Fast Skeletal Troponin Activator Research Articles

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.

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.

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.

Pathogenic variants in TNNC2 cause congenital myopathy due to an impaired force response to calcium.

Troponin C (TnC) is a critical regulator of skeletal muscle contraction; it binds Ca2+ to activate muscle contraction. Surprisingly, the gene encoding fast skeletal TnC (TNNC2) has not yet been implicated in muscle disease. Here, we report 2 families with pathogenic variants in TNNC2. Patients present with a distinct, dominantly inherited congenital muscle disease. Molecular dynamics simulations suggested that the pathomechanisms by which the variants cause muscle disease include disruption of the binding sites for Ca2+ and for troponin I. In line with these findings, physiological studies in myofibers isolated from patients' biopsies revealed a markedly reduced force response of the sarcomeres to [Ca2+]. This pathomechanism was further confirmed in experiments in which contractile dysfunction was evoked by replacing TnC in myofibers from healthy control subjects with recombinant, mutant TnC. Conversely, the contractile dysfunction of myofibers from patients was repaired by replacing endogenous, mutant TnC with recombinant, wild-type TnC. Finally, we tested the therapeutic potential of the fast skeletal muscle troponin activator tirasemtiv in patients' myofibers and showed that the contractile dysfunction was repaired. Thus, our data reveal that pathogenic variants in TNNC2 cause congenital muscle disease, and they provide therapeutic angles to repair muscle contractility.

Acute and chronic tirasemtiv treatment improves in vivo and in vitro muscle performance in actin-based nemaline myopathy mice.

Nemaline myopathy, a disease of the actin-based thin filament, is one of the most frequent congenital myopathies. To date, no specific therapy is available to treat muscle weakness in nemaline myopathy. We tested the ability of tirasemtiv, a fast skeletal troponin activator that targets the thin filament, to augment muscle force—both in vivo and in vitro—in a nemaline myopathy mouse model with a mutation (H40Y) in Acta1. In Acta1H40Y mice, treatment with tirasemtiv increased the force response of muscles to submaximal stimulation frequencies. This resulted in a reduced energetic cost of force generation, which increases the force production during a fatigue protocol. The inotropic effects of tirasemtiv were present in locomotor muscles and, albeit to a lesser extent, in respiratory muscles, and they persisted during chronic treatment, an important finding as respiratory failure is the main cause of death in patients with congenital myopathy. Finally, translational studies on permeabilized muscle fibers isolated from a biopsy of a patient with the ACTA1H40Y mutation revealed that at physiological Ca2+ concentrations, tirasemtiv increased force generation to values that were close to those generated in muscle fibers of healthy subjects. These findings indicate the therapeutic potential of fast skeletal muscle troponin activators to improve muscle function in nemaline myopathy due to the ACTA1H40Y mutation, and future studies should assess their merit for other forms of nemaline myopathy and for other congenital myopathies.

Structural Basis of Tirasemtiv Activation of Fast Skeletal Muscle.

Troponin regulates the calcium-mediated activation of skeletal muscle. Muscle weakness in diseases such as amyotrophic lateral sclerosis and spinal muscular atrophy occurs from diminished neuromuscular output. The first direct fast skeletal troponin activator, tirasemtiv, amplifies the response of muscle to neuromuscular input. Tirasemtiv binds selectively and strongly to fast skeletal troponin, slowing the rate of calcium release and sensitizing muscle to calcium. We report the solution NMR structure of tirasemtiv bound to a fast skeletal troponin C-troponin I chimera. The structure reveals that tirasemtiv binds in a hydrophobic pocket between the regulatory domain of troponin C and the switch region of troponin I, which overlaps with that of Anapoe in the X-ray structure of skeletal troponin. Multiple interactions stabilize the troponin C-troponin I interface, increase the affinity of troponin C for the switch region of fast skeletal troponin I, and drive the equilibrium toward the active state.

