Investigational Treatment Strategies in Glioblastoma: Progress Made and Barriers to Success

Abstract

Introduction:

Glioblastoma, isocitrate dehydrogenase wildtype (IDHwt), remains an incurable disease despite considerable research effort. The current standard of care since 2005 comprises maximal safe resection followed by radiation with concurrent and adjuvant temozolomide; more recently, the addition of tumor treating fields was approved in the newly diagnosed and recurrent disease settings.

Areas Covered:

Searches of PubMed, Cochrane Library, and ClinicalTrials.gov provided a foundation for this review. We first describe early research including carmustine wafers, brachytherapy, anti-angiogenesis, and immune checkpoint inhibition for glioblastoma. Next, we discuss challenges precluding the translation of preclinical successes. This is followed by a description of promising treatments such as chimeric antigen receptor T-cell therapy as well as the recent qualified successes of cancer vaccinations. Non-immunotherapy trials are also highlighted, and ongoing or pending phase 2 and 3 clinical trials are codified in study tables.

Expert Opinion:

Unfortunately, hundreds of trials, including of agents effective in systemic malignancy, have not drastically changed management of glioblastoma. This may reflect unique resistance mechanisms and highlights a need for multimodality treatments beyond surgery, radiation, and conventional chemotherapy. Novel techniques, such as those in the emerging field of cancer neuroscience, may help uncover tolerable and effective regimens for this lethal malignancy.

4.0. Introduction

Glioblastoma IDHwt (GBM) is the most common malignant primary brain tumor of adulthood affecting more than 12,000 individuals per year in the United States (US) and with an estimated worldwide increasing incidence of 0.5-5 per 100,000 (1-3). Existing therapies are not curative. Efforts to develop more effective treatments have yielded only marginal change to survival rates with multimodal therapies. At present, upfront maximal safe resection; concurrent chemoradiation with the alkylating chemotherapy agent TMZ; and 6 adjuvant 28-day cycles of TMZ has been the accepted standard of care for newly diagnosed GBM for the past two decades (4). This regimen demonstrated the value of chemotherapy to extend median overall survival (OS; 12.1 months with RT alone versus 14.6 months with the combination) in 2005, and it remains the backbone of treatment for most patients (4). In 2011, the US Food and Drug Administration (FDA) approved the Novocure Optune ^® device, which uses alternating electric fields (tumor treating fields, TTF) to disrupt cancer proliferation, for recurrent GBM given comparable OS to physician’s choice chemotherapy (5). This modality was subsequently approved in 2015 as an adjunctive therapy for newly diagnosed GBM after demonstrating a median OS of 20.9 months with TTF + TMZ vs 16.0 months with TMZ alone (6). However, adoption of TTF has not been universal due to concerns about trial design, patient adherence, and convenience, and many providers elect to offer this treatment on a case-by-case basis (7). Accordingly, novel therapeutic targets and modalities supported by a deeper understanding of the molecular underpinnings of the disease and the importance of the tumor microenvironment in GBM evolution and progression are needed to reshape the landscape of treatment options in coming years.

Here we briefly describe recent work that has informed the current understanding of GBM and its management. We discuss recent advances and the existing challenges faced by clinicians and researchers in identifying new strategies and in bringing new discoveries from the research bench to a clinical application. We also explore promising efforts that take advantage of personalized treatment approaches and incorporate recent discoveries made in the growing field of cancer neuroscience, which have the potential to transform the management of GBM in the near future.

5.0. Prior Investigations and Historical Context

5.1. In-situ Treatment: Carmustine (BCNU) Wafers and Brachytherapy

For most of the 20th century, GBM was managed with resection alone. In the 1980s, radiotherapy became commonplace, but the adoption of chemotherapy for GBM was slow due to uncertainty of benefit, timing, dose, and preferred agents as well as associated toxicities (8). Once preclinical models in the 1990s demonstrated in vitro efficacy with agents such as carmustine, trials were undertaken to explore multiple systemic treatments (9,10). However, short drug half-life, low intra-tumoral drug concentration, limited drug penetrance across the blood-brain barrier, and systemic toxicities were felt to be limiting factors for widespread use. To address these issues, Brem et al explored the safety and efficacy of locally delivered carmustine, administered in the form of wafers implantable into the resection cavity at the time of surgery (11,12). Early data revealed an improvement in overall survival (OS), which was corroborated in a phase 3 trial that demonstrated a slight, but statistically significant, median OS benefit at 13.9 months with carmustine wafers vs 11.6 months with placebo; there was no improvement in progression-free survival (PFS) (11-13). However, while FDA-approved in GBM, concerns about generalizability due to strict patient selection, a requirement for institutional experience with implantation, associated toxicities, and an elevated risk for wound infection have limited the use of this treatment in current clinical practice. Conceptually similar to carmustine wafers is brachytherapy, which employs ionizing radiation delivered locally and continuously through beads or wafers placed into the tumor bed. Trials of varying total radiation dose, dose delivery rates, and placement techniques have yielded inconsistent benefit in GBM, and concerns about this modality mirror those expressed for carmustine wafers (14).

