What CAR-M are being developed?

Introduction to CAR-M

Definition and Basic Concepts
Chimeric Antigen Receptor Macrophages (CAR‑Ms) represent an innovative class of cellular immunotherapies in which primary or stem‐cell–derived macrophages are genetically engineered to express a synthetic receptor that endows them with the ability to specifically recognize antigens on tumor cells. Unlike conventional therapies, CAR‑Ms harness the innate ability of macrophages to infiltrate the tumor microenvironment (TME), phagocytose cancer cells, and modulate local immune responses. In the construction of these receptors, the extracellular domain—usually a single-chain variable fragment (scFv) derived from an antibody—confers antigen specificity; meanwhile, the transmembrane and intracellular signaling domains (such as CD3ζ, Fc receptor gamma FcRγ, and Megf10 domains) trigger phagocytic functions and pro-inflammatory cytokine production once the CAR engages its target antigen. CAR‑Ms are designed to overcome some of the limitations of T‑cell–based therapies (CAR‑T cells), particularly by capitalizing on macrophages’ inherent tendency to accumulate within solid tumors and remodel an otherwise immunosuppressive TME. These cells can additionally act as potent antigen-presenting cells, thereby linking innate and adaptive immunity and potentially “educating” other immune effectors against the tumor.

Comparison with CAR‑T Therapies
While both CAR‑T and CAR‑M therapies involve the genetic modification of immune cells to express synthetic receptors for targeting cancer, they differ significantly in their biologic properties and clinical applications. CAR‑T cells have demonstrated remarkable success in hematological malignancies but face challenges in penetrating the solid TME and sometimes trigger severe toxicities such as cytokine release syndrome (CRS) and neurotoxicity. In contrast, CAR‑Ms can assimilate into the TME due to the natural migratory ability of macrophages and are less likely to circulate systemically for long periods, which may translate into a lower toxicity risk. Furthermore, whereas CAR‑T cells primarily function by inducing direct cytotoxicity via granzyme and perforin release, CAR‑Ms can mediate tumor clearance through phagocytosis and by secreting cytokines that convert immunosuppressive M2 macrophages into tumoricidal M1 macrophages. This dual role—not only in the direct elimination of tumor cells but also in modulating the microenvironment—potentially allows CAR‑Ms to synergize with other modalities, including immune checkpoint inhibitors and adoptively transferred T cells.

Current CAR-M Therapies in Development

Leading CAR-M Therapies
The landscape of CAR‑M development is rapidly expanding. Several leading CAR‑M therapies have been designed with different target antigens and intracellular signaling domains to address various types of solid tumors.

One of the most notable CAR‑M therapies under development is the anti‑HER2 CAR‑M, exemplified by Carisma Therapeutics’ CT‑0508. This candidate uses a chimeric construct targeting the human epidermal growth factor receptor 2 (HER2), which is overexpressed on many solid tumors, including certain breast and gastric cancers. CT‑0508 has been engineered using an adenoviral vector (Ad5f35) to efficiently transduce patient‑derived macrophages, which, upon reinfusion, infiltrate the tumor tissue, mediate antigen‑specific phagocytosis, and are capable of remodeling the TME toward a pro‑inflammatory state that attracts cytotoxic T cells.

Another promising candidate in this arena is the mesothelin‑targeting CAR‑M, commonly referred to as CT‑1119. Mesothelin is expressed on various solid tumors, such as mesothelioma and ovarian cancers. Preclinical studies have demonstrated that CT‑1119 can mediate effective tumor cell phagocytosis, release pro‑inflammatory cytokines, and control tumor growth in lung cancer models.

Innovative approaches are also exploring the derivation of CAR‑Ms from induced pluripotent stem cells (iPSCs), leading to the development of CAR‑iMacs. This strategy allows for the generation of large numbers of CAR‑expressing macrophages from a single donor sample, potentially lowering manufacturing costs and streamlining production logistics. Preliminary in vitro and in vivo investigations have indicated that these iPSC‑derived CAR‑Macrophages demonstrate significant antibody‑dependent phagocytosis, tumor burden shrinkage, and persistence lasting several weeks in tumor models.

In addition, several preclinical studies have evaluated CAR‑Ms with different intracellular signaling domain configurations. For example, constructs incorporating the CD3ζ domain or the FcRγ signaling domain have been compared, with some studies indicating that CAR‑M with FcRγ may exhibit enhanced phagocytic and tumor‑killing capacity relative to those with alternative signaling domains. Researchers are further optimizing these constructs by adding domains that recruit PI3K or other signaling molecules that can amplify macrophage activation and sustenance of the desired M1 phenotype.

