Targeting the Nerve Growth Factor (NGF) Pathway in Drug Discovery. Potential Applications to New Therapies for Chronic Pain

The History of Nerve Growth Factor: From Molecule to Drug

Nerve growth factor (NGF), the first neurotrophin to be discovered, has a long and eventful research journey with a series of turning points, setbacks, and achievements. Since the groundbreaking investigations led by Nobel Prize winner Rita Levi-Montalcini, advancements in the comprehension of NGF's functions have revolutionized the field of neuroscience, offering new insights and opportunities for therapeutic innovation. However, the clinical application of NGF has historically been hindered by challenges in determining appropriate dosing, administration strategies, and complications related to the production process. Recent advances in the production and scientific knowledge of recombinant NGF have enabled its clinical development, and in 2018, the United States Food and Drug Administration approved cenegermin-bkbj, a recombinant human NGF, for the treatment of all stages of neurotrophic keratitis. This review traces the evolutionary path that transformed NGF from a biological molecule into a novel therapy with potential research applications beyond the eye. Special emphasis is put on the studies that advanced NGF from discovery to the first medicinal product approved to treat a human disease.

Macrophage memories of early-life injury drive neonatal nociceptive priming

The developing peripheral nervous and immune systems are functionally distinct from those of adults. These systems are vulnerable to early-life injury, which influences outcomes related to nociception following subsequent injury later in life (i.e., "neonatal nociceptive priming"). The underpinnings of this phenomenon are unclear, although previous work indicates that macrophages are trained by inflammation and injury. Our findings show that macrophages are both necessary and partially sufficient to drive neonatal nociceptive priming, possibly due to a long-lasting remodeling in chromatin structure. The p75 neurotrophic factor receptor is an important effector in regulating neonatal nociceptive priming through modulation of the inflammatory profile of rodent and human macrophages. This "pain memory" is long lasting in females and can be transferred to a naive host to alter sex-specific pain-related behaviors. This study reveals a mechanism by which acute, neonatal post-surgical pain drives a peripheral immune-related predisposition to persistent pain following a subsequent injury.

Effects of the nerve growth factor and its carrier protein on the inflammatory response from human monocytes

BACKGROUND

The nerve growth factor (NGF) has been previously shown to be involved in cellular proliferation, differentiation, survival, or wound healing. This factor displays a variety of biological effects that yet remain to be explored. Previous data on cell lines show a pro-inflammatory role of NGF on monocytes.

OBJECTIVES

The objective of the study was to investigate the pro-inflammatory effect of NGF, using a model of fresh human monocytes.

METHODS

Monocytes obtained from PBMC were exposed to NGF at various concentrations. Alternatively, monocytes were exposed to BSA, the NGF carrier protein without the NGF. Gene expression and cytokine release in the supernatant were monitored.

RESULTS

We found that NGF increased the expression of pro-inflammatory, chemotactic, and remodeling genes such as interleukin (IL)-1β, IL-6, tumor necrosis factor (TNF)-α, and C-X-C motif ligand (CXCL)8. The protein levels of CXCL8 and matrix metalloproteinase (MMP)-9 were also increased in the cell supernatants following NGF exposure. BSA alone was found to drive part of this response, bringing nuance to the inflammatory potential of the NGF.

CONCLUSION

These data suggest that NGF is able to enhance monocyte inflammatory responses once cells are stimulated with another signal but is possibly not able to directly activate it. This could have implications for example in patients with bacterial infections, where NGF could worsen the local inflammation by over-activating immune cells.

Type II & III inhibitors of tropomyosin receptor kinase (Trk): a 2020-2022 patent update

INTRODUCTION

The Trk family proteins are membrane-bound kinases predominantly expressed in neuronal tissues. Activated by neurotrophins, they regulate critical cellular processes through downstream signaling pathways. Dysregulation of Trk signaling can drive a range of diseases, making the design and study of Trk inhibitors a vital area of research. This review explores recent advances in the development of type II and III Trk inhibitors, with implications for various therapeutic applications.

AREAS COVERED

Patents covering type II and III inhibitors targeting the Trk family are discussed as a complement of the previous review, Type I inhibitors of tropomyosin receptor kinase (Trk): a 2020-2022 patent update. Relevant patents were identified using the Web of Science database, Google, and Google Patents.

EXPERT OPINION

While type II and III Trk inhibitor development has advanced more gradually compared to their type I counterparts, they hold significant promise in overcoming resistance mutations and achieving enhanced subtype selectivity - a critical factor in reducing adverse effects associated with pan-Trk inhibition. Recent interdisciplinary endeavors have marked substantial progress in the design of subtype selective Trk inhibitors, with impressive success heralded by the type III inhibitors. Notably, the emergence of mutant-selective Trk inhibitors introduces an intriguing dimension to the field, offering precise treatment possibilities.

NGF Signaling Exacerbates KOA Peripheral Hyperalgesia via the Increased TRPV1-Labeled Synovial Sensory Innervation in KOA Rats

Objectives

Knee osteoarthritis (KOA) pain is caused by nociceptors, which are actually sensory nerve fiber endings that can detect stimuli to produce and transmit pain signals, and high levels of NGF in synovial tissue led to peripheral hyperalgesia in KOA. The purpose of this study is to investigate how sensory nerve fibers respond to the NGF/TrKA signal pathway and mediate the peripheral hyperalgesia in KOA rats.

Methods

Forty SD male rats were randomly divided into 4 groups: normal, KOA, KOA + NGF, and KOA + siRNA TrKA. KOA model rats were induced by anterior cruciate ligament transection (ACLT). Mechanical and cold withdrawal thresholds (MWT and CWT) were measured 4 times in each group. The synovial tissues were harvested on day 28, and the expressions of NGF, TrKA, TRPV1, IL-1β, and PGP9.5 were determined using western blot, qPCR, and immunofluorescence staining. The primary rat fibroblast-like synoviocytes (FLSs) and DRG cells were divided into 4 groups as in vivo. The expressions of NGF, TrKA, TRPV1, and CGRP in vitro were determined using western blot and qPCR.

Results

KOA and intra-articular injection with NGF protein increased both mRNA and protein levels, not only TRPV1, PGP 9.5, and IL-1β in the synovial tissue, but also TRPV1, PGP 9.5, and S100 in the DRG tissue, while above changes were partly reversed after siRNA TrKA intervention. Besides, siRNA TrKA could improve peripheral hyperalgesia and decreased the TRPV1 positive nerve fiber innervation in synovial tissue. The results in vitro were consistent with those in vivo.

