Description

	• Various drugs are administered through different routes for action on the nervous system.
	• Most of these drugs need to cross or bypass the blood-brain barrier, and various technologies are used to facilitate delivery.

Table 1 shows a classification of available strategies for CNS drug delivery. Most of these strategies involve circumventing the blood-brain barrier.

Table 1. Strategies for Drug Delivery to the CNS

• Systemic administration of therapeutic substances for CNS action
	- Intravenous injection for targeted action in the CNS - Transdermal delivery for introduction into systemic circulation for action on CNS
• Direct administration of therapeutic substances to the CNS
	- Introduction into cerebrospinal fluid pathways: intraventricular, subarachnoid pathways - Introduction into the cerebral arterial circulation - Introduction into the brain substance - Direct positive pressure infusion - Intranasal instillation for introduction into the brain along the olfactory tract
• Drug delivery by manipulation of the blood-brain barrier (see Table 2)
• Drug delivery using novel formulations
	- Conjugates - Gels - Liposomes - Microspheres - Nanoparticles
• Chemical delivery systems
• Drug delivery devices
	- Pumps - Catheters - Implants releasing drugs - Controlled-release microchip
• Use of microorganisms for drug delivery to the brain
	- Bacteriophages for brain penetration - Bacterial vectors
• Cell therapy
	- CNS implants of live cells secreting therapeutic substances - CNS implants of encapsulated genetically engineered cells producing therapeutic substances - Cells for facilitating crossing of the blood brain barrier
• Gene transfer
	- Direct injection into the brain substance - Targeting of CNS by retrograde axonal transport

Table 2. Strategies For Drug Delivery Across the Blood-Brain Barrier

• Increasing the permeability (opening) of the blood-brain barrier
	- Osmotic opening of the blood-brain barrier - Chemical opening - Cerebral vasodilatation: stimulation of the sphenopalatine ganglion, nitric oxide inhalation - Disruption of blood-brain barrier by focused ultrasound - Neurostimulation for opening of the blood-brain barrier
• Pharmacological strategies to facilitate transport across the blood-brain barrier
	- Modification of the drug to enhance its lipid solubility - Use of transport/carrier systems - Inhibition of efflux transporters that impede drug delivery - Kinin analogs for modification of blood-brain barrier - Trojan horse approach - Chimeric peptides - Monoclonal antibody fusion proteins - Prodrug bioconversion strategies - Nanoparticle-based technologies - Neuroimmunophilins - Sphingosine 1–phosphate (S1P) receptor-1 inhibitors
• Bypassing the blood-brain barrier
	- Using an alternative route of delivery, eg, injection into epidural venous plexus - Direct introduction of drugs into the brain or the CSF

Some of the recommendations by participants of a workshop on the topic of crossing the blood-brain barrier for CNS drug delivery included the following issues that are relevant for approval of drugs and devices (33):

	• Areas of major concern to regulatory agencies include off-target effects, safety of long-term dosing, immunogenicity, and unintended adverse effects.
	• Technologies that disrupt the blood-brain barrier raise safety concerns regarding exposure of the brain to chemicals or toxins.
	• Evaluation of the safety profile of biologics can be particularly challenging as they may combine multiple components that are delivered to the brain.
	• The acceptable safety threshold and the risk-benefit ratio for a therapeutic will vary depending on the condition being treated.
	• Focused ultrasound introduces many regulatory challenges because it combines a device, microbubbles, the therapeutic agent, and an imaging agent.

Osmotic opening of the blood-brain barrier. Hypertonic solutions can disrupt the blood-brain barrier both when applied directly to the surface of the brain and when infused on the blood side of the blood-brain barrier. Electron microscopic studies in experimental animals show that such reversible opening is mediated by increased permeability of the interendothelial tight junctions to tracers such as horseradish peroxidase. It is possible that such opening is due to a shrinking of endothelial cells, leading to a widening of spaces between the cells. Entry of neutral, water-soluble drugs into the brain can be predicted by a model that takes into consideration the effective pore size, pore density, and time course for bulk water flow. In addition to this, the molecular size and molecular charge of the drug also play a role in the entry of drugs into the brain. A negative surface charge associated with the pores can restrict the passage of negatively charged molecules more than their neutral or positively charged counterparts.

This strategy may be useful to facilitate the entry into the brain of drugs that are bound tightly to plasma proteins. A temporary increase of blood-brain barrier permeability would allow the entry of both the free as well as the bound forms of the drug. Whereas the free part of the drug would have an immediate effect, the bound portion would have to dissociate from the protein before producing its effect, which would cause a delay. In experimental animals, the osmotic opening method has been used to obtain wide access of water-soluble drugs, such as peptides, boron compounds for neutron capture therapy, and viral vectors for gene therapy, to the brain.

Chemical opening of the blood-brain barrier. This is more selective and controllable than osmotic opening. Bradykinin given by intracarotid injection opens the brain tumor capillaries without affecting the capillaries in the normal brain.

Disruption of blood-brain barrier by focused ultrasound. Low-frequency, MRI-guided, focused ultrasound has been shown to induce localized and reversible disruption of the blood-brain barrier without undesired long-term effects in experimental animals. A frequency range of approximately 260 kHz enables the passage of ultrasound through the skull and produced focal disruption of blood-brain barrier without extravasation of red blood cells (44). The exact mechanism is not known, but ultrasound-induced microbubbles may temporarily expand the capillary wall and open the tight junctions. This noninvasive technique offers a potential method for targeted drug delivery in the brain aided by a relatively simple low-frequency device.

Neurostimulation. This produces an indirect effect, ie, opening of the blood-brain barrier to facilitate drug delivery to the brain. One device, Vascular Enabled Integrated Nanosecond (VEIN) pulse, involves insertion of minimally invasive electrodes into diseased brain tissue and application of multiple bursts of nanosecond pulses with alternating polarity that disrupt tight junction proteins responsible for maintaining the integrity of the blood-brain barrier without causing damage to the surrounding tissue (01).

Modification of the drug to enhance its lipid solubility. There is a good correlation between the lipid solubility of a drug and the blood-brain barrier penetration in vivo. The lipophilic pathway also provides a large surface area for drug delivery. It is approximately 12 m2 in an average human brain. Therefore, the addition of hydrophobic groups to molecules increases their ability to penetrate the blood-brain barrier. The addition of methyl groups in a series of barbiturates improves lipophilicity and brain penetration, leading to increased hypnotic action. It is also possible to generate a lipophilic prodrug that is broken down to release the more active drug within the brain. An example of this is heroin, which enters the brain readily due to its lipophilicity but, after entry, hydrolyses to morphine, which is less lipophilic and less likely to diffuse back across the blood-brain barrier, leading to prolongation of duration of its action on the brain. Ester formation is another approach for increasing the lipophilicity of polar molecules exhibiting poor CNS penetration. Several investigators have explored the lipophilic-ester concept for improving the CNS delivery of antiviral agents. An example of this is improvement of the blood-brain barrier penetration of GABA (gamma amino butyric acid), an anticonvulsant agent with poor CNS penetration, by use of lipophilic esters.

