Clinical aspects

Description

	• Clinical trials of different neurotrophic factors for neurodegenerative disorders have been conducted, but no approved method for clinical use has emerged.
	• A major problem in neurotrophic factor therapy is delivery to the nervous system.
	• The most promising method of application to the CNS is gene therapy, including antisense approaches and implantation of genetically modified cells secreting neurotrophic factors.

Over the past quarter of a century, numerous clinical trials of different neurotrophic factors for neurodegenerative disorders employing various routes of administration have been conducted, but no approved method for clinical use has emerged. Some of the reasons for this are the advanced stages of diseases at the time of treatment and lack of a suitable method of delivery to the central nervous system (03).

A major problem in neurotrophic factor therapy is the method of administration. It is unlikely that neurotrophic factors will be available as pills, as they are proteins and are liable to be rapidly destroyed in the digestive tract before they can be absorbed. Delivery of neurotrophic factors to the CNS is problematic because the large molecules cannot cross the blood-brain barrier. Various methods of administration are shown in Table 1.

Table 1. Delivery Systems for Neurotrophic Factors

Systemic administration
	• Subcutaneous injection • Intramuscular injection • Combined with blood-brain barrier permeabilizers
Carrier-based delivery systems to facilitate crossing of the blood-brain and nerve barrier
	• Lipid-binding carrier systems • Chimeric drugs: coupling the neurotrophic factor to a peptide • Conjugated prodrug systems • Short fragments of nerve growth factor (small molecules) as nerve growth factor-analogs • Antiferritin receptor antibodies • Activated T-lymphocytes
Opening of blood-brain barrier by focused ultrasound to facilitate delivery
Intracerebral administration
	• Injection into cerebral parenchyma
		- localized infusion - convention-enhanced delivery to large areas of the brain
	• Intracerebroventricular injection
		- single injection - via an implanted cannula
	• Pumps
Intrathecal administration
	• Injection into subarachnoid space by lumbar puncture • Chronic delivery into subarachnoid space by catheter
Direct application to the CNS surface: nitrocellulose strips or foams impregnated with neurotrophic factors
Transplantation of cells secreting neurotrophic factors
	• Implantation of polymer encapsulated cells in a specific brain region • Immortalized cell: neural cells and glial lines
Use of neurotrophic factor mimics
	• Peptide mimetics • Neuroimmunophilins
Antisense technique
Gene therapy
	• Engineered stem cells secreting neurotrophic factors

Systemic methods of administration are convenient because the dosage and treatment schedules can be controlled efficiently. Subcutaneous injections can be effective for peripheral neuropathies and motor neuron disease because neurotrophic factors travel by retrograde transport along the peripheral nervous system. Disadvantages of systemic administration for treatment of CNS disorders are:

• High system doses are required to reach significant CNS levels, which produce undesirable peripheral side effects • Repeated injections are required • Inadequate blood-brain barrier penetration • Nonlocalized action throughout the CNS

Several methods are being developed to overcome the difficulty of blood-brain barrier penetration. Direct application to the CNS requires neurosurgical approaches. Knowledge of the pattern of trafficking of a neurotrophic factor within the brain parenchyma can be used to select the route of delivery. For example, spread of brain-derived neurotrophic factor following intraventricular injection is minimal due to the binding to the TrkB receptor along the ventricular wall, but intraparenchymal injections result in widespread retrograde transport throughout the brain. Encapsulated cell biodelivery is an efficient tool for the delivery of nerve growth factor in Alzheimer disease (14). Focused ultrasound opens the blood-brain barrier locally, reversibly, and noninvasively to allow intravenous administration of neurotrophic factors, which ameliorates the damage to the nigrostriatal dopaminergic pathway in the subacute mouse model of early-stage Parkinson disease (10).

Currently, the most promising method of application to the CNS is gene therapy. Antisense and implantation of genetically modified cells that secrete neurotrophic factors are also technically a gene therapy approach and are discussed in other clinical summaries on gene therapy for neurologic disorders.

The goal is to successfully apply neurotrophic factor therapy in the management of degenerative neurologic disorders.

