Pediatric clinical trials for pilocytic astrocytomas using targeted therapies
The outstanding progress in advanced molecular technologies has provided a tremendous amount of data that has altered the way in which we classify and categorize pediatric brain tumors. From the initial identification of chromosomal aberrations by karyotyping and comparative genomic hybridization, we have rapidly moved to expression array studies and to integrative genomic approaches which allowed the stratification of several pediatric brain tumors into molecular subgroups. These data have not only increased our understanding of the molecular pathogenesis of pediatric brain tumors, but have also identified prognostic markers and opened new avenues for targeted therapies.
One of the most important discoveries in pediatric astrocytomas was the duplication and the mutation of the v-raf murine sarcoma viral oncogene homolog B1
Until recently, the biology of diffuse intrinsic pontine glioma (DIPG) was poorly understood. Overexpression of epidermal growth factor receptor (
Integrated genomics has shown that medulloblastoma, the most common malignant pediatric brain tumor, comprises four distinct molecular and clinical variants. The stratification of patients into the Wnt subgroup, the sonic hedgehog (SHH) subgroup, Group 3 and Group 4 may lead to the identification of those patients that will most likely benefit from targeted therapies, like SMO inhibitors. Moreover, this molecular classification may help identify patients predicted to have a poor prognosis, who may benefit from intensified therapies, and patients with a favorable prognosis that potentially benefit from reduced radiation and chemotherapy regimens. It is already known from four independent studies that the Wnt subgroup patients have a very good prognosis while patients in Group 3 have a high incidence of metastasis and a dismal prognosis.
The transcriptional profiling of two large independent cohorts of posterior fossa ependymomas also identified two groups with distinct molecular and clinical features. Group A ependymomas have a balanced genome, occur in younger patients and tumors are located laterally. These patients have a worse clinical outcome and higher incidence of metastasis and recurrence. In contrast, Group B ependymomas occur in the midline and have a better prognosis. Since ependymomas are usually refractory to current chemotherapeutic agents, these discoveries may shed some light into the pathways involved in tumor initiation and progression and also identify new targets for therapy.
We are now facing the next-generation (Next-Gen) sequencing era which will provide further insights into the dysregulated signaling pathways in each tumor type. The identification of driver mutations will allow a better understanding of tumorigenesis and lead to the development of more accurate preclinical models. Moreover, the profiling of transcriptomes, genetic and epigenetic events of large cohorts of tumors will eventually shed some light into the cells of origin of specific subgroups and also the important driver events for tumor initiation, maintenance and progression. On the clinical side, this knowledge will allow the stratification of patients into appropriate risk groups and the tailoring of treatments according to each tumor’s genomic landscape. In this chapter we describe the latest advances in molecular genomics of the most common pediatric brain tumors as well as its prognostic significance and relevance to the clinic. We also highlight the most recent clinical trials using molecular targeted therapies and discuss further possible avenues in the treatment of pediatric brain cancer.
Astrocytomas are a common brain tumor in children. According to the 2007 classification of the World Health Organization (WHO) they can be classified into four grades (WHO grade I-IV), which reflect their biological and clinical behavior. Pilocytic astrocytomas (WHO grade I) represent the most frequent brain tumor in the pediatric population and have excellent survival rates over 95% . On the other hand, glioblastomas (WHO grade IV) are aggressive tumors, often non-responsive to treatments and with survival rates ranging from 10% to 30% . We will focus on the molecular biology of these two types of pediatric astrocytomas.
2.1. Pilocytic astrocytoma
The first-line treatment for pilocytic astrocytoma (PA) is surgical resection. Although most children with these tumors survive for a long time, some will experience tumor recurrence, especially if the tumor resection is incomplete. Recurrent and progressive tumors are treated with radiation and/or chemotherapy but, until recently, the mechanisms of recurrence and malignant transformation were poorly understood.