Conclusion

This study showed the activation of the NGF/TrKA signaling pathway in KOA promoted the release of pain mediators, increased the innervation of sensory nerve fibers in the synovium, and worsened peripheral hyperalgesia. It also showed increased TRPV1 positive sensory innervation in KOA was mediated by NGF/TrKA signaling and exacerbated peripheral hyperalgesia.

A bone to pick-cellular and molecular mechanisms of bone pain in sickle cell disease

The bone is one of the most commonly affected organs in sickle cell disease (SCD). Repeated ischemia, oxidative stress and inflammation within the bone is largely responsible for promoting bone pain. As more individuals with SCD survive into adulthood, they are likely to experience a synergistic impact of both aging and SCD on their bone health. As bone health deteriorates, bone pain will likely exacerbate. Recent mechanistic and observational studies emphasize an intricate relationship between bone remodeling and the peripheral nervous system. Under pathological conditions, abnormal bone remodeling plays a key role in the propagation of bone pain. In this review, we first summarize mechanisms and burden of select bone complications in SCD. We then discuss processes that contribute to pathological bone pain that have been described in both SCD as well as non-sickle cell animal models. We emphasize the role of bone-nervous system interactions and pitfalls when designing new therapies especially for the sickle cell population. Lastly, we also discuss future basic and translational research in addressing questions about the complex role of stress erythropoiesis and inflammation in the development of SCD bone complications, which may lead to promising therapies and reduce morbidity in this vulnerable population.

Electroacupuncture attenuates nociceptive behaviors in a mouse model of cancer pain

Pain is a major symptom in cancer patients, and cancer-induced bone pain (CIBP) is the most common type of moderate and severe cancer-related pain. The current available analgesic treatments for CIBP have adverse effects as well as limited therapeutic effects. Acupuncture is proved effective in pain management as a safe alternative therapy. We evaluated the analgesic effect of acupuncture in treatment of cancer pain and try to explore the underlying analgesic mechanisms. Nude mice were inoculated with cancer cells into the left distal femur to establish cancer pain model. Electroacupuncture (EA) treatment was applied for the xenograft animals. Pain behaviors of mice were evaluated, followed by the detections of neuropeptide-related and inflammation-related indicators in peripheral and central levels. EA treatment alleviated cancer-induced pain behaviors covering mechanical allodynia, thermal hyperalgesia and spontaneous pain, and also down-regulated immunofluorescence expressions of neuropeptide CGRP and p75 in the skin of affected plantar area in xenograft mice, and inhibited expressions of overexpressed neuropeptide-related and inflammation-related protein in the lumbar spinal cord of xenograft mice. Overall, our findings suggest that EA treatment ameliorated cancer-induced pain behaviors in the mouse xenograft model of cancer pain, possibly through inhibiting the expressions of neuropeptide-related and inflammation-related protein in central level following tumor cell xenografts.

N-Acetylcysteine Antagonizes NGF Activation of TrkA through Disulfide Bridge Interaction, an Effect Which May Contribute to Its Analgesic Activity

N-acetylcysteine (NAC), a mucolytic agent and an antidote to acetaminophen intoxication, has been studied in experimental conditions and trials exploring its analgesic activity based on its antioxidant and anti-inflammatory properties. The purpose of this study is to investigate additional mechanisms, namely, the inhibition of nerve growth factor (NGF) and the activation of the Tropomyosin receptor kinase A (TrkA) receptor, which is responsible for nociception. In silico studies were conducted to evaluate dithiothreitol and NAC's interaction with TrkA. We also measured the autophosphorylation of TrkA in SH-SY5Y cells via ELISA to assess NAC's in vitro activity against NGF-induced TrkA activation. The in silico and in vitro tests show that NAC interferes with NGF-induced TrkA activation. In particular, NAC breaks the disulfide-bound Cys 300-345 of TrkA, perturbing the NGF-TrkA interaction and producing a rearrangement of the binding site, inducing a consequent loss of their molecular recognition and spatial reorganization, which are necessary for the induction of the autophosphorylation process. The latter was inhibited by 40% using 20 mM NAC. These findings suggest that NAC could have a role as a TrkA antagonist, an action that may contribute to the activity and use of NAC in various pain states (acute, chronic, nociplastic) sustained by NGF hyperactivity and/or accompanied by spinal cord sensitization.

Macrophage epigenetic memories of early life injury drive neonatal nociceptive priming

The developing peripheral nervous and immune systems are functionally distinct from adults. These systems are vulnerable to early life injury, which influences outcomes related to nociception following subsequent injury later in life (neonatal nociceptive priming). The underpinnings of this phenomenon are largely unknown, although previous work indicates that macrophages are epigenetically trained by inflammation and injury. We found that macrophages are both necessary and partially sufficient to drive neonatal nociceptive priming possibly due to a long-lasting epigenetic remodeling. The p75 neurotrophic factor receptor (NTR) was an important effector in regulating neonatal nociceptive priming through modulation of the inflammatory profile of rodent and human macrophages. This pain memory was long lasting in females and could be transferred to a naive host to alter sex-specific pain-related behaviors. This study reveals a novel mechanism by which acute, neonatal post-surgical pain drives a peripheral immune-related predisposition to persistent pain following a subsequent injury.

Up-Regulation of ProBDNF/p75NTR Signaling in Spinal Cord Drives Inflammatory Pain in Male Rats

Background

The spinal cord expresses brain-derived neurotrophic factor precursor (proBDNF) and its receptor pan neurotrophin receptor 75 (p75^NTR). However, the role of spinal proBDNF signaling in the pathogenesis of inflammatory pain remains unknown.

Methods

Rats were locally injected with complete Freund's adjuvant (CFA) to induce inflammatory pain. The proBDNF signal expression was detected by double-labeled immunofluorescence. ProBDNF protein, p75^NTR extracellular domain (p75^NTR-ECD), or monoclonal anti-proBDNF (McAb-proB) were administrated by intrathecal injection to investigate their effects on pain behavior. Paw withdrawal thermal latency (PWL) and paw withdrawal mechanical threshold (PWT) were performed to evaluate pain behavior. Immunoblotting, immunohistochemistry, and immunofluorescence were used to assess inflammation-induced biochemical changes.

