Characterization of NEB pathogenic variants in patients reveals novel nemaline myopathy disease mechanisms and omecamtiv mecarbil force effects

Nebulin, a critical protein of the skeletal muscle thin filament, plays important roles in physiological processes such as regulating thin filament length (TFL), cross-bridge cycling, and myofibril alignment. Pathogenic variants in the nebulin gene (NEB) cause NEB-based nemaline myopathy (NEM2), a genetically heterogeneous disorder characterized by hypotonia and muscle weakness, currently lacking curative therapies. In this study, we examined a cohort of ten NEM2 patients, each with unique pathogenic variants, aiming to understand their impact on mRNA, protein, and functional levels. Results show that pathogenic truncation variants affect NEB mRNA stability and lead to nonsense-mediated decay of the mutated transcript. Moreover, a high incidence of cryptic splice site activation was found in patients with pathogenic splicing variants that are expected to disrupt the actin-binding sites of nebulin. Determination of protein levels revealed patients with either relatively normal or markedly reduced nebulin. We observed a positive relation between the reduction in nebulin and a reduction in TFL, or reduction in tension (both maximal and submaximal tension). Interestingly, our study revealed a pathogenic duplication variant in nebulin that resulted in a four-copy gain in the triplicate region of NEB and a much larger nebulin protein and longer TFL. Additionally, we investigated the effect of Omecamtiv mecarbil (OM), a small-molecule activator of cardiac myosin, on force production of type 1 muscle fibers of NEM2 patients. OM treatment substantially increased submaximal tension across all NEM2 patients ranging from 87 to 318%, with the largest effects in patients with the lowest level of nebulin. In summary, this study indicates that post-transcriptional or post-translational mechanisms regulate nebulin expression. Moreover, we propose that the pathomechanism of NEM2 involves not only shortened but also elongated thin filaments, along with the disruption of actin-binding sites resulting from pathogenic splicing variants. Significantly, our findings highlight the potential of OM treatment to improve skeletal muscle function in NEM2 patients, especially those with large reductions in nebulin levels.

Mechanical and signaling responses of unloaded rat soleus muscle to chronically elevated β-myosin activity

It has been reported that muscle functional unloading is accompanied by an increase in motoneuronal excitability despite the elimination of afferent input. Thus, we hypothesized that pharmacological potentiation of spontaneous contractile soleus muscle activity during hindlimb unloading could activate anabolic signaling pathways and prevent the loss of muscle mass and strength. To investigate these aspects and underlying molecular mechanisms, we used β-myosin allosteric effector Omecamtiv Mekarbil (OM). We found that OM partially prevented the loss of isometric strength and intrinsic stiffness of the soleus muscle after two weeks of disuse. Notably, OM was able to attenuate the unloading-induced decrease in the rate of muscle protein synthesis (MPS). At the same time, the use of drug neither prevented the reduction in the markers of translational capacity (18S and 28S rRNA) nor activation of the ubiquitin-proteosomal system, which is evidenced by a decrease in the cross-sectional area of fast and slow muscle fibers. These results suggest that chemically-induced increase in low-intensity spontaneous contractions of the soleus muscle during functional unloading creates prerequisites for protein synthesis. At the same time, it should be assumed that the use of OM is advisable with pharmacological drugs that inhibit the expression of ubiquitin ligases.

KCCQ total symptom score, clinical outcome measures, and adverse events associated with omecamtiv mecarbil for heart failure with reduced ejection fraction: a systematic review and meta-analysis of randomized controlled trials

BACKGROUND

Omecamtiv mecarbil (OM) is a direct myosin activator that augments left ventricular systolic function. This review compares OM to placebo by evaluating its effect on clinical outcomes and adverse events in patients with heart failure with reduced left ventricular ejection fraction.