Companies and Research Institutions Involved
CAR‑M development is being driven by both academic research institutions and biotech companies. Prominent among these is Carisma Therapeutics, which has spearheaded the clinical development of CT‑0508, an anti‑HER2 CAR‑M currently in early‑phase human trials. The University of Pennsylvania, which has been a key collaborator with Carisma Therapeutics, has contributed significantly to the preclinical work that established the feasibility of using adenovirus‑transduced CAR‑Macrophages. Moreover, research groups at other leading institutions are also involved in advancing CAR‑M technology—for instance, pioneering work by Klichinsky et al. has provided foundational insights into CAR‑M biology and mechanism of action.

On the commercial front, additional companies have filed patents relating to CAR‑M and multifunctional immune cell therapies. For instance, patents describe combination approaches that integrate CAR‑M with other immunotherapeutic modalities, such as CAR‑T or CAR‑NK cells, hinting at future strategies wherein multimodal immune cell therapies may be employed together to overcome the immunosuppressive TME in solid tumors. These intellectual property documents indicate that the field is not only advancing from an academic standpoint but is also garnering commercial interest aimed at producing off‑the‑shelf, scalable CAR‑M products.

Mechanism of Action and Benefits

How CAR-M Works
CAR‑Macrophages are engineered by introducing a CAR construct into macrophages via viral vectors (commonly adenoviral vectors such as Ad5f35) or increasingly by non‑viral and genome‑editing techniques. The CAR construct typically comprises:
• An extracellular antigen‑binding domain that allows the engineered macrophages to recognize tumor‑associated antigens like HER2 and mesothelin with high specificity.
• A transmembrane domain that anchors the CAR into the macrophage membrane.
• An intracellular signaling domain that may include motifs such as CD3ζ, FcRγ, or combinations with PI3K/p85 recruitment domains. These domains trigger a cascade of signals upon antigen binding that result in the activation of downstream pathways responsible for phagocytosis, cytokine production, and antigen presentation.

Upon engaging their target antigen, CAR‑Ms initiate an antigen‑specific phagocytic response that leads to the internalization and digestion of tumor cells. Concurrently, these engineered macrophages secrete pro‑inflammatory cytokines and chemokines that help convert the immunosuppressive M2 macrophages present in the TME into more tumoricidal M1 phenotypes. In doing so, CAR‑Ms not only directly reduce the tumor cellular mass but also promote the recruitment and activation of other immune effectors, such as T cells and natural killer (NK) cells, thus amplifying the overall antitumor response.

Furthermore, this process of antigen uptake and subsequent antigen cross‑presentation can potentially “educate” the adaptive immune system, thereby generating a more durable and systemic immune response against the tumor. This capacity to bridge innate and adaptive immunity uniquely positions CAR‑Ms as a versatile option for combating cancers that are otherwise resistant to conventional therapies.

Potential Advantages over Other Therapies
CAR‑M therapy presents several distinct advantages when compared to other forms of immunotherapy, particularly CAR‑T cell therapy:
• Superior Infiltration of Solid Tumors: Macrophages have an inherent propensity to home to and accumulate within solid tumors. Their natural ability to traffic into the TME enables them to overcome the physical barriers—such as dense extracellular matrices and abnormal vasculature—that often impede T‑cell entry.
• Reduced Systemic Toxicity: Because macrophages generally have a limited lifespan in circulation and tend to remain localized once they infiltrate the tumor, there is a reduced risk of systemic side effects such as severe CRS, which is a prominent issue with CAR‑T therapies.
• Phagocytosis and Immune Modulation: Beyond merely killing tumor cells by cytotoxic mechanisms, CAR‑Ms actively engulf and digest tumor cells. In addition, they secrete a milieu of cytokines that can transform the TME from immunosuppressive to immunostimulatory, thereby promoting the activation of other immune cells including T cells and NK cells.
• Antigen Cross-Presentation: The ability of macrophages to process and present tumor antigens to T cells may facilitate a secondary wave of immune activation, potentially leading to long‑term immune memory and reducing the likelihood of tumor relapse.
• Potential for Off‑the‑Shelf Application: With innovations such as iPSC‑derived CAR‑Ms (CAR‑iMacs), there is the potential to manufacture standardized, off‑the‑shelf products rather than relying solely on autologous cells. This could streamline manufacturing, reduce production costs, and ultimately broaden patient access to these therapies.

Collectively, these advantages suggest that CAR‑M therapies might be particularly well suited for treating solid tumors—a domain where conventional CAR‑T therapies have, to date, shown limited success.

