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Gallium-68 Labeling of Albumin and Albumin Microspheres

Edward Mallinckrodt Institute of Radiology, St. Louis, Missouri

Correspondence: For reprints contact: Michael Welch, Div. of Radiation Sciences, Edward Mallinckrodt Institute of Radiology, 510 South Kingshighway, St. Louis, MO 63110.

ABSTRACT

Because of the high stability constant of gallium transferrin, the formation of a protein that will be stable in vivo and labeled with gallium-68 (a positron emitter) requires preliminary coupling of a strong chelating group to the protein. In the present study, we have used a reaction developed by Krejcarek and Tucker, in which DTPA is coupled to proteins by the formation of an amide bond. Using human serum albumin (HSA) as a model, we have studied the efficiency of the reaction of HSA with the mixed acid anhydride of the quarternary triethyl ammonium salt of DTPA and butyl formate, as a function of the ratio of albumin to DTPA. After purification of the DTPA-labeled HSA, it is possible to prepare Ga-68-labeled albumin in high yield by chelation of the Ga-68 with the DTPA-labeled protein. In vitro and in vivo stability studies showed that the labeled protein was stable over a period of several hours. The same type of bifunctional chelate has been used to attach Ga-68 to HSA microspheres.