Indications

Neurotrophic factors are being investigated for potential clinical applications in the following neurodegenerative disorders:

• Alzheimer disease • Parkinson disease • Amyotrophic lateral sclerosis • Huntington disease

Contraindications

None

Outcomes

Alzheimer disease. The first Alzheimer disease patient treated with intracerebroventricular nerve growth factor showed improvement in episodic memory but not of cognitive impairment and had no adverse effects. This approach has been discontinued.

A phase 1 clinical study for Alzheimer disease using cells extracted from the patient's skin and genetically modified to express nerve growth factor has been completed. The cells were administered by a stereotactic procedure into the nucleus basalis of Meynert (25). The subjects showed a reduced rate of decline that persisted throughout the period of the study. A commercial product (Cere-110) that carries the nerve growth factor (NGF) gene encased in an adeno-associated virus coating for protection and facilitates its delivery to brain cells was prepared for phase 2 clinical trials (24). A double-blind, placebo-controlled, randomized, multicenter study is evaluating Cere-110 gene delivery in subjects with mild to moderate Alzheimer disease.

Postmortem examination of brains of Alzheimer disease patients from clinical trials of NGF gene therapy conducted between 2001 and 2012 and survival times ranging from 1 to 10 years after treatment showed a trophic response to NGF in the form of axonal sprouting (26). NGF therapy for Alzheimer disease appears safe and effective over extended periods.

Parkinson disease. Glial cell line-derived neurotrophic factor (GDNF), cerebral dopamine neurotrophic factor (CDNF), and mesencephalic astrocyte-derived neurotrophic factor (MANF) have shown neuroprotective and restorative effects on nigral dopaminergic neurons in various animal models of Parkinson disease. Results of microdialysis studies show that that GDNF, CDNF, and MANF have divergent effects on dopaminergic neurotransmission, as well as on dopamine synthetizing and metabolizing enzymes (16).

Intracerebral administration of glia cell-derived neurotrophic factor or brain-derived neurotrophic factor to rats with selective destruction of the dopamine-producing cells in substantia nigra has been shown to improve motor function. Growth factor infusion stimulates endogenous stem cells and causes their migration toward the striatum. Improvement is due to delayed retrograde degeneration of the nigrostriatal nerve fibers. Lentiviral delivery of glial cell line-derived neurotrophic factor using a lentiviral vector system can prevent nigrostriatal degeneration and induce regeneration in primate models of Parkinson disease. This procedure might be a viable therapeutic strategy for Parkinson disease patients.

The effectiveness of glial cell line-derived neurotrophic factor in human patients, however, needs to be demonstrated. Clinical trials with intracerebroventricular infusion of glial cell line-derived neurotrophic factor in Parkinson disease patients are have been carried out. Further steps in the development of neurotrophic factor therapy of Parkinson disease include design of a small molecule that mimics the protein's effect but that can cross the blood-brain barrier and can be administered systemically. FK-binding protein neuroimmunophilin, a neuroprotective neurotrophic factor-like compound, is in phase 1 clinical trials for Parkinson disease.

Future transplantation approaches to Parkinson disease will focus on the use of genetically modified cells carrying genes for dopamine-synthesizing enzymes or neurotrophic factors.

Glial cell line-derived neurotrophic factor was delivered directly into the putamen of 5 Parkinson disease patients in a phase 1 safety trial (08). There was improvement and no serious clinical side effects. Medication-induced dyskinesias were reduced by 64%.

Currently, the use of neurotrophic factors remains the best option for the prevention of the progression of Parkinson disease. It is superior to enzyme replacement strategies. The choice of neurotrophic factor is open, and glial cell-derived neurotrophic factor alone or in combination with other neurotrophic factors is under consideration. Cerebral dopamine neurotrophic factor (CDNF) and mesencephalic astrocyte-derived neurotrophic factor (MANF) are gaining attention for Parkinson disease due to their neurotrophic effects on dopaminergic neurons. In various animal models of Parkinson disease, CDNF is efficient in protecting and repairing dopaminergic neurons, and it inhibits endoplasmic reticulum stress, neuroinflammation, and apoptosis (22). Beneficial effects of CDNF and MANF in in vitro and in vivo experimental models of Parkinson disease justify further investigation (19).