Early studies on chromosomal aberrations in PAs showed normal karyotypes in most cases. Trisomy of chromosomes 5 and 7 and 7q gains were some of the few cytogenetic abnormalities found . Using an array-based comparative genomic hybridization (array-CGH), Pfister
With the discovery of constitutive activation of MAPK pathway and BRAF alterations in most PAs, there has been an increasing interest in using these targets for novel therapeutic approaches. A number of phase I and phase II clinical trials are ongoing to test small molecule inhibitors of MAPK and related pathways (Table 1).
|MEK inhibitor||AZD6244||Phase I||NCT01386450, NCT01089101|
|Sorafenib||Phase II||NCT01338857 (suspended)|
|mTOR inhibitor||Everolimus||Phase II||NCT01158651, NCT00782626|
2.2. Glioblastoma multiforme
Glioblastoma multiforme (GBM) is the most common brain tumor in adults and is less frequent in the pediatric population. Despite improvements in neurosurgery and neuro-oncology, children diagnosed with GBM still have a dismal outcome with low survival rates even with aggressive therapeutic regimens. Although the histology of pediatric and adult GBMs appears identical, the molecular biology of these tumors is different. Therefore, the development of effective therapies for GBM in children should probably not rely on advances made in adult tumors.
Mutations in phosphatase and tensin homolog (
It has been shown that overexpression of O6-methylguanine-DNA methyltransferase (MGMT) is rare in HGGs but it has also been reported that it is associated with a poor overall survival in children. The
Finaly, a very recent study uncovered an interesting interplay between genetic and epigenetic events in pediatric GBM. The authors performed whole-exome sequencing in 48 pediatric GBMs and identified 44% of tumors harbouring somatic mutations in genes involved in the chromatin remodeling pathway, including the histone H3 gene (
The novel targeted therapies that are currently under clinical investigation for high-grade gliomas, including GBM, are reviewed in reference .
3. Diffuse intrinsic pontine glioma
Despite decades of clinical research, the prognosis of diffuse intrinsic pontine glioma (DIPG) remains dismal with more than 90% of affected children dying within two years of diagnosis . The only known effective treatment is radiation therapy although in most patients it has a transient effect. Different combinations of chemotherapy, radiation and radiosensitizers were attempted but failed to improve long-term survival . The indication for biopsy in DIPG has been reserved for atypical tumors (prominent enhancement in magnetic ressonance imaging (MRI), low T2/FLAIR signal and/or high signal on diffuse imaging). However, the recognition that it is crucial to understand the biology of DIPG in order to design more rational drug treatments, led to a plea for routine biopsies in these patients. Some groups are already performing regular image-guided stereotactic biopsies in patients with DIPGs [18,19] and, furthermore, biopsies have been included in a clinical trial to decide which patients with DIPG would benefit from targeted therapy with erlotinib, an
The paradigm shift to obtain tissue from patients with DIPG not only increased our knowledge in the molecular biology of the disease but also raised important clinical considerations. Paugh
Another level of gene expression regulation in DIPG occurs through epigenetic mechanisms, including histone modifications. Very recent sudies used whole-genome sequencing in large cohorts of DIPGs to identify somatic mutations in the
The discoveries from the molecular biology of DIPGs identified several drugable targets allowing the development of molecular target-based trials that are summarized in Table 2. Some of these trials have shown a subset of patients with survival longer than expected and, therefore, several other trials are ongoing, some of them including combined therapies [17,31].
Finally, DIPG treatment involves another challenge which is drug distribution. Probably due to an intact blood-brain barrier, penetration of drugs in the pons seems to be poor, a fact demonstrated by the lack of gadolinium enhancement in the MRI in most DIPGs. Therefore, it is crucial to improve drug delivery either by disrupting the blood-brain barrier (e.g. focused ultrasound or drugs that increase permeability such as manitol), by local delivery (tumor injection or convection enhanced delivery) or even nanoparticles . These techniques may promote high drug concentrations in the pons, including agents that normally don’t cross the blood-brain barrier.
|Imatinib||Phase I||45.5%||2007 |
|Tipifarnib||Phase I||36.4%||2008 |
|Gefitinib||Phase I||48%||2010 |
|Vandetanib||Phase I||37%||2010 |
|Erlotinib||Phase I||50%||2011 |
|Gefitinib||Phase II||56%||2011 |
|Nimotuzumab||Phase II||Median OS 9.6 months||2011 |
Medulloblastoma, the commonest malignant brain tumor in the pediatric population, is no longer considered a single disease. Recent efforts of multiple independent groups have reached to a consensus that medulloblastoma comprises four distinct molecular variants named WNT, SHH, Group 3 and Group 4 . The subgroups have different demographics, genetic profiles and prognosis . This may explain why patients with the same histological disease have different clinical outcomes and a variable response to current treatments including surgery, whole-brain radiation and intensive chemotherapy. Figure 1 summarizes the main features of the four medulloblastoma subgroups.