Although increasing lipophilicity generally increases penetration across the blood-brain barrier, it may also result in reduction of biological action due to drug-receptor interaction, drug metabolism, or binding to plasma proteins, as in the case of barbiturates. Therefore, optimization rather than maximization of both lipophilicity and rate of bioconversion are required.

Lipid-binding carriers may reduce the binding of neurotrophic factors to serum lipids and increase transport across the blood-brain barrier. Liposomes have been considered, but their size is too large to cross the blood-brain barrier.

Use of transport or carrier systems. Of the various carrier systems, those for glucose and neutral amino acids have high enough transport capacity to hold promise for significant drug delivery to the brain. Approaches for drug delivery across the blood-brain barrier include the prodrug strategy whereby drug molecules are conjugated to glucose transporter substrates or encapsulated in nano-enabled delivery systems (eg, liposomes, micelles, nanoparticles) that are functionalized to target glucose transporters (37). Glucose transporters have the limitation that only molecules closely resembling D-glucose are transported. Neutral amino acids are less specific. Entry via this carrier may explain the central effects of the muscle relaxant baclofen. Transport systems for peptides may prove to be effective targets for peptide drugs required to control natural peptide hormones.

Drugs used to treat neurologic disorders appear to cross the blood-brain barrier more easily when an ascorbic acid molecule is attached. Ascorbic acid works like a shuttle and, theoretically, could transport any compound into the brain. The ascorbic acid SVCT2 transporter, which is believed to play a major role in regulating the transport of ascorbic acid into the brain, provides a targeted delivery to the brain. Potential applications include drugs to treat neurodegenerative diseases, such as Alzheimer disease, Parkinson disease, and epilepsy.

Receptor-mediated transport. Receptor-mediated transcytosis employs the vesicular trafficking machinery of the endothelium to transport substrates between blood and brain. If appropriately targeted, this approach can also be used to shuttle a wide range of therapeutics into the brain in a noninvasive manner. Sophisticated cell culture models of the blood-brain barrier have enabled the identification, characterization, and validation of a novel targeted drug delivery technology, designated 2B-Trans, for the receptor-mediated uptake and transport of drugs across the blood-brain barrier (20). Differential drug targeting or delivery approach based on receptor-mediated transport is recommended for the treatment of CNS diseases that are related to the blood-brain barrier alone as well as for CNS diseases that are related to both the brain and the blood-brain barrier.

A “Brain Shuttle module” has been developed by manipulating the binding mode of an antibody fragment to the transferrin receptor (TfR), which can be engineered into a standard therapeutic antibody for successful transcytosis across the blood-brain barrier (34). A monovalent binding mode to the TfR anti-amyloid beta antibody increases delivery to beta amyloid target in a mouse model of Alzheimer disease by 55-fold compared to the parent antibody.

Thermoresponsive hydrogels as drug delivery systems. Current understanding and challenges in the design of thermoresponsive hydrogels for CNS therapy that hinder mass and drug transport to the CNS highlight the distinct features of each barrier (03). Advantages and limitations of each design and application have been described with the objective of identifying general rules that could enhance the effective translation of thermoresponsive hydrogel-based therapies for the treatment of CNS disorders.

Inhibition of efflux transporters. Efflux transport system at the blood-brain barrier provides a protective barrier function by removing drugs from the brain or CSF and transferring them to the systemic circulation. Therefore, modulation of these efflux transporters by design of inhibitors and/or design of compounds that have minimal affinity for these transporters may enhance the treatment of neurologic disorders. Concomitant use of inhibitors of P-glycoprotein, an efflux transporter, also enhances the delivery of drugs to the CNS. In a clinical experimental study, administration of the experimental oncology drug tariquidar (a potent P-glycoprotein inhibitor) along with radiolabeled verapamil in healthy volunteers showed that total distribution volume of verapamil in whole brain grey matter increased about 300% on PET scans (02).

Trojan horse approach. Attaching an active drug molecule to a vector that accesses a specific catalyzed transporter mechanism creates a Trojan horse-like deception that tricks the blood-brain barrier into welcoming the drug through its gates. Transport vectors, such as endogenous peptides, modified proteins, or peptidomimetic monoclonal antibodies are a way of tricking the brain into allowing these molecules to pass. The therapeutic peptide or protein drug is fused to a molecular Trojan horse, which may be a monoclonal antibody that binds to a specific receptor on the blood-brain barrier and enables receptor-mediated delivery of the fusion protein across the barrier to exert the desired pharmacological effect on the brain (36). The Trojan horse approach has been applied to delivery of nonviral gene and RNAi therapeutics, particularly in lysosomal storage disorders and Parkinson disease (04).

Inhibition of carriers that impede drug delivery. P-glycoprotein active drug efflux transporter is present at high densities in the luminal membranes of brain endothelium. It pumps out some cytotoxic agents used to treat brain tumors and excludes them from the brain. P-glycoprotein inhibitors enhance the effects of cytotoxic agents and have the potential of enhancing chemotherapeutic effects on the brain.

Transport of small molecules across the blood-brain barrier. Lipid-soluble small molecules with a molecular mass of less than 400 to 600 d are transported readily through the blood-brain barrier in vivo via lipid-mediated transport. However, other small molecules lacking these molecular properties, antisense drugs, and peptide-based pharmaceuticals ordinarily undergo negligible transport through the blood-brain barrier in pharmacologically significant amounts. Some small-molecule neuroprotective agents have failed in human trials due to poor transport of these agents across the blood-brain barrier. Strategies that enable drug transport through the blood-brain barrier arise from knowledge of the molecular and cellular biology of blood-brain barrier transport processes.

Receptor-mediated endocytosis. In this approach, the nontransportable drug is attached to a protein or peptide vector, which is accepted by the receptor at the luminal side of the blood-brain barrier and endocytosed. After exocytosis in the brain interstitial fluid, the chemical link binding the drug to the peptide is cleaved, and the drug binds to a receptor at the neuron. The high level of expression of transferrin receptors on the surface of endothelial cells of the blood-brain barrier have been widely utilized to deliver drugs to the brain. This approach has been explored for the delivery of citicoline, a neuroprotective drug for stroke, which does not readily cross the blood-brain barrier because of its strong polar nature.