The secretion of neurotrophic factors combined with activation of specific receptors of transplanted human mesenchymal stem cells is an alternative mechanism for neuroprotection of degenerating neurons in Parkinson disease.

An adeno-associated viral vector carrying the gene for neurturin, CERE-120, is in phase II clinical trials for Parkinson disease (NCT00985517). The initial data from phase I studies supported the safety, tolerability, and potential efficacy of CERE-120. A double-blind, controlled, phase 2a trial showed clinically meaningful benefit at the end of 18 months (02). The NIH is also conducting a phase 1 open-label dose escalation safety study of convection-enhanced delivery of adeno-associated virus encoding glial cell line-derived neurotrophic factor in subjects with advanced Parkinson disease (NCT01621581).

Amyotrophic lateral sclerosis. Ciliary neurotrophic factor was the first neurotrophic factor to be considered for the treatment of amyotrophic lateral sclerosis. Subsequently, several other neurotrophic factors were tested.

Downregulation of genes involved in axon outgrowth and synapse formation in adult mice suggests the role of vascular endothelial growth factor (VEGF) in the maintenance of neuronal circuitry. Dysregulation of VEGF may lead to neurodegeneration through synaptic regression and forms the basis for its use in amyotrophic lateral sclerosis.

Clinical trials of neurotrophic factors in amyotrophic lateral sclerosis. The use of neurotrophic factors for amyotrophic lateral sclerosis remains promising but with guarded optimism because of several failures in clinical trials. A phase 2 randomized, double-blind, placebo-controlled multicenter study has been completed evaluating the safety, tolerability, and efficacy of autologous cultured mesenchymal bone marrow stromal cells secreting neurotrophic factors (MSC-NTF) as a treatment for patients with amyotrophic lateral sclerosis (NCT02017912). Phase 3 of this trial is testing intrathecal MSC-NTF by standard lumbar puncture (NCT03280056).

Another phase 2 clinical trial was done to test the efficacy of a new neurotrophic factor, GM604, that controls and modulates several amyotrophic lateral sclerosis genes with positive effects interactively and dynamically through multiple pathways. It is involved in neuroprotection, neurogenesis, neural development, neuronal signaling, neural transport, and other processes. The efficacy as well as disease progression was monitored by biomarkers (NCT01854294).

Huntington disease. Nerve growth factor may be useful in the treatment of Huntington disease. Preclinical studies in animal models indicate that brain-derived neurotrophic factor may have beneficial effects in the treatment of Huntington disease. Direct intrastriatal ciliary neurotrophic factor infusion in a primate model of Huntington disease has been demonstrated to restore cognitive and motor functions. Based on studies in animal models, adenovirus-mediated ciliary neurotrophic factor gene transfer appears to be a potentially useful delivery system for widespread, long-term neuroprotection in Huntington disease patients.

Transplantation of fetal neuroblast cells secreting ciliary neurotrophic factor into a few patients with Huntington disease appeared to improve motor and cognitive function in patients several years ago, but confirmation of these results in a larger group of patients is required but has not been carried out.

There is loss of brain-derived neurotrophic factor in the striatum of Huntington disease patients, which raises the possibility that 1 of the mechanisms triggering the reduction in this neurotrophic factor may also affect the activity of many other neuronal proteins, and attempts to restore these proteins should be of therapeutic interest. Bone marrow-derived mesenchymal stem cells can be induced to secrete neurotrophic factors and ameliorate behavioral symptoms, as well as reduce striatal lesions when transplanted into brain of a quinolinic acid rat model of excitotoxicity (18). This is a promising basis for developing treatment for Huntington disease. There are no ongoing clinical trials of neurotrophic factors in Huntington disease.

Adverse effects

Safety studies of neurotrophic factors in experimental animals have shown that weight loss and hyperalgesia are 2 main adverse effects.

Weight loss. Weight loss is seen frequently with neurotrophins such as nerve growth factor and brain-derived neurotrophic factor. Animal experimental studies suggest that brain-derived neurotrophic factor may induce appetite suppression and weight loss through a central mechanism.

