Weight-based dosing of Yttrium 90 ibritumomab tiuxetan in patients with relapsed or refractory B-cell non-Hodgkin lymphoma

Background: Ibritumomab tiuxetan radioimmunotherapy produces durable remissions in patients with relapsed/refractory indolent non-Hodgkin lymphoma. The dosing of Yttrium 90 (⁹⁰Y)-ibritumomab tiuxetan is based on patient weight and platelet count: 0.4 mCi/kg in patients with a count ≥ 150 × 10⁹/L, to a maximum dose of 32 mCi. Patients weighing > 80 kg with platelet counts of ≥ 150 × 10⁹/L receive a lower dose per unit of body weight. We evaluated whether this influences the safety or efficacy of treatment. Patients and Methods: Data on efficacy and safety in patients in 3 registration trials who were treated with ⁹⁰Y ibritumomab tiuxetan at 0.4 mCi/kg (patients weighing ≤ 80 kg) or 32 mCi (patients weighing > 80 kg) were collected. Results: Clinical responses were evaluated in 103 patients weighing < 80 kg (median, 70 kg) and 67 patients weighing > 80 kg (median, 95 kg). Sex (41% female vs. 73% male) was the only significantly different characteristic between the patients weighing ≤ 80 kg and > 80 kg, respectively. The overall response rates were 79% (≤ 80 kg) and 70% (> 80 kg), and the complete response rates were 28% and 34%, respectively. Median times to progression were 8.9 months (≤ 80 kg) and 9.5 months (> 80 kg). There were no significant differences in efficacy measures or in the incidences of grade 3/4 nonhematologic adverse events, neutropenia, thrombocytopenia, or anemia. Conclusion: The dose of ⁹⁰Y ibritumomab tiuxetan was < 0.4 mCi/kg in 39% of patients, owing to the 32-mCi dose cap, but the efficacy or safety of ⁹⁰Y ibritumomab tiuxetan in patients > 80 kg was not affected.