Tirasemtiv amplifies skeletal muscle response to nerve activation in humans

Tirasemtiv enhances submaximal muscle tension in an Acta1:p.Asp286Gly mouse model of nemaline myopathy

Nemaline myopathies are the most common form of congenital myopathies. Variants in ACTA1 (NEM3) comprise 15-25% of all nemaline myopathy cases. Patients harboring variants in ACTA1 present with a heterogeneous disease course characterized by stable or progressive muscle weakness and, in severe cases, respiratory failure and death. To date, no specific treatments are available. Since NEM3 is an actin-based thin filament disease, we tested the ability of tirasemtiv, a fast skeletal muscle troponin activator, to improve skeletal muscle function in a mouse model of NEM3, harboring the patient-based p.Asp286Gly variant in Acta1. Acute and long-term tirasemtiv treatment significantly increased muscle contractile capacity at submaximal stimulation frequencies in both fast-twitch extensor digitorum longus and gastrocnemius muscle, and intermediate-twitch diaphragm muscle in vitro and in vivo. Additionally, long-term tirasemtiv treatment in NEM3 mice resulted in a decreased respiratory rate with preserved minute volume, suggesting more efficient respiration. Altogether, our data support the therapeutic potential of fast skeletal muscle troponin activators in alleviating skeletal muscle weakness in a mouse model of NEM3 caused by the Acta1:p.Asp286Gly variant.

Small molecule drugs to improve sarcomere function in those with acquired and inherited myopathies

Muscle weakness is a hallmark of inherited or acquired myopathies. It is a major cause of functional impairment and can advance to life-threatening respiratory insufficiency. During the past decade, several small-molecule drugs that improve the contractility of skeletal muscle fibers have been developed. In this review, we provide an overview of the available literature and the mechanisms of action of small-molecule drugs that modulate the contractility of sarcomeres, the smallest contractile units in striated muscle, by acting on myosin and troponin. We also discuss their use in the treatment of skeletal myopathies. The first of three classes of drugs discussed here increase contractility by decreasing the dissociation rate of calcium from troponin and thereby sensitizing the muscle to calcium. The second two classes of drugs directly act on myosin and stimulate or inhibit the kinetics of myosin-actin interactions, which may be useful in patients with muscle weakness or stiffness.NEW & NOTEWORTHY During the past decade, several small molecule drugs that improve the contractility of skeletal muscle fibers have been developed. In this review, we provide an overview of the available literature and the mechanisms of action of small molecule drugs that modulate the contractility of sarcomeres, the smallest contractile units in striated muscle, by acting on myosin and troponin.

Molecular Mechanisms of Deregulation of Muscle Contractility Caused by the R168H Mutation in TPM3 and Its Attenuation by Therapeutic Agents

The substitution for Arg168His (R168H) in γ-tropomyosin (TPM3 gene, Tpm3.12 isoform) is associated with congenital muscle fiber type disproportion (CFTD) and muscle weakness. It is still unclear what molecular mechanisms underlie the muscle dysfunction seen in CFTD. The aim of this work was to study the effect of the R168H mutation in Tpm3.12 on the critical conformational changes that myosin, actin, troponin, and tropomyosin undergo during the ATPase cycle. We used polarized fluorescence microscopy and ghost muscle fibers containing regulated thin filaments and myosin heads (myosin subfragment-1) modified with the 1,5-IAEDANS fluorescent probe. Analysis of the data obtained revealed that a sequential interdependent conformational-functional rearrangement of tropomyosin, actin and myosin heads takes place when modeling the ATPase cycle in the presence of wild-type tropomyosin. A multistep shift of the tropomyosin strands from the outer to the inner domain of actin occurs during the transition from weak to strong binding of myosin to actin. Each tropomyosin position determines the corresponding balance between switched-on and switched-off actin monomers and between the strongly and weakly bound myosin heads. At low Ca²⁺, the R168H mutation was shown to switch some extra actin monomers on and increase the persistence length of tropomyosin, demonstrating the freezing of the R168HTpm strands close to the open position and disruption of the regulatory function of troponin. Instead of reducing the formation of strong bonds between myosin heads and F-actin, troponin activated it. However, at high Ca²⁺, troponin decreased the amount of strongly bound myosin heads instead of promoting their formation. Abnormally high sensitivity of thin filaments to Ca²⁺, inhibition of muscle fiber relaxation due to the appearance of the myosin heads strongly associated with F-actin, and distinct activation of the contractile system at submaximal concentrations of Ca²⁺ can lead to muscle inefficiency and weakness. Modulators of troponin (tirasemtiv and epigallocatechin-3-gallate) and myosin (omecamtiv mecarbil and 2,3-butanedione monoxime) have been shown to more or less attenuate the negative effects of the tropomyosin R168H mutant. Tirasemtiv and epigallocatechin-3-gallate may be used to prevent muscle dysfunction.