Results

CFA induced a rapid increase in proBDNF in the ipsilateral spinal cord, and immunofluorescence revealed that CFA-enhanced proBDNF was expressed in NeuN positive neurons and GFAP positive astrocytes. The administration of furin cleavage-resistant proBDNF via intrathecal injection (I.t.) significantly decreased the PWT and PWL, whereas McAb-proB by I.t. alleviated CFA-induced pain-like hypersensitivity in rats. Meanwhile, CFA administration triggered the activation of p75^NTR and its downstream signaling extracellular signal-regulated kinase 1/2 (ERK1/2) and nuclear factor (NF)-kappaB p65 in the spinal cord. I.t. administration of p75^NTR-ECD suppressed CFA-induced pain and neuroinflammation, including the expression of p-ERK1/2, p-p65, and the gene expression of tumor necrosis factor-α (TNF-α), and interleukin 6 (IL-6).

Conclusion

Our study reveals that the activated proBDNF/p75^NTRsignaling in the spinal cord contributes to the development of CFA-induced inflammatory pain. McAb-proB and p75^NTR-ECD appear to be promising therapeutic agents for inflammatory pain.

Current understanding of osteoarthritis pathogenesis and relevant new approaches

Osteoarthritis (OA) is the most common degenerative joint disease that causes painful swelling and permanent damage to the joints in the body. The molecular mechanisms of OA are currently unknown. OA is a heterogeneous disease that affects the entire joint, and multiple tissues are altered during OA development. To better understand the pathological mechanisms of OA, new approaches, methods, and techniques need to be used to understand OA pathogenesis. In this review, we first focus on the epigenetic regulation of OA, with a particular focus on DNA methylation, histone modification, and microRNA regulation, followed by a summary of several key mediators in OA-associated pain. We then introduce several innovative techniques that have been and will continue to be used in the fields of OA and OA-associated pain, such as CRISPR, scRNA sequencing, and lineage tracing. Next, we discuss the timely updates concerning cell death regulation in OA pathology, including pyroptosis, ferroptosis, and autophagy, as well as their individual roles in OA and potential molecular targets in treating OA. Finally, our review highlights new directions on the role of the synovial lymphatic system in OA. An improved understanding of OA pathogenesis will aid in the development of more specific and effective therapeutic interventions for OA.

Structural Optimization and Structure-Activity Relationship Studies of 6,6-Dimethyl-4-(phenylamino)-6H-pyrimido[5,4-b][1,4]oxazin-7(8H)-one Derivatives as A New Class of Potent Inhibitors of Pan-Trk and Their Drug-Resistant Mutants

Tropomyosin receptor kinases (TrkA, TrkB, and TrkC) are attractive therapeutic targets for multiple cancers. Two first-generation small-molecule Trks inhibitors, larotrectinib and entrectinib, have just been approved to use clinically. However, the drug-resistance mutations of Trks have already emerged, which calls for new-generation Trks inhibitors. Herein, we report the structural optimization and structure-activity relationship studies of 6,6-dimethyl-4-(phenylamino)-6H-pyrimido[5,4-b][1,4]oxazin-7(8H)-one derivatives as a new class of pan-Trk inhibitors. The prioritized compound 11g exhibited low nanomolar IC₅₀ values against TrkA, TrkB, and TrkC and various drug-resistant mutants. It also showed good kinase selectivity. 11g displayed excellent in vitro antitumor activity and strongly suppressed Trk-mediated signaling pathways in intact cells. In in vivo studies, compound 11g exhibited good antitumor activity in BaF3-TEL-TrkA and BaF3-TEL-TrkC^G623R allograft mouse models without exhibiting apparent toxicity. Collectively, 11g could be a promising lead compound for drug discovery targeting Trks and deserves further investigation.

In Pursuit of an Allosteric Human Tropomyosin Kinase A (hTrkA) Inhibitor for Chronic Pain

Human β-nerve growth factor (β-NGF) and its associated receptor, human tropomyosin receptor kinase A (hTrkA), have been demonstrated to be key factors in the perception of pain. However, efficacious small molecule therapies targeting the intracellularly located hTrkA kinase have not been explored thoroughly for pain management. Herein, we report the pharmacological properties of a selective hTrkA allosteric inhibitor, 1. 1 was shown to be active against the full length hTrkA, showing preferential binding for the inactive kinase, and was confirmed through the X-ray of hTrkA···1 bound complex. 1 was also found to inhibit β-NGF induced neurite outgrowth in rat PC12 cells. Daily oral administration of 1 improved the joint compression threshold of rats injected intra-articularly with monoiodoacetate over a 14-day period. The efficacy of 1 in a relevant chronic pain model of osteoarthritis coupled with in vitro confirmation of target mediation makes allosteric hTrkA inhibitors potential candidates for modulating pain.

Anisotropic scaffolds for peripheral nerve and spinal cord regeneration

The treatment of long-gap (>10 mm) peripheral nerve injury (PNI) and spinal cord injury (SCI) remains a continuous challenge due to limited native tissue regeneration capabilities. The current clinical strategy of using autografts for PNI suffers from a source shortage, while the pharmacological treatment for SCI presents dissatisfactory results. Tissue engineering, as an alternative, is a promising approach for regenerating peripheral nerves and spinal cords. Through providing a beneficial environment, a scaffold is the primary element in tissue engineering. In particular, scaffolds with anisotropic structures resembling the native extracellular matrix (ECM) can effectively guide neural outgrowth and reconnection. In this review, the anatomy of peripheral nerves and spinal cords, as well as current clinical treatments for PNI and SCI, is first summarized. An overview of the critical components in peripheral nerve and spinal cord tissue engineering and the current status of regeneration approaches are also discussed. Recent advances in the fabrication of anisotropic surface patterns, aligned fibrous substrates, and 3D hydrogel scaffolds, as well as their in vitro and in vivo effects are highlighted. Finally, we summarize potential mechanisms underlying the anisotropic architectures in orienting axonal and glial cell growth, along with their challenges and prospects.

