Regulatory Considerations for the Development of Novel Antibody-Related Products

• FDA

Abstract

Monoclonal antibodies (mAbs) are widely recognized as invaluable biologic tools for diagnostic and research investigations and have also become a leading class of therapeutic molecules. Tremendous efforts have been made to improve the safety and efficacy of antibody based therapeutics by obtaining a better understanding of the molecular basis of diseases and selecting appropriate targets, as well as developing novel antibody related products as new options for clinical applications. Although a great amount of knowledge regarding the quality attributes, manufacture and control strategies for antibody products is gained as a result of these efforts, the regulation of therapeutic antibody related products continues to be challenging for both the pharmaceutical industry and regulatory authorities. This article provides an overview of the general regulatory considerations from a quality perspective for mAb products, and outlines the current thinking regarding special considerations for several types of novel antibody related products.

Introduction

Since the development of anti-diphtheria antitoxins in the 19th century, researchers and physicians have investigated antibodies due to their remarkable characteristics and great potential for clinical applications. After hybridoma technology was introduced in the 1970’s, the first therapeutic uses of hybridoma-derived mAbs in the 1980’s showed sporadic promise, but overall resulted in disappointment. The first mAb, Orthoclone, approved in the US in 1986, was a murine mAb. Although Orthoclone has been withdrawn from the market, its approval represents the beginning of an era in the development of a new class of therapeutic proteins for clinical applications.

Early therapeutic mAbs derived from rodents were largely unsuccessful because they were immunogenic, had a short half-life and were poor at inducing effector functions in humans. In addition, inadequate preclinical and clinical product development contributed to these failures. Despite these disappointing clinical outcomes, what was learned from these failures was used to develop chimeric, humanized and human antibodies, which reduced immunogenicity, led to adequate half-lives and the ability to carry out effector function. Since then, there have been significant changes and rapid advancements in the analytical technologies to characterize the structure and function of mAbs, in expression systems and manufacturing processes, and in understanding the underlying mechanisms of various diseases¹.

Modifications to mAbs and related products can be divided into two categories: product modifications and product variations. Product modifications involve genetic engineering to alter the amino acid sequence and/or glycoforms to enhance or diminish Fc mediated effector functions, and protein engineering to link radioactive isotopes or drugs (e.g., small molecule or protein drugs) to the antibody to achieve targeted delivery². Product variation involves genetic engineering to achieve antibody related products of various sizes ranging from a domain antibody of approximately 20 Kd to dual variable domain immunoglobulin (DVD-IgG) of more than 200 Kd. A more recent concept uses antibody fragments (e.g., Fab, scFv and VH) as “building blocks” to achieve desirable multivalent antibody structures³. Each of these antibody related products offers unique characteristics that could have advantages in different clinical settings³. In some cases, antibody related products could involve both product modifications and product variations (e.g., pegylation of antibody fragments).

To date, over 50 mAbs and antibody related products have been approved by the FDA as in vivo diagnostics or for treating various diseases in humans including cancer, autoimmune diseases, transplantation, infectious diseases and cardio-vascular disease. Table 1 outlines a brief regulatory history and the milestones of “firsts” in mAb development in the US.

Table 1.

General Considerations for Antibody Products

The principles for the regulation of the quality of antibody related products is largely dependent on the understanding of the structure, function, manufacturing process and control strategy of the product. Typical quality attributes for mAbs include intrinsic (product-related) and extrinsic (process-related) characteristics. In general, the processrelated characteristics for a mAb include, but are not limited to, host cell protein, host cell DNA, residual Protein A (if Protein A chromatography is used in the purification process), cell culture medium components, purification buffer components, selective agents (if used in production), viral and microbiological purity. The product-related characteristics include, but are not limited to, color, clarity, osmolality, visible and subvisible particulates, pH, product concentration, potency, size variants (aggregates and fragments), charge variants (acidic, main and basic isoforms), glycosylation, oxidation, deamidation, free thiol, primary amino acid sequence, and secondary and tertiary structure.

Based on these intrinsic and extrinsic characteristics, the general regulatory considerations are to control the potential contaminations from bacteria, mycoplasma, fungi, viruses and transmissible spongiform encephalopathy (TSE/BSE) agents, and to minimize process (e.g., HCP, HDNA, methotrexate, soy hydrolysate, recombinant insulin) and product-related impurities (e.g., size, charge and glycoform variants), all of which could potentially impede clinical outcomes (e.g., efficacy, PK/PD, immunogenicity, allergenicity, adverse events).

Product Specific Considerations for Novel Antibody Related Products

Most approved mAbs and mAbs in development are intact IgG1 molecules, but IgG2 and IgG4 mAbs are in clinical development and several have been approved. The advantages of using intact mAbs is that the general knowledge of quality attributes of IgG1, IgG2 and IgG4 mAbs are generally applicable to other mAbs and platform manufacturing and analytical methods can be applied to different molecules sharing the same isotype. Special considerations for IgG2 and IgG4 mAbs include the different disulfide isoforms of IgG2 and the half antibody formation of IgG4. IgG4 mAbs are frequently engineered to prevent half antibody formation.

An advantage of intact mAbs is that a rational design can be applied to result in the desired activity. However, for the treatment of solid tumors, a major disadvantage is that due to the large size of an intact mAb and other features of the tumor, the penetration of the mAb into solid tumors is limited. This is one of the reasons that novel antibodyrelated products have been developed⁴.

