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Table of Contents
Year : 2019  |  Volume : 2  |  Issue : 2  |  Page : 88-95

Chimeric antigen receptor T-cells for glioblastoma: The journey ahead

Center for Cell and Gene Therapy, Texas Children’s Hospital, Houston Methodist Hospital, Baylor College of Medicine; Departments of Pediatrics, Baylor College of Medicine; Texas Children's Cancer and Hematology Centers, Texas Children's Hospital, Houston, TX, USA

Date of Web Publication27-Jun-2019

Correspondence Address:
Dr. Meenakshi Hegde
Departments of Pediatrics, Baylor College of Medicine, 1102 Bates Avenue, Houston, TX 77030
Dr. Sujith Kurian Joseph
Departments of Pediatrics, Baylor College of Medicine, 1102 Bates Avenue, Houston, TX 77030
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/glioma.glioma_6_19

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Chimeric antigen receptors (CARs) are genetically engineered transmembrane cell receptors consisting of an antigen-binding ectodomain fused to an activating intracellular T-cell signaling chain. Grafting CAR molecules on T-cells enables targeted killing of tumors. The gene therapy approach of modifying autologous patient immune cells with CARs has now moved from the research bench to early-phase clinical trials in patients with refractory, recurrent, and nonresectable glioblastoma. This is a review of the state of the science in the field of CAR T-cells for glioblastoma and an update on the completed and ongoing clinical trials available at the Clinical Trials Registry (www.clinicaltrials.gov). Here, we also discuss insights gained from the clinical trials of CAR T-cells against glioblastoma and innovative approaches to improve their efficacy.

Keywords: Chemokine, chimeric antigen receptor T-cells, chimeric antigen receptors, clinical trials, glioblastoma, immune cells, immunotherapy, microenvironment

How to cite this article:
Joseph SK, Ahmed NM, Hegde M. Chimeric antigen receptor T-cells for glioblastoma: The journey ahead. Glioma 2019;2:88-95

How to cite this URL:
Joseph SK, Ahmed NM, Hegde M. Chimeric antigen receptor T-cells for glioblastoma: The journey ahead. Glioma [serial online] 2019 [cited 2023 Oct 3];2:88-95. Available from: http://www.jglioma.com/text.asp?2019/2/2/88/261676

  Introduction Top

Chimeric antigen receptors (CARs) are synthetic fusion molecules consisting of an ectodomain that binds to a tumor-associated antigen and a signal transducing endodomain that triggers an intracellular T-cell signaling cascade.[1] The most abundantly used antigen-recognizing ectodomains are single-chain variable fragments of antibodies.[2] Multiple preclinical studies have also successfully used peptide ligands, muteins, and centyrins, though of these ectodomains, only interleukin (IL) 13 receptor alpha 2-targeted mutein has been tested in humans (NCT03288493).[3],[4],[5],[6] In most iterations, the ζ chain of the T-cell receptor complex provides signal 1 for phosphorylation of immunoreceptor tyrosine-based activation motifs, which is required for T-cell activation and killing.[1] To provide a co-stimulatory T-cell activation signal (Signal 2) in order to prevent T-cell anergy when engaging the target antigen, improved CARs have incorporated co-stimulatory signaling endodomains such as CD28 (phosphoinositide 3-kinase pathway), CD134/OX40, and CD137/4-1BB (tumor necrosis factor receptor-associated factor pathway) alone or in combination.[7],[8] Further improvements to provide Signal 3 using constitutive cytokine expression of IL-2, IL-7, or IL-15 have also been developed. The cytokine-expressing CAR T-cells are referred to as TRUCK T-cells, indicating that these cells deliver a payload to the target site.[9]

CAR T-cells, directed against the pan B-cell antigen CD19, Kymriah™ (tisagenlecleucel) and Yescarta™ (axicabtagene ciloleucel), recently became the first US Food and Drug Administration-approved gene therapy in the clinic. A relatively large cohort, long-term persistence Phase I/IIA study of CD19 CAR T-cells against refractory/relapsed acute lymphoblastic leukemia reported complete remission in 27/30 patients (90%), with a 6-month event-free survival rate of 67% and an overall survival rate of 78%.[10],[11] This trial was followed by the successful use of CD19 CAR T-cells against other advanced B-cell malignancies, including diffuse large B-cell lymphoma, advanced lymphomas, and chronic lymphocytic leukemia. In this study, 8/15 patients achieved complete remission, four achieved partial remission, and one had stable disease at the time of reporting.[12]