Prescription and acceptance of durable medical equipment in FORTITUDE-ALS, a study of reldesemtiv in ALS: post hoc analyses of a randomized, double-blind, placebo-controlled clinical trial

Objective: To evaluate the possible effect of reldesemtiv, a fast skeletal muscle troponin activator, on prescription and acceptance of durable medical equipment (DME) in the FORTITUDE-ALS trial. Methods: Health economic outcome information was collected in FORTITUDE-ALS (NCT03160898); sites recorded if and when DME, specifically manual or power wheelchairs, gastrostomy tubes, noninvasive ventilators, or augmentative language devices, was prescribed by a physician and accepted by the patient (DME-PAP) during the trial. Acceptance was defined as the patient agreeing the item was needed. Cox regression analysis compared time to DME-PAP for each reldesemtiv dose with placebo. Post hoc analyses evaluated all reldesemtiv doses compared with placebo. Results: At least one DME item was prescribed and accepted by 33/114 (28.9%) of placebo patients, 19/112 (17.0%) of patients receiving reldesemtiv 150 mg bid, 24/113 (21.2%) receiving 300 mg bid, and 29/117 (24.8%) receiving 450 mg bid. The proportion of new DME-PAP was significantly lower in patients receiving reldesemtiv 150 mg bid vs placebo (17.0% vs 28.9%, p = 0.032). The hazard ratio versus placebo for accepting at least one DME item for all reldesemtiv doses combined was 0.61 (confidence interval: 0.39, 0.96, p = 0.032). 25% of placebo patients were prescribed and agreed to obtain a DME item by 84 days; this threshold was met for reldesemtiv-treated patients at 120 days. Conclusions: Results suggest ALS patients receiving reldesemtiv may have lower risk of and delayed need for DME related to impaired mobility, breathing, swallowing, or speaking; this delay is consistent with other measures indicating delay in disease progression.

Discovery of Reldesemtiv, a Fast Skeletal Muscle Troponin Activator for the Treatment of Impaired Muscle Function

The discovery of reldesemtiv, a second-generation fast skeletal muscle troponin activator (FSTA) that increases force production at submaximal stimulation frequencies, is reported. Property-based optimization of high throughput screening hit 1 led to compounds with improved free exposure and in vivo muscle activation potency compared to the first-generation FSTA, tirasemtiv. Reldesemtiv demonstrated increased muscle force generation in a phase 1 clinical trial and is currently being evaluated in clinical trials for the treatment of amyotrophic lateral sclerosis.

Small Molecules acting on Myofilaments as Treatments for Heart and Skeletal Muscle Diseases

Hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) are the most prevalent forms of the chronic and progressive pathological condition known as cardiomyopathy. These diseases have different aetiologies; however, they share the feature of haemodynamic abnormalities, which is mainly due to dysfunction in the contractile proteins that make up the contractile unit known as the sarcomere. To date, pharmacological treatment options are not disease-specific and rather focus on managing the symptoms, without addressing the disease mechanism. Earliest attempts at improving cardiac contractility by modulating the sarcomere indirectly (inotropes) resulted in unwanted effects. In contrast, targeting the sarcomere directly, aided by high-throughput screening systems, could identify small molecules with a superior therapeutic value in cardiac muscle disorders. Herein, an extensive literature review of 21 small molecules directed to five different targets was conducted. A simple scoring system was created to assess the suitability of small molecules for therapy by evaluating them in eight different criteria. Most of the compounds failed due to lack of target specificity or poor physicochemical properties. Six compounds stood out, showing a potential therapeutic value in HCM, DCM or heart failure (HF). Omecamtiv Mecarbil and Danicamtiv (myosin activators), Mavacamten, CK-274 and MYK-581 (myosin inhibitors) and AMG 594 (Ca²⁺-sensitiser) are all small molecules that allosterically modulate troponin or myosin. Omecamtiv Mecarbil showed limited efficacy in phase III GALACTIC-HF trial, while, results from phase III EXPLORER-HCM trial were recently published, indicating that Mavacamten reduced left ventricular outflow tract (LVOT) obstruction and diastolic dysfunction and improved the health status of patients with HCM. A novel category of small molecules known as “recouplers” was reported to target a phenomenon termed uncoupling commonly found in familial cardiomyopathies but has not progressed beyond preclinical work. In conclusion, the contractile apparatus is a promising target for new drug development.

Fast skeletal muscle troponin activator CK-2066260 mitigates skeletal muscle weakness independently of the underlying cause

BACKGROUND

Muscle weakness is a common symptom in numerous diseases and a regularly occurring problem associated with ageing. Prolonged low-frequency force depression (PLFFD) is a form of exercise-induced skeletal muscle weakness observed after exercise. Three different intramuscular mechanisms underlying PLFFD have been identified: decreased sarcoplasmic reticulum Ca²⁺ release, decreased myofibrillar Ca²⁺ sensitivity, and myofibrillar dysfunction. We here used these three forms of PLFFD as models to study the effectiveness of a fast skeletal muscle troponin activator, CK-2066260, to mitigate muscle weakness.

METHODS

Experiments were performed on intact single muscle fibres or fibre bundles from mouse flexor digitorum brevis, which were stimulated with electrical current pulses, while force and the free cytosolic [Ca²⁺ ] ([Ca²⁺ ]_i ) were measured. PLFFD was induced by three different stimulation protocols: (i) repeated isometric contractions at low intensity (350 ms tetani given every 5 s for 100 contractions); (ii) repeated isometric contractions at high intensity (250 ms tetani given every 0.5 s for 300 contractions); and (iii) repeated eccentric contractions (350 ms tetani with 20% length increase given every 20 s for 10 contractions). The extent and cause of PLFFD were assessed by comparing the force-[Ca²⁺ ]_i relationship at low (30 Hz) and high (120 Hz) stimulation frequencies before (control) and 30 min after induction of PLFFD, and after an additional 5 min of rest in the presence of CK-2066260 (10 μM).

RESULTS

Prolonged low-frequency force depression following low-intensity and high-intensity fatiguing contractions was predominantly due to decreased sarcoplasmic reticulum Ca²⁺ release and decreased myofibrillar Ca²⁺ sensitivity, respectively. CK-2066260 exposure resulted in marked increases in 30 Hz force from 52 ± 16% to 151 ± 13% and from 6 ± 4% to 98 ± 40% of controls with low-intensity and high-intensity contractions, respectively. Following repeated eccentric contractions, PLFFD was mainly due to myofibrillar dysfunction, and it was not fully reversed by CK-2066260 with 30 Hz force increasing from 48 ± 8% to 76 ± 6% of the control.

CONCLUSIONS

The fast skeletal muscle troponin activator CK-2066260 effectively mitigates muscle weakness, especially when it is caused by impaired activation of the myofibrillar contractile machinery due to either decreased sarcoplasmic reticulum Ca²⁺ release or reduced myofibrillar Ca²⁺ sensitivity.

A Phase 2, Double-Blind, Randomized, Dose-Ranging Trial Of Reldesemtiv In Patients With ALS

Objective:

To evaluate safety, dose response, and preliminary efficacy of reldesemtiv over 12 weeks in patients with amyotrophic lateral sclerosis (ALS).

