Innovative Cancer Therapies: The Promise of Targeting CD47 in Immunotherapy

Cancer continues to be a major global health challenge, driving the need for innovative approaches to address its complexities and unresolved issues. Traditional cancer treatments have undoubtedly improved patient outcomes, but they often come with significant side effects and limited effectiveness for certain cancer types. Recently, immunotherapy has emerged as a highly promising alternative, especially for patients with advanced or metastatic cancers that are resistant to conventional treatments like chemotherapy and radiation therapy. Immunotherapy harnesses the immune system’s ability to recognize and eliminate cancer cells, offering more targeted and effective treatment options with fewer side effects.

While immunotherapy has shown remarkable success in many cancer patients, response rates can vary, and not everyone benefits from the current treatments. The tumor microenvironment (TME), consisting of various cells, molecules, and factors interacting with the tumor, plays a crucial role in shaping the immune response to cancer. Within the TME, cancer cells employ various strategies to evade immune detection, such as expressing immune checkpoint proteins like programmed death ligand-1 (PD-L1), which suppress immune activity by binding to receptors on their surface. Moreover, the TME can create an immunosuppressive environment that encourages the development of regulatory immune cells, such as regulatory T cells (Tregs), which inhibit anti-tumor immune responses. Thus, the TME significantly influences the growth and progression of cancer cells.

CD47, a cell surface protein found on many cell types, including cancer cells, is a key regulator of the TME and presents a promising target for cancer therapy. The interaction between CD47 on cancer cells and signal regulatory protein alpha (SIRPα) on myeloid cells sends a signal that prevents the cancer cells from being detected and destroyed by the immune system, allowing them to proliferate unchecked. Recently, blocking CD47 has emerged as a potential therapeutic strategy in cancer immunotherapy. By inhibiting CD47, the “don’t eat me” signal is disrupted, enabling immune cells to recognize and eliminate cancer cells. Preclinical studies have shown that blocking CD47 enhances the phagocytosis of cancer cells by macrophages and promotes anti-tumor immune responses. Various CD47-targeted therapies, including monoclonal antibodies, small molecule inhibitors, and nanotechnology-based delivery systems, are currently under development. Promising results from preclinical studies and early-phase clinical trials suggest that CD47-targeted therapy could be a significant breakthrough in cancer immunotherapy.

The success of immunotherapy approaches, such as immune checkpoint inhibitors (ICIs) and CD47-CAR-T cell therapy, depends on a favorable TME, characterized by the presence of immune cells and the absence of immunosuppressive factors. However, the functions and implications of CD47 in the TME of cancer patients undergoing immunotherapy are not yet fully understood. Additionally, there are potential challenges and limitations associated with CD47-targeted therapy, including off-target effects and resistance mechanisms.

This article aims to provide an overview of CD47’s role and will explore CD47’s interactions with other TME components, its impact on various immunotherapy outcomes, and the potential implications of CD47-targeted therapies in cancer treatment.

Understanding CD47: Structure and Function

CD47, initially identified as integrin-associated protein due to its co-purification with integrins in various cell types, has gained attention for its role in immune regulation. It was also recognized as an ovarian tumor marker, referred to as OA3, due to its high expression in ovarian carcinomas and limited presence in normal tissues. However, subsequent studies revealed a broader distribution of CD47 across various normal adult tissues and numerous non-hematopoietic cells, including melanoma and breast cancer cells. Elevated CD47 expression is associated with decreased progression-free survival in several cancers, making it a valuable prognostic marker.

CD47 in Cancer: Mechanisms and Therapeutic Potential

Preclinical and clinical studies have demonstrated the potential of targeting the CD47/SIRPα axis in cancer treatment. CD47 serves as a "don't eat me" signal to macrophages, helping cancer cells evade phagocytosis. Blocking CD47 with antibodies activates innate immunity, leading to pronounced antitumor effects and tumor cell death. This interaction also influences caspase-independent cell death, cell proliferation, adaptive immunity regulation, and NK cell homeostasis.

CD47 Structure and Ligands

CD47 is a member of the immunoglobulin superfamily of membrane proteins and comprises three main components: the NH2-terminal extracellular immunoglobulin variable domain (IgV)-like domain, a five-domain transmembrane region, and COOH-terminal splice variant cytoplasmic tails. The extracellular and transmembrane domains are critical for CD47-mediated signal transduction. The primary binding partners for CD47 include SIRPα, SIRPγ, TSP-1, and integrins. SIRPα binds to CD47 with high affinity, significantly stronger than SIRPγ, and plays a crucial role in transmitting inhibitory signals.

CD47-SIRPα Axis: Inhibition of Phagocytosis

The inhibition of myeloid cell-mediated innate immunity is a key function of the CD47-SIRPα axis. In macrophages, the phagocytosis process involves priming, synergy, and actin cytoskeleton remodeling. The binding of CD47 to SIRPα results in the phosphorylation of SIRPα tyrosine residues, recruiting SHP-1 and SHP-2 phosphatases. These phosphatases deactivate downstream signaling pathways, preventing macrophage phagocytosis triggered by immune activation receptors.