Novel Approaches, Drug Candidates, and Targets in Pain Drug Discovery

Because of the problems associated with opioids, drug discovery efforts have been employed to develop opioids with reduced side effects using approaches such as biased opioid agonism, multifunctional opioids, and allosteric modulation of opioid receptors. Receptor targets such as adrenergic, cannabinoid, P2X3 and P2X7, NMDA, serotonin, and sigma, as well as ion channels like the voltage-gated sodium channels Nav1.7 and Nav1.8 have been targeted to develop novel analgesics. Several enzymes, such as soluble epoxide hydrolase, sepiapterin reductase, and MAGL/FAAH, have also been targeted to develop novel analgesics. In this review, old and recent targets involved in pain signaling and compounds acting at these targets are summarized. In addition, strategies employed to reduce side effects, increase potency, and efficacy of opioids are also elaborated. This review should aid in propelling drug discovery efforts to discover novel analgesics.

NGF nanoparticles enhance the potency of transplanted human umbilical cord mesenchymal stem cells for myocardial repair

In this study, we investigated whether human umbilical cord mesenchymal stem cell (hUCMSC) fibrin patches loaded with nerve growth factor (NGF) poly(lactic-co-glycolic acid) (PLGA) nanoparticles could enhance the therapeutic potency of hUCMSCs for myocardial infarction (MI). In vitro, NGF significantly improved the proliferation of hUCMSCs and mitigated cytotoxicity and apoptosis under hypoxic injury. NGF also promoted the paracrine effects of hUCMSCs on angiogenesis and cardiomyocyte protection. The tyrosine kinase A (TrkA) and phosphoinositide 3-kinase (PI3K)-serine/threonine protein kinase (Akt) signaling pathways in hUCMSCs were involved in the NGF-induced protection. NGF PLGA nanoparticles continued to release NGF for at least 1 mo and also exerted a protective effect on hUCMSCs, the same with free NGF. In vivo, we treated MI mice with nothing (MI group), a cell-free fibrin patch with blank PLGA nanoparticles (MI + OP group), a cell-free fibrin patch with NGF nanoparticles (MI + NGF group), and hUCMSC fibrin patches with blank PLGA nanoparticles (MI + MSC group) or NGF PLGA nanoparticles (MSC + NGF group). Among these groups, the MSC + NGF group exhibited the best cardiac contractile function, the smallest infarct size, and the thickest ventricular wall. The application of NGF PLGA nanoparticles significantly improved the retention of transplanted hUCMSCs and enhanced their ability to reduce myocardial apoptosis and promote angiogenesis in the mouse heart after MI. These findings demonstrate the promising therapeutic potential of hUCMSC fibrin cardiac patches loaded with NGF PLGA nanoparticles.NEW & NOTEWORTHY NGF PLGA nanoparticles can exert a protective effect on hUCMSCs and promote the paracrine effects of hUCMSCs on angiogenesis and cardiomyocyte protection through TrkA-PI3K/Akt signaling pathway, the same with free NGF. The application of NGF PLGA nanoparticles in the hUCMSC fibrin cardiac patches can significantly improve the retention of transplanted hUCMSCs and enhance their ability to reduce myocardial apoptosis and promote angiogenesis in the mouse heart after MI.

In silico development of potential therapeutic for the pain treatment by inhibiting voltage-gated sodium channel 1.7

The voltage-gated sodium channel Nav1.7 can be considered as a promising target for the treatment of pain. This research presents conformational-independent and 3D field-based QSAR modeling for a series of aryl sulfonamide acting as Nav1.7 inhibitors. As descriptors used for building conformation-independent QSAR models, SMILES notation and local invariants of the molecular graph were used with the Monte Carlo optimization method as a model developer. Different statistical methods, including the index of ideality of correlation, were used to test the quality of the developed models, robustness and predictability and obtained results were good. Obtained results indicate that there is a very good correlation between 3D QSAR and conformation-independent models. Molecular fragments that account for the increase/decrease of a studied activity were defined and used for the computer-aided design of new compounds as potential analgesics. The final evaluation of the developed QSAR models and designed inhibitors were carried out using molecular docking studies, bringing to light an excellent correlation with the QSAR modeling results.

Repeated Injections of Low-Dose Nerve Growth Factor (NGF) in Healthy Humans Maintain Muscle Pain and Facilitate Ischemic Contraction-Evoked Pain

OBJECTIVE

Nerve growth factor (NGF) is essential for generating and potentiating pain responses. This double-blinded crossover study assessed NGF-evoked pain in healthy humans after repeated NGF injections in the tibialis anterior (TA) muscle compared with control injections of isotonic saline.

SUBJECTS

Twenty healthy subjects participated in two experimental phases; each consisted of seven sessions over 21 days.

METHODS

At day 0, day 2, and day 4, a low-dose NGF (1 µg) was injected. Data on daily self-reported muscle pain (using a Likert scale) were collected. Data on pressure pain thresholds (PPTs), pain evoked by nonischemic and ischemic muscle contractions (using a numerical rating scale [NRS]), pressure pain detection (PDT), and pain tolerance thresholds (PTTs) to cuff algometry were recorded before day 0 and at 1, 2, 4, 7, 10, and 21 days after the first injection. Temporal summation of pain (TSP) and conditioned pain modulation (CPM) were recorded to assess central pain mechanisms.

RESULTS

Likert scores remained elevated for 9 days after NGF injection (P<0.05). PPTs at the TA muscle were decreased at day 1 until day 7 after NGF injection compared with day 0 (P=0.05). In subjects presenting with NGF-induced muscle hyperalgesia, pain NRS scores evoked by nonischemic contractions were higher after NGF injection at day 4 and day 7 (P<0.04) compared with the control condition. At all time points, higher pain NRS scores were found with ischemic compared with nonischemic contractions (P<0.05). The pain NRS after ischemic contractions was elevated following prolonged NGF hyperalgesia at day 7 compared with the control condition and day 0 (P<0.04). The PDT, PTT, TSP, and CPM remained unchanged during the period of NGF-induced hyperalgesia.

CONCLUSIONS

Repeated low-dose NGF injections maintain muscle pain and potentiate pain evoked by ischemic contractions during prolonged NGF hyperalgesia.

