High-Valency Receptor Crosslinking:
A New Therapeutic Approach
Antibodies bound to Fc-receptor-positive (FcR+) immune effector cells destroy cancer cells in two ways: Antibody-Dependent Cellular Cytotoxicity (ADCC) or receptor crosslinking agonism. ADCC relies on the release of cytokines from immune effector cells to kill cancer cells, while receptor crosslinking agonism forces apoptosis (programmed cell suicide) of the target cell through internal cell signaling. Activity of ADCC far outpaces receptor crosslinking in observable scale and off-target effects due to the massive release of cytokines by receptor-bound immune cells. Because of this, antibody therapies depend on ADCC as their primary mechanism to treat cancer.
Despite the success of antibody therapies, ADCC has notable problems due to a dependence on FcR+ cell binding. Cancer cells can develop decoy receptors, deactivate immune effector cells, or prevent entry of immune effector cells into the tumor microenvironment altogether, and prevent therapeutic activity. ADCC is also associated with intense, and potentially fatal, side effects from cytokine storm. Such side effects are common with any immunotherapies that rely on immune system activation to be effective. Patients face perilous circumstances enduring therapies that can seriously threaten their lives. Solutions are needed that can overcome these issues.
Apoptosis: Programmed cell death incurred by extrinsic or intrinsic biological pathways.
Polymer Antibody Conjugate (PAC): A therapeutic molecule, developed by Bastion Biologics, composed of a polymer backbone attached to multiple antibody fragments.
Receptor Crosslinking Agonism: When certain receptors are forced to cluster, they bind together at the cell surface, causing specific cell signals, e.g. apoptosis, cell proliferation, among others.
Valence: The number of targets that a molecule, like an antibody, can bind to simultaneously. Standard antibodies are bivalent, meaning they can bind only two targets at once.
A paradigm-shifting solution is the therapeutic application of receptor crosslinking agonism. Antibodies alone, however, are poor receptor crosslinking agonists due to their limited valence. Even with recruitment of the FcR+ cell, antibodies are ineffective receptor crosslinking agonists. Several receptor crosslinking antibodies have failed in the clinic with the failures attributed to insufficient crosslinking. At Bastion Biologics, we have shown that if receptor crosslinking is enhanced appropriately, it can potentially surpass ADCC in the clearing of cancer cells, while avoiding cytokine storm. Given the number of resistances to antibody therapy and intense side effects associated with ADCC, an approach that enhances receptor crosslinking agonism without activating ADCC could reduce cytokine storm and improve treatment response for patients.
Bastion’s Polymer Antibody Conjugates (PAC) employ high-valency surface receptor crosslinking agonism to eliminate cancer cells without ADCC. In the body, receptor crosslinking is initiated by multiple receptor-bound antibodies at the cell surface binding to FcR+ immune effector cells to force clustering and crosslinking of specific antibody-bound surface receptors. This agonist crosslinking triggers the apoptosis signal in the cell. Antibodies alone are unable to effectively crosslink receptors without FcR+ immune effector cells. These cells reduce the ability to capitalize on receptor crosslinking activity, due to massive ADCC activity when FcR+ cells bind to antibodies on the cell surface. Receptor crosslinking is therefore underutilized as a tool for cancer cell destruction.
The PAC approach harnesses the power of receptor crosslinking to treat cancer, eliminates the need for ADCC in antibody therapy, and eliminates the need for immune effector cells in crosslinking. This is done by synthetically imitating and enhancing immune effector cell crosslinking functions. PAC’s multivalent, polymer scaffold drastically improves the crosslinking agonism of antibodies without activating ADCC, making it a safer alternative with superior efficacy to other agonist antibody therapies.
Increasing Valence causes Cancer Cell Elimination
The highly valent approach of PAC enhances crosslinking that is weak, or nonexistent, with today’s antibody therapies. PAC can crosslink more than 10 receptors simultaneously, resulting in greatly improved rates of cancer cell apoptosis due to increased crosslinking.
Mechanistic studies showed a clear relationship between increasing PAC valence and increased rates of cancer cell apoptosis in Raji B-cells in vivo.
