Is the blood-brain barrier really disrupted in all glioblastomas? A critical assessment of existing clinical data

Intranasal Perillyl Alcohol for Glioma Therapy: Molecular Mechanisms and Clinical Development

Intracranial malignancies, such as primary brain cancers and brain-localized metastases derived from peripheral cancers, are particularly difficult to treat with therapeutic agents, because the blood-brain barrier (BBB) effectively minimizes brain entry of the vast majority of agents arriving from the systemic circulation. Intranasal administration of cancer drugs has the potential to reach the brain via direct nose-to-brain transport, thereby circumventing the obstacle posed by the BBB. However, in the field of cancer therapy, there is a paucity of studies reporting positive results with this type of approach. A remarkable exception is the natural compound perillyl alcohol (POH). Its potent anticancer activity was convincingly established in preclinical studies, but it nonetheless failed in subsequent clinical trials, where it was given orally and displayed hard-to-tolerate gastrointestinal side effects. Intriguingly, when switched to intranasal delivery, POH yielded highly promising activity in recurrent glioma patients and was well tolerated. As of 2018, POH is the only intranasally delivered compound in the field of cancer therapy (outside of cancer pain) that has advanced to active clinical trials. In the following, we will introduce this compound, summarize its molecular mechanisms of action, and present the latest data on its clinical evaluation as an intranasally administered agent for glioma.

Radiation-Induced Alterations in the Recurrent Glioblastoma Microenvironment: Therapeutic Implications

Glioblastoma (GBM) is uniformly fatal with a median survival of just over 1 year, despite best available treatment including radiotherapy (RT). Impacts of prior brain RT on recurrent tumors are poorly understood, though increasing evidence suggests RT-induced changes in the brain microenvironment contribute to recurrent GBM aggressiveness. The tumor microenvironment impacts malignant cells directly and indirectly through stromal cells that support tumor growth. Changes in extracellular matrix (ECM), abnormal vasculature, hypoxia, and inflammation have been reported to promote tumor aggressiveness that could be exacerbated by prior RT. Prior radiation may have long-term impacts on microglia and brain-infiltrating monocytes, leading to lasting alterations in cytokine signaling and ECM. Tumor-promoting CNS injury responses are recapitulated in the tumor microenvironment and augmented following prior radiation, impacting cell phenotype, proliferation, and infiltration in the CNS. Since RT is vital to GBM management, but substantially alters the tumor microenvironment, we here review challenges, knowledge gaps, and therapeutic opportunities relevant to targeting pro-tumorigenic features of the GBM microenvironment. We suggest that insights from RT-induced changes in the tumor microenvironment may provide opportunities to target mechanisms, such as cellular senescence, that may promote GBM aggressiveness amplified in previously radiated microenvironment.

ABCB1 Attenuates the Brain Penetration of the PARP Inhibitor AZD2461

Poly (ADP-ribose) polymerase (PARP) inhibitors are a relatively new class of anticancer agents that have attracted attention for treatment of glioblastoma because of their ability to potentiate temozolomide chemotherapy. Previous studies have demonstrated that sufficient brain penetration is a prerequisite for efficacy of PARP inhibitors in glioma mouse models. Unfortunately, however, most of the PARP inhibitors developed to date have a limited brain penetration due to the presence of P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) at the blood-brain barrier. AZD2461 is a novel PARP inhibitor that is unaffected by P-gp mediated resistance in breast cancer models and thus appears to have promising characteristics for brain penetration. We here use a comprehensive set of in vitro and in vivo models to study the brain penetration and oral bioavailability of AZD2461. We report that AZD2461 has a good membrane permeability. However, it is a substrate of P-gp and BCRP, and P-gp in particular limits its brain penetration in vivo. We show that AZD2461 has a low oral bioavailability, although it is not affected by P-gp and BCRP. Together, these findings are not in favor of further development of AZD2461 for treatment of glioblastoma.

In vivo delineation of glioblastoma by targeting tumor-associated macrophages with near-infrared fluorescent silica coated iron oxide nanoparticles in orthotopic xenografts for surgical guidance

Glioblastoma multiforme (GBM) is the most aggressive and lethal type of human brain cancer. Surgery is a current gold standard for GBM treatment but the complete surgical resection of GBM is almost impossible due to their diffusive characteristics into surrounded normal brain tissues. There is an urgent need to develop a sensitive imaging tool for accurate delineation of GBM in the operating room to guide surgeons. Here we illustrate the feasibility of using near-infrared fluorescent silica coated iron oxide nanoparticles (NF-SIONs) with high water dispersion capacity and strong fluorescence stability for intraoperative imaging of GBM by targeting tumor-associated macrophages. Abundant macrophage infiltration is a key feature of GBM margins and it is well associated with poor prognosis. We synthesized NF-SIONs of about 37 nm to maximize endocytosis activity for macrophage uptake. The NF-SIONs selectively visualized tumor-associated macrophage populations by in vitro live-cell imaging and in vivo fluorescence imaging. In the orthotopic GBM xenograft models, the NF-SIONs could successfully penetrate blood-brain barrier and delineated tumor burden specifically. Taken together, this study showcased the potential applications in GBM treatment for improved intraoperative staging and more radical surgery as well as dual modality benefit in order to circumvent previous clinical failure.

