Clinical aspects

Description

	• Several neurotrophic factors and similar drugs have been developed for of treatment of various peripheral neuropathies.
	• Neurotrophic factors can be injected locally into peripheral nerves, but cell and gene therapies provide targeted and safer delivery.

The clinical results of the use of neurotrophic factors in peripheral neuropathies are shown in Table 1 and described in the following text.

Table 1. Neurotrophic Factors and Similar Drugs in Development for Peripheral Neuropathies

Neurotrophic factor/similar drug	Mode of action	Current status	Comments
Gangliosides	Promote nerve repair by increasing collateral sprouting, as well as correct deficits of nerve conduction velocity	Results of studies on human diabetic neuropathy are controversial. It has also been used for treatment of uremic and compressive neuropathies.	No controlled trials
Acetyl-L-carnitine	Neurotrophic effect by enhancement of regeneration of lesioned nerves	It has been proposed as treatment for diabetic neuropathy but is not supported by clinical trials (17).	Marketed in Italy for treatment of Alzheimer disease
Insulin-like growth factor	Ameliorates experimental diabetic neuropathy	Phase 3 was discontinued because of the concern for aggravation of diabetic retinopathy.	None
Neurotrophin-3 receptor-agonist monoclonal antibodies	Neurotrophin-3, an axonal guidance molecule, stimulates neurite outgrowth and nerve regeneration. Neurotrophin-3 promotes nerve regeneration in sural nerves from patients with Charcot-Marie-Tooth 1A, but its relatively short plasma half-life poses a practical difficulty in its clinical application. Therapeutic antibodies that are agonists for the neurotrophin-3 receptors circumvent this obstacle due to their high specificity and long half-life.	Recovery of neurologic function and regeneration has been demonstrated in animal models of Charcot-Marie-Tooth 1A disease.	Monoclonal antibody agonists of neurotrophin-3 receptors have therapeutic potential for the treatment of Charcot-Marie-Tooth disease.
Recombinant human nerve growth factor	Nerve growth factor is trophic for small sensory neurons and stimulates the regeneration of damaged nerve fibers. It is an axon guidance molecule.	There is a phase 2 study on neuropathic pain and pain sensitivity in HIV-associated sensory neuropathy.	Positive effect. Recombinant human nerve growth factor was safe and well tolerated, but injection site pain was frequent. No further development.
Brain-derived neurotrophic factor	Stimulates nerve generation and protects neurons from axonal degeneration	Pilot randomized, controlled trial in Guillain-Barré syndrome.	Safety established but no further clinical development.
Glial cell line-derived neurotrophic factor (GDNF)	Protein neublastin (artemin) in the GDNF is a selective ligand for the receptor alpha‐3 that normalizes cellular changes resulting from damage or disease and relieves neuropathic pain.	A phase 2 randomized, blinded, placebo‐controlled study of intravenous neublastin showed some benefit for patients with painful lumbosacral radiculopathy (01).	No further clinical development has been reported.

Delivery of neurotrophic factors. Delivery of neurotrophic factors to the peripheral nerves is easier than delivery to the brain. Although neurotrophic factors can be injected locally, cell and gene therapies provide targeted and safer delivery.

Gene transfer using herpes virus vectors to target delivery of neurotrophic factors to the primary sensory afferent for treatment of neuropathy resulting from diabetes or use of anticancer drugs provides highly selective targeted release of bioactive molecules within the peripheral nervous system (19). Preclinical studies with nonreplicating herpes simplex virus (HSV)-based vectors injected into the skin to transduce neurons in the dorsal root ganglion have demonstrated efficacy in preventing progression of sensory neuropathy (02).

Bone marrow-derived cells such as mononuclear cells or endothelial progenitor cells can produce neurotrophic factors and have been used for treating experimental diabetic neuropathy with reversal of manifestations of the disease (10).

Diabetic neuropathy. It is pathophysiologically related to both impaired angiogenesis and a deficiency of neurotrophic factors in the nerves. Neuronal damage leads to sensory disturbances and peripheral neuropathic pain. Neurotrophic factors and drugs with neurotrophic activity used for the treatment of diabetic neuropathy are as follows:

Gangliosides. Open studies of use of gangliosides in patients with various types of neuropathies have reported relief of sensory symptoms and motor deficits, but no controlled trial has been carried out.

