radiolabeling

Radiolabeling is the process of attaching a radioactive isotope to a molecule, such as a drug, peptide, antibody, nucleic acid, or nanoparticle, to enable tracking or detection by radiation-based methods. Radiolabeled compounds are used as tracers in research and medicine, allowing in vivo imaging, ex vivo analysis, and pharmacokinetic studies.

Labeling approaches fall into direct and indirect methods. Direct labeling incorporates the radioisotope into the molecule itself, which can be simple but may alter activity or stability. Indirect labeling uses a chelating agent or prosthetic group to hold the isotope and is then linked to the target molecule; common chelators include DOTA, DTPA, and others for metals such as gallium-68, indium-111, or lutetium-177. For non-metal isotopes, prosthetic groups or isotopic exchange can be used to attach isotopes like fluorine-18.

Common isotopes and modalities include positron emitters such as 18F, 11C, 68Ga, and 124I for PET imaging, and gamma emitters such as 99mTc, 111In, and 123I for SPECT. The choice of isotope depends on half-life, decay mode, radiochemical stability, and the biology of the targeting vector.

Applications span diagnostic imaging, receptor or antigen targeting, biodistribution studies, and radiotherapy planning. Theranostics combines diagnostic radiolabeling with therapeutic isotopes (e.g., 177Lu- or 90Y-labeled agents). In research, radiolabeled tracers support autoradiography and PK studies.

Safety and regulation are central: radiopharmaceuticals are produced under radiopharmacy practices and GMP, with attention to radiochemical purity, stability, specific activity, and sterility. Handling requires radiation protection and proper waste management. The field evolved from early 20th-century radiolabeling to modern PET/SPECT radiochemistry and generator-based work.