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 Palynziq in the studies described below with the incidence of antibodies in other studies or to other products may be misleading.

All patients treated with Palynziq developed a sustained total anti-drug antibody (TAb) response with a majority of patients (91%; N = 235/258) developing that response by Week 4 of treatment. Mean TAb titers peaked 2 weeks after Palynziq initiation and remained elevated throughout treatment (greater than 3 years after treatment initiation). Anti-phenylalanine ammonia lyase (PAL) IgM antibodies were detected in a majority of patients (98%; N = 265/270) by 2 months after treatment initiation, with incidence declining over time to 67% at 36 months (N = 114/171). Anti‑PAL IgG antibodies were detected in almost all patients (N = 226/227) by 4 months after treatment initiation. Mean anti-PAL IgM and IgG titers peaked at approximately 3 and 6 months, respectively, after treatment initiation and remained elevated throughout treatment (greater than 3 years after treatment initiation). Drug-induced anti-PEG IgM and IgG antibodies were detected in the majority of patients (98%; N = 277/284 for IgM; and 278/284 for IgG) with mean titers for both peaking at 1 to 3 months after treatment initiation [see Drug Interactions (7.1)]. Neutralizing antibodies (NAb) capable of inhibiting PAL enzyme activity were detected on at least one measurement in the majority of patients (89%; N = 253/284) over time. Mean NAb titers peaked and reached a plateau at 16 to 20 weeks of treatment and then remained present throughout treatment (greater than 3 years after treatment initiation). 

Twenty-seven of 29 patients who had anaphylaxis were tested for anti‑pegvaliase‑pqpz IgE antibodies, which recognize the PEGylated protein product. Of the 27 patients tested for anti‑pegvaliase‑pqpz IgE antibodies, 26 patients tested negative. The one patient who tested positive for anti‑pegvaliase‑pqpz IgE antibodies on the screening test did not have sufficient sample to confirm IgE positivity. This patient tested negative for anti‑pegvaliase‑pqpz IgE at routine visits prior to and after the anaphylaxis episode (not at times of anaphylaxis). Sixty‑seven of 285 patients in clinical trials were tested for both anti‑PAL IgE antibodies, which recognize the recombinant PAL protein, and for anti‑pegvaliase‑pqpz IgE antibodies during routine study visits (not at times of anaphylaxis episodes) or during additional visits for hypersensitivity reactions. Of those 67 patients, 5 (8%) tested positive at least once for anti‑PAL IgE antibodies but negative for anti‑pegvaliase‑pqpz IgE antibodies.

The highest frequency of hypersensitivity reactions (consistent with a Type III immune complex-mediated hypersensitivity mechanism) occurred within the first 6 months of Palynziq treatment when the mean circulating immune complex (CIC) concentrations were at their highest and mean complement C3 and C4 concentrations were at their lowest. Mean CIC concentrations decreased and complement levels increased over time as the exposure-adjusted rate of hypersensitivity reactions decreased. IgG and IgM CIC concentrations were above the upper limit of normal in 63% (N = 164/259) and 42% of patients (N = 109/259), respectively, at 12 weeks of Palynziq treatment and returned towards baseline with long-term treatment (greater than 3 years after treatment initiation). 61% of patients (N = 110/180) had complement C3 concentrations less than lower limit of normal (LLN) at 6 months after treatment initiation and 38% of patients (N = 94/248) had complement C4 concentrations less than LLN at 3 months after treatment initiation. The incidence of low complement C3 and C4 concentrations decreased over time, but approximately 35% (N = 34/96) and 12% (N = 11/96) of patients had low C3 and C4 concentrations, respectively, at 36 months after treatment initiation.

Higher antibody responses for all antibody analytes, including NAb, were associated with lower mean trough pegvaliase‑pqpz concentrations and with higher blood phenylalanine concentrations. Hypersensitivity reactions occurred more frequently in patients with higher antibody titers for some but not all antibody analytes. Patients with higher mean change in IgG CIC concentrations from pre-treatment baseline tended to have higher discontinuation rates than patients with lower mean change in IgG CIC concentrations. Mean antibody titers for anti‑PAL IgG and IgM, TAb, and NAb remained relatively stable with long-term treatment.