What is the role of monoclonal antibodies in immunotherapeutic targeting in pediatric oncology?

Updated: Mar 20, 2018
  • Author: Crystal L Mackall, MD; Chief Editor: Jennifer Reikes Willert, MD  more...
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When Kohler and Milstein reported the technology for generating monoclonal antibodies (mAbs) in 1975, many tumor biologists expected the rapid evolution of various antibodies that could act as “magic bullets” to target and kill tumors. [103] Although progress in the field of mAb therapy for neoplasia has proceeded much slower than was initially anticipated, recent clinical trials have demonstrated antitumor activity in various malignancies.

Effective mAb therapy for cancer requires the identification of appropriate tumor-specific targets expressed on the surface of the cancer cells. For effective targeting, the molecule should be relatively tumor-specific (eg, show substantially less binding to normal tissues or binding to only “dispensable tissue” (eg, B cells) and should be highly expressed in tumors (compared with normal tissues).

In addition, the target of the mAb should not be shed from the tumor following mAb binding; rather, the antigen-mAb complex should be internalized by the tumor cell. Importantly, many antigens are down-regulated after antibody binding, which can limit the effectiveness of the therapy.

Furthermore, because mAbs are theoretically “foreign proteins” and therefore could induce an immune response that might limit their efficacy, they must be rendered sufficiently nonimmunogenic to prevent development of neutralizing antibodies.

mAbs were initially produced by fusing murine myeloma cells with B cells from mice immunized with specific antigens. Because mAbs generated in this manner are murine proteins, they are recognized as foreign in immunocompetent humans and, thus, generate neutralizing antimurine antibodies termed human antimouse antibodies (HAMAs). After development of these antibodies in patients, the half-life of mAbs is greatly reduced, which significantly reduces their biologic activity.

Advances in genetic engineering subsequently allowed the creation of chimeric antibodies, which use human constant regions and retain the entire murine variable region, and humanized antibodies, which retain only the murine hypervariable region responsible for epitope binding. The rest of the variable region and the entire constant region are human derived. More recently, fully human mAbs have become readily available and further limit the likelihood of neutralizing antibody development.

The overall success of mAb therapy in cancer is determined by the ability of antibody binding to result in tumor cell death. This can occur via 1 of the following 3 pathways, which are not mutually exclusive:

  • Immune effects through antibody-dependent cellular cytotoxicity (ADCC) and complement-mediated cytolysis - Binding the antibody recruits cells with Fc receptors to the site of the tumor, which then kill the tumor cell, or complement is fixed and the tumor cell is killed.

  • Direct killing - Key process include interruption of a critical cell signaling cascade by inhibition of ligand binding; downregulation of a receptor tyrosine kinase, which transmits a necessary life signal; and induction of an apoptotic signal following ligation of the target by the mAb.

  • Targeting via a conjugated antibody of antibody receptor (eg, radionuclide, immunotoxin, cell-based genetic fusion) - This targets a lethal “hit” to the tumor cell.

Ongoing work in this field is focused on identifying approaches to increase ADCC by using agents such as granulocyte-macrophage colony-stimulating factor (GM-CSF) to improve recruitment of effector cells in combination with an mAb for refractory osteosarcoma and neuroblastoma. [104, 105, 106] Indeed, differences in the relative potency of ADCC effects with differing antibodies [107] and with genetic variation appear to be a primary determining factor in whether mAb therapy for cancer is effective. [28]

ADCC appears to be the primary mechanism of action for mAbs targeting the GD2 disialoganglioside in neuroblastoma. Studies have shown that neuroblastoma is susceptible to ADCC via lymphocytes, neutrophils, and activated macrophages. GD2 is expressed at high densities on nearly all neuroblastoma cells, is not shed from the cell surface, and is restricted to neuroectodermal tissues, thus representing a good potential target for mAb therapy. [108] Several antibodies directed against GD2 on the surface of neuroblastoma cells have been developed.

Initial studies using 3F8, an anti-GD2 mAb, demonstrated that the primary adverse effects of therapy were limited to results of acute toxicity. [109] Despite toxicity, 3F8 mAb can be administered in the outpatient setting with symptomatic management of the toxic effects. Results of initial nonrandomized clinical trials report a long-term disease-free survival rate in patients with stage IV neuroblastoma that is comparable to the rate in historical control subjects without mAb exposure. [109]

Interestingly, evidence suggests that low levels of HAMAs and the development of nonneutralizing anti-idiotypic antibodies (antibodies directed to the variable region of the immunizing antibody rather than to the constant region, which is the target of most HAMAs) correlate with improved survival rates after adjuvant 3F8 mAb therapy. [110] Trials using another anti-GD2 mAb, hu14.18, linked with either interleukin (IL)-2 or GM-CSF, also had toxicity that was reversible, but no clinical responses were documented. [111, 105]

mAb therapy may also effectively kill tumors when the mAb induces death in tumor cells after antibody binding. [112] Examples of this approach include the anti-CD20 mAb rituximab in lymphoma [113] and anti-CD99 in Ewing sarcoma. [114, 115] Unfortunately, CD99 is also expressed on hematopoietic progenitors and T cells, thus limiting its clinical potential for tumor targeting.