5.2. Anti-angiogenesis

Prior to routine use of molecular characterization for establishing a diagnosis of GBM, histologic evaluation served as the gold standard. One important factor separating low- and high-grade glioma is the presence of microvascular proliferation, which implicates angiogenesis in disease development. Genomic evaluations of GBM early in the 21st century confirmed the role of vascular endothelial growth factor (VEGF) in disease pathogenesis, and use of anti-VEGF treatment garnered significant interest in the neuro-oncology community (15-17). Unfortunately, multiple trials using VEGF targeting in GBM - as monotherapy or in combination, and at both diagnosis or disease recurrence - ultimately demonstrated no survival benefit (18-23). Specifically, no trial to date has conclusively shown prolongation of OS following administration of bevacizumab, the most commonly used VEGF targeting agent, and increases in PFS are likely attributable to reduction in contrast enhancement due to vascular normalization (24,25). Despite minor effects on disease progression, bevacizumab maintains an important role in patient management due to frequent treatment-associated symptomatic improvement and potential for reduced corticosteroid dependence (26).

5.3. Immune Checkpoint Inhibition

Harnessing the immune system to treat malignancy and systemic cancer has ushered tremendous progress in previously recalcitrant disease, including metastatic melanoma, renal cell cancer, and non-small cell lung cancer (27-31). Response to immune checkpoint inhibition (ICI) therapy has also been encouraging for central nervous system (CNS) metastases, opening exploration of primary brain tumor treatment (32-34). Unfortunately, ICI treatment either in head-to-head randomization against or in combination with standard therapy for GBM has not yielded a clear survival benefit (35-37). Notably, a subset of patients with GBM may experience prolonged and durable responses to ICI treatment; the reasons for disparate response are unclear but likely have to do with the immune milieu in the tumor microenvironment, and mutations infrequently enriched in GBM, such as BRAF or PTPN11, seem to be associated with improved outcomes (38). Work is underway to determine additional factors contributing to this inconsistent response and to assist in the identification and selection of patients who may benefit from ICI use, particularly in the recurrent setting. At present, ICI therapy is primarily available under the auspices of a clinical trial (e.g., NCT04977375, NCT03961971, NCT03491683, NCT04396860, and NCT04145115; see also: Table 1).

Table 1.

Recruiting and not yet recruiting international immunotherapy-based phase 2 or 3 clinical trials registered with ClinicalTrials.gov. Included trials have either OS or PFS as primary or secondary outcomes. The above trials exclude exclusively intra-operative or radiation therapy-based as well as purely diagnostic or observational studies. Terms: B7-H3 – B7 homolog 3 protein; CD – cluster of differentiation; EGFR – epidermal growth factor receptor; GBM - glioblastoma; ICI – immune checkpoint inhibitor; IGF - insulin-like growth factor; IL – interleukin; IL13Rα2 – IL-13 receptor subunit alpha 2; MMP2 – matrix metallopeptidase 2; NKG2D – CD129-R for natural killer cells type 2D; RT - radiotherapy; SoC - standard of care; TMZ - temozolomide

