and image analysis. Assessment of vessel injury as well as the biodistribution of the radiolabeled scFv was studied. RESULTS: After incubation with increasing concentrations of (18)F-scFv(anti-LIBS) clots had retained significantly higher amounts of radioactivity compared to clots incubated with radiolabeled (18)F-mut-scFv (13.3 ± 3.8 vs. 3.6 ± 1 KBq, p < 0.05, n = 9, decay corrected). In the in vivo experiments we found an high uptake of the tracer in the injured vessel compared with the non-injured vessel, with 12.6 ± 4.7% injected dose per gram (ID/g) uptake in the injured vessel and 3.7 ± 0.9% ID/g in the non-injured vessel 5 minutes after injection (p < 0.05, n = 6). CONCLUSIONS: Our results show that the novel antibody radiotracer (18)F-scFv(anti-LIBS) is useful for the sensitive detection of activated platelets and thrombosis. ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: We describe the first (18)F variant of a scFv(anti-LIBS) against activated platelets. This diagnostic agent could provide a powerful tool for the assessment of acute thrombosis and inflammation in patients in the future.

Radiolabelling and evaluation of a novel sulfoxide as a PET imaging agent for tumor hypoxia.

[¹8F]FMISO is the most widely validated PET radiotracer for imaging hypoxic tissue. However, as a result of the pharmacokinetics of [¹8F]FMISO a 2h wait between tracer administration and patient scanning is required for optimal image acquisition. In order to develop hypoxia imaging agents with faster kinetics, we have synthesised and evaluated several F-18 labelled anilino sulfoxides. In this manuscript we report on the synthesis, in vitro and in vivo evaluation of a novel fluoroethyltriazolyl propargyl anilino sulfoxide. The radiolabelling of the novel tracer was achieved via 2-[¹8F]fluoroethyl azide click chemistry. Radiochemical yields were 23 ± 4% based on 2-[¹8F]fluoroethyl azide and 7 ± 2% based on K[¹8F]F. The radiotracer did not undergo metabolism or defluorination in an in vitro assay using S9 liver fractions. Imaging studies using SK-RC-52 tumors in BALB/c nude mice have indicated that the tracer may have a higher pO2 threshold than [¹8F]FMISO for uptake in hypoxic tumors. Although clearance from muscle was faster than [¹8F]FMISO, uptake in hypoxic tumors was slower. The average tumor to muscle ratio at 2h post injection in large, hypoxic tumors with a volume greater than 686 mm³ was 1.7, which was similar to the observed ratio of 1.75 for [¹8F]FMISO. Although the new tracer showed improved pharmacokinetics when compared with the previously synthesised sulfoxides, further modifications to the chemical structure need to be made in order to offer significant in vivo imaging advantages over [¹8F]FMISO.

Radiolabelling and evaluation of novel haloethylsulfoxides as PET imaging agents for tumor hypoxia.

The significance of imaging hypoxia with the PET ligand [(18)F]FMISO has been demonstrated in a variety of cancers. However, the slow kinetics of [(18)F]FMISO require a 2-h delay between tracer administration and patient scanning. Labelled chloroethyl sulfoxides have shown faster kinetics and higher contrast than [(18)F]FMISO in a rat model of ischemic stroke. However, these nitrogen mustard analogues are unsuitable for routine production and use in humans. Here we report on the synthesis and in vitro and in vivo evaluation of two novel sulfoxides which we synthesised from a single precursor molecule via either 2-[(18)F]fluoroethyl azide click chemistry or conventional nucleophilic displacement of a chloride leaving group. The yields of the click chemistry approach were 90±5% of [(18)F]2 based on 2-[(18)F]fluoroethyl azide, and the yields for the S(N) reaction were 15±5% of [(18)F]1 based on K[(18)F]F. Both radiotracers underwent metabolism in an in vitro assay using S9 liver fractions with biological half-lives of 32.39 and 43.32 min, respectively. Imaging studies using an SK-RC-52 tumor model in BALB/c nude mice have revealed that only [(18)F]1 is retained in hypoxic tumors, whereas [(18)F]2 is cleared from those tumors at a rate similar to that of muscle tissue. [(18)F]1 has emerged as a promising new lead structure for further development of sulfoxide-based hypoxia imaging agents. In particular, the mechanism of uptake needs to be elucidated and changes to the chemical structure need to be made in order to reduce metabolism and improve radiotracer kinetics.