Role of Nerve Growth Factor in Orofacial Pain

Some chronic pain conditions in the orofacial region are common and the mechanisms underlying orofacial pain are unresolved. Nerve growth factor (NGF) is a member of a family of neurotrophins and regulates the growth, maintenance and development of neurons. Increasing evidence suggests that NGF plays a crucial role in the generation of pain and hyperalgesia in different pain states. This review investigates the role of NGF in orofacial pain and their underlying cellular mechanisms, which may provide essential guidance to drug-discovery programmes. A systemic literature search was conducted in Pubmed focusing on NGF and orofacial pain. Articles were reviewed, and those discussing in vitro studies, animal evidence, clinical course, and possible mechanisms were summarized. We found a hyperalgesic effect of NGF in peripheral sensitization in orofacial pain models. We also summarize the current knowledge regarding NGF-dependent pain mechanism, which is initiated by retrograde transport of the ligand-receptor complex, ensuing transcriptional regulation of many important nociceptor genes involved in nociceptive processing. Phase III trials suggest that anti-NGF drug is endorsed with anti-inflammatory and pain-relieving effects with good tolerance in a variety of pain conditions, including pain associated with osteoarthritis and chronic lower back pain. Based on the data reviewed herein, NGF is believed to be an important hyperalgesic mediator in orofacial pain. The identification of underlying mechanisms and pathways of orofacial pain opens new frontiers for pain management.

Anti-nerve growth factor antibodies for the treatment of low back pain

INTRODUCTION

The treatment of chronic low back pain (cLBP) often involves multimodal pharmacologic and non-pharmacologic strategies. There remain shortcomings with these tools with regards to both effect size and side effects.

AREAS COVERED

In an effort to better address cLBP, anti-nerve growth factor (NGF) monoclonal antibodies (mAbs) are nearing marketing approval. This class of medications has been primarily evaluated for osteoarthritis, but are being examined at higher doses for use in cLBP. We review the efficacy of this class in treating LBP as well as their potential side effects based on nine phase II or III published clinical trials. Five trials evaluated Tanezumab and four trials evaluated Fasinumab, with seven trials evaluating nonspecific LBP, one evaluating sciatica related cLBP, and one evaluating vertebral fracture related cLBP.

EXPERT OPINION

The results of available clinical trials indicate modest effectiveness with regard to reduction of pain in the low back, and improved functionality, compared to placebo in keeping with the effect size of other pharmacologic treatment modalities. Rapidly progressive osteoarthritis was infrequently reported. However, the continued observation of this serious side effects warrants careful patient selection and balancing the risks and benefits of anti-NGF mAbs in treating cLBP.

Spinal Mobilization Prevents NGF-Induced Trunk Mechanical Hyperalgesia and Attenuates Expression of CGRP

Introduction

Low back pain (LBP) is a complex and growing global health problem in need of more effective pain management strategies. Spinal mobilization (SM) is a non-pharmacological approach recommended by most clinical guidelines for LBP, but greater utilization and treatment optimization are hampered by a lack of mechanistic knowledge underlying its hypoalgesic clinical effects.

Methods

Groups of female Sprague-Dawley rats received unilateral trunk (L5 vertebral level) injections (50 μl) of either vehicle (phosphate-buffer solution, PBS; VEH) or nerve growth factor (NGF; 0.8 μM) on Days 0 and 5 with or without daily L5 SM (VEH, NGF, VEH + SM, VEH + SM). Daily passive SM (10 min) was delivered by a feedback motor (1.2 Hz, 0.9N) from Days 1 to 12. Changes in pain assays were determined for mechanical and thermal reflexive behavior, exploratory behavior (open field events) and spontaneous pain behavior (rat grimace scale). On Day 12, lumbar (L1–L6) dorsal root ganglia (DRG) were harvested bilaterally and calcitonin gene-related peptide (CGRP) positive immunoreactive neurons were quantified from 3 animals (1 DRG tissue section per segmental level) per experimental group.

Results

NGF induced bilateral trunk (left P = 0.006, right P = 0.001) mechanical hyperalgesia and unilateral hindpaw allodynia (P = 0.006) compared to the vehicle group by Day 12. Additionally, we found for the first time that NGF animals demonstrated decreased exploratory behaviors (total distance traveled) and increased grimace scale scoring compared to the VEH group. Passive SM prevented this development of local (trunk) mechanical hyperalgesia and distant (hindpaw) allodynia, and normalized grimace scale scores. NGF increased CGRP positive immunoreactive neurons in ipsilateral lumbar DRGs compared to the VEH group ([L1]P = 0.02; [L2]P = 0.007) and SM effectively negated this increase in pain-related neuropeptide CGRP expression.

Conclusion

SM prevents the development of local (trunk) NGF-induced mechanical hyperalgesia and distant (hindpaw) allodynia, in part, through attenuation of CGRP expression in lumbar DRG sensory neurons. NGF decreases rat exploratory behavior and increases spontaneous pain for which passive SM acts to mitigate these pain-related behavioral changes. These initial study findings suggest that beginning daily SM soon after injury onset might act to minimize or prevent the development of LBP by reducing production of pain-related neuropeptides.

Therapeutic approaches of trophic factors in animal models and in patients with spinal cord injury

Trophic factors are naturally produced by different tissues that participate in several functions such as the intercellular communication, in the development, stability, differentiation and regeneration at the cellular level. Specifically, in the case of spinal injuries, these factors can stimulate neuronal recovery. They are applied both in experimental models and in clinical trials in patients. The trophic factors analysed in this review include gonadotropin-releasing hormone (GnRH), thyrotropin-releasing hormone (TRH), growth hormone (GH), melatonin, oestrogens, the family of fibroblast growth factors (FGFs), the family of neurotrophins and the glial cell-derived neurotrophic factor (GDNF). There are some trophic (neurotrophic) factors that already been tested in patients with spinal cord injury (SCI), but only shown partial recovery effect. It is possible that, the administration of these trophic factors together with physical rehabilitation, act synergistically and, therefore, significantly improve the quality of life of patients with SCI.

Analgesic Effects of Directed Repetitive Transcranial Magnetic Stimulation in Acute Neuropathic Pain After Spinal Cord Injury

Objectives

Central neuropathic pain (CNP) often appears following spinal cord injury (SCI), but current treatments are not always successful. In this study, we evaluated the analgesic effects of repetitive transcranial magnetic stimulation (rTMS) applied over the hand area of the motor cortex in patients with acute CNP after SCI.

Methods

A total of 48 patients with complete or incomplete SCI and acute CNP participated in this study and were randomized to receive either rTMS (10 Hz, 1,500 stimuli; N = 24) or a sham intervention (N = 24) for three weeks. The numeric rating scale (NRS) and Short-Form McGill Pain Questionnaire-2 (Chinese Edition; SF-MPQ-2-CN) were analyzed to assess the degree of pain. Brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) were collected to explore expression influenced by rTMS. Motor-evoked potential (MEP) latency and maximal amplitude were measured to determine neurophysiological changes. The assessments were carried out at baseline (T0), three days (T1), one week (T2), two weeks (T3), and three weeks (T4) after onset of treatment.