Reldesemtiv in Patients with Spinal Muscular Atrophy: a Phase 2 Hypothesis-Generating Study

This phase 2, double-blind, placebo-controlled, hypothesis-generating study evaluated the effects of oral reldesemtiv, a fast skeletal muscle troponin activator, in patients with spinal muscular atrophy (SMA). Patients ≥ 12 years of age with type II, III, or IV SMA were randomized into 2 sequential, ascending reldesemtiv dosing cohorts (cohort 1: 150 mg bid or placebo [2:1]; cohort 2: 450 mg bid or placebo [2:1]). The primary objective was to determine potential pharmacodynamic effects of reldesemtiv on 8 outcome measures in SMA, including 6-minute walk distance (6MWD) and maximum expiratory pressure (MEP). Changes from baseline to weeks 4 and 8 were determined. Pharmacokinetics and safety were also evaluated. Patients were randomized to reldesemtiv 150 mg, 450 mg, or placebo (24, 20, and 26, respectively). The change from baseline in 6MWD was greater for reldesemtiv 450 mg than for placebo at weeks 4 and 8 (least squares [LS] mean difference, 35.6 m [p = 0.0037] and 24.9 m [p = 0.058], respectively). Changes from baseline in MEP at week 8 on reldesemtiv 150 and 450 mg were significantly greater than those on placebo (LS mean differences, 11.7 [p = 0.038] and 13.2 cm H2O [p = 0.03], respectively). For 6MWD and MEP, significant changes from placebo were seen in the highest reldesemtiv peak plasma concentration quartile (Cmax > 3.29 μg/mL; LS mean differences, 43.3 m [p = 0.010] and 28.8 cm H2O [p = 0.0002], respectively). Both dose levels of reldesemtiv were well tolerated. Results suggest reldesemtiv may offer clinical benefit and support evaluation in larger SMA patient populations.

LATE BREAKING NEWS ORAL PRESENTATION: LBO 1 Pathogenic variants in TNNC2 cause congenital myopathy due to an impaired force response to calcium

Troponin C (TnC) is a critical regulator of skeletal muscle contraction: it binds Ca2+ to activate muscle contraction. Surprisingly, the gene encoding fast skeletal TnC (TNNC2) has never been implicated in muscle disease. Here, we report two families with pathogenic variants in TNNC2. Patients present with a distinct, dominantly inherited congenital muscle disease. Molecular dynamics simulations indicate that the pathomechanisms by which the variants cause muscle disease include disruption of the bindings sites for Ca2+ and for troponin I. In line with these findings, physiological studies in myofibers isolated from patients’ biopsies revealed a markedly reduced force response of the sarcomeres to [Ca2+]. This pathomechanism was further confirmed in experiments in which contractile dysfunction was evoked by replacing TnC in myofibers from healthy control subjects with recombinant, mutant TnC. Conversely, the contractile dysfunction of myofibers from patients was repaired by replacing endogenous, mutant TnC with recombinant, healthy TnC. Finally, we tested the therapeutic potential of the fast skeletal muscle troponin activator tirasemtiv in patients’ myofibers and showed that the contractile dysfunction was repaired. Thus, our data reveal that pathogenic variants in TNNC2 cause congenital muscle disease, and they provide therapeutic angles to repair muscle contractility.

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

BackgroundMuscle 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.MethodsExperiments 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).ResultsProlonged 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.ConclusionsThe 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.

Functional Characterization of the Intact Diaphragm in a Nebulin-Based Nemaline Myopathy (NM) Model-Effects of the Fast Skeletal Muscle Troponin Activator tirasemtiv.

Respiratory failure due to diaphragm dysfunction is considered a main cause of death in nemaline myopathy (NM) and we studied both isometric force and isotonic shortening of diaphragm muscle in a mouse model of nebulin-based NM (Neb cKO). A large contractile deficit was found in nebulin-deficient intact muscle that is frequency dependent, with the largest deficits at low–intermediate stimulation frequencies (e.g., a deficit of 72% at a stimulation frequency of 20 Hz). The effect of the fast skeletal muscle troponin activator (FSTA) tirasemtiv on force was examined. Tirasemtiv had a negligible effect at maximal stimulation frequencies, but greatly reduced the force deficit of the diaphragm at sub-maximal stimulation levels with an effect that was largest in Neb cKO diaphragm. As a result, the force deficit of Neb cKO diaphragm fell (from 72% to 29% at 20 Hz). Similar effects were found in in vivo experiments on the nerve-stimulated gastrocnemius muscle complex. Load-clamp experiments on diaphragm muscle showed that tirasemtiv increased the shortening velocity, and reduced the deficit in mechanical power by 33%. Thus, tirasemtiv significantly improves muscle function in a mouse model of nebulin-based nemaline myopathy.