Type	Intervention (Target)	Country (Site)	Clinical Trial Number	Status	Estimated Completion
CAR T cell-based	CAR T cells (NKG2D)	China (Jiangxi) and Taiwan	NCT05131763 and NCT04717999	Recruiting and not yet recruiting (NCT04717999)	December 2022 (NCT05131763) and September 2023 (NCT04717999)
	CAR T cells (IL13Rα2)	US (Duarte, CA)	NCT04003649	Recruiting	December 2023
	CAR T cells (B7-H3)	Multiple	NCT04077866, NCT05241392, and NCT04385173 (China); NCT05366179, NCT05474378, and NCT05835687 (US)	Recruiting	March 2024 (NCT04385173) through March 2028 (NCT05835687)
	CAR T cells (MMP2)	US (Duarte, CA and Austin, TX)	NCT04214392 and NCT05627323	Recruiting (NCT04214392) and not yet recruiting (NCT05627323)	January and October 2024
	CARv3-TEAM-E (EGFR)	US (Boston, MA)	NCT05660369	Recruiting	June 2025
	CAR T cells (IL-8R/CD70)	US (Gainesville, FL)	NCT05353530	Recruiting	December 2027
	Tris-CAR T cells (IL7Ra)	China (Beijing)	NCT05577091	Not yet recruiting	November 2024
	CAR T cells (EGFRvIII)	China (Beijing)	NCT05802693	Not yet recruiting	November 2025
ICI-based	AK105, anlotinib, and SoC	China (Nanjing)	NCT05033587	Recruiting	May 2023
	Pembrolizumab with and without NGM707 (myeloid target)	International (17 sites)	NCT04913337	Recruiting	February 2025
	Atezolizumab and cabozantinib plus SoC	US (Houston, TX)	NCT05039281s	Recruiting	December 2025
	Sintilimab (anti-PD-1) and bevacizumab	China (Zhengzhou)	NCT05502991	Not yet recruiting	December 2025
Other Cellular Therapies	hSTAR (hematopoietic stem cell rescue)	US (Bethesda, MD and Cleveland, OH)	NCT05052957	Recruiting	June 2024
	IGV-001 combined agent (autologous GBM cells and ASO against IGF-1)	US (23 sites)	NCT04485949	Recruiting	January 2025
Other T cell-based	Activated autologous T cells	US (Birmingham, AL, Louisville, KY, and Los Angeles, CA)	NCT05664243 and NCT05341947	Recruiting (NCT05664243) and not yet recruiting (NCT05341947)	June 2023 (NCT05341947) and December 2025 (NCT05664243)
	Efineptakin alfa (long-acting IL-7) and pembrolizumab	US (Rochester, MN)	NCT05465954	Recruiting	October 2024
	DeltEx (gamma-delta T cells) plus SoC	US (Birmingham, AL and Louisville, KY)	NCT05664243	Recruiting	December 2025
	L19TNF (L19 antibody) plus SoC	Switzerland (Zurich)	NCT04443010	Recruiting	October 2026
Peri-operative Immunotherapy	Pembrolizumab with and without ACT001 plus surgery	US (Houston, TX)	NCT05053880	Recruiting	November 2022
	Pre-surgical pembrolizumab and radiation therapy	US (Los Angeles, CA)	NCT05879250	Recruiting	December 2023
	Surgical nivolumab and ipilimumab	US (CA, MA, and NY)	NCT04606316	Recruiting	December 2023
	Surgical pembrolizumab with or without olaparib	US (Boston)	NCT05463848	Recruiting	December 2024
Vaccine	Dendritic cell	Italy (Milan), Germany (multiple sites), and Taiwan	NCT04801147, NCT03395587, and NCT04115761	Recruiting	December 2022 (NCT04115761) to September 2024 (NCT03395587)
	SurVaxM (survivin)	US (11 sites)	NCT05163080	Recruiting	August 2023
	HSV-tk, valacyclovir, and SoC	US (Houston, TX)	NCT03596086	Recruiting	December 2023
	VBI-1901 (gB and pp65)	US (10 sites)	NCT03382977	Recruiting	July 2025
	TVI-Brain-1 (vaccination and activated T-cell enrichment)	US (New Brunswick, NJ)	NCT05685004	Not yet recruiting	December 2025

6.0. Challenges

Despite identification of novel treatments with success in preclinical or animal models, translation of such promising strategies into patient care has consistently failed, and clinical trial results have been disappointing. There are several possible reasons why clinical studies fail to reproduce preclinical findings. One is that in vitro models and patient-derived xenografts, which are generated from cells cultured from a human tumor specimen and injected into an immunocompromised animal, by definition, are not spontaneously formed in vivo. Following xenografting, this newly introduced cellular material proliferates and integrates in a neural environment that inherently differs from a naturally occurring tumor. Microenvironmental factors that are believed to play a critical role in disease progression - such as glioblastoma stem cells (39), tumor-associated macrophages (40), and novel functional neural-tumor synapses (41-43) - may be entirely absent or affect the xenograft in unpredictable ways. Ongoing work is exploring how immune cells within the brain contribute to tumor growth, but it follows logically that this complex interplay is altered in immunodeficient recipient animals. A second factor is the striking cellular and molecular heterogeneity seen in GBM; most preclinical models use a limited number of commercially available or institutional cell lines that may not faithfully recapitulate tumoral divergence (44). Third is an incomplete understanding of the driver mutations of GBM that may be dynamic during disease evolution.

