Given the infiltrative characteristics and molecular heterogeneity of glioblastoma (GBM), the ongoing difficulty in discovering a cure persists. Despite advancements in technology and biological research, the median overall survival (OS) time of patients with GBM remains low, with a median OS time of 14–18 months (1–3). Even with advances in genetics and surgical techniques, neuro-oncology research has shown generally disappointing outcomes since the Stupp regimen was implemented. Although there have been advancements in surgical techniques, imaging protocols and radiotherapy (RT), as well as the use of intraoperative mapping to guide macroscopic complete resection, the median survival time has only increased by a few months (4). The current gold standard treatment after surgery for newly diagnosed patients with GBM dates back to 2005, with the EORTC/NCIC 26981 study demonstrating the survival benefits of combining radiation with temozolomide (TMZ) over radiation alone (2). Despite subsequent efforts, progress has remained constrained, as therapies including bevacizumab, everolimus and dose-dense TMZ have not demonstrated a significant survival benefit in randomized clinical trials when compared with the standard regimen of radiation and adjuvant TMZ. Challenges in developing new therapies include the need to penetrate the blood-brain barrier (BBB), tumor heterogeneity, and the widespread distribution of microscopic disease (1–4).

The resistance of GBM to treatment is widely known and can be explained by numerous different aspects of the tumor: i) GBM spatial heterogeneity limits the options and efficacy of target therapies; ii) GBM resistance to radiation and chemotherapy is facilitated by the pro-tumorigenic activity of the tumor microenvironment (TME); and iii) the limited immunogenicity of GBM inhibits a robust immunological reaction (5–7). In total, ~80% of GBM recurrences manifest within or on the periphery of the radiation field, highlighting the significant influence of local variables on the recurrence of the tumor. The localized recurrence significantly correlates with a notable reduction in progression-free survival (PFS), underscoring the imperative need for effective local treatment interventions (8). Additionally, the isolation of the brain by the BBB provides a distinctive chance for aggressive local treatment with minimal risk of systemic toxicity. In recent decades, several local therapy (LT) strategies have been developed. These include local thermal or laser therapy, local injection of immunotoxins via convection-enhanced delivery systems and implantation of Carmustine Wafers (CWs) into the resection cavity (9,10). Intracavitary radioimmunotherapy and Tumor Treating Fields (TTF) represent other examples of localized therapeutic strategy aimed at delaying or potentially preventing local tumor recurrence (11).

The 2021 World Health Organization (WHO) classification (12) of central nervous system (CNS) tumors brought significant revisions, particularly concerning GBM. These updates involved molecular parameters, refining GBM subtypes such as isocitrate dehydrogenase [NADP(+)] (IDH)-wild-type and IDH-mutant. It is crucial to note that each of these subtypes has distinct clinical and prognostic implications. For instance, IDH-wild-type GBM is associated with a more aggressive disease course and poorer prognosis compared with IDH-mutant GBM. In addition, the emergence of new entities, such as diffuse midline glioma with histone H3 K27M mutation further complicates historical comparisons, emphasizing the need for updated diagnostic criteria and standardized reporting for consistency across studies (12). The new classification represents a significant temporal landmark, which can make it challenging and potentially misleading to compare studies conducted before and after this time point. Therefore, it is crucial to be aware of these complexities when interpreting and applying research findings.

Despite the limited or marginal improvements demonstrated by LT over current standard radio-chemotherapy, its future potential is promising. This is particularly true in light of recent advancements in gene-editing technologies and novel molecular and genetics discoveries, which are opening up new possibilities and avenues for treatment. The present review aimed to provide a historical review of LT and summarize the LTs that are currently being investigated or explored in GBM management.

GBM persists as one of the most resistant malignant tumors in the CNS, characterized by inevitable recurrence despite progressions in neurosurgery, chemotherapy and RT. Recurrences predominantly occur within or proximal to the resection cavity, typically within regions exposed to the highest radiation doses (2,3,9,10). There is thus an urgent requirement for novel therapeutic techniques to improve patient outcomes. In this clinical setting, LTs have gained significant interest as a developing approach. This is due to their potential to overcome the constraints associated with conventional glioma treatment protocols.

