Evaluation of Baseline Body-Weight Dosing of Yttrium 90 (⁹⁰Y) Ibritumomab Tiuxetan (Zevalin®) Radioimmunotherapy in Patients with Relapsed or Refractory B-Cell Non-Hodgkin’s Lymphoma (NHL).

Background: As the first radiolabled monoclonal antibody approved for the treatment for the treatment of cancer, yttrium 90 (⁹⁰Y) ibritumomab tiuxetan (Zevalin®) achieves both high response rates and durable remissions in patients with relapsed or refractory, low-grade, follicular, or transformed B-cell NHL. Because of the stability of ⁹⁰Y ibritumomab tiuxetan and its favorable pharmacokinetic profile (ie, predictable urinary clearance and absence of dehalogenation), dosing is based on patient weight and platelet counts. Yttrium 90 ibritumomab tiuxetan is typically delivered at a dose of 0.4 mCi/kg (in patients with platelets >150,000/mm³) or 0.3 mCi/kg (in patients with platelets ≤150,000/mm³) to a maximum recommended dose (MRD) of 32 mCi. However, patients >80 kg with platelet counts exceeding 150,000/mm³ may receive a lower drug concentration (ie, <0.4 mCi/kg ⁹⁰Y) as a result of dose capping. We evaluated whether the 32 mCi dose cap influences the safety or efficacy of ibritumomab tiuxetan therapy in patients >80 kg.

Methods: Efficacy and safety data were collected for patients from 3 registrational trials that received either 0.4 mCi/kg ⁹⁰Y (patients ≤80 kg) or 32 mCi ⁹⁰Y (patients >80 kg).

Results: Clinical responses in 170 patients were evaluated. Patients ≤80 kg (n = 103) had a median weight of 70 kg (range, 45–80 kg) versus a median weight of 95 kg (range, 81–159) for patients >80 kg (n = 67). Gender (41% M vs 73% M, respectively; P <.01) was the only significantly different baseline characteristic between the ≤80 kg and >80 kg groups. Similar efficacy and safety results were reported for both subsets of patients. Overall response rates of 79% and 70% were observed for patients ≤80 kg and >80 kg, respectively (P =.27); and no significant differences were seen in complete response rates (28% vs 34%, respectively; P =.40). Median TTP (8.9 months vs 9.5 months; P =.53) and median DR (8.5 months vs 11.5 months; P =.34) were equivalent between the 2 weight-based groups. In addition, there were no statistically significant differences in safety measures including grade 3/4 nonhematologic adverse events, neutropenia, thrombocytopenia, or anemia.

Conclusions: As a consequence of dose capping at an MRD of 32 mCi, 39% of patients received a lower dose/kg of ⁹⁰Y ibritumomab tiuxetan. However, despite the difference in dose administered on a unit of body weight basis, the efficacy and safety results were similar between patients ≤80 kg and >80 kg. We conclude that the 32 mCi dose cap does not influence the efficacy or safety profile of ⁹⁰Y ibritumomab tiuxetan for patients >80 kg.