Action of a CD24-specific deglycosylated ricin-A-chain immunotoxin in conventional and novel models of small-cell-lung-cancer xenograft

From mechanism to therapy: the journey of CD24 in cancer

CD24 is a glycosylphosphatidylinositol-anchored protein that is expressed in a wide range of tissues and cell types. It is involved in a variety of physiological and pathological processes, including cell adhesion, migration, differentiation, and apoptosis. Additionally, CD24 has been studied extensively in the context of cancer, where it has been found to play a role in tumor growth, invasion, and metastasis. In recent years, there has been growing interest in CD24 as a potential therapeutic target for cancer treatment. This review summarizes the current knowledge of CD24, including its structure, function, and its role in cancer. Finally, we provide insights into potential clinical application of CD24 and discuss possible approaches for the development of targeted cancer therapies.

Targeting CD24 in Cancer Immunotherapy

Immunotherapy is a hot area in cancer treatment, and one of the keys to this therapy is the identification of the right tumour-associated or tumour-specific antigen. Cluster of differentiation 24 (CD24) is an emerging tumour-associated antigen that is commonly and highly expressed in various tumours. In addition, CD24 is associated with several cancer-related signalling pathways and closely interacts with other molecules and immune cells to influence tumour progression. Monoclonal antibodies, antibody-drug conjugates (ADCs), chimeric antigen receptor (CAR) T-cell therapy, and CAR-NK cell therapy are currently available for the treatment of CD24. In this review, we summarise the existing therapeutic approaches and possible future directions targeting CD24.

CD24 blockade as a novel strategy for cancer treatment

The CD24 protein is a heat-stable protein with a small core that undergoes extensive glycosylation. It is expressed on the surface of various normal cells, including lymphocytes, epithelial cells, and inflammatory cells. CD24 exerts its function by binding to different ligands. Numerous studies have demonstrated the close association of CD24 with tumor occurrence and progression. CD24 not only facilitates tumor cell proliferation, metastasis, and immune evasion but also plays a role in tumor initiation, thus, serving as a marker on the surface of cancer stem cells (CSCs). Additionally, CD24 induces drug resistance in various tumor cells following chemotherapy. To counteract the tumor-promoting effects of CD24, several treatment strategies targeting CD24 have been explored, such as the use of CD24 monoclonal antibodies (mAb) alone, the combination of CD24 and chemotoxic drugs, or the combination of these drugs with other targeted immunotherapeutic techniques. Regardless of the approach, targeting CD24 has demonstrated significant anti-tumor effects. Therefore, the present study focuses on anti-tumor therapy and provides a comprehensive review of the structure and fundamental physiological function of CD24 and its impact on tumor development, and suggests that targeting CD24 may represent an effective strategy for treating malignant tumors.

CD24 as a Potential Therapeutic Target in Patients with B-Cell Leukemia and Lymphoma: Current Insights

CD24 is a highly glycosylated glycophosphatidylinositol (GPI)-anchored protein that is expressed in many types of differentiating cells and some mature cells of the immune system as well as the central nervous system. CD24 has been extensively used as a biomarker for developing B cells as its expression levels change over the course of B cell development. Functionally, engagement of CD24 induces apoptosis in developing B cells and restricts cell growth in more mature cell types. Interestingly, CD24 is also expressed on many hematological and solid tumors. As such, it has been investigated as a therapeutic target in many solid tumors including ovarian, colorectal, pancreatic, lung and others. Most of the B-cell leukemias and lymphomas studied to date express CD24 but its role as a therapeutic target in these malignancies has, thus far, been understudied. Here, I review what is known about CD24 biology with a focus on B cell development and activation followed by a brief overview of how CD24 is being targeted in solid tumors. This is followed by an assessment of the value of CD24 as a therapeutic target in B cell leukemia and lymphoma in humans, including an evaluation of the challenges in using CD24 as a target considering its pattern of expression on normal cells.

CD24: A Novel Target for Cancer Immunotherapy

Cluster of differentiation 24 (CD24) is a small, highly glycosylated cell adhesion protein that is normally expressed by immune as well as epithelial, neural, and muscle cells. Tumor CD24 expression has been linked with alterations in several oncogenic signaling pathways. In addition, the CD24/Siglec-10 interaction has been implicated in tumor immune evasion, inhibiting macrophage-mediated phagocytosis as well as natural killer (NK) cell cytotoxicity. CD24 blockade has shown promising results in preclinical studies. Although there are limited data on efficacy, monoclonal antibodies against CD24 have demonstrated clinical safety and tolerability in two clinical trials. Other treatment modalities evaluated in the preclinical setting include antibody–drug conjugates and chimeric antigen receptor (CAR) T cell therapy. In this review, we summarize current evidence and future perspectives on CD24 as a potential target for cancer immunotherapy.

