Drug delivery across the blood-brain barrier for the treatment of pediatric brain tumors - An update

Current status and advances to improving drug delivery in diffuse intrinsic pontine glioma

Diffuse midline glioma (DMG), including tumors diagnosed in the brainstem (diffuse intrinsic pontine glioma - DIPG), is the primary cause of brain tumor-related death in pediatric patients. DIPG is characterized by a median survival of <12 months from diagnosis, harboring the worst 5-year survival rate of any cancer. Corticosteroids and radiation are the mainstay of therapy; however, they only provide transient relief from the devastating neurological symptoms. Numerous therapies have been investigated for DIPG, but the majority have been unsuccessful in demonstrating a survival benefit beyond radiation alone. Although many barriers hinder brain drug delivery in DIPG, one of the most significant challenges is the blood-brain barrier (BBB). Therapeutic compounds must possess specific properties to enable efficient passage across the BBB. In brain cancer, the BBB is referred to as the blood-brain tumor barrier (BBTB), where tumors disrupt the structure and function of the BBB, which may provide opportunities for drug delivery. However, the biological characteristics of the brainstem's BBB/BBTB, both under normal physiological conditions and in response to DIPG, are poorly understood, which further complicates treatment. Better characterization of the changes that occur in the BBB/BBTB of DIPG patients is essential, as this informs future treatment strategies. Many novel drug delivery technologies have been investigated to bypass or disrupt the BBB/BBTB, including convection enhanced delivery, focused ultrasound, nanoparticle-mediated delivery, and intranasal delivery, all of which are yet to be clinically established for the treatment of DIPG. Herein, we review what is known about the BBB/BBTB and discuss the current status, limitations, and advances of conventional and novel treatments to improving brain drug delivery in DIPG.

OV Modulators of the Paediatric Brain TIME: Current Status, Combination Strategies, Limitations and Future Directions

Oncolytic viruses (OVs) are characterised by their preference for infecting and replicating in tumour cells either naturally or after genetic modification, resulting in oncolysis. Furthermore, OVs can elicit both local and systemic anticancer immune responses while specifically infecting and lysing tumour cells. These characteristics render them a promising therapeutic approach for paediatric brain tumours (PBTs). PBTs are frequently marked by a cold tumour immune microenvironment (TIME), which suppresses immunotherapies. Recent preclinical and clinical studies have demonstrated the capability of OVs to induce a proinflammatory immune response, thereby modifying the TIME. In-depth insights into the effect of OVs on different cell types in the TIME may therefore provide a compelling basis for using OVs in combination with other immunotherapy modalities. However, certain limitations persist in our understanding of oncolytic viruses' ability to regulate the TIME to enhance anti-tumour activity. These limitations primarily stem from the translational limitations of model systems, the difficulties associated with tracking reliable markers of efficacy throughout the course of treatment and the role of pre-existing viral immunity. In this review, we describe the different alterations observed in the TIME in PBTs due to OV treatment, combination therapies of OVs with different immunotherapies and the hurdles limiting the development of effective OV therapies while suggesting future directions based on existing evidence.

State of the art in pediatric nanomedicines

In recent years, the continuous development of innovative nanopharmaceuticals is expanding their biomedical and clinical applications. Nanomedicines are being revolutionized to circumvent the limitations of unbound therapeutic agents as well as overcome barriers posed by biological interfaces at the cellular, organ, system, and microenvironment levels. In many ways, the use of nanoconfigured delivery systems has eased challenges associated with patient differences, and in our opinion, this forms the foundation for their potential usefulness in developing innovative medicines and diagnostics for special patient populations. Here, we present a comprehensive review of nanomedicines specifically designed and evaluated for disease management in the pediatric population. Typically, the pediatric population has distinguishing needs relative to those of adults majorly because of their constantly growing bodies and age-related physiological changes, which often need specialized drug formulation interventions to provide desirable therapeutic effects and outcomes. Besides, child-centric drug carriers have unique delivery routes, dosing flexibility, organoleptic properties (e.g., taste, flavor), and caregiver requirements that are often not met by traditional formulations and can impact adherence to therapy. Engineering pediatric medicines as nanoconfigured structures can potentially resolve these limitations stemming from traditional drug carriers because of their unique capabilities. Consequently, researchers from different specialties relentlessly and creatively investigate the usefulness of nanomedicines for pediatric disease management as extensively captured in this compilation. Some examples of nanomedicines covered include nanoparticles, liposomes, and nanomicelles for cancer; solid lipid and lipid-based nanostructured carriers for hypertension; self-nanoemulsifying lipid-based systems and niosomes for infections; and nanocapsules for asthma pharmacotherapy.