4.1. WNT medulloblastomas
WNT medulloblastomas are frequent in older children and teenagers and are rarely seen in infants. Patients within this subgroup have usually an excellent outcome with survival rates over 90%. However, Remke
This subgroup is enriched in genes of the WNT pathway. Although few gains and losses were reported in the WNT genome, mutations in
4.2. SHH medulloblastomas
SHH medulloblastomas are frequently found in infants and adults (approximately 60% of cases in each age group) but are rare in childhood. In the SHH subgroup, prognostic factors such as M-stage and desmoplasia are age-dependent. Metastasis at presentation represents a negative prognostic factor only in adults while desmoplasia is associated with worse outcome only in pediatric cases . From a histological point of view this subgroup is unique since it includes tumors of the four main variants (classic, nodular desmoplastic, large-cell anaplastic and medulloblastoma with extensive nodularity).
SHH medulloblastomas are characterized by aberrant expression of SHH pathway genes including
The clinical and molecular distinction of infant and adult SHH medulloblastomas suggests a disparate underlying biology and raises the question of possible different responses to current targeted therapies.
4.3. Group 3 medulloblastomas
Group 3 medulloblastomas are restricted to pediatric patients. Indeed, two recent studies concluded that Group 3 tumors are extremely rare in adults [40,48]. Another feature of this subgroup is its aggressive behavior with high incidence of metastasis, frequent large-cell anaplastic histology and an invariable dismal prognosis (approximately 20 to 30% overall survival). It has been shown that Group 3 medulloblastomas consist of two distinct subtypes one of which harbors frequent amplifications of the
Until recently, there were no known targetable pathways in Group 3 medulloblastomas and the suggested intensification of treatment for these patients would necessarily result in increased toxicity and morbidity (see below).
4.4. Group 4 medulloblastomas
Group 4 tumors represent simultaneously the most common and the less well-understood subgroup of medulloblastomas. They are found across all age groups and have an intermediate prognosis although the adult patients show a reduced survival when compared to their pediatric counterparts. Group 4 medulloblastomas are usually of the classic variant and they rarely present with metastasis. The most frequent genetic aberration in Group 4 tumors, present in up to 80% of cases, is isochromosome 17q (i17q) although
4.5. From genomic revolution to clinical trials in medulloblastoma
Despite important advances in medulloblastoma treatment, approximately 40% of children will have recurrence and 30% will die from the disease. Moreover, the survivors are often left with significant disabilities due to cytotoxic side effects of chemotherapy and radiation to the developing central nervous system (CNS). Identifying the genetic events that drive medulloblastoma is, therefore, critical to develop more effective and less toxic therapies.
Except for SHH inhibitors that have shown some promise in SHH patients , there were no other targetable genes or pathways for WNT, Group 3 and Group 4 medulloblastomas. However, very recently published studies from four independent groups dissected the genomic landscape of medulloblastoma using large cohorts of patients and the latest high-throughput technology.