The treatment of non-CNS tumors has dramatically improved in recent years in no small part due to targeted and immunotherapy-based approaches, which have not yet demonstrated robust or durable responses in human trials for GBM. Despite evidence of a likely evolution of GBM from neural stem or progenitor cells assumed to be associated with early TERT promoter mutations, it is unclear which subsequent alterations may consistently contribute to gliomagenesis to serve as critical therapeutic targets (45); the accumulation of multiple somatic mutations and avoidance of replicative senescence appear key for GBM formation. It is also likely that tumoral exploitation of multiple direct and indirect pathways for progression, including peritumoral neural hyperactivity and neurovascular decoupling, allow for subversion of therapeutic benefit (43,46,47).

Further complicating the treatment landscape is a challenge in interpreting existing clinical trial data. In 2021, the World Health Organization released the fifth edition of its Classification of Central Nervous System Tumors, an update to the 2016 schema. The most notable change for GBM is a requirement that tumors are isocitrate dehydrogenase (IDH) wild-type (wt); prior versions allowed for IDH mutations in the setting of characteristic histological findings such as pseudopalisading necrosis. This distinction is important as the behavior of IDH mutant tumors is significantly different from their wild-type counterparts. More specifically, overall survival (~1-2 years in IDH-wt GBM patients vs 3-4 years in the as-previously-classified IDH-mutant GBM patients) reflects a more indolent, albeit ultimately life-limiting, disease course (48,49). Therefore, interpretation of data predating this reclassification may be confounded by uneven distributions of IDH mutational status between treatment groups, especially from trials prior to the identification of this molecular alteration in 2008 (50).

As our knowledge of the molecular and genetic underpinnings and complexities of GBM evolves, it is relevant to contextualize our discoveries and incorporate genomic, molecular, histologic, and microenvironmental data for rational clinical trial design. A Society for Neuro-oncology (SNO) Think Tank was convened in 2020 to identify barriers to trial design. Some key limitations identified included overly restrictive inclusion and exclusion criteria that diminish both recruitment and generalizability; a paucity of appropriate control arms for phase 2 trials; and the absence of positive phase 2 data prior to initiation of a phase 3 design (51). The development of adaptive trials, such as GBM AGILE (NCT03970447), are attempts to address some of these issues, and umbrella or basket design trials will likely increase in popularity as well.

Lastly, it is important to consider the tolerability of novel treatments. The safety of any new intervention is scrutinized at all phases of trial design, though this is the primary focus of early phase or phase 1 trials; intuitively, highly cytotoxic chemotherapies may be efficacious in vitro but fail early into clinical investigation. However, this is true not only for novel drug design but also for radiation-, immunotherapy-, or device-based treatments.

7.0. On the Horizon

Acknowledging the challenges described above and the so far limited success with ICI therapy, significant interest remains in co-opting the immune system to treat primary brain tumors. Specifically, chimeric antigen receptor (CAR) T-cell therapy, bispecific antibodies, and tumor-directed vaccines are active areas of investigation. Table 1 highlights some ongoing studies of interest.

7.1. CAR T-cell Therapy

CAR T-cell therapy is a cellular immunotherapy approach in which an adaptive immune response is generated to cancer-associated surface antigens independent of major histocompatibility complex activity, thereby allowing recognition of a wider range of targets than natural T-cells (52). Following extraction of the patient’s or donor’s native T-cells, synthetic antigen receptors are added ex vivo that in turn allow for T-cell recognition and attack of the tumor. This technique has heralded a new era of treatment for hematologic malignancies and was first approved for advanced/refractory B-cell lymphoma and refractory pediatric B-cell acute lymphoblastic leukemia in 2017 (53,54). There is considerable interest in applying this modality to solid tumors as well, including those involving the CNS. However, selection of an appropriate, specific, and conserved neoantigen is difficult, and candidate cell-surface antigens are rare for tumors with low mutational burden such as GBM (55).

Fortunately, identification of novel tumor targets has permitted the development of new CAR T-cell products under investigation. One such antigen is the EGFRv3 alteration, seen in up to 30% patients with GBM, which has led to a phase 1 trial (NCT05660369) of CARv3-TEAM-E T cell therapy. Other neoantigens with recruiting clinical trials include B7-H3, an immunoregulatory protein of the B7 family possibly associated with tumor infiltration (NCT04077866, NCT05241392, and NCT04385173 in China and NCT05366179, NCT05474378, and NCT05835687 in the US) (56); a basket trial of NKG2D ligand (NCT05131763 in China); IL-13 receptor ɑ2, a high-affinity receptor expressed by the majority of GBMs with low non-tumor expression, with and without ICIs (NCT04003649 in the US) (57); and matrix metalloproteinase-2 (MMP2) that appears to contribute to chlorotoxin binding to GBM (NCT04214392 in the US) (58). Additional studies of these and other CAR T-cell and activated T-cell therapies that are not yet recruiting, including NCT05341947, NCT05353530, NCT05577091, NCT05627323, NCT05664243, NCT05802693, NCT04717999, and NCT05685004, highlighting the ongoing global interest in these treatments while other neoantigens are being discovered. Table 1 lists ongoing and not yet recruiting studies using immunotherapy approaches for the management of GBM.