Meta-analysis of the impact of laser interstitial hyperthermia on wound healing complications in brain tumors

High-grade gliomas (HGGs) may be amenable to the neurosurgical technique known as laser interstitial thermal therapy (LITT), which delivers thermal energy to interstitial brain injuries and wounds with pinpoint accuracy. The purpose of this extensive meta-analysis was to evaluate the effects of LITT on wound complications among patients who have brain tumours. Diverse conclusions emerge from a systematic review of pertinent studies, necessitating a comprehensive examination. The meta-analysis, performed utilizing the meta library provided by the R package meta, reveals an initial significant overall effect (RR: -2.1262, 95% CI [-2.7466, -1.5059], p < 0.0001) accompanied by considerable heterogeneity among studies (I2  = 61.13%). Following analyses that specifically examined the incidence of wounds, a complex correlation was found (RR: 0.0471, 95% CI [0.0264, 0.0842], p < 0.0001), indicating that LITT has a discernible but insignificant effect on the occurrence of wounds. Although the meta-analysis emphasizes a notable decrease in wound complications subsequent to LITT treatment, additional research is warranted due to constraints in standardized reporting, data accessibility, and small sample sizes. The results of this study underscore the need for exhaustive protocols to analyse wound complications in patients with brain tumours undergoing LITT.

Intrathecal or intraventricular antimicrobial therapy for post-neurosurgical Gram-negative bacillary meningitis or ventriculitis: a systematic review and meta-analysis

PURPOSE

Extensively-drug-resistant Gram-negative bacteria (XDR GNB)-related post-neurosurgical infection is closely related to mortality, which represents a major challenge for neurosurgeons. There is an urgent need to review and evaluate methods to reduce mortality.

METHODS

Both international and Chinese databases were searched independently from their inception to 15 June 2023. A meta-analysis was conducted using RevMan 5.4 to compare the efficacy and safety of intravenous (IV) treatment in combination with intrathecal or intraventricular (ITH/IVT) treatment with IV treatment alone for post-neurosurgical meningitis or ventriculitis due to GNB. Mortality, microbiological clearance and adverse events were considered as primary outcomes.

RESULTS

In total, 18 eligible studies involving 602 patients were included in the meta-analysis. The IV + ITH/IVT group was associated with significantly lower mortality (especially in the XDR GNB subgroup) and acceptable safety. In terms of microbiological clearance, a significant decrease was shown in the XDR GNB subgroup. Significant benefits were shown in laboratory parameters and clinical symptoms after patients were treated with ITH/IVT.

CONCLUSION

Additional ITH/IVT treatment may promote XDR GNB clearance and reduce mortality. In addition, ITH/IVT administration can improve clinical symptoms and cerebrospinal fluid indicators of patients with post-neurosurgical infections. Significantly, ITH/IVT treatment does not increase the incidence of adverse events at the recommended dose.

Co-biopolymer of chitosan/carboxymethyl cellulose hydrogel improved by zinc oxide and graphene quantum dots nanoparticles as pH-sensitive nanocomposite for quercetin delivery to brain cancer treatment

Brain cancer is the major reason of cancer-relevant deaths every year, as it is the most challenging cancer to treat and drug delivery. Quercetin (QUR), as a flavonoid substance found in plants and fruits, has good anticancer and medicinal effects on brain tumors, but its low stability and bioavailability as well as the blood-brain barrier (BBB), prevent it from reaching brain tumors. This research has introduced a nanocomposite made of biocompatible polymers, chitosan, and carboxymethyl cellulose. This co- biopolymer's mechanical and chemical properties and drug-loading capacity have been improved by adding zinc oxide nanoparticles (ZnO NPs). In addition, graphene quantum dots (GQDs) were used to improve the chemical properties as well as the ability to penetrate the BBB. The CS/CMC/GQDs/ZnO@QUR nanocomposites have nanoneedle structures with an average size of 219.38 ± 5.21 nm and a zeta potential of -53 mV. The morphology, chemical bonds, and crystallinity of the nanocomposite were examined by FE-SEM, FTIR, and XRD analyses, respectively. By examining the release of QUR, it became apparent that the half-drug release takes about 72 h, which has a much more controlled release than other QUR carriers. Further, the MTT test on U-87 MG and L929 cell lines suggested that this nanocomposite has good anticancer properties and low cytotoxicity compared to the free QUR.

