Overview of SEEDbody

What is SEEDbody?

SEEDbody represents a groundbreaking fusion protein built on the innovative Fc analogue platform, leveraging the strand-exchange engineered domain (SEED) technique. SEED is a humanized CH3 domain, comprising alternating segments of human IgG and IgA CH3 sequences, forming complementary heterodimers. By fusing with various partners, SEEDbody generates asymmetric binders, immunofusions, and bispecific antibodies. SEEDbody can achieve various combinations of targets and functions, expanding the application range of Fc fusion proteins and bispecific antibodies. Its exceptional expression, purification, stability, pharmacokinetics, and immune effects through Fc receptor-mediated cytotoxicity and complement-dependent cytotoxicity further bolster its potential therapeutic applications in areas such as cancer, autoimmune diseases, and infectious diseases.

Structural Features of SEEDbody

The essence of SEEDbody lies in its meticulously engineered CH3 domain, incorporating the strand exchange technique. This method seamlessly combines human IgG and IgA CH3 sequences to form two complementary SEED CH3 domains, AG and GA, respectively. In mammalian cells, these SEED CH3 domains preferentially assemble into heterodimers rather than homodimers. The fusion of SEED CH3 with the hinge and CH2 regions of IgG1 yields a powerful heterodimerized Fc analog platform. This platform can then be genetically linked to a range of fusion partners such as Fabs, scFvs or cytokines, in a manner that caters to the desired configuration and functionality.

The meticulous sequence design of SEED CH3 domains centers around a three-dimensional structural alignment of human IgG and IgA C(H)3 domains. This process meticulously selects exchange sites that preserve structural integrity and functionality. SEED CH3 domains achieve heterodimerization through two mechanisms: a hydrogen bond network at the exchange sites, bolstering heterodimer stability, and the elimination of the key residues required for homodimer formation, thereby reducing the likelihood of homodimerization.

Fig.1 Sequence Comparison of SEED CH3 Domains (Jonathan, 2010)

SEEDbody distinguishes itself from conventional Fc fusion proteins and bispecific antibodies through its exceptional features:

Diverse Target Combinations and Functions: SEEDbody enables the creation of mono-Fab-SEEDbody, bi-Fab-SEEDbody, scFv-SEEDbody, and bi-scFv-SEEDbody, offering unparalleled versatility in targeting and functionality. 
Retention of Fc Region Characteristics: SEEDbody retains essential characteristics of the Fc region, including interactions with Fc receptors and complement factors, as well as the  extension of serum half-life, enhancing its therapeutic potential.
Efficient Expression and Purification: SEEDbody can be efficiently expressed and purified in mammalian cells, circumventing the complexities and challenges associated with mixed heavy or light chains.

SEEDbody Generation Methods

SEEDbody is generated through a sophisticated engineering process that involves modifying the CH3 domains of human IgG and IgA to create complementary SEED domains, which exhibit a preference for forming heterodimers. These SEED domains consist of alternating segments of IgG and IgA CH3 sequences, carefully designed to exchange strands at specific positions. Subsequently, the SEED domains are fused to the CH2 and hinge regions of IgG1, resulting in the formation of a heterodimeric Fc analogue platform. This platform can then be genetically linked to one or more fusion partners, such as Fab, scFv, or cytokines. Depending on the intended configuration and functionality, these fusion partners can be attached to either the N-terminus or C-terminus of the SEEDbody chains. To ensure the proper folding, glycosylation, and heterodimerization of SEEDbody, its expression and purification rely on a mammalian cell system and recombinant protein A resin. The mammalian cell system facilitates the precise assembly of SEEDbody, while the recombinant protein A resin capitalizes on the high affinity of SEEDbody's Fc region to protein A, facilitating efficient purification. Various methods can be employed to characterize SEEDbody, such as SDS-PAGE, size-exclusion chromatography, mass spectrometry, and binding assays. These techniques allow for the assessment of molecular weight, purity, stability, affinity and functionality of SEEDbody.

Advantages and Disadvantages of SEEDbody

As a fusion protein based on the Fc analog platform, SEEDbody possesses distinct that must be carefully considered based on specific application scenarios and goals. Notable advantages of SEEDbody include:

Multifunctional Targeting: SEEDbody can achieve the simultaneous targeting of multiple antigens or tumor cells and immune effector cells, thus broadening the application scope of Fc fusion proteins and bispecific antibodies. For example, SEEDbody can simultaneously target two or more tumor-associated antigens, or target tumor cells and immune effector cells to enhance the anti-tumor effect.
Preservation of Fc Region Properties: SEEDbody retains crucial Fc region properties, such as interactions with Fc receptors and complement factors, as well as the prolongation of serum half-life. These attributes augment SEEDbody's immune effects, encompassing antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and antibody-dependent cell-mediated phagocytosis (ADCP), while also improving its pharmacokinetics. 
Efficient Expression and Purification: SEEDbody can be efficiently expressed and purified in mammalian cells, avoiding the complexities associated with producing mixed heavy or light chains. The heterodimerization technology employed in SEED CH3 domain ensures proper pairing between heavy chains, streamlining the expression and purification process.