Of note, unique toxicities related to tumor-associated inflammation may be seen with CAR T-cell therapies targeting the CNS and likely pose additional challenges to patients with glioblastoma (59,60). The most worrisome CNS adverse event is cerebral edema, which can be fatal; other short- and long-term effects may include delayed neurologic toxicities or non-neurologic symptoms that can be severe enough to halt a trial. As cancer immunotherapeutics are explored for solid tumors, strategies to abrogate or mitigate these on-target, off-tumor effects will be crucial (59).

7.2. Bispecific Antibodies

Similar in concept to CAR T-cell therapy are bispecific antibodies. These compounds build upon success seen with monoclonal antibodies - such as bevacizumab and rituximab - but bind to two different antigens or two epitopes of the same antigen simultaneously; the most common application is to engage both the CD3 receptor and tumor-specific antigens (61). Binding of these targets can recruit and activate immune cells, primarily T-cells, to a tissue of interest and eradicate specific cell populations, which is not possible using monoclonal antibodies alone. The development of these antibodies is technically challenging, and it took from 1985, when the first preclinical bispecific antibody model was created, until 2014 for a clinically accessible agent - blinatumomab - to receive FDA approval (62,63). However, notable progress has been made at antibody generation by applying novel hybridization and “knob-in-hole” techniques, yielding a more extensible and generalizable pathway for production of new agents (64,65). At the time of this publication, nine bispecific antibodies are FDA approved with seven used for malignancies such as lymphoma, multiple myeloma, and non-small cell lung cancer with many more in active clinical trials. The utility of this technique extends beyond malignancy, but one phase 1 trial (NCT03344250) is investigating the use of EGFR-CD3 bi-armed activated T-cells - administered during adjuvant TMZ cycles - following standard of care chemoradiation.

7.3. Vaccines

A longstanding and popular investigational cancer immunotherapy for GBM is tumor-directed vaccination. Although this technique also exploits an adaptive immune response, vaccines differ from CAR T-cell and bispecific antibody therapy due to the administration of an exogenous antigen meant to elicit a response to tumor. An important exception to this paradigm is dendritic cell (DC) vaccination, which requires isolation of DCs from the patient followed by exposure to tumor antigen and exogenous DC maturation prior to reinjection (66). As with the previously discussed immunotherapies, selection of antigen is the key step for vaccine treatment, and multiple targets (such as peptides to EGFRv3 and IDH-1) have been explored. As of this publication, there are 53 completed vaccine trials of which 43 had study sites in the United States; the first study was initiated in 1997. Treatment regimens have included vaccine monotherapy or combinations with standard of care or other experimental agents (67,68).

Although early vaccine trials demonstrated mixed results, two recent trials merit discussion. First is a single arm phase 2a study of SurVaxM, a peptide vaccine conjugate that targets the molecule survivin. This trial enrolled 64 patients with newly diagnosed GBM to receive four “priming” doses of the vaccine product plus sargramostim at two week intervals following resection and concurrent chemoradiation with TMZ (69). After the study treatment period, patients received adjuvant TMZ and maintenance SurVaxM until disease progression. Sixty-three patients were evaluable - as one patient received only one of four “priming” doses - with a six-month progression-free survival (PFS) of 95.2%, median PFS of 11.4 months, and median OS of 25.9 months (69). Within their population, there was a notable difference when stratified by O-6-methylguanine-DNA methyltransferase (MGMT) promoter methylation status (MGMT promoter methylated n=33 and unmethylated n=29): median PFS was 17.9 vs 7.0 months and median OS was 41.4 vs 25.9 months, both favoring patients with methylated MGMT status (69). These results compare favorably with historical controls, but the lack of a dedicated control arm is a reasonable criticism for this trial. Recruitment is ongoing in a multicenter, randomized, double-blind clinical trial of SurVaxM (NCT05163080).

