Rates and equilibria for probe capture by an antibody with infinite affinity

Probe-capture systems based on proteins and synthetic ligands have become important for new analytical and imaging applications. We have used kinetic measurements of luminescence and measurements of binding by isothermal calorimetry to determine essential rate and equilibrium constants for a system that permanently captures modified DOTA chelates for positron imaging. We used that information along with previous results to quantitatively characterize the behavior of this system in vitro and in vivo. Under physiological conditions at 37 degrees C, the equilibrium dissociation constant for yttrium S-2-(4-aminobenzyl)-1,4,7,10-tetraazacyclododecanetetraacetate from antibody 2D12.5 is 2.0 (+/- 0.4) x 10(-9) M and the dissociation rate constant is 7.0 (+/- 0.7) x 10(-3) s(-1), leading to an inferred association rate constant of 3.5 x 10(6) M(-1) s(-1). Using these values to interpret data from earlier experiments leads to the rate constant 2.5 x 10(-2) s(-1) for covalent attachment of bound yttrium S-2-(4-acrylamidobenzyl)-1,4,7,10-tetraazacyclododecanetetraacetate to the G54C mutant of antibody 2D12.5. These values lead to a model for the detailed behavior of the latter system for tumor imaging in vivo that is consistent with experimental observations.