Current non-invasive strategies for brain drug delivery: overcoming blood-brain barrier transport

BACKGROUND

The blood-brain barrier (BBB) is a complex and dynamic structure that serves as a gatekeeper, restricting the migrations of most compounds and molecules from blood into the central nervous system (CNS). The BBB plays a crucial role in maintaining CNS physiological function and brain homeostasis. It can protect the CNS from the entrance of toxic and infectious agents, however, it also restricts the drug permeation into brain to play a therapeutic role. The BBB has been the biggest limiting hurdle to medications entering the brain excluding from the brain about 100% of large-molecule and more than 98% of all small-molecule neurotherapeutics. As a result, it is of inability for drug molecule to reach requisite concentrations within the brain.

OBJECTIVE

With the aim of enhancing drug permeability and efficacy, a variety of strategies have been developed: invasive approaches, such as intraarterial delivery, intrathecal delivery, or administrating directly the drug intraventricularly and intracerebrally; non-invasive approaches that take advantage of innate BBB functions, using prodrugs, focused ultrasound, intranasal administration or nanotechnology.

CONCLUSIONS

Here we mainly review recent developments and challenges related to non-invasive BBB-crossing techniques, whose benefits include higher efficacy, easier application, less treatment burden, better patient acceptability, and adherence. Additionally, we also analyze the potential of non-invasive methods in the treatment of CNS disorders and render them as a most suitable platform for the management of neurological diseases.

Chemotherapy in pediatric brain tumor and the challenge of the blood-brain barrier

BACKGROUND

Pediatric brain tumors (PBT) stand as the leading cause of cancer-related deaths in children. Chemoradiation protocols have improved survival rates, even for non-resectable tumors. Nonetheless, radiation therapy carries the risk of numerous adverse effects that can have long-lasting, detrimental effects on the quality of life for survivors. The pursuit of chemotherapeutics that could obviate the need for radiotherapy remains ongoing. Several anti-tumor agents, including sunitinib, valproic acid, carboplatin, and panobinostat, have shown effectiveness in various malignancies but have not proven effective in treating PBT. The presence of the blood-brain barrier (BBB) plays a pivotal role in maintaining suboptimal concentrations of anti-cancer drugs in the central nervous system (CNS). Ongoing research aims to modulate the integrity of the BBB to attain clinically effective drug concentrations in the CNS. However, current findings on the interaction of exogenous chemical agents with the BBB remain limited and do not provide a comprehensive explanation for the ineffectiveness of established anti-cancer drugs in PBT.

METHODS

We conducted our search for chemotherapeutic agents associated with the blood-brain barrier (BBB) using the following keywords: Chemotherapy in Cancer, Chemotherapy in Brain Cancer, Chemotherapy in PBT, BBB Inhibition of Drugs into CNS, Suboptimal Concentration of CNS Drugs, PBT Drugs and BBB, and Potential PBT Drugs. We reviewed each relevant article before compiling the information in our manuscript. For the generation of figures, we utilized BioRender software.

FOCUS

We focused our article search on chemical agents for PBT and subsequently investigated the role of the BBB in this context. Our search criteria included clinical trials, both randomized and non-randomized studies, preclinical research, review articles, and research papers.

FINDING

Our research suggests that, despite the availability of potent chemotherapeutic agents for several types of cancer, the effectiveness of these chemical agents in treating PBT has not been comprehensively explored. Additionally, there is a scarcity of studies examining the role of the BBB in the suboptimal outcomes of PBT treatment, despite the effectiveness of these drugs for other types of tumors.