Strategies to generate antibodies equipped with new capabilities are being developed at an accelerated pace. Some of these novel antibody related products resemble traditional mAb constructs, whereas others are structurally quite different. While the general regulatory principles regarding product- and process-related characteristics are applicable to these novel antibody related structures, specific considerations are necessary due to the unique characteristics of the novel mAb-related constructs. Currently, these novel mAb-related constructs can be divided into three classes: antibody drug conjugates (ADCs), bispecific antibodies (BsAb), and antibody cocktails.

Antibody Drug Conjugates

Conjugation of mAbs with cytotoxic entities or radioactive isotopes is aimed at enhancing the efficacy of mAb based therapy. Antibody conjugation can be generated via natural (lysine or cysteine) or engineered amino acid residues (e.g., site-specific engineered cysteine, non-natural amino acids, aldehyde tagging, etc), carbohydrates, and small molecule or peptide linkers⁵. The structure of an ADC is complex and consists of three components: the naked antibody, the linker and the drug. Therefore, the regulatory considerations for ADCs are derived from the chemistry, manufacture and control (CMC) considerations associated with the individual components (mAb, linker and drug) and the ADC drug substance and drug product as a whole molecule. Generally, the CMC expectations for the mAb intermediate are the same as those for a typical mAb drug substance. In addition to the control of identity, purity, potency and stability of the individual components and the ADC, other unique characteristics that could contribute to the safety and efficacy of the ADC, including drug to antibody ratio (DAR), free drug/mAb and potential conjugation sites, should be characterized throughout development and might need to be controlled for release and stability. The impact of conjugation on the mAb, such as potential interference with its binding to target and Fc receptors, and possible changes in purity (e.g., size and charge variants), should be assessed during development and controlled as needed. With respect to developing an appropriate method to control potency, multiple assays may be needed to ensure that all aspects of the mechanism of action (MOA) are properly controlled. Due to the complex nature of the immune response that could be developed against this class of products, the immunogenicity assessment may need to include an evaluation of potential anti-drug-antibody (ADA) responses to the mAb, linker and drug, as well as the impact on safety (e.g., potential high toxicity due to loss of specific binding activity of the mAb) and efficacy.

Bispecific Antibodies

Although the first BsAb was approved in 2014, the concept of therapeutic BsAbs can be traced back three decades. The early development of bispecific IgGs was hampered by technical limitations in the manufacturing processes, which relied on the production in quadromas and the need to isolate the desired bispecific product from all the possible variants. However, with progress in protein engineering, there are currently over 60 unique BsAb constructs in development, assuring a spot in the list of nextgeneration therapeutic antibodies. Structurally, bispecific antibodies can be categorized into five major classes⁶: IgG with additional antigen binding domain, bispecific IgG, fragments, fusion proteins and antibody conjugates. Additional BsAb formats include BsAb fusion proteins and conjugates and multivalent constructs3. Due to the dual or multivalent binding nature of these constructs, the binding affinity of each domain to its individual target should be characterized and optimized at the Research and Development (R&D) stage to achieve a desirable safety, PK and efficacy profile.

Multiple potency assays may be needed to demonstrate the binding of the bispecific antibody to both targets simultaneously and applicable effector functions that could be involved in the MOA. The potential for success of the candidate molecule is also a concern and may be unique for each type of BsAb platform. For some bispecific constructs, stability could be an issue with the formation of aggregates over time during storage; therefore, extensive formulation studies may be needed at early stages to avoid the derailment of product development. For small bispecific constructs, typically without the Fc region, microbial control during prolonged administration, which is necessary due to the short half-life of these constructs, could be problematic. Strategies such as the addition of antibiotics in the intravenous injection solution at the time of administration may be considered under certain circumstances to ensure patient safety. For those BsAb with unique structures that are significantly different from a conventional human immunoglobulin, immunogenicity could be a challenge if these BsAbs are intended to be administered chronically. Special considerations should be given to BsAbs used to achieve targeted cytotoxicity and assays to distinguish ADA developed against either or both variable regions may need to be elucidated for patient safety reasons. This knowledge is particularly useful if one of the variable regions (e.g., anti-CD3) is designed as a platform for the development of BsAbs towards different tumor antigens.

Antibody Cocktails

Antibody cocktails (i.e., two or more mAb products packaged in a final container closure system) are representative of antigen specific polyclonal antibody preparations, which have been in use since the anti-diphtheria anti-toxins were introduced in the 1890’s. Antibody cocktails are in development predominantly to treat viral diseases and some oncology indications. Cocktails of recombinant mAbs maintain specificity for an overall target, but can be designed recognize different antigenic variants (anti-Rabies virus cocktails as a replacement for anti-rabies immune globulin⁷) or different epitopes of the same molecule (Ebola virus⁸ and Her-2⁹). Recent development has expanded the investigation of using antibody cocktails for autoimmune diseases¹⁰. Although the information from existing submissions for individual mAbs may be used to support the development of the mixture, mAb cocktails are recognized and regulated as new products. The general considerations for mAb cocktails would be applicable to the control of each individual antibody component prior to pooling, including controls for the identity, purity/impurity, potency and immunogenicity. During early development, the binding specificity of each mAb component to its target should be characterized and optimized at the R&D stage to avoid any competition between each mAb that could lead to safety concerns and impede the clinical efficacy. The total dose of the mAb cocktail, not the maximum amount of one mAb component, would need to be accounted for when dosing patients.

Conclusion

In summary, the general regulatory expectations and considerations for mAb products are applicable to novel antibody related products. However, some special considerations may be applicable based on physiochemical and biological property, and unique quality attributes for each type of novel antibody products.

Disclaimer

This article reflects the views of the authors and should not be construed to represent FDA’s views or policies.

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