Hyperalgesia. Subcutaneous injections of nerve growth factor can cause mechanical and heat hyperalgesia. Various explanations for this are:

	• Nerve growth factor is increased at sites of inflammation, indicating that it has a role in mediating inflammatory pain. Anti-nerve growth factor antibody administration can inhibit inflammatory hyperalgesia.
	• Nerve growth factor is trophic for neurons most involved with pain sensation: small fiber sensory neurons and sympathetic neurons.
	• Nerve growth factor stimulates the expression of substance P and calcitonin gene-related peptide, both of which potentiate pain transmission.
	• Nerve growth factor induces degranulation of mast cells, resulting in release of activators of nociception: histamine and serotonin.
	• A more likely explanation for this adverse effect is sprouting of nerve fibers induced by nerve growth factor.

Adverse reactions observed in clinical trials.

	• Subcutaneous brain-derived neurotrophic factor: injection site reactions (erythema, pruritus, and edema) and gastrointestinal symptoms (bowel urgency and diarrhea).
	• Subcutaneous ciliary neurotrophic factor SC: asthenia, fever, chills, nausea, weight loss, and increased cough or sputum production.
	• Intrathecal ciliary neurotrophic factor: CSF pleocytosis (primarily lymphocytic) and rises in protein levels. Severe headache and leg and back cramping have been reported.
	• Intracerebroventricular injection of nerve growth factor: intractable limb pain due to aberrant sprouting of nerve fibers in dorsal root ganglia, which are bathed with CSF and accessible to the nerve growth factor. Other events have been reported are:
		- Weight loss - Sudden electrical pain sensations - Bent posture indicative of severe pain - Confusion - Insomnia

No relevant information is available pertaining to the prognosis.

Special considerations

No experience exists concerning the use of neurotrophic factors during pregnancy. This is not a usual condition in patients with the neurodegenerative disorders discussed in this article.

Scientific basis

	• Neurotrophic factors provide a promising therapeutic approach to neurodegenerative diseases because of their potential for neurorestoration and neuroprotection.
	• Neurotrophic factors also mediate the beneficial effects of other therapies for neurodegenerative disorders.

Neuronal specificity is relevant, and some neurotrophic factors have a trophic effect on more than one type of neuron and are potentially effective for more than one type of disorder as shown in Table 2.

Table 2. Neuronal Specificity of Neurotrophic Factors in Relation to Neurodegenerative Disorders

Disease	Neurons involved	NGF	BDNF	NT-3	NT-4/5	CNTF	IGF-1	FGF	GDNF
PD	Nigral dopamine neurons	0	++	++	+++	+	0	++	+++
HD	Striatal GABA neurons	0	+++	++	+++	0	0	0	0
AD	Cholinergic neurons	+++	++	+	+++	+	0	++	++
	Hippocampal neurons	0	++	++	++	0	0	0	0
	Cortical neurons	0	+++	0	0	0	0	0	+
ALS	Spinal motor neurons	0	+++	++	+++	+++	+	++	+++
0 = ineffective or no evidence available + = slight effect ++ = moderate effect +++ = pronounced effect

Brain-derived neurotrophic factor, neurotrophin-4/5, and glial growth factor are versatile in this respect.

Failed axonal transport of neurotrophic signals is a common property of degenerating neurons. Although it is downstream from the main pathogenetic events, it is important for understanding as well as treating these disorders.

Rationale for therapeutic use of neurotrophic factors. Neurotrophic factors provide a promising therapeutic approach to neurodegenerative diseases because of their potential for neurorestoration and neuroprotection. Neurotrophic factor therapy is based on the evidence that growth factors stimulate the survival of neurons that degenerate in neurologic disorders.

The concept of neuroprotection, originally applied to acute neurologic conditions, has now been extended to chronic diseases of the brain characterized by neurodegeneration because some of the basic mechanisms of damage to the central nervous system are similar in these conditions. Neurotrophic factors have significant neuroprotective effects in neurodegenerative disorders.