CD47 and Cancer Immune Evasion

Cancer cells exploit the CD47-SIRPα axis to evade immune detection. Studies have shown that CD47-deficient cells are rapidly cleared by macrophages, while high CD47 expression on leukemic cells correlates with increased tumorigenic potential. This immune evasion mechanism highlights the importance of CD47 as a therapeutic target. By blocking CD47, researchers can enhance macrophage-mediated phagocytosis and activate adaptive immune responses, providing a dual approach to combat cancer.

Integrin-Mediated Signaling and CD47

In addition to SIRPα, CD47 interacts with integrins and TSP-1, mediating various cellular functions. Integrins, widely expressed on different cell types, are particularly abundant on tumor cells and tumor-derived endothelial cells. These interactions regulate cell adhesion, migration, and proliferation. The extracellular domain of CD47 binds to the C-terminal domain of TSP-1, influencing cell signaling pathways.

Intercellular Signal Transduction by CD47

The interaction between CD47 and SIRPα is essential for maintaining immune homeostasis. It acts as a self-regulatory module under normal conditions, while cancer cells hijack this process to evade immune responses. Blocking CD47 can enhance antigen presentation by dendritic cells and activate T cells, bridging innate and adaptive immunity. This dual activation underscores the therapeutic potential of targeting CD47 in cancer treatment.

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Immunotherapies targeting the CD47-SIRPα Axis

The CD47-SIRPα axis is a prominent focus in the realm of cancer immunotherapy, representing a vital checkpoint for phagocytosis. Multiple therapies targeting this axis are in various stages of development, showing significant promise in enhancing the body’s immune response to cancer.

Antibodies and Recombinant Proteins: Blocking CD47-SIRPα Interactions

CD47 and SIRPα antibodies function by preventing their interaction, thereby promoting macrophage phagocytosis and inhibiting tumor growth. Several antibodies are currently undergoing clinical trials, such as Hu5F9-G4, ALX?148, TTI?621, and CC-90002. Among these, Hu5F9-G4, also known as magrolimab, is the most advanced. This humanized IgG4 monoclonal antibody binds with high affinity to human CD47, effectively targeting tumor cells.

Clinical Insights:

• Hu5F9-G4 (Magrolimab): Prominent in treating aggressive lymphoma, myelodysplastic syndrome (MDS), and acute myeloid leukemia (AML). Side effects like anemia were mitigated through a prime and maintenance dose strategy.
• Recombinant Proteins: ALX 148, TTI-621, and TTI-622 stand out for their enhanced affinity and lower molecular weight, allowing better tumor infiltration and higher efficacy in blocking CD47.

Advanced Therapeutic Approaches: Bispecific Antibodies and Engineered Bacteria

Bispecific Antibodies: Bispecific antibodies simultaneously target CD47 and tumor-specific antigens, leveraging macrophage recognition and antibody-dependent cellular phagocytosis (ADCP) to eradicate tumor cells. Examples include CD47/CD33 for AML and CD47/CD19 for B-cell lymphoma, showcasing improved tumor growth inhibition.

Engineered Bacteria: Bacteria can specifically target tumors through both passive entrapment in the tumor vasculature and active penetration due to their motility and the hypoxic tumor environment. Researchers have engineered non-pathogenic strains of Escherichia coli to express nanobody antagonists of CD47, resulting in durable phagocytosis, T cell activation, and systemic antitumor immunity.

Oncolytic Viruses: Multifaceted Cancer Therapy

Oncolytic viruses (OV) offer a unique therapeutic platform by lysing tumor cells, recruiting T cell tumor-infiltrating lymphocytes, and priming immune responses. Engineered OVs targeting the CD47-SIRPα axis have shown durable responses in preclinical models. Notably, OV-αCD47-IgG1 and OV-αCD47-IgG4 have been developed, with the former showing a stronger tumor-killing effect due to additional ADCP and antibody-dependent cellular cytotoxicity (ADCC) by NK cells.

Innovative Strategies: CAR Macrophages and Peptides

CAR Macrophages: Given that macrophages are abundant tumor-infiltrating cells, CAR-macrophages are increasingly researched over CAR-T cell therapy. CAR-macrophages can be engineered to target CD47, delivering CD47 blockers or silencing SIRPα to enhance antitumor activity. This approach exploits macrophages' natural tumor infiltration capabilities for more effective cancer treatment.

Peptides: Peptides offer advantages such as low immunogenicity, toxicity, and cost, as well as ease of manufacturing and storage. Specific peptides like D4-2 and Pep-20 have shown to effectively inhibit CD47-SIRPα interactions and promote macrophage-mediated phagocytosis, proving beneficial in cancer immunotherapy.

Small Molecules: Targeting CD47-SIRPα Axis

Small molecule inhibitors demonstrate antitumor effects through various mechanisms, including direct binding to CD47 and inhibition at transcriptional and translational levels. Myc inhibitor RRx-001 and QPCTL inhibitors like SEN177 and PQ912 have shown potential by reducing CD47-SIRPα interactions and enhancing phagocytosis.