Using SNP arrays in a large cohort of over 1,000 medulloblastoma samples, Northcott
Another interesting observation, consistent with the first published study using genome sequencing in medulloblastoma , is the low number of somatic mutations found in these tumors when compared to adult solid tumors and the increased mutation frequency with age [52,55-57]. Jones
The genome sequencing of independent cohorts of medulloblastoma samples and matched blood, identified previously known mutated genes (
The ongoing genomic revolution in medulloblastoma is moving the next generation of clinical trials towards targeted treatments according to the molecular subgroup. SHH inhibitors, including GDC-0449 and NVP-LDE225, already proved its efficacy in tumor growth reduction but the reported acquired resistance suggests that a combination of targeted therapies may be a key approach to improve response . Phase II clinical trials using GDC-0449 are now recruiting pediatric and adult patients with recurrent or refractory medulloblastoma and phase I trials with NVP-LDE225 are recruiting patients with advanced solid tumors including medulloblastoma. Under debate is the de-escalation of therapy in WNT patients and the intensification of treatment and/or targeted therapy for Group 3 patients [59,60]. Finaly, the discovery of a significant number of chromatin modifier genes across medulloblastoma subgroups suggests that histone deacetylase inhibitors may constitute a good therapeutic option in the future.
Ependymoma, the third most common brain tumor in childhood, is still incurable in up to 45% of patients . The gold standard of treatment is maximal safe surgical resection followed by radiation since chemotherapy is usually ineffective. Ependymomas can arise in different regions of the CNS including the cerebral hemispheres, the posterior fossa and the spinal cord. This diversity is extended to its demographic, genetic, clinical and prognostic characteristics. Both children and adults can be affected although posterior fossa tumors are more common in children and supratentorial and spinal tumors occur more frequently in adults. The clinical behavior of ependymoma is variable with some patients experiencing a fatal clinical course while others have a long recurrence-free survival. The lack of novel targeted treatments for ependymoma can be explained by the paucity of cell lines and animal models of the disease.
The genetic heterogeneity of ependymoma has been highlighted by different studies with some cohorts of tumors showing frequent chromosomal alterations and others displaying a balanced genome. Korshunov
The interesting observation that up to 50% of pediatric ependymomas have a balanced genome  raises the possibility that epigenetic mechanisms may play a role in ependymoma pathogenesis. Most studies have focused on promoter hypermethylation of candidate genes known to be tumor suppressor genes in ependymoma or frequently methylated in other cancers.
Another less frequent epigenetic event in cancer genomes is hypomethylation. This loss of DNA methylation occurs mainly in repetitive elements (Alu repeats). In a recent study, Xie
The increased knowledge of the genetic and epigenetic events that drive ependymoma may lead to more effective targeted therapies aimed to repair molecular functions and dysregulated pathways. However, there are currently no clinical trials evaluating specific molecular therapies in ependymoma. Despite the recent achievements in ependymoma research, greater progress is needed to decifer the molecular and biological mechanisms of this disease and, ultimately, to improve patient’s clinical outcome.
Major steps have been made to a better understanding of the molecular genetics underlying the most common pediatric brain tumors. An important advance was to recognize that adult and pediatric brain tumors are distinct and, therefore, need different therapeutic approaches. This knowledge opened new avenues for targeted therapies and clinical trials based on tumor-specific molecular subgrouping are currently ongoing. When compared to standard chemotherapy and radiation, the use of biological agents has several advantages. They can target cancer cells and spare normal cells in the developing CNS of children and also be used to delay radiotherapy, which is responsible for long-term side effects of treatment. Many of the newer agents are small molecules, with low molecular weight, which facilitates blood-brain barrier penetration. However, despite the enthusiasm with the phase I and phase II clinical trials using biological agents as monotherapy, mainly for progressive and recurrent brain tumors, efficacy has not yet been proven. In the future, combination therapies will likely be needed to target multiple pathways involved in tumorigenesis and to overcome the cytostatic effect of several biological agents. As the amount of data generated by high-throughput studies increases the drugable targets for each pediatric brain tumor, the number of clinical trials will continue to expand aiming a better control of the disease with less morbidity and extended survival.
Claudia Faria was supported by a fellowship from The Hospital for Sick Children Research Training Centre and the Garron Family Cancer Centre. She is a PhD candidate from The Programme for Advanced Medical Education, supported by Fundação Gulbenkian, Fundação Champalimaud, Ministério da Saúde e Fundação para a Ciência e Tecnologia, Portugal. This work was also supported by grants from the Canadian Cancer Society Research Institute (grant no. 019073), the Wiley fund, the Laurie Berman Fund for Brain Tumour Research, Pediatric Brain Tumour Foundation of the United States, and B.r.a.i.n.child.