Nerve growth factor. The efficacy of intradermal recombinant human nerve growth factor in inducing pressure allodynia and lowering heat-pain threshold in healthy human volunteers has been demonstrated. The onset of action was too rapid to be explained by uptake of nerve growth factor by nociceptive terminals, retrograde transport, and up-regulation of pain modulators. Local mechanisms were implicated in this effect.

Development of nerve growth factor for diabetic neuropathy was discontinued because phase 3 results did not confirm efficacy.

Neurotrophin-3. As a neurotrophin like the brain-derived neurotrophic factor, neurotrophin-3 is a candidate for clinical trials in diabetic neuropathy. A double-blind, placebo-controlled clinical trial testing the safety and tolerability of neurotrophin-3 in diabetic patients was conducted at five clinical sites, four in the United States and one in Canada. From the available evidence it appears that neurotrophin-3 is likely to be the best neurotrophic factor for the treatment of diabetic neuropathy where sensory and autonomic dysfunction is involved. When combined with brain-derived neurotrophic factors, it can cover the entire spectrum of types of diabetic neuropathy. Further development is on hold on this project.

Insulin-like growth factor-1. Based on the demonstration that insulin-like growth factor-1 can ameliorate experimental diabetic neuropathy, clinical trials have been progressed to phase 2. Development was discontinued during phase 3 in 1997. The reason was that a planned interim evaluation revealed the potential for insulin-like growth factor-1 to exert an angiogenic effect in the eye that could aggravate diabetic retinopathy. Because of the role of insulin-like growth factors during peripheral nervous system development, the insulin-like growth factor signaling system remains a potential therapeutic target for the treatment of peripheral neuropathy and motor neuron diseases despite mixed therapeutic results in clinical trials.

Recombinant human brain-derived neurotrophic factor. A randomized, double-blind, placebo-controlled study of brain-derived neurotrophic factor showed improvement of detectable cool detection when compared to baseline in the treated group, but not in the placebo group. Further clinical trials were recommended but have not been carried out.

Hepatocyte growth factor. Nonviral gene transfer of human hepatocyte growth factor improves streptozotocin-induced diabetic neuropathy in rats.

Vascular endothelial growth factor (VEGF). This factor has been suggested to be a useful treatment for prevention of neurodegeneration in diabetic peripheral neuropathy and for the treatment of neuropathic pain. One isoform, VEGF-A165a, is proangiogenic and neuroprotective, but pronociceptive and increases vascular permeability in animal models. However, VEGF-A splice variant, VEGF-A165b, is not only antiangiogenic, but also a potential analgesic for diabetic neuropathy possibly through an effect on transient receptor potential ankyrin 1 (TRPA1) channel activity (07).

Future treatment of diabetic neuropathy with neurotrophic factors. Advances in directly assessing the progression of nerve damage in diabetic patients will hopefully facilitate renewed clinical evaluation of neurotrophic factors as treatments for diabetic neuropathy.

Mesenchymal stem cells (MSCs) are being used as a novel emerging regenerative therapy for diabetic neuropathy because they secrete neurotrophic factors and immunomodulatory substances to ameliorate diabetic neuropathy (20). However, there are still several challenges to the clinical translation of MSC therapy, such as safety, optimal dose of administration, optimal mode of delivery, engraftment, and choices of autologous versus allogeneic cells.

Antineoplastic agents-induced neuropathy. Most of the investigations and studies in this area include cisplatin-induced neuropathy as an indication. Preclinical data suggest that several neurotrophic factors, such as nerve growth factor, insulin-like growth factor-1, and neurotrophin-3, merit further investigation in the management of cisplatin-induced neuropathy. Neurotrophin-3 delivery, using direct gene transfer into muscle by in vivo electroporation in a mouse model of cisplatin-induced neuropathy, has shown some beneficial effect, and clinical development has been considered but is not being pursued currently.