METHODS AND RESULTS

A literature search of multiple databases for randomized controlled trials (RCTs) investigating OM versus placebo was undertaken. Six RCTs comprising 9596 patients were included. Use of OM was associated with a reduced risk of stroke (RR: 0.69; 95% CI 0.52-0.92). There was no significant mean difference (MD) change in the KCCQ total symptom score (MD: 1.82, 95% CI - 1.33 to 4.97), all-cause death (RR: 1.00; 95% CI 0.93-1.07), hospital readmissions (RR: 0.96; 95% CI 0.90-1.03), myocardial infarction (RR: 1.05; 95% CI 0.83-1.33), cardiovascular death (RR: 1.01; 95% CI 0.92-1.10), heart failure (HF) events (RR: 0.95; 95% CI 0.89-1.02), or a composite of cardiovascular death or HF events (RR: 0.97; 95% CI 0.93-1.02). In addition, OM was associated with an increased risk of dizziness (RR: 1.25; 95% CI 1.04-1.50) and hypotension (RR: 1.17; 95% CI 1.01-1.36). Other adverse events including ventricular tachyarrhythmias, (RR: 0.95; 95% CI 0.82-1.11), supraventricular tachyarrhythmias and atrial fibrillation/flutter (RR: 0.73; 95% CI 0.46-1.18), dyspnea (RR: 1.00; 95% CI 0.86-1.18), and acute renal injury (RR: 0.88; 95% CI 0.60-1.27) were not significant.

CONCLUSION

OM is generally well tolerated. We identified a reduced risk of stroke with use of OM. However, there was no improvement in other clinical outcomes or quality of life. Study protocol was registered in PROSPERO international prospective register of systematic reviews (CRD42022348423).

Omecamtiv mecarbil augments cardiomyocyte contractile activity both at resting and systolic Ca2+ levels

AIMS

Heart failure with reduced ejection fraction (HFrEF) is a disease with high mortality and morbidity. Recent positive inotropic drug developments focused on cardiac myofilaments, that is, direct activators of the myosin molecule and Ca²⁺ sensitizers for patients with advanced HFrEF. Omecamtiv mecarbil (OM) is the first direct myosin activator with promising results in clinical studies. Here, we aimed to elucidate the cellular mechanisms of the positive inotropic effect of OM in a comparative in vitro investigation where Ca²⁺ -sensitizing positive inotropic agents with distinct mechanisms of action [EMD 53998 (EMD), which also docks on the myosin molecule, and levosimendan (Levo), which binds to troponin C] were included.

METHODS

Enzymatically isolated canine cardiomyocytes with intact cell membranes were loaded with Fura-2AM, a Ca²⁺ -sensitive, ratiometric, fluorescent dye. Changes in sarcomere length (SL) and intracellular Ca²⁺ concentration were recorded in parallel at room temperature, whereas cardiomyocyte contractions were evoked by field stimulation at 0.1 Hz in the presence of different OM, EMD, or Levo concentrations.

RESULTS

SL was reduced by about 23% or 9% in the presence of 1 μM OM or 1 μM EMD in the absence of electrical stimulation, whereas 1 μM Levo had no effect on resting SL. Fractional sarcomere shortening was increased by 1 μM EMD or 1 μM Levo to about 152%, but only to about 128% in the presence of 0.03 μM OM. At higher OM concentrations, no significant increase in fractional sarcomere shortening could be recorded. Contraction durations largely increased, whereas the kinetics of contractions and relaxations decreased with increasing OM concentrations. One-micromole EMD or 1 μM Levo had no effects on contraction durations. One-micromole Levo, but not 1 μM EMD, accelerated the kinetics of cardiomyocyte contractions and relaxations. Ca²⁺ transient amplitudes were unaffected by all treatments.

CONCLUSIONS

Our data revealed major distinctions between the cellular effects of myofilament targeted agents (OM, EMD, or Levo) depending on their target proteins and binding sites, although they were compatible with the involvement of Ca²⁺ -sensitizing mechanisms for all three drugs. Significant part of the cardiotonic effect of OM relates to the prolongation of systolic contraction in combination with its Ca²⁺ -sensitizing effect.

Effects of omecamtiv mecarbil in heart failure with reduced ejection fraction according to blood pressure: the GALACTIC-HF trial

AIM

Patients with heart failure with reduced ejection fraction and low systolic blood pressure (SBP) have high mortality, hospitalizations, and poorly tolerate evidence-based medical treatment. Omecamtiv mecarbil may be particularly helpful in such patients. This study examined its efficacy and tolerability in patients with SBP ≤100 mmHg enrolled in the Global Approach to Lowering Adverse Cardiac outcomes Through Improving Contractility in Heart Failure (GALACTIC-HF).

