Comparison of ⁹⁰Y-ibritumomab tiuxetan and ¹³¹I- tositumomab in clinical practice

We retrospectively evaluated our single-center clinical experience with ⁹⁰Y-ibritumomab tiuxetan and ¹³¹I-tositumomab for therapy of refractory non-Hodgkin's lymphoma (NHL). We evaluated the hypothesis that the patient-specific dosing regimen used with ¹³¹I-tositumomab results in less bone marrow toxicity than does the weight-based dosing regimen used with ⁹⁰Y-ibritumomab tiuxetan. Methods: Thirty-eight patients (25 male and 13 female; median age, 64 y) received radioimmunotherapy for NHL (20 received ⁹⁰Y-ibritumomab tiuxetan; 18 received ¹³¹I- tositumomab). Patient and disease characteristics were evaluated to determine whether any were prognostic indicators of short- or long-term clinical response. The 12-wk response rate and clinical and hematologic toxicities attributable to each therapy were assessed. The response rate at 12 wk was correlated with long-term overall survival. Results: Twenty-six patients received full-radiation-dose radioimmunotherapy and 12 received attenuated doses because of hematologic concerns. The 12-wk overall response rate for all patients was 47%, and the complete response rate was 13%. The 12-wk overall response rate did not significantly differ between the ⁹⁰Y-ibritumomab tiuxetan and ¹³¹I-tositumomab groups. Responses at 12 wk were more frequent in patients with normal levels of serum lactate dehydrogenase, no bone marrow involvement, and International Prognostic Index scores of no more than 2 (P ≤ 0.04). Grade 3 or 4 thrombocytopenia occurred in 57% and 56% of patients treated with ⁹⁰Y-ibritumomab tiuxetan and ¹³¹I- tositumomab, respectively. Grade 3 or 4 neutropenia was observed in 57% and 50%, respectively. The time to the absolute neutrophil count nadir was shorter for the ⁹⁰Y-ibritumomab tiuxetan group than for the ¹³¹I- tositumomab group (36 ± 9 vs. 46 ± 14 d, P = 0.01). The mean percentage decline in platelet count after radioimmunotherapy was greater in the ⁹⁰Y-ibritumomab tiuxetan group than in the ¹³¹I- tositumomab group (79% ± 17% vs. 63% ± 28%, P = 0.04). Overall survival was longer in responders than in nonresponders 12 wk after therapy (P ≤ 0.05). Conclusion: Both ⁹⁰Y-ibritumomab tiuxetan and ¹³¹I-tositumomab were well tolerated. We observed response rates at the lower range of those reported in the literature, possibly because of referral bias, dose attenuation, and reasonably liberal acceptance criteria for a patient to receive therapy. Initial response assessments 12 wk after radioimmunotherapy predict longer-term response. ¹³¹I-tositumomab caused significantly less severe declines in platelet counts than did ⁹⁰Y-ibritumomab tiuxetan and may be a more appropriate choice for patients with limited bone marrow reserve, but large, randomized, prospective trials are needed to better compare the performance of these 2 treatments.