Results

The analysis showed significant treatment–time interactions for the quality and intensity of pain, as measured by NRS (P &lt; 0.001, η2 = 0.441) and SF-MPQ-2 (P &lt; 0.001, η2 = 0.590). Compared with the sham group, the NRS and SF-MPQ2-CN scores were significantly lower on the third day (P &lt; 0.001, Cohen’s d = 1.135; P = 0.006, Cohen’s d = 0.616) and after one week (P &lt; 0.001, Cohen’s d = 0.846; P = 0.012, Cohen’s d = 0.557) of treatment. In addition, the serum levels of BDNF and NGF were significantly higher in the treated group after three weeks (P = 0.015, Cohen’s d = 0.539; P = 0.009, Cohen’s d = 0.580), and the MEP amplitude increased by 109.59% (P = 0.033, Cohen’s d = 0.464).

Conclusions

These findings indicate that 10 Hz rTMS over the hand area of the motor cortex could alleviate acute CNP in the early phase of SCI and could enhance MEP parameters and modulate BDNF and NGF secretion.

The NGFR100W Mutation Specifically Impairs Nociception without Affecting Cognitive Performance in a Mouse Model of Hereditary Sensory and Autonomic Neuropathy Type V

Nerve growth factor (NGF) is a key mediator of nociception, acting during the development and differentiation of dorsal root ganglion (DRG) neurons, and on adult DRG neuron sensitization to painful stimuli. NGF also has central actions in the brain, where it regulates the phenotypic maintenance of cholinergic neurons. The physiological function of NGF as a pain mediator is altered in patients with Hereditary Sensory and Autonomic Neuropathy type V (HSAN V), caused by the 661C>T transition in the Ngf gene, resulting in the R100W missense mutation in mature NGF. Homozygous HSAN V patients present with congenital pain insensitivity, but are cognitively normal. This led us to hypothesize that the R100W mutation may differentially affect the central and peripheral actions of NGF. To test this hypothesis and provide a mechanistic basis to the HSAN V phenotype, we generated transgenic mice harboring the human 661C>T mutation in the Ngf gene and studied both males and females. We demonstrate that heterozygous NGF^R100W/wt mice display impaired nociception. DRG neurons of NGF^R100W/wt mice are morphologically normal, with no alteration in the different DRG subpopulations, whereas skin innervation is reduced. The NGF^R100W protein has reduced capability to activate pain-specific signaling, paralleling its reduced ability to induce mechanical allodynia. Surprisingly, however, NGF^R100W/wt mice, unlike heterozygous mNGF^+/- mice, show no learning or memory deficits, despite a reduction in secretion and brain levels of NGF. The results exclude haploinsufficiency of NGF as a mechanistic cause for heterozygous HSAN V mice and demonstrate a specific effect of the R100W mutation on nociception.SIGNIFICANCE STATEMENT The R100W mutation in nerve growth factor (NGF) causes Hereditary Sensory and Autonomic Neuropathy type V, a rare disease characterized by impaired nociception, even in apparently clinically silent heterozygotes. For the first time, we generated and characterized heterozygous knock-in mice carrying the human R100W-mutated allele (NGF^R100W/wt). Mutant mice have normal nociceptor populations, which, however, display decreased activation of pain transduction pathways. NGF^R100W interferes with peripheral and central NGF bioavailability, but this does not impact on CNS function, as demonstrated by normal learning and memory, in contrast with heterozygous NGF knock-out mice. Thus, a point mutation allows neurotrophic and pronociceptive functions of NGF to be split, with interesting implications for the treatment of chronic pain.

Molecular determinants for ligand binding at Nav1.4 and Nav1.7 channels: Experimental affinity results analyzed by molecular modeling

Here, we focused on exploring the selectivity mechanism against Nav1.7 over Nav1.4 due to different binding modes of two selected inhibitors. By the superposition of Nav1.7 and Nav1.4 proteins, we selected the most homologous chain of Nav1.7 with Nav1.4, defining the active site of Nav1.4-VSD4 based on the aryl sulfonamide binding site of Nav1.7-VSD4. Comparison of the conformations exhibited by Tyr1386 (Nav1.4) and Tyr1537 (Nav1.7) suggested that the steric hindrance caused by Tyr1386 owned primary influence on inhibition selectivity, which was further verified through molecular docking and MD simulation of two representative inhibitors. Our finding would be helpful for discovery of selective Nav1.7 inhibitors over Nav1.4.

Deciphering the Allosteric Binding Mechanism of the Human Tropomyosin Receptor Kinase A ( hTrkA) Inhibitors

Access to cryptic binding pockets or allosteric sites on a kinase that present themselves when the enzyme is in a specific conformational state offers a paradigm shift in designing the next generation small molecule kinase inhibitors. The current work showcases an extensive and exhaustive array of in vitro biochemical and biophysical tools and techniques deployed along with structural biology efforts of inhibitor-bound kinase complexes to characterize and confirm the cryptic allosteric binding pocket and docking mode of the small molecule actives identified for hTrkA. Specifically, assays were designed and implemented to lock the kinase in a predominantly active or inactive conformation and the effect of the kinase inhibitor probed to understand the hTrkA binding and hTrkB selectivity. The current outcome suggests that inhibitors with a fast association rate take advantage of the inactive protein conformation and lock the kinase state by also exhibiting a slow off-rate. This in turn shifts the inactive/active state protein conformational equilibrium cycle, affecting the subsequent downstream signaling.

Targeting tropomyosin receptor kinase for cancer therapy

NTRKs and their expression product tropomyosin receptor kinases (Trks) are widely distributed in mammals. While neural growth factor (NGF)-induced normal Trk activation plays a key role in nerve growth, NTRK alternations occurring in tumor cells were highly correlated to tumor progression and invasion. Recent clinical data from several pan-Trk inhibitors have demonstrated potential and broad applications in various cancers. This intrigues us to summarize the development of inhibitors targeting Trks with different mechanisms of action and their applications in cancer therapy. We believe that this perspective would be of great help in investigating novel anticancer drugs with better therapeutic index.