The brain presents a substantial challenge for the efficient delivery of pharmacological compounds owing to the presence of specialized interfaces governing the exchange between the peripheral blood circulation and the cerebrospinal fluid (CSF) circulatory system. These interfaces include the choroid plexus epithelium (regulating blood-ventricular CSF), the arachnoid epithelium (regulating blood-subarachnoid CSF) and the blood-brain interstitial fluid. The BBB, formed by endothelial cells, limits the paracellular flux of hydrophilic molecules through tight junctions (TJs). The endothelial cells are surrounded by a basal lamina, and astrocytic glial cells provide biochemical support, regulate blood flow, supply nutrients, maintain ion balance and contribute to repair processes. Essential elements of the BBB supporting system include brain capillary endothelial cells, extracellular base membrane, pericytes, astrocytes and microglia. In detail, TJs are complex networks of transmembrane and cytoplasmic strands that are found on the apical portion of endothelial cells. They are made up of integral membrane proteins termed claudin, occludin and junction adhesion molecules. In addition, adherens junctions (AJs), situated below TJs in the basal region of the lateral plasma membrane, involve transmembrane glycoproteins (cadherins) linked to the cytoskeleton, enhancing the structural integrity between adjacent endothelial cells at the BBB. The ability of the BBB to control the passage of solutes and other substances is thus greatly supported by both TJs and AJs (26,54,56,57).

Overall, the BBB is a specialized structure that tightly regulates the molecular transit into the CNS. Typically, the relative impermeability of the BBB protects the brain from circulating toxins, maintaining an optimal microenvironment for neuronal function. Nevertheless, these obstacles impede efficient medication administration in disorders of the CNS, such as tumors (56,57). Hence, contemporary therapeutic strategies emphasize the endeavor to overcome BBB obstacles by administering treatments directly into or in close proximity to the tumor cavity. The present review explores the spectrum of currently accessible and emerging techniques, including both LTs and the latest advancements in immunotherapeutic drugs and genetically engineered cell therapies (Fig. 4).

Over time, a multitude of instruments have been developed to aid brain tumor surgery (9,10). Instruments such as surgical microscopes, high-resolution imaging, fluorescence-guided surgery and neuro-navigation are extensively employed in neurosurgery but fall outside the scope of the present review. At present, four main approaches, laser interstitial thermal therapy (LITT), magnetic hyperthermia (MH), (TTF) and focused ultrasound (FUS), are undergoing regulatory approval at different stages for the LT of GBM (Fig. 5).

Given the notable drawbacks of systemic drug delivery options, local drug delivery techniques are seen as viable substitutes. The drawbacks of systemic methods include long delivery routes, which increase the chance of medication absorption by unwanted organs or clearance during blood circulation. Furthermore, the potential for systemic toxicity remains a notable concern. By contrast, local drug delivery techniques provide a focused and efficient approach, delivering drugs directly into the brain bypassing the limits of the BBB (9,78–82). Local drug delivery approaches encompass a growing number of research topics, including the continuous discovery of therapeutic nanomaterials and the continued development of pharmaceutical molecular design (9,10,78–82). Numerous intracranial implant-based delivery techniques are now under investigation (83–94).

RT is currently the primary method for controlling the growth of GBM. Since publication of the study by Stupp et al (1) in 2005, the dose of 60 Gy in 30 sessions has been delivered along with chemotherapy in the postoperative setting to the residual disease (when present) and the surgical bed with an adequate clinical margin (97). Unfortunately, even after RT, disease recurrence is inevitable, and in 70–80% of cases, it occurs within the treatment field (98). Several clinical trials have attempted to increase the dose of ionizing radiation in combination with TMZ using external beams RT (99). Nevertheless, the advantages in terms of survival have been marginal, while the adverse effects induced by radiation have significantly exacerbated.

