Monoclonal antibodies (mAbs), as the name implies, are clonal antibodies that bind to the same antigen. mAbs are broadly used as diagnostic or therapeutic tools for neoplasms, autoimmune diseases, allergic conditions, and infections.
The concept of antibodies dates back to the 18th century when Edward Jenner discovered their role in generating immunity by injecting fluid from a smallpox pustule into recipients [1]. Antibodies (Abs) are glycoproteins belonging to the immunoglobulin superfamily, secreted by plasma cells after B cell lymphocytes encounter foreign antigens. Abs are highly specific and can target various chemical compositions on living/non-living substrates or cell surfaces. Abs are typically produced in labs from animals like rodents, rats, rabbits, goats, horses, camels, or chickens. Using the hybridoma (hybrid between antibody-producing spleen cells and myeloma cells) technique [4], primed clonal B cells can be made immortal thereby continuing to produce mAbs “forever”. By RNA or protein sequencing, the amino acid codes of the binding domains (VH and VL) of any Ab can be defined. With sequences in hand, X-ray crystallography can dissect their 3D structures with precision. In fact, using next-generation sequencing, the entire B cell receptor (BCR) repertoire, i.e., the VH and VL domains of all the B cells can be deduced from a B-cell pool, and the predominant clones (matched VH and VL sequences) predicted using bioinformatics. Today, people can build phage libraries from VH and VL sequences derived from naïve or primed B cells from most species that people can use to fish out mAbs without the need for a living mouse. People even have transgenic rodents carrying human VH and VL sequences that people can immunize to make fully human mAbs. With the current level of sophistication, Abs can be taken apart and built back like legos into forms that fit the purpose of use. Maneuvers such as isotype switch (from IgG1 to IgA), Fc enhancement (to increase FcR affinity), or Fc silencing (to reduce cytokine release syndrome), affinity maturation, multi-specificities, and multi-functionalities are actively being used, and explored. Increasingly, these artificial Ab forms are used to genetically modify the target specificity of both T cells and NK cells in cytotherapy [5][6].
The invention of the hybridoma technique in 1975 [4] marked a milestone in producing mAbs for human use. Since then, over 100 therapeutic mAbs have been authorized by the FDA for various indications in adults[7]. mAbs are clinically valuable in children for both disease diagnosis and treatment. As in vitro diagnostics, they accurately identify normal cells, determine their lineage, and activation status, and detect tumor cells with specific surface antigens like CD3 on T-lymphocytes. Lineage-specific mAbs enable precise diagnosis of immunodeficiencies and hematological malignancies. In treatment, mAbs have shown high efficacy against autoimmune diseases and infectious agents, serving as primary therapy or salvage options for drug-resistant cases[8]. They can be tailored to reduce cytokines, excess Abs, and alloreactive T cells, or enhance immune functions. Classic examples include palivizumab for respiratory syncytial virus [9] and COVID-19 mAbs under emergency use authorization [10][11][12].
Pediatric cancer privileges a survival rate of ≥80% in high-income countries, reaching 95% in acute lymphoblastic leukemia (ALL) or Wilms tumor [11][12][13]. However, the chances of survival are highly variable within tumor entities and among geographic areas in the world. Metastatic or relapsed sarcomas, high-grade brain tumors, and some rare pediatric cancers have a dismal prognosis with no relevant therapeutic advances in decades. Unintended deaths from childhood cancers in low- and middle-income countries, once diagnosed, result from the abandonment of treatment in the setting of complex and intensive treatment regimens, death from toxicity because of insufficient supportive care options, and relapse [12]. The high frequency of long-term sequelae among childhood cancer survivors (nearly 50% with moderate to high multi-organ late effects) [13] has also overshadowed improvement in survival, demanding a reconsideration of treatment intensification typically saddled with toxicities driven to their limits. mAbs are attractive therapeutic alternatives and have already demonstrated efficacy in a variety of childhood cancers [14].
The application of mAb therapy in childhood cancer is restricted due to several biological barriers. This section focuses on anti-GD2 mAbs, which have the most clinical experience in pediatric solid tumors.
Pediatric tumors differ significantly from adult cancers as they arise from embryonal cells with distinct genetic and epigenetic aberrations. Pediatric tumors have a low mutational burden, resulting in few neo-antigens and druggable targets. This reduces the frequency of anti-tumor T and B cells and hampers the immune response[83][84]. Targeting alternative tumor-associated antigens, such as GD2, B7H3, L1CAM, GPC3, polysialic acid, DLL3, and HER2, may hold promise for B cell-specific therapies such as mAbs[85][86][87][88]. For B cell targets, tissue distribution of the target is key. The density and the heterogeneity of the target will determine which tumor will escape. Insufficient GD2 density, plus both intratumoral and intertumoral heterogeneity, can account for the failure of anti-GD2 mAbs in tumors other than neuroblastoma (see Figure 1) [47][48][58][59][60].