Chimeric peptides. Under normal conditions, vesicular transport that involves receptor-mediated endocytosis is responsible for only a small amount of molecular trafficking across the blood-brain barrier, but it may be suitable for the delivery of agents that are too large to use other routes. This approach involves forming a chimeric peptide by coupling an otherwise nontransportable drug to a blood-brain barrier transporter vector by a disulfide bond. The chimera is then endocytosed by the capillary endothelial cells and transported to the brain where it can be cleaved by disulfide reductase to release the pharmacologically active compound. Blood-brain barrier peptide receptor systems include those for insulin, insulin-like growth factor, transferrin, and leptin.

Kinin analogs for modification of blood-brain barrier. In vivo imaging studies in animal models of glioma have shown that kinin B1 (B1R) and B2 receptor (B2R) agonists increase the blood-brain barrier penetration of chemotherapeutic doxorubicin to glioma sites, with additive effects when applied in combination (39). B2R agonist also promoted the selective delivery of co-injected diagnostic MRI agent gadopentetate dimeglumine in irradiated brain areas, depicting increased vascular B2R expression. These findings suggest potential use of kinin analogs to facilitate access of drugs to the brain.

Monoclonal antibody fusion proteins. These involve conjugation of a drug to a transport vector. These have diagnostic and therapeutic applications for the treatment of brain tumors. Nontransportable specific antigen-binding monoclonal antibodies such as IgG3 have been attached to a transport vector such as insulin-like growth factor. The bifunctional molecule can cross the blood-brain barrier through interaction with the receptor for insulin-like growth factor.

Transferrin is a specific receptor for molecules that are not synthesized in the brain but play an essential biological role. This transfer mechanism can be exploited in an approach in which antiferritin receptor antibodies are covalently linked to nerve growth factor, resulting in a substantial transfer of biologically active nerve growth factor across the blood-brain barrier into the CNS. Nerve growth factor can be transported across the blood-brain barrier by conjugating with OX-2, an antibody directed against the transferrin receptor.

Prodrug bioconversion strategies and their CNS selectivity. A wide range of prodrug options can be explored in improving the CNS delivery of low molecular weight drugs. Site-specific delivery to the CNS via the prodrug approach can be achieved only if the basic criteria of this approach are fulfilled. Prodrug strategies for increasing the blood-brain barrier transport of small hydrophilic molecules include chemical modification to more lipophilic derivatives, carrier-mediated prodrug transport, and prodrug bioconversion strategies.

Carrier-mediated prodrug transport involves the use of a carrier that is a substrate for one of the transporters within the blood-brain barrier. A classic example of this approach is the improvement of CNS delivery of the neurotransmitter dopamine, which is poorly transported across the blood-brain barrier and is rapidly degraded in the endothelial cells by the action of the enzyme monoamine oxidase. Prodrug L-dopa is readily transported across the blood-brain barrier via the large neutral amino acid transporter and is decarboxylated to dopamine by L-amino acid decarboxylase, which is localized in the capillary endothelium. Several other drugs are transported to the brain via the neutral amino acid transporter (L-system) such as baclofen, melphalan, and buthionine sulfoximine. Similarly, many beta-lactim antibiotics including benzylpenicillin, ceftriaxone, and the anticonvulsant valproic acid are transported by the monocarboxylic acid carrier. Several peptide transporter systems have also been described.

Parent drug efflux from the CNS and metabolism in brain tissue are important factors limiting drug efficacy. Combined prodrugs containing both the active agent and an inhibitor of the enzyme that metabolizes the parent compound is a more effective approach. Additional research is necessary before CNS delivery can be truly designed into a molecule via the prodrug approach.

Activated T lymphocytes. T lymphocytes can cross the blood-brain barrier and can be engineered to produce nerve growth factor in quantities comparable to those produced by genetically engineered fibroblasts. Engineered T lymphocytes have been used as vehicles to deliver nerve growth factor across the endothelial blood-nerve barrier to attenuate experimental autoimmune neuritis.

Neuroimmunophilins. These small-molecule neurotrophic factors can be administered orally and can cross the blood-brain barrier. Neuroimmunophilin ligands are small molecules that can repair and regenerate damaged nerves without affecting normal, healthy nerves and may have application in the treatment of a broad range of diseases, including Parkinson disease, spinal cord injury, traumatic brain injury, and peripheral nerve injuries. The immunosuppressants tacrolismus (FK-506) and cyclosporin are in clinical use for the treatment of allograft rejection following organ transplantation. Immunophilins can regulate neuronal survival and nerve regeneration although the molecular mechanisms are poorly understood.

Endothelial sphingosine 1–phosphate (S1P) receptor-1. S1P receptor-1, a G protein-coupled receptor, promotes the barrier function in peripheral vessels. Experimental studies in mice suggest that brain endothelial S1P1 maintains the blood-brain barrier by regulating the proper localization of tight junction proteins (47). Thus, endothelial S1P1 inhibition may be a strategy for transient blood-brain barrier opening and delivery of small molecules into the CNS.

Delivery of drugs to the brain via the nasal route. The intranasal route consists of two pathways (12):

(1) The intracellular pathway starts with endocytosis by olfactory sensory cells, followed by axonal transport to their synaptic clefts in the olfactory bulb where the drug is exocytosed. This process is repeated by olfactory neurons, thereby distributing the drug to other brain regions.

(2) In the extracellular mechanism, drugs are transported directly into the cerebral spinal fluid by first passing through the paracellular space across the nasal epithelium, then through the perineural space to the subarachnoid space of the brain.

The olfactory nerve is the target when direct absorption into the brain is the goal because it is the only site in the human body where the CNS is directly expressed on the nasal mucosal surface (21). Although the traditional blood-brain barrier is not present at the interface between nasal epithelium and brain, P-glycoprotein and other barrier transporters are expressed at this interface, which can be modulated with nasal administration of appropriate inhibitors. Nasal administration of two prodrugs (L-dopa butyl esters) has been reported to result in higher CSF levels of L-dopa than those observed after intravenous administration. The percentage of the applied dose that passes to the brain and CSF is about 2% to 3%. This indicates that a nasal route may be a viable method for the delivery of peptides, analgesics, and other drugs for the treatment of CNS disorders. Preferential uptake of intranasally administered apomorphine directly into the cerebral spinal fluid has been demonstrated in a phase I study, and this opens the possibility of treating neurologic disorders with intranasal apomorphine.

