Cost and Time Effective Approach to Early Stage Biosimilar Development: A Case Study of Tocilizumab
19th August 2015
By: Dr Terry Gray, Field Marketing Manager
Introduction to Tocilizumab (Actemra)
Tocilizumab (Actemra (US)/RoActemra (EU)) is a therapeutic monoclonal antibody that targets the Interleukin-6 receptor (IL-6R). Interleukin-6 (IL-6) is a multifunctional cytokine which is central to the regulation of the immune and nervous systems. This molecule has been linked to pathogenesis of several autoimmune diseases including rheumatoid arthritis (RA) due to aberrations in cell signalling. Co-developed and marketed by Chugai and Roche, Actemra/RoActemra is indicated for the treatment of rheumatoid arthritis (in combinational therapy with Methotrexate). With sales in 2013, reaching $1.1bn this has become another blockbuster biologic. The marketing exclusivity for Tocilizumab is expected to expire by early 2019. Therefore, early interest in follow-on biologic capabilities has already generated a swell of interest in the molecule and also in analytical approaches to show similarity.
Key to the clinical potency of Tocilizumab, is the drugs ability to affect the IL-6 signalling pathway. A simple complex between IL-6 and IL-6R alone is not sufficient for signal transduction; the IL-6/IL-6R complex must associate with the gp130 glycoprotein, driving its dimerization and subsequent downstream intracellular signalling via the tightly regulated JAK/STAT pathway. Transmembrane IL-6R (mIL-6R) can be shed from the cell membrane either through enzymatic cleavage from the transmembrane domain by a metalloprotease, ADAM17, or by alternative splicing of the mRNA, excising the exon coding for the transmembrane domain, to provide the soluble form of the receptor (sIL6-R). Thus, IL-6R exhibits distinct subcellular distribution, localised to the plasma membrane or as a soluble moiety in the extracellular matrix. This differential localisation of IL-6R presents two signalling pathways which can be targeted by Tocilizumab:
- Classical Signalling Pathway(Diagram A) The classical signalling pathway relies on the IL-6 cytokine complexing with membrane bound IL-6R with the subsequent recruitment of gp130 and induction of JAK/STAT signalling cascade. This mechanism is thought to be limited to hepatocytes, selected epithelial cells and leukocytes which have the propensity to express IL-6R on the cell surface. (Diagram B) Tocilizumab targeting of IL-6R directly blocks IL-6 binding to the receptor and gp130 homodimerisation. Thus, the JAK/STAT pathway activation is attenuated, and as a result the immunogenic response reduced.
- Trans Signalling Pathway(Diagram C) The alternate signalling mechanism allows IL-6 to interact with soluble IL-6R (i) in the extracellular milieu, denoted by (ii). This complex can then recruit the membrane expressed gp130 on the surface of a sensitive cell driving downstream signalling (iii). Trans signalling is reported to be more widespread since gp130 is ubiquitously expressed throughout, therefore theoretically, stimulate most cells in the body. (Diagram D) The affinity of Tocilizumab for soluble IL-6R is comparable to the affinity for membrane bound antigen (i), therefore competes with IL-6 for IL-6R (ii) preventing the formation of the functional quaternary protein structure. As a result, downstream signalling is suppressed.
Analytical Similarity Approaches
Analysis of monoclonal antibody therapeutics is inherently complex due to the multifactorial effector functions of the molecule. An orthogonal approach is required to balance an understanding of the Fab-driven antibody-antigen interaction, affinities therein, receptor off and on rates and blocking capabilities, coupled to related functional aspects associated with the Fc domain- interactions such as affinities for distinct effector molecules related to either the complement system or the induction of Antibody Directed Cell Cytotoxicity (ADCC).
When selecting the suite of methodologies that form the analytical similarity assessments, it is important to understand both the molecular basis of the mechanism of action (MOA) of the biologic itself, as well as the biological significance of the MOA. As a reduced number of the available methods will be employed at an early stages of follow-on biologic development, those that best represent the pathology of the indicated disease state should be prioritised. With Tocilizumab, the choice between methods that measure blockade of either the Classical or Trans pathways of IL-6 signalling represents an interesting decision.
The expression of mIL-6R is limited to hepatocytes and immune cells, therefore, anti-IL-6R therapies may specifically block Classical IL-6 signalling and local immune cell activation. However, sIL-6R can, theoretically, assert IL-6-responsive signalling in any cell line expressing the co-activator gp130. Therefore, blocking the Trans pathway may allow alleviation of systemic presentations of disease. As IL-6 elevation in the disease state is evident, in both synovial fluid and serum, both approaches could be considered appropriate. It may then be appropriate to consider the accuracy, efficiency and ease-of-use of a novel methodology for anti-IL-6R functional activity to provide meaningful, rapid and accurate data. Consequently, it is imperative to understand both signalling mechanisms to fully characterize a novel Tocilizumab biosimilar, as both will impact the in vivo function; but which branch of signalling to pursue primarily is open for discussion. Therefore, proof-of-concept studies are in development at BioOutsource to specifically characterize the role of Tocilizumab in both the Classical and Trans signalling pathways. These studies will provide the key basis for the analytical similarity studies of Tocilizumab biosimilar materials.
Measurement of potency represents a small but critical component of the analytical similarity package that will contain a broad variety of methodologies to elucidate any potential differences that may be present between a biosimilar molecule and the reference innovator product. Utilizing orthogonal, redundant approaches will add value to the biosimilarity assessment and has the potential to reduce the overall number of assessments that are required, at least at the early phases. Therein, representing a far more powerful approach as a balanced, comprehensive and relevant analysis at an early stage allowing efficient, economical and concise characterization of a follow-on biologic.