Immunogenicity- From the desk of Henry Leng – In a previous blog I have stated that the purpose of the comparative clinical study between a candidate biosimilar and its reference product (RP) is only to confirm biosimilarity. The results from such a trial should not be used for making conclusions about the safety and efficacy of the biosimilar. Once biosimilarity has been convincingly established it can be accepted that the biosimilar will be as safe and effective as the reference product, and in addition, will perform the same clinically in all approved indications of the RP. With advancements in analytical methods that are now sufficiently sensitive and robust to detect even small differences in structure and potency between a biosimilar and its RP, and from which deductions can be made about their clinical impact, many have argued that animal studies and comparative human phase III efficacy trials have become obsolete. Regulators encourage the use of alternatives to animal testing but have not relented on the requirement for a comparative trial, not so much for the need to demonstrate similarity in safety and efficacy profiles, but to show a lack of, or comparative, immunogenicity.
Immunogenicity is one of the major considerations in the assessment of a biosimilar application, simply because it is an inherent property of proteins and, as we all know, most biopharmaceuticals, especially biosimilars, are proteins. It took many years after the development of monoclonal antibodies (mAb) in 1975 before mAb drugs entered the market. This was in spite of the fact that their utility as highly specific therapeutic agents were recognised almost immediately after their discovery. The reason for the delay was because the first mAb agents were all murine in origin, and hence, would elicit severe immune reactions in patients as they would be recognised as foreign by the human immune system. Furthermore, the neutralising antibodies that would be produced as part of the immune response would render them ineffective soon after administration. Through the development of chimeric (two-thirds human, e.g., rituximab), humanized (95% human, e.g., bevacizumab) and finally, human (e.g., adalimumab) mAb drugs, the immunogenicity of these protein pharmaceuticals was greatly reduced. Although humanisation eliminated the extreme immunogenicity (anaphylaxis and serum sickness) seen with murine mAb products, it did not eliminate immunogenicity. Neutralising antibodies have been seen in patients treated with human mAb therapeutics.
Two types of immunogenicity reactions have been associated with biopharmaceutical products: those due to differences in structure (amino acid sequence, secondary and tertiary conformations), which is referred to as intrinsic immunogenicity, and those due to differences in process and product-related impurities, known as extrinsic immunogenicity. Since biosimilars must have the same amino acid sequence and higher order structures as the reference products, the immunogenicity reactions observed have all been of the extrinsic type, i.e., due to impurities such as endotoxins, host cell proteins (HCPs) and the most important, aggregates, which result from denatured product protein molecules that have self-aggregated. For example, in a somatropin comparative trial, the biosimilar was shown to meet the efficacy endpoints, but failed to obtain authorisation because of higher immunogenicity compared to the reference product. The higher immunogenicity was shown to be due to higher HCP levels in the biosimilar product. Improved purification, followed by another comparative trial, showed similar rates of immunogenicity development as the innovator. The product was eventually approved by the EMA in 2006. Another example involved an erythropoietin biosimilar that elicited neutralising antibodies in two patients in a clinical trial. The cause was traced to tungsten ions released from the syringe that catalysed the formation of aggregates. No such antibodies were observed in participants in a subsequent trial with a low-tungsten syringe.
The question that we need to consider is whether the immunogenicity risk associated with all protein-containing biopharmaceutical products justify a comparative phase III clinical trial for biosimilar approvals. We have seen that intrinsic immunogenicity, which is responsible for severe immune responses, such as anaphylactoid reactions, is not a factor as biosimilars are required to have the same primary, secondary, and higher order structures as the innovator. Thus, we only need to consider those factors that may cause extrinsic immunogenicity, i.e., process and product-related impurities that are dependent on purification. Since biosimilars were first approved in 2006, state-of-the-art purification methods and assays have been developed, which are now basically standardised across the industry. I have often found when evaluating biosimilar applications that they have superior impurity profiles compared to their reference innovator products. In addition, regulatory agencies now have sufficient experience to set specification limits for most, if not all, impurities (HCP, host cell DNA, aggregates, etc.) that ensure they will not cause untoward effects in patients. The need for a phase III comparative trial to determine the immunogenicity potential of a biosimilar is, thus, questionable, especially since it can be assessed in a comparative pharmacokinetic and safety/immunogenicity study.
Bibliography
Doevendans, E and Schellekens, H (2019). Immunogenicity of Innovative and Biosimilar Monoclonal Antibodies. Antibodies 8, 21: 1 – 10.
Schiestl, M., et al., (2020). The Path Towards a Tailored Clinical Biosimilar Development. BioDrugs 34: 297 – 306.