**3.4 Radiolabeling with 111In**

The SPECT isotope indium-111 is a frequently used radionuclides in diagnostic nuclear medicine. It has a physical half-life of 67 hours and is produced by a

*Strategies for Site-Specific Radiolabeling of Peptides and Proteins DOI: http://dx.doi.org/10.5772/intechopen.99422*

cyclotron. The principle photons are 173 keV (89%) and 247 keV (94%). The most commonly used chelators for 111In-labeling of peptides and proteins are DTPA and DOTA. DTPA chelates 111In at room temperature with sufficient efficiency and stability. This DTPA chelator is used in the commercially available somatostatin analog OctreoScan®. DOTA forms a more stable complex with 111In, but requires heating of the reaction mixture which can lead to protein denaturation, especially for larger proteins. Labeling of DTPA- and DOTA-conjugated peptides and proteins is a one pot, one step procedure in which the compound is incubated with 111InCl3 at a pH between 4 and 6. Ammonium and sodium acetate buffers are commonly used as buffer for labeling of DTPA- and DOTA-conjugated compounds with 111In. However, it was shown that 111In-labeling efficiency and specific activity of DTPA- and DOTA-conjugated peptides was significantly improved in MES (2-(*N*-morpholino)ethanesulfonic acid) and HEPES buffer compared to acetate buffers [63]. The enhanced labeling efficiency appeared to be due to the reduced competitive chelation of cadmium, the decay product of 111In. These observations made MES and HEPES the buffers of choice for 111In-labeling.