Methods:

Patients (≤2 years since diagnosis) with slow upright vital capacity (SVC) of ≥60% were randomized 1:1:1:1 to reldesemtiv 150, 300, or 450 mg twice daily (bid) or placebo; active treatment was 12 weeks with 4-week follow-up. Primary endpoint was change in percent predicted SVC at 12 weeks; secondary measures included ALS Functional Rating Scale-Revised (ALSFRS-R) and muscle strength mega-score.

Results:

Patients (N = 458) were enrolled; 85% completed 12-week treatment. The primary analysis failed to reach statistical significance (p = 0.11); secondary endpoints showed no statistically significant effects (ALSFRS-R, p = 0.09; muscle strength megascore, p = 0.31). Post hoc analyses pooling all active reldesemtiv-treated patients compared against placebo showed trends toward benefit in all endpoints (progression rate for SVC, ALSFRS-R, and muscle strength mega-score (nominal p values of 0.10, 0.01 and 0.20 respectively)). Reldesemtiv was well tolerated, with nausea and fatigue being the most common side effects. A dose-dependent decrease in estimated glomerular filtration rate was noted, and transaminase elevations were seen in approximately 5% of patients. Both hepatic and renal abnormalities trended toward resolution after study drug discontinuation.

Conclusions:

Although the primary efficacy analysis did not demonstrate statistical significance, there were trends favoring reldesemtiv for all three endpoints, with effect sizes generally regarded as clinically important. Tolerability was good; modest hepatic and renal abnormalities were reversible. The impact of reldesemtiv on patients with ALS should be assessed in a pivotal Phase 3 trial. (ClinicalTrials.gov Identifier: NCT03160898)

Potential of the Cardiovascular Drug Levosimendan in the Management of Amyotrophic Lateral Sclerosis: An Overview of a Working Hypothesis

Levosimendan is a calcium sensitizer that promotes myocyte contractility through its calcium-dependent interaction with cardiac troponin C. Administered intravenously, it has been used for nearly 2 decades to treat acute and advanced heart failure and to support the heart function in various therapy settings characterized by low cardiac output. Effects of levosimendan on noncardiac muscle suggest a possible new application in the treatment of people with amyotrophic lateral sclerosis (ALS), a neuromuscular disorder characterized by progressive weakness, and eventual paralysis. Previous attempts to improve the muscle response in ALS patients and thereby maintain respiratory function and delay progression of disability have produced some mixed results. Continuing this line of investigation, levosimendan has been shown to enhance in vitro the contractility of the diaphragm muscle fibers of non-ALS patients and to improve in vivo diaphragm neuromuscular efficiency in healthy subjects. Possible positive effects on respiratory function in people with ALS were seen in an exploratory phase 2 study, and a phase 3 clinical trial is now underway to evaluate the potential benefit of an oral form of levosimendan on both respiratory and overall functions in patients with ALS. Here, we will review the various known pharmacologic effects of levosimendan, considering their relevance to people living with ALS.

Fast skeletal muscle troponin activator CK-2066260 increases fatigue resistance by reducing the energetic cost of muscle contraction

Key points

Skeletal muscle fatigue limits performance in various physical activities, with exercise intolerance being a key symptom in a broad spectrum of diseases.

We investigated whether a small molecule fast skeletal troponin activator (FSTA), CK‐2066260, can mitigate muscle fatigue by reducing the cytosolic free [Ca²⁺] required to produce a given submaximal force and hence decreasing the energy requirement.

Isolated intact single mouse muscle fibres and rat muscles in‐situ treated with CK‐2066260 showed improved muscle endurance., which was accompanied by decreased ATP demand and reduced glycogen usage.

CK‐2066260 treatment improved in‐vivo exercise capacity in healthy rats and in a rat model of peripheral artery insufficiency.

In conclusion, we show that the FSTA CK‐2066260 effectively counteracts muscle fatigue in rodent skeletal muscle in vitro, in situ, and in vivo. This may translate to humans and provide a promising pharmacological treatment to patients suffering from severe muscle weakness and exercise intolerance.

