Label stability in serum of four radionuclides on DTPA-coupled antibodies--an evaluation

Although DTPA forms strong chelates with many metals, the harsh chemical environment of the serum may nevertheless dissociate metallic radionuclides from DTPA attached to proteins. We have investigated the stability in 37 degrees C serum of 111In, 99mTc, 90Y and 153Gd chelated to DTPA-coupled antibodies. Stability was evaluated primarily by affinity chromatography and HPLC analysis of serum incubates; however, the stability of 111In was determined in vivo in patient studies. Analysis of patient urine showed no evidence for dissociation of DTPA from antibody whereas analysis of serum demonstrated exchange rates of 111In to transferrin of about 9%/day. When labeled by the described method, 99mTc on DTPA coupled antibodies shows instability in serum but superior stability than 99mTc on antibodies without the attached DTPA. Both 90Y and 153Gd dissociate in serum at rates which are comparable to that of 111In.