A novel approach to improve cardiac performance: cardiac myosin activators

Challenging and target-based shifting strategies for heart failure treatment: An update from the last decades

Heart failure (HF) is a major global health problem afflicting millions worldwide. Despite the significant advances in therapies and prevention, HF still carries very high morbidity and mortality, requiring enormous healthcare-related expenditure, and the search for new weapons goes on. Following initial treatment strategies targeting inotropism and congestion, attention has focused on offsetting the neurohormonal overactivation and three main therapies, including angiotensin-converting enzyme inhibitors or angiotensin II type 1 receptor antagonists, β-adrenoceptor antagonists, and mineralocorticoid receptor antagonists, have been the foundation of standard treatment for patients with HF. Recently, a paradigm shift, including angiotensin receptor-neprilysin inhibitor, sodium glucose co-transporter 2 inhibitor, and ivabradine, has been added. Moreover, soluble guanylate cyclase stimulator, elamipretide, and omecamtiv mecarbil have come out as a next-generation therapeutic agent for patients with HF. Although these pharmacologic therapies have been significantly successful in relieving symptoms, there is still no complete cure for HF. We may be currently entering a new era of treatment for HF with animal experiments and human clinical trials assessing the value of antibody-based immunotherapy and gene therapy as a novel therapeutic strategy. Such tempting therapies still have some challenges to be addressed but may become a weighty option for treatment of HF. This review article will compile the paradigm shifts in HF treatment over the past dozen years or so and illustrate current landscape of antibody-based immunotherapy and gene therapy as a new therapeutic algorithm for patients with HF.

Estimation of crossbridge-state during cardiomyocyte beating using second harmonic generation

Estimation of dynamic change of crossbridge formation in living cardiomyocytes is expected to provide crucial information for elucidating cardiomyopathy mechanisms, efficacy of an intervention, and others. Here, we established an assay system to dynamically measure second harmonic generation (SHG) anisotropy derived from myosin filaments depended on their crossbridge status in pulsating cardiomyocytes. Experiments utilizing an inheritable mutation that induces excessive myosin-actin interactions revealed that the correlation between sarcomere length and SHG anisotropy represents crossbridge formation ratio during pulsation. Furthermore, the present method found that ultraviolet irradiation induced an increased population of attached crossbridges that lost the force-generating ability upon myocardial differentiation. Taking an advantage of infrared two-photon excitation in SHG microscopy, myocardial dysfunction could be intravitally evaluated in a Drosophila disease model. Thus, we successfully demonstrated the applicability and effectiveness of the present method to evaluate the actomyosin activity of a drug or genetic defect on cardiomyocytes. Because genomic inspection alone may not catch the risk of cardiomyopathy in some cases, our study demonstrated herein would be of help in the risk assessment of future heart failure.

Characterization of Stimulatory Action on Voltage-Gated Na+ Currents Caused by Omecamtiv Mecarbil, Known to Be a Myosin Activator

Omecamtiv mecarbil (OM, CK-1827452) is recognized as an activator of myosin and has been demonstrated to be beneficial for the treatment of systolic heart failure. However, the mechanisms by which this compound interacts with ionic currents in electrically excitable cells remain largely unknown. The objective of this study was to investigate the effects of OM on ionic currents in GH3 pituitary cells and Neuro-2a neuroblastoma cells. In GH₃ cells, whole-cell current recordings showed that the addition of OM had different potencies in stimulating the transient (I_Na(T)) and late components (I_Na(L)) of the voltage-gated Na⁺ current (I_Na) with different potencies in GH₃ cells. The EC₅₀ value required to observe the stimulatory effect of this compound on I_Na(T) or I_Na(L) in GH₃ cells was found to be 15.8 and 2.3 µM, respectively. Exposure to OM did not affect the current versus voltage relationship of I_Na(T). However, the steady-state inactivation curve of the current was observed to shift towards a depolarized potential of approximately 11 mV, with no changes in the slope factor of the curve. The addition of OM resulted in an increase in the decaying time constant during the cumulative inhibition of I_Na(T) in response to pulse-train depolarizing stimuli. Furthermore, the presence of OM led to a shortening of the recovery time constant in the slow inactivation of I_Na(T). Adding OM also resulted in an augmentation of the strength of the window Na⁺ current, which was evoked by a short ascending ramp voltage. However, the OM exposure had little to no effect on the magnitude of L-type Ca²⁺ currents in GH₃ cells. On the other hand, the delayed-rectifier K⁺ currents in GH₃ cells were observed to be mildly suppressed in its presence. Neuro-2a cells also showed a susceptibility to the differential stimulation of I_Na(T) or I_Na(L) upon the addition of OM. Molecular analysis revealed potential interactions between the OM molecule and hNa_V1.7 channels. Overall, the direct stimulation of I_Na(T) and I_Na(L) by OM is assumed to not be mediated by an interaction with myosin, and this has potential implications for its pharmacological or therapeutic actions occurring in vivo.

