As with all therapeutic proteins, there is potential for immunogenicity. The detection of antibody formation is highly dependent on the sensitivity and specificity of the assay. Additionally, the observed incidence of antibody (including neutralizing antibody) positivity in an assay may be influenced by several factors including assay methodology, sample handling, timing of sample collection, concomitant medications, and underlying disease. For these reasons, comparison of the incidence of antibodies to FABRAZYME in the studies described below with the incidence of antibodies in other studies or to other agalsidase beta products may be misleading.

Patients with classic Fabry disease in Study 1, Study 2, and extension studies were tested at multiple time points for antibodies to agalsidase beta during the 55 to 58-month period. Approximately 83% (110 of 133) of adult patients receiving agalsidase beta developed antibodies; 77% (102/133) of patients developed neutralizing antibody (NAb) that inhibited in vitro agalsidase beta catalytic activity, which declined over time, and 6% (8/133) of patients developed NAb that inhibited cellular uptake. In pediatric patients with Fabry disease in Study 3 receiving the recommended dose who were 8 to <16 years of age, antibodies to agalsidase beta were detected in approximately 69% (11/16) of patients. Most patients who developed antibodies did so within the first 3 months of treatment. Antibody titers generally declined over time. Approximately 18% of adult patients who developed antibodies became antibody negative by 74 weeks (median time) from the time of seroconversion; however, none of the pediatric patients became antibody negative. Female patients generally had lower incidence of antibodies and lower antibody titers compared to male patients. In Study 5, patients with truncating GLA mutations had higher incidence of antibodies and higher antibody titers compared to patients with nontruncating GLA mutations. Patients with plasma α-galactosidase A activity ≤1.5 nmol/hr/mL had higher incidence of antibodies and higher antibody titers compared to patients with plasma α-galactosidase A activity >1.5 nmol/hr/mL.

In general, over 90% of adult and pediatric patients treated with agalsidase beta achieved and maintained normalization of plasma globotriaosylceramide (GL-3) levels irrespective of developing antibodies to agalsidase beta.

Study 4 was an open-label, rechallenge study to evaluate the safety of FABRAZYME treatment in patients who had a positive skin test to FABRAZYME or who had tested positive for FABRAZYME-specific IgE antibodies. In this study, six adult male patients, who had experienced multiple or recurrent infusion-associated reactions during previous clinical trials of FABRAZYME, were rechallenged with FABRAZYME administered as a graded infusion for up to 52 weeks of treatment. The initial two rechallenge doses of FABRAZYME were administered as a 0.5 mg/kg dose per week at an initial infusion rate of 0.01 mg/min for the first 30 minutes (1/25th the usually recommended maximum infusion rate). The infusion rate was doubled every 30 minutes thereafter, as tolerated, for the remainder of the infusion up to a maximum rate of 0.25 mg/min. If the patient tolerated the infusion, the dose was increased to 1 mg/kg every two weeks and the infusion rate was increased by slow upwards titration [see Dosage and Administration (2.1)]. Pretreatment was not permitted for at least the first 4 infusions in order to allow early recognition of acute systemic hypersensitivity reactions. Four of the six patients treated in this study received at least 26 weeks of FABRAZYME (2 patients received 26 weeks and 2 patients received 52 weeks), and two patients discontinued prematurely due to recurrent infusion-associated reactions [see Warnings and Precautions (5.1, 5.2)].

Testing for IgE antibodies was performed in approximately 60 patients in clinical trials who experienced moderate to severe infusion-associated reactions or in whom mast cell activation was suspected. Seven of these patients tested positive for FABRAZYME-specific IgE antibodies or had a positive skin test to FABRAZYME. Patients who have had a positive skin test to FABRAZYME, or who have tested positive for FABRAZYME-specific IgE antibodies in clinical trials with FABRAZYME have been rechallenged [see Dosage and Administration (2.1) and Warnings and Precautions (5.1, 5.2)].

The incidences of hypersensitivity reactions were 51% (41/80) and 60% (25/42) in adult patients with persistent anti-FABRAZYME antibodies and in adult patients with high antibody titer, respectively, compared to 30% (7/23) in antibody-negative adult patients [see Warnings and Precautions (5.1)].

The incidence of infusion-associated reactions was 76% (84/110) in antibody-positive adult patients compared to 30% (7/23) in antibody-negative adult patients. The incidence of infusion-associated reactions was 46% (5/11) in antibody positive pediatric patients compared to 20% (1/5) in antibody negative pediatric patients [see Warnings and Precautions (5.2)].