Visualization of gene expression in the live subject using the Na/I symporter as a reporter gene: applications in biotherapy

Abstract

Biotherapies involve the utilization of antibodies, genetically modified viruses, bacteria or cells for therapeutic purposes. Molecular imaging has the potential to provide unique information that will guarantee their biosafety in humans and provide a rationale for the future development of new generations of reagents. In this context, non-invasive imaging of gene expression is an attractive prospect, allowing precise, spacio-temporal measurements of gene expression in longitudinal studies involving gene transfer vectors. With the emergence of cell therapies in regenerative medicine, it is also possible to track cells injected into subjects. In this context, the Na/I symporter (NIS) has been used in preclinical studies. Associated with a relevant radiotracer ((123)I(-), (124)I(-), (99m)TcO4(-)), NIS can be used to monitor gene transfer and the spread of selectively replicative viruses in tumours as well as in cells with a therapeutic potential. In addition to its imaging potential, NIS can be used as a therapeutic transgene through its ability to concentrate therapeutic doses of radionuclides in target cells. This dual property has applications in cancer treatment and could also be used to eradicate cells with therapeutic potential in the case of adverse events. Through experience acquired in preclinical studies, we can expect that non-invasive molecular imaging using NIS as a transgene will be pivotal for monitoring in vivo the exact distribution and pharmacodynamics of gene expression in a precise and quantitative way. This review highlights the applications of NIS in biotherapy, with a particular emphasis on image-guided radiotherapy, monitoring of gene and vector biodistribution and trafficking of stem cells.

Figure 1

Schematic representation of the principle of image-guided radiotherapy mediated by NIS gene transfer in the living animal. Viral or synthetic gene transfer vectors encoding hNIS are injected intravenously into tumour-bearing mice. For kinetic studies, mice are anaesthetized, injected intravenously with radioiodide [¹²⁴I] and positioned in a µPET/CT scanner. PET images are obtained from various acquisition times depending on the specificity of radioactivity levels in mouse. A CT scan is taken at the same time. The reconstruction of images and fusion between PET and CT images are processed with adequate software such MEDISO and PMOD (Medical Imaging Systems). Final images are shown in coronal, axial or sagittal view in the plane of the tumour or, alternatively, as seen in this figure in three-dimensions (3D). Coloured dot points reveal radiotracer uptake in the whole body of mice while CT images allow the precise determination of the localization of the radiotracer emission source. In this example, radiotracer uptakes is detectable in the thyroid (Thy) and the stomach (St) as the result of endogenous expression of mNIS, and in the tumour (Tu) as the result of hNIS gene transfer. The kinetics of hNIS expression at the tumour site could be determined by quantification of the radiotracer activity in tumour tissues at different times in a longitudinal study carried out on the same animal. When expression of hNIS in the tumour is found to be maximal, a dosimetry study is then performed. Dosimetric calculation determines accurately the minimal therapeutic dose to administrate for selectively irradiated the tumour volume with minimal side effects. Such protocol fully validates the principle of individualized image-guided radiotherapy.