The single-chain relaxin mimetic, B7-33, maintains the cardioprotective effects of relaxin and more rapidly reduces left ventricular fibrosis compared to perindopril in an experimental model of cardiomyopathy

The hormone, relaxin (RLX), exerts various organ-protective effects independently of etiology. However, its complex two-chain and three disulphide bonded structure is a limitation to its preparation and affordability. Hence, a single chain-derivative of RLX, B7-33, was developed and shown to retain the anti-fibrotic effects of RLX in vitro and in vivo. Here, we determined whether B7-33 could retain the other cardioprotective effects of RLX, and also compared its therapeutic efficacy to the ACE inhibitor, perindopril. Adult male 129sv mice were subjected to isoprenaline (ISO; 25 mg/kg/day, s.c)-induced cardiomyopathy, then s.c-treated with either RLX (0.5 mg/kg/day), B7-33 (0.25 mg/kg/day; equivalent dose corrected for MW) or perindopril (1 mg/kg/day) from days 7-14 post-injury. Control mice received saline instead of ISO. Changes in animal body weight (BW) and systolic blood pressure (SBP) were measured weekly, whilst cardiomyocyte hypertrophy and measures of vascular dysfunction and rarefaction, left ventricular (LV) inflammation and fibrosis were assessed at day 14 post-injury. ISO-injured mice had significantly increased LV inflammation, cardiomyocyte hypertrophy, fibrosis, vascular rarefaction and aortic contractility in the absence of any changes in BW or SBP at day 14 post-injury. Both B7-33 and RLX equivalently reduced LV fibrosis and normalised the ISO-induced LV inflammation and cardiomyocyte hypertrophy, whilst restoring blood vessel density and aortic contractility. Comparatively, perindopril lowered SBP and the ISO-induced LV inflammation and vascular rarefaction, but not fibrosis or hypertrophy. As B7-33 retained the cardioprotective effects of RLX and provided rapid-occurring anti-fibrotic effects compared to perindopril, it could be considered as a cost-effective cardioprotective therapy.

Lipids: a Potential Molecular Pathway Towards Diastolic Dysfunction in Youth-Onset Type 2 Diabetes

PURPOSE OF THE REVIEW

Obesity and type 2 diabetes (T2D) with onset in youth are emerging public health concerns. Youth with obesity and T2D are at risk for the development of heart failure with preserved ejection fraction (HFpEF) due to diabetes-related cardiomyopathy with evidence of precursor stages, namely diastolic dysfunction, present in youth. We review the literature regarding diastolic dysfunction in youth with obesity and T2D; discuss the potential mechanisms including the role of lipids, contractile proteins and their post-translational modifications, and conclude with studies to guide future treatments.

RECENT FINDINGS

The diabetes milieu namely hyperglycemia, hyperinsulinemia, and lipotoxicity favor development of diastolic dysfunction and HFpEF. Recent studies show HFpEF is associated with slow left ventricular relaxation and sarcomere stiffness induced by reduced calcium (Ca2+) and β-adrenergic responses. There are currently no effective therapies available for treating HFpEF. Targeting the sarcomere is an area of ongoing research.

Contemporary Pillars of Heart Failure with Reduced Ejection Fraction Medical Therapy