However, SEEDbody also faces certain challenges and limitations:

Structural and Functional Considerations: SEEDbody may encounter issues such as insufficient stability, affinity, and functionality, necessitating optimization and improvement through engineering methods. 
Safety and Immunogenicity: As a genetically engineered product, SEEDbody might raise public concerns regarding safety, effectiveness, acceptability. These concerns need to be addressed and resolved through scientific evidence, policy norms, and effective communication.

Clinical Application of SEEDbody

SEEDbody, as a fusion protein boasting a wide array of target combinations and functionalities, has demonstrated remarkable therapeutic potential across diverse disease areas, including cancer, autoimmune diseases, infectious diseases, and more. At present, several SEEDbody products have either entered clinical trials or obtained marketing approval. For instance, SEEDbody exhibits the ability to inhibit tumor growth, invasion and metastasis by effectively targeting tumor-associated antigens or tumor microenvironment factors. One noteworthy example is the bispecific SEEDbody, M7824, developed by EMD Serono, which targets epidermal growth factor receptor (EGFR) and programmed death ligand 1 (PD-L1) simultaneously. By doing so, it inhibits tumor cell proliferation and their evasion of immune surveillance. M7824 is presently undergoing clinical trials for the treatment of various solid tumors, including non-small cell lung cancer, gastric cancer, esophageal cancer, gallbladder cancer, pancreatic cancer, and more. Moreover, SEEDbody proves highly effective in modulating immune balance and suppressing inflammatory responses by targeting key mediators or cells involved in autoimmune reactions. EMD Serono's bispecific SEEDbody, M1095, for instance, targets interleukin 17A (IL-17A) and interleukin 17F (IL-17F), effectively inhibiting the pro-inflammatory effects of these two cytokines in autoimmune diseases. M1095 is presently undergoing clinical trials for treating psoriasis and psoriatic arthritis. Additionally, SEEDbody exhibits the potential to clear infections and neutralize toxins by targeting pathogenic microorganisms or their toxins. An example of this is Creative Biolabs' bispecific SEEDbody, CB-SEED-001, which targets the surface antigen 85B (Ag85B) of Mycobacterium tuberculosis (Mtb) and the human macrophage surface receptor CD14, thereby enhancing macrophage phagocytosis and killing ability against Mtb. CB-SEED-001 is currently undergoing validation and optimization in animal models.

Future Prospects of SEEDbody

SEEDbody, being a novel fusion protein platform based on SEED C(H)3 heterodimers, ha olds immense promise for developing of bispecific and asymmetric binders or immunofusions across various therapeutic applications. However, certain challenges and opportunities for improvement and innovation in SEEDbody technology remain. Some of the future prospects include:

Optimization of SEEDbody products: Engineering the SEED C(H)3 domains, hinge region, fusion partners, and glycosylation patterns to optimize stability, affinity, functionality, and immunogenicity. For example,  introducing specific mutations or deletions in SEED C(H)3 domains can enhance their heterodimerization specificity and stability. 
Expansion of product diversity: Incorporating different types of fusion partners, such as scFv, Fab, cytokines, enzymes, and toxins, to enhance the diversity and versatility of SEEDbody products. Utilizing scFv or Fab as fusion partners, for instance, can increase the valency and avidity of SEEDbody products. 
Combination with other modalities: Exploring the combination and synergy of SEEDbody products with other treatment modalities, such as small molecules, vaccines, cell therapies, and gene therapies. Combining SEEDbody products with small molecules, for instance, can enhance pharmacological effects by improving cellular uptake or inhibiting degradation. 
Streamlining clinical development: Accelerating the clinical development and approval of SEEDbody products by establishing robust and scalable manufacturing processes, adhering to regulatory standards and guidelines, and conducting rigorous and comprehensive clinical trials. Utilizing mammalian cell systems like CHO cells, for instance, ensures high-level expression and proper folding of SEEDbody products. 
Enhancing accessibility and affordability: Increasing the accessibility and affordability of SEEDbody products through cost reduction, heightened market competition and collaboration, and addressing unmet medical needs and social demands. Using alternative expression systems, such as bacteria or plants can lower production costs by simplifying culture conditions and scaling up yield.

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