SYNTHESIS, METAL CHELATE STABILITY STUDIES, AND ENZYME DIGESTION OF A PEPTIDE-LINKED DOTA DERIVATIVE AND ITS CORRESPONDING RADIOLABELED IMMUNOCONJUGATES

By directly coupling a tetrapeptide to DOTA through an amide bond, we synthesized a novel DOTA derivative, DOTA-glycylglycylglycyl-L-p-nitrophenylalanine amide We converted this new precursor bifunctional chelating agent to DOTA-glycylglycylglycyl-L-P-idothiocyanatophenylalanine and conjugated it to monoclonal antibody Lym-1. Serum stability studies show that the radiolabeled conjugates are kinetically inert under physiological conditions. The rates of loss of radiometals in human serum are 0.1 +/- 0.1% per day for In(III), 0.02 +/- 0.15% per day for Y(III), and 0.3 +/- 0.2% per day for Cu(II) labeled immunoconjugates. In the presence of the liver enzyme cathepsin B, an in vitro digestion of In-114m-labeled conjugate yields a small fragment containing In-114m. Characterization of the cleavage products shows that this liver enzyme hydrolyzes the peptide linkage before the phenylalanine residue, freeing the In-DOTA-triglycine complex from the conjugate. However, the liver enzyme cathepsin D does not cleave the linkage over the span of 7 days.