The Drosophila blood-brain barrier regulates sleep via Moody G protein-coupled receptor signaling

Sleep is vital for most animals, yet its mechanism and function remain unclear. We found that permeability of the BBB (blood-brain barrier)-the organ required for the maintenance of homeostatic levels of nutrients, ions, and other molecules in the brain-is modulated by sleep deprivation (SD) and can cell-autonomously effect sleep changes. We observed increased BBB permeability in known sleep mutants as well as in acutely sleep-deprived animals. In addition to molecular tracers, SD-induced BBB changes also increased the penetration of drugs used in the treatment of brain pathologies. After chronic/genetic or acute SD, rebound sleep or administration of the sleeping aid gaboxadol normalized BBB permeability, showing that SD effects on the BBB are reversible. Along with BBB permeability, RNA levels of the BBB master regulator moody are modulated by sleep. Conversely, altering BBB permeability alone through glia-specific modulation of moody, gαo, loco, lachesin, or neuroglian-each a well-studied regulator of BBB function-was sufficient to induce robust sleep phenotypes. These studies demonstrate a tight link between BBB permeability and sleep and indicate a unique role for the BBB in the regulation of sleep.

Bench-to-bedside investigations of H3 K27-altered diffuse midline glioma: drug targets and potential pharmacotherapies

INTRODUCTION

H3 K27-altered diffuse midline glioma (DMG) is the most common malignant brainstem tumor in the pediatric population. Despite enormous preclinical and clinical efforts, the prognosis remains dismal, with fewer than 10% of patients surviving for two years after diagnosis. Fractionated radiation remains the only standard treatment options for DMG. Developing novel treatments and therapeutic delivery methods is critical to improving outcomes in this devastating disease.

AREAS COVERED

This review addresses recent advances in molecularly targeted pharmacotherapy and immunotherapy in DMG. The clinical presentation, diagnostic workup, unique pathological challenges, and current clinical trials are highlighted throughout.

EXPERT OPINION

Promising pharmacotherapies targeting various components of DMG pathology and the application of immunotherapies have the potential to improve patient outcomes. However, novel approaches are needed to truly revolutionize treatment for this tumor. First, combinational therapy should be employed, as DMG can develop resistance to single-agent approaches and many therapies are susceptible to rapid clearance from the brain. Second, drug-tumor residence time, i.e. the time for which a therapeutic is present at efficacious concentrations within the tumor, must be maximized to facilitate a durable treatment response. Engineering extended drug delivery methods with minimal off-tumor toxicity should be a focus of future studies.

Oncolytic Virus-Driven Biotherapies from Bench to Bedside

With advances in cancer biology and an ever-deepening understanding of molecular virology, oncolytic virus (OV)-driven therapies have developed rapidly and become a promising alternative to traditional cancer therapies. In recent years, satisfactory results for oncolytic virus therapy (OVT) are achieved at both the cellular and organismal levels, and efforts are being increasingly directed toward clinical trials. Unfortunately, OVT remains ineffective in these trials, especially when performed using only a single OV reagent. In contrast, integrated approaches, such as using immunotherapy, chemotherapy, or radiotherapy, alongside OVT have demonstrated considerable efficacy. The challenges of OVT in clinical efficacy include the restricted scope of intratumoral injections and poor targeting of intravenous administration. Further optimization of OVT delivery is needed before OVs become a viable therapy for tumor treatment. In this review, the development process and antitumor mechanisms of OVs are introduced. The advances in OVT delivery routes to provide perspectives and directions for the improvement of OVT delivery are highlighted. This review also discusses the advantages and limitations of OVT monotherapy and combination therapy through the lens of recent clinical trials and aims to chart a course toward safer and more effective OVT strategies.

Exploring the Potential of Nanotechnology in Pediatric Healthcare: Advances, Challenges, and Future Directions

The utilization of nanotechnology has brought about notable advancements in the field of pediatric medicine, providing novel approaches for drug delivery, disease diagnosis, and tissue engineering. Nanotechnology involves the manipulation of materials at the nanoscale, resulting in improved drug effectiveness and decreased toxicity. Numerous nanosystems, including nanoparticles, nanocapsules, and nanotubes, have been explored for their therapeutic potential in addressing pediatric diseases such as HIV, leukemia, and neuroblastoma. Nanotechnology has also shown promise in enhancing disease diagnosis accuracy, drug availability, and overcoming the blood-brain barrier obstacle in treating medulloblastoma. It is important to acknowledge that while nanotechnology offers significant opportunities, there are inherent risks and limitations associated with the use of nanoparticles. This review provides a comprehensive summary of the existing literature on nanotechnology in pediatric medicine, highlighting its potential to revolutionize pediatric healthcare while also recognizing the challenges and limitations that need to be addressed.