Heart failure with reduced ejection fraction (HFrEF) is a clinical condition associated with cardiac contractility impairment. HFrEF is a significant public health issue with a high morbidity and mortality burden. Pathological left ventricular (LV) remodeling and progressive dilatation are hallmarks of HFrEF pathogenesis, ultimately leading to adverse clinical outcomes. Therefore, cardiac remodeling attenuation has become a treatment goal and a standard of care over the last three decades. Guideline-directed medical therapy mainly targeting the sympathetic nervous system and the renin-angiotensin-aldosterone system (RAAS) has led to improved survival and a reduction in HF hospitalization in this population. More recently, novel pharmacological therapies targeting other pathways implicated in the pathophysiology of HFrEF have emerged at an exciting rate, with landmark clinical trials demonstrating additive clinical benefits in patients with HFrEF. Among these novel therapies, angiotensin receptor-neprilysin inhibitors (ARNI), sodium-glucose cotransporter-2 inhibitors (SGLT2i), vericiguat (a novel oral guanylate cyclase stimulator), and omecamtiv mecarbil (a selective cardiac myosin activator) have shown improved clinical benefit when added to the traditional standard-of-care medical therapy in HFrEF. These new comprehensive data have led to a remarkable change in the medical therapy paradigm in the setting of HFrEF. This article will review the pivotal studies involving these novel agents and present a suggestive paradigm of pharmacological therapy representing the 2021 European Society of Cardiology (ESC) guidelines for the treatment of chronic HFrEF.

Omecamtiv mecarbil attenuates length-tension relationship in healthy rat myocardium and preserves it in monocrotaline-induced pulmonary heart failure

The cardiac-specific myosin activator, omecamtiv mecarbil (OM), is an effective inotrope for treating heart failure but its effects on active force and Ca²⁺ kinetics in healthy and diseased myocardium remain poorly studied. We tested the effect of two concentrations of OM (0.2 and 1 µmol/L in saline) on isometric contraction and Ca-transient (CaT) in right ventricular trabeculae of healthy rats (CONT, n = 8) and rats with monocrotaline-induced pulmonary heart failure (MCT, n = 8). The contractions were obtained under preload of 75%-100% of optimal length (tension-length relationship). The 0.2 µmol/L OM did not affect the diastolic level, amplitude, or kinetics of isometric contraction and CaT, irrespective of the group of rats or preload. The 1 µmol/L OM significantly suppressed active tension-length relationships in CONT but not in MCT, while leading in both groups to a significantly prolonged relaxation. CaT time-to-peak was unaffected in CONT and MCT, but CaT decay was slightly accelerated in its early phase and considerably prolonged in its late phase to a similar extent in both groups. We conclude that the substantial prolongation of CaT decay is due to enhanced Ca²⁺ utilisation by troponin C mediated by the direct effect of OM on the cooperative activation of myofilaments. The lack of beneficial effect of OM in the healthy rat myocardium may be due to a relatively high level of activating Ca²⁺ in cells with normal Ca²⁺ handling, whereas the preservation of the tension-length relationship in the failing heart may relate to the diminished Ca²⁺ levels of sarcoplasmic reticulum.

Evaluation of drug-drug interaction potential between omecamtiv mecarbil and rosuvastatin, a BCRP substrate, with a clinical study in healthy subjects and using a physiologically-based pharmacokinetic model

Omecamtiv mecarbil (OM) is a novel cardiac myosin activator in development for the treatment of heart failure. In vitro, OM is an inhibitor of BCRP. Rosuvastatin, a BCRP substrate, is one of the most commonly prescribed medications in patients with heart failure. The potential for a pharmacokinetic (PK) drug‐drug interaction (DDI) was investigated, specifically to determine whether a single 50 mg dose of OM would impact the PKs of a single 10 mg dose of rosuvastatin in an open‐label study in 14 healthy subjects. The ratios of the geometric least‐square means (90% confidence intervals [CIs]) of rosuvastatin co‐administered with OM compared to rosuvastatin alone were 127.1% (90% CI 113.8–141.9), 132.8% (90% CI 120.7–146.1), and 154.2% (90% CI 132.8–179.1) for area under the plasma‐concentration time curve from time zero to infinity (AUC_inf), area under the plasma‐concentration time curve from time zero to time of last quantifiable concentration (AUC_last), and maximum observed plasma concentration (C_max), respectively. Whereas the DDI study with rosuvastatin was conducted with the co‐administration of a single dose of OM, in the clinical setting, patients receive OM at doses of 25, 37.5, or 50 mg twice daily (b.i.d.). Hence, to extrapolate the results of the DDI study to a clinically relevant scenario of continuous b.i.d. dosing with OM, physiologically‐based pharmacokinetic (PBPK) modeling was performed to explore the potential of BCRP inhibition following continuous b.i.d. dosing of OM at the highest 50 mg dose. Modeling results indicated that following 50 mg b.i.d. dosing of OM, the predicted ratios of the geometric means (90% CIs) for rosuvastatin AUC_inf and C_max were 1.18 (90% CI 1.16–1.20) and 2.04 (90% CI 1.99–2.10), respectively. Therefore, these results suggest that OM, following multiple dose administration, is a weak inhibitor of BCRP substrates and is in accordance with that observed in the single dose OM DDI clinical study.

