Zevalin (Page 5 of 6)

12 CLINICAL PHARMACOLOGY

12.1 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.

12.2 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-months post therapy.

12.3 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.

13 NONCLINICAL TOXICOLOGY

13.1 Carcinogenesis, Mutagenesis, Impairment of Fertility

Carcinogenicity and mutogenicity studies have not been conducted. However, radiation is a potential carcinogen and mutagen.

No animal studies have been performed to determine the effects of Zevalin on fertility in males or females. In clinical studies, the Zevalin therapeutic regimen results in a significant radiation dose to the testes: the radiation dose to the ovaries has not been established [see Dosage and Administration (2.5)]. There is a potential risk that the Zevalin therapeutic regimen could cause toxic effects on the male and female gonads. Effective contraceptive methods should be used during treatment and for up to 12 months following the Zevalin therapeutic regimen.

13.2 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.

14 CLINICAL STUDIES

14.1 Relapsed or Refractory, Low-grade or Follicular Lymphoma

A study to evaluate the efficacy and safety of Zevalin (referred to as Study 106-06) 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 (IWRC). Other efficacy outcome measures included time-to-disease-progression (TTP) and duration of response (DR). Table 7 summarizes efficacy data from Study 106-06.

Another study to evaluate the efficacy and safety of Zevalin (referred to as Study 106-04) 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 106-04.

Table 7. Summary of Efficacy Data *
*
IWRC: International Workshop Response Criteria
CRu and CR: Unconfirmed and confirm complete response
Estimated with observed range
§
Duration of response: interval from the onset of response to disease progression
“+” indicates an ongoing response
#
Time to Disease Progression: interval from the first infusion to disease progression
Study 106-06 Study 106-04
Zevalin therapeutic regimen N = 54 Zevalin therapeutic regimen N = 64 Rituximab N = 66
Overall Response Rate (%) 74 83 55
Complete Response Rate (%) 15 38 18
Median DR §(Months)[Range ] 6.4[0.5-49.9+] 14.3[1.8-47.6+] 11.5[1.2-49.7+]
Median TTP # (Months)[[Range ] 6.8[1.1-50.9+] 12.1[2.1-49.0+] 10.1[0.7-51.3+]

A single-arm study to evaluate the efficacy and safety of Zevalin (referred to as Study 106-05) was conducted in 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 106-05. 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)].

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