A wide variety of therapeutic compounds can be delivered intranasally, including relatively large molecules such as peptides and proteins, particularly in the presence of permeation enhancers (11). Direct nose-to-brain delivery is still limited by low efficiency and low volume of delivery. However, future studies are expected to achieve a detailed understanding of pharmacokinetics and mechanisms of delivery to optimize formulations and fully exploit the nose-to-brain interface to deliver proteins for the treatment of neurologic diseases (30).

Intranasal route facilitates the delivery of large molecules, which fail to effectively cross the blood-brain barrier. Therefore, intranasal delivery is an efficient method of administration for neurotrophic factors, hormones, neuropeptides, and even stem cells (07).

Retrograde axonal transport. In experimental animals with transected peripheral nerve roots, targeted axonal import has been shown to deliver protein cargo into spinal cord motor neurons after intramuscular injection (38). Nonviral-mediated delivery of functional proteins into the spinal cord indicates the clinical potential of this technology.

Invasive neurosurgical approaches. This can be achieved by special delivery methods that involve direct introduction of the therapeutic substances into the brain substance or into the CSF. These methods involve neurosurgical procedures, implantation of cells or tissues, or the use of special devices.

Injection into the arterial circulation of the brain. There is considerable clinical experience with injections into the arterial circulation of the brain (25). Neuroradiologists manipulate catheters in the circulation and inject contrast agents for angiography.

Direct injection into the CNS substance or CNS lesions. Stereotactic techniques used in modern neurosurgery enable placement of electrodes or probes in precise locations in the brain with minimal invasion. Drugs injected this way can spread over a wider area by convection currents and achieve a high and uniform concentration in the interstitial space of the brain. Direct positive pressure-controlled infusion has potential for the delivery of pharmacologic agents, viral vectors, and oligonucleotides to the brain. In the case of brain tumors, surgical removal of the tumor provides an opportunity to pack the tumor cavity with therapeutic substances in such a way that they do not escape into the systemic circulation. Brain abscesses can be aspirated through a cannula inserted by a minimally invasive technique, and antibiotics can be injected after aspiration of the infected material.

Convention-enhanced delivery. This is a potentially useful method for significantly circumventing the blood-brain barrier and increasing delivery of water-soluble drugs to the brain. Important advances have been made to optimize the unique ability of convention-enhanced delivery to locally deliver high doses of powerful chemotherapeutics to gliomas for maximal tumor destruction with minimal neurologic and systemic side effects. Direct convection delivery can also be used to deliver an infusate and distribute macromolecules in a predictable, homogeneous manner over significant volumes of naive and traumatized spinal cord. These characteristics make direct convection delivery a valuable tool in the investigation of the therapeutic potential of various compounds for the treatment of spinal cord injury. Development of surrogate imaging tracers that are co-infused during drug delivery now permit accurate, noninvasive real-time tracking of convective infusate flow in nervous system tissues (29).

Injections into the CSF. Delivery of drugs by lumbar puncture or direct intraventricular injection can bypass the blood-brain barrier by direct introduction into the CSF. The layers of cells that line the fluid spaces of the brain are permeable to molecules introduced this way. Various problems with this approach are as follows:

	• The procedure of lumbar puncture is relatively simple, but intraventricular injection is a neurosurgical procedure.
	• The diffusion distance from the CSF to the drug target can be several centimeters, and an insufficient amount of the drug may reach the intended site.
	• The flow of CSF is from the microvessels of the brain toward the CSF spaces, opposite to the direction of the drug diffusion.
	• CSF has a high turnover rate, and it is replaced within few hours when the injected drug is continuously cleared back into the blood.

Controlled-release formulations and drug-delivery devices are required for overcoming these disadvantages. Depot cytarabine (DTC 101) contains cytarabine encapsulated in microscopic spherical particles and has extended use of the therapeutic drug concentrations after a single dose given by lumbar puncture or by intraventricular injection.

Delivery of drugs into CSF via implanted catheters, reservoirs, and pumps. Various catheters are inserted into the CSF compartments (lumbar subarachnoid space, cisterns, and ventricles). These are connected to reservoirs and pumps and left in place during the duration of therapy for continuous or pulsatile drug infusions. The main objectives of these devices are as follows:

• To provide an access to various compartments of the CNS without repeated neurosurgical procedures • To prevent leakage of active principle to other organs • To obtain samples of CSF for monitoring the results of therapy • To enable treatment on an outpatient basis

The earliest of such devices is Ommaya™ reservoir (constructed of a synthetic material). It is implanted subcutaneously and connected to an intraventricular catheter. Drugs can be injected into the reservoir (access port) at required intervals. Several improvements on this have appeared since then. These can be continuous flow pumps or programmable pumps.

Continuous flow pumps. The delivery mechanism of the pump is based on the expansion of Freon gas at 37°C that pushes a diaphragm "plunger/pusher" plate. Usually, the pump reservoir is implanted subcutaneously and is connected to the catheter implanted into the nervous system to deliver the therapeutic molecules. The reservoirs are refilled by subcutaneous injection of the solution containing the bioactive molecules. A well-known example of this technique is the intrathecal pump delivery of the GABAergic drug baclofen for spasticity.

Programmable pumps. These are electromechanical pumps of peristaltic type powered by batteries. Their built-in electronics can be remotely controlled from an external programming unit. An example is SynchroMed system (Medtronic Inc.). The infusion can be programmed in various modes: continuous hourly infusions, repeated bolus infusions with a specified delay, multiple doses over a programmed interval, or a single bolus infusion. The use is restricted to a hospital with a relevant programming unit. The patient cannot switch the pump off and on.

Types of drugs and cell delivery to the nervous system. In addition to the usual advances in drug delivery, biotechnology has been used for improving drug delivery. Products of biotechnology also face drug-delivery problems. Techniques such as gene therapy, which includes implantation of genetically engineered cells, provide an opportunity to produce proteins in vivo. These technologies have been described elsewhere (24), and their application to the diseases of the nervous system will be discussed in later sections.