Neurotrophic factors also mediate the beneficial effects of other therapies for neurodegenerative disorders. Investigations have been mainly performed in vitro and in animal models by using mesenchymal stem cells generally obtained from umbilical cord, bone marrow, or adipose tissue. Positive results have led to several clinical trials, which have shown safety and efficacy. Although the exact mechanisms of mesenchymal stem cell-induced beneficial effects are not quite clear, they include stimulation of neurogenesis and angiogenesis and antiapoptotic, immunomodulatory, and antiinflammatory actions, which are mostly exerted through their paracrine expression of neurotrophic factors and cytokines (13). Mesenchymal stem cell transplantation can alleviate motor impairments and cerebellar atrophy in the ataxic mouse model, probably via promoting certain neurotrophic factors (31). Therapeutic benefits of stem cell therapy for neurodegenerative disorders can be provided by genetic modification of these cells to transfer genes for neurotrophic factors (11).

Clinical trials with neurotrophic factors in neurodegenerative disorders. As of August 2021, 83 clinical trials of neurotrophic factors for neurodegenerative diseases are listed on the U.S. Government website for clinical trials, which can be accessed at the following link:clinicaltrials.gov. These include 25 for Parkinson disease, 29 for Alzheimer disease, 16 for amyotrophic lateral sclerosis, and 5 for Huntington disease. None of the trials for Huntington disease are testing neurotrophic factor administration but other drugs that may promote in vivo production of neurotrophic factors.

Several clinical trials of therapeutic intervention have failed, and it remains to be shown if neurotrophic factors can rescue damaged cells in the brain and spinal cord of patients with these disorders. The inability in crossing the blood-brain barrier and the lack of selectivity are some of the most highlighted limits of neurotrophic factor-based therapy.

Alzheimer disease. Levels of several neurotrophic factors are altered in cerebrospinal fluid and peripheral blood from Alzheimer disease patients as well as in animal models of this disease. Several tests for diagnostic as well as prognosis of Alzheimer disease are based on these changes, and neurotrophic factors are also a promising therapeutic approach (21).

Neurotrophic factors play a role in the pathogenesis of Alzheimer disease because neuropil threads can spread throughout the cerebral cortex in this condition and are the evidence for neuronal sprouting. Dystrophic neurites are a common element of senile plaques, which are also considered to stimulate sprouting. Hippocampal cholinergic sprouting occurs in Alzheimer disease as a response to gradual denervation. An increase in neurotrophic activity takes place in the brains of patients with Alzheimer disease as demonstrated by the ability of extracts of these brains to increase neuronal survival. However, decreased amounts of nerve growth factor at the level of basal forebrain cholinergic neurons are largely due to failed retrograde transport rather than decreased synthesis, binding, or expression of nerve growth factor receptors. Levels of a precursor of nerve growth factor increase during the preclinical stage of Alzheimer disease and may reflect an early biological marker for the onset of the disease.

Beta-amyloid and its precursor beta-amyloid precursor protein have also been shown to promote neuronal survival, or neurite growth, or both in vitro, although other evidence suggests that these proteins may be neurotoxic. Despite the controversy, it is certain that Alzheimer disease is accompanied by an abnormal growth-promoting activity. Cortical neuritic dystrophy in Alzheimer disease represents a regenerating, sprouting response of neurons to partial deafferentation following cell loss or may represent a primary aberrant growth phenomenon leading to cell loss. Such changes could result from derangement of neurite-promoting and growth-inhibitory factors in an Alzheimer disease brain.

Nerve growth factor, platelet-derived growth factor (NGF), brain-derived neurotrophic factor (BDNF), and other neurotrophic factors have been implicated in the pathogenesis of Alzheimer disease (09). BDNF is involved in the regulation of age-related processes in the hippocampus, and disturbances in the BDNF-system also affect hippocampal dysfunctions as seen in Alzheimer disease. A selective decline of the BDNF/TrkB neurotrophic signaling pathway occurs in the frontal cortex and hippocampus in Alzheimer disease, which raises the possibility of use of BDNF as a therapeutic agent in these patients. BDNF is implicated in the mechanism of action of drugs that improve cognitive deficits in Alzheimer disease patients.

In Alzheimer disease, a decline in nerve growth factor levels occurs with a loss of nerve growth factor receptors in basal forebrain structures that are involved in memory and learning.

Nerve growth factor acts primarily through activation of its specific high-affinity TrkA tyrosine kinase receptor (p75NGFR). Reduced expression of TrkA has been shown in cholinergic neurons in Alzheimer disease patients, and restoration of TrkA expression in the surviving neurons may represent a target for developing neuroprotective therapies for Alzheimer disease.

