To understand the influence of formulation on the clinical performance of botulinum toxin (BoNT) products.

Conventional BoNT products used to treat cervical dystonia (CD) tend to have a similar clinical profile with a duration of efficacy of approximately 12 weeks and are formulated using the common excipient human serum albumin (HSA) in addition to a salt or sugar. DaxibotulinumtoxinA for injection (DAXI) is a newly approved BoNT/A product with a novel formulation without HSA. Rather DAXI contains a proprietary 35-amino acid peptide (RTP004) which is known to increase thermostability and help prevent surface adsorption. In clinical trials for CD DAXI at a dose of 250U, which contains approximately half the amount of core neurotoxin as the 236U approved dose for onabotulinumtoxinA for CD, showed a median time to loss of efficacy of 20 weeks.

The effect of RTP004 and HSA on BoNT/A binding to neuronal cells was assessed via cell-binding assays in N2a cells (mouse) and SiMa cells (human). SNAP-25 cleavage was assessed by western blot after incubation with a fixed concentration of BoNT and increasing concentrations of RTP004 or HSA.

The 150kD BoNT/A binding to the surface of neuronal cells was increased by 2- to 3-fold with increasing amounts of RTP004 compared with BoNT/A binding alone. HSA did not increase the amount of toxin bound to cells at any concentration tested. Additionally, RTP004 produced a dose-dependent increase in the cleavage of SNAP-25 in cells exposed to a constant concentration of BoNT/A compared to BoNT/A alone or BoNT with HSA.

We have demonstrated that the interaction of RTP004 with the 150-kDa BoNT/A can enhance binding to neurons and cleavage of the intracellular substrate, SNAP-25. This data illustrates the importance of formulation on the clinical performance of a BoNT product and the impact of the novel peptide excipient in DAXI.