To augment the dose directed at the tumor bed and mitigate harm to adjacent tissue, diverse BCT approaches have been explored, yielding differing levels of success (100). BCT and intraoperative RT (IORT) are the two main types of localized RT. BCT consists of placing a source of ionizing radiation into the surgical cavity, and IORRT treats the surgical bed immediately after surgery using a dedicated linear accelerator. Both upfront and recurrent settings have been assessed in clinical trials. Nevertheless, to date, there are a limited number of published clinical studies, and most studies are retrospective and single-centered in design. In 2007, Chen et al (43) reported the results of a phase I study on 18 patients with GBM treated with BCT after the first surgical intervention (all patients had undergone radical resection). The median dose of ionizing radiation delivered through the placement of permanent iodine-125 (I-125) seeds at a depth of 0.5 cm was 400 Gy, followed by postoperative external beam RT. Despite PFS and OS results consistent with those reported by the aforementioned Stupp study, 11 patients underwent surgery for the development of radionecrosis (without evidence of disease progression). As a result of the high rate of toxicity, the trial was prematurely stopped. Similar results in terms of OS following BCT treatment at first diagnosis were also reported by Welsh et al (44). In addition, Fabrini et al (46) evaluated the viability and effectiveness of perioperative high-dose-rate BCT in 2009, administering an 18 Gy radiation at a 5 mm depth to 21 patients with recurrent GBM. The median OS time was 21.7 and 8 months from diagnosis and tumor recurrence, respectively. Chan et al (40) reported similar survival outcomes for 24 patients who received BCT at the time of the second surgical intervention, which likely indicates selection bias by the neurosurgeon affected the data. This was due to patient selection (ideal patients eligible for a second surgery were selected), a prolonged time since the end of RT and a tumor recurrence that is easily resectable and preferably localized within the RT treatment fields. The study reported a median survival time of 23.3 months from diagnosis and 9.1 months from the date of recurrence.

In a study by Chino et al (45), the survival time of 26 patients treated with BCT after the second surgical intervention was 7.1 months. Gabayan et al (42) observed that patients who received BCT at the time of recurrence had a median survival time of 36.3 weeks from the date of BCT, which was consistent with earlier studies. In 2014, Kickingereder et al (101) reported a retrospective case series of patients with inoperable GBM treated with BCT at diagnosis and recurrence (103 patients treated at diagnosis and 98 at disease recurrence) between 1990 and 2012. A median dose of 60 Gy was administered through low-dose-rate stereotactic I-125 BCT. The treatment-related mortality was 0% and toxicities occurred in <7.5% of patients. It was found that patients treated with BCT at diagnosis and at recurrence had the same length of disease control (6.2 vs. 5.9 months, respectively; P=0.11). This result was likely affected by the patient selection (102,103). In 2015, Schwartz et al (48) published the results of a retrospective study on 68 patients with recurrent GBM who had been treated with I-125 BCT (the reference dose was 50.0 Gy, calculated to the boundary of the tumor). The median survival time was 41.8 months (95% CI, 29.2–55.9) and the perioperative morbidity was 2.9%. However, this study exhibited notable bias in patient selection, rendering the evaluation of OS data challenging.

Based on the available published studies, it can be concluded that BCT is likely safe, meriting further investigation in dedicated prospective clinical trials. However, the retrospective nature of currently published BCT studies, the variability in patient selection criteria, the prolonged enrollment periods resulting in heterogeneity according to various WHO classifications and the differing techniques and doses utilized present significant challenges to definitively determining the impact of this technique on patient survival. Future prospective studies are anticipated to provide critical insights.