Of the various neurotrophic factors, nerve growth factor is the most promising for the treatment of Alzheimer disease. Nerve growth factor is produced in the neocortex and the hippocampus and then retrogradely transported to the cholinergic neurons of the basal forebrain; Alzheimer disease is linked to deficits in this axonal transport. Implantation of genetically engineered cells secreting nerve growth factor in the brains of aging primates has been shown to reverse the reduction of cortical cholinergic innervation. The rationale of the use of nerve growth factor in Alzheimer disease is based on the following:

	• Degeneration of neurons in cholinergic and noradrenergic systems with loss of cell bodies of these neurons in the basal forebrain, which selectively express nerve growth factor receptors.
	• In animals, experimental lesions of cholinergic neurons innervating the hippocampus and the cerebral cortex result in pronounced deficits in memory and cognitive function, which are hallmarks of human Alzheimer disease. These deficits can be improved by administration of cholinergic drugs.
	• Nerve growth factor administration can prevent cholinergic denervation following experimental injury or associated with aging.

Delivery of nerve growth factor to the brain is a challenge. Bone marrow stem cells have provided an effective carrier for delivery of nerve growth factor into rat models of Alzheimer disease, and the administration of nerve growth factor-gene-modified stem cells may be considered as a potential strategy for the development of effective therapies for the treatment of Alzheimer disease.

Based on the demonstration of potential benefit in Alzheimer disease of amyloid beta clearance as well as nerve growth factor administration in separate studies, an approach combining both may be an effective therapeutic strategy (12).

Neurotrophic impairment during early development can be one of the pathogenic factors in the etiology of Alzheimer disease. In 3xTg-AD, a transgenic mouse model of Alzheimer disease, oral treatment during prenatal to early postnatal development with a neurotrophic compound, P021 (Ac-DGGLAG-NH2), prevented cognitive deficits at 4 months, reduced accumulation of tau at neurofibrillary pathology-associated sites, and decreased amyloid beta plaque load at 22 months (30). Therefore, this neurotrophic peptide mimetic is a potential early prevention strategy for Alzheimer disease. Peptides derived from nerve growth factor, brain-derived neurotrophic factor, and fibroblast growth factor have been developed to replace the deficient or dysregulated growth factors in Alzheimer disease with the following advantages (07):

	• Their structure can be designed to better interact with the growth factor receptors and to activate a specific downstream pathway.
	• Blood stability and pharmacokinetic properties of these peptides can also be improved by chemical modification.
	• Their small size facilitates the penetration of the blood-brain barrier to reach neuronal cells.

Parkinson disease. Lack of neurotrophic factor production may constitute the common underlying mechanism of various degenerative disorders of the nervous system. Neurotrophins play a role in the pathogenesis of Parkinson disease. Several neurotrophic factors have been investigated for possible application in the treatment of Parkinson disease.

Glial cell line–derived neurotrophic factor (GDNF). Promising results have been obtained with fibroblasts engineered to secrete GDNF or brain-derived neurotrophic factor or viral vectors expressing GDNF. Intrastriatal grafts of chromaffin cells exert beneficial effects in parkinsonian rats and release trophic factors such as GDNF and transforming growth factor-beta1. Of the various neurotrophic factors, GDNF is considered most suitable for the treatment of Parkinson disease. The aim of this therapy is to rescue the patient's own dopaminergic neurons, making neuron grafting unnecessary. Neuroprotection by endogenous GDNF stimulation may be a potential preventive therapy for Parkinson disease patients (06).

A major barrier to clinical translation has been delivery of GDNF. When administered by intracerebroventricular injection, it does not reach the target -- the putamen -- and intraputaminal infusion is ineffective, probably because of limited distribution within the putamen.

