Mosunetuzumab: Dual-Targeting Antibody Innovations

Advancements in antibody-based therapies have transformed cancer treatment, offering more precise ways to target malignant cells. Mosunetuzumab is a bispecific antibody that engages CD3 on T cells and CD20 on B cells, enhancing immune-mediated destruction of cancerous B cells. This dual-targeting approach marks a significant step in immunotherapy, particularly for patients with relapsed or refractory B-cell malignancies.

Understanding its function requires examining its molecular structure, binding mechanisms, and the immune responses it activates. Comparing it to traditional single-target antibodies highlights its unique therapeutic potential.

Structural Composition

Mosunetuzumab is a bispecific monoclonal antibody engineered to recognize CD3 on T cells and CD20 on B cells. Its structure is based on a modified immunoglobulin G (IgG) framework with two distinct antigen-binding sites. Unlike conventional monoclonal antibodies that target a single epitope, mosunetuzumab’s dual-binding configuration allows it to bridge T cells and B cells with high specificity. This design, achieved through recombinant DNA technology, ensures optimal spatial orientation of the binding domains for therapeutic efficacy.

The antibody’s heterodimeric format is essential for stability and function. Traditional IgG antibodies rely on symmetric heavy and light chain pairing, but bispecific antibodies require modifications to prevent chain mispairing. This is achieved using knob-into-hole (KiH) technology, where a “knob” mutation in one heavy chain fits into a “hole” mutation in the other, ensuring proper assembly and reducing unwanted byproducts during production. This refinement enhances manufacturability while preserving the antibody’s ability to engage both targets effectively.

To improve pharmacokinetics, mosunetuzumab incorporates Fc region modifications that limit interactions with Fc gamma receptors (FcγRs) and complement proteins, reducing off-target immune activation and prolonging serum half-life. Additionally, humanized antibody sequences help minimize immunogenicity, lowering the risk of anti-drug antibodies (ADAs) that could compromise treatment efficacy.

Receptor Binding Mechanisms

Mosunetuzumab’s function relies on its ability to simultaneously engage CD3 on T cells and CD20 on B cells. The CD3-binding domain targets the ε-chain of the CD3 complex, a key component of the T-cell receptor (TCR). By binding this region, mosunetuzumab facilitates T-cell engagement without requiring antigen presentation via major histocompatibility complex (MHC) molecules. This direct tethering bypasses the need for co-stimulatory signals, enabling a controlled immune response.

The CD20-binding domain targets an extracellular epitope of the CD20 molecule, expressed on both malignant and normal B cells. Unlike monoclonal antibodies that compete for the same CD20 epitope, mosunetuzumab binds a distinct site, allowing compatibility with prior or concurrent CD20-directed therapies. The antibody’s affinity for CD20 is optimized to maintain stable engagement without inducing Fc-mediated effector functions, as its therapeutic action relies on T-cell cytotoxicity rather than direct immune effector recruitment. This specificity ensures activity even in patients resistant to conventional anti-CD20 monoclonal antibodies like rituximab or obinutuzumab.

Once bound to both receptors, mosunetuzumab forms a bridge between T cells and B cells, bringing them into close proximity. This enforced synapse mimics the natural immunological synapse but functions independently of antigen presentation. The spatial arrangement enhances T-cell receptor engagement and downstream signaling, leading to activation. The antibody’s binding kinetics promote transient yet effective interactions, ensuring T cells exert cytotoxic effects without becoming exhausted.

Immunological Pathways

The immune response triggered by mosunetuzumab begins with CD3 engagement on T cells, causing a conformational shift in the TCR complex and phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs). This phosphorylation recruits signaling molecules like Lck and ZAP-70, which propagate activation through the LAT-SLP76 signaling axis. This cascade amplifies calcium flux and activates the nuclear factor of activated T cells (NFAT), driving cytokine production and effector differentiation.

Activated T cells secrete pro-inflammatory cytokines, including interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α), and interleukin-2 (IL-2). These cytokines enhance T-cell proliferation and modulate the tumor microenvironment. IFN-γ increases MHC expression on antigen-presenting cells, while TNF-α promotes apoptosis in malignant B cells. IL-2 sustains T-cell activation through autocrine and paracrine signaling. However, excessive cytokine release can lead to immune-related adverse events, necessitating careful dosing strategies.

Beyond cytokine signaling, mosunetuzumab affects T-cell metabolism, shifting energy utilization toward glycolysis. This adaptation supports sustained effector function by providing ATP for cytoskeletal rearrangements needed for immune synapse formation. Upregulation of glucose transporters like GLUT1 ensures adequate energy reserves, helping T cells function in the nutrient-deprived tumor microenvironment.

B-Cell Depletion Processes

Mosunetuzumab depletes B cells by triggering proximity-dependent cytolytic activity. Once the antibody bridges a T cell to a B cell, cytotoxic T cells release perforin and granzymes. Perforin facilitates granzyme entry into the target cell, where they induce apoptosis. This mechanism is particularly effective in hematologic malignancies, where systemic B-cell reduction is needed for disease control.

A key advantage of mosunetuzumab is its ability to overcome resistance mechanisms seen with traditional anti-CD20 monoclonal antibodies. Some B-cell malignancies evade treatment by downregulating CD20 expression, reducing susceptibility to single-target therapies. Mosunetuzumab circumvents this by engaging T cells directly rather than relying on Fc-mediated effector functions, maintaining efficacy even in relapsed patients.

Distinctions From Single-Target Therapeutics

Single-target monoclonal antibodies like rituximab and obinutuzumab have long been standard for B-cell malignancies, but their reliance on Fc-mediated mechanisms presents limitations. Tumor cells can evade these therapies by downregulating CD20 or through Fc receptor polymorphisms that reduce immune cell engagement. Mosunetuzumab offers an alternative by directly recruiting T cells, bypassing Fc receptor interactions and enabling sustained cytotoxic activity even in resistant patients.

Unlike single-target antibodies, mosunetuzumab induces immune synapse formation between T cells and B cells, enhancing T-cell activation and enabling serial killing, where a single T cell eliminates multiple malignant B cells sequentially. Traditional monoclonal antibodies depend on innate immune effectors like natural killer (NK) cells and macrophages, which may be less effective in immunosuppressive tumor environments. Mosunetuzumab’s mechanism reduces reliance on circulating effector cells, expanding its therapeutic potential for patients with compromised innate immunity.