PAC outperforms standard-of-care in mouse models of Non-Hodgkin’s Lymphoma
Multiple mouse models showed long term, curative survival with PAC treatment in Non-Hodgkin’s Lymphoma (NHL) when tested in xenograft SCID mice targeting CD20 on Raji cells.
In vivo studies directly comparing PAC to standard-of-care anti-CD20 antibodies showed near, or total, elimination of tumors in SCID mice when using PAC, while mice treated with anti-CD20 antibodies all showed fatal signs of tumor progression.
PAC decreases side effects compared to Antibody Therapies and Chemotherapy
PAC works independently of the host immune system while keeping the advantages of antibody specificity. Cancer cells are targeted for specific destruction without activation of the immune system and the consequent cytokine release. This creates an advantage over other targeted and immunotherapies as PAC works independently of the patient immune system to be effective. Overstimulated immune activation side effects, such as cytokine storm, are commonplace with antibody therapy and other immunotherapies. Patients can potentially avoid these dangerous and fatal side effects.
The targeted nature of the PAC also reduces off-target effects common to chemotherapy and radiation. Fab’-MORF1 and P-MORF2, the two conjugates of PAC, are both inactive on their own. They cannot cause off-target effects. Only when the conjugates are bound together on the target receptor at the cell surface is agonistic crosslinking activity observed. Because of this, off-target effects are extremely limited when compared to other therapies.
An antibody fragment is attached to a DNA-like molecule (MORF1) to create a targeting scaffold (Fab’-MORF1).
Fab‘-MORF1 binds the target receptor on the malfunctioning cell.
A polymer is attached to multiple MORF2s (P-MORF2). MORF2 is complementary to MORF1.
P-MORF2 attaches to multiple Fab’-MORF1 at the cell surface. This causes target receptors to crosslink. Binding of P-MORF2 to Fab’-MORF1 is the therapeutically active step. Fab'-MORF1 can be premixed with P-MORF2 prior to administration or administered separately.
Crosslinking of (3 to 10+) cell receptors sends a signal downstream to engage pathways within the cell. These signals can cause desired downstream events depending on the target receptor, e.g. cell suicide, cell proliferation, immune system activation, among others.
PAC Can Overcome Common Resistances to Targeted Therapies
Endocytosis of Receptor-Antibody Construct
A common antibody resistance mechanism in NHL is for the cancer cell to engulf and eject antibodies bound to CD20 via endocytosis. This interferes with the ability of anti-CD20 antibodies to eliminate cancer cells. The polymer scaffold of PAC disrupts this process by not allowing target cancer cells to eject the PAC bound to CD20 while still initiating apoptosis of the cell, thus defeating the resistance mechanism in early studies. CD20+ Rituximab resistant cell lines have shown a failure to eject PAC bound to CD20 after endocytosis, leading to significantly increased apoptotic efficacy.
Bypassing the Intrinsic Apoptosis Pathway
Crosslinking of CD20 and the TNF receptor DR5 leads to apoptosis that bypasses the p53-dependent intrinsic pathway. This allows for potential treatment of cancers that show standard resistances along the common apoptosis pathways.
An Adaptable Platform: PAC Turns Failed Antibodies into New Therapies by Enhancing Crosslinking
The modular nature of the PAC platform provides the opportunity to make failed antibodies viable by applying them in the highly valent PAC scaffold. Many antibodies that have failed in the clinic have shown therapeutic promise using receptor crosslinking in preclinical studies. These approaches have shown insufficient crosslinking in vivo to be effective clinically. Inserting these failed antibodies in the PAC platform as targeting molecules could resurrect assets and provide value to patients where none existed previously.
Cellular Communication: Multiple Applications for Receptor Crosslinking
Receptor crosslinking is more than just a tool for apoptosis induction. Cells are using receptor crosslinking to communicate all the time. PAC has potential application across a myriad of receptors and cell types involved in apoptosis, immune activation, cell proliferation, among others. Bastion is currently exploring potential use in these applications to enhance today’s groundbreaking immunotherapies.
At Bastion Biologics, we have a unique solution to the agonism problem. Our PAC platform provides a powerful approach to defeat cancer.