METHODS AND RESULTS

The GALACTIC-HF enrolled patients with baseline SBP ≥85 mmHg with a primary outcome of time to cardiovascular death or first heart failure event. In this analysis, patients were divided according to their baseline SBP (≤100 vs. >100 mmHg). Among the 8232 analysed patients, 1473 (17.9%) had baseline SBP ≤100 mmHg and 6759 (82.1%) had SBP >100 mmHg. The primary outcome occurred in 715 (48.5%) and 2415 (35.7%) patients with SBP ≤100 and >100 mmHg, respectively. Patients with lower SBP were at higher risk of adverse outcomes. Omecamtiv mecarbil, compared with placebo, appeared to be more effective in reducing the primary composite endpoint in patients with SBP ≤100 mmHg [hazard ratio (HR), 0.81; 95% confidence interval (CI), 0.70-0.94] compared with those with SBP >100 mmHg (HR, 0.95; 95% CI, 0.88-1.03; P-value for interaction = 0.051). In both groups, omecamtiv mecarbil did not change SBP values over time and did not increase the risk of adverse events, when compared with placebo.

CONCLUSION

In GALACTIC-HF, risk reduction of heart failure outcomes with omecamtiv mecarbil compared with placebo was large and significant in patients with low SBP. Omecamtiv mecarbil did not affect SBP and was well tolerated independent of SBP values.

Advanced heart failure: guideline-directed medical therapy, diuretics, inotropes, and palliative care

Heart failure (HF) is a major cause of mortality, hospitalizations, and reduced quality of life and a major burden for the healthcare system. The number of patients that progress to an advanced stage of HF is growing. Only a limited proportion of these patients can undergo heart transplantation or mechanical circulatory support. The purpose of this review is to summarize medical management of patients with advanced HF. First, evidence-based oral treatment must be implemented although it is often not tolerated. New therapeutic options may soon become possible for these patients. The second goal is to lessen the symptomatic burden through both decongestion and haemodynamic improvement. Some new treatments acting on cardiac function may fulfil both these needs. Inotropic agents acting through an increase in intracellular calcium have often increased risk of death. However, in the recent Global Approach to Lowering Adverse Cardiac Outcomes Through Improving Contractility in Heart Failure (GALACTIC-HF) trial, omecamtiv mecarbil was safe and effective in the reduction of the primary outcome of cardiovascular death or HF event compared with placebo (hazard ratio, 0.92; 95% confidence interval, 0.86-0.99; P = 0.03) and its effects were larger in those patients with more severe left ventricular dysfunction. Patients with severe HF who received omecamtiv mecarbil experienced a significant treatment benefit, whereas patients without severe HF did not (P = 0.005 for interaction). Lastly, clinicians should take care of the end of life with an appropriate multidisciplinary approach. Medical treatment of advanced HF therefore remains a major challenge and a wide open area for further research.

Safety and Efficacy of Omecamtiv Mecarbil for Heart Failure: A Systematic Review and Meta-Analysis

Aim

To assess the safety and efficacy of omecamtiv mecarbil compared with placebo in heart failure (HF) patients.

Methods

We searched PubMed, Web of Science, Cochrane Library, and SCOPUS until August 15th, 2021. We included all randomized controlled studies comparing omecamtiv mecarbil with placebo in heart failure patients. The meta-analysis was carried out using Rev Man software V5.4.

Results

A total of eight studies were included in our systematic review. Pooled analysis showed that omecamtiv mecarbil is not associated with increased incidence of death, any adverse events, hypotension, heart failure, ventricular tachyarrhythmia, dyspnea, dizziness, and serious adverse events. Regarding the efficacy, omecamtiv mecarbil significantly reduced heart rate with some studies demonstrating its significant improvement in left ventricular ejection fraction and systolic function.

Conclusion: Omecamtiv mecarbil is a well-tolerated drug in heart failure patients. The limited data regarding the efficacy suggested that it may improve ejection fraction and systolic function.

Comprehensive analyses of the inotropic compound omecamtiv mecarbil in rat and human cardiac preparations

