Immunogenicity of Botulinum Toxin Formulations: Potential Therapeutic Implications

Abstract

Botulinum neurotoxins (BoNTs) are proteins produced by bacteria of the Clostridium family. Upon oral ingestion, BoNT causes the neuroparalytic syndrome botulism. There are seven serotypes of BoNT (serotypes A-G); BoNT-A and BoNT-B are the botulinum toxin serotypes utilized for therapeutic applications. Treatment with BoNT injections is used to manage chronic medical conditions across multiple indications. As with other biologic drugs, immunogenicity after long-term treatment with BoNT formulations may occur, and repeated use can elicit antibody formation leading to clinical nonresponsiveness. Thus, approaching BoNT treatment of chronic conditions with therapeutic formulations that minimize stimulating the host immune response while balancing patient responsiveness to therapy is ideal. Immunogenicity is a clinical limitation in many settings that use biologic drugs for treatment, and clinically relevant immunogenicity reduction has been achieved through engineering smaller protein constructs and reducing unnecessary formulation components. A similar approach has influenced the evolution of BoNT formulations. Three BoNT-A products and one BoNT-B product have been approved by the Food and Drug Administration (FDA) for therapeutic use: onabotulinumtoxinA, abobotulinumtoxinA, incobotulinumtoxinA, and rimabotulinumtoxinB; a fourth BoNT-A product, daxibotulinumtoxinA, is currently under regulatory review. Additionally, prabotulinumtoxinA is a BoNT-A product that has been approved for aesthetic indications but not therapeutic use. Here, we discuss the preclinical and clinical immunogenicity data that exist within the scientific literature and provide a perspective for considering immunogenicity as a key factor in choice of BoNT formulation.

Keywords: AbobotulinumtoxinA; Antibodies; Biologics; Clinical response; IncobotulinumtoxinA; Neutralizing; OnabotulinumtoxinA; Second generation.

Fig. 1

Immunogenicity in response to biologic drugs. Dendritic cells engulf biologics that bind to various cell surface receptors [27, 31]. Biologics are processed and presented as peptide fragments on the cell surface via the major histocompatibility complex (MHC) [30, 31]. Peptide-MHC complexes are presented to and recognized by T cells, which stimulate B cells to clonally expand and produce antigen-specific antibodies [30, 31]. Costimulation by molecules such as cytokines is required for complete activation of T cells. Subsequent expansion of B cells results in long-lasting and abundant production of antibodies [30, 31]. TCR, T-cell receptor

Fig. 2

Immunogenicity drives clinical nonresponsiveness. Repeated clinical exposure to biologic drugs can provoke a continual immune response and formation of nAbs leading to clinical resistance [30, 31, 39]. Common signs of clinical resistance include increasing the frequency or dose of the drug administered to elicit results. Over time, this can lead to clinical nonresponse [25, 42, 43]. nAb, neutralizing antibody

Fig. 3

Molecular structures of botulinum neurotoxin type A in A supramolecular complex and B purified core neurotoxin. A BoNT-A is a supramolecular complex consisting of hemagglutinin proteins, a nontoxic nonhemagglutinin protein, and the core neurotoxin [54, 64], with a molecular weight of up to ~ 900 kDa as produced by the Clostridum bacteria [57]. The structural model shown here (representing ~ 760 kDa) was determined using recombinant protein and cryogenic electron microscopy [57] and is currently the largest assembly for which three-dimensional structural information is available. B Purified BoNT-A formulations contain only the core neurotoxin (~ 150 kDa) [54, 64]. Images rendered with PyMol (Schrödinger, Inc.) using atomic coordinates from the Protein Data Bank with the following accession codes: 3WIN, 3V0A, 3V0B, 3V0C, and 4LO7 [56, 57, 120]. BoNT-A, botulinum neurotoxin type A

Fig. 4

Total clostridial protein and active core neurotoxin in BoNT-A formulations. A Changes in total clostridial protein content in onaBoNT-A [1, 121], aboBoNT-A [1, 74], and incoBoNT-A [1] formulations over time. Current formulations are presented in the 2005 panel. B Total core BoNT-A neurotoxin in onaBoNT-A, aboBoNT-A, and incoBoNT-A formulations separated into denatured/inactive toxin and active neurotoxin components [64, 74, 75]. aboBoNT-A, abobotulinumtoxinA; BoNT-A, botulinum neurotoxin type A; incoBoNT-A, incobotulinumtoxinA; NA, not available; onaBoNT-A, onabotulinumtoxinA