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      Evolving Drug Delivery Strategies to Overcome the Blood Brain Barrier

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          Abstract

          The blood-brain barrier (BBB) poses a unique challenge for drug delivery to the central nervous system (CNS). The BBB consists of a continuous layer of specialized endothelial cells linked together by tight junctions, pericytes, nonfenestrated basal lamina, and astrocytic foot processes. This complex barrier controls and limits the systemic delivery of therapeutics to the CNS. Several innovative strategies have been explored to enhance the transport of therapeutics across the BBB, each with individual advantages and disadvantages. Ongoing advances in delivery approaches that overcome the BBB are enabling more effective therapies for CNS diseases. In this review, we discuss: (1) the physiological properties of the BBB, (2) conventional strategies to enhance paracellular and transcellular transport through the BBB, (3) emerging concepts to overcome the BBB, and (4) alternative CNS drug delivery strategies that bypass the BBB entirely. Based on these exciting advances, we anticipate that in the near future, drug delivery research efforts will lead to more effective therapeutic interventions for diseases of the CNS.

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          Most cited references 201

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          Blood-brain barrier delivery.

          Neuropharmaceutics is the largest potential growth sector of the pharmaceutical industry. However, this growth is blocked by the problem of the blood-brain barrier (BBB). Essentially 100% of large-molecule drugs and >98% of small-molecule drugs do not cross the BBB. The BBB can be traversed because there are multiple endogenous transporters within this barrier. Therefore, brain drug development programs of the future need to be re-configured so that drugs are formulated to enable transport into the brain via endogenous BBB transporters.
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            Convection-enhanced delivery of macromolecules in the brain.

            For many compounds (neurotrophic factors, antibodies, growth factors, genetic vectors, enzymes) slow diffusion in the brain severely limits drug distribution and effect after direct drug administration into brain parenchyma. We investigated convection as a means to enhance the distribution of the large and small molecules 111In-labeled transferrin (111In-Tf; M(r), 80,000) and [14C]sucrose (M(r), 359) over centimeter distances by maintaining a pressure gradient during interstitial infusion into white matter to generate bulk flow through the brain interstitium. The volume of distribution (Vd) containing > or = 1% concentration of infusion solution increased linearly with the infusion volume (Vi) for 111In-Tf(Vd/Vi, 6:1) and [14C]sucrose (Vd/Vi, 13:1). Twenty-four hours after infusion, the distribution of 111In-Tf was increased and more homogeneous, and penetration into gray matter had occurred. By using convection to supplement simple diffusion, enhanced distribution of large and small molecules can be obtained in the brain while achieving drug concentrations orders of magnitude greater than systemic levels.
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              Neural stem cells display extensive tropism for pathology in adult brain: evidence from intracranial gliomas.

              One of the impediments to the treatment of brain tumors (e.g., gliomas) has been the degree to which they expand, infiltrate surrounding tissue, and migrate widely into normal brain, usually rendering them "elusive" to effective resection, irradiation, chemotherapy, or gene therapy. We demonstrate that neural stem cells (NSCs), when implanted into experimental intracranial gliomas in vivo in adult rodents, distribute themselves quickly and extensively throughout the tumor bed and migrate uniquely in juxtaposition to widely expanding and aggressively advancing tumor cells, while continuing to stably express a foreign gene. The NSCs "surround" the invading tumor border while "chasing down" infiltrating tumor cells. When implanted intracranially at distant sites from the tumor (e.g., into normal tissue, into the contralateral hemisphere, or into the cerebral ventricles), the donor cells migrate through normal tissue targeting the tumor cells (including human glioblastomas). When implanted outside the CNS intravascularly, NSCs will target an intracranial tumor. NSCs can deliver a therapeutically relevant molecule-cytosine deaminase-such that quantifiable reduction in tumor burden results. These data suggest the adjunctive use of inherently migratory NSCs as a delivery vehicle for targeting therapeutic genes and vectors to refractory, migratory, invasive brain tumors. More broadly, they suggest that NSC migration can be extensive, even in the adult brain and along nonstereotypical routes, if pathology (as modeled here by tumor) is present.
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                Author and article information

                Journal
                Curr Med Chem
                Curr. Med. Chem
                CMC
                Current Medicinal Chemistry
                Bentham Science Publishers
                0929-8673
                1875-533X
                March 2016
                March 2016
                : 22
                : 9
                : 1177-1193
                Affiliations
                Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, MD 21201 USA; Marlene and Stewart Greenebaum Cancer Center, University of Maryland School of Medicine, Baltimore, MD 21201 USA; Department of Diagnostic Radiology and Nuclear Medicine, University of Maryland School of Medicine, Baltimore, MD 21201 USA; Department of Surgery, University of Maryland School of Medicine, Baltimore, MD 21201 USA; Center for Vascular and Inflammatory Diseases, University of Maryland School of Medicine, Baltimore, MD 21201 USA; Department of Pharmaceutical Sciences, University of Maryland School of Pharmacy, Baltimore, MD 21201 USA; Center for Biomedical Engineering and Technology, University of Maryland School of Medicine, Baltimore, MD 21201 USA
                Author notes
                [* ]Address correspondence to these authors at the Department of Neurosurgery, University of Maryland School of Medicine, 22 South Greene Street, Baltimore, MD 21201; E-mail: gwoodworth@ 123456smail.umaryland.edu , Departments of Neurosurgery and Pharmaceutical Sciences, University of Maryland, Baltimore, 655 W. Baltimore Street, Baltimore, MD 21201;, E-mail: akim@ 123456smail.umaryland.edu
                [#]

                These authors contributed equally.

                Article
                CPD-22-1177
                10.2174/1381612822666151221150733
                4997920
                © 2016 Bentham Science Publishers

                This is an open access article licensed under the terms of the Creative Commons Attribution-Non-Commercial 4.0 International Public License (CC BY-NC 4.0) ( https://creativecommons.org/licenses/by-nc/4.0/legalcode ), which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

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