mTOR: a new therapeutic target for pediatric low-grade glioma?

Unlocking the power of precision medicine for pediatric low-grade gliomas: molecular characterization for targeted therapies with enhanced safety and efficacy

In the past decade significant advancements have been made in the discovery of targetable lesions in pediatric low-grade gliomas (pLGGs). These tumors account for 30-50% of all pediatric brain tumors with generally a favorable prognosis. The latest 2021 WHO classification of pLGGs places a strong emphasis on molecular characterization for significant implications on prognosis, diagnosis, management, and the potential target treatment. With the technological advances and new applications in molecular diagnostics, the molecular characterization of pLGGs has revealed that tumors that appear similar under a microscope can have different genetic and molecular characteristics. Therefore, the new classification system divides pLGGs into several distinct subtypes based on these characteristics, enabling a more accurate strategy for diagnosis and personalized therapy based on the specific genetic and molecular abnormalities present in each tumor. This approach holds great promise for improving outcomes for patients with pLGGs, highlighting the importance of the recent breakthroughs in the discovery of targetable lesions.

Benign Glioma

Benign glioma broadly refers to a heterogeneous group of slow-growing glial tumors with low proliferative rates and a more indolent clinical course. These tumors may also be described as "low-grade" glioma (LGG) and are classified as WHO grade I or II lesions according to the Classification of Tumors of the Central Nervous System (CNS) (Louis et al. in Acta Neuropathol 114:97-109, 2007). Advances in molecular genetics have improved understanding of glioma tumorigenesis, leading to the identification of common mutation profiles with significant treatment and prognostic implications. The most recent WHO 2016 classification system has introduced several notable changes in the way that gliomas are diagnosed, with a new emphasis on molecular features as key factors in differentiation (Wesseling and Capper in Neuropathol Appl Neurobiol 44:139-150, 2018). Benign gliomas have a predilection for younger patients and are among the most frequently diagnosed tumors in children and young adults (Ostrom et al. in Neuro Oncol 22:iv1-iv96, 2020). These tumors can be separated into two clinically distinct subgroups. The first group is of focal, well-circumscribed lesions that notably are not associated with an increased risk of malignant transformation. Primarily diagnosed in pediatric patients, these WHO grade I tumors may be cured with surgical resection alone (Sturm et al. in J Clin Oncol 35:2370-2377, 2017). Recurrence rates are low, and the prognosis for these patients is excellent (Ostrom et al. in Neuro Oncol 22:iv1-iv96, 2020). Diffuse gliomas are WHO grade II lesions with a more infiltrative pattern of growth and high propensity for recurrence. These tumors are primarily diagnosed in young adult patients, and classically present with seizures (Pallud et al. Brain 137:449-462, 2014). The term "benign" is a misnomer in many cases, as the natural history of these tumors is with malignant transformation and recurrence as grade III or grade IV tumors (Jooma et al. in J Neurosurg 14:356-363, 2019). For all LGG, surgery with maximal safe resection is the treatment of choice for both primary and recurrent tumors. The goal of surgery should be for gross total resection (GTR), as complete tumor removal is associated with higher rates of tumor control and seizure freedom. Chemotherapy and radiation therapy (RT), while not typically a component of first-line treatment in most cases, may be employed as adjunctive therapy in high-risk or recurrent tumors and in some select cases. The prognosis of benign gliomas varies widely; non-infiltrative tumor subtypes generally have an excellent prognosis, while diffusely infiltrative tumors, although slow-growing, are eventually fatal (Sturm et al. in J Clin Oncol 35:2370-2377, 2017). This chapter reviews the shared and unique individual features of the benign glioma including diffuse glioma, pilocytic astrocytoma and pilomyxoid astrocytoma (PMA), subependymal giant cell astrocytoma (SEGA), pleomorphic xanthoastrocytoma (PXA), subependymoma (SE), angiocentric glioma (AG), and chordoid glioma (CG). Also discussed is ganglioglioma (GG), a mixed neuronal-glial tumor that represents a notable diagnosis in the differential for other LGG (Wesseling and Capper 2018). Ependymomas of the brain and spinal cord, including major histologic subtypes, are discussed in other chapters.

A POETIC Phase II study of continuous oral everolimus in recurrent, radiographically progressive pediatric low-grade glioma

BACKGROUND

To evaluate efficacy, pharmacokinetics (PK) and pharmacodynamics of single-agent everolimus in pediatric patients with radiographically progressive low-grade glioma (LGG).

METHODS

Everolimus was administered at 5 mg/m² once daily as a tablet or liquid for a planned 48-week duration or until unacceptable toxicity or disease progression. Patients with neurofibromatosis type 1 were excluded. PK and pharmacodynamic endpoints were assessed in consenting patients.

RESULTS

Twenty-three eligible patients (median age 9.2 years) were enrolled. All patients received prior chemotherapy (median number of prior regimens two) and/or radiotherapy (two patients). By week 48, two patients had a partial response, 10 stable disease, and 11 clinical or radiographic progression; two discontinued study prior to 1 year (toxicity: 1, physician determination: 1). With a median follow up of 1.8 years (range 0.2-6.7 years), the 2-, 3-, and 5-year progression-free survivals (PFS) were 39 ± 11%, 26 ± 11%, and 26 ± 11%, respectively; two patients died of disease. The 2-, 3-, and 5-year overall survival (OS) were all 93 ± 6%. Grade 1 and 2 toxicities predominated; two definitively related grade 3 toxicities (mucositis and neutropenia) occurred. Grade 4 elevation of liver enzymes was possibly related in one patient. Predose blood levels showed substantial variability between patients with 45.5% below and 18.2% above the target range of 5-15 ng/mL. Pharmacodynamic analysis demonstrated significant inhibition in phospho-S6, 4E-BP1, and modulation of c-Myc expression.

