One of the major goals of cell biology is to investigate the dynamic interactions of molecules in a living cell as they execute the reactions of a particular metabolic pathway. Until recently, the experimental methods used in this research were based on in vitro studies of molecular interactions in model systems, such as aqueous solutions, and isolated or artificial membranes. Alternatively, they relied on data obtained at certain time points in the respective process by disrupting cells and analyzing the resulting sample by traditional biochemical methods. With this approach, the question always remains as to whether the resulting data accurately reflect the processes in the live cell. Another disadvantage is the time factor, because it is impossible to investigate fast reactions occurring in the cell with such an approach. Molecular imaging, including reporter gene methods, provides a unique opportunity to study biology in a living subject with minimal disturbances of metabolic pathways. For many years researchers used reporter genes encoding enzymes, such as bacterial β-galactosidase (lacZ gene) or chloramphenicol acetyltransferase (CAT gene), to study various intracellular events. To assay these enzymes, however, postmortem tissue sampling and processing was necessary.
Molecular imaging in living organisms is a new technique that has grown exponentially during the last decade. Three currently used strategies (nuclear, magnetic resonance, and optical imaging) for in vivo monitoring of gene expression include so-called 'direct' and 'indirect' imaging strategies.
Direct molecular imaging involves using a specific target and a target-specific probe. One of the examples of 'direct' imaging is detection of specific antigens with radiolabeled antibodies. Though some of the developed probes have wide clinical applications (e.g., [18F]-fluoro-2-deoxy-D-glucose for imaging glucose utilization in tumors), time and cost for new probe development seems to be prohibitive for a wider extension of this approach.