Clinical experiences assessing the application of IORT in patients with GBM have revealed comparable limitations. In 2005, Schueller et al (41) reported the results of a retrospective study conducted on 71 patients with glioma treated with IORT. IORT demonstrated feasibility, with perioperative complication rates remaining unchanged. However, the survival outcomes generally did not exhibit improvement when compared with a historical control group. Disease recurrence exhibited similar survival rates as primary tumors, and GBM displayed a slightly elevated survival, suggesting potential indications for the use of IORT. Investigations by Usychkin et al (47) and Sarria et al (49) described findings indicating less favorable outcomes in terms of safety. The two studies assessed the viability of IORT in new and recurrent GBM.. Radionecrosis occurred in 35 and 25.5% of patients in the respective studies. In addition, Usychkin et al (47) evaluated the impact of IORT in terms of survival and toxicity in a retrospective single-center study of 17 patients with GBM treated between 1992 and 2002. Each patient received high-dose IORT (20 Gy), followed by post-operative external beam RT. For the whole group, the median OS time was 13 months, consisting of 10.4 months for recurrent cases and 14 months for primary cases. Of the complications recorded, 3 patients presented with radionecrosis, 1 with osteomyelitis at the craniotomy bone flap, 1 with intracerebral hemorrhage and 1 with pulmonary embolism. In addition, 2 patients had a fatal outcome. Sarria et al (49) evaluated 51 patients with GBM. IORT was performed in a single session immediately after surgery (10–40 Gy prescribed at the applicator surface using low-energy photon) and was followed by standard radiochemotherapy treatment. Although no grade 4 radionecrosis was recorded, G1-G3 radionecrosis occurred in 25.4% of patients. At present, there is a lack of data supporting the use of IORT or BCT for diffuse glioma. Furthermore, the main clinical randomized trial investigating the efficacy of I-125 has failed to demonstrate a survival benefit (104).

The FDA clearance of GammaTile, incorporating Cs-131 titanium seeds within a resorbable collagen-based tile, has renewed the interest in BCT and introduced the concept of surgically targeted RT. Early clinical reports suggest a reasonable safety profile, but potential delayed seed settling during collagen absorption raises uncertainty about efficacy. Ongoing clinical trials are actively exploring the safety and efficacy of GammaTile for CNS tumors (105,106).

Immunotherapy stands out as one of the extensively explored novel approaches for GBM treatment. The low immunogenicity of the tumor, along with an immunosuppressive TME, allows it to evade an immune response. For a number of reasons, including its low tumor mutational burden, low number of tumor-infiltrating T cells and low programmed cell death protein 1 (PD-1)/programmed death-ligand 1 expression, GBM is recognized as an immunologically inert tumor, particularly when compared with other cancer types that have responded favorably to immunotherapy. The high heterogeneity of GBM further facilitates immune evasion (7). In addition, although steroids are often essential for managing peritumoral edema, they can compromise the effectiveness of immunotherapies (7,107,108). Until now, several phase III clinical trials that focused on immune therapy for GBM have encountered difficulties, mainly related to individual components of the antitumor immune response. Learning from these setbacks, the potential success of immunotherapy for GBM appears most optimistic when utilizing a combination of immunotherapies to address the significant immunosuppression disease-related. It is thus crucial to detect reliable biomarkers both for appropriate patient selection and tumor evolution monitoring (9,10).

In this section, current findings and continuing clinical research in the field of immunotherapy for GBM will be discussed, which includes immune checkpoint inhibitors (ICIs), vaccines, chimeric antigen receptor T cell (CAR-T) treatment and viral therapy. In total, two different approaches of immunotherapy are described in literature: Passive immunotherapy (using antibodies and immune checkpoint modulators) and active immunotherapy (using tumor vaccination with viral vectors or dendritic cells and CAR-T treatments). A complete list of trials with clinical relevance using different immunotherapy strategies in the treatment of glioma are reported in Table III.

Despite efforts in the genomic, transcriptomic, epigenetic and proteomic characterization of GBM specimens, little progress has been made in the survival of patients with GBM. The failure of treatments can be attributed to a number of GBM gene alterations, including mutations in KRAS, c-MET, PI3KCA, BRAF, telomerase reverse transcriptase, TP53 and PTEN, as well as the mutational status of the EGFR and platelet derived growth factor receptor α genes (7). Furthermore, the inability of tyrosine kinase inhibitors to cross the BBB reduces the efficacy of chemotherapy (116). Consequently, accurate tumor antigen targeting and effective intracranial drug delivery are essential for the success of GBM treatments (117). Comprehensive single-cell RNA-sequencing analysis of cancer stem cells, cells that remain in recurrent and resistant GBM inside the TME, may provide valuable data for future research on effective targeting strategies for this fatal illness (118). At present, several clinical trials are being conducted to treat GBM. These trials include vaccine therapy, immunotherapy and CAR-T cell therapy (119).