In pediatric studies, mAbs targeting vascular endothelial growth factor (VEGF) as well as the immunoglobulin (Ig)F-1 receptor have shown progress in preliminary reports, presumably via interruption of signaling pathways critical for tumor survival. [116, 117]

Nonconjugated antibodies can also induce cell death if they crosslink a cell surface receptor that can initiate a downstream death cascade. Antibodies to the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors show significant activity in vitro in pediatric tumors by initiating such a death pathway. [118] Antibodies to CD40 can induce similar effects in melanoma and some carcinomas. [119, 120]

Recent evidence has suggested that the capacity to induce cell death as a result of mAb binding can be synergistic with cytotoxic chemotherapy. Whether this synergism results from enhanced ADCC, from enhanced effects of an interrupted growth signal, or from both is not clear in every case. One example of this is trastuzumab, which targets the HER2 or NEU protooncogene in breast cancer and has substantially increased the chemotherapy response rate in various clinical trials. [121]

On the basis of the association between HER2 expression and worse survival in osteosarcoma, [122] trastuzumab has recently been used in conjunction with standard chemotherapy for HER2 -positive osteosarcoma. However, a concern with this approach is that the HER2 expression in osteosarcoma does not result from gene amplification, as it does in breast carcinoma; therefore, interrupting HER2 signaling is not likely to result in death in this tumor.

A third mechanism by which antibodies can kill tumor cells involves targeted delivery of a lethal agent, such as a toxin or radionuclide. With rituximab, an unconjugated mAb against CD20, a 96% overall response rate was reported in one phase II trial for lymphocyte-predominant Hodgkin lymphoma, with 75% remaining in remission after one year. [123] Another phase II trial showed similar results. [124]

A phase II pilot study is underway through the Children’s Oncology Group to assess the toxicity of adding rituximab to upfront chemotherapy for B-cell leukemia and lymphoma (see Clinicaltrials.gov).

Conjugating anti-CD20 to radionuclides has shown success in children with Hodgkin lymphoma, although the bone marrow toxicity is substantial; therefore, this approach is generally undertaken in the setting of marrow rescue. [125] Other mAbs that have been conjugated to toxins in pediatric oncology include anti-CD25 and anti-CD30, conjugated with ricin for Hodgkin lymphoma, [126, 127] and anti-CD22, conjugated with pseudomonal exotoxin for acute lymphoblastic leukemia (ALL). [128]

In pediatric acute myeloid leukemia (AML), the differentiation antigen CD33 is expressed in almost all patients. The US Food and Drug Administration (FDA) approved anti-CD33 conjugated with calicheamicin (gemtuzumab [Mylotarg]) for treatment of adult AML in 2000, but the agent was withdrawn from the US market on June 21, 2010.

Apart from some infusional allergic reactions, the primary toxicity of this approach has been bone marrow suppression caused by binding the mAb-toxin conjugate to normal hematopoietic precursors that express CD33. Another still unexplained toxicity of anti-CD33–calicheamicin conjugates is hepatic damage, which is characterized by transient increases in liver enzymes in approximately 25% of patients and, occasionally, a more severe complication consistent with veno-occlusive disease. [129]

Currently, the agent is being tested both as a single agent and in combination with chemotherapy in children with AML; however, results of phase I and II clinical trials indicate promise, with an overall remission response rate of 45% and a 1-year event-free survival and overall survival estimates of 38% and 53%, respectively. [130, 131]

Genetic engineering has recently opened serious prospects of using the cytolytic machinery of T cells or natural killer cells with the effective targeting properties of antibodies via creation of so-called chimeric antigen receptors (CARs). In this case, a single-chain Fv-Fc fragment of an mAb is fused to the signaling chain of the T-cell receptor and is introduced genetically into T cells or natural killer cells.

The cells are then systemically delivered or delivered at the site of the tumor; following engagement of the chimeric receptor with its target, T-cell activation or natural killer cell activation leads to tumor cytolysis. Promising preclinical results have been recently reported using direct administration of such T cells expressing CARs in a murine model of medulloblastoma. [132]

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