Buparlisib is a brain penetrable pan-PI3K inhibitor

Characterization of the genomic landscapes of intracranial tumours has revealed a clear role for the PI3K-AKT-mTOR pathway in tumorigenesis and tumour maintenance of these malignancies, making phosphatidylinositol 3-kinase (PI3K) inhibition a promising therapeutic strategy for these tumours. Buparlisib is a novel pan-PI3K inhibitor that is currently in clinical development for various cancers, including primary and secondary brain tumours. Importantly however, earlier studies have revealed that sufficient brain penetration is a prerequisite for antitumor efficacy against intracranial tumours. We therefore investigated the brain penetration of buparlisib using a comprehensive set of in vitro and in vivo mouse models. We demonstrate that buparlisib has an excellent brain penetration that is unaffected by efflux transporters at the blood-brain barrier, complete oral bioavailability and efficient intracranial target inhibition at clinically achievable plasma concentrations. Together, these characteristics make buparlisib the ideal candidate for intracranially-targeted therapeutic strategies that involve PI3K inhibition.

Highlighting the need for reliable clinical trials in glioblastoma

INTRODUCTION

Several recent phase III studies have attempted to improve the dismal survival seen in glioblastoma patients, with disappointing results despite prior promising phase II data. Areas covered: A literature review of prior phase II and phase III studied in glioblastoma was performed to help identify possible areas of concern. Numerous issues in previous phase II trials for glioblastoma were found that may have contributed to these discouraging outcomes and discordant results. Expert commentary: These concerns include the improper selection of therapeutics warranting investigation in a phase III trial, suboptimal design of phase II studies (often lacking a control arm), absence of molecular data, the use of imaging criteria as a surrogate endpoint, and a lack of pharmacodynamic testing. Hopefully, by recognizing prior phase II trial limitations that contributed to failed phase III trials, we can adapt quickly to improve our ability to accurately discover survival-prolonging treatments for glioblastoma patients.

Beyond the Blood:Brain Barrier: The Importance of Central Nervous System (CNS) Pharmacokinetics for the Treatment of CNS Tumors, Including Diffuse Intrinsic Pontine Glioma

Over the past decade, we have made considerable progress in establishing diffuse intrinsic pontine glioma (DIPG) as a disease entity and developing preclinical tools to interrogate potential therapeutics. However, translation to improved clinical outcomes in children with DIPG has not yet been realized. This is in part due to difficulties encountered in delivering active drugs adequately to the tumor site. However, most preclinical evaluations gloss over the fundamental concepts of central nervous system (CNS) pharmacokinetics and requirements needed to optimize drug delivery and exposure and translate this into efficacious therapy. This article discusses not only the blood:brain barriers but additional barriers to drug delivery for CNS tumors and pharmacokinetic principles that need to be addressed and considered.

Improved Brain Penetration and Antitumor Efficacy of Temozolomide by Inhibition of ABCB1 and ABCG2

The anticancer drug temozolomide is the only drug with proven activity against high-grade gliomas and has therefore become a part of the standard treatment of these tumors. P-glycoprotein (P-gp; ABCB1) and breast cancer resistance protein (BCRP; ABCG2) are transport proteins, which are present at the blood-brain barrier and limit the brain uptake of substrate drugs. We have studied the effect of P-gp and BCRP on the pharmacokinetics and pharmacodynamics of temozolomide, making use of a comprehensive set of in vitro transport experiments and in vivo pharmacokinetic and antitumor efficacy experiments using wild-type, Abcg2^−/−, Abcb1a/b^−/−, and Abcb1a/b;Abcg2^−/− mice. We here show that the combined deletion of Abcb1a/b and Abcg2 increases the brain penetration of temozolomide by 1.5-fold compared to wild-type controls (P < .001) without changing the systemic drug exposure. Moreover, the same increase was achieved when temozolomide was given to wild-type mice in combination with the dual P-gp/BCRP inhibitor elacridar (GF120918). The antitumor efficacy of temozolomide against three different intracranial tumor models was significantly enhanced when Abcb1a/b and Abcg2 were genetically deficient or pharmacologically inhibited in recipient mice. These findings call for further clinical testing of temozolomide in combination with elacridar for the treatment of gliomas, as this offers the perspective of further improving the antitumor efficacy of this already active agent.

Role of Macrophages in Brain Tumor Growth and Progression

The role of macrophages in the growth and the progression of tumors has been extensively studied in recent years. A large body of data demonstrates that macrophage polarization plays an essential role in the growth and progression of brain tumors, such as gliomas, meningiomas, and medulloblastomas. The brain neoplasm cells have the ability to influence the polarization state of the tumor associated macrophages. In turn, innate immunity cells have a decisive role through regulation of the acquired immune response, but also through humoral cross-talking with cancer cells in the tumor microenvironment. Neoangiogenesis, which is an essential element in glial tumor progression, is even regulated by the tumor associated macrophages, whose activity is linked to other factors, such as hypoxia. In addition, macrophages play a decisive role in establishing the entry into the bloodstream of cancer cells. As is well known, the latter phenomenon is also present in brain tumors, even if they only rarely metastasize. Looking ahead in the future, we can imagine that characterizing the relationships between tumor and tumor associated macrophage, as well as the study of circulating tumor cells, could give us useful tools in prognostic evaluation and therapy. More generally, the study of innate immunity in brain tumors can boost the development of new forms of immunotherapy.