Figure 1. Biologic Resistance to anti-GD2 mAbs in neuroblastoma. Antigen loss or antigen internalization; epigenetic down-regulation of gangliosides synthesis; inter/intratumoral antigen heterogeneity. Poor penetration of mAbs because of mechanical barriers; mAbs disposal through internalization. Impaired effector cells because of chemo-radiotherapy; FcR polymorphisms or KIR mismatch; ineffective tumor infiltration by effector cells due to an inhibitory microenvironment (MDSCs, TAM, TAF); direct immunosuppression by tumors and their released products; paucity of mutations and neoantigens, absent or downregulation of HLA expression, low immunogenicity escaping tumor surveillance. DCs, dendritic cells; FcR, Fc receptor; KIR, killer immunoglobulin-like receptor; MDSCs, myeloid-derived suppressor cells; Polysia, polysialic acid; Treg, regulatory T cells; TAF, tumor-associated fibroblast; TAM, tumor-associated macrophage; TCR, T cell receptor; TMB, tumor mutational burden; ? means absent or downregulation of HLA expression, low immunogenicity escaping tumor surveillance.
Repeated exposure to sub-optimal doses of antigen-specific targeting can lead to antigen modulation. Antigens can be lost by release, internalization, or trogocytosis [89] [90][91][1](Figure 1). Acquired genetic alterations as seen in adults, are probably rare in pediatric tumors [3]. Strategies to address this issue include dual-targeting therapies and induction of antigen re-expression. [2][92] (Table 1).
Table 1.
Resistant mechanisms to anti-GD2 therapy and potential alternatives.
Mechanisms of Anti-GD2 Resistance | Strategies to Overcome Them |
---|---|
Antigen loss or downregulation by epigenetic modulation | EZH2 inhibition |
Poor tumor penetration | Increased payload: Antibody-drug-conjugates Radio-immunotherapy conjugates Drug delivery platforms |
Impaired effector functions | Fc engineering Engaging T-cells by bispecific antibodies Co-administration with certain cytokines or immuno-conjugates Co-administration with granulocyte-macrophage colony-stimulating factor GM-CSF Early administration of antibodies within the cytotoxic therapeutic plan GD2 conjugated vaccines |
Solid tumors pose challenges for mAb therapy penetration due to leaky vessels, limited lymphatics, altered interstitial pressure, and tumor stroma[93] (Figure 1). Other factors influencing Ab distribution and retention in the tumor include Ab size, affinity, and specificity. Engineered Ab fragments could penetrate better, but their small size below the renal threshold forces their rapid clearance into the urine rendering them sub-therapeutic [94][95]. Higher Ab affinity and higher antigen expression could mitigate the poor retention of small Ab fragments while increasing the cytotoxic payload could amplify the therapeutic effect. Payload optimization has been successful in at least three approaches: (a) drug conjugates, (b) radio-immuno-conjugates, and (c) drug delivery platforms.
Drug conjugates, specifically antibody-drug conjugates (ADCs), were developed to increase drug selectivity and reduce unintended systemic toxicity. ADCs utilize antibodies (Abs) to deliver toxic agents precisely to tumor sites, aiming to widen the safety margins between efficacy and side effects. The therapeutic index (TI), which compares drug exposure in tumors to that in normal organs, plays a crucial role. While ADCs show promise, challenges remain, including myelotoxicity and on-target off-tumor effects. Linker chemistry improvements help ensure plasma stability, preventing premature release of cytotoxic payloads. However, the development of anti-drug antibodies (ADA) remains a hurdle when modifying human IgGs. Despite passing efficacy assays, ADCs are expected to encounter toxicities limiting dose escalation in patients [96]. Anti-GD2 ADCs have shown potent cytotoxicity in vitro across various tumor cell lines, but their clinical use will depend on managing toxicity in children [97].
Radio-immuno-conjugates use radioisotopes for cancer treatment, but their development has been hindered by suboptimal therapeutic indices and supply chain issues. This limits their application in children. Testing anti-GD2 131I-3F8 in children with metastatic neuroblastoma showed responses in soft tissue and bone marrow, but survival did not improve compared to non-radiolabeled 3F8. Compartmental delivery with intra-Ommaya 131I-3F8 aimed to reduce systemic toxicity and achieved modest success in certain pediatric patients with relapsed neuroblastoma or metastatic medulloblastoma, leading to long-term remissions in some cases (NCT00445965)[98][99].
Refinement of drug delivery platforms remains the key challenge if toxic payloads need further dose escalation to achieve cures. Multi-step Targeting (MST) separates antibody delivery from payload administration, reducing unintended toxicity. Pretargeted strategies (PRIT) using bispecific Abs (BsAbs) offer highly specific targeting, achieving tumor-to-blood ratios not previously possible[100]. BsAbs accumulate in the tumor before payload administration, ensuring precise engagement. Self-assembling and disassembling Abs (SADAs) are designed to stay large in the tumor for penetration and small in the bloodstream to minimize immunogenicity. SADAs successfully deliver radioisotopes without organ toxicity in preclinical models[101]. First human trials of SADA in GD2-positive tumors are ongoing (NCT05130255).
