Improving Anti-CD45 Antibody Radioimmunotherapy Using a Physiologically Based Pharmacokinetic Model

Radioimmunotherapy is a method to selectively deliver radioactivity to cancer cells via specific antibodies. A strategy to enhance the efficacy of radioimmunotherapy is the prior application of unlabeled antibody, resulting in an increase in the dose to the target tissue and a decrease in the burden to other organs. It was suggested that optimizing this approach might considerably improve radioimmunotherapy with anti-CD45 antibody. The present work develops a physiologically based pharmacokinetic model to individually determine the optimal preload for radioimmunotherapy with the YAML568 anti-CD45 antibody for each patient. Methods: A physiologically based pharmacokinetic model was developed to describe the biodistribution of anti-CD45 antibody. The transport of antibody to the organs of interest via blood flow, competitive binding of unlabeled and labeled antibody, degradation and excretion of antibody, and physical decay were included in the model. The model was fitted to the biokinetics data of 5 patients with acute myeloid leukemia. On the basis of the estimated parameters, simulations for a 0- to 534-nmol preload of unlabeled antibody were conducted and the organ residence times were calculated. Results: The measured data could be adequately described by the constructed model. The estimated numbers of accessible antigens in the respective organ, in nanomoles, were 97 ± 33 for red marrow, 49 ± 24 for liver, 34 ± 18 for spleen, 38 ± 31 for lymph nodes, and 0.9 ± 0.4 for blood. These ranges indicate high interpatient variability. The optimal amount of unlabeled antibody identified by simulations would improve the ratio of residence time in red marrow to residence time in liver by a factor of 1.6–2.4. Conclusion: The efficacy of radioimmunotherapy using anti-CD45 antibody can be considerably increased with the presented model. A more selective delivery of radioactivity to the target organ and a reduction in the toxicity to normal tissue are achieved by determining the optimal preload. Furthermore, the adverse effects of radioimmunotherapy might be drastically reduced while saving antibody expenses. The validation of the model is ongoing. The model is easily extendible and therefore most probably applicable to radioimmunotherapy of other hematologic malignancies, such as antibodies targeted to CD20, CD33, or CD66.