The management of antineoplastic agent-induced neuropathy is like that of diabetic neuropathy, with the difference that prophylactic therapy is considered along with chemotherapy to prevent the development of peripheral neuropathy. Neurotrophic factors, among other neuroprotective agents, are promising for preventing neurotoxicity resulting from taxanes exposure, but further confirmatory trials are warranted. Only nerve growth factor has been studied in clinical trials:

Nerve growth factor. This is the only agent reported to prevent, rather than partially protect, cisplatin-induced neuropathy in an experimental model. The basis for this therapeutic approach is that nerve growth factor impairment has been shown to play a role in the neurotoxicity of oxaliplatin. It was studied in phase 2 clinical trials in patients with peripheral neuropathy due to antineoplastic agents, but no further clinical development has taken place.

HIV-associated sensory neuropathy. Treatment for HIV-associated painful sensory neuropathy is usually symptomatic using pain-modifying agents or membrane-stabilizing drugs. Because nerve growth factor is important for the development and maintenance of sympathetic and sensory neurons as well as their outgrowths, a randomized, placebo-controlled clinical trial of recombinant human nerve growth factor to provide a specific restorative treatment for this neuropathy was done. Human nerve growth factor was found to be safe and well tolerated, but injection site pain was frequent. However, in the long-term phase of the study, there was no improvement in severity of neuropathy as assessed by neurologic examination, quantitative sensory testing, and epidermal nerve fiber density. Further development was discontinued.

Goals of treatment and indications. The goals of treatment of neuropathy are relief of pain, arrest of the disease process, and recovery of neurologic deficits. No evidence exists that neurotrophic factors relieve pain, but they help in the stabilization of the disease.

Indications

• Diabetic neuropathy • Antineoplastic agent-induced neuropathies • Compressive neuropathies, such as carpal tunnel syndrome

Contraindications

No specified contraindications exist for the use of neurotrophic factors. The following are precautions to be exercised in planning clinical trials:

	(1) Patients with diabetic retinopathy should not be treated with neurotrophic factors that have an angiogenic effect, such as insulin-like growth factor-1.
	(2) Expression of brain-derived neurotrophic factor and its receptor TrkB in neuroblastoma cells can partially blunt the cytotoxic effect of vinblastine, and this has been offered as an explanation whereby neuroectodermal tumor cells escape damage by a chemotherapeutic agent. Brain-derived neurotrophic factor should be used with caution for antineoplastic-induced neuropathy.

Outcomes

Although there is a good rationale for the use of neurotrophic factors in peripheral neuropathies, many clinical trials have failed to show significant improvement. A major problem is the delivery of neurotrophic factors, which are recombinant proteins, to the desired site of action. Administration by gene therapy has shown more favorable results in animal models, and clinical trials using this method are expected to show improved results.

Adverse effects

There were no serious or life-threatening adverse reactions. The most frequently reported events were myalgia and injection site pain.

Myalgia. Mild to moderate myalgic pain was dose related and occurred with intravenous as well as subcutaneous administration. The pain resolved without any treatment.

Injection site pain. This was the most frequent event. There was tenderness and increased sensitivity at the site of injection. Those who received saline placebo did not report this event. An interesting observation in clinical trials of the use of nerve growth factor for neuropathic pain is the relief of pain even though the patients complained of injection site hyperalgesia.

Pain induced by NGF. An interesting observation in clinical trials is the use of intradermal injection of nerve growth factor for inducing neuropathic pain to test drugs for relief of pain in volunteers.

No relevant information is available regarding prognosis.

Special considerations

There is no experience with the use of neurotrophic factors during pregnancy.

Scientific basis

	• Antibodies against nerve growth factors improve chronic neuropathic pain by acting directly on peripheral sensitization as well as indirectly on central sensitization.
	• Deficiency of neurotrophic factors forms the basis of neurotrophic factor therapy in diabetic neuropathy.
	• Basis of beneficial effect on anticancer drug-induced peripheral neuropathy is likely due to lack of neurotrophic factors.

Role of neurotrophic factors in peripheral neuropathic pain. Pain is a manifestation of several neuropathies, and there is no satisfactory relief for neuropathic pain. Because of the role of nerve growth factors in pain, several anti- nerve growth factor approaches have been tested. A study on the rat sciatic model of constriction injury provides evidence that antibodies against nerve growth factors improve chronic neuropathic pain by acting directly on peripheral sensitization as well as indirectly on central sensitization (03).

