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A systematic analysis of various factors involved in the CDI-mediated coupling of DTPA to IgG have been carried out, in order to optimize the labeling yield of a monoclonal antibody labeled to high specific activity. Various CDI-to-DTPA ratios were tested, followed by a range of DTPA (activated) to IgG ratios. Specific activities as high as 600 Ci/mmol could be obtained following labeling with 113mIn, when an IgG : DTPA : CDI ratio of 0.01 : 1:20 was used. Other aspects, such as the volume of each of the reactants, the pH and the reaction times, were also standardized to yield a labeled IgG2a (KS1/4) that could be consistently tested for biodistribution and tumor-binding.

Desferrioxamine-human serum albumin conjugate was prepared at pH 4.7 using a water-soluble carbodiimide and at pH 7.8 using glutaraldehyde. The crude conjugate was purified by dialysis and gel filtration. The efficiency of 67Ga labeling of the purified conjugate was greater than 95%, with good in vitro stability. The specific activity ranged from 5 to 50 microCi/mg. Treatment of the conjugate with urea resulted in a 20--30-fold increase in specific activity, suggesting that a number of chelon groups are unavailable for Ga binding due to non-covalent intramolecular cross-linking.