Challenges and Regulatory Considerations

Development Challenges
Notwithstanding the promising preclinical and early clinical data, CAR‑M therapies face several substantial development challenges that must be surmounted before they can achieve widespread clinical adoption:
• Manufacturing Hurdles: One of the primary technical challenges is the efficient gene delivery into primary macrophages. Macrophages are notoriously resistant to conventional viral vector transduction due to their intrinsic innate immune defenses. Although the use of adenoviral vectors such as Ad5f35 has shown promise in overcoming these barriers, ensuring consistent transduction efficiency and cell viability remains challenging. Furthermore, primary macrophages do not proliferate ex vivo, which creates a bottleneck in obtaining sufficient cell numbers for therapy—a problem that is driving interest in developing iPSC-derived CAR‑Ms.
• Phenotype Stability: There is a concern about the plasticity of macrophages. Once engineered, CAR‑Ms must maintain a tumoricidal M1 phenotype rather than reverting to an M2 phenotype, which can inadvertently promote tumor growth. Optimizing the CAR construct—including the choice of intracellular signaling domains—and the ex vivo culture conditions is crucial to ensure long‑term phenotype stability.
• Safety Concerns: Although the risk of systemic toxicity such as CRS appears lower than with CAR‑T cells, potential off‑target effects and the possibility of insertional mutagenesis (especially when using viral vectors) must be rigorously evaluated. The scope of inflammatory cytokine release by activated macrophages also requires careful modulation to prevent local tissue damage.
• Scalability and Reproducibility: Meeting regulatory standards for cell therapy manufacturing is complex. Given that CAR‑M therapy is in its early stages, establishing scalable processes that yield reproducible cell products meeting good manufacturing practice (GMP) standards is a significant hurdle.

Regulatory Pathways
Given that CAR‑M therapy is a relatively new frontier in cellular immunotherapy, regulatory pathways remain in evolution. Current clinical trials, such as the phase I trial for CT‑0508 targeting HER2‑overexpressing tumors, are pivotal in establishing the safety and efficacy profiles of these therapies. Regulatory agencies such as the U.S. Food and Drug Administration (FDA) or the European Medicines Agency (EMA) require extensive characterization of the cell product, demonstration of manufacturing consistency, and rigorous preclinical safety data before granting investigational new drug (IND) applications. Patents detailing the CAR‑M construct design and combination therapy approaches further highlight an ongoing effort to protect intellectual property while complying with regulatory standards. Collaborative efforts between academic institutions and biotech companies are crucial in navigating these regulatory challenges, ensuring that preclinical successes translate effectively into clinical benefit.

Future Directions and Research

Innovations in CAR-M Technology
The next generation of CAR‑M technology is expected to benefit from multiple lines of innovative research. Current efforts are focused on refining the CAR design to maximize therapeutic efficacy while minimizing adverse effects. For instance, some researchers are exploring “tandem” CAR constructs that incorporate multiple intracellular signaling domains (e.g., combining the PI3K recruitment domain with FcRγ or CD3ζ domains) to enhance phagocytosis and sustain macrophage activation. Such structural innovations are aimed at tripling the cellular phagocytic capacity and reinforcing antigen cross‑presentation, thereby amplifying the downstream activation of adaptive immune responses.

Beyond receptor engineering, there is growing interest in alternative gene delivery methods. Non‑viral delivery techniques, including electroporation of mRNA or the use of nanoparticle systems, are being investigated to overcome the innate resistance of primary macrophages to viral vectors and to avoid risks associated with insertional mutagenesis. For example, some groups have successfully utilized lipid‑nanoparticle platforms for in vivo CAR expression in myeloid cells, offering a potential off‑the‑shelf solution that bypasses cumbersome ex vivo manufacturing processes.

Moreover, the field is embracing the potential of induced pluripotent stem cell (iPSC) technology to generate CAR‑Ms. iPSC‑derived CAR‑Macrophages, often termed CAR‑iMacs, enable the generation of large quantities of functionally consistent, genetically engineered macrophages that can be produced from a single donor sample. This approach not only addresses scalability concerns but also opens avenues for creating “universal” or allogeneic CAR‑M therapies that could be administered to multiple patients without provoking graft‑versus‑host disease (GVHD).

Finally, the integration of CAR‑M therapy with other immunomodulatory strategies is a vibrant area of research. As patents suggest, combination therapies that leverage the advantages of both adaptive and innate immune systems—such as co‑administration with CAR‑T cells or checkpoint‑inhibitors—could lead to a synergistic attack on tumors. These combination strategies are designed to prime the TME, enhance tumor antigen presentation, and facilitate the sustained activation of a broad immune response against the tumor.

Potential Applications and Clinical Trials
CAR‑M therapy holds promise across a variety of cancer types, particularly solid tumors where conventional cellular therapies have encountered significant challenges. The primary targets currently under investigation include HER2‑positive tumors—a subset that benefits from the robust preclinical data associated with CT‑0508—as well as mesothelin‑expressing malignancies such as ovarian cancer, mesothelioma, and lung cancer. Early clinical trials, such as those investigating CT‑0508 in HER2‑overexpressing solid tumors, are already underway, with initial data suggesting favorable safety profiles and evidence of TME remodeling.

