As the most common brain tumor in adults, gliomas have sustained the focus of scientific research for the past several decades. Recently, more attention has been drawn to the diagnostic criteria of gliomas with the restructured 2021 WHO Classification of Tumors of the Central Nervous System, specifically focusing more on molecular biomarkers as a means of categorization 
. Within this classification adult-type diffuse gliomas are the most prevalent tumor types, defined on the basis of molecular expression of isocitrate dehydrogenase (IDH) and the 1p/19q codeletion. These glioma subtypes include astrocytoma (IDH-mutant astrocytoma), oligodendroglioma (IDH-mutant and 1p19q-codeleted), and glioblastoma (GBM) (IDH-wildtype) 
. The typical management of adult-type diffuse glioma begins with a resection or biopsy, followed by possible radiotherapy and/or chemotherapy with the alkylating agent, temozolomide, or the combination procarbazine, lomustine, and vincristine (PCV) 
. Even with this regimen, recurrence is prevalent, and the prognosis is dismal, particularly in GBM, which has an average survival of 14–16 months 
As gliomas are becoming more molecularly defined, so too is their treatment progressing more towards the targeting of molecular pathways 
. Compared with traditional chemotherapeutic drugs, molecularly targeted antitumor therapy has the advantage of strong specificity with minimal damage to normal tissues. Molecular-targeted glioma therapies have gained traction in the scientific literature, with many analyses centered on identifying mechanisms pertinent to glioma growth 
. The Raf/MEK/Erk pathway has been of particular interest as a targetable pathway due to its preponderance among gliomas 
. Additionally, a systematic review by Da Silva et al. highlighted the molecular targeted therapies in clinical trials for GBM, identifying four categories of targets: targeting the potential for unlimited replication, growth autonomy and migration, cell cycle and apoptosis, and angiogenesis 
. The figure below is a visual summary of some of the most common pathways where targeted therapies act.(Figure 1
Figure 1. Summary of molecularly targeted pathways in adult-type diffuse glioma.
2. Adult-Type Diffuse Glioma Subtypes
Though the overwhelming majority of studies centered on GBM, the literature shows that adult gliomas found more frequently in practice tend to harbor IDH mutations 
. The reason for the overrepresentation of GBM-focused studies and the underrepresentation of IDH-mutant astrocytoma or oligodendroglioma is multifactorial. First off, the updated WHO classification is a recent development as of 2021; because the majority of the works occurred prior to the molecular subtype differentiation, there were likely studies that self-identified as GBM studies that may have included tumors with an IDH mutation or 1p19q co-deletion. Additionally, it is likely that GBM has received more research funding and scientific attention than other brain tumors, perhaps due to its more aggressive nature and mortality rates. Therefore, the funding for studies investigating IDH-mutant astrocytoma or oligodendroglioma may be less robust. Of note, the ongoing clinical trials for glioma vastly favor GBM as well, receiving the majority of funding from industry sources. Further studies to quantify the distribution of research funding between glioma subsets would be necessary to confirm this association. Lastly, the standard cell lines for all glioma research tend to be glioblastoma models, particularly U87, U373, and U251 
3. Protein Kinase Pathways
In terms of molecular targets, protein kinase pathways—especially PI3K/Akt/mTOR and Ras/BRAF/Mek/Erk—were the most prevalent in the clinical and laboratory studies analyzing existing therapies and novel targets to treat adult-type diffuse glioma. These results are consistent with previous studies that have demonstrated a predominance in the PI3K/Akt/mTOR and Ras/BRAF/Mek/Erk protein kinase pathways in molecularly targeted glioma treatment 
. The importance of these pathways in glioma has been well-described in the literature; ultimately, these tumors harbor mutations that continuously activate these protein kinase signaling pathways, leading to increased tumorigenesis and progression 
Both the PI3K/Akt/mTOR and Ras/BRAF/Mek/Erk protein kinase pathways are also downstream of receptors such as EGFR, one of the most significant signaling pathways clinically implicated in glioma 
. A systematic review of molecular-targeted therapy clinical trials for GBM identified EGFR as the most prevalent molecular target 
. Nonetheless, studies have demonstrated limited clinical benefit of anti-EGFR therapies, theorized to be secondary to PTEN-mediated resistance of GBM to this therapy type 
Similar to the published clinical studies on this topic, protein kinase pathways were by far the most predominant molecular targets tested in ongoing clinical trials. Interestingly, these therapeutics were also investigated much earlier on average. This finding is likely due to the fact that protein kinase inhibitors are some of the earliest molecular target therapies in the field of targeted oncologic interventions, thus being able to start clinical trials for the treatment of glioma as early as 2001 
. Perhaps, in the coming years, as the analysis of existing molecularly targeted therapies progresses from earlier stage clinical testing or laboratory testing, there will be a shift favoring more of the scientifically novel approaches—such as immunotherapeutics, cell cycle inhibitors, or more specifically localized targeting—in clinical trials.