Pharmacology of nerve growth factor and discovery of tanezumab, an anti-nerve growth factor antibody and pain therapeutic

The pharmacology of Nerve Growth Factor (NGF) and the discovery and development of tanezumab, a monoclonal anti-NGF antibody for the treatment of pain illustrate the complex and unpredictable nature of modern drug development. Initial efforts attempted to use NGF agonistically for Alzheimer's disease and neuropathies. Most unexpectedly, clinical studies unmasked hyperalgesic effects. These observations together with new data emerging from molecular and animal model studies stimulated the idea of using an NGF antagonist for chronic pain. These events also reflect the advances of neuropharmacology from classical small molecule efforts directed at neurotransmitter receptors to modern biotechnology with significant integration in molecular biology, biochemistry, and protein engineering.

Physiologic facet capsule stretch can induce pain & upregulate matrix metalloproteinase-3 in the dorsal root ganglia when preceded by a physiological mechanical or nonpainful chemical exposure

BACKGROUND

Neck pain from cervical facet loading is common and induces inflammation and upregulation of nerve growth factor (NGF) that can sensitize the joint afferents. Yet, the mechanisms by which these occur and whether afferents can be pre-conditioned by certain nonpainful stimuli are unknown. This study tested the hypothesis that a nonpainful mechanical or chemical insult predisposes a facet joint to generate pain after a later exposure to typically nonpainful distraction.

METHODS

Rats were exposed to either a nonpainful distraction or an intra-articular subthreshold dose of NGF followed by a nonpainful distraction two days later. Mechanical hyperalgesia was measured daily and C6 dorsal root ganglia (DRG) tissue was assayed for NGF and matrix metalloproteinase-3 (MMP-3) expression on day 7.

FINDINGS

The second distraction increased joint displacement and strains compared to its first application (p = 0.0011). None of the initial exposures altered behavioral sensitivity in either of the groups being pre-conditioned or in controls; but, sensitivity was established in both groups receiving a second distraction within one day that lasted until day 7 (p < 0.024). NGF expression in the DRG was increased in both groups undergoing a pre-conditioning exposure (p < 0.0232). Similar findings were observed for MMP-3 expression, with a pre-conditioning exposure increasing levels after an otherwise nonpainful facet distraction.

INTERPRETATION

These findings suggest that nonpainful insults to the facet joint, when combined, can generate painful outcomes, possibly mediated by upregulation of MMP-3 and mature NGF.

Emerging therapeutic agents in osteoarthritis

Osteoarthritis (OA) is the most common joint disorder and a leading cause of disability. Current treatments for OA can improve symptoms but do not delay the progression of disease. In the last years, much effort has been devoted to developing new treatments for OA focused on pain control, inflammatory mediators or degradation of articular tissues. Although promising results have been obtained in ex vivo studies and animal models of OA, few of these agents have completed clinical trials. Available clinical data support the interest of nerve growth factor as a target in pain control as well as the disease-modifying potential of inhibitors of Wnt signaling or catabolic enzymes such as aggrecanases and cathepsin K, and anabolic strategies like fibroblast growth factor-18 or cellular therapies. Carefully controlled studies in patients selected according to OA phenotypes and with a long follow-up will help to confirm the relevance of these new approaches as emerging therapeutic treatments in OA.

A New Model of Sensorial Neuron-Like Cells for HTS of Novel Analgesics for Neuropathic Pain

In this study we developed a new translational phenotypic in vitro model for high-throughput screening (HTS) of novel analgesics for treating neuropathic pain, in order to address the poor translation of traditional recombinant models. The immortalized dorsal root ganglia (DRG) neuron-like F11 cell line was selected based on its phenotype after differentiation. The acquisition of neuronal characteristics was evaluated by measuring the expression of TrkA as a DRG neuron marker ( p < 0.01) as well as by measuring the global neurite length ( p < 0.001). The response of F11 cells to ATP and KCl was obtained by measuring intracellular calcium concentration, dynamic mass redistribution, and membrane potential. A KCl-induced increase of intracellular calcium levels was chosen as the readout because of the better signal quality, higher reproducibility, and greater compatibility with HTS assay requirements compared with other methods. The response to KCl differed significantly between differentiated and undifferentiated cells ( p < 0.05), with an EC₅₀ value of 5 mM in differentiated cells. The model was validated by screening the Prestwick Chemical Library. Five hits already proposed for neuropathic-related pain were identified, with IC₅₀ values between 1 and 7 µM. This cell model provides a new tool for screening novel analgesics for the relief of neuropathic pain.

In silico exploration of aryl sulfonamide analogs as voltage-gated sodium channel 1.7 inhibitors by using 3D-QSAR, molecular docking study, and molecular dynamics simulations

It has been demonstrated by human genetics that the voltage-gated sodium channel Nav1.7 is currently a promising target for the treatment of pain. In this research, we performed molecular simulation works on a series of classic aryl sulfonamide Nav1.7 inhibitors using three-dimensional quantitative structure-activity relationships (3D-QSAR), molecular docking and molecular dynamics (MD) simulations for the first time to explore the correlation between their structures and activities. The results of the relevant statistical parameters of comparative molecular field analyses (CoMFA) and comparative molecular similarity indices analyses (CoMSIA) had been verified to be reasonable, and the deep relationship between the structures and activities of these inhibitors was obtained by analyzing the contour maps. The generated 3D-QSAR model showed a good predictive ability and provided valuable clues for the rational modification of molecules. The interactions between compounds and proteins were modeled by molecular docking studies. Finally, accuracy of the docking results and stability of the complexes were verified by 100 ns MD simulations. Detailed information on the key residues at the binding site and the types of interactions they participate in involved was obtained. The van der Waals energy contributed the most in the molecular binding process according to the calculation of binding free energy. All research results provided a good basis for further research on novel and effective Nav1.7 inhibitors.

Nerve Growth Factor Antagonists: Is the Future of Monoclonal Antibodies Becoming Clearer?