Chronic intraputaminal delivery of GDNF in Parkinson disease has been reported to result in clinical improvement, an increase in putamen 18F-dopa uptake on PET imaging, and neuropathologic evidence of sprouting of dopaminergic fibers. Long-term follow-up has provided clinical and PET evidence of persistent efficacy lasting for at least 3 years following cessation of GDNF infusion in a patient with Parkinson disease (15). Delivery of glial cell line-derived neurotrophic factor by gene therapy is a viable option. GDNF family ligands have not yet been fully perfected, and problems of biological activity, dosage, distribution, and the method of delivery need to be carefully reevaluated in new trials that follow patients long enough and with the appropriate protocols to separate symptomatic relief from a disease-modifying effect (17).

Brain-derived neurotrophic factor (BDNF). Exercise increases serum BDNF, which crosses the blood-brain barrier, and improves cognitive scores in patients with Parkinson disease (01). It has not been determined if exercise influences dementia risk in Parkinson disease. BDNF combined with GDNF has a synergistic effect on injured neurons. Evidence based on a current review of literature indicates that blood-borne biomolecules, such as BDNF, may be powerful agents mediating the benefits of exercise on cognitive function in neurodegenerative disorders (23). Physical exercise should be included in strategies to prevent and treat dementia.

Neurturin. This is a homologue of GDNF, and when used in a microencapsulated form, it is able to preserve tyrosine hydroxylase immunoreactivity in a rat model of Parkinson disease and might have therapeutic value for the treatment of the disease in humans. Neurturin and GDNF, both members of the GDNF family of growth factors, exert similar biological activities in different systems, including the substantia nigra. In vivo gene therapy with a modified neurturin construct is a possible treatment option for Parkinson disease.

Glial growth factors. Glial growth factors are members of the neuregulin family of growth factors and are known to stimulate the growth and differentiation of glial cells, the support cells of the nervous system. They act on glial or Schwann cells, and as a part of a signaling pathway, they facilitate production of nerve growth factors. These glial cells form the myelin sheath that insulates nerve cells and is essential for their survival and proper functioning. In demyelinating diseases, such as multiple sclerosis, the myelin sheath is damaged or lost, leading to the degeneration of the nerve cells. Experiments in animal models have demonstrated that glial growth factor-2 can stimulate the cell growth necessary to protect and potentially regenerate the damaged myelin sheath. Glial growth factors are in a preclinical stage of development for several neurologic indications, including Parkinson disease and multiple sclerosis.

Cerebral dopamine/mesencephalic astrocyte-derived family of neurotrophic factors (CDNF/MANF). These have a unique structure and a dual mode of action that differs from other known neurotrophic factors. In animal models of Parkinson disease, CDNF protects and repairs dopamine neurons, regulates endoplasmic reticulum stress, and improves motor function more efficiently than other neurotrophic factors (28).

A review of the properties of various molecules with neurotrophic properties for dopaminergic neurons indicates that a neurotrophic strategy can be developed for management of patients with Parkinson disease; however, techniques for targeted delivery of these compounds will be a problem.

A review of literature on preclinical studies and clinical trials of neurotrophic factor based therapies for Parkinson disease point towards dysfunctions of mitochondria, autophagy-lysosomal pathway, unfolded protein response, and prion protein-like spreading of misfolded alpha-synuclein, which is the major component of Lewy bodies, but the exact chain of events leading to the death of dopamine neurons is unclear and may differ among subpopulations of patients (05). Limitations in our understanding of underlying disease etiology have hindered our attempts to find neurotrophic factor-based treatments that are neuroprotective in Parkinson disease.

Amyotrophic lateral sclerosis. Neurotrophic factors that have been investigated for treatment of amyotrophic lateral sclerosis include ciliary neurotrophic factor, brain-derived neurotrophic factor, pigment epithelium-derived factor, and insulin-like growth factor-1. Although there is a good rationale and experimental evidence for the usefulness of neurotrophic factors in this condition, clinical trials so far have not been successful.

Insulin-like growth factor-1. The preclinical rationale for the use of insulin-like growth factor-1 in amyotrophic lateral sclerosis is that it affects all components of the motor unit: spinal cord motor neuron, axon, neuromuscular synapse, and muscle fiber.