Omecamtiv mecarbil (OM), a myosin activator, was reported to induce complex concentration- and species-dependent effects on contractile function and clinical studies indicated a low therapeutic index with diastolic dysfunction at concentrations above 1 µM. To further characterize effects of OM in a human context and under different preload conditions, we constructed a setup that allows isometric contractility analyses of human induced pluripotent stem cell (hiPSC)-derived engineered heart tissues (EHTs). The results were compared to effects of OM on the very same EHTs measured under auxotonic conditions. OM induced a sustained, concentration-dependent increase in time-to-peak under all conditions (maximally 2-3 fold). Peak force, in contrast, was increased by OM only in human, but not rat EHTs and only under isometric conditions, varied between hiPSC lines and showed a biphasic concentration-dependency with maximal effects at 1 µM. Relaxation time tended to fall under auxotonic and strongly increase under isometric conditions, again with biphasic concentration-dependency. Diastolic tension concentration-dependently increased under all conditions. The latter was reduced by an inhibitor of the mitochondrial sodium calcium exchanger (CGP-37157). OM induced increases in mitochondrial oxidation in isolated cardiomyocytes, indicating that OM, an inotrope that does not increase intracellular and mitochondrial Ca²⁺, can induce mismatch between an increase in ATP and ROS production and unstimulated mitochondrial redox capacity. Taken together, we developed a novel setup well suitable for isometric measurements of EHTs. The effects of OM on contractility and diastolic tension are complex with concentration-, time-, species- and loading-dependent differences. Effects on mitochondrial function require further studies.

Where are we in 2021 with heart failure with reduced ejection fraction?-current outlook and expectations from new promising clinical trials

Guideline-directed optimal medical therapy is a well-established therapy in treating patients with heart failure with reduced ejection fraction (HFrEF). Despite clear recommendations, the prognosis in this group of patients is still poor with high mortality. After publishing results of the PARADIGM-HF trial (Prospective Comparison of ARNI-Angiotensin Receptor/Neprilysin Inhibitors-with ACEI-Angiotensin-Converting Enzyme Inhibitor-to Determine Impact on Global Mortality and Morbidity in Heart Failure) clinical investigators accelerated their research. Recently, many new trials have been designed to evaluate the efficacy and safety of promising management, taking into account heterogeneity of population with chronic HFrEF. Determining target doses still poses the biggest problem in standard pharmacotherapy. Implementation of new substances for the HFrEF therapy makes it possible to formulate simple rules of treatment-in most cases, administering a dose of drug in one tablet provides a faster therapeutic effect. The aim of this article is to summarize current knowledge on recently announced findings on novel molecules and to propose a new revolutionary and individualised approach to treatment of HFrEF patients.

Omecamtiv mecarbil evokes diastolic dysfunction and leads to periodic electromechanical alternans

Omecamtiv mecarbil (OM) is a promising novel drug for improving cardiac contractility. We tested the therapeutic range of OM and identified previously unrecognized side effects. The Ca2+ sensitivity of isometric force production (pCa50) and force at low Ca2+ levels increased with OM concentration in human permeabilized cardiomyocytes. OM (1 µM) slowed the kinetics of contractions and relaxations and evoked an oscillation between normal and reduced intracellular Ca2+ transients, action potential lengths and contractions in isolated canine cardiomyocytes. Echocardiographic studies and left ventricular pressure-volume analyses demonstrated concentration-dependent improvements in cardiac systolic function at OM concentrations of 600-1200 µg/kg in rats. Administration of OM at a concentration of 1200 µg/kg was associated with hypotension, while doses of 600-1200 µg/kg were associated with the following aspects of diastolic dysfunction: decreases in E/A ratio and the maximal rate of diastolic pressure decrement (dP/dtmin) and increases in isovolumic relaxation time, left atrial diameter, the isovolumic relaxation constant Tau, left ventricular end-diastolic pressure and the slope of the end-diastolic pressure-volume relationship. Moreover, OM 1200 µg/kg frequently evoked transient electromechanical alternans in the rat in vivo in which normal systoles were followed by smaller contractions (and T-wave amplitudes) without major differences on the QRS complexes. Besides improving systolic function, OM evoked diastolic dysfunction and pulsus alternans. The narrow therapeutic window for OM may necessitate the monitoring of additional clinical safety parameters in clinical application.

American Heart Association (AHA) Scientific Sessions 2020 (November 13-17, 2020 - Virtual Meeting)

Participants from industry and academia attended the American Heart Association Scientific Sessions 2020 (AHA 2020) Annual Meeting held over November 13-17, 2020. AHA 2020 was originally scheduled to be held in Dallas, Texas, but due to public health concerns surrounding the SARS-CoV-2 (COVID-19) crisis, it was instead presented as a virtual summit. The virtual online program included oral, poster and poster discussion presentations, as well as track-based clinical science symposia throughout the conference.