Second is a phase 3, randomized and double-blind trial of DCVax-L, a personalized vaccination engineered from patient DCs and antigens derived from their tumor sample (70). Patients with newly diagnosed (n=232) or recurrent (n=64) GBM who underwent near or gross total resection were enrolled in a 2:1 fashion to post-chemoradiation, upfront active treatment or placebo with all groups receiving standard of care adjuvant TMZ. The original primary endpoint was designed to be PFS. Importantly, this was a crossover from placebo trial in which every patient eventually received DCVax-L treatment. The initial report in 2018 did not describe PFS outcomes but emphasized OS (23.1 months), particularly in patients with MGMT promoter methylation (34.7 months) (71). When final results were published in 2023, the primary endpoint was changed from PFS to OS under the arguments that placebo arm loss due to crossover and difficulty in delineating pseudoprogression from true progression would render PFS a less useful measure. Overall, the newly diagnosed cohort experienced a median OS of 19.3 months vs 16.5 months in the control group, and in recurrent GBM there was a median OS of 13.2 vs 7.8 months in the control group (70). Both the newly diagnosed and recurrent cohorts therefore demonstrated a prolonged survival compared to existing treatment. To address concerns about the effects of crossover into the active treatment, the research team compared their outcomes to external controls comprising 15 outside trials in which patients received standard of care treatment. However, criticism has been leveled regarding the selection of trials as some with prolonged OS, including EF-14, were not incorporated. Furthermore, the exclusion of patients who underwent biopsy alone reduces the generalizability of these results. Despite the limitations to this study, the use of DC vaccination for GBM is promising, but future studies with formal randomization will need to confirm a true survival benefit with this approach.

Globally, recruitment is ongoing for 18 trials with at least one arm of vaccine-based therapy for adults with GBM: 6 in the United States, 4 in Germany, 4 in China, 2 each in Belgium and Italy, and 1 each in France and the Netherlands.

7.4. Other Agents

New chemotherapies, targeted therapies, repurposed medicines, and devices are under investigation around the world. These include inhibitors of EGFR, exportin 1, fatty acid synthesis, fibroblast growth factor receptor, glutamate, histone deacetylase, Janus kinase, phosphoinositide 3-kinase, poly (ADP-ribose) polymerase, protein arginine methyltransferase 5, receptor for advanced glycation end-products, sphingomyelin synthase 1, topoisomerase 2, and tyrosine kinase; botanical products including polyphenols and cannabinoids; chemotherapies used in other systemic malignancies; multiagent basket and umbrella trials (GBM AGILE, INSIGhT, and N²M² NOA-20); sonodynamic therapies; nanoparticle delivery; and dietary interventions such as ketogenic diet. Table 2 highlights ongoing and not yet recruiting studies of these and other regimens.

Table 2.

Recruiting and not yet recruiting international phase 2 or 3 clinical trials registered with ClinicalTrials.gov. Included trials have either OS or PFS as primary or secondary outcomes. The above trials exclude exclusively intra-operative or radiation therapy-based as well as purely diagnostic or observational studies. Terms: CBD – cannabidiol; EGFR – epidermal growth factor receptor; FGFR – fibroblast growth factor receptor; GBM - glioblastoma; HDAC – histone deacetylase; JAK – Janus kinase; PARP - poly (ADP-ribose) polymerase; PI3K – phosphoinositide 3-kinase; PRMT5 - protein arginine methyltransferase 5; RAGE - receptor for advanced glycation endproducts; RT - radiotherapy; SoC - standard of care; STAT3 - signal transducer and activator of transcription 3; TCA – tricyclic antidepressant; THC – tetrahydrocannabinol; TKI – tyrosine kinase inhibitor; TMZ - temozolomide