Table 3. Various Types of Drug and Cell Delivery to the Nervous System

	• Drug delivery using novel formulations
		- Conjugates - Gels - Injectable thermoresponsive hydrogels - Liposomes - Microspheres - Nanoparticles
	• Chemical delivery systems • Biomaterial-based drug delivery systems
	• Drug delivery devices
		- Pumps - Catheters - Implants releasing drugs
	• Controlled-release microchip
	• Cell therapy
		- CNS implants of live cells secreting therapeutic substances - CNS implants of genetically engineered cells producing therapeutic substances - Cells for facilitating crossing of the blood-brain barrier
	• Gene transfer
		- Direct injection into the brain substance - Intranasal instillation for introduction into the brain along the olfactory tract - Targeting of CNS by retrograde axonal transport - Gene transfer by viral vectors - Gene transfer by nonviral vectors

Methods of delivery of peptides for neurologic disorders. Peptides regulate most physiological processes, acting at some sites as endocrine signals and, at others, as neurotransmitters or growth factors. Alterations in the storage, release, and metabolism of neuropeptides are associated with many diseases of the CNS. Various peptides native to the CNS, as well as synthetic peptides and peptide analogs, are potentially useful for the treatment of neurologic disorders. Peptides act by binding to specific cell surface receptors. The ideal therapeutic peptide should have a small molecular mass, should be a chemical mimic of the receptor, and should get to the site of action after oral administration.

Most peptides, however, cannot be administered orally, as gastrointestinal enzymes rapidly inactivate them, so subcutaneous or intravenous administration is required. Therefore, research is focusing on alternative routes of delivery, including inhaled, buccal transmucosal, intranasal, and transdermal routes, as well as novel formulations with liposomes. Another approach is to make the peptides more stable and longer acting so that injections are required infrequently. Various strategies for delivering peptides into the CNS are like approaches for delivery of proteins and applied to neurotrophic factors. The delivery of neurotrophic factors for CNS disorders involves microencapsulation and gene therapy.

For targets in the brain, delivering neuropeptides to the site of action is problematic because of the blood-brain barrier. However, polypeptides not only have the potential to serve as carriers for selective therapeutic agents, but they themselves may directly cross the blood-brain barrier after delivery into the bloodstream to become potential treatments for a variety of CNS disorders, including neurodegeneration, autoimmune diseases, stroke, depression, and obesity. Artificial hydrophobization of peptides can facilitate their delivery across the blood-brain barrier.

Liposome-mediated drug delivery to the CNS. Liposomes have been used as nonviral vectors for gene therapy. Currently, the in vivo use of liposome particles is limited to the nanometric range, and the circulation depends on the size. Larger liposomes are cleared faster. The action of neuroactive agents (eg, neurotrophic factors) is mostly achieved by their coupling to a receptor on the plasma membrane, causing an intracellular cascade of events. Therefore, the formulation of these molecules for delivery must allow for sustained extracellular release. Sterically stabilized liposomes, which are taken up by endocytosis, are unsuitable for this purpose. Because of the frequent potent nature of the neuroactive agents, sustained drug release requires extended in vivo stability of the delivery system, which is difficult to achieve with liposomes. Therefore, liposomes are currently unsuitable for long-term administration of drugs to the nervous system unless they are adequately stabilized.

Multivesicular liposomes enable encapsulation of drugs with greater efficiency, ensuring reliable, steady, and prolonged drug release. DepoDur (morphine sulfate extended-release liposome injection), an approved formulation, contains morphine for single epidural injection in the treatment of postoperative pain. Although animal studies confirm that epidural injection of DepoDur results in the sustained release of morphine into CSF, the CSF pharmacokinetics have not been determined in humans.

Implantation of biodegradable microspheres. Controlled drug release in the CNS has been carried out by using implantable polymeric vectors. The biocompatibility of Poly (DL-lactide-co-glycolide), a biodegradable polymer, implanted into the CNS provides support for its use in a wide range of new therapeutic applications for sustained and localized drug delivery to the brain. Polymeric controlled release can provide high, localized doses of rhNGF in the brain. Microencapsulation methods allow the production of microparticles or nanoparticles loaded with neuroactive drugs. Biodegradable microspheres can be implanted stereotactically in the brain. Stereotactic procedures on the brain involve guiding a probe into discrete and precise target areas based on anatomical and functional landmarks without causing damage to the surrounding structures. Currently, this method is most frequently applied for the treatment of brain tumors and neurodegenerative disorders such as Parkinson disease. However, the potential applications of drug targeting by stereotactic implantation of drug-loaded particles are unlimited.

Nanoparticles. The use of nanoparticles to deliver drugs to the brain across the blood-brain barrier may provide a significant advantage over current strategies (41; 23). Very small nanoparticles may simply pass through the blood-brain barrier, but this uncontrolled passage is not desirable. Controlled passage of drugs across the blood-brain barrier can be enhanced by nanotechnology. Nanoparticles open the tight junctions between endothelial cells and enable the drug to penetrate the blood-brain barrier either in free form or together with the nanocarrier.

By enabling controlled drug release in the brain, nanoparticle carrier technology decreases neurotoxicity. Various factors that influence transport include the type of polymer or surfactant used, nanoparticle size, and the drug molecule.

Nanoparticles conjugated with specific ligands that can target receptors in the brain microvasculature can carry the drugs to the brain through the receptor-mediated transcytosis. Organically modified silica nanoparticles can be used as nonviral vectors for efficient in vivo gene delivery without toxic effects and with efficacy equaling or exceeding that obtained by using a viral vector. Different polymer nanoparticles can be targeted to specific ligands to enhance the specificity of drugs delivered to the CNS.

Glucocorticoids have shown efficacy in neuroinflammatory disorders such as multiple sclerosis. However, their side effects are dose limiting. CNS delivery of methylprednisolone across the blood-brain barrier has been enhanced by use of targeted pegylated liposomes conjugated to the brain-targeting ligand glutathione, and this method has potential for clinical development (18).

Superparamagnetic nanoparticles used for magnetic resonance navigation, when exposed to a low radiofrequency field, release energy in the form of heat to their surroundings and temporarily open the blood-brain barrier (43). These nanoparticles can be chemically engineered to specifically target the surface of the vascular endothelium by means of adhesion antibodies. This strategy could deliver only the required drug-nanoparticle concentration at the target site and reduce side effects. Increased permeability of the blood-brain is reversible and is not associated with an increase in brain immune response.

Chemical delivery systems. Improvements in the properties of the drugs as related to their brain specificity and prolonged effects are required. Drug targeting to specific receptors has been one of the main objectives of medicinal chemistry for several years. Chemical delivery systems consist of biologically inert molecules that enhance drug delivery to a certain organ and require several steps of chemical or enzymatic reactions. Chemical delivery systems are unique in that after reaching the brain, the dihydropyridine carrier portion of the molecule oxidizes enzymatically to give rise to a quaternary pyridinium salt. This substance, because of its hydrophilic character, is trapped in the CNS and releases the drug by hydrolysis. Pyridinium salt is rapidly excreted. The chemical delivery systems concept is a step ahead of the prodrug concept. Although prodrug is a direct chemical predecessor of the drug, it cannot solve the organ and site specificity problems. This concept has been applied to several drugs including antibiotics, neurotransmitters, anticancer drugs, antidepressants, and antiepileptic drugs.