The antitumor efficacy of CAR T-cells against solid tumors to date is not as impressive as against hematological malignancies. A Phase 1 study testing Epstein–Barr virus-specific T-cell receptor T-cells or activated T-cells engrafted with CARs targeting the surface disialoganglioside GD2 in neuroblastoma patients demonstrated the therapy's safety and moderate efficacy (NCT00085930). A single infusion of the GD2-specific CAR T-cells led to complete remission in three out of 11 patients with active disease (27%); however, the expansion of infused cells at the periphery could not be detected even in responders.[13] In another Phase 1 study in sarcoma patients, human epidermal growth factor receptor 2 (HER-2)-targeted CAR T-cells demonstrated safety in all patients, but the best response was stable disease in 4 out of 19 evaluable patients for 12–14 months (NCT00902044).[14] CAR T-cells targeting carcinoembryonic antigen (CEA) in colorectal cancers, carcinoembryonic antigen related cell adhesion molecule 5 (CEACAM5) in CEACAM5+ malignancies, PMSA in prostate cancer, c-Met in metastatic breast cancer, carboxy-anhydrase-IX in metastatic renal cell carcinoma, tumor-associated glycoprotein-72 in colorectal cancer, and mesothelin in pancreatic ductal adenocarcinoma have demonstrated variable safety and antitumor efficacy profiles in early clinical trials.[15],[16],[17],[18],[19]

In glioblastoma (GBM), CAR T-cells redirected against antigens such as the epidermal growth factor receptor variant III (EGFRvIII), HER-2 (ErbB2/neu), and IL-13 receptor alpha 2 (IL-13Rα2) have shown safety and some efficacy in early-phase clinical trials.[20],[21],[22],[23] A Phase 1 trial of CAR-modified cytomegalovirus-specific T-cells targeting HER-2 by Ahmed et al.[21] demonstrated safety and objective response in 8 out of 16 evaluable GBM patients (9 adults and 7 children; NCT01109095). Of these patients, one had a partial response for over 9 months, 7 had stable disease (including three patients that survived >3 years), and 8 progressed after T-cell infusion. Similarly, Brown et al.[22] demonstrated that multiple infusions of IL-13Rα2 CAR T-cells were safe and led to a complete response in one patient with recurrent multifocal GBM that lasted for 7.5 months (NCT02208362). A summary of completed and ongoing clinical trials against GBM using CAR T-cells is provided in [Table 1].
Table 1: List of chimeric antigen receptor T-cell clinical trials registered at the Clinical Trials Registry for patients with glioblastoma

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CAR T-cell therapy still faces multiple challenges against GBM. While the challenges posed by the unique anatomical location and presence of the blood–brain barrier (BBB) prevent most infused CAR T-cells from reaching their destination, tumor-intrinsic challenges such as antigen heterogeneity and escape reduce the effectiveness of CAR T-cells at their destinations. A suppressive tumor microenvironment (TEM) further adds to the poor performance of these cells at target sites. In this review, we will address advancements that will allow the CAR T-cells to reach the destination and boost their performance at the glioma site.

  Database Search Strategy Top

The literature selected for this review was obtained from articles indexed on MEDLINE®/PubMed®, a US National Library of Medicine database of abstracts and citations (https://www.ncbi.nlm.nih.gov/). All completed and ongoing clinical trials cited for this review were registered in ClinicalTrials.gov, a US government database registry of all clinical trials funded both privately and publicly around the world (https://clinicaltrials.gov/). The medical subject headings used for searching the databases included Glioblastoma AND CAR T cells AND clinical trials, CNS tumors and blood brain barrier, GBM AND microenvironment, CNS AND T cells, GBM AND heterogeneity, CNS AND Immune cells, CAR AND cytokine release syndrome (CRS), CAR AND safety, GBM AND chemokine, transendothelial migration AND GBM access. The selection of peer-reviewed literature was based on direct relevance to the topics covered in this review and on the expert judgment of the authors as to the impact of the research article in the field of GBM immunotherapy.