Evolving Diagnostic and Treatment Strategies for Pediatric CNS Tumors: The Impact of Lipid Metabolism

Pediatric neurological tumors are a heterogeneous group of cancers, many of which carry a poor prognosis and lack a "standard of care" therapy. While they have similar anatomic locations, pediatric neurological tumors harbor specific molecular signatures that distinguish them from adult brain and other neurological cancers. Recent advances through the application of genetics and imaging tools have reshaped the molecular classification and treatment of pediatric neurological tumors, specifically considering the molecular alterations involved. A multidisciplinary effort is ongoing to develop new therapeutic strategies for these tumors, employing innovative and established approaches. Strikingly, there is increasing evidence that lipid metabolism is altered during the development of these types of tumors. Thus, in addition to targeted therapies focusing on classical oncogenes, new treatments are being developed based on a broad spectrum of strategies, ranging from vaccines to viral vectors, and melitherapy. This work reviews the current therapeutic landscape for pediatric brain tumors, considering new emerging treatments and ongoing clinical trials. In addition, the role of lipid metabolism in these neoplasms and its relevance for the development of novel therapies are discussed.

H3K27-altered diffuse midline glioma: a paradigm shifting opportunity in direct delivery of targeted therapeutics

INTRODUCTION

Despite much progress, the prognosis for H3K27-altered diffuse midline glioma (DMG), previously known as diffuse intrinsic pontine glioma when located in the brainstem, remains dark and dismal.

AREAS COVERED

A wealth of research over the past decade has revolutionized our understanding of the molecular basis of DMG, revealing potential targetable vulnerabilities for treatment of this lethal childhood cancer. However, obstacles to successful clinical implementation of novel therapies remain, including effective delivery across the blood-brain barrier (BBB) to the tumor site. Here, we review relevant literature and clinical trials and discuss direct drug delivery via convection-enhanced delivery (CED) as a promising treatment modality for DMG. We outline a comprehensive molecular, pharmacological, and procedural approach that may offer hope for afflicted patients and their families.

EXPERT OPINION

Challenges remain in successful drug delivery to DMG. While CED and other techniques offer a chance to bypass the BBB, the variables influencing successful intratumoral targeting are numerous and complex. We discuss these variables and potential solutions that could lead to the successful clinical implementation of preclinically promising therapeutic agents.

Tyrosine Kinase Inhibitors for Glioblastoma Multiforme: Challenges and Opportunities for Drug Delivery

Glioblastoma multiforme (GBM) is an aggressive brain tumor with high mortality rates. Due to its invasiveness, heterogeneity, and incomplete resection, the treatment is very challenging. Targeted therapies such as tyrosine kinase inhibitors (TKIs) have great potential for GBM treatment, however, their efficacy is primarily limited by poor brain distribution due to the presence of the blood-brain barrier (BBB). This review focuses on the potential of TKIs in GBM therapy and provides an insight into the reasons behind unsuccessful clinical trials of TKIs in GBM despite the success in treating other cancer types. The main section is dedicated to the use of promising drug delivery strategies for targeted delivery to brain tumors. Use of brain targeted delivery strategies can help enhance the efficacy of TKIs in GBM. Among various drug delivery approaches used to bypass or cross BBB, utilizing nanocarriers is a promising strategy to augment the pharmacokinetic properties of TKIs and overcome their limitations. This is because of their advantages such as the ability to cross BBB, chemical stabilization of drug in circulation, passive or active targeting of tumor, modulation of drug release from the carrier, and the possibility to be delivered via non-invasive intranasal route.

Controllable blood–brain barrier (BBB) regulation based on gigahertz acoustic streaming

The blood–brain barrier (BBB) is a structural and functional barrier necessary for brain homeostasis, and it plays an important role in the realization of neural function and in protecting the brain from damage by circulating toxins and pathogens. However, the extremely dense BBB also severely limits the transport of molecules across it, which is a great hindrance to the diagnosis and treatment of central nervous system (CNS) diseases. This paper reports a new method for controllable opening of the BBB, based on the gigahertz acoustic streaming (AS) generated by a bulk acoustic wave resonant device. By adjusting the input power and working distance of the device, AS with tunable flow rate can be generated to disrupt tight junction proteins (TJs) between endothelial cells. The results obtained with this method show that the gigahertz AS promotes the penetration of dextran molecules with different molecular weights across the BBB. This work provides a new platform for studying the mechanical regulation of BBB by fluid shear forces and a new method for improving the efficiency of drug delivery.

