Table I.

Overview of streptavidin/avidin- and bsMAb-based pretargeting systems

Streptavidin/avidin-biotin	bsMAb-hapten
Advantages Up to 4 biotin-binding sites on avidin Binding affinity 10−15 M Current biotin-chelate conjugation methods improve stability from serum biotinylases	Advantages Humanized bsMAbs are less likely to be immunogenic Clearing step is not required Divalent hapten (AES) enhances local retention in tumor. D-amino acids used to help stabilize hapten-peptide structures.
Disadvantages Immunogenicity of avidin/streptavidin Clearing step required Endogenous biotin	Disadvantages bsMAb is allowed to clear naturally from blood, usually requiring several days before radiolabeled hapten is given

• Retention in the target for both procedures is ultimately governed by the affinity of the antibody for the target antigen.
• Pretargeting systems are best applied for targets that would not be internalized rapidly as a consequence of the first antibody-binding step.