Omecamtiv mecarbil in chronic heart failure with reduced ejection fraction: GALACTIC-HF baseline characteristics and comparison with contemporary clinical trials

AIMS

The safety and efficacy of the novel selective cardiac myosin activator, omecamtiv mecarbil, in patients with heart failure with reduced ejection fraction (HFrEF) is being tested in the Global Approach to Lowering Adverse Cardiac outcomes Through Improving Contractility in Heart Failure (GALACTIC-HF) trial. Here we describe the baseline characteristics of participants in GALACTIC-HF and how these compare with other contemporary trials.

METHODS AND RESULTS

Adults with established HFrEF, New York Heart Association (NYHA) functional class ≥II, ejection fraction ≤35%, elevated natriuretic peptides and either current hospitalization for heart failure or history of hospitalization/emergency department visit for heart failure within a year were randomized to either placebo or omecamtiv mecarbil (pharmacokinetic-guided dosing: 25, 37.5, or 50 mg bid). A total of 8256 patients [male (79%), non-white (22%), mean age 65 years] were enrolled with a mean ejection fraction 27%, ischaemic aetiology in 54%, NYHA class II 53% and III/IV 47%, and median N-terminal pro-B-type natriuretic peptide 1971 pg/mL. Heart failure therapies at baseline were among the most effectively employed in contemporary heart failure trials. GALACTIC-HF randomized patients representative of recent heart failure registries and trials with substantial numbers of patients also having characteristics understudied in previous trials including more from North America (n = 1386), enrolled as inpatients (n = 2084), systolic blood pressure <100 mmHg (n = 1127), estimated glomerular filtration rate <30 mL/min/1.73 m² (n = 528), and treated with sacubitril/valsartan at baseline (n = 1594).

CONCLUSIONS

GALACTIC-HF enrolled a well-treated, high-risk population from both inpatient and outpatient settings, which will provide a definitive evaluation of the efficacy and safety of this novel therapy, as well as informing its potential future implementation.

Treatments targeting inotropy

Acute heart failure (HF) and in particular, cardiogenic shock are associated with high morbidity and mortality. A therapeutic dilemma is that the use of positive inotropic agents, such as catecholamines or phosphodiesterase-inhibitors, is associated with increased mortality. Newer drugs, such as levosimendan or omecamtiv mecarbil, target sarcomeres to improve systolic function putatively without elevating intracellular Ca2+. Although meta-analyses of smaller trials suggested that levosimendan is associated with a better outcome than dobutamine, larger comparative trials failed to confirm this observation. For omecamtiv mecarbil, Phase II clinical trials suggest a favourable haemodynamic profile in patients with acute and chronic HF, and a Phase III morbidity/mortality trial in patients with chronic HF has recently begun. Here, we review the pathophysiological basis of systolic dysfunction in patients with HF and the mechanisms through which different inotropic agents improve cardiac function. Since adenosine triphosphate and reactive oxygen species production in mitochondria are intimately linked to the processes of excitation-contraction coupling, we also discuss the impact of inotropic agents on mitochondrial bioenergetics and redox regulation. Therefore, this position paper should help identify novel targets for treatments that could not only safely improve systolic and diastolic function acutely, but potentially also myocardial structure and function over a longer-term.

Myosin from the ventricle is more sensitive to omecamtiv mecarbil than myosin from the atrium

Omecamtiv mecarbil (OM), an activator of cardiac myosin, strongly affects contractile characteristics of the ventricles and, to a much lesser extent, the characteristics of atrial contraction. We compared the molecular mechanism of action of OM on the interaction of atrial and ventricular myosin with actin using an optical trap and an in vitro motility assay. In concentrations up to 0.5 μM, OM did not affect the step size of a myosin molecule but reduced it at a higher OM level. OM substantially prolonged the interaction of both isoforms of myosin with actin. However, the interaction characteristics of ventricular myosin with actin were more sensitive to OM than those of atrial myosin. Our results, obtained at the level of isolated proteins, can explain why the impact of OM in therapeutic concentrations on the contractile function of the atrium is less significant as compared to those of the ventricle.

Omecamtiv Mecarbil: A Myosin Motor Activator Agent with Promising Clinical Performance and New in vitro Results

BACKGROUND

Clinical treatment of heart failure is still suffering from limited efficacy and unfavorable side effects. The recently developed group of agents, the myosin motor activators, act directly on cardiac myosin resulting in an increased force generation and prolongation of contraction. The lead molecule, omecamtiv mecarbil is now in human 3 stage. In addition to the promising clinical data published so far, there are new in vitro results indicating that the effect of omecamtiv mecarbil on contractility is rate-dependent. Furthermore, omecamtiv mecarbil was shown to activate cardiac ryanodine receptors, an effect that may carry proarrhythmic risk.