CD24, A Review of its Role in Tumor Diagnosis, Progression and Therapy

CD24, is a mucin-like GPI-anchored molecules. By immunohistochemistry, it is widely detected in many solid tumors, such as breast cancers, genital system cancers, digestive system cancers, neural system cancers and so on. The functional roles of CD24 are either fulfilled by combination with ligands or participate in signal transduction, which mediate the initiation and progression of neoplasms. However, the character of CD24 remains to be intriguing because there are still opposite voices about the impact of CD24 on tumors. In preclinical studies, CD24 target therapies, including monoclonal antibodies, target silencing by RNA interference and immunotherapy, have shown us brighten futures on the anti-tumor application. Nevertheless, evidences based on clinical studies are urgently needed. Here, with expectancy to spark new ideas, we summarize the relevant studies about CD24 from a tumor perspective.

Novel insights into the function of CD24: A driving force in cancer

CD24 is a highly glycosylated protein with a small protein core that is linked to the plasma membrane via a glycosyl-phosphatidylinositol anchor. CD24 is primarily expressed by immune cells but is often overexpressed in human tumors. In cancer, CD24 is a regulator of cell migration, invasion and proliferation. Its expression is associated with poor prognosis and it is used as cancer stemness marker. Recently, CD24 on tumor cells was identified as a phagocytic inhibitor ("do not eat me" signal) having a suppressive role in tumor immunity via binding to Siglec-10 on macrophages. This finding is reminiscent of the demonstration that soluble CD24-Fc can dampen the immune system in autoimmune disease. In the present review, we summarize recent progress on the role of the CD24-Siglec-10 binding axis at the interface between tumor cells and the immune system, and the role of CD24 genetic polymorphisms in cancer. We describe the specific function of cytoplasmic CD24 and discuss the presence of CD24 on tumor-released extracellular vesicles. Finally, we evaluate the potential of CD24-based immunotherapy.

Targets in small cell lung cancer

Recurrent small cell lung cancer is a recalcitrant malgnancy. The application of genomic technologies has begun to elucidate the large number of genetic abnormalities in SCLC. Several cell surface receptors are known to be overexpressed by SCLC in clinic specimens and cell in culture including GPCRs such as the bradykinin receptor, the chemokine receptor CXCR4, the vasopression receeptor and the three bomebsin receptors. The glucose transporter GLUT1, the tetraspanin family member PETA/CD151 and the immunoglobulin superfamily member ALCAM/CD166 are also overexpressed by SCLC. NCAM/CD56 is overexpressed by nearly all SCLC and is currently the target for an antibody drug conjugate in Phase II trial. Although SCLC is not considered a RTK driven disease, IGF1R and FGFRs are often overexpressed by SCLC. SCLC abberantly expresses several developmental transcription factors including ASCL1, SOX2, 4, and 11, OCT4, NANOG, PAX5; however, overexpression of MYC may be a driver in SCLC. Like other cancers, SCLC expresses survival factors and uses aerobic glycolysis as a major source of ATP. The drawback of many potential targets overexpressed by SCLC is expression of the same proteins by normal tissues. We are slowly learning more about the molecular abnormalities that occur in SCLC; however, therapeutic impact from new findings remains a goal to work toward.

Targeted immunotherapy for colorectal cancer: monoclonal antibodies and immunotoxins

Colorectal cancer (CRC) is a major health concern worldwide. It is the third most frequently diagnosed cancer and the second leading cause of cancer death. There currently are a number of treatment options for CRC, however many of them have failed to demonstrate desired therapeutic benefit. Therefore, significant efforts are being directed towards the development of new biological therapies with improved efficacy. Immunotherapy is an emerging treatment modality for a variety of cancers. Several promising treatments have already been approved by the US FDA and are being tested in clinical trials. Antibodies have been proved to be useful in cancer therapy due to their ability to recognize tumor-associated antigens expressed at higher density on malignant cells in comparison with those that are normal. Antibodies can be used as a single therapy or in combination with other therapies. A large variety of monoclonal antibodies have been developed. However, only a very few are able to kill a sufficient number of malignant cells and cause tumor regression. Hence, it is often necessary to arm the antibody with a cytotoxic agent to enhance the efficacy of the anti-tumor activity. This review provides a brief overview of some of the current agents being employed in targeted immunotherapy for CRC.