  Navigating Chimeric Antigen Receptor T-Cells to the Tumor Site Top

The efficacy of CAR T-cell therapy is dependent on the ability of adoptively transferred cells to reach brain and induce a potent antitumor response. The central nervous system compartment tightly controls lymphocyte infiltration because any inflammation is highly detrimental to its function. The glia limitans and BBB tightly regulate lymphocyte traffic, allowing activated T-cells alone and not naïve T-cells to enter the brain parenchyma and its interstitial fluid.[24],[25] Moreover, the brain lacks a conventional lymphatic system and hence poses a major challenge to infiltrating cells of the adaptive immune system, in turn preventing effective antigen presentation and antitumor response.[26],[27]

Locoregional delivery of CAR T-cells is one promising strategy to overcome this limitation.[2],[28],[29] Along with overcoming trafficking restrictions across the BBB, this approach may also reduce toxicities associated with systemic delivery of CAR T-cells and achieve antitumor effects with a lower cell dose. Locoregional delivery was safely demonstrated in the clinical trial using IL-13Rα2 CAR T-cells in patients with recurrent or refractory malignant glioma. In this trial, the T-cells were administered through the intratumoral and/or intraventricular route (NCT02208362).[22] Additional trials utilizing this strategy with CARs targeting EGFRvIII (NCT03283631) and HER-2 (NCT02442297) are ongoing. Patients who have an intracranial catheter (Rickham or Ommaya) in place will benefit this approach especially because multiple infusions are likely necessary for effective antitumor response. While this approach has clear advantages over delivering CAR T-cells through the systemic route, delivery to the ventricles or thecal sac does not necessarily ensure that these cells reach the brain parenchyma where tumors are present.

A second strategy is to engineer T-cells to bind and migrate through the postcapillary vessels into the perivascular space to reach the tumor site. Transendothelial migration involves binding of cell adhesion molecules such as integrins α4 β1 and lymphocyte-associated antigen-1 on the T-cells and their cognate receptors, namely vascular cell adhesion molecule 1 and intracellular cell adhesion molecule 1 on the endothelial cells in a coordinated manner.[30],[31] The binding leads to rolling and arrest of the T-cells on the vascular endothelium, followed by conformational changes that allow the T-cells to transmigrate and reach the perivascular space. Understanding the changes in pathology-associated binding molecule expression such as activated cell adhesion molecule and signaling activation can be utilized to preferentially home CAR T-cells to the tumor sites.[31],[32] Alternately, genetically engineering T-cells to express chemokine receptor CCR4 may help direct these T-cells to the brain in response to high chemokine (C-C motif) ligand 17 or chemokine (C-C motif) ligand 22 chemokine secretion by GBM, originally intended to promote T regulatory cell infiltration in GBM.[33],[34],[35],[36],[37] Others have demonstrated a role for C-X-C chemokine receptor 3, along with its ligands C-X-C chemokine ligand 9 and C-X-C chemokine ligand 10, in T-cell homing to the tumor site and thus can be useful to redirect CAR T-cells to the brain.[38]

  Steering Chimeric Antigen Receptor T-Cells Through a Heterogeneous Tumor Landscape Top

Intrapatient and interpatient tumor heterogeneity is a hallmark of GBM.[39] Along with causing variability and resistance in treatment responses to standard therapy, this heterogeneity has also made targeted approaches difficult in GBM. The altered expression or downregulation of tumor-associated antigens, often termed antigen escape, following CAR T-cell treatment is another mechanism tumors use in immune evasion.[20],[40] The immune escape observed in GBM following CAR T-cell treatment occurs at epigenetic, genetic, and transcriptional levels with variations observed in O[6]-methylguanine-DNA methyltransferase (MGMT) methylation status, DNA repair mechanisms, and gene expression.[41],[42] Preclinically, researchers have demonstrated antigen downregulation in xenograft models of GBM where CAR T-cells specific for only one antigen were used.[40],[43] In the Phase 1 trial using EGFRvIII-specific CAR T-cells, the only neoantigen-based therapy to date in GBM, a decrease in EGFRvIII-expression likely accounted for the low objective responses, in spite of the presence of CAR T-cells in the tumor biopsies (NCT02209376).[20]

CAR T-cells targeting two or more antigens can be an effective way to overcome this limitation. Hegde et al.[40],[44] previously demonstrated that simultaneously targeting HER-2 and IL-13Rα2 using CAR T-cells where the binding CAR ectodomains to the tumor-associated antigens are arranged in tandem (called tandem CAR T-cells) can circumvent escape in a xenograft model of GBM. Further, the same team demonstrated that including a third antigen, Ephrin type A receptor 2, with HER-2 and IL-13Rα2 could overcome the interpatient heterogeneity by broadening the target spectrum.[45] Trivalent CAR T-cell-targeting multiple antigens may be well poised to be effective against a heterogeneous GBM target profile, making them applicable to a wider cohort of patients. An alternate method developed by Cho et al.[46] uses a split, universal, and programmable (SUPRA) CAR system for T-cell therapy, where exodomains can be switched based on the affinity between leucine zipper pairs that attach the single-chain variable fragment portion to the CAR molecule. While this approach overcomes the limitation of manufacturing different CAR T-cells based on antigen variability in the patient, it still poses the bioavailability challenge of the zip-Fvs at the glioma site. Finally, epigenetic mechanisms responsible for antigen loss may be inhibited by concomitant treatment with epigenetic modifiers, such as DNA methyltransferase inhibitors or histone deacetylase inhibitors, which are already approved for other disease indications.[47]

