Abstract Mosunetuzumab (Mosun) is a CD20xCD3 T‐cell engaging bispecific antibody that redirects T cells to eliminate malignant B cells. The approved step‐up dose regimen of 1/2/60/30 mg IV is designed to mitigate cytokine release syndrome (CRS) and maximize efficacy in early cycles. A population pharmacokinetic (popPK) model was developed from 439 patients with relapsed/refractory B‐Cell Non‐Hodgkin lymphoma receiving Mosun IV monotherapy, including fixed dosing (0.05–2.8 mg IV every 3 weeks (q3w)) and Cycle 1 step‐up dosing groups (0.4/1/2.8–1/2/60/30 mg IV q3w). Prior to Mosun treatment, ~50% of patients had residual levels of anti‐CD20 drugs (e.g., rituximab or obinutuzumab) from prior treatment. CD20 receptor binding dynamics and rituximab/obinutuzumab PK were incorporated into the model to calculate the Mosun CD20 receptor occupancy percentage (RO%) over time. A two‐compartment model with time‐dependent clearance (CL) best described the data. The typical patient had an initial CL of 1.08 L/day, transitioning to a steady‐state CL of 0.584 L/day. Statistically relevant covariates on PK parameters included body weight, albumin, sex, tumor burden, and baseline anti‐CD20 drug concentration; no covariate was found to have a clinically relevant impact on exposure at the approved dose. Mosun CD20 RO% was highly variable, attributed to the large variability in residual baseline anti‐CD20 drug concentration (median = 10 μg/mL). The 60 mg loading doses increased Mosun CD20 RO% in Cycle 1, providing efficacious exposures in the presence of the competing anti‐CD20 drugs. PopPK model simulations, investigating Mosun dose delays, informed treatment resumption protocols to ensure CRS mitigation.

Population pharmacokinetics and CD20 binding dynamics for mosunetuzumab in relapsed/refractory B‐cell non‐Hodgkin lymphoma