Immunotoxin therapy of hematologic malignancies

Patients with chemotherapy relapsed or refractory hematologic malignancies may be effectively treated with allogeneic or autologous stem cell transplants. However, many patients cannot be transplanted due to age, comorbidities, or lack of suitable donors. Further, a fraction of patients relapse post-transplant. Novel therapeutic agents that can kill multidrug-resistant malignant stem cells and are not myelosuppressive are needed. One class of such agents is immunotoxins. Immunotoxins consist of cell-selective ligands covalently linked to peptide toxins. The ligand delivers the molecule to specific cell surface receptors on malignant cells. The toxin triggers cell death either by reaching the cytosol and catalytically inactivating vital cell processes or by modifying the tumor cell surface membrane. We have synthesized immunotoxins for therapy of chemoresistant hematologic diseases. In this review, we will detail the synthesis of a number of these drugs and describe their preclinical and clinical activity. Several of these agents have shown dramatic antitumor effects in patients with hematologic neoplasms, and one immunotoxin has been approved for use by the US Food and Drug Administration (FDA). Over the next several decades, a growing number of these agents should reach the clinic.

Potent anti-tumor effects of an anti-CD24 ricin A-chain immunotoxin in vitro and in a disseminated human Burkitt's lymphoma model in SCID mice

A new anti-CD24 immunotoxin (IT), SWA11.dgA, was constructed by coupling the MAb SWA11 via the bivalent linker SMPT to deglycosylated ricin A-chain (dgA). The effects of SWA11.dgA were evaluated in vitro against the B-precursor leukemia cell line REH, the non-B-non-T acute lymphoblastic leukemia cell line NALM-6 and the Burkitt's lymphoma cell lines BL-2 and BL-38. Binding of SWA11 to the CD24 antigen was assessed by flow cytometry demonstrating high affinity of the MAb for all cell lines tested. SWA11.dgA inhibited the protein synthesis of BL-38, NALM-6, REH and BL-2 cells by 50% at concentrations (IC50) of 4.0 x 10(-11) M, 6.0 x 10(-11) M, 8.0 x 10(-11) M and 3.0 x 10(-9) M, respectively. SWA11.dgA was subsequently used for the treatment of disseminated human BL-38 Burkitt's lymphoma in a newly developed SCID mouse model. The mean survival time (MST) of BL-38-bearing SCID mice was extended from 23 days in untreated controls to more than 230 days when 6 microg SWA11.dgA was applied intraperitoneally one day after tumor challenge. All of the animals achieved continuous complete remissions. SCID mice treated with SWA11.dgA 4 days after tumor cell challenge or a reduced dose of SWA11.dgA (67%) also had a significantly extended MST (45.0 and 51.4 days, respectively, as compared to 22.7 and 23.1 days in the controls). We conclude that SWA11.dgA might be of potential use for the treatment of lymphoma in man.

Expression of a B-cell marker, CD24, on nasopharyngeal carcinoma cells

Random sequencing of clones from a lambda gt10 cDNA library, made from mRNA expressed in an Epstein-Barr virus (EBV)-associated nasopharyngeal carcinoma (NPC) has revealed the gene transcript of human CD24. The CD24 antigen, a glycosylphosphatidylinositol-anchored cell surface molecule, has been identified as a B-cell marker that is lost during cell maturation. We show here that it is expressed on 3 NPC xenografts, previously defined as consisting of poorly differentiated epithelial cells, and on an NPC biopsy. In the case of the former, the level of expression of CD24 corresponds to the EBV load. A B-lymphoblastoid cell line carrying the same EBV genome as one of the tumours, C15, and an EBV-negative Burkitt's lymphoma cell line do not display the antigen, but epithelial-like cells of a laryngeal tumour cell line (Hep2) do express it. Our data suggest that CD24 may be a marker of cell differentiation not only for B cells but also for epithelial cells and may have an indirect association with EBV gene expression.