  Armoring Chimeric Antigen Receptor T-Cells to Overcome Microenvironment Inhibition Top

The use of CAR T-cells overcomes one of the major concerns encountered during cell-mediated immune response, the low expression of human leukocyte antigen Class I molecules on the surface of tumor cells. The human leukocyte antigen Class I downregulation or loss of function mutation allows GBM to escape detection by immune cells.[48] Here, CAR T-cells offer advantage over autologous tumor lysate-pulsed dendritic cell vaccine (DCVax®-L), where antitumor responses depend on CD8+ cells in a Class I major histocompatibility complex-restricted manner.[49] Still, the highly suppressive TME poses major challenges to the function of CAR T-cells, including the recruitment of T regulatory cells, the presence of suppressive myeloid cell population, and upregulation of immune checkpoint molecules and immunosuppressive cytokines.[50] For instance, myeloid-derived suppressor cells enable tumor growth and support gliomagenesis.[51],[52] Tumor-infiltrating T-cells also express checkpoint inhibitors such as programmed cell death protein 1 (PD-1), cytotoxic T-lymphocyte-associated protein 4, lymphocyte-activation gene 3, T-cell immunoreceptor with immunoglobulin and ITIM domains, and T-cell immunoglobulin and mucin domain containing-3.[53] Upregulation of these molecules occurs in response to the high expression of programmed death ligand receptor-1 (PD-L1), indoleamine-2,3-deoxygenase 1, and other suppressive molecules on GBM and other components of the TEM, functionally exhausting CAR T-cells that reach tumor site. In the EGFRvIII clinical trial, the authors observed increased expression of immunosuppressive molecules such as indoleamine-2,3-deoxygenase 1 and FoxP3 in the TEM. Further, patients whose tumors were surgically resected after CAR T-cell infusion showed loss of EGFRvIII expression.[20] Moreover, GBMs can generate extracellular vesicles in patients that express PD-L1 on their surface that can cause profound local and systemic immunosuppression.[54]

The use of checkpoint inhibitor antibodies targeted against PD-1 (pembrolizumab, nivolumab, and cemiplimab), PD-L1 (atezolizumab, avelumab, durvalumab, and ipilimumab), and cytotoxic T-lymphocyte-associated protein 4 (ipilimumab) alone or in combination has demonstrated varying degrees of efficacy against GBM and other solid tumors.[55],[56],[57] Along this line, a Phase 1 trial at the University of Pennsylvania (NCT03726515) is currently testing pembrolizumab (a PD-1 blocking antibody) with EGFRvIII CAR T-cells in GBM patients whose tumors have EGFRvIII expression and unmethylated MGMT. Two major challenges associated with using antibodies directly are ensuring their bioavailability at the tumor site due to low BBB penetrance and the associated systemic toxicity observed with the treatment. Using PD-1/PD-L1 blocking antibody-secreting T-cells or T-cells grafted with PD-1-dominant negative receptor or a checkpoint reversal receptor is one way to circumvent these issues. In the checkpoint reversal strategy, the PD-1 signals are truncated or reversed by a co-stimulatory signaling domain such as CD28 or 4-1BB.[58],[59] A Phase 1 clinical trial is ongoing at the Beijing Sanbo Brain Hospital, China, where the T-cells carry a chimeric switch receptor consisting of PD-1 extracellular domain and a CD28 endodomain (NCT02937844). The same effect could also be achieved by knocking out PD-1 in CAR T-cells by genome editing techniques that use zinc finger nucleases, transcription activator-like effector nucleases, or Cas9 nucleases.[60] As the checkpoint inhibition is enabled by redundant pathways, blocking or knocking out one receptor alone may not achieve the desired effects, rather blocking nodes of regulation would prove beneficial with this approach. Similar to the concept of dominant negative receptors for PD-1, a dominant negative transforming growth factor beta (TGF-β) type II receptor can render CAR T-cells resistant to tumor-secreted TGF-β, improving the antitumor function of these cells.[61],[62] A recent study demonstrated that CRISPR/Cas9-mediated knockout of diacylglycerol kinase, an enzyme metabolizing diacylglycerol to phosphatidic acid, rendered CAR T-cells resistant to TGF-β and prostaglandin E2, released to the TEM.[63]