METHODS

These new results, together with the controversial effects of the drug on cardiac oxygen consumption, are critically discussed in this review in light of the current literature on omecamtiv mecarbil.

RESULTS

In therapeutically relevant concentrations the beneficial inotropic effect of the agent is not likely affected by these new results - in accordance with the good clinical data. At supratherapeutic concentrations, however, activation of cardiac ryanodine receptors may increase arrhythmia propensity, and the stronger effect on diastolic than systolic cell shortening, observed at higher pacing frequencies, may decrease or offset the inotropic effect of omecamtiv mecarbil.

CONCLUSION

Further studies with definitely supratherapeutical concentrations of omecamtiv mecarbil should be designed to map the actual risk of these potentially harmful side-effects.

Cardiac Myosin Activators in Systolic Heart Failure: More Friend than Foe?

Despite the rising prevalence of HF, new evidence-based novel therapies for patients with worsening HF remain lacking, e.g., safe inotropic therapies. Traditional inotropes increase contractility by altering intracellular calcium flux, a pathway that may be responsible for the multitude of adverse effects associated with current options. Omecamtiv mecarbil, a direct myosin activator, increases contractility through a distinct pathway by increasing the proportion of myosin heads that are bound to actin in a high-affinity state. Phase II clinical trials in patients with chronic HF with this agent seem promising. A phase III trial investigating this therapy has not yet been pursued to date.

Frequency-dependent effects of omecamtiv mecarbil on cell shortening of isolated canine ventricular cardiomyocytes

Omecamtiv mecarbil (OM) is a myosin activator agent developed for the treatment of heart failure. OM was reported to increase left ventricular ejection fraction and systolic ejection time, but little is known about the effect of heart rate on the action of OM. The present study, therefore, was designed to investigate the effects of OM on unloaded cell shortening and intracellular Ca²⁺ ([Ca²⁺]_i) transients as a function of the pacing frequency. Isolated cardiomyocytes were stimulated at various frequencies under steady-state conditions. Cell length was monitored by an optical edge detector and changes in [Ca²⁺]_i were followed using the Ca²⁺-sensitive dye Fura-2. At the pacing frequency of 1 Hz, OM (1-10 μM) significantly decreased both diastolic and systolic cell length, however, fractional shortening was augmented only by 1 μM OM. Time to peak tension and time of 90% relaxation were progressively increased by OM. At the frequency of 2 Hz, diastolic cell length was reduced by 10 μM OM to a larger extent than systolic cell length, resulting in a significantly decreased fractional shortening under these conditions. OM had no effect on the parameters of the [Ca²⁺]_i transient at any pacing frequency. The results suggest that supratherapeutic concentrations of OM may decrease rather than increase the force of cardiac contraction especially in tachycardic patients.

Omecamtiv mecarbil activates ryanodine receptors from canine cardiac but not skeletal muscle

Due to the limited results achieved in the clinical treatment of heart failure, a new inotropic strategy of myosin motor activation has been developed. The lead molecule of myosin activator agents is omecamtiv mecarbil, which binds directly to the heavy chain of the cardiac β-myosin and enhances cardiac contractility by lengthening the lifetime of the acto-myosin complex and increasing the number of the active force-generating cross-bridges. In the absence of relevant data, the effect of omecamtiv mecarbil on canine cardiac ryanodine receptors (RyR 2) has been investigated in the present study by measuring the electrical activity of single RyR 2 channels incorporated into planar lipid bilayer. When applying 100nM Ca²⁺ concentration on the cis side ([Ca²⁺]_cis) omecamtiv mecarbil (1-10µM) significantly increased the open probability and opening frequency of RyR 2, while the mean closed time was reduced. Mean open time was increased moderately by 10µM omecamtiv mecarbil. When [Ca²⁺]_cis was elevated to 322 and 735nM, the effect of omecamtiv mecarbil on open probability was evident only at higher (3-10µM) concentrations. All effects of omecamtiv mecarbil were fully reversible upon washout. Omecamtiv mecarbil (up to 10µM) had no effect on the open probability of RyR 1, isolated from either canine or rabbit skeletal muscles. It is concluded that the direct stimulatory action of omecamtiv mecarbil on RyR 2 has to be taken into account when discussing the mechanism of action or the potential side effects of the compound.

