Preparation of Cr-51 RBC's is achieved by incubation of 20-30 ml of anticoagulated whole blood with 50-100 µCi of sodium chromate Na251CrO4. The blood is typically anticoagulated with heparin or ACD solution. After 15 min incubation at room temperature, the reaction is terminated by the addition of a small amount of ascorbic acid, which converts unreacted (CrO4)2-to Cr3+ (chromous ion). This prevents the continuation of cell labeling after the material is injected into the patient. The Cr-51 binds avidly and irreversibly to the b-globin chains on the hemoglobin molecule, forming labeled cells with excellent in vivo stability. The labeling of RBC's with Tc-99m also involves the binding of the radioisotope to the b-globin chains on the hemoglobin molecule. Cells may be labeled in vivo or in vitro by a variety of different procedures, described in detail in the chapter on Cell Labeling.
I-123 mIBG and I-131 mIBG can be easily prepared by heating a mixture containing 0.5-2.0 mg of mIBG hemisulfate, 12 mg of ammonium sulfate, and the appropriate radioiodide. After two 45-60 min heating cycles in the dry state, radioiodinated mIBG is formed in high yield and with an average radiochemical purity in excess of 97%. The I-131 compound is currently commercially available; the I-123 compound must be manufactured on-site under a Physician-sponsored Investigational New Drug Exemption.
Radiopharmaceuticals may also be produced by biological synthesis. Before the chemical synthesis of Vitamin B12 was elucidated, 57Co labeled vitamin B12 was made by placing 57CoCl2 in a broth containing streptomyces griseus. This resulted in the biological production of 57Co labeled vitamin B12. By a similar process, yeast growing in a medium high in 75Se and low in sulfur produced 75Se selenomethionine, a radiopharmaceutical formerly used for pancreatic imaging.
|Stephen Karesh, PhD.||
Last Updated: August 14, 1996