Low-dose radiation has been shown to be beneficial in a preclinical model against pancreatic adenocarcinoma with heterogeneous expression of sialyl Lewis-A (sLeA) recently.[64] The study demonstrated that low-dose radiation made both sLeA + and sLeA-cells susceptible to CAR therapy, reducing antigen-negative tumor relapse through a TRAIL-receptor 2 (also known as death receptor 5)-mediated mechanism. Moreover, increased leakage through the BBB and microvascular collapse have been observed flowing radiation in mice and humans and should allow more lymphocyte infiltration to the tumor site.[64] Preconditioning with lymphodepletion chemotherapy with agents such as cyclophosphamide and fludarabine has been shown to improve the expansion and persistence of infused CD19 CAR T-cells.[11],[12] There are two Phase 1 trials currently recruiting that have incorporated lymphodepletion chemotherapy with fludarabine and cyclophosphamide into the treatment regimen before introducing EGFRvIII targeting CAR T-cells against GBM (NCT02844062 and NCT03170141). Cyclophosphamide can cross the BBB and may allow lymphatic space for the infused CAR T-cells to expand along with depleting suppressive cells at the tumor site.[65] A similar approach using an intensified dose of temozolomide, a drug currently included in the standard of care for GBM, demonstrated improved engraftment and proliferation of EGFRvIII-targeted CAR T-cells. Currently, this observation is being tested in a Phase 1 trial at Duke University (NCT02664363).[66]

  Torqueing Chimeric Antigen Receptor T-Cell Performance and Endurance Top

The GBM microenvironment is composed of glial cells, vascular cells, and immune cells, especially tumor-associated macrophages of the M2 phenotype.[50],[67] Tumor-associated macrophages make up ~30%–40% of cells in GBM and can promote tumor growth and progression. Additionally, tumor-associated macrophages can secrete immunosuppressive cytokines such as IL-10 and TGF-β that hinder the function of CAR T-cells and limit their persistence at the tumor site.[68]

CAR T-cells armed with stimulatory cytokines such as IL-12, IL-15, and IL-21 can counteract the suppressive milieu of the TEM and provide the boost necessary to induce a potent antitumor response alone or in combination. The cytokine IL-12 can improve the activation and proliferation of CD8+ T-cells and provide a type I signal that can impact the innate immune cell infiltrates.[69] Further, IL-12 can also improve the cytotoxicity associated with natural killer cells through interferon-gamma production and subsequent Fas (CD95) and Fas-L upregulation on tumor cells. Preclinical evaluation of IL-12-secreting CAR T-cells indicated improved activation and persistence that translated to better antitumor effects compared to regular CAR T-cells.[70] A recent preclinical study evaluating IL-13Rα2 targeting CAR T-cells that co-express IL-15 demonstrated superior persistence and antitumor activity against GBM xenografts when compared to IL-13Rα2 CAR T-cells.[43] Additionally, IL-15 can activate natural killer cells through components of the IL-2 receptor.[71] IL-21 is another cytokine that can be co-expressed on CAR T-cells because it mediates Foxp3 suppression and enhances the generation of CD8+ cytotoxic T-cells.[72] However, systemic toxicities have been associated with intravenous administration of recombinant human IL-12 or other cytokines. Thus, a high degree of caution is recommended when administering CAR T-cells with constitutive cytokine expression to the brain.[73] An alternate approach would be to “backpack” cytokines such as IL-15 or IL-21 on T-cells using protein nanogels and release them gradually to reduce bystander activation or use membrane tethering to prevent secretion.[74]

The use of central memory T-cells has demonstrated improved persistence for infused CD19 CAR T-cells in patients with relapsed B-cell non-Hodgkin lymphoma.[75] Currently, a Phase 1 clinical trial is ongoing at the City of Hope Medical Center where HER-2-directed CAR T-cells with central memory phenotype are infused in patients with recurrent GBM (NCT03389230).