Combining CD47-SIRPα Blockades with Other Therapies

Adaptive Immunotherapies: Combining CD47 blockades with adaptive immune checkpoint inhibitors like PD-1 enhances antitumor responses. Studies have shown synergistic effects in tumor models, resulting in improved tumor elimination and immune cell activation.

Macrophage Phagocytosis Activators: Combining CD47-SIRPα blockades with macrophage phagocytosis activators, such as tumor-specific monoclonal antibodies, enhances ADCP activity. This approach polarizes macrophages into a pro-inflammatory phenotype, promoting antitumor effects.

Reprogramming Cancer Cells: Chemotherapeutics and radiotherapy can stimulate cancer cells into immunogenic cell death (ICD), enhancing the efficacy of CD47-SIRPα blockades. Combining these treatments increases CRT expression and macrophage-mediated phagocytosis, providing a synergistic effect in cancer therapy.

Radiotherapy: CD47 blockade in combination with radiotherapy has shown to protect normal tissues while increasing tumor radiosensitivity. This combination promotes efficient tumor antigen cross-presentation, CD8+ T-cell expansion, and activation, enhancing the overall antitumor response.

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Challenges in targeting CD47

CD47-targeted therapies have emerged as promising cancer immunotherapy approaches due to their potential to enhance anti-tumor immunity by promoting the phagocytosis of cancer cells and reducing immune suppression in the tumor microenvironment (TME). Despite these opportunities, several significant challenges need to be addressed to optimize their effectiveness and safety.

One primary challenge is the expression of CD47 on both cancerous and healthy cells. CD47 acts as a "don't eat me" signal, preventing macrophages from phagocytosing cells that express it. Consequently, blocking CD47 can lead to unintended effects such as anemia and other hematological toxicities. This occurs because CD47 blockade removes the protective signal on healthy red blood cells (RBCs), making them targets for phagocytosis by macrophages in the spleen and liver. The extent of these toxicities can vary based on the dose, timing, and the patient's baseline health. To mitigate these effects, strategies such as dose optimization, co-administration with erythropoietin, and the development of modified or alternative CD47-targeting agents are being explored.

Another challenge lies in the fact that CD47 is not the only checkpoint molecule involved in immune evasion by tumors. Other molecules like PD-L1, CTLA-4, LAG-3, TIM-3, and IDO also play significant roles in suppressing the immune response to tumors. Therefore, targeting CD47 alone may not be sufficient for a durable anti-tumor response. This has led to interest in combination therapies that target multiple checkpoints simultaneously. Preclinical and clinical studies have shown promise for these approaches, suggesting that combining CD47 inhibitors with other immune checkpoint inhibitors can enhance the overall anti-tumor response.

Resistance to CD47-targeted therapies has also been observed, which presents another significant obstacle. Tumors may adapt by upregulating other immune checkpoint molecules or through mechanisms such as tumor heterogeneity, where varying expression levels of CD47 among cancer cells can lead to differences in susceptibility to therapy. This underscores the need for combination approaches and further research to understand and overcome resistance mechanisms. Additionally, the presence of immune-suppressive cells in the TME, such as myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs), tumor-associated dendritic cells (TADCs), and tumor-associated neutrophils (TANs), can interfere with the effectiveness of CD47-targeted therapies by upregulating other immune checkpoints and producing immune-suppressive cytokines.

To overcome these challenges, several strategies are being pursued. One promising approach is to combine CD47-targeted therapies with other immune checkpoint inhibitors, such as anti-PD-1/PD-L1 antibodies. This combination aims to tackle multiple immune evasion mechanisms employed by tumor cells simultaneously. Researchers are also developing bispecific antibodies that can target both CD47 and another checkpoint molecule, which may enhance therapeutic efficacy and reduce resistance.

Modulating the TME to enhance the efficacy of CD47-targeted therapies is another area of active research. This includes strategies to reduce the presence of immune-suppressive cells and factors within the TME, thereby improving the overall immune response against the tumor.

Despite these challenges, CD47-targeted therapies present significant opportunities for improving cancer treatment. They have the potential to enhance anti-tumor immunity by promoting the phagocytosis of cancer cells and reducing immune suppression in the TME. These therapies may also work synergistically with other immunotherapies, such as checkpoint inhibitors, to further enhance anti-tumor immunity and improve clinical outcomes. Furthermore, CD47 is expressed on many different types of cancer cells, making it a potentially broad target for cancer treatment.

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Future Directions

The CD47-SIRPα axis represents a promising target for cancer immunotherapy, with various strategies showing potential in preclinical and clinical studies. Ongoing research aims to optimize these therapies' efficacy, safety, and delivery methods, ensuring they become a mainstay in cancer treatment protocols. By combining CD47-SIRPα blockades with other therapeutic strategies, we can enhance immune responses and achieve more effective, long-lasting cancer treatment outcomes.

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