Extended rituximab therapy in Waldenström's macroglobulinemia

Background: Waldenstrom's macroglobulinemia (WM) is a CD20 expressing B-cell malignancy represented by the pathological diagnosis of IgM secreting lymphoplasmacytic lymphoma. Major response rates of 30% have been reported in most studies with standard dose rituximab, i.e. 4 weekly infusions at 375 mg/m 2 /week. Methods: In an effort to increase rituximab activity in WM, an extended dose schedule employing two sets of four (375 mg/m 2 /week) infusions at weeks 1-4 and 12-16 was evaluated. Expression of the complement resistance antigens CD46, CD55 and CD59 was also evaluated on tumor cells pre- and post-therapy to determine impact on response. Results: Twenty-nine patients were enrolled and 26 patients completed the intended therapy. On an intent to treat analysis, 14 (48.3%) patients achieved a partial response, and 5 (17.2%) patients achieved a minor response. Responses were observed in 18/24 (75%) patients with a serum IgM level of 6000mg/dl (P=0.03). The median time to best response was 17 months, and only 2 of 19 responding patients progressed with a median follow-up of 29 months. No differences in baseline expression of the complement resistance antigens CD46, CD55 and CD59 were observed among responding and non-responding patients, although post-therapy CD55 expression was higher in non-responding patients (P= 0.002). Conclusions: These data show that extended rituximab therapy is active and may lead to more major responses over standard dose rituximab in WM. WM patients with serum IgM levels of <6000 mg/dl are more likely to benefit from extended rituximab therapy.