Population PK‐PD Modeling of Circulating Lymphocyte Dynamics in Chronic Lymphocytic Leukemia Patients Under Ibrutinib Treatment

Ibrutinib is indicated for the treatment of chronic lymphocytic leukemia (CLL). Absolute lymphocyte count (ALC) is a clinical criterion used for the monitoring of CLL. Ibrutinib has several effects on lymphocytes, and has highly variable pharmacokinetics (PK). The objective of this work was to build a PK-pharmacodynamic (PD) model describing ALC dynamics under ibrutinib treatment in patients with CLL. ALC observations before and after ibrutinib treatment initiation in patients with CLL were included in the analysis. A population PK-PD model was developed based on physio-pharmacological knowledge. Individual PK concentrations at each hospital visit were included in the model. The association between PD parameters and lymphocytosis, and between PD parameters and response to treatment were assessed. A total of 94 patients, 658 ALC and 1,501 PK observations were included in model development. The final PK-PD model accurately described ALC dynamics for different patient profiles. It consisted in two compartments (tissues and blood circulation) with ibrutinib plasmatic concentration inducing two drug effects: stimulation of lymphocyte redistribution and death. Patients with hyperlymphocytosis had significantly higher tissues to circulation baseline lymphocyte count ratio, and lower death effect. Patients who progressed under ibrutinib had significantly lower baseline lymphocyte counts in tissues (2-fold lower) and blood (3-fold lower). The first PK-PD model for ALC in patients with CLL under ibrutinib treatment was developed. This model suggests that estimated lymphocyte counts in tissues and blood could be used as an early predictor of response in patients with CLL.