Biomaterials for local drug delivery in the central nervous system. Local drug delivery in the tissues of the central nervous system provides a solution for the problems of physiological barriers and systematic toxicity of drugs. Local drug delivery systems using biomaterials are being developed using biomaterials such as biodegradable polymers for sustained release, local parameter-responsible release, and regional cell-selective active targeting release in the central nervous system (08).

Cell-based drug delivery. Cell therapy of neurologic disorders is the topic of another Medlink Neurology article. Apart from their use as therapeutic agents, cells are also used as carriers of therapeutic agents for delivery. A special example is that of genetically engineered cells that continue to release therapeutic proteins after implantation in the body.

Cell delivery is an important component of tissue engineering for regenerative therapy of CNS disorders. Controlled release of biomolecules can facilitate cell engraftment, modulate inflammatory response or otherwise benefit the behavior of the delivered cells. Technologies include encapsulated cell therapy, biomolecule delivery in polymeric nanospheres for nerve regeneration as well as endogenous cell stimulation, and combination of neural stem cell or mesenchymal stem cell with biomolecule delivery for tissue regeneration and repair (17).

Mesenchymal stem cells. Mesenchymal stem cells are promising candidates for the development of cell-based drug delivery systems for autoimmune inflammatory diseases, such as multiple sclerosis. A study has investigated the effect of Ro-31-8425, an ATP-competitive kinase inhibitor, on the therapeutic properties of mesenchymal stem cells (28).

Gene transfer. See article on gene therapy.

Controlled-release microchip. The conventional controlled drug release from polymeric materials is in response to specific stimuli such as electric and magnetic fields, ultrasound, light, enzymes, etc. Microchip technology has been applied to achieve pulsatile release of liquid solutions. Solid-state silicon microchips have been developed that incorporate micrometer-scale pumps and flow channels to provide controlled release of single or multiple chemical substances on demand. The release mechanism is based on the electrochemical dissolution of thin anode membranes covering microreservoirs filled with chemicals in various forms. Various amounts of chemical substances in solid, liquid, or gel form can be released either in a pulsatile or continuous manner or a combination of both. The entire device can be mounted on the tip of a small probe or implanted subcutaneously in the body. In the future, proper selection of a biocompatible material may enable the development of an autonomous controlled-release implant that has been dubbed as “pharmacy-on-a-chip” or a highly controlled tablet (“smart tablet”) for drug delivery in CNS disorders. A successful human clinical trial with an implantable, wirelessly controlled, and programmable microchip-based drug delivery device has been conducted in osteoporosis patients receiving the drug teriparatide. Drug delivery for multiple sclerosis with the same device is in preclinical development. Such a device would also be useful in the treatment of conditions such as Parkinson disease.

Clinical applications

	• Knowledge of the anatomy and physiology of the blood-brain barrier has been exploited to develop strategies for drug delivery across the barrier to the brain.
	• The aim is controlled and targeted drug delivery to the brain.
	• Most of the methods are experimental, but some have been used clinically.

Of the various approaches described, the following have been used clinically:

Introduction of therapeutic substances into the CSF.

	• An intrathecal approach, with various types of implanted pumps, has been used for administration into the CSF of morphine for cancer pain and baclofen for spasticity. Targeted intrathecal drug delivery systems are an option for the treatment of patients with moderate to severe chronic refractory pain when more conservative approaches fail (13).
	• Therapeutic recombinant proteins can be introduced into the brain via this route.
	• Implantation of genetically engineered encapsulated cells producing ciliary neurotrophic factor in the spinal subarachnoid space is an example of gene delivery into the CNS via this route. Deposition of transgenic constructs into the subarachnoid space or the ventricular system is likely to be an efficient route for transducing not only the subependymal region but also for disseminating products of gene expression into the brain.

Osmotic opening of the blood-brain barrier. The clinical experience with osmotic opening of the blood-brain barrier is based on intracarotid injections of an inert hypertonic solution (generally mannitol). The osmotic method has been shown to be clinically effective in humans and has been used to facilitate the entry of anticancer agents into brain tumors in phase 3 trials. Although this approach increases the efficacy of the cytotoxic drugs, it also increases their neurotoxicity by increasing the permeability of the blood-brain barrier of the normal brain. This approach has also been used to facilitate the delivery of adenoviral vectors for gene therapy of brain tumors and for the administration of bifunctional fusion proteins of tumor-specific monoclonal antibodies for the treatment of brain tumors. Opening of the blood-brain barrier facilitates the entry of superparamagnetic iron oxide conjugates used as adjuncts to MRI for diagnosis of brain metastases.

Intra-arterial administration of therapeutic substances for CNS disorders.

	• Acrylic and thrombosis inducing material for occluding arteriovenous malformations
	• Thrombolytic agents such as tissue plasminogen activator for treatment of arterial thrombosis in stroke patients. The approved method of administration is intravenous, but the intra-arterial route is under investigation.
	• Vasodilators such as papaverine for treatment of vasospasm of cerebral arteries
	• Diazepam has been administered intra-arterially as an anesthetic during cerebral angiography and was found to be safe.
	• Direct injection of gene vectors into the arterial circulation of the brain
	• Intra-arterial administration of chemotherapy for brain tumors

Intra-arterial administration of tissue plasminogen activator in stroke patients. The method of administration of thrombolytic agents such as tissue plasminogen activator is important for evaluating the results of treatment in stroke patients. Although the approved method is intravenous administration, considerable experience exists with the use of intra-arterial thrombolysis in patients in whom the lesions have been demonstrated by angiography prior to thrombolysis. Angiography is necessary for monitoring clot lysis during treatment, and a specially trained physician is required to maneuver the catheter to the site of occlusion. Despite this limitation and the risk of the complications of angiography, hemorrhages are less common with intra-arterial treatment because less of a drug is generally used. Physical manipulation of the clot may aid thrombolysis, and more of a drug can be delivered to and within the clot. Occlusion of the internal carotid artery in the neck does not respond well to intravenous thrombolysis if the stroke is due to embolization of the carotid clot to block the middle cerebral artery. Neck occlusion diminishes the amount of intravenously administered drug that can reach the middle cerebral artery clots under these circumstances. The intra-arterial approach may be more successful in these situations.

