Background: Radioimmunotherapy (RIT) is an emerging clinical treatment option for patients with non-Hodgkin’s lymphoma (NHL). Yttrium 90 (90Y) ibritumomab tiuxetan (Zevalin®) RIT was approved by the FDA in February 2002 for the treatment of patients with relapsed or refractory low-grade, follicular, or transformed B-cell NHL, including patients with rituximab-refractory follicular NHL. In 4 registrational trials of 90Y ibritumomab tiuxetan conducted between 1996 and 1999, 211 patients with B-cell NHL were treated. Of these 211 patients, 153 patients (73%) had follicular lymphoma (FL). With ongoing follow-up, long-term durable responses have been observed, but until now have not been more fully characterized.

Methods: Responding patients with time to progression (TTP) of ≥12 months were identified and characterized as long-term responding (LTR) patients.

Results: In the 4 registrational trials, 78 of the 211 patients (37%) were identified as LTR patients. Characteristics of these LTR patients were as follows: median age 58 years (range, 24 to 80), 44% >60 years old, 55% male, 76% with follicular lymphoma, and 41% with lymphomatous marrow involvement. LTR patients had a median of 2 prior regimens (range, 1–9): 59% had ≥2 prior therapies, 33% had ≥3 prior therapies, and 37% had no response to their last prior therapy. Thirty percent of LTR patients had bulky disease (tumor size >5 cm) and 83% had stage III/IV disease. At the time of this analysis, the median duration of response (DR) and TTP for LTR patients were 28 months (range 11–80) and 29 months (range 12–82), respectively, with a median follow-up of 50 months (range 13–82). The median DR to the last prior therapy for LTR patients was 12 months. The complete response rate (confirmed [CR] and unconfirmed [CRu]) among LTR patients was 65%, and the median DR and TTP were 29 and 31 months, respectively, for CR/CRu patients. In ongoing responders the median DR is 52 months (range 48–80). Among the 153 patients with FL, 59 (39%) were identified as LTR patients. Compared to the overall LTR patients, LTR patients with FL had similar disease characteristics, DR, TTP, and CR/CRu rates.

Conclusions: Ongoing follow-up indicates that 90Y ibritumomab tiuxetan frequently produces durable long-term responses (TTP ≥12 months) in patients with relapsed or refractory B-cell NHL. Failure to respond to the therapy immediately prior to treatment with 90Y ibritumomab tiuxetan does not appear to affect the ability to achieve long-term responses with 90Y ibritumomab tiuxetan. Durable long-term responses were achieved in 37% of all patients and 39% of patients with FL treated in 4 registrational trials of 90Y ibritumomab tiuxetan at 30 centers in the US. Of these LTR patients, a high proportion were >60 years old and had received ≥3 prior therapies.

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