Abstract

Skeletal muscle fatigue limits performance during physical exercise and exacerbated muscle fatigue is a prominent symptom among a broad spectrum of diseases. The present study investigated whether skeletal muscle fatigue is affected by the fast skeletal muscle troponin activator (FSTA) CK‐2066260, which increases myofibrillar Ca²⁺ sensitivity and amplifies the submaximal force response. Because more force is produced for a given Ca²⁺, we hypothesized that CK‐2066260 could mitigate muscle fatigue by reducing the energetic cost of muscle activation. Isolated single mouse muscle fibres were fatigued by 100 repeated 350 ms contractions while measuring force and the cytosolic free [Ca²⁺] or [Mg²⁺] ([Mg²⁺]_i). When starting fatiguing stimulation at matching forces (i.e. lower stimulation frequency with CK‐2066260): force was decreased by ∼50% with and by ∼75% without CK‐2066260; [Mg²⁺]_i was increased by ∼10% with and ∼32% without CK‐2066260, reflecting a larger decrease in [ATP] in the latter. The glycogen content in in situ stimulated rat muscles fatigued by repeated contractions at matching forces was about two times higher with than without CK‐2066260. Voluntary exercise capacity, assessed by rats performing rotarod exercise and treadmill running, was improved in the presence of CK‐2066260. CK‐2066260 treatment also increased skeletal muscle fatigue resistance and exercise performance in a rat model of peripheral artery insufficiency. In conclusion, we demonstrate that the FSTA CK‐2066260 mitigates skeletal muscle fatigue by reducing the metabolic cost of force generation.

Skeletal muscle fatigue limits performance in various physical activities, with exercise intolerance being a key symptom in a broad spectrum of diseases.

We investigated whether a small molecule fast skeletal troponin activator (FSTA), CK‐2066260, can mitigate muscle fatigue by reducing the cytosolic free [Ca²⁺] required to produce a given submaximal force and hence decreasing the energy requirement.

Isolated intact single mouse muscle fibres and rat muscles in‐situ treated with CK‐2066260 showed improved muscle endurance., which was accompanied by decreased ATP demand and reduced glycogen usage.

CK‐2066260 treatment improved in‐vivo exercise capacity in healthy rats and in a rat model of peripheral artery insufficiency.

In conclusion, we show that the FSTA CK‐2066260 effectively counteracts muscle fatigue in rodent skeletal muscle in vitro, in situ, and in vivo. This may translate to humans and provide a promising pharmacological treatment to patients suffering from severe muscle weakness and exercise intolerance.

Sarcomere Dysfunction in Nemaline Myopathy

Nemaline myopathy (NM) is among the most common non-dystrophic congenital myopathies (incidence 1:50.000). Hallmark features of NM are skeletal muscle weakness and the presence of nemaline bodies in the muscle fiber. The clinical phenotype of NM patients is quite diverse, ranging from neonatal death to normal lifespan with almost normal motor function. As the respiratory muscles are involved as well, severely affected patients are ventilator-dependent. The mechanisms underlying muscle weakness in NM are currently poorly understood. Therefore, no therapeutic treatment is available yet.

Eleven implicated genes have been identified: ten genes encode proteins that are either components of thin filament, or are thought to contribute to stability or turnover of thin filament proteins. The thin filament is a major constituent of the sarcomere, the smallest contractile unit in muscle. It is at this level of contraction – thin-thick filament interaction – where muscle weakness originates in NM patients.

This review focusses on how sarcomeric gene mutations directly compromise sarcomere function in NM. Insight into the contribution of sarcomeric dysfunction to muscle weakness in NM, across the genes involved, will direct towards the development of targeted therapeutic strategies.

