What are Gliomas?
Gliomas are the most common type of malignant tumor originating in the brain, comprising approximately 80% of all malignant brain tumors and 30% of all central nervous system tumors (Goodenberger et al., 2012). They are split into four grades of severity by the World Health Organization: grade I are non-malignant tumors, grade II are relatively non-malignant tumors, grade III are low-grade malignant tumors, and grade IV are the most malignant tumors, including glioblastoma multiforme (GBM) (Urbanska et al., 2014). Grade I tumors typically have the best prognosis, while grade II tumors are associated with hypercellularity, which has a 5-8 year median survival once diagnosed. Grade III tumors, or anaplastic astrocytoma tumors, have a median of 3 year patient survival. Lastly, patients diagnosed with GBMs have a median survival of 12-18 months (Gladson et al., 2010). In addition, compared to other types of cancer, the 5-year relative survival rate of high grade gliomas is much smaller, ranging between 5-15% (Khan et al., 2020). The figure below outlines the average five-year survival rates from common cancer types in the United States (Figure 1).
Figure 1. Five Year Survival Rates of High Grade Glioma vs. Other Common Cancer Types (Duggan et al., 2016)
Two common and widely researched mutations associated with gliomas are in the isocitrate dehydrogenase 1 and 2 (IDH1/2) genes and in the telomerase reverse transcriptase (TERT) promoter. IDH1/2 mutations alter the function of metabolic enzymes in the tricarboxylic acid (TCA) cycle, causing 2-hydroxyglutarate (2-HG) to be produced instead of nicotinamide adenine dinucleotide phosphate (NADPH). These mutations occur in 70% of grade II and III astrocytomas and oligodendrogliomas and in secondary grade IV GBMs (Hai Yan et al., 2009; Myung et al., 2012). Likewise, TERT promoter mutations cause tumor cells to grow unrestrictedly by elongation of telomere length because of TERT activation, playing a vital role in anti-senescence and immortal cancer development (Yujin et al., 2017). These mutations occur in up to 75% of GBM patients (Vuong et al., 2020). Growing evidence indicates that both of these mutations play a causal role in gliomagenesis, promoting glioma initiation and progression through epigenetic and metabolic reprogramming (Cohen et al., 2013; Huang et al., 2019) Despite the low survival rate of patients diagnosed with gliomas and the many studies done on their associated pathways and relevant mutations, current treatment methods offer little hope in extending the lifespan of patients (Alexander et al., 2017).
Current Glioma Treatments
The current standard of care for glioma patients is tumor resection surgery, followed by radiation therapy and chemotherapy (Minniti et al., 2009). Surgery alone puts the median survival of GBM at 6 months and is rarely curative. The addition of radiation therapy increases survival to 12.1 months and can temporarily control tumor growth (Harter et al., 2014; Warren et al., 2008). Further use of chemotherapeutics, such as temozolomide — the most common drug that acts as an agent that delivers a methyl group to purine bases of DNA — further increases survival timespan to 14.6 months (Jihong et al., 2012).
Figure 2: The Current Gold Standard of Glioma Treatment
Despite being the gold standard therapeutic for glioma, temozolomide is still limited in efficacy due to high rates of recurrence and damaging neurological effects (Kim et al., 2015; Lin et al., 2015). Other candidates, such as alkylating agents carmustine and lomustine, and bevacizumab, a VEGF-A-targeting monoclonal antibody, have also shown potential in glioma treatment, yet studies have also found no additional benefits with increased toxicity. (Rahman et al., 2014) Therefore, the search for more effective and targeted drug candidates as glioma chemotherapeutic agents remains vital for extending patient survival and health.
Lack of Accurate Models for Glioma Drug Screening
Current model systems for drug screening can be grouped into two categories: in vitro utilizing model or patient-derived cell lines and in vivo utilizing animal models (Rybin et al., 2021). However, both systems fail to address the complex features of the brain and are unsuccessful in reflecting human conditions (Majc et al., 2021).
Even the most established in vitro cell lines are controversial due to key differences when compared to in situ gliomas (Gillet et al., 2011). Furthermore, establishing model cell lines for gliomas with specific mutations, like IDH1, is especially difficult (Luchman et al., 2012). To address the limitations of traditional in vivo systems, transgenic mouse models and patient-derived xenografts (PDX) were developed. Although transgenic models have been instrumental in helping researchers understand oncogenic mutation mechanisms, they, like in vitro models, fail to fully represent the complex genetic and phenotypic characteristics of glioma and the brain (Rybin et al., 2021). PDX, while they capture the histological markers and tumor invasiveness, also present limitations, as they are expensive and laborious to establish and maintain (Wang et al., 2017). Thus, clinical treatments of gliomas are limited by the lack of a scalable, physiologically-relevant model for testing therapeutics (Pine et al., 2020).
Figure 3: Different Glioma Drug Testing Methods
To address such limitations of in vitro and in vivo glioma models, sophisticated organoid-glioma systems have been recently developed. Organoids have shown promising results in capturing key features of the brain, including cellular morphology and spatial distribution (Rybin et al., 2021). In addition, such organoids, also known as minibrains, have proven to be increasingly similar to patient derived samples and are capable of accurately modeling patient-specific responses to treatment. However, scalability and compound screening still exist as problems with growing organoids, perpetuating the need for a more efficient organoid development method (Yuhong et al., 2020). If developed, a high-throughput, minibrain model presents promising new avenues for glioma drug discovery research.
Our Approach
In a two phase process, we are developing a novel organoid-dependent drug efficacy system (NODES) as an improved economical and high-throughput method that screens therapeutic drugs before billions of dollars and multiple years are used to evaluate them through animal testing. NODES can be used by basic science researchers to test novel compounds, pharmaceutical companies to verify the impacts of therapeutics before committing them to clinical trials, and as a personalized medicine platform for patient-specific therapy. By recapitulating the brain microenvironment, NODES has the potential to accurately characterize drug responses, offering new hope to patients in their fight against this lethal disease.
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