Role of neurotrophic factors in peripheral nerve regeneration. Experimental studies on peripheral nerve regeneration have shown that the total neurite length and tortuosity are differently influenced by trophic factors. Nerve growth factor and, indirectly, brain-derived neurotrophic factor stimulate the tortuous growth of sensory fibers and the formation of cell clusters, whereas neurotrophin-3 enhances neurite growth in terms of length and linearity allowing for a more organized and directed axonal elongation toward a peripheral target compared to the other growth factors (04). These findings are important for clinical application of neurotrophic factors for peripheral nerve regeneration.

A study has concluded that each growth factor is able to bind to its respective receptor to exert its biological effects by activating different kinds of downstream signaling pathways (11). Although growth factors possess the capability of enhancing the survival of stem cells and promoting axon regrowth and remyelination after peripheral nerve injuries, there are many shortcomings of growth factors that must be overcome to optimize their actions.

Diabetic neuropathy. The cause of diabetic neuropathy is not clear, but many factors play a role, such as metabolic disturbances, ischemia, hypoxia, autoimmune mechanisms, altered protein synthesis, and axon transport. Reduced fiber regeneration, a characteristic of diabetic neuropathy, is probably due to impairment of the local synthesis of neurotrophic factors. Deficiency of neurotrophic factors forms the basis of neurotrophic factor therapy in diabetic neuropathy.

Several neurotrophic factors have been implicated in diabetic neuropathy, like nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and insulin-like growth factors. All these neurotrophic factors promote regenerative activity in the nervous system. Both collateral and regenerative sprouting occur in polyneuropathy; repair of the neurologic deficits depends on the effectiveness of these mechanisms. Collateral sprouting may slow the progression of the disease in subacute neuropathy.

Evidence supporting the role of neurotrophic factors in diabetic neuropathy is as follows:

	• Neurotrophic factors support regenerative or collateral sprouting.
	• There are alterations in neurotrophic factor content in neurons, target tissues, and muscles in patients with diabetic neuropathy.
	• There is an increase in cytokine expression and a decrease in expression of neurotrophic factors (BDNF, NGF, NT-3) in nerve and skin biopsies of patients with neuropathies (18).
	• Specific neurotrophic factors improve nerve conduction in experimental diabetes models.
	• Mesenchymal stem cells ameliorate diabetic neuropathy in a mouse model by increasing the expression of neurotrophic factors in addition to angiogenesis and restoration of myelination following local transplantation (09; 05).

A biodegradable coacervate, for regulating the combined release of bFGF and NGF in a steady fashion, administered as a single injection, improved motor and sensory functions, restored morphometric structure and decreased apoptosis of Schwann cells in a rat model of diabetic peripheral neuropathy (12). The neuroprotective effect of the growth factors is likely due to suppression of endocytoplasmic reticulum stress.

Tissue nanotransfection (TNT) technology has been successfully used for delivery of Ascl1, Brn2, and Myt1l (TNTABM) to directly convert skin fibroblasts into electrophysiologically active induced neuronal cells in vivo and also causes neurotrophic enrichment of the skin stroma in the cutaneous form of diabetic neuropathy (14). Furthermore, topical application of TNTABM causes elevation of endogenous NGF and other neurotrophic factors such as NT-3 to spare loss of cutaneous mature nerve fibers in db/db diabetic mice.

Evidence against the role of neurotrophic factors in diabetic neuropathy is:

• Several factors other than neurotrophic factors are involved in the repair of nerve deficits in diabetes. A considerable portion of adult sensory neurons (mostly C fibers) innervating cutaneous tissues do not express any known Trk receptor. This implies the existence of an unknown growth factor required for the maintenance of C-fiber type neurons in adult skin. This factor may also be implicated in diabetic neuropathy.

• Neurotrophic factors do not correct metabolic defects such as altered polyol flux into diabetic nerves or microangiopathy resulting in ischemia.

The roles of specific neurotrophic factors are discussed below.

Glial cell line–derived neurotrophic factor (GDNF). Nonpeptidergic unmyelinated sensory neurons are also vulnerable to diabetes; glial cell line–derived neurotrophic factor administration has been shown to selectively reverse deficits caused by experimental diabetes in rats.

