Figure 6.

Schematic overview of the main methods for direct cell radiolabeling. (A) Radio-ionophore complexes. The ionophore ligand forms a complex with a radionuclide which allows it to cross cell membranes. Once inside the cell, the radioisotope is released and trapped by binding to intracellular macromolecules. (B) Surface of cells can be radiolabeled using stable radiopharmaceuticals which can bind covalently to components of the cell surface (e.g., proteins) or via compounds which can interact with the lipid membrane. (C) Radiolabeled small molecules can be used for direct cell labeling. They can enter cells through passive or active transport mechanisms and subsequently be converted into hydrophilic forms which are unable to diffuse out of cells. (D) Radiolabeled particles, such as colloids and nanoparticles, can be taken up by cells through phagocytic processes.