3. Discussion
This analysis of a large phase 3 clinical development program provides a comprehensive assessment of the immunogenicity of the novel BoNTA formulation, DAXI, and is the first large-scale evaluation of clinical immunogenicity for a PTD-containing formulation. The findings reported herein indicate a low incidence of treatment-induced binding antibodies to daxibotulinumtoxinA (0.7%) and the PTD-containing excipient RTP004 (1.3%). Importantly, no formation of neutralizing antibodies was detected in this study. In subjects with treatment-induced binding antibodies, titers were generally low (<1:200) and no subject had binding antibodies to both daxibotulinumtoxinA and RTP004, suggesting that the RTP004 peptide did not have adjuvant activity. Most binding antibodies to daxibotulinumtoxinA were shown to be transient and most positive results to RTP004 were singular with a titer of <1:200. The few subjects who showed persistent anti-RTP004 antibodies demonstrated a clinical response to treatment, suggesting that the binding did not alter the effect of the excipient. Multiple treatments or prior exposure to BoNTA did not show any apparent relationship with the occurrence of binding antibodies to either daxibotulinumtoxinA or RTP004. Additionally, treatment response and duration of response were unaffected by the presence of binding antibodies to daxibotulinumtoxinA or RTP004. There was no evidence that immune-related AEs were associated with the presence of binding antibodies to daxibotulinumtoxinA or RTP004. The one subject who had neutralizing antibodies at baseline and after treatment showed no clinical response to DAXI. Interestingly, the subject had received treatment with another BoNTA up to 32 months before the study, suggesting that the subject had developed an antibody response to the previously administered BoNTA. The presence of baseline reactivity to RTP004 in a small percentage of subjects is believed to reflect interaction of the highly positively charged RTP004 with other serum immunoglobulins rather than being due to the binding of specific antibodies.The immunogenicity of commercial preparations of BoNTA will be influenced by the dose and re-treatment frequency required for specific indications [Immunogenicity rates are not directly comparable between BoNTA products owing to differences in multiple factors such as assay sensitivity, sampling procedures, duration of the treatment period, and prior exposure to BoNTA [A limitation of this analysis is the relatively small number of subjects who received multiple doses compared with the longer-term repeated treatment generally seen in clinical practice. However, the large sample size pooled from three SAKURA trials was considered sufficient to assess the absence of daxibotulinumtoxinA neutralizing antibodies. Additionally, we believe this is the first time the three-tier anti-drug antibody testing methodology recommended by the US Food and Drug Administration has been applied to the assessment of BoNTA.