Brain-derived neurotrophic factor (BDNF). Changes in brain-derived neurotrophic factor expression can be detrimental to a wide range of sensory neurons. Brain-derived neurotrophic factor has an advantage over recombinant human nerve growth factor (rhNGF) in that after injection at the site of crush injuries it is found in the motor neurons, whereas nerve growth factor is not. This would be an advantage in treating motor dysfunction in diabetic peripheral neuropathy.

Ciliary neurotrophic factor (CNTF). Ciliary neurotrophic factor bioactivity is reduced in the peripheral nerves of hyperglycemic rats. Ciliary neurotrophic factor improves nerve conduction and ameliorates regeneration deficits in diabetic rats. The ability of CNTF to enhance axon regeneration and protect peripheral nerves from diabetic peripheral neuropathy is associated with targeting of mitochondrial function and improvement of cellular bioenergetics (15).

Neurotrophin-3. This is a survival factor for large, presumably proprioceptive, sensory neurons. Neurotrophin-3, like nerve growth factor, promotes survival of sympathetic neurons in vitro but, unlike nerve growth factor, neurotrophin-3 does not cause mast cell degranulation, a phenomenon that may be responsible for hyperalgesia reported in neuropathy patients receiving nerve growth factor. This effect is of importance in using neurotrophin-3 for treating diabetic neuropathy patients with autonomic dysfunction. In preclinical studies neurotrophin-3 has been shown to protect against progression of diabetic neuropathy in mice, but no clinical trials have been conducted in humans.

Glial growth factors. These factors play a significant part in regulating the function of Schwann cells, which are a source of trophic factors and have an important function in the regeneration of peripheral nerves following injury. Recombinant human glial growth factor 2 (rhGGF 2) has been shown to improve functional recovery of crushed sciatic nerve in rats and has potential application in pathologic processes of peripheral neuropathies.

Acetyl-L-carnitine. Acetyl-L-carnitine has gained clinical interest for its analgesic effect in diabetic neuropathy. The rationale for the potential of acetyl-L-carnitine as treatment for diabetic neuropathy is based on the following findings:

	• There is a depletion of free carnitine in the nerves of diabetic rats as compared with controls.
	• Acetyl-L-carnitine can correct the biochemical abnormalities of low protein kinase C levels that result from low levels of free carnitine in the rats.
	• Acetyl-L-carnitine improves the diminished nerve conduction velocity in rats with streptozotocin-induced diabetes.
	• The ability of acetyl-L-carnitine to delay nerve myoinositol depletion independently of the levels of other organic nerve osmolytes is particularly useful. A combination of acetyl-L-carnitine with the cyclo-oxygenase inhibitor flurbiprofen is being studied to gain a better understanding of role of cyclo-oxygenase products in regulation of nerve myoinositol and ATPase levels.

Gangliosides. Peripheral nerves contain gangliosides, although they constitute only 1% of the total lipid content. Exogenous gangliosides have a role in promoting nerve repair in mammalian cells by enhancing neurite outgrowth.

Antineoplastic agents-induced neuropathy. Peripheral neuropathy is well known with several chemotherapeutic agents. No treatment has been proven to prevent this so far. Most of the investigations and studies include cisplatin-induced neuropathy as an indication. Preclinical data suggest that several neurotrophic factors (nerve growth factor, insulin-like growth factor-1, and neurotrophin-3) merit further investigation in the management of cisplatin-induced neuropathy (08).

Nerve growth factor. In vivo studies in experimental animals have shown protective effects of the nerve growth factor against paclitaxel-induced sensory neuropathic changes. Nerve growth factor protected a dorsal root ganglion model from oxaliplatin toxicity by modulating JNK/Sapk and ERK1/2, which are mitogen-activated protein kinases (16).

Neurotrophins and angiogenic factors. These factors play a role in the proliferation, differentiation, survival, and death of neuronal and nonneuronal cells. The mechanism of the development of drug-induced peripheral neuropathy during treatment of multiple myeloma with thalidomide and bortezomib is likely due to lack of crucial trophic factors, including neurotrophic factors, or angiogenic factors, or both (13).

Neurotrophin-3. It has the following effects in cisplatin-induced neuropathy in experimental animals:

	• Restored to normal levels the reduced H-reflex-related sensory nerve conduction velocity.
	• Correction of an abnormal cytoplasmic distribution of neurofilament protein in large sensory neurons in dorsal root ganglia.
	• Reduction in the number of myelinated fibers in sural nerves caused by cisplatin.

