Enzymatic cleavage of peptide-linked radiolabels from immunoconjugates

We have incorporated peptides selected by combinatorial library [Peterson, J. J., and Meares, C. F. (1998) Bioconjugate Chem. 9, 618-626) into peptide -linked radiolabeled immunoconjugates of the form DOTA-peptide-antibody. De capeptide linkers -GFQGVQFAGF- and -GFGSVQFAGF-, selected for cleavage by h uman liver cathepsin B, were rapidly digested in vitro when compared to the simple model tetrapeptide motif of the prototype -GGGF- [Li, M., and Meare s, C. F. (1993) Bioconjugate Chem. 4, 275-283]. Cleavage properties of thes e library-selected substrates for cathepsin B compared favorably with decap eptide linkers -GLVGGAGAGF- and -GGFLGLGAGF-, which incorporate two of the most labile extended cathepsin B substrates from the literature. The decape ptide linker -GFGSTFFAGF-, selected from the library for cleavage by human liver cathepsin D, was rapidly digested by cathepsin D while the others wer e not.