Additional protein kinase pathways targeted in laboratory studies included HER2 receptors, epithelial membrane protein-2 (EMP2), and STAT3, to name a few 
. HER2 expression tends to be low in GBM, and though one clinical trial examining a HER2 inhibitor has yet to show therapeutic gain, laboratory studies have promising evidence for efficacy 
. EMP2 has been implicated in bevacizumab resistance and thus shows promise as a molecular target for preventing resistance in conjunction with this common therapeutic 
. STAT3 plays a role in astrocyte development and has tumor-suppressive roles in glial malignancies; this target shows promise in laboratory research using tetrandrine as an inhibitor 
. Despite varying clinical evidence of efficacy, protein kinase-targeted therapies remain a prevalent area of study for both individual inhibitors and combined therapies.
4. Cell Cycle/Apoptosis Pathways
Interestingly, a prevalent molecular target in laboratory studies—both testing existing therapies and identifying novel targets—were cell cycle/apoptosis pathways. This difference may be attributed to the fact that clinical studies tend to focus on targets with existing FDA-approved therapies or targets that are more well-established in the literature, while studies with the goal of establishing new targets or testing newly developed therapies can explore a wider range of targets with less established evidence.
The use of cell cycle or apoptosis pathways as targets stems from the use of these pathways in the treatment of other tumors, in particular. In the present study, only four clinical studies included cell cycle/apoptosis pathway inhibitors, namely the cyclin-dependent kinase (CDK) 4/6 inhibitor palbociclib, the mouse double minute 2 (MDM2) inhibitor idasanutlin, the ribonucleotide reductase inhibitor Motexafin Gadolinium, and the 26S proteasome inhibitor bortezomib 
. CDK and MDM2 inhibitors were also prevalent in laboratory studies testing existing therapies 
. Two CDKs were identified as novel molecular targets—namely CDK 5 and 10—and other novel targets include other apoptosis regulators such as E2F1, trichothiodystrophy group A protein (TTDA), and protease-activated receptor 2 (PAR2) 
5. Microenvironmental Pathways (Angiogenesis, Cell-Cell Adhesion, Ιron/Cation Regulation)
Anti-angiogenic therapies aim to compensate for the robust vascularity of gliomas, particularly GBM 
. Specifically, vascular endothelial growth factor (VEGF) is overexpressed in GBM, providing the rationale for the thirteen published clinical trials investigating VEGF inhibitors. Specific inhibitors studied include cediranib, cabozantinib, apatinib, and bevacizumab, which appear to be well-tolerated by patients and, in many cases, portend progression-free survival 
. Many laboratory studies also tested VEGF inhibitors—namely bevacizumab, axitinib, and apatinib—and all found promising results in vivo 
. Other microenvironmental targets included mitochondrial transcription factor A (TFAM), transient receptor potential cation channel subfamily V member 4 (TRPV4), and HIF2α. These targets were acted on by melatonin, cannabidiol, and PT2385, respectively, all of which demonstrated antitumor effects 
. Promising novel microenvironmental targets include miR-497, TWIST transcription factor, and tenascin-W, among others 
6. Immunotherapy Pathways
The immune checkpoint blockade adopted in the glioma therapeutics model follows treatment paradigms for melanoma, lung cancer, colon cancer, and hepatocellular carcinoma; the therapies used to treat these tumors tend to block programmed cell death protein 1 (PD1), a protein known for attenuating the host immune response to tumor cells, or cytotoxic T lymphocyte antigen-4 (CTLA-4), a molecule that inhibits T-cell activation 
. The clinical studies identified in the present study investigating immunotherapeutic pathways targeted PD1 using the inhibitor nivolumab 
. Other immunotherapy targets that were found to be effective in vivo included the inhibition of CD73 with antibodies, extracellular matrix metalloproteinase (EMMPRIN) with icaritin, and NFκB with BAY117082 
. Novel immunotherapeutics for GBM and oligodendroglioma include cluster of differentiation 204 (CD204), S100A, and the CE7 epitope of the L1-CAM adhesion molecule 
7. Wnt/β-Catenin Pathway
The wnt/β-catenin pathway was much more prevalent in earlier stages of laboratory research identifying new targets, likely because the role of wnt/β-catenin in glioma progression is a more recent scientific advancement 
. It is likely that in the upcoming years, the distribution of molecular targets may shift from protein kinase pathway-targeted therapies towards the wnt/β-catenin pathway or a combinatory approach of the two. Ongoing clinical trials have yet to target these pathways, but it is likely that this will soon change.
8. Study Design
The majority of laboratory studies utilized GBM cell lines or GBM patient samples. The frequent use of the U87 cell line in laboratory studies may be attributed to its widely accepted use as a model for GBM 
. The use of technology such as spheroid or 3D cell culture is highly relevant in the context of therapies for gliomas. These technologies more accurately represent the tumor microenvironment and allow for better design of patient-specific treatments. Nearly one-third of laboratory studies testing existing therapies utilized this technology, implying that these studies are likely closer to translation to human studies.
The use of patient-derived GBM and glioma samples also highlights the importance of personalized medicine approaches in glioma treatment; nonetheless, this use also limits the generalizability of the conclusions, as most of these studies did not investigate molecular subtypes.