Molecular Mechanisms of TDP-43 Misfolding and Pathology in Amyotrophic Lateral Sclerosis

TAR DNA binding protein 43 (TDP-43) is a versatile RNA/DNA binding protein involved in RNA-related metabolism. Hyper-phosphorylated and ubiquitinated TDP-43 deposits act as inclusion bodies in the brain and spinal cord of patients with the motor neuron diseases: amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). While the majority of ALS cases (90-95%) are sporadic (sALS), among familial ALS cases 5-10% involve the inheritance of mutations in the TARDBP gene and the remaining (90-95%) are due to mutations in other genes such as: C9ORF72, SOD1, FUS, and NEK1 etc. Strikingly however, the majority of sporadic ALS patients (up to 97%) also contain the TDP-43 protein deposited in the neuronal inclusions, which suggests of its pivotal role in the ALS pathology. Thus, unraveling the molecular mechanisms of the TDP-43 pathology seems central to the ALS therapeutics, hence, we comprehensively review the current understanding of the TDP-43's pathology in ALS. We discuss the roles of TDP-43's mutations, its cytoplasmic mis-localization and aberrant post-translational modifications in ALS. Also, we evaluate TDP-43's amyloid-like in vitro aggregation, its physiological vs. pathological oligomerization in vivo, liquid-liquid phase separation (LLPS), and potential prion-like propagation propensity of the TDP-43 inclusions. Finally, we describe the various evolving TDP-43-induced toxicity mechanisms, such as the impairment of endocytosis and mitotoxicity etc. and also discuss the emerging strategies toward TDP-43 disaggregation and ALS therapeutics.

Myasthenia gravis: new developments in research and treatment

PURPOSE OF REVIEW

Myasthenia gravis, a rare disorder of the neuromuscular transmission, is increasingly acknowledged as a syndrome more than as a single disease. This review summarizes recent advances in pathophysiology which confirm the disease heterogeneity, and may help find disease-targeted and patient-targeted therapies.

RECENT FINDINGS

Antibodies to the acetylcholine receptor, the muscle-specific tyrosine kinase and the lipoprotein receptor protein 4, characterize disease subtypes with distinct clinical traits and immune-pathogenic mechanisms. Genome-wide approaches have identified susceptibility loci within genes that participate in the immune response. Regulatory T and B cells appear to be defective in myasthenia gravis. In patients with acetylcholine receptor antibodies, thymectomy associated with prednisone proved more effective than prednisone alone in a multicenter randomized trial. New therapeutic options target B cells, B-cell growth factors and complement inhibition, and are currently reserved for patients with refractory disease.

SUMMARY

In the recent past, there has been an active search for new antigens in myasthenia gravis, whereas clinical and experimental studies have provided new insights of crucial pathways in immune regulation, which might become the targets of future therapeutic interventions.

Discovery of Tirasemtiv, the First Direct Fast Skeletal Muscle Troponin Activator

The identification and optimization of the first activators of fast skeletal muscle are reported. Compound 1 was identified from high-throughput screening (HTS) and subsequently found to improve muscle function via interaction with the troponin complex. Optimization of 1 for potency, metabolic stability, and physical properties led to the discovery of tirasemtiv (25), which has been extensively characterized in clinical trials for the treatment of amyotrophic lateral sclerosis.

VITALITY-ALS, a phase III trial of tirasemtiv, a selective fast skeletal muscle troponin activator, as a potential treatment for patients with amyotrophic lateral sclerosis: study design and baseline characteristics

OBJECTIVE

To assess the efficacy of tirasemtiv, a fast skeletal muscle troponin activator, vs. placebo on respiratory function and other functional measures in patients with amyotrophic lateral sclerosis (ALS). This study was designed to confirm and extend results from a large phase IIb trial and maximize tolerability with a slower dose escalation.

METHODS

VITALITY-ALS (NCT02496767) was a multinational, double-blind, randomized, placebo-controlled, parallel-group study in ALS patients. Participants who tolerated two weeks of open-label tirasemtiv (125 mg twice a day) were randomized 3:2:2:2 to placebo or one of three target total daily dose levels of tirasemtiv (250, 375, or 500 mg). Participants randomized to tirasemtiv escalated their dose every two weeks to their target dose level or maximum tolerated dose. The primary outcome measure was change in slow vital capacity from baseline to 24 weeks. Secondary endpoints assessed the effect of tirasemtiv on muscle strength and certain respiratory milestones of disease progression. A four-week randomized withdrawal phase followed 48 weeks of treatment to evaluate the possibility of sustained benefit or rebound decline.