Immunotoxins recognising a new epitope on the neural cell adhesion molecule have potent cytotoxic effects against small cell lung cancer

The present study describes a comparison of two potent immunotoxins which utilise an identical targeting component, a monoclonal antibody (SEN7) specific for small cell lung cancer (SCLC), conjugated to two different effector components, blocked ricin (bR) and Pseudomonas exotoxin A (PE). SEN7 recognises a novel epitope on the neural cell adhesion molecule (NCAM) which is highly associated with SCLC. The immunotoxins SEN7-PE and SEN7-bR were selectively and potently active against a number of SCLC cell lines, of both classic and variant morphologies, inhibiting the incorporation of [3H]leucine with IC50 values ranging between 22 pM and 85 pM and between 7 pM and 62 pM for SEN7-PE and SEN7-bR respectively. Intoxication by both immunotoxins proceeded rapidly following short 2 h lag phases; the initial rates of protein synthesis inhibition occurred with t50 values of 6.5 h for SEN7-PE and 5.5 h for SEN7-bR. Monensin drastically enhanced the cytotoxic activity of the weakly active SEN7-ricin A-chain by 2,100-fold and of SEN7-bR by 80-fold but had no effect on SEN7-PE. In limiting dilution assays, four and more than 4.5 logs of clonogenic SW2 tumour cells were selectively eliminated from the cultures during continuous exposure to the immunotoxins SEN7-PE and SEN7-bR respectively, while antigen-negative cells required up to 1,000-fold more drug for a similar cell kill. SW2 cells surviving SEN7-bR treatment in the cultures did not express NCAM and consequently were not selectively killed by SEN7 immunotoxins. SW2 cells surviving continuous exposure to SEN7-PE showed no alteration in NCAM expression but were more resistant to intoxication mediated by PE. These cells were still sensitive to SEN7-bR.

Antibodies to the protein core of the small cell lung cancer workshop antigen cluster-w4 and to the leucocyte workshop antigen CD24 recognize the same short protein sequence leucine-alanine-proline

We recently described the identity of the small cell lung cancer (SCLC) cluster-w4 antigen and the human B cell differentiation marker CD24, a glycosylphosphatidylinositol (GPI)-anchored, highly glycosylated surface molecule of only 31-35 amino acids [15]. The specificities of three anti-cluster-w4 and of eleven anti-CD24 MoAbs have been investigated with respect to their binding capacity to the protein core of cluster-w4/CD24 antigen. Four overlapping peptides spanning this protein core were synthesized. MoAbs shown to bind to two overlapping peptides by antibody binding inhibition using the cluster-w4/CD24-positive SCLC cell line SW2 and by direct peptide binding detected in an ELISA were investigated in more detail. To determine the exact epitopes recognized by these MoAbs, an epitope mapping assay using peptides synthesized onto polyethylene pins was established. The three anti-cluster-w4 MoAbs SWA11, SWA21 and SWA22 and the anti-CD24 MoAbs OKB2 and ALB9 recognized the same short leucine-alanine-proline (LAP) sequence in an area without potential glycosylation sites close to the GPI anchor of the protein core of the cluster-w4/CD24 antigen.

Anti-tumor activity of a blocked ricin immunotoxin with specificity against the cluster-5A antigen associated with human small-cell lung cancer

The monoclonal antibody (MAb SEN31, a mouse IgG1 which recognizes the cluster-5a antigen on small-cell lung cancer (SCLC) cells, was used to prepare a selective and potent blocked ricin immunotoxin. In a series of experiments in vitro and in a SCLC xenograft model in nude mice, the tumor localization potential of the radiolabeled antibody SEN31 and the anti-tumor activity of the immunotoxin SEN31-bR, the non-specific binding activity of which had been greatly reduced by blocking of the galactose binding domains of the B-chain, was determined. Radiolabeling of SEN31 was performed by linking a 67Ga-labeled desferrioxamine moiety to the oligosaccharide side chains of the antibody in order to preserve the specific cell-binding activity. 67Ga-SEN31 bound to the antigenic sites on cells of the SW2 SCLC cell line, with a dissociation constant of 3.5 nM and, when injected i.v., selectively localized at the site of s.c.-growing SW2 tumor xenografts in nude mice, with a tumor-to-blood ratio of 3.5. The immunotoxin SEN31-bR was potently and selectively active against SCLC cell lines both of classic and of variant morphologies. At a concentration of 300 pM the immunotoxin selectively eliminated 4.5 logs of clonogenic tumor cells. In nude mice, SEN31-bR was cleared from the blood with biphasic kinetics following i.v. injection and maintained a stable serum level during continuous i.p. infusion. The growth of s.c. SW2 solid-tumor xenografts was delayed following a single i.v. injection or a continuous i.p. infusion, each at a non-toxic dose.

Immunotoxins to human small-cell lung cancer

Ricin A chain ITs directed against a variety of the common cell-surface antigens associated with SCLC exerted selective toxic effects on SCLC cell lines. The potency of the cytotoxic effects matched or exceeded that previously reported for ricin A chain ITs directed against identical or similar antigens on other types of carcinoma, suggesting that SCLC may be uniquely sensitive to this type of IT.