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. 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.

A phase III trial of tirasemtiv as a potential treatment for amyotrophic lateral sclerosis

Objective: To assess the efficacy of tirasemtiv, a fast skeletal muscle troponin activator, vs. placebo in patients with amyotrophic lateral sclerosis. Methods: VITALITY-ALS (NCT02496767) was a multinational, double-blind, randomized, placebo-controlled clinical trial. Participants tolerating 2 weeks of open-label tirasemtiv (125 mg twice daily) were randomized 3:2:2:2 to placebo or one of three target tirasemtiv dose levels, using an escalating dosage protocol lasting 28 days. The primary outcome measure was changed in slow vital capacity (SVC) at 24 weeks. Secondary endpoints included a change in muscle strength and time to respiratory milestones of disease progression. Results: Of 744 participants, 565 tolerated open-label tirasemtiv and received randomized treatment. By 24 weeks, 23 (12.2%) placebo-treated participants discontinued study treatment vs. 129 (34.2%) randomized to tirasemtiv. SVC declined by 14.4% (95% CI: −16.8, −11.9) in the placebo group and 13.4% (95% CI: −15.3, −11.6) in the tirasemtiv group (p = 0.56). Secondary endpoints did not show significant differences. However, participants who tolerated tirasemtiv at their randomized dose showed a numeric trend toward a dose-related slowing of decline in SVC (p = 0.11). Dizziness, fatigue, nausea, weight loss, and insomnia occurred more frequently on tirasemtiv. Serious adverse events were similar across groups. Conclusions: Tirasemtiv did not alter the decline of SVC or significantly impact secondary outcome measures. Poor tolerability of tirasemtiv may have contributed to this result. However, participants tolerating their intended dose exhibited a trend toward treatment benefit on SVC, suggesting the underlying mechanism of action may still hold promise, as is being tested with a different fast skeletal muscle troponin activator (NCT03160898).Trial registration: ClinicalTrials.gov identifier: NCT02496767.

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

ABSTRACT 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 Nerve 57: 729–734, 2018

The Ca2+ sensitizer CK-2066260 increases myofibrillar Ca2+ sensitivity and submaximal force selectively in fast skeletal muscle.

Key points We report that the small molecule CK‐2066260 selectively slows the off‐rate of Ca2 + from fast skeletal muscle troponin, leading to increased myofibrillar Ca2 + sensitivity in fast skeletal muscle.Rodents dosed with CK‐2066260 show increased hindlimb muscle force and power in response to submaximal rates of nerve stimulation in situ.CK‐2066260 has no effect on free cytosolic [Ca2 +] during contractions of isolated muscle fibres.We conclude that fast skeletal muscle troponin sensitizers constitute a potential therapy to address an unmet need of improving muscle function in conditions of weakness and premature muscle fatigue. Skeletal muscle dysfunction occurs in many diseases and can lead to muscle weakness and premature muscle fatigue. Here we show that the fast skeletal troponin activator, CK‐2066260, counteracts muscle weakness by increasing troponin Ca2+ affinity, thereby increasing myofibrillar Ca2+ sensitivity. Exposure to CK‐2066260 resulted in a concentration‐dependent increase in the Ca2+ sensitivity of ATPase activity in isolated myofibrils and reconstituted hybrid sarcomeres containing fast skeletal muscle troponin C. Stopped‐flow experiments revealed a ∼2.7‐fold decrease in the Ca2+ off‐rate of isolated troponin complexes in the presence of CK‐2066260 (6 vs. 17 s−1 under control conditions). Isolated mouse flexor digitorum brevis fibres showed a rapidly developing, reversible and concentration‐dependent force increase at submaximal stimulation frequencies. This force increase was not accompanied by any changes in the free cytosolic [Ca2+] or its kinetics. CK‐2066260 induced a slowing of relaxation, which was markedly larger at 26°C than at 31°C and could be linked to the decreased Ca2+ off‐rate of troponin C. Rats dosed with CK‐2066260 showed increased hindlimb isometric and isokinetic force in response to submaximal rates of nerve stimulation in situ producing significantly higher absolute forces at low isokinetic velocities, whereas there was no difference in force at the highest velocities. Overall muscle power was increased and the findings are consistent with a lack of effect on crossbridge kinetics. In conclusion, CK‐2066260 acts as a fast skeletal troponin activator that may be used to increase muscle force and power in conditions of muscle weakness.