Although there is an unmet need for pain medications that are both effective and safe, virtually no novel analgesics have been approved over the past two decades. In view of both experimental and clinical evidence of a major role for nerve growth factor (NGF) in the generation and maintenance of a wide range of pain states, the clinical development of humanised anti-nerve growth factor monoclonal antibodies (anti-NGF mAbs) aroused particular interest. However, the US Food and Drug Administration (FDA) placed a clinical hold on anti-NGF mAb clinical studies in late 2010, first because of reports of serious joint-related adverse events, and afterwards because of sympathetic nervous system safety concerns. The development programmes of tanezumab and fasinumab resumed after the FDA lifted its hold in March 2015, whereas other anti-NGF mAbs were dropped by their sponsors. This article provides an updated review on the analgesic efficacy and safety of anti-NGF agents based on data from fully published studies and public information from websites, and discusses the possible future role of these agents in managing chronic pain. The efficacy of anti-NGF mAbs was highly variable depending on the chronic pain condition studied. The most consistent and convincing results were obtained in patients with symptomatic osteoarthritis of the knee and/or hip. Conversely, studies in non-specific lower back pain and peripheral neuropathic pain generated mixed results. Finally, there was no conclusive evidence of the effectiveness of anti-NGF mAbs in cancer pain and urological chronic pelvic pain syndromes. Treatment-emergent adverse events were similar across anti-NGF mAbs, thus being suggestive of 'class-specific effects'. Although most patients tolerated anti-NGF agents well, neurosensory symptoms occurred frequently, and some patients developed new or worsened peripheral neuropathies. However, the most problematic safety issue was rapidly destructive arthropathies, leading to joint replacement surgery. To date, the aetiologies of joint-related side effects and their pathophysiology have not been clearly elucidated. However, some risk factors have been identified, such as higher doses of anti-NGF mAbs and longer drug exposure, concurrent nonsteroidal anti-inflammatory drug use and pre-existing subchondral insufficiency fractures. Taken together, the present data suggest that low-dose anti-NGF mABs may exhibit a favourable risk-benefit ratio in selected patients with certain chronic pain conditions, especially symptomatic osteoarthritis.

From Cancer to Pain Target by Automated Selectivity Inversion of a Clinical Candidate

Elimination of inadvertent binding is crucial for inhibitor design targeting conserved protein classes like kinases. Compounds in clinical trials provide a rich source for initiating drug design efforts by exploiting such secondary binding events. Considering both aspects, we shifted the selectivity of tozasertib, originally developed against AurA as cancer target, toward the pain target TrkA. First, selectivity-determining features in binding pockets were identified by fusing interaction grids of several key and off-target conformations. A focused library was subsequently created and prioritized using a multiobjective selection scheme that filters for selective and highly active compounds based on orthogonal methods grounded in computational chemistry and machine learning. Eighteen high-ranking compounds were synthesized and experimentally tested. The top-ranked compound has 10000-fold improved selectivity versus AurA, nanomolar cellular activity, and is highly selective in a kinase panel. This was achieved in a single round of automated in silico optimization, highlighting the power of recent advances in computer-aided drug design to automate design and selection processes.

Activation of the NGF/TrkA signaling pathway attenuates diabetic erectile dysfunction

Erectile dysfunction (ED) is a common complication of diabetes mellitus (DM). The exact role of the NGF/TrkA signaling pathway in the pathogenesis of diabetic ED is largely unknown. In the present study, we investigated the role of the NGF/TrkA signaling pathway in Sprague-Dawley rats with diabetic ED. Animals were divided into 2 groups: the normal group and the DM ED model group. The model group included the blank subgroup, the negative control (NC) subgroup, the TrkA subgroup and the TrkA + NGF subgroup. Erectile function, intracavernous pressure (ICP) and mean arterial pressure were measured respectively. Immunohistochemistry was used to examine the number of neuronal nitric oxide synthase (nNOS) expressing nerve fibers. The quantitative real-time polymerase chain reaction was applied to detect the mRNA expressions of NGF and TrkA in the cavernous tissue. Further, Western blotting was conducted to detect the expressions of NGF, TrkA and its downstream ERK pathway-related proteins. Higher erectile frequency, ICP values and diastolic function, more nNOS-positive nerve fibers, and decreased systolic function of the corpus cavernosum smooth muscle were found in the TrkA and TrkA+NGF groups when compared with the blank and the NC groups. Moreover, significantly higher mRNA expressions of NGF and TrkA, and upregulated protein expressions of NGF, TrkA, c-raf, ERK1/2 and CREB1 were found in the TrkA and the TrkA + NGF groups. In conclusion, downregulation in the NGF/TrkA signaling pathway may contribute to the pathogenesis of diabetic ED.

Nerve growth factor is elevated in the CSF of patients with multiple sclerosis and central neuropathic pain

Central neuropathic pain (CNP) is common and disabling among patients with multiple sclerosis (MS). The pathological mechanisms underlying CNP in MS are not well understood. We explored whether NGF is implicated in the pathogenesis of CNP in MS. We measured NGF concentration in the CSF of 73 patients affected by MS, 15 with and 58 without CNP and 14 controls. We found increased levels of NGF in the CSF of patients with CNP compared to patients without and to controls. This finding supports the hypothesis that NGF plays a role in MS related CNP.

Evaluation and Characterization of Trk Kinase Inhibitors for the Treatment of Pain: Reliable Binding Affinity Predictions from Theory and Computation

Optimization of ligand binding affinity to the target protein of interest is a primary objective in small-molecule drug discovery. Until now, the prediction of binding affinities by computational methods has not been widely applied in the drug discovery process, mainly because of its lack of accuracy and reproducibility as well as the long turnaround times required to obtain results. Herein we report on a collaborative study that compares tropomyosin receptor kinase A (TrkA) binding affinity predictions using two recently formulated fast computational approaches, namely, Enhanced Sampling of Molecular dynamics with Approximation of Continuum Solvent (ESMACS) and Thermodynamic Integration with Enhanced Sampling (TIES), to experimentally derived TrkA binding affinities for a set of Pfizer pan-Trk compounds. ESMACS gives precise and reproducible results and is applicable to highly diverse sets of compounds. It also provides detailed chemical insight into the nature of ligand-protein binding. TIES can predict and thus optimize more subtle changes in binding affinities between compounds of similar structure. Individual binding affinities were calculated in a few hours, exhibiting good correlations with the experimental data of 0.79 and 0.88 from the ESMACS and TIES approaches, respectively. The speed, level of accuracy, and precision of the calculations are such that the affinity predictions can be used to rapidly explain the effects of compound modifications on TrkA binding affinity. The methods could therefore be used as tools to guide lead optimization efforts across multiple prospective structurally enabled programs in the drug discovery setting for a wide range of compounds and targets.