Comparison of 90Y-Ibritumomab Tiuxetan and 131I-Tositumomab in Clinical Practice

UNLABELLED: We retrospectively evaluated our single-center clinical experience with (90)Y-ibritumomab tiuxetan and (131)I-tositumomab for therapy of refractory non-Hodgkin's lymphoma (NHL). We evaluated the hypothesis that the patient-specific dosing regimen used with (131)I-tositumomab results in less bone marrow toxicity than does the weight-based dosing regimen used with (90)Y-ibritumomab tiuxetan. METHODS: Thirty-eight patients (25 male and 13 female; median age, 64 y) received radioimmunotherapy for NHL (20 received (90)Y-ibritumomab tiuxetan; 18 received (131)I-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 (90)Y-ibritumomab tiuxetan and (131)I-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 < or = 0.04). Grade 3 or 4 thrombocytopenia occurred in 57% and 56% of patients treated with (90)Y-ibritumomab tiuxetan and (131)I-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 (90)Y-ibritumomab tiuxetan group than for the (131)I-tositumomab group (36 +/- 9 vs. 46 +/- 14 d, P = 0.01). The mean percentage decline in platelet count after radioimmunotherapy was greater in the (90)Y-ibritumomab tiuxetan group than in the (131)I-tositumomab group (79% +/- 17% vs. 63% +/- 28%, P = 0.04). Overall survival was longer in responders than in nonresponders 12 wk after therapy (P < or = 0.05). CONCLUSION: Both (90)Y-ibritumomab tiuxetan and (131)I-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. (131)I-tositumomab caused significantly less severe declines in platelet counts than did (90)Y-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.