Insulin-like growth factor. It improves electrophysiological measurements in a mouse model of vincristine neuropathy.

Prosaposin. This facilitates nerve regeneration in paclitaxel-induced neurotoxicity.

Recombinant human glial growth factor 2. This has been shown to protect against cisplatin neuropathy in rats.

Acetyl-L-carnitine. This prevents and reduces paclitaxel-induced painful peripheral neuropathy in rat pain models. It may be useful in the prevention and treatment of chemotherapy-induced painful peripheral neuropathy.

Recurrent compressive neuropathies. Autologous vein wrapping is a useful technique for reducing neuropathic pain in recurrent compressive neuropathy, but the mechanism of beneficial effect is not well understood. In a rat model of chronic constriction injury, vein-derived bFGF was shown to contribute to therapeutic benefit of vein wrapping through the induction of heme oxygenase-1 expression in the sciatic nerve (06). This may lead to the development of new therapeutic approaches using recombinant neurotrophic factors for treating recurrent compressive neuropathy and traumatic peripheral nerve injury.

Motor neuropathies. Neurotrophic factors with a demonstrated effect on motor neurons include brain-derived neurotrophic factors, neurotrophin-3, neurotrophin-4/5, ciliary neurotrophic factor, and insulin-like growth factor-1. Insulin-like growth factor-1 is an effective motor neuron sprouting factor and promotes motor neuron regeneration following sciatic nerve crush.

Charcot-Marie-Tooth type 1A neuropathy. Because production of CNTF by Schwann cells is markedly reduced in Charcot-Marie-Tooth type 1A neuropathy, further studies on the therapeutic use of neurotrophic factors are suggested for this condition.

Selection of neurotrophic factors for peripheral neuropathies. It is unlikely that any one neurotrophin can be useful for all types of neuropathies, let alone a single type such as diabetic neuropathy that has a multifactorial etiology. A combination of neurotrophic factors may be a strategy for consideration. Neuropathies may be described according to the type of neuron involved: motor, sensory, or autonomic. Each of these may be divided further into subpopulations of neurons that respond differently to various neurotrophic factors, as shown in Table 2. The use of fibroblast growth factor and glial cell line–derived neurotrophic factor in peripheral neuropathies has not been explored.

Neurotrophic factors-induced neuropathic pain. Nerve growth factor is implicated in the pain pathways, as it is required for the development of sympathetic and small fiber sensory neurons that serve as nociceptors. Nerve growth factor stimulates the expression and release of neuropeptides involved in pain transmission. Blockade with nerve growth factor antiserum abolishes the effect of nerve growth factor in mediating inflammatory hyperalgesia. Administration of nerve growth factor in human subjects has been associated with neuropathic pain as a complication.

Table 2. Neuronal Specificity of Neurotrophic Factors in Relation to Peripheral Neuropathies

Neurons involved:

Large sensory neurons
	• nerve growth factor: + • brain-derived neurotrophic factor: + • neurotrophin-3: +++ • neurotrophin-4/5: ++ • ciliary neurotrophic factor: 0 • insulin-like growth factor-1: ++ • glial cell line–derived neurotrophic factor: 0
Small sensory neurons
	• nerve growth factor: ++ • brain-derived neurotrophic factor: + • neurotrophin-3: + • neurotrophin-4/5: + • ciliary neurotrophic factor: + • insulin-like growth factor-1: + • glial cell line–derived neurotrophic factor: 0
Motor neurons
	• nerve growth factor: + • brain-derived neurotrophic factor: ++ • neurotrophin-3: ++ • neurotrophin-4/5: + • ciliary neurotrophic factor: 0 • insulin-like growth factor-1: 0 • glial cell line–derived neurotrophic factor: ++
Autonomic neurons
	• nerve growth factor: ++ • brain-derived neurotrophic factor: + • neurotrophin-3: ++ • neurotrophin-4/5: 0 • ciliary neurotrophic factor: ++ • insulin-like growth factor-1: + • glial cell line–derived neurotrophic factor: +
0=ineffective or no evidence available, + = slight effect, ++ =moderate effect, +++ =pronounced effect. *Neurturin and persephin are structurally similar to glial cell line–derived neurotrophic factor and have similar response on motor neurons.