Bexxar consists of a monoclonal antibody, tositumomab, linked to the radioactive isotope iodine iodine-131. The monoclonal antibody targets the CD20 antigen, which is found on the surface of mature B cells and B cell tumors.
The Bexxar therapeutic regimen is administered in two steps: the dosimetric and therapeutic steps. Each step consists of a sequential infusion of tositumomab followed by iodine-131 (I 131) tositumomab. The therapeutic step is administered 7 – 14 days after the dosimetric step.
The purpose of the dosimetric step is to provide a consistent radiation dose by adjusting for the individual patient’s rate of clearance of the drug. Clinical studies found that patients with high tumor burden, splenomegaly, or bone marrow involvement have a faster clearance, shorter terminal half-life, and larger volume of distribution. Patient-specific dosing, based on total body clearance, has been found to provide a consistent radiation dose, despite variable pharmacokinetics, by allowing each patient’s administered activity to be adjusted for individual patient variables.
The efficacy of the Bexxar therapeutic regimen was evaluated in a multi-center, single-arm study in forty patients with low grade or transformed low-grade or follicular large-cell lymphoma whose disease had not responded to or had progressed after rituximab therapy. Determination of clinical benefit of the Bexxar therapeutic regimen was based on evidence of durable responses. All patients in the study were required to have received prior treatment with at least four doses of rituximab without an objective response, or to have progressed following treatment.
Overall response rate was 68 percent, with a median duration of response of 16 months. One third of patients demonstrated a complete response. Among a subset of patients who were refractory to rituximab, overall response rate was 63 percent, with a median duration of response of 25 months. Twenty-nine percent of patients who were refractory to rituximab exhibited a complete response to the Bexxar therapeutic regimen.
The results of this study were supported by demonstration of durable objective responses in four single arm studies enrolling 190 patients evaluable for efficacy with Rituximab-naïve, follicular non-Hodgkin’s lymphoma with or without transformation, who had relapsed following or were refractory to chemotherapy. In these studies, the overall response rates ranged from 47% to 64% and the median durations of response ranged from 12 to 18 months.
It is not known whether the Bexxar therapeutic regimen improves overall survival.
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Vose JM, Wahl RL, Saleh M, et al. Multicenter phase II study of iodine-131 tositumomab for chemotherapy-relapsed/refractory low-grade and transformed low-grade B-cell non-Hodgkin’s lymphomas. J Clin Oncol. 2000;18(6):1316-1323.