Another promising application lies in the use of CAR‑Ms for inflammatory and even neurodegenerative conditions. Recent preclinical work suggests that by modifying the intracellular domains of the CAR construct, it is possible to impart anti‑inflammatory functions that mimic the M2 phenotype. For example, studies have reported that CAR‑M cells engineered to target pro‑inflammatory M1‑type microglia can reduce neuroinflammation and may be applied to treat inflammation-associated depression. This expands the potential clinical applications of CAR‑M technology beyond oncology to include a range of inflammatory systemic and neurological diseases.

In parallel, combination therapies using CAR‑M with CAR‑T or CAR‑NK cells are being explored. By recruiting different components of the immune system, these approaches aim to overcome barriers inherent to single‑modality treatments. Such combination strategies may prove especially effective in “cold” tumors that lack sufficient immune cell infiltration, as the inclusion of CAR‑M cells could help reprogram and “heat up” the tumor, paving the way for a more robust antitumor immune response.

In summary, ongoing and planned clinical trials, together with active patent filings, indicate that the pipeline for CAR‑M therapy is rich and diverse, with multifaceted approaches in both design and application that hold the promise of addressing unmet medical needs in oncology and beyond.

Conclusion
In conclusion, the development of CAR‑Macrophage therapies represents a notable evolution in the field of cellular immunotherapy. These engineered macrophages are being developed to overcome challenges that have limited the efficacy of conventional CAR‑T cell therapies in solid tumors. Leading candidates such as Carisma Therapeutics’ CT‑0508 (targeting HER2) and CT‑1119 (targeting mesothelin) are already showing considerable promise in preclinical models and early phase clinical trials. The innovative use of different intracellular signaling domains—ranging from CD3ζ and FcRγ to tandem recruitment of PI3K—has opened new avenues to enhance phagocytosis, antigen presentation, and sustained immune activation.

The mechanisms of CAR‑M action, rooted in the inherent capacity of macrophages to infiltrate the tumor microenvironment and orchestrate immune responses, provide multiple potential advantages over CAR‑T cells. These include superior tumor infiltration, reduced systemic toxicities, enhanced phagocytic function, and the unique ability to reeducate the TME by converting immunosuppressive M2 macrophages into pro‑inflammatory M1 cells. Despite these promising aspects, significant challenges remain, including issues related to efficient gene transduction, phenotypic stability, manufacturing scalability, and regulatory approval. Regulatory considerations are particularly pertinent as these are emerging therapies, with a limited number of clinical trials validating their safety and efficacy.

Future directions in CAR‑M research are encouraging and multifaceted. There is a strong impetus to refine CAR design by integrating multiple intracellular signaling domains and exploring non‑viral gene delivery systems that promise enhanced safety and efficacy. The development of iPSC‑derived CAR‑Ms (CAR‑iMacs) opens the door to scalable, off‑the‑shelf therapies applicable to a broad patient population. Moreover, combination strategies that integrate CAR‑M therapy with other immunomodulatory treatments, such as CAR‑T cells, CAR‑NK cells, and immune checkpoint inhibitors, may yield synergistic clinical benefits by simultaneously targeting both the tumor and its immunosuppressive microenvironment.

Overall, CAR‑M therapies are positioned as a transformative approach in the treatment of solid tumors and potentially other diseases beyond oncology, such as inflammatory and neurodegenerative conditions. The continued collaborative efforts among academic institutions, biotech companies, and regulatory agencies will be essential to overcoming current challenges and translating these innovations into effective, safe, and scalable clinical products. As research progresses, the integration of novel engineering techniques, advances in gene delivery, and strategic combination therapies will likely propel CAR‑M technology to become one of the key pillars in next-generation cancer immunotherapy.

Detailed Conclusion:
The breadth and depth of CAR‑M development underscore that this field is in a dynamic phase of evolution. With a strong foundation built on the basic principles of antigen recognition and macrophage biology, researchers have successfully engineered CAR‑Ms that not only directly attack tumor cells via phagocytosis but also modulate the TME to enhance overall antitumor immunity. Leading candidates such as CT‑0508 and CT‑1119 are being validated in early clinical settings, while innovative approaches using iPSC‑derived macrophages offer scalable solutions for future clinical applications. Despite significant developmental and regulatory challenges, including gene transduction, phenotype maintenance, and ensuring manufacturing consistency under GMP conditions, continuous advancements in CAR design and combination therapies promise to overcome these obstacles. In essence, CAR‑M therapies hold tremendous potential to redefine treatment paradigms for solid tumors and possibly a range of inflammatory conditions, representing an exciting frontier in cellular immunotherapy that is expected to mature significantly in the coming years.