RESULTS

Data collection will be complete in the fourth quarter of 2017.

CONCLUSIONS

VITALITY-ALS was a phase III trial designed to evaluate the efficacy, safety, and tolerability of tirasemtiv in ALS.

CK-2127107 amplifies skeletal muscle response to nerve activation in humans

Introduction

Three studies evaluated safety, tolerability, pharmacokinetics, and pharmacodynamics of CK‐2127107 (CK‐107), a next‐generation fast skeletal muscle troponin activator (FSTA), in healthy participants. We tested the hypothesis that CK‐107 would amplify the force‐frequency response of muscle in humans. Methods: To assess the force‐frequency response, participants received single doses of CK‐107 and placebo in a randomized, double‐blind, 4‐period, crossover study. The force‐frequency response of foot dorsiflexion following stimulation of the deep fibular nerve to activate the tibialis anterior muscle was assessed. Results: CK‐107 significantly increased tibialis anterior muscle response with increasing dose and plasma concentration in a frequency‐dependent manner; the largest increase in peak force was ∼60% at 10 Hz. Discussion: CK‐107 appears more potent and produced larger increases in force than tirasemtiv—a first‐generation FSTA—in a similar pharmacodynamic study, thereby supporting its development for improvement of muscle function of patients. Muscle Nerve57: 729–734, 2018

Optogenetic approach for targeted activation of global calcium transients in differentiated C2C12 myotubes

Excitation-contraction coupling in muscle cells is initiated by a restricted membrane depolarization delimited within the neuromuscular junction. This targeted depolarization triggers an action potential that propagates and induces a global cellular calcium response and a consequent contraction. To date, numerous studies have investigated this excitation-calcium response coupling by using different techniques to depolarize muscle cells. However, none of these techniques mimic the temporal and spatial resolution of membrane depolarization observed in the neuromuscular junction. By using optogenetics in C2C12 muscle cells, we developed a technique to study the calcium response following membrane depolarization induced by photostimulations of membrane surface similar or narrower than the neuromuscular junction area. These stimulations coupled to confocal calcium imaging generate a global cellular calcium response that is the consequence of a membrane depolarization propagation. In this context, this technique provides an interesting, contactless and relatively easy way of investigation of calcium increase/release as well as calcium decrease/re-uptake triggered by a propagated membrane depolarization.

Drugs in clinical development for the treatment of amyotrophic lateral sclerosis

INTRODUCTION

Amyotrophic Lateral Sclerosis (ALS) is a fatal motor neuron progressive disorder for which no treatment exists to date. However, there are other investigational drugs and therapies currently under clinical development may offer hope in the near future. Areas covered: We have reviewed all the ALS ongoing clinical trials (until November 2016) and collected in Clinicaltrials.gov or EudraCT. We have described them in a comprehensive way and have grouped them in the following sections: biomarkers, biological therapies, cell therapy, drug repurposing and new drugs. Expert opinion: Despite multiple obstacles that explain the absence of effective drugs for the treatment of ALS, joint efforts among patient's associations, public and private sectors have fueled innovative research in this field, resulting in several compounds that are in the late stages of clinical trials. Drug repositioning is also playing an important role, having achieved the approval of some orphan drug applications, in late phases of clinical development. Endaravone has been recently approved in Japan and is pending in USA.

Sarcopenia--The search for emerging biomarkers

Sarcopenia, an age-related decline in skeletal muscle mass and function, dramatically affects the life quality of elder people. In view of increasing life expectancy, sarcopenia renders a heavy burden on the health care system. However, although there is a consensus that sarcopenia is a multifactorial syndrome, its etiology, underlying mechanisms, and even definition remain poorly delineated, thus, preventing development of a precise treatment strategy. The main aim of our review is to critically analyze potential sarcopenia biomarkers in light of the molecular mechanisms of their involvement in sarcopenia pathogenesis. Normal muscle mass and function maintenance are proposed to be dependent on the dynamic balance between the positive regulators of muscle growth such as bone morphogenetic proteins (BMPs), brain-derived neurotrophic factor (BDNF), follistatin (FST) and irisin, and negative regulators including TGFβ, myostatin, activins A and B, and growth and differentiation factor-15 (GDF-15). We hypothesize that the shift in this balance to muscle growth inhibitors, along with increased expression of the C- terminal agrin fragment (CAF) associated with age-dependent neuromuscular junction (NMJ) dysfunction, as well as skeletal muscle-specific troponin T (sTnT), a key component of contractile machinery, is a main mechanism underlying sarcopenia pathogenesis. Thus, this review proposes and emphasizes that these molecules are the emerging sarcopenia biomarkers.

