Synthesis and biological analysis of novel Fluorine-18 positron emission tomography (PET) imaging agents for hypoxic tissues in tumours

© 2012 Dr. Evelyn LaurensThe significance of imaging hypoxia with the PET ligand [18F]FMISO (1.1) has been demonstrated in a variety of cancers. However, as a result of the slow kinetics of [18F]FMISO (1.1), a 2 h delay between tracer administration and patient scanning is required. Labelled chloroethyl sulfoxides have shown faster kinetics and higher contrast than [18F]FMISO (1.1) in a rat model of ischemic stroke. However, these nitrogen mustard analogues are unsuitable for routine production and use in humans. Development of new PET imaging agents was achieved by attempting structural modifications of parent sulfoxides. Five novel radiotracers [18F]1.18, [18F]1.20, [18F]1.21, [18F]1.23 and [18F]1.25 representing various of structural modifications of the parent sulfoxide-based imaging agents were synthesized. These modifications included the introduction of the propargyl group, acetic ester group and trizole-ethyl group to the parent sulfoxides. Radiotracers [18F]1.18, [18F]1.20, [18F]1.21, [18F]1.23 and [18F]1.25 were successfully prepared (with RCY of 2.5%, 15%, 28%, 2.5%, and 3.5% respectively) from the corresponding precursor molecules 1.17, 1.19, 1.22 and 1.24 either via 2-[18F]fluoroethyl azide click chemistry or conventional nucleophilic displacement of a chloride leaving group. Radiotracers [18F]1.20 and [18F]1.21 were synthesized from a single precursor compound 1.19, utilizing both radiolabelling methods. In-vitro and in-vivo evaluation of radiotracers [18F]1.18, [18F]1.20, [18F]1.21, [18F]1.23 and [18F]1.25 were accomplished. Only radiotracer [18F]1.25 was stable in both mice and rat S9 liver fraction during phase I aerobic metabolism studies. Phase I aerobic metabolism studies of tracer [18F]1.20 and [18F]1.21 showed that both tracers were stable in mice S9 liver fraction but undergo metabolism in rat S9 liver fraction, with biological half-lives of 32.39 min and 43.32 min respectively. While, tracer [18F]1.23 underwent minor phase I metabolism in mice and rat S9 liver fractions. Imaging studies using an SK-RC-52 tumour model in BALB/c nude mice have revealed that [18F]1.23 is the most promising hypoxia radiotracer in the series. This tracer shown high t/m ratio of 3.3 in large sized tumours with a pO2 value less than 5 mmHg. Thus, [18F]1.23 has emerged as an exciting new lead compound for further development. Radiotracers [18F]1.18 and [18F]1.20 are also retained in hypoxic tissues at highly hypoxic condition and mildly hypoxic conditions respectively, showing different hypoxia selectivity. [18F]1.18 and [18F]1.20 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. Whereas [18F]1.21 and [18F]1.25 are cleared from those tumours at a rate similar to that of muscle tissue, showing great promise in future development of hypoxic radiotracer. Investigation of new PET imaging agent with diazobenzene as a bioreducible functional group, replacing the sulfoxide moiety was achieved and radiotracer [18F]1.27 was synthesized from precursor 1.26 in RCY 6.6%. Radiotracer [18F]1.27 underwent metabolism in faster rate in rat liver fraction in comparison to mice liver fraction, with the biological half-life of [18F]1.27 being 118.5 min for rat S9 liver fraction, while the biological half-life in mice S9 liver fraction was calculated to pass the 2 h incubation period (139 min). The imaging studies of [18F]1.27 shows some promises as a platform for new development of hypoxia radiotracer. However, structure modification of this aniline yellow derivative needs to be undertaken to improve this derivative further towards hypoxia imaging agents