Reduced force of diaphragm muscle fibers in patients with chronic thromboembolic pulmonary hypertension.

Patients with pulmonary hypertension (PH) suffer from inspiratory muscle weakness. However, the pathophysiology of inspiratory muscle dysfunction in PH is unknown. We hypothesized that weakness of the diaphragm, the main inspiratory muscle, is an important contributor to inspiratory muscle dysfunction in PH patients. Our objective was to combine ex vivo diaphragm muscle fiber contractility measurements with measures of in vivo inspiratory muscle function in chronic thromboembolic pulmonary hypertension (CTEPH) patients. To assess diaphragm muscle contractility, function was studied in vivo by maximum inspiratory pressure (MIP) and ex vivo in diaphragm biopsies of the same CTEPH patients (N = 13) obtained during pulmonary endarterectomy. Patients undergoing elective lung surgery served as controls (N = 15). Muscle fiber cross-sectional area (CSA) was determined in cryosections and contractility in permeabilized muscle fibers. Diaphragm muscle fiber CSA was not significantly different between control and CTEPH patients in both slow-twitch and fast-twitch fibers. Maximal force-generating capacity was significantly lower in slow-twitch muscle fibers of CTEPH patients, whereas no difference was observed in fast-twitch muscle fibers. The maximal force of diaphragm muscle fibers correlated significantly with MIP. The calcium sensitivity of force generation was significantly reduced in fast-twitch muscle fibers of CTEPH patients, resulting in a ∼40% reduction of submaximal force generation. The fast skeletal troponin activator CK-2066260 (5 μM) restored submaximal force generation to levels exceeding those observed in control subjects. In conclusion, diaphragm muscle fiber contractility is hampered in CTEPH patients and contributes to the reduced function of the inspiratory muscles in CTEPH patients.

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.

Tirasemtiv amplifies skeletal muscle response to nerve activation in humans

Introduction: In this study we tested the hypothesis that tirasemtiv, a selective fast skeletal muscle troponin activator that sensitizes the sarcomere to calcium, could amplify the response of muscle to neuromuscular input in humans. Methods: Healthy men received tirasemtiv and placebo in a randomized, double-blind, 4-period, crossover design. The deep fibular nerve was stimulated transcutaneously to activate the tibialis anterior muscle and produce dorsiflexion of the foot. The force–frequency relationship of tibialis anterior dorsiflexion was assessed after dosing. Results: Tirasemtiv increased force produced by the tibialis anterior in a dose-, concentration-, and frequency-dependent manner with the largest increases [up to 24.5% (SE 3.1), P < 0.0001] produced at subtetanic nerve stimulation frequencies (10 Hz). Conclusions: The data confirm that tirasemtiv amplifies the response of skeletal muscle to nerve input in humans. This outcome provides support for further studies of tirasemtiv as a potential therapy in conditions marked by diminished neuromuscular input. Muscle Nerve 50: 925–931, 2014

Fast skeletal muscle troponin activator tirasemtiv increases muscle function and performance in the B6SJL-SOD1G93A ALS mouse model.

Amyotrophic Lateral Sclerosis (ALS) is a motor neuron disease characterized by progressive motor neuron loss resulting in muscle atrophy, declining muscle function, and eventual paralysis. Patients typically die from respiratory failure 3 to 5 years from the onset of symptoms. Tirasemtiv is a fast skeletal troponin activator that sensitizes the sarcomere to calcium; this mechanism of action amplifies the response of muscle to neuromuscular input producing greater force when nerve input is reduced. Here, we demonstrate that a single dose of tirasemtiv significantly increases submaximal isometric force, forelimb grip strength, grid hang time, and rotarod performance in a female transgenic mouse model (B6SJL-SOD1G93A) of ALS with functional deficits. Additionally, diaphragm force and tidal volume are significantly higher in tirasemtiv-treated female B6SJL-SOD1G93A mice. These results support the potential of fast skeletal troponin activators to improve muscle function in neuromuscular diseases.

Diaphragm Fiber Strength Is Reduced in Critically Ill Patients and Restored by a Troponin Activator

Diaphragm Fiber Strength Is Reduced in Critically Ill Patients and Restored by a Troponin Activator