A Double-Blinded, Randomized, Placebo-Controlled Trial to Evaluate Efficacy, Safety, and Tolerability of Single Doses of Tirasemtiv in Patients with Acetylcholine Receptor-Binding Antibody-Positive Myasthenia Gravis

Tirasemtiv is a fast skeletal troponin activator that sensitizes the sarcomere to calcium and increases muscle force following subtetanic nerve input. In an animal model of myasthenia gravis (MG), single oral doses of tirasemtiv improved muscle force and reduced fatigability. The purpose of this study was to determine the effect of single doses of tirasemtiv on skeletal muscle function and fatigability in patients with generalized MG. Thirty-two patients with acetylcholine receptor-antibody positive MG and muscle weakness received single doses of tirasemtiv (250 mg or 500 mg) or placebo in a double-blind, randomized treatment sequence with each treatment separated by at least 1 week. Outcome measures included the Quantitative MG Score (QMG), MG Composite, Manual Muscle Testing, and forced vital capacity. At 6 h after dosing, tirasemtiv produced dose-related improvements from baseline in the QMG score (slope: -0.49 QMG point per 250 mg; p = 0.02) and in percent predicted forced vital capacity (slope: 2.2% per 250 mg; p = 0.04). QMG improved >3 points in twice as many patients after 500 mg tirasemtiv than after placebo. Both doses of tirasemtiv were well tolerated; there were no premature terminations or serious adverse events. The results of this study suggest that tirasemtiv may improve muscle function in MG and will be used to support further development of tirasemtiv in neuromuscular diseases.

The small-molecule fast skeletal troponin activator, CK-2127107, improves exercise tolerance in a rat model of heart failure

Heart failure-mediated skeletal myopathy, which is characterized by muscle atrophy and muscle metabolism dysfunction, often manifests as dyspnea and limb muscle fatigue. We have previously demonstrated that increasing Ca(2+) sensitivity of the sarcomere by a small-molecule fast skeletal troponin activator improves skeletal muscle force and exercise performance in healthy rats and models of neuromuscular disease. The objective of this study was to investigate the effect of a novel fast skeletal troponin activator, CK-2127107 (2-aminoalkyl-5-N-heteroarylpyrimidine), on skeletal muscle function and exercise performance in rats exhibiting heart failure-mediated skeletal myopathy. Rats underwent a left anterior descending coronary artery ligation, resulting in myocardial infarction and a progressive decline in cardiac function [left anterior descending coronary artery heart failure (LAD-HF)]. Compared with sham-operated control rats, LAD-HF rat hindlimb and diaphragm muscles exhibited significant muscle atrophy. Fatigability was increased during repeated in situ isokinetic plantar flexor muscle contractions. CK-2127107 produced a leftward shift in the force-Ca(2+) relationship of skinned, single diaphragm, and extensor digitorum longus fibers. Exercise performance, which was assessed by rotarod running, was lower in vehicle-treated LAD-HF rats than in sham controls (116 ± 22 versus 193 ± 31 seconds, respectively; mean ± S.E.M.; P = 0.04). In the LAD-HF rats, a single oral dose of CK-2127107 (10 mg/kg p.o.) increased running time compared with vehicle treatment (283 ± 47 versus 116 ± 22 seconds; P = 0.0004). In summary, CK-2127107 substantially increases exercise performance in this heart failure model, suggesting that modulation of skeletal muscle function by a fast skeletal troponin activator may be a useful therapeutic in heart failure-associated exercise intolerance.