Induced host immunity is crucial for durable remission in mAb-treated patients. Anti-idiotypic networks and human antimouse antibodies (HAMA) response may play roles in the anti-tumor response [124][125]. Early administration of anti-GD2 mAbs, combined with induction chemotherapy and immunomodulatory agents, has shown improved objective responses in pediatric patients[126] (Table 1).
The limited success of mAbs in clinical trials may be attributed in part to inadequate patient selection criteria, including risk stratification and target antigen expression. Clinical trials often do not require confirmation of the target antigen expression before enrollment, leading to potential inefficacy in cases where the target antigen is absent or low-density. Biomarkers, such as Fc receptor polymorphisms[127], KIR mismatch[128], and minimal residual disease [129], have been associated with clinical outcomes and response to mAb therapy, but need further validation.
Theranostics, which uses the same mAb for in vivo diagnostic imaging and therapy, has emerged as an appealing drug platform in antibody therapy. Pretargeted radio-immuno-diagnosis is the companion diagnostics for PRIT. It utilizes 177Lu for SPECT and 86Y for PET with high precision; at the same time, 177Lu is used for beta therapy, and 225Ac for alpha therapy [130]. Using whole IgG as a carrier, 68Ga and 64Cu have been used to monitor neuroblastoma during treatment with anti-GD2 [131][132]. Liquid biopsies based on the patient's tumor genotype and circulating GD2 levels have shown potential for detecting residual disease, but further clinical validation is needed at predefined times during treatment[133][134].
mAbs exhibit limited antitumor activity as monotherapy, but their efficacy improves when combined with cytokines, chemotherapy, radiotherapy, and kinase inhibitors. Anti-GD2 mAbs show clinical benefits primarily in patients with minimal residual disease (MRD) or exclusive bone/bone marrow involvement, with limited response in soft tissue bulky tumors [11][55][56][57]. However, their combination with chemotherapy, as demonstrated in studies like ANBL1221 and HITS, has proven to be safe and effective for refractory/relapsed neuroblastoma patients. The BEACON study also supports the benefit of adding dinutuximab beta to the standard salvage chemotherapy regimen for relapsed neuroblastoma. Currently, various clinical trials, backed by COG and St. Jude, are investigating the potential benefits of introducing anti-GD2 mAbs in induction therapy[126].
Managing the unique toxicity profile of mAbs poses challenges and requires specialized care teams and dedicated facilities. Visceral pain is a characteristic side effect that demands careful administration and pain management with analgesics and sedation. Efforts to reduce pain side effects through antibody engineering, like the K322A mutation, have had limited success[126]. Prolonged infusion times and desensitization strategies are being explored to minimize side effects and enhance patient acceptance [135][136].
Pharmaceutical companies' interest in rare diseases, including pediatric cancer, is growing. However, the profit-driven model often leads to exorbitant drug prices, making mAbs unavailable in developing countries where most children with cancer live (Figure 2). To improve access to life-saving treatments, a cost-driven model similar to handling pandemic vaccines could be adopted, prioritizing need over wealth. Governments can play a crucial role by prioritizing pediatric cancer as a national concern and supporting initiatives that incentivize pharmaceutical companies to invest in pediatric research.
Figure 2.
Authorized anti-GD2 mAbs by national regulatory agencies.
Streamlining regulatory processes and approval procedures is essential to expedite the availability of pediatric drugs, ensuring timely access to life-saving treatments for children [137][138]. Cost-effectiveness considerations must be adapted to suit the pediatric population's unique needs, accounting for long-term consequences and the impact on family members and caregivers.
Innovative approaches such as basket trials based on shared targets and expanded age eligibility are crucial to advance pediatric cancer treatments, given the limited number of available pediatric patients for early-phase trials [139]. Specialized centers of excellence can improve drug delivery efficiency, accessibility, and patient outcomes by unifying care plans and treatment sites, reducing paperwork, and providing training and education for healthcare professionals.
All these barriers are summarized in Figure 3.
Figure 3.
Identified barriers to mAbs access and proposed recommendations. HCP: health care professionals.
Significant progress has been made in mAb-based anticancer therapies in the past decade, and ongoing innovations in protein engineering and immunobiology offer promising avenues for the future. However, challenges persist, including target heterogeneity, tumor microenvironments, and the need for accurate biomarkers. Novel strategies are being explored to overcome these limitations, improving responses and survival in children with cancer.
The projected growth of the mAb market indicates its potential impact on cancer treatment. Academic researchers hold the responsibility to advance new therapeutics, while regulatory agencies should facilitate their translation into clinical practice. Economists and social scientists can promote health policies and international collaboration to maximize the benefits of mAbs as life-changing therapeutics. With concerted efforts, mAbs can bring about transformative change in cancer treatment.