Anti-HIV drug delivery for treatment of AIDS encephalopathy. The blood-brain barrier hinders the penetration of anti-HIV drugs into the brain, promoting viral replication, the development of drug resistance, and, ultimately, subtherapeutic concentrations of drugs reaching the brain, leading to therapeutic failure. A penetration-effectiveness ranking concept for quantifying antiretroviral drug penetration into the CNS clearly could be considered as a guide for selecting relatively more effective treatment strategies from the available drugs. The specificity and efficiency of anti-HIV drug delivery can be further enhanced by using nanocarriers with specific brain-targeting, cell-penetrating ligands (45).

Drug delivery to the brain in Parkinson disease. Glial cell line-derived neurotrophic factor (GDNF) is potentially useful in the treatment of Parkinson disease, but penetration into brain tissue from either the blood or the cerebrospinal fluid is limited. Although safety and potential efficacy of unilateral intraputaminal glial cell line-derived neurotrophic factor infusion via a catheter in patients with advanced Parkinson disease has been demonstrated in pilot clinical studies, the method has not been adopted in clinical practice because of the invasive nature.

Drug delivery to the brain in Alzheimer disease. Several routes of drug delivery other than oral have been explored for the management of Alzheimer disease. Transdermal rivastigmine maintains steady drug levels in the bloodstream, improving tolerability and allowing a higher proportion of patients to receive therapeutic doses compared to the capsule form of the medication. Several studies are exploring the nasal route of drug delivery for Alzheimer disease.

Biological therapies for the treatment of Alzheimer disease that exploit mechanisms of penetration of the blood-brain barrier include peptides, vaccines, antibodies, and antisense oligonucleotides. Currently, the use of nanotechnology-based drug delivery systems appears to be a promising direction for therapy of Alzheimer disease (46).

An experimental study demonstrated that the brain-targeting efficiency of intravenously injected rivastigmine encapsulated in hybrid polymer nanoparticles can be enhanced 5.4-fold compared to the drug solution (35). The results suggest that the fabricated biohybrid delivery system was able to circumvent the blood-brain barrier and is expected to minimize systemic side effects of rivastigmine.

Drug delivery in epilepsy. Special methods of drug delivery would improve the control of seizures, reduce toxic effects, and increase compliance in patients with epilepsy, such as by use of long-acting formulations and subcutaneous implants. In emergency situations, administration via intravenous, rectal, nasal, or buccal mucosa can deliver the drug more quickly than oral administration. Early termination of prolonged seizures with intravenous administration of benzodiazepines is known to improve outcomes. Results of a double-blind, randomized noninferiority trial show that for subjects in status epilepticus, intramuscular midazolam is at least as safe and effective as intravenous lorazepam for prehospital seizure cessation (40). Antiepileptic drugs have been delivered effectively to seizure foci through programmed infusion pumps, which are triggered to act in response to computerized EEG seizure detection. Cell and gene therapies may prove to be more effective forms of drug delivery in epilepsy.

Overexpression of P-glycoprotein and other efflux transporters in the cerebrovascular endothelium, in the region of the epileptic focus, may also lead to drug resistance in epilepsy. This hypothesis is supported by the findings of elevated expression of efflux transporters in epileptic foci in patients with drug-resistant epilepsy, induction of expression by seizures in animal models, and experimental evidence that some commonly used antiepileptic drugs are substrates. Further studies to delineate the exact role, if any, of P-glycoprotein and other efflux transporters in drug-resistant epilepsy are warranted.

Innovative methods of drug delivery for glioblastoma multiforme. Drug delivery to brain tumors can be facilitated by targeting receptor- or carrier-mediated transport mechanisms across the blood-brain barrier used by drug conjugates, nanoparticles, polymer-based nanocarriers, siRNA, and antibodies (31). Various methods for drug delivery to the glioblastoma multiforme are shown in Table 4.

Table 4. Methods of Drug Delivery to Glioblastoma Multiforme

	• Design of anticancer drugs with higher penetration across the blood-brain barrier
		- Nitrogen mustard/nicotinic acid conjugate - Pegylated liposomal doxorubicin - Hypothalamic growth hormone-releasing hormone antagonists
	• Intravascular delivery of anticancer agents
		- Intravenous transferrin-bearing anticancer agents for targeted delivery - Intra-arterial chemotherapy
	• Local delivery of chemotherapeutic agents to the tumor
		- Biodegradable polymer wafers: carmustine implants - Biodegradable microspheres containing 5-fluorouracil - Magnetically controlled microspheres - Intraoperative local chemotherapy with fibrin glue containing anticancer agents
	• Introduction of the chemotherapeutic agent into the CSF pathways
		- Intraventricular injection - Intrathecal injection
	• Increasing the permeability of the blood-brain barrier
		- Focused ultrasound guided by MRI - Intracarotid infusion of the bradykinin analog lobradimil (formerly RMP-7) - Synthetic form of bradykinin -- NS-1619
	• Chemotherapy sensitization
		- Use of thermosensitive liposomes and localized hyperthermia - Photodynamic therapy for chemosensitization
	• Brachytherapy: implantation of interstitial radiation-emitting seeds into the tumor • Convection-enhanced drug delivery • Interstitial delivery of dexamethasone for treatment of peritumoral edema • Targeted monoclonal antibodies conjugated with liposomes • Photodynamic therapy for chemosensitization • Boron capture therapy • Cell therapy
		- Chimeric antigen receptor (CAR) T cell therapy into tumor or ventricles - Mesenchymal stem cells engineered to deliver therapeutic substances - Neural stem cells engineered to deliver therapeutic substances
	• Gene therapy

Biodegradable polymer implants containing anticancer drugs. Polymer-based drug delivery to the brain has special applications for the delivery of anticancer agents to malignant brain tumors. One example is the use of carmustine implants. One of the problems with surgical excision of malignant glioblastoma is local recurrence within 2 cm of the primary lesion. Strategies to prevent local recurrence include implantation of delivery devices containing chemotherapeutic agents. Biodegradable polymer impregnated with carmustine (Gliadel) has been implanted into the tumor cavity after surgery. This product has now been approved by the United States Food and Drug Administration as second-line therapy for glioblastoma multiforme.