Pigment epithelium-derived factor. Pigment epithelium-derived factor aids the development, differentiation, and survival of neural retina and motor neurons of the human spinal cord. Pigment epithelium-derived factor has been tested in a postnatal culture model of motor neuron degeneration and shown to be highly neuroprotective. The morphological appearance and number of motor neurons was preserved. Motor neuron choline acetyltransferase was maintained but not increased, indicating that the observed effect was neuroprotective and not merely an upregulation of motor neuron choline acetyltransferase. Pigment epithelium-derived factor has the potential for further development as a therapeutic agent for motor neuron diseases such as amyotrophic lateral sclerosis.

Adenovirus-mediated gene transfer of neurotrophic factors. Adenovirus-mediated gene transfer of neurotrophic factors offers new prospects for the treatment of motor neuron disease. Adenovirus-mediated gene transfer of neurotrophin-3 was demonstrated to produce substantial therapeutic effects in the mouse mutant progressive motor neuronopathy. After intramuscular injection of the neurotrophin-3 adenoviral vector, progressive motor neuronopathy mice showed a 50% increase in life span, reduced loss of motor axons, and improved neuromuscular function as assessed by electromyography. These results were further improved by co-injecting an adenoviral vector coding for ciliary neurotrophic factor.

Combination of multiple neurotrophic factors. Experimental studies in the developing spinal cord suggest that a combination of neurotrophic factors promote the survival of motor neurons. The data reveal potent additive action of 3 neurotrophic factors due to their specific survival effects on distinct classes of motor neurons innervating different targets: hepatocyte growth factor, ciliary neurotrophic factor, and GDNF (20). These findings indicate the potential therapeutic use of a combination of neurotrophic factors in amyotrophic lateral sclerosis.

Stem cell transplantation. Stem cells offer both cellular replacement for the lost neurons and trophic support for the rest. Combination therapy consisting of stem cells expressing beneficial growth factors may provide a comprehensive treatment for amyotrophic lateral sclerosis.

Multiple approaches. Neurotrophins and histamine are dysregulated at different omics levels in patients with amyotrophic lateral sclerosis, and both might become indicators of disease prognosis as well as progression. An integrated approach combining omics technologies with neurotrophic factors may provide guidance for personalization of management of amyotrophic lateral sclerosis (27).

Huntington disease. The rationale of neurotrophic factor therapy for Huntington disease is linked to the excitotoxicity hypothesis of etiology of Huntington disease. Intrastriatal injection of glutamate receptor agonists reproduces some of the neuropathological features of this disorder. Expression of neurotrophins, glial cell line-derived neurotrophic factor, neurturin, and their receptors in the striatum show a selective trophic response against excitotoxic insults. Neurturin only protects GABA neurons that project to the external segment of the globus pallidus, whereas glial cell line-derived neurotrophic factor exerts its effects on GABA/substance P positive neurons, which project to the substantia nigra pars compacta and the internal segment of the globus pallidus. The findings of a study have highlighted the involvement of low brain-derived neurotrophic factor expression in pathogenesis of Huntington disease, potentially mediated by cyclic adenosine monophosphate, mitogen-activated protein kinase, and renin-angiotensin system signaling pathways (32).

Ciliary neurotrophic factor/Axokine. The protective effect of ciliary neurotrophic factor in Huntington disease has been investigated by pretreating rats with these prior to induction of excitotoxic lesions by quinolinic acid. Possible mechanisms of the protective effect of ciliary neurotrophic factor are:

• Increased glial sequestration of quinolinic acid or glutamate • Decreased expression of glutamate receptors on striatal neurons • Increased Ca2+ buffering capacity in striatal neurons • Decreased levels of the release of glutamate from cortical projections

However, a phase 1 study two decades ago suggested that the technique needed improvement, but nothing further has been reported.

Concluding remarks and the future of neurotrophic factor therapy. The scientific basis for the use of neurotrophic factors has been established, but the clinical efficacy has not been enough to obtain approval for routine clinical use. The problem of delivery to the nervous system is being addressed by cell and gene therapies. Weak neurotrophic responses in human patients as compared to primate models of neurodegenerative disorders can be improved by enrolling less advanced cases. In Alzheimer disease, the trend is shifting to much earlier-stage (even prodromal) patients in trials intended to modify disease progression. Other neurodegenerative diseases should consider similar changes in approach (04). Neurotrophic factors can be combined with mesenchymal stem cells-derived exosomes for treatment of neurodegenerative diseases (29).