CONCLUSION

Daily oral everolimus provides a well-tolerated, alternative treatment for multiple recurrent, radiographically progressive pediatric LGG. Based on these results, everolimus is being investigated further for this patient population.

Inhibition of mTORC1 in pediatric low-grade glioma depletes glutathione and therapeutically synergizes with carboplatin

BACKGROUND

Pediatric low-grade glioma (pLGG) often initially responds to front-line therapies such as carboplatin, but more than 50% of treated tumors eventually progress and require additional therapy. With the discovery that pLGG often contains mammalian target of rapamycin (mTOR) activation, new treatment modalities and combinations are now possible for patients. The purpose of this study was to determine if carboplatin is synergistic with the mTOR complex 1 inhibitor everolimus in pLGG.

METHODS

We treated 4 pLGG cell lines and 1 patient-derived xenograft line representing various pLGG genotypes, including neurofibromatosis type 1 loss, proto-oncogene B-Raf (BRAF)-KIAA1549 fusion, and BRAFV600E mutation, with carboplatin and/or everolimus and performed assays for growth, cell proliferation, and cell death. Immunohistochemistry as well as in vivo and in vitro metabolomics studies were also performed.

RESULTS

Carboplatin synergized with everolimus in all of our 4 pLGG cell lines (combination index <1 at Fa 0.5). Combination therapy was superior at inhibiting tumor growth in vivo. Combination treatment increased levels of apoptosis as well as gamma-H2AX phosphorylation compared with either agent alone. Everolimus treatment suppressed the conversion of glutamine and glutamate into glutathione both in vitro and in vivo. Exogenous glutathione reversed the effects of carboplatin and everolimus.

CONCLUSIONS

The combination of carboplatin and everolimus was effective at inducing cell death and slowing tumor growth in pLGG models. Everolimus decreased the amount of available glutathione inside the cell, preventing the detoxification of carboplatin and inducing increased DNA damage and apoptosis.

High LC3/Beclin Expression Correlates with Poor Survival in Glioma: a Definitive Role for Autophagy as Evidenced by In Vitro Autophagic Flux

Recent studies suggest the role of autophagy, an evolutionarily conserved catabolic process, in determining the response of gliomas to treatment either positively or negatively. The study attempts to characterize autophagy in low and high-grade glioma by investigating the autophagic flux and clinical significance of autophagy proteins (LC3 and beclin 1) in a group of glioma patients. We evaluated the expression of autophagic markers in resected specimens of low-grade glioma (LGG) and high-grade glioma (HGG) tissues, by immunohistochemistry and Western blotting. Our results show that expression of autophagy proteins were more prominent in HGG than in LGG. Increased level of autophagic proteins in HGG can be due to an increased rate of autophagy or can be because of blockage in the final degradation step of autophagy (defective autophagy). To distinguish these possibilities, the autophagic flux assay which helps to determine the rate of degradation/synthesis of autophagic proteins (LC3-II and p62) over a period of time by blocking the final degradation step of autophagy using bafilomycin A1 was used . The assessment of autophagic flux in ex vivo culture of primary glioma cells revealed for the first time increased turnover of autophagy in high grade compared to low grade-glioma. Though autophagic markers were reduced in LGG, functionally autophagy was non defective in both grades of glioma. We then investigated whether autophagy in gliomas is regulated by nutrient sensing pathways including mTOR and promote cell survival by providing an alternate energy source in response to metabolic stress. The results depicted that the role of autophagy during stress varies with tissue and has a negative correlation with mTOR substrate phosphorylation. We also evaluated the expression of LC3 and beclin 1 with progression free survival (PFS) using Kaplan-Meier survival analysis and have found that patients with low LC3/beclin 1 expression had better PFS than those with high expression of LC3/beclin 1 in their tumors. Together, we provide evidence that autophagy is non-defective in glioma and also show that high LC3/beclin 1 expression correlates with poor PFS in both LGG and HGG.

mTOR activation is increased in pilocytic astrocytomas from older adults compared with children

BACKGROUND

Recent studies suggest that the behavior and biology of WHO grade I pilocytic astrocytomas (PAs) in adults is different than that associated with grade I PAs in children.

METHODS

We evaluated Ki-67 labeling, BRAF abnormalities, isocitrate dehydrogenase R132 immunoreactivity phosphorylation (activation) of p44/42 mitogen activated protein kinase (MAPK), and mammalian target of rapamycin (mTOR) in formalin-fixed tissue from 21 adult (18 years or older, mean age 37 years) and 10 children (mean age 9.4 years) WHO grade I PAs.

RESULTS

The mean Ki-67 labeling was 4.8% in adults and 3.8% in children. There was no significant difference between Ki-67 labeling in children and adults or either subgroups of adults. No differences were found in phospho p44/42MAPK in adult subgroups (18-33 years and 34 and older) compared to children. Activation/phosphorylation of mTOR was biphasic in adults being significantly lower than children in young adults but significantly higher than children in older adults (age 34 and older).

CONCLUSIONS

Identifying mTOR phosphorylation/activation may represent a difference in biology and a new marker to guide chemotherapy with recently approved mTOR inhibitors.