Drug delivery to brain tumors

A Computational Framework for the Administration of 5-Aminovulinic Acid Before Glioblastoma Surgery

5-Aminolevulinic Acid (5-ALA) is the only fluorophore approved by the FDA as an intraoperative optical imaging agent for fluorescence-guided surgery in patients with glioblastoma. The dosing regimen is based on rodent tests where a maximum signal occurs around 6 h after drug administration. Here, we construct a computational framework to simulate the transport of 5-ALA through the stomach, blood, and brain, and the subsequent conversion to the fluorescent agent protoporphyrin IX at the tumor site. The framework combines compartmental models with spatially-resolved partial differential equations, enabling one to address questions regarding quantity and timing of 5-ALA administration before surgery. Numerical tests in two spatial dimensions indicate that, for tumors exceeding the detection threshold, the time to peak fluorescent concentration is 2-7 h, broadly consistent with the current surgical guidelines. Moreover, the framework enables one to examine the specific effects of tumor size and location on the required dose and timing of 5-ALA administration before glioblastoma surgery.

A mathematical model to elucidate brain tumor abrogation by immunotherapy with T11 target structure

T11 Target structure (T11TS), a membrane glycoprotein isolated from sheep erythrocytes, reverses the immune suppressed state of brain tumor induced animals by boosting the functional status of the immune cells. This study aims at aiding in the design of more efficacious brain tumor therapies with T11 target structure. We propose a mathematical model for brain tumor (glioma) and the immune system interactions, which aims in designing efficacious brain tumor therapy. The model encompasses considerations of the interactive dynamics of glioma cells, macrophages, cytotoxic T-lymphocytes (CD8+ T-cells), TGF-β, IFN-γ and the T11TS. The system undergoes sensitivity analysis, that determines which state variables are sensitive to the given parameters and the parameters are estimated from the published data. Computer simulations were used for model verification and validation, which highlight the importance of T11 target structure in brain tumor therapy.

Targeted delivery of antibody-based therapeutic and imaging agents to CNS tumors: crossing the blood-brain barrier divide

INTRODUCTION

Brain tumors are inherently difficult to treat in large part due to the cellular blood-brain barriers (BBBs) that limit the delivery of therapeutics to the tumor tissue from the systemic circulation. Virtually no large molecules, including antibody-based proteins, can penetrate the BBB. With antibodies fast becoming attractive ligands for highly specific molecular targeting to tumor antigens, a variety of methods are being investigated to enhance the access of these agents to intracranial tumors for imaging or therapeutic applications.

AREAS COVERED

This review describes the characteristics of the BBB and the vasculature in brain tumors, described as the blood-brain tumor barrier (BBTB). Antibodies targeted to molecular markers of central nervous system (CNS) tumors will be highlighted, and current strategies for enhancing the delivery of antibodies across these cellular barriers into the brain parenchyma to the tumor will be discussed. Noninvasive imaging approaches to assess BBB/BBTB permeability and/or antibody targeting will be presented as a means of guiding the optimal delivery of targeted agents to brain tumors.

EXPERT OPINION

Preclinical and clinical studies highlight the potential of several approaches in increasing brain tumor delivery across the BBB divide. However, each carries its own risks and challenges. There is tremendous potential in using neuroimaging strategies to assist in understanding and defining the challenges to translating and optimizing molecularly targeted antibody delivery to CNS tumors to improve clinical outcomes.

Targeted drug delivery for treatment and imaging of glioblastoma multiforme

Glioblastoma multiforme is a grade IV astrocytic tumor with a very high mortality rate. Although current treatment often includes surgical resection, this rarely removes all primary tumor cells, so is usually followed by radiation and/or chemotherapy. Remaining migratory tumor cells invade surrounding healthy tissue and contribute to secondary and tertiary tumor recurrence; therefore, despite significant research into glioma removal and treatment, prognosis remains poor. A variety of treatment modalities have been investigated to deliver drug to these cells, including systemic, diffusive and convection-enhanced delivery (CED). As systemic delivery is limited by molecules larger than approximately 500 Da being unable to cross the blood-brain barrier (BBB), therapeutic concentrations are difficult to attain; thus, localized delivery options relying on diffusion and CED have been used to circumvent the BBB. Although CED enables delivery to a greater volume of tissue than diffusive delivery alone, limitations still exist, requiring that these delivery strategies be improved. This review enumerates the strengths and weaknesses of these currently used strategies and details how predictive mathematical modeling can be used to aid investigators in optimizing these delivery modalities for clinical application.