CAR-T cell technology envisages the production of T lymphocytes redirected to express a single-chain variable fragment (scFv) of an antibody to target and eliminate tumor cells that overexpress a specific tumor-associated antigen (TA) (120). CAR antigens that have been primarily targeted in patients with GBM include EGFR variant III (vIII), HER2, IL-13Rα2, B7-H3 and NKG2D (51,121–123). EGFRvIII CAR-T has been explored in several trials but has shown little OS benefit. Since CAR-T cell abundance in the blood is correlated with tumor regression, Suryadevara et al (124) have demonstrated that greater lymphodepletion induced by TMZ-dose intensified (TMZ-DI) is required to stimulate the proliferation and persistence of EGFRvIII CAR-T cells in a murine model. In light of this, a phase I clinical trial was established that involved individuals diagnosed with newly onset GBM, utilizing TMZ-DI as a preconditioning regimen preceding CAR-T cell immunotherapy (NCT02664363). According to Brown et al (51), intracranial infusion of IL-13Rα2 CAR-T cells initially demonstrated GBM regression but ultimately resulted in recurrence. In line with this, an ongoing phase I trial has been established to study the side effects and mechanism of IL-13Rα2 CAR-T cells when administered alone or in combination with the ICIs, nivolumab (anti-PD-1 mAb) and ipilimumab (anti-CTLA-4 mAb), in treating recurrent GBM cases (NCT04003649). As a result, CAR-T cell therapy for GBM will develop; however, several issues must be addressed, including TA heterogeneity, T-cell exhaustion, T-cell infiltration and the tumor immunosuppressive microenvironment.

Specifically, as demonstrated in the aforementioned studies, EGFRvIII CAR-T cell therapy led to a reduction in the number of EGFRvIII-expressing cells in patients with GBM. However, examination of post-infusion surgical resections demonstrated that, although EGFRvIII CAR-T cells effectively penetrated brain tumors, a high level of wild-type EGFR (wt EGFR) expression persisted in the residual tumor (51,123,124). To overcome this antigen heterogeneity, Choi et al (125) developed a sophisticated modified EGFRvIII CAR-T cells to produce a bispecific T-cell engager (BiTE) with the aim of targeting the residual EGFRvIII⁻ GBM cells in mice. BiTE was conceived to incorporate two scFvs, one directed against wt EGFR and the other to engage and activate T cells by binding CD3. BiTEs secreted by EGFRvIII CAR-T cells were also able to recruit local bystander T-cell effector activity. This platform may also apply to other tumor types that have demonstrated heterogenous EGFRvIII expression, including medulloblastomas and breast and ovarian carcinoma.

To overcome the limited efficacy of CAR-T cells, multitargeting CAR-T cells were also tested in preclinical GBM models. Novel CAR-T cells were developed to simultaneously target IL-13Rα2 and EphA2 or HER2 and IL-13Rα2 or EGFRvIII and IL-13Rα2 (126–128) and have shown additive T cell activation and antitumor activity. Trivalent CAR-T (IL-13Rα2, HER2 and EphA2) products were also developed. These CAR-T cells demonstrated a notably improved tumor clearance of autologous orthotopic glioma [patient-derived xenografts (PDXs)] compared with univalent and bivalent T-cell products (129). However, to the best of our knowledge, no first-in-human clinical trials of the latter proposed innovation have been listed on clinicaltrials.gov at present.

One of the cutting-edge therapeutic approaches to stimulate the immune system against solid tumors is the use of T-cell bispecific antibodies (TCB). TCBs are engineered to incorporate binding sites for CD3ε of the T-cell receptor and a TA. For instance, EGFRvIII-TCB was developed against EGFRvIII, which is upregulated solely in GBM and is absent in healthy tissues (130). Peripheral infusion of EGFRvIII-TCB induced a strong antitumor activity in orthotopic humanized and PDX GBM models (130). The favorable preclinical data supported the use of this molecule in a first-in-human clinical trial (NCT05187624) in patients with newly diagnosed or recurrent GBM.

There are also currently ongoing clinical trials using the immune checkpoint inhibitor, lymphocyte activating 3 (LAG-3) mAb, alone or in combination regimens (NCT02658981 and NCT03493932). Unfortunately, immunotherapy using ICIs for the treatment of GBM still has drawbacks, such as high toxicity and poor efficacy (131).

