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Mechanism of Action
Ibritumomab tiuxetan binds specifically to the CD20 antigen (human B-lymphocyte-restricted differentiation antigen, Bp35). The apparent affinity (KD) 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 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.
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-months post therapy.
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 therapeutic regimen have not been conducted. Because the Zevalin therapeutic regimen includes the use of rituximab, also see prescribing information for rituximab.
Relapsed or Refractory, Low-grade or Follicular Lymphoma
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 main efficacy outcome measure of the study was the overall response rate (ORR) using the International Workshop Response Criteria (rWRC). Other efficacy outcome measures included time-to-disease-progression (TTP) and duration of response (DR). Table 7 summarizes efficacy data from Study 1.
Study 2 was a randomized (1:1), open-label, multicenter study comparing the Zevalin therapeutic regimen with rituximab. The trial was conducted in 130 patients with relapsed or refractory low-grade or follicular non-Hodgkin's lymphoma (NHL); no patient had received prior rituximab. Patients had histologically confirmed NHL requiring therapy, a WHO PS 0-2, < 25% bone marrow involvement by NHL, no prior bone marrow transplantation, and acceptable hematologic function. Sixty-four patients received the Zevalin therapeutic regimen, and 66 patients received rituximab given as an IV infusion at 375 mg per m2 weekly times 4 doses. The main efficacy outcome measure of the study was ORR using the IWRC. The ORR was significantly higher for patients receiving the Zevalin therapeutic regimen (83% vs. 55%, p < 0.001). Time-to-disease-progression was not significantly different between study arms. Table 7 summarizes efficacy data from Study 2.
Table 7. Summary of Efficacy Dataa
|Study 1||Study 2|
N = 54
N = 64
N = 66
|Overall Response Rate (%)||74||83||55|
|Complete Response Rateb (%)||15||38||18|
| Median TTPc,f (Months)
|6.8 [1.1-50.9+]||12.1 [2.1-49.0+]||10.1[0.7-51.3+]|
|a) IWRC: International Workshop Response Criteria
b) CRu and CR: Unconfirmed and confirm complete response
c) Estimated with observed range
d) Duration of response: interval from the onset of response to disease progression
e) "+" indicates an ongoing response
f) Time to Disease Progression: interval from the first infusion to disease progression
Study 3 was a single arm study of 30 patients of whom 27 had relapsed or refractory low-grade, follicular NHL and a platelet count 100,000 to 149,000/mm3. Patients with ≥ 25% lymphomatous marrow involvement, prior myeloablative therapy with stem cell support, prior external beam radiation to > 25% of active marrow or neutrophil count < 1,500/mm3 were ineligible for Study 3. All patients received Y-90 Zevalin [0.3 mCi per kg (11.1 MBq per kg)]. Objective, durable clinical responses were observed [89% ORR (95% CI: 70-97%) with a median duration of response of 11.6 months (range: 1.0-42.4+months)].
Follicular, B-Cell NHL Upon Completion of First-Line Chemotherapy
Study 4 was a multi-center, randomized, open-label study conducted in patients with follicular NHL with a partial (PR) or complete response (CR/CRu) upon completion of first-line chemotherapy. Randomization was stratified by center and response to first-line therapy (CR or PR). Key eligibility criteria were < 25% bone marrow involvement, no prior external beam radiation or myeloablative therapy, and recovery of platelets to normal levels. Patients were randomized to receive Zevalin (n=208) or no further therapy (n=206). Y-90 Zevalin was administered at least 6 weeks but no more than 12 weeks following the last dose of chemotherapy. The main efficacy outcome measure was progression-free survival (PFS) assessed by study investigators using the International Workshop to Standardize Response Criteria for nqn-Hodgkin's Lymphoma (1999).
Among the 414 patients, 49% were male, 99% were Caucasian, 12% were ≥ 65 years old, 83% had a WHO performance status of 0, and 65% had Stage IV disease. Thirty-nine (9.5%) patients received single agent chlorambucil, 22 (5%) patients received fludarabine or a fludarabine-containing regimen, 294 (71%) patients received cyclophosphamide-containing combination chemotherapy [CHOP (31%); CHOP-like (15%); CVP/COP (26%)] and 59 (14%) patients received rituximab-containing combination chemotherapy as first-line treatment.
Progression-free survival was significantly prolonged among Zevalin-treated patients compared to those receiving no further treatment [median PFS 38 months vs. 18 months; HR 0.46 (95% CI: 0.35, 0.60) p < 0.0001 Cox model stratified by response to first-line therapy and initial treatment strategy (immediate vs. watch-and-wait)]. The number of patients who died was too small to permit a reliable comparison on survival.
The results for PFS are presented in Figure 1.
Figure 1. Study 4: Kaplan-Meier Estimator for Investigator-Assessed
Progression Free Survival Time
Last reviewed on RxList: 12/21/2011
This monograph has been modified to include the generic and brand name in many instances.
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