Temporal and spatial control of neural effects following intracerebral microinfusion

Spatial and temporal control of neural drug delivery is critical for many therapeutic applications and analyses of brain patterns and behavior. Specifically, for localized injections that serve to deliver drug or inactivate an isolated tissue region in order to observe changes in neural activity at that site, excess distribution into surrounding regions may confound analysis or adversely affect healthy tissue. Here, we develop a mass transport model that simulates a short period of initial infusion of inactivating drug, followed by a successive convective wash with artificial cerebrospinal fluid (aCSF), while tracking the regions of tissue that are above a certain threshold concentration of inactivating agent. We analyze the effect of parameters such as effective diffusion coefficient, extracellular volume fraction, and injectate concentration upon spatiotemporal distribution profiles. Further, we observe the effects of following the initial injection with a wash-out period with aCSF upon the breadth of the volume affected by the injectate. These simulations indicate that, by injecting small volumes of drug at low concentrations and following them with an aCSF flush, a well-delineated region of tissue can be altered for a controlled duration.

Comparing monoclonal antibodies and small peptidic hormones for local targeting of malignant gliomas

Monoclonal antibodies, F(ab')2 fragments and peptidic vectors have been clinically tested for systemic and locoregional treatment of malignant gliomas. Since these brain-intrinsic neoplasms are characterized by relentless tumor cell infiltration of normal brain parenchyma, targeting agents require diffusive properties in order to reach invading tumor cell clusters that migrate along vascular clefts and axonal pathways. Tumor uptake was significantly improved by using small peptidic hormone receptors, e.g. modified octreotide, following systemic injections as compared to macromolecules which only led to limited stabilization of the disease. More importantly, biodistribution was found to be superior following direct intratumoral injection by using these small drug-like radioconjugates. Rapid and extensive distribution within 30 minutes was observed in large tumors, even crossing the corpus callosum in bihemispheric lesions following injection of 2-3 ml of the radiopharmakon injected into the center of non-resected tumors. Distribution was far more extensive after direct intratumoral injection as compared to intracavitary injection after surgical debulking. Increased interstitial pressure gradients and the much larger and chaotic structure of the interstitial space of a tumor compared to the extremely tight architecture of normal brain tissue might explain this unexpected biodistribution pattern. Peptidic hormone vectors might become useful agents to deliver radiopharmaceuticals into human invasive gliomas.

Cytotoxic chemotherapy: advances in delivery, pharmacology, and testing

Adjuvant treatment of malignant gliomas, the most common types of primary brain tumors, with intravenous (iv) chemotherapy has not significantly improved survival for patients with these forms of cancer. A major factor in the failure of iv chemotherapy is the blood-brain barrier (BBB), a physiologic impediment to the delivery of cytotoxic chemotherapeutic drugs to the central nervous system (CNS). Intra-arterial and intrathecal infusion, blood-brain barrier disruption, high-dose chemotherapy, intratumoral administration, and convection-enhanced delivery are methods developed to overcome the BBB. Although some of these methods may increase the local concentration-time profile, improvement in clinical outcomes has yet to be definitively established. New methods for assessment of drug delivery to the brain tumor will assume increasing importance in the study of new cytotoxic chemotherapeutic drugs for these types of cancer. Pharmacokinetic studies are critical components of these clinical trials and allow assessment of drug delivery to the CNS and brain tumor. Additionally, pharmacokinetic studies will remain an important component of early clinical trials, particularly for identifying significant drug interactions involving the various supporting medications that are typically used in this patient population.

Mathematical and computational challenges in population biology and ecosystems science

Mathematical and computational approaches provide powerful tools in the study of problems in population biology and ecosystems science. The subject has a rich history intertwined with the development of statistics and dynamical systems theory, but recent analytical advances, coupled with the enhanced potential of high-speed computation, have opened up new vistas and presented new challenges. Key challenges involve ways to deal with the collective dynamics of heterogeneous ensembles of individuals, and to scale from small spatial regions to large ones. The central issues-understanding how detail at one scale makes its signature felt at other scales, and how to relate phenomena across scales-cut across scientific disciplines and go to the heart of algorithmic development of approaches to high-speed computation. Examples are given from ecology, genetics, epidemiology, and immunology.