Type	Intervention or Trial (Target)	Country (Site)	Clinical Trial Number	Status	Estimated Completion
Chemotherapy (Targeted and Traditional)	Glutamate inhibition - multiple agents	Switzerland (Zurich)	NCT05664464	Recruiting	June 2023
	Denifanstat (fatty acid synthesis inhibitor) and bevacizumab	China (Beijing)	NCT05118776	Recruiting	July 2023
	Multiple regimens (N2M2 NOA-20, Neuro Master Match)	Germany (13 sites)	NCT03158389	Recruiting	September 2023
	Paxalisib (PI3Kα inhibitor) with ketogenic diet and metformin	US (New York, NY)	NCT05183204	Recruiting	December 2023
	Cetuximab (EGFR) plus SoC	US (New York, NY)	NCT02861898	Recruiting	December 2023
	Endostatin, TMZ, and irinotecan	China (Beijing)	NCT04267978	Recruiting	February 2024
	LAM561 (sphingomyelin synthase 1) plus SoC	International (20 sites)	NCT04250922	Recruiting	February 2024
	Capecitabine plus SoC	US (New York, NY)	NCT03213002	Recruiting	June 2024
	Verteporfin (EGFR)	US (Atlanta, GA)	NCT04590664	Recruiting	August 2024
	Imipramine (TCA) and lomustine	US (San Antonio, TX)	NCT04863950	Recruiting	September 2024
	SKL27969 (PRMT5 inhibitor)	US (MI, NC, and TX)	NCT05388435	Recruiting	September 2024
	Antisecretory factor	Sweden (Lund)	NCT05669820	Recruiting	January 2025
	Debio 0123 (Wee1 kinase inhibitor) plus SoC	International (10 sites)	NCT05765812	Recruiting	January 2025
	INSIGhT – multiple regimens	US (12 sites)	NCT05095376	Recruiting	March 2025
	WP1066 (JAK2/STAT3 inhibitor) with RT	US (Chicago)	NCT05879250	Not yet recruiting	March 2025
	Selinexor (exportin 1) plus SoC	US (21 sites)	NCT05432804	Recruiting	April 2025
	NMS-03305293 (PARP inhibitor) plus TMZ	International (5 sites)	NCT04910022	Recruiting	June 2025
	Pazopanib (TKI) with TMZ	France (Nice)	NCT02331498	Recruiting	August 2025
	Berubicin (topoisomerase 2 inhibitor) plus SoC	Poland (Gdansk and Warszawa)	NCT04915404	Recruiting	October 2025
	BPM31510 (shifts glycolysis to oxidative phosphorylation) with RT and TMZ	US (CA, NY, VA, and WA)	NCT04752813	Recruiting	December 2025
	Pemigatinib (FGFR inhibitor)	International (87 sites)	NCT05267106	Recruiting	January 2026
	GBM AGILE - multiple regimens	International (50 sites)	NCT03970447	Recruiting	June 2026
	Lomustine plus SoC	US (223 sites)	NCT05095376	Recruiting	August 2026
	Lomustine with or without RT	Europe	NCT05904119	Recruiting	September 2027
	NBM-BMX (HDAC-8 inhibitor)	Taiwan	NCT06012695	Recruiting	May 2028
	Azeliragon (RAGE inhibitor) plus RT	Pending	NCT05986851	Not yet recruiting	September 2024
	APG-157 (multi-polyphenol botanical) and bevacizumab	US (Rochester, MN and Omaha, NE)	NCT06011109	Not yet recruiting	December 2024
	NG101m	US (Houston, TX)	NCT04373785	Not yet recruiting	July 2027
Other	Tofacitinib (JAK inhibitor)	US (Dallas, TX)	NCT05326464	Recruiting	June 2024
	Valganciclovir plus SoC	Sweden (Stockholm)	NCT04116411	Recruiting	August 2024
	AGuIX nanoparticles plus SoC	France (6 sites)	NCT04881032	Recruiting	September 2024
	Ultrasound with paclitaxel and carboplatin	US (Chicago, IL)	NCT04528680	Recruiting	September 2024
	Rhenium nanoliposomes	Texas (3 sites)	NCT01906385	Recruiting	January 2025
	TN-TC11G (THC and CBD)	Spain (8 sites)	NCT03529448	Recruiting	March 2025
	Liposomal curcumin plus SoC	US (Baltimore, MD)	NCT05768919	Recruiting	February 2026
	Nabiximols (cannabinoids)	UK (18 sites)	NCT05629702	Recruiting	February 2026
	Sonodynamic therapy (SONALA-001 and Exablate 4000 type 2)	US (6 sites)	NCT05370508	Recruiting	April 2026
	Ketogenic diet plus SoC	US (CA, NC, and TX)	NCT05708352	Recruiting	June 2028
	SonoCloud-9 ultrasound with carboplatin	International (8 sites)	NCT05902169	Not yet recruiting	January 2028

The advent of novel treatment pathways will likely complement existing and experimental agents, as it has become clear that a personalized but multifaceted approach to GBM treatment is needed. For instance, dietary interventions and neuromodulatory techniques may be used in conjunction with vaccines and standard of care to address treatment evasion by GBM. As novel insights are gained from studying the tumor micro- and macroenvironment, additional targets may be identified that are relevant to disease development and progression.

8.0. Conclusion

Despite decades of difficulty in identifying breakthrough treatments for GBM, recent advances are cause for cautious optimism in the field of neuro-oncology. Immunotherapy, which has yielded noteworthy progress in systemic and hematologic malignancies, initially did not appear to have similar success in intracranial tumors. However, CAR T-cell therapy, bispecific antibodies, and tumor vaccines all have shown promise that indicate - with optimal target selection - new treatments may be within our reach. It has become evident that the identification and development of new therapies requires collaborative integration of preclinical and clinical investigations in an institutional environment conducive to and supportive of this scientific mission.

