A pharmacokinetic model describing the removal of circulating radiolabeled antibody by extracorporeal immunoadsorption.

Extracorporeal immunoadsorption is a new technique for removal of circulating radiolabeled antibody from the peripheral blood (1) to reduce background activity for improved tumor imaging, and (2) to reduce whole-body and marrow toxicity when high doses of radiolabeled antibodies are used for antitumor therapy. A pharmacokinetic model was developed to describe plasma disappearance of 111In-KC-4G3 prior to, during, and after immunoadsorption in humans. The model is developed based on a two-compartment open model, and during immunoadsorption a third compartment is added for removed radioactivity by the immunoadsorption column. Goodness-of-fit statistics indicate a good fit of the model to the data. The resulting pharmacokinetic parameters for a selected patient are V1 = 2.64 L, VSS = 3.64 L, t 1/2 alpha = 3.77 hr, and t 1/2 beta = 48.5 hr. The immunoadsorption clearance (CLIA = 19.3 ml/min) was 21-fold greater than the patient's plasma clearance (CL10 = 0.899 ml/min), indicating a very effective immunoadsorption process. The model predicts an increase in plasma radioactivity upon termination of immunoadsorption, probably due to redistribution of radioactivity from the extravascular compartment to the plasma in response to the rapid decline in plasma radioactivity during immunoadsorption. Two series of simulations were performed to examine the influence of onset time and duration of immunoadsorption. In series one the onset time was varied and in series two immunoadsorption duration was varied. In series one, the predicted radioactivity amounts adsorbed by the immunoadsorption column ranged from 75% of the injected dose (4-hr onset) to 52% of the injected dose (24-hr onset). In series two, immunoadsorbed radioactivity ranged from 32% (2-hr duration) to 64% of the injected dose (12-hr duration). When instituted as early as 4 hr, the predictions suggest that earlier immunoadsorption onset improves the effectiveness of radioactivity removal, relating to higher early circulation concentrations, and longer immunoadsorption periods remove more radioactivity, but also result in larger predicted radioactivity redistribution form tissue to plasma. To employ the immunoadsorption procedure for tumor imaging and therapy optimally, the data and our predictions indicate that a compromise must be made that will balance immunoadsorption onset and duration against tumor radioactivity uptake and subsequent radioactivity redistribution from tissues back to plasma. Together with biologic considerations providing sufficient antigen-antibody interaction and dependent on the utilized radioisotope, these data support the utility of extracorporeal immunoadsorption during the radioimmunodetection of cancer and for future therapeutic applications.