Novel PET Tracers

A significant late side effect in lung and breast radiotherapy is the development of fibrosis and a common and debilitating acute toxicity associated with lung radiotherapy is pneumonitis, caused by radiation-induced inflammation. Radiotherapy generates reactive oxygen species (ROS), resulting in oxidative damage that kills tumour cells but also affects surrounding tissues. A current focus is the secondary inflammatory processes that influence outcome. This can lead to angiogenesis or targeted immune response, depending on which cells dominate the inflammatory response. Drug modulation of this response has exciting therapeutic potential and is being actively pursued by our collaborating groups in Manchester.

Currently, there are no sensitive imaging biomarkers for oxidative damage. ROS form transient free radical sites on proteins, lipids or nucleic acids, impairing biochemical function and potentially leading to cell death. Free radical scavengers bind very rapidly to these free radical sites and development of positron emission tomography (PET) or single photon emission computed tomography (SPECT) tracers based on these molecules will allow, therefore, the distribution and concentration of free radical sites in tissues to be imaged. Imaging of these sites would be a valuable tool in radiotherapy planning.

 

There is a large variety of potential tracer molecules for detecting radical species, a selection of these will be radiolabelled at Manchester as potential radiotracers. Imaging radical species in vitro and in vivo has been demonstrated by electron spin resonance (ESR) spectroscopy, magnetic resonance imaging and spin immuno-histochemistry, using stable nitroxide radicals. These studies support the proposed mechanism of action of these nitroxides, which have not shown significant toxicity. However, ESR lacks sensitivity, requiring administration of very high doses and is totally insensitive to the closed electron shell protein-, lipid- and nucleic acid- spin scavenger adducts that are the interesting biomarkers of oxidative damage. Radio-labelling these same molecules for in vivo PET or SPECT will allow free radical damage to be imaged at tracer doses, whilst unbound tracer and tracer adducts are cleared.

The mechanism of binding of radical scavengers or radical traps to immobilised free radicals has been demonstrated by mass spectrometry (MS) and  the proposed mechanism of action of the radical scavenger TEMPO has been confirmed in a superoxide dismutase knockout rat model and shown that its rate of reaction is sufficiently high to react with the transient protein-bound radical sites. We have shown increased binding of a fluorescently-tagged radical scavenger following x-ray irradiation, or doxorubicin treatment of cells in culture. Although focusing here on ROS-damage in tumour cells, clearly monitoring of deposition within adjacent normal tissues would have clinical utility in monitoring of damage in normal tissues.

We propose to use labelled antibody tracers to allow PET imaging of the inflammatory responses following the initial ROS insult. Manchester is currently developing methods to radiolabel an antibody fragment specific for receptor sites on activated macrophages. We have access to antibody fragments specifically developed for imaging and will apply our existing [18F]fluoroacetaldehyde chemistry expertise to  label these with [89Zr] labelling for slower-clearing macromolecular antibodies. 

The use of these antibody tracers would complement existing programmes in Manchester investigating inflammatory processes that use neutrophil labelling and use small molecule tracers binding to the TSPO receptor (formerly known as the peripheral benzodiazepine receptor, PBR) which is expressed in activated macrophages and microglia.

The overall objective is to perform a first in man experiment to investigate tracer use in irradiated lung tumours. This will build on Manchester’s extensive experience in developing GMP PET tracers for clinical application.

 

 

Dr Adam McMahon

Head of Analytical Chemistry at the Wolfson Molecular Imaging Centre at The University of Manchester.



Dr Franklin Aigbirhio

Director of PET Chemistry at the Wolfson Brain Imaging Centre.


Dr Marie-Claude Asselin

Senior Lecturer in Quantitative Imaging at the Wolfson Molecular Imaging Centre at The University of Manchester.


 


Professor Tim Illidge

Chair in Targeted Therapy at The University of Manchester.




Dr Dmitry Soloviev

Senior Staff Scientist at Cancer Research UK Cambridge Institute


Professor Kaye Williams

Head of the Hypoxia and Therapeutics group in the Manchester Pharmacy School at The University of Manchester