Fibrin glue implants containing anticancer drugs. Fibrin glue, which is widely used in neurosurgery, is an intriguing substrate for local chemotherapy of brain tumors. Fibrin glue is used for reinforcing dura mater to prevent CSF leaks and can be prepared in the operating room. The biological glue consists of a heat-treated and lyophilized human fibrinogen component including coagulation factor X III and human thrombin, a solution of aprotinin and a solution of calcium. The material is available commercially. Local intraoperative chemotherapy in the brain tumor cavity using fibrin glue containing nitrosourea has been shown to be useful for the local control of malignant gliomas in animal models, but this has not proceeded to clinical development because of the introduction of Gliadel, which has a similar principle of local application of chemotherapy and is now established in clinical use.

Magnetically controlled microspheres. Polylactic acid microspheres containing iron oxide labeled with yttrium-90 have been administered intraperitoneally in rats and guided to the tumor site by magnets placed over the tumor site. Radioactivity emitted by the microspheres is mostly confined to the tumor. There has been no clinical application of this technique in humans.

Liposomes. A brain-targeted doxorubicin liposome, 2B3-101A, coated with the endogenous antioxidant glutathione at the tips of polyethylene glycol can safely enhance the delivery of free drug to the brain (19). In preclinical studies, treatment with 2B3-101 resulted in a significant inhibition of brain tumor growth, which was stronger than the effect of pegylated liposomal doxorubicin alone. 2B3-101A is now in phase I/IIa clinical trials in the Netherlands.

Transferrin-bearing anticancer agents. Intravenous delivery of transferrin-bearing anticancer agents is targeted to gliomas that have abundant transferrin receptors on the surface (15).

Thermoliposomes containing cytotoxic drugs. Thermosensitive liposomes are microscopic vesicles that can contain drugs and release them in response to hyperthermia. Thermosensitive liposomes have been used to deliver the anticancer agent cisplatinum in conjunction with localized brain heating in experimental malignant gliomas in rats.

Meningeal cancer. Chemotherapy is given by intraventricular and intrathecal injection but has serious toxic effects on the brain. A randomized study concluded that there was no difference for patients treated with sustained-release cytarabine, whether the route used was intrathecal or intraventricular, but for patients treated with short-acting drugs such as methotrexate, there was a statistically significant difference favoring patients receiving intraventricular therapy (22).

The development of monoclonal antibodies that recognize tumor-associated antigens has raised the possibility of creating more tumor-specific therapeutic agents. Antibodies have been conjugated with radionucleotides and various peptide toxins to create new drugs with high tumor selectivity in vitro. Attempts to develop these compounds for clinical use are limited by transcapillary and interstitial barriers encountered during delivery to solid tumors. In meningeal cancer, the tumor cells often grow as thin sheets based in CSF, which reduces the problem of drug delivery and tissue penetration.

Intrathecal therapy with DepoCyt. DepoCyt, an injectable sustained-release formulation of cytarabine encapsulated in multivesicular lipid-based particles, is available for the treatment of lymphomatous meningitis. DepoCyt is administered intrathecally to overcome the poor penetration of most anticancer agents into the brain. This is direct administration of anticancer agents into the CSF pathways either by a lumbar puncture or by a reservoir catheter implanted into the ventricular cavities of the brain.

Increasing the permeability of blood-brain barrier to anticancer drugs. Calcium-activated potassium channels are overexpressed in brain tumor endothelial cells compared with normal brain tissue and play a pivotal role in blood-brain tumor barrier permeability regulation. Intravenous infusion of potassium channel agonists and bradykinin selectively enhance blood-brain tumor barrier permeability to enhance selective delivery of chemotherapeutic drugs to metastatic brain tumors in rat models, but this method has not been used clinically.

Intra-arterial chemotherapy. The blood-brain barrier is damaged in glioblastoma, and chemotherapeutic agents can reach the tumor directly by an intra-arterial route. Despite the theoretical advantage, the benefit of intra-arterial chemotherapy remains unproven.

Interstitial delivery of dexamethasone for reduction of peritumor edema. Dexamethasone is used for reducing cerebral edema due to a number of causes, including brain injury. Malignant tumors are surrounded by a normal brain that swells and increases intracranial pressure. High-dose systemic dexamethasone has been used to relieve this edema but can be associated with side effects. Controlled-release polymers implanted directly into the brain for interstitial delivery of dexamethasone have been used to achieve high, local concentrations of the drug while minimizing the plasma concentrations.

Photodynamic therapy for chemosensitization. The more traditional name for this therapy is photoradiation therapy. It depends on the selective retention of a photosensitizer within a tumor followed by activation of the sensitizer by irradiating the tumor with the light of an appropriate wavelength, such as a laser. This method has been applied to the treatment of malignant gliomas of the brain. Porphyrin compounds have been used for this purpose historically as well. Photodynamic properties of porphyrins were known to German scientists at the turn of the last century. Several hematoporphyrin derivatives are selectively retained by the malignant tissues.

The second generation of photodynamic therapy is more selective for tumor cells than earlier forms. A boronate porphyrin compound is used as a chemosensitizing agent for malignant brain tumors, and nonthermal light is used to achieve a selective photochemical destruction of cancer cells with minimal effect on the surrounding tissues. The compound is concentrated 200 times more in the tumor than in the surrounding brain. Boronated porphyrin may be effective in targeting the metastatic nests of brain cancer that remain embedded in the periphery of the primary tumor but are not detectable or accessible by surgery.

Boronated porphyrin requires much less light for activation than do other photodynamic agents, which results in greater tumor destruction in less time. This reduction, in time, is expected to speed up neurosurgical procedures in which boronated porphyrin is used. Boronated porphyrin may also be useful for boron neutron capture therapy. The neutron beam can cause the disintegration of boron atoms within boronated porphyrin with the release of alpha particles and gamma rays, which are lethal to cancer cells.

Cell therapy. This is described in an article on cell therapy. One example relevant to glioblastoma is that of chimeric antigen receptors (CAR) T cells, which combine the antigen binding site of a monoclonal antibody with the signal activating machinery of a T cell, freeing antigen recognition from MHC restriction and, thus, breaking one of the barriers to more widespread application of cell therapy. Multiple infusions of CAR-T cells targeting the tumor-associated antigen interleukin-13 receptor alpha 2 (IL13Rα2) were administered to a patient with recurrent multifocal glioblastoma through two intracranial delivery routes—infusions into the resected tumor cavity followed by infusions into the ventricular system (06). There were no serious toxic effects and regression of all intracranial and as well as spinal tumors was observed, along with corresponding increases in levels of cytokines and immune cells in the CSF.

Gene therapy for glioblastoma multiforme. See article on gene therapy of glioblastoma multiforme.

Innovative methods of drug delivery for neurodegenerative disorders. Various innovative methods for drug delivery for neurodegenerative disorders include cell therapy, gene therapy, and neurotrophic factors.