To ensure preclinical work successfully moves into the translational and clinical domains, there is a need to design both animal studies and clinical trials in rational but creative ways. The adoption and repurposing of techniques used in other scientific disciplines, such as electrophysiologic monitoring and network mapping, has the potential to broaden our understanding of GBM at all levels; genomic analyses and the burgeoning discipline of cancer neurosciences have begun to reveal new insights into factors facilitating tumor development, infiltration, and progression. Using these revelations to improve patient outcomes will likely require near-native recapitulation of the tumor microenvironment, diversity in cell lines studied, cross-institutional collaboration, expanded inclusion criteria for clinical trials, and adaptive study design. Although there is a long way to go, prospects for better treatment of GBM are on the horizon.

9.0. Expert Opinion

In recent years, there has been unprecedented activity to identify pathologic mechanisms of and novel treatments for GBM. Despite this furor, no major therapeutic breakthroughs have come to fruition. Shortcomings in clinical and translational efforts are attributed to the inherent biological sequestration of CNS neoplasms, which are immunologically privileged and consequently less responsive to immunotherapy; inter- and intratumoral heterogeneity that both underscores the molecular diversity of GBM and potentially drives resistance to treatment; biological disparities between animal models and in situ tumors; and the response of GBM to external stressors, such as chemotherapy or radiotherapy, which may produce a hypermutated phenotype and/or activate alternative bioenergetic pathways. These mechanisms all likely contribute meaningfully, but we feel that another key limitation is the prevalence of interventions or regimens targeting a single aspect or mechanism of disease.

Neuroscience, genomic, and molecular techniques continue to reveal new mechanistic knowledge, highlighting the extent to which GBM hijacks existing metabolic and proliferative pathways. In turn, this work has uncovered new treatment targets, leading to an ever-increasing arsenal of agents with compelling efficacy in the laboratory. We then eagerly test these new medicines or devices only to find discordance between preclinical and in-human results, but not as a consequence of poor academic rigor. Science requires logical and methodical testing to facilitate interpretation of results and ensure reproducibility; there is little doubt that most GBM research adheres to this expectation. However, the paradigm of the scientific method is typically applied by evaluating a single investigational agent per arm or clinical trial, which is meant to reduce the number of controllable variables. Unfortunately, as historical efforts have shown, it is unlikely that a single therapy will work completely in isolation, and the rational selection and testing of multiple agents - each affecting a different aspect of GBM growth - is likely to yield the greatest efficacy.

Any potentially curative therapy for GBM will acknowledge and address multiple intra- and extra-tumoral factors. These may include de novo functional and hyperactive synapses between tumor and surrounding brain, which co-opt AMPA and/or NMDA receptors and non-activity dependent ionic dysregulation (43); tumoral preference for - but not dependence on - glycolysis (72); paracrine signaling, such as through neuroligin-3 and brain-derived neurotrophic factor (73,74); neuro-immunologic cross-talk; blood vessel dysregulation and angiogenesis; and cellular hyperproliferation, arising at least in part from insulin growth factor 1, PI3K, and MAPK (75-77). The repurposing of existing medications, such as those reducing glutamate activity or neurotrophins, may complement new targeted therapies or established agents. Furthermore, recent clinical trials have demonstrated great potential for immunotherapy, with cancer vaccines and CAR T-cell treatment as active areas of research with multiple ongoing trials. Lastly, the development or incorporation of neuromodulatory techniques may also dramatically affect our understanding and treatment of GBM, and these devices or agents may represent a growing field of exciting research. Importantly, assessment of treatment effect must take into account that even a modest benefit, when used in concert with other therapies affecting different pathways, may merit further investigation.

We therefore posit that a contemporary and highly efficacious treatment regimen will require a multimodality approach extending beyond surgery, radiation, and classical chemotherapy. Although it will be necessary to balance the toxicity or tolerability of any prospective treatment(s) with the potential benefit, the advances in GBM research in recent years is a source of hope for future patients.

3.0. Highlights.

• Glioblastoma, IDH wildtype, (GBM) affects more than 12,000 individuals per year in the US with an estimated worldwide increasing incidence of 0.5-5 per 100,000, and existing therapies are not curative

• Trials of brachytherapy, chemotherapy-impregnated wafers, high-dose chemotherapy, molecular-targeted therapies, anti-angiogenesis, and immune checkpoint inhibition have not resulted in changes to present management

• Evolving understanding of GBM due to molecular characterization, genomics, and cancer neuroscience has the potential to improve outcomes

• Recent work has demonstrated the potential benefit of dendritic cell and peptide GBM vaccines as well as CAR T-cell therapies

• Many trials are underway of immunotherapy- and non-immunotherapy-based regimens, including those targeting neural hyperexcitability, tumor metabolism, and neuro-immune signaling

• A multimodality, multi-pathway treatment regimen has the greatest likelihood of durable response in GBM but will require creative trial design on a solid scientific foundation