Direct Myosin Activation by Omecamtiv Mecarbil for Heart Failure with Reduced Ejection Fraction

Myosin is the indispensable molecular motor that utilizes chemical energy to produce force for contraction within the cardiac myocyte. Myosin activity is gated by intracellular calcium levels which are regulated by multiple upstream signaling cascades that can be altered for clinical utility using inotropic medications. In contrast to clinically available cardiac inotropes, omecamtiv mecarbil is a novel direct myosin activator developed to augment left ventricular systolic function without the undesirable secondary effects of altered calcium homeostasis. Its identification and synthesis followed high-throughput screening of a reconstituted sarcomere, deliberate optimization, exquisite biochemical evaluation, and subsequently promising effects in animal models were demonstrated. Physiologically, it prolonged the duration of left ventricular systole in animal models, healthy adults, and patients with heart failure with reduced ejection fraction (HFrEF) without changing the velocity of pressure development, as assessed in animal models. It has been formulated for both intravenous and oral administration, and in both acute and chronic settings produced similar alterations in the duration of systole associated with beneficial increases in cardiac output, improvements in left ventricular volumes, and reductions in heart rate and often of natriuretic peptides. Small, asymptomatic increases in troponin were also observed in the absence of clinically evident ischemia. Clinically, the question remains as to whether the possible harm of this minimal troponin release is outweighed by the potential benefits of reduced neurohormonal activation, increased stroke volume and cardiac output, and improved ventricular remodeling in patients treated with omecamtiv mecarbil. The resolution of this question is being addressed by a phase III outcomes trial of this potential novel therapy for heart failure.

Incorporating development of a patient-reported outcome instrument in a clinical drug development program: examples from a heart failure program

BACKGROUND

Patient-reported outcome (PRO) measures can be used to support label claims if they adhere to US Food & Drug Administration guidance. The process of developing a new PRO measure is expensive and time-consuming. We report the results of qualitative studies to develop new PRO measures for use in clinical trials of omecamtiv mecarbil (a selective, small molecule activator of cardiac myosin) for patients with heart failure (HF), as well as the lessons learned from the development process.

METHODS

Concept elicitation focus groups and individual interviews were conducted with patients with HF to identify concepts for the instrument. Cognitive interviews with HF patients were used to confirm that no essential concepts were missing and to assess patient comprehension of the instrument and items.

RESULTS

During concept elicitation, the most frequently reported HF symptoms were shortness of breath, tiredness, fluid retention, fatigue, dizziness/light-headedness, swelling, weight fluctuation, and trouble sleeping. Two measures were developed based on the concepts: the Heart Failure Symptom Diary (HF-SD) and the Heart Failure Impact Scale (HFIS). Findings from cognitive interviews suggested that the items in the HF-SD and HFIS were relevant and well understood by patients. Multiple iterations of concept elicitation and cognitive interviews were needed based on FDA request for a broader patient population in the qualitative study. Lessons learned from the omecamtiv mecarbil PRO/clinical development program are discussed, including challenges of qualitative studies, patient recruitment, expected and actual timelines, cost, and engagement with various stakeholders.

CONCLUSION

Development of a new PRO measure to support a label claim requires significant investment and early planning, as demonstrated by the omecamtiv mecarbil program.

Pharmacology of heart failure: From basic science to novel therapies

Chronic heart failure is one of the leading causes for hospitalization in the United States and Europe, and is accompanied by high mortality. Current pharmacological therapy of chronic heart failure with reduced ejection fraction is largely based on compounds that inhibit the detrimental action of the adrenergic and the renin-angiotensin-aldosterone systems on the heart. More than one decade after spironolactone, two novel therapeutic principles have been added to the very recently released guidelines on heart failure therapy: the HCN-channel inhibitor ivabradine and the combined angiotensin and neprilysin inhibitor valsartan/sacubitril. New compounds that are in phase II or III clinical evaluation include novel non-steroidal mineralocorticoid receptor antagonists, guanylate cyclase activators or myosine activators. A variety of novel candidate targets have been identified and the availability of gene transfer has just begun to accelerate translation from basic science to clinical application. This review provides an overview of current pharmacology and pharmacotherapy in chronic heart failure at three stages: the updated clinical guidelines of the American Heart Association and the European Society of Cardiology, new drugs which are in clinical development, and finally innovative drug targets and their mechanisms in heart failure which are emerging from preclinical studies will be discussed.