A Nanomedicine Structure-Activity Framework for Research, Development, and Regulation of Future Cancer Therapies

Despite their clinical success in drug delivery applications, the potential of theranostic nanomedicines is hampered by mechanistic uncertainty and a lack of science-informed regulatory guidance. Both the therapeutic efficacy and the toxicity of nanoformulations are tightly controlled by the complex interplay of the nanoparticle's physicochemical properties and the individual patient/tumor biology; however, it can be difficult to correlate such information with observed outcomes. Additionally, as nanomedicine research attempts to gradually move away from large-scale animal testing, the need for computer-assisted solutions for evaluation will increase. Such models will depend on a clear understanding of structure-activity relationships. This review provides a comprehensive overview of the field of cancer nanomedicine and provides a knowledge framework and foundational interaction maps that can facilitate future research, assessments, and regulation. By forming three complementary maps profiling nanobio interactions and pathways at different levels of biological complexity, a clear picture of a nanoparticle's journey through the body and the therapeutic and adverse consequences of each potential interaction are presented.

Recent advances in drug delivery and targeting to the brain

Our body keeps separating the toxic chemicals in the blood from the brain. A significant number of drugs do not enter the central nervous system (CNS) due to the blood-brain barrier (BBB). Certain diseases, such as tumor growth and stroke, are known to increase the permeability of the BBB. However, the heterogeneity of this permeation makes it difficult and unpredictable to transport drugs to the brain. In recent years, research has been directed toward increasing drug penetration inside the brain, and nanomedicine has emerged as a promising approach. Active targeting requires one or more specific ligands on the surface of nanoparticles (NPs), which brain endothelial cells (ECs) recognize, allowing controlled drug delivery compared to conventional targeting strategies. This review highlights the mechanistic insights about different cell types contributing to the development and maintenance of the BBB and summarizes the recent advancement in brain-specific NPs for different pathological conditions. Furthermore, fundamental properties of brain-targeted NPs will be discussed, and the standard lesion features classified by neurological pathology are summarized.

Therapeutic exercise interventions in pediatric survivors of brain cancer and other solid tumors: A scoping review

Background

Improved survival rates for children with solid tumors presents an ongoing challenge of how to maximize quality of survivorship and effectively manage the short- and long-term complications of disease and treatment. To gain an understanding of the extent and nature of research pertaining to therapeutic exercise interventions and identify knowledge gaps, we conducted a scoping review of exercise training studies conducted in pediatric survivors of brain cancer and other solid tumors.

Method

A systematic literature search was performed across four electronic databases. Papers were selected for full-text review if they included participants treated for brain cancer or other solid tumors, with at least 50% of participants aged ≤ 21 years, evaluated an exercise intervention ≥2-weeks in duration, and were published in an English, peer-reviewed journal. We included the following quantitative study designs; randomized controlled trials, non-randomized trials, and single-arm pre-test-post-test.

Results

Of the 7,482 citations identified, 17 papers met the inclusion criteria (presenting findings from eleven studies). Two studies were randomized controlled trials, five studies were non-randomized controlled trials, and four studies were a single-arm pre-test post-test design. Average age of participants ranged from 7.3-15.5 years, and time since diagnosis ranged from 3 to 70 months. Five studies included participants with brain tumors exclusively, three studies included other solid tumors, and three studies included a mixed sample (brain and other solid tumors). A wide range of exercise modalities were employed, including cycle ergometry, resistance training, sport, yoga, and active gaming. The length of the exercise program ranged from 3-40 weeks and frequency from 3-11 sessions per week. Exercise session duration ranged from 15-180 min, with most studies reporting 30-90-min sessions. Adherence ranged from 77 to 100%, with none of the studies reporting adverse events. Studies reported improvements in cardiorespiratory fitness, functional strength, physical activity, and quality of life.

Conclusions

A small number of mostly low methodological quality studies have examined the effects of therapeutic exercise in pediatric survivors of solid tumors. Although limited, the extant literature supports the feasibility and safety of therapeutic exercise interventions for pediatric survivors of brain cancer and other solid tumors.