Novel Dual PET and Fluorescent Labelling Reagents for Multiscale Cell Tracking

Emerging as the fourth pillar of healthcare, cell-based immunotherapies offer a novel and rapidly developing technology that has great potential to ameliorate human disease. Exemplifying this, chimeric antigen receptor (CAR) technology is increasingly being harnessed for the treatment of cancer and other disease types.1 Maher (biomedical/clinical co-supervisor) developed a CAR named T1E28z that targets 8 of 9 ErbB homo- and heterodimers upregulated in several solid tumours.2 Immunotherapy using T1E28z-engineered T-cells is currently undergoing Phase I clinical trial evaluation in patients with locally advanced/ recurrent head and neck squamous cell carcinoma (clinicaltrials.gov NCT01818323).3 One fundamental challenge in both medical research and clinical applications of cell therapy is to understand the in vivo behaviours of the infused cells. Imaging studies can dynamically track the migration, proliferation, and final fate of the administered cells. It will provide definitive evidence of successful targeting and allow quantification of the degree of cell migration to the target and non-target sites. Thus, it is vital to incorporate cell tracking studies at the earliest stage of preclinical and clinical development of such experimental treatments to provide early insight into their safety, mechanism of action, and efficacy. 4

Imaging methods employed single modality such as positron emission tomography (PET), MRI, or fluorescence etc. have been explored to dynamically track the persistence, migration, and proliferation of the infused therapeutic cells.4 However, our understanding of the fate of the therapeutic cells in vivo remains limited because no single imaging modality meets the need for high resolution, high sensitivity, and deep tissue penetration. The ability to study the in vivo behaviour of the administered cells across cellular to macroscopic scales therefore remains a fundamental goal for cell tracking. Cell labelling reagents equipped with dual reporters can overcome some of these restraints by allowing combinational use of the strengths from both imaging modalities.5 For example, PET is highly sensitive and non-invasive. It can produce real-time images of the radiolabelled cells and quantitatively measure their whole-body distribution without the limitation of detection depth. By selecting an appropriate radionuclide, sequential images can be acquired over a clinically relevant timeframe. Meanwhile, the distribution of the labelled cells in the target and non-target tissues can be examined by ex vivo high-resolution fluorescence imaging.
This interdisciplinary collaborative project aims to develop a novel dual PET and fluorescent labelling reagent to track the therapeutic cells from the whole body to the microscopic scale. The commercially available iodine-124 will be employed as it has the longest half-life among the clinically used PET radioisotopes. In principle, iodine-124 can provide the longest possible tracking time for directly labelled cells with PET. In addition, the thyroid and stomach uptake of any free iodine-124 generated through the catabolism of the labeling reagent can be readily blocked by the potassium iodide pre-treatment. Thus, it will provide a low background to visualise the labelled cells. However, the major challenge to use iodine-124 based organic bioconjugation reagents for cell labelling is how to overcome the deiodinases mediated deiodination. As the deiodinases mainly present intracellularly, we envisage that the deiodination would be minimised by coupling the iodine-124 based dual labelling reagent on the cell surface.