The shortcomings in clinical outcomes underscore the imperative to pinpoint targets that can enhance the antitumor activity of CD8⁺ T-cells, suggesting that T-cell dysfunction may impact the GBM microenvironment. Ye et al (132) reported that the adoptive transfer of CD8⁺ T cells with protein disulfide isomerase family A member 3 (PDIA3), α-1,6-mannosylglycoprotein 6-β-N-acetylglucosaminyltransferase, epithelial membrane protein 1 or LAG-3 gene editing enhances the survival of GBM-bearing mice. In summary, it was shown that mutant human CD8⁺ T cells PDIA3⁻/⁻EGFRvIII CAR-T cells compared with wt EGFRvIII CAR-T cells enhanced killing of the GBM cell line, U87-EGFRvIII.

Although myeloid cell infiltration in the colorectal carcinoma (CRC) TME does not appear to be immunosuppressive, but instead is associated with a favorable clinical course of the disease (133,134), the presence of myeloid cells in the TME of GBM and most epithelial tumors, including renal cell carcinoma, is immunosuppressive (135,136).

Marked T cell dysfunction in GBM has also been demonstrated in two studies, which indicates why there is little antitumor immunity in GBM. Gangoso et al (136) demonstrated that GBM cells develop myeloid-like transcriptional and epigenetic programs in response to the immune microenvironment, which serves as a means of immune evasion. Another notable study by Ravi et al (137), explained the relationship between the release of the immunosuppressive cytokine, IL-10, from CD163⁺HMOX1⁺ myeloid cells in the GBM microenvironment and effector T-cell exhaustion and dysfunction. Ravi et al (137) discovered that chemically depleted myeloid cells caused T cells to produce more Granzyme B and less IL-10 and, based on these findings, the authors treated a patient with recurrent GBM with ruxolitinib, an inhibitor of the Janus kinase/STAT pathway, in a neoadjuvant setting in an effort to partially rescue the immunosuppressive environment. These studies provide insights into potential future therapeutic strategies that enhance T cell activation by decreasing immunosuppressive programs, beyond the use of CAR-T cell technology.

Multiple factors contribute to the poor prognosis of GBM and its resistance to current treatments, which include: i) The heterogeneity of GBM, which limits the options and efficacy of targeted therapies; ii) the pro-tumorigenic role of the TME, which activates resistance to radiation and chemotherapy in GBM; and iii) the low immunogenicity of GBM, which impedes a robust immunological response. In addition, the treatment for GBM incurs significant costs without providing an effective cure. Consequently, there is a pressing medical need for more effective treatments. To effectively address this lethal disease, future treatment approaches are needed that involve a combination of targeted local and systemic therapies, rather than relying on a single strategy.

Recurrences of GBM most typically occur within or near the resection cavity. Focusing therapeutic interventions directly on the tumor cavity has the potential of enhancing treatment effectiveness. Although LT for GBM has garnered attention, numerous innovative strategies are still in development. A combination of different approaches, such as dual targeting, should be considered. The exploration of post-operative implants holds particular appeal when compared with conventional chemotherapies for four reasons: i) LT allows the start of bridging therapy between surgery and conventional standard treatments; ii) LT implants offer the advantage of circumventing the BBB, allowing consideration of various chemotherapeutic agents and establishing a reservoir of active molecules in close proximity to the pathology; iii) the localized administration of active molecules results in limited systemic toxicity; and iii) given the complexity of GBM and the potential for multi-targeting, employing ligands specific to various components of the TME could yield a synergistic effect in treatments. This innovative approach could revolutionize GBM treatment, offering new hope to patients and medical professionals alike.

Before the WHO 2021 classification, all investigations regarding LT and GBM were partially representative, adding bias and reducing the generalizability of results in the context of subsequent molecular discoveries. Prior classifications did not include extensive molecular and genetic profiling that currently separates a number of GBM subtypes. This oversight could mean that the patients or data selected for downstream analysis did not fairly represent the range of GBM cases, potentially distorting study findings. To ensure more accurate and inclusive research, it is beneficial and essential to reinterpret historical data in light of the new classification and define future prospective studies in this challenging clinical setting.