Mechanism of Action

Ibritumomab tiuxetan binds specifically to the CD20 antigen (human B-lymphocyte-restricted differentiation antigen, Bp35). The apparent affinity (K_D) of ibritumomab tiuxetan for the CD20 antigen ranges between approximately 14 to 18 nM. The CD20 antigen is expressed on pre-B and mature B lymphocytes and on > 90% of B-cell non-Hodgkin's lymphomas (NHL). The CD20 antigen is not shed from the cell surface and does not internalize upon antibody binding.

The chelate tiuxetan, which tightly binds In-111 or Y-90, is covalently linked to ibritumomab. The beta emission from Y-90 induces cellular damage by the formation of free radicals in the target and neighboring cells.

Ibritumomab tiuxetan binding was observed in vitro on lymphoid cells of the bone marrow, lymph node, thymus, red and white pulp of the spleen, and lymphoid follicles of the tonsil, as well as lymphoid nodules of other organs such as the large and small intestines.

Pharmacodynamics

In clinical studies, administration of the Zevalin therapeutic regimen resulted in sustained depletion of circulating B cells. At four weeks, the median number of circulating B cells was zero (range, 0-1084/mm³). B-cell recovery began at approximately 12 weeks following treatment, and the median level of B cells was within the normal range (32 to 341/mm³) by 9 months after treatment. Median serum levels of IgG and IgA remained within the normal range throughout the period of B-cell depletion. Median IgM serum levels dropped below normal (median 49 mg/dL, range 13-3990 mg/dL) after treatment and recovered to normal values by 6-month post therapy.

Pharmacokinetics

Pharmacokinetic and biodistribution studies were performed using In-111 Zevalin (5 mCi [185 MBq] In-111, 1.6 mg ibritumomab tiuxetan). In an early study designed to assess the need for pre-administration of unlabeled antibody, only 18% of known sites of disease were imaged when In-111 Zevalin was administered without unlabeled ibritumomab. When preceded by unlabeled ibritumomab (1.0 mg/kg or 2.5 mg/kg), In-111 Zevalin detected 56% and 92% of known disease sites, respectively. These studies were conducted with a Zevalin therapeutic regimen that included unlabeled ibritumomab.

In pharmacokinetic studies of patients receiving the Zevalin therapeutic regimen, the mean effective half-life for Y-90 activity in blood was 30 hours, and the mean area under the fraction of injected activity (FIA) vs. time curve in blood was 39 hours. Over 7 days, a median of 7.2% of the injected activity was excreted in urine.

Animal Toxicology and/or Pharmacology

Animal reproductive toxicology studies of the Zevalin regimen have not been conducted. Because the Zevalin therapeutic regimen includes the use of rituximab, also see prescribing information for rituximab.

Clinical Studies

The safety and efficacy of the Zevalin therapeutic regimen were evaluated in 2 multi-center trials, enrolling a total of 197 subjects and a third study enrolling a total of 30 subjects.

Relapsed Follicular Lymphoma Refractory to Rituximab

Study 1 was a single arm study of 54 patients with relapsed follicular lymphoma, who were refractory to rituximab treatment. Patients had a World Health Organization (WHO) Performance Status (PS) 0-2, <25% bone marrow involvement by NHL, no prior bone marrow transplantation, and acceptable hematologic, renal, and hepatic function. Refractoriness to rituximab was defined as failure to achieve a complete or partial response or time to disease progression (TTP) of < 6 months. The primary efficacy endpoint of the study was the overall response rate (ORR) using the International Workshop Response Criteria (IWRC). Secondary efficacy endpoints included time to disease progression (TTP) and duration of response (DR). Table 8 summarizes efficacy data from Study 1.

Relapsed or Refractory, Rituximab-Naïve, Low-Grade or Follicular NHL

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