PET imaging for gene & cell therapy

As the interest in gene therapy increases, the development of an efficient and reliable means to monitor gene delivery and expression in patients is becoming more important. An ideal imaging modality would be non-invasive, allowing for repeated imaging, thus validating stages subsequent to vector administration and allowing for the improvement of clinical protocols. Positron Emission Tomography (PET) has been employed for some time in clinical imaging and has in more recent years been adapted to enable imaging in small animal models, including gene therapy models for a range of diseases. PET imaging is based on the detection of trace quantities of positron-emitting molecular probe within cells postadministration, permitting imaging of target molecules in vivo, and numerous tracers have been developed for a wide range of applications, including imaging of reporter gene activity. Use of radiolabelled substrates that interact with specific transgene proteins, has identified a number of reporter genes that are suitable for imaging vector mediated gene delivery and expression in both pre-clinical and clinical situations. These reporter genes enable non-invasive analysis of the location, level and kinetics of transgene activity. Among the various imaging modalities in existence, the PET approach displays arguably the optimum characteristics in terms of sensitivity and quantitation for in vivo gene expression measurements. Given the existing availability of PET scanning equipment and expertise in hospitals, this imaging modality represents the most clinically applicable means of analysing gene therapy in patients. This review outlines the principles of PET imaging in the context of gene and cell therapy at both pre-clinical and clinical levels, comparing PET with other relevant modalities, and describes the progress to date in this field.