  Enhancing Chimeric Antigen Receptor T-Cell Safety Top

Any enhancement to CAR T-cells should not increase the toxicity associated with the therapy. The safety challenge of treating brain tumors with CAR T-cells includes the fact that elements of a strong immune response such as inflammatory infiltration and edema near the neural tissues can be detrimental. There have been several reported adverse events associated with CRS and neurotoxicity after infusion of CD-19 CAR T-cells.[10],[11],[76] Symptoms associated with CRS included high fever, hypotension, and respiratory difficulties caused by serum IL-6, and those with neurotoxicity included encephalopathy, somnolence, and aphasia. High-grade adverse events after CAR T-cell infusion have not been reported to date in clinical trials for GBM, possibly due to the low expansion and persistence observed. In the study administering HER-2 CAR T-cells for GBM, the researchers did not observe any dose-limiting toxicity, except for Grade-II toxicity in two of the 17 patients treated.[21]

CAR T-cells can be selectively depleted at the tumor site or in systemic circulation through incorporation of a dimerizable death molecule-inducible Fas or Caspase 9 (iCasp9), a safety switch that can be turned on using inert bivalent small molecules.[77],[78] The dimerization of the components of the binding protein activates downstream caspases, inducing T-cell apoptosis. Studies in murine CRS models have elegantly demonstrated that the severity of CRS can be attributed to IL-6, IL-1, and nitric oxide produced by macrophages and not by the CAR T-cells themselves.[79] This study highlights IL-1 as an actionable target, and use of anakinra, an IL-1 receptor antagonist, or a soluble IL-1RA-secreting cell modification to block the effects of IL-1, which can prevent CRS without affecting the antitumor activity of the CD19 CAR T-cells. Other approaches to manage and mitigate the symptoms of CRS include IL-6 antibody, steroids, and prophylactic anti-epileptics.

  Customizing Manufacturing and Delivery of Chimeric Antigen Receptor T-Cells Top

CAR T-cell therapy entails manufacturing and distribution of large numbers of patient-specific products. The major step in this manufacturing involves gene modification of the patients' cells, starting with T-cell selection to cell formulation. Manufacturing further requires dedicated clean rooms and highly trained personnel in a good manufacturing practice environment. Recent advances such as CliniMACS® Prodigy Platform have made possible automation of all cell processing steps, additionally ensuring a highly reproducible and standardized manufacturing process.[80],[81] An alternate approach to clean rooms could be the use of modular hood systems, with independent programmable control of oxygen, carbon dioxide, and temperature in modular incubator and cell-handling chambers like the Xvivo System manufactured by BioSpherix (Lacona, NY, USA). These closed manufacturing methods can reduce open process steps, minimizing the risks for cross-contamination and can be cost-effective in the long run.

Currently, CAR T-cell therapy is restricted to transplant centers that have the expertise, professional teams, accreditation, and infrastructure to administer the therapy safely and effectively. While CAR T-cells for GBM presently remain in an experimental phase, if cell therapy is to go frontline, it will require collaboration between oncologists, neurosurgeons, and transplant specialists to decide on an algorithm for treating patients. An automated manufacturing platform can decentralize manufacturing to or near hospitals and transplant centers where patients are treated. The decentralized manufacturing process for patient T-cell modification can use viral vectors and other nonpatient-specific components made using the currently approved pharmaceutical manufacturing models. Such decentralization may reduce distribution logistics and make planning of treatment infusions easier.

  Conclusion Top

Adoptive cell therapy with autologous CAR T-cells holds great promise in treating refractory or recurring brain tumors, unresectable and residual GBMs, as well as secondary neoplasms that have metastasized to the brain from other primary tumors. While there is no one strategy that can enable CAR T-cells to reach tumor site and enhance their cytotoxicity once there, a combination of targeted delivery along with checkpoint reversal and cytokine boost should allow multitargeted T-cells to persist and produce robust responses in treated patients. The successful journey of CAR T-cells to the clinic will not be a lone one, but will occur in the company of new technological advancements and standard of care approaches.


We thank Catherine Gillespie for language editing and proofreading of the manuscript.

Financial support and sponsorship

The authors received funding from the Alex's Lemonade Stand Foundation for Childhood Cancer, Cookies for Kids' Cancer™ Childhood Cancer Foundation, and St. Baldrick's-Stand Up to Cancer Dream Team Translational Research Grant (SU2C-AACR-DT-27-17). Stand Up to Cancer is a division of the Entertainment Industry Foundation. Research Grants are administered by the American Association for Cancer Research, the Scientific Partner of SU2C.

Conflicts of interest

NMA and MH have patent applications in the field of T-cell and gene-modified T-cell therapy for cancer. NMA is on the scientific advisory board of Adaptimmune LLC. SKJ has no conflict of interest.

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