Influence of metabolic dysfunction on cardiac mechanics in decompensated hypertrophy and heart failure

Alterations in energetic state of the myocardium are associated with decompensated heart failure in humans and in animal models. However, the functional consequences of the observed changes in energetic state on mechanical function are not known. The primary aim of the study was to quantify mechanical/energetic coupling in the heart and to determine if energetic dysfunction can contribute to mechanical failure. A secondary aim was to apply a quantitative systems pharmacology analysis to investigate the effects of drugs that target cross-bridge cycling kinetics in heart failure-associated energetic dysfunction. Herein, a model of metabolite- and calcium-dependent myocardial mechanics was developed from calcium concentration and tension time courses in rat cardiac muscle obtained at different lengths and stimulation frequencies. The muscle dynamics model accounting for the effect of metabolites was integrated into a model of the cardiac ventricles to simulate pressure-volume dynamics in the heart. This cardiac model was integrated into a simple model of the circulation to investigate the effects of metabolic state on whole-body function. Simulations predict that reductions in metabolite pools observed in canine models of heart failure can cause systolic dysfunction, blood volume expansion, venous congestion, and ventricular dilation. Simulations also predict that myosin-activating drugs may partially counteract the effects of energetic state on cross-bridge mechanics in heart failure while increasing myocardial oxygen consumption. Our model analysis demonstrates how metabolic changes observed in heart failure are alone sufficient to cause systolic dysfunction and whole-body heart failure symptoms.

Molecular effects of the myosin activator omecamtiv mecarbil on contractile properties of skinned myocardium lacking cardiac myosin binding protein-C

Decreased expression of cardiac myosin binding protein-C (cMyBP-C) in the myocardium is thought to be a contributing factor to hypertrophic cardiomyopathy in humans, and the initial molecular defect is likely abnormal cross-bridge (XB) function which leads to impaired force generation, decreased contractile performance, and hypertrophy in vivo. The myosin activator omecamtiv mecarbil (OM) is a pharmacological drug that specifically targets the myosin XB and recent evidence suggests that OM induces a significant decrease in the in vivo motility velocity and an increase in the XB duty cycle. Thus, the molecular effects of OM maybe beneficial in improving contractile function in skinned myocardium lacking cMyBP-C because absence of cMyBP-C in the sarcomere accelerates XB kinetics and enhances XB turnover rate, which presumably reduces contractile efficiency. Therefore, parameters of XB function were measured in skinned myocardium lacking cMyBP-C prior to and following OM incubation. We measured ktr, the rate of force redevelopment as an index of XB transition from both the weakly- to strongly-bound state and from the strongly- to weakly-bound states and performed stretch activation experiments to measure the rates of XB detachment (krel) and XB recruitment (kdf) in detergent-skinned ventricular preparations isolated from hearts of wild-type (WT) and cMyBP-C knockout (KO) mice. Samples from donor human hearts were also used to assess the effects of OM in cardiac muscle expressing a slow β-myosin heavy chain (β-MHC). Incubation of skinned myocardium with OM produced large enhancements in steady-state force generation which were most pronounced at low levels of [Ca(2+)] activations, suggesting that OM cooperatively recruits additional XB's into force generating states. Despite a large increase in steady-state force generation following OM incubation, parallel accelerations in XB kinetics as measured by ktr were not observed, and there was a significant OM-induced decrease in krel which was more pronounced in the KO skinned myocardium compared to WT skinned myocardium (58% in WT vs. 76% in KO at pCa 6.1), such that baseline differences in krel between KO and WT skinned myocardium were no longer apparent following OM-incubation. A significant decrease in the kdf was also observed following OM incubation in all groups, which may be related to the increase in the number of cooperatively recruited XB's at low Ca(2+)-activations which slows the overall rate of force generation. Our results indicate that OM may be a useful pharmacological approach to normalize hypercontractile XB kinetics in myocardium with decreased cMyBP-C expression due to its molecular effects on XB behavior.

Discovery of omecamtiv mecarbil the first, selective, small molecule activator of cardiac Myosin

We report the design, synthesis, and optimization of the first, selective activators of cardiac myosin. Starting with a poorly soluble, nitro-aromatic hit compound (1), potent, selective, and soluble myosin activators were designed culminating in the discovery of omecamtiv mecarbil (24). Compound 24 is currently in clinical trials for the treatment of systolic heart failure.