Pharmacokinetic Drug-Drug Interaction Study of Omecamtiv Mecarbil With Omeprazole, a Proton Pump Inhibitor, in Healthy Subjects

Omecamtiv mecarbil (OM) is a novel cardiac myosin activator in development for the treatment of heart failure (HF) with reduced ejection fraction. OM is administered as a 25-, 37.5-, or 50-mg modified-release formulation in patients with HF. Proton pump inhibitors are one of the most commonly prescribed drugs in this patient population. Given the potential for coadministration of both drugs in patients with HF, we evaluated the potential for omeprazole to affect the pharmacokinetics of OM in an open-label study in 14 healthy subjects. Subjects received a single 50-mg dose of OM on day 1, followed by 40-mg once-daily doses of omeprazole on days 4 to 8. On day 9, a single 40-mg dose of omeprazole was administered first and immediately followed by 50-mg of OM. Blood samples were collected up to 144 hours after dosing following administration of OM on days 1 and 9 to characterize plasma concentrations of OM. The ratios of the geometric least-square means (90% confidence intervals) of OM coadministered with omeprazole compared to OM alone were 94.5% (81.7%-109.3%), 94.3% (81.5%-109.1%), and 101.2% (95.4%-107.3%) for area under the plasma concentration-time curve from time 0 to infinity, area under the plasma concentration-time curve from time 0 to the last measurable concentration, and maximum observed plasma concentration, respectively. Coadministration of OM with omeprazole was not associated with any clinically significant pharmacokinetic drug interactions. Single doses of OM were safe and well tolerated when coadministered with omeprazole.

Switchability and minimal effect of food on pharmacokinetics of modified release tablet strengths of omecamtiv mecarbil, a cardiac myosin activator

Omecamtiv mecarbil (OM) is a cardiac myosin activator in clinical development for the treatment of heart failure. The effect of food on the pharmacokinetics (PK) of 25, 37.5, and 50 mg strength modified release (MR) tablets and the bioequivalence of two 25 mg tablets versus one 50 mg MR tablet were evaluated in two open-label, randomized, cross-over studies in healthy subjects. Subjects received two 25 mg tablets or one 50 mg OM MR tablet under fed or fasted states in Study 1 (n = 39), and single oral doses of 25 and 37.5 mg OM MR tablets and to assess its relative bioavailability to the 25 mg MR tablet, a 25 mg oral solution under fed or fasted states in Study 2 (n = 34). The area under the concentration-time curve (AUC) and the maximum observed concentration (C_max ) of 25, 37.5, or 50 mg OM MR tablets were approximately 13%-22% higher and 31%-40% higher, respectively, when taken with food. The two 25 mg and one 50 mg OM MR tablets were bioequivalent (90% confidence intervals) of the geometric mean ratios for C_max and AUC of OM were within 0.8-1.25 under the fasted or fed state. OM was well tolerated and all treatment-emergent events were mild in severity and resolved by the end of the study. In conclusion, these studies demonstrated that the effect of food on the PK of OM was minimal at all three studied strengths of the MR tablets, and two 25 mg MR tablets may be switched for one 50 mg MR tablet (EudraCT Number: 2019-003683-44).

Omecamtiv mecarbil does not prolong QTc intervals at therapeutic concentrations

AIMS

Omecamtiv mecarbil (OM) is a novel selective cardiac myosin activator under investigation for the treatment of heart failure. This study aimed to evaluate the effect of therapeutic concentrations of OM on electrocardiogram (ECG) parameters and exclude a clinically concerning effect on the rate-corrected QT (QTc) interval.

METHODS

In part A, 70 healthy subjects received a 25 mg oral dose of OM, and pharmacokinetics were assessed. Only subjects with maximum observed plasma concentration ≤ 350 ng/mL (n = 60) were randomized into part B, where they received a single oral dose of placebo, 50 mg OM and 400 mg moxifloxacin in a 3-period, 3-treatment, 6-sequence crossover study with continuous ECG collection.

RESULTS

After a 50-mg dose of OM, mean placebo-corrected change from baseline QTcF (∆∆QTcF; Fridericia correction) ranged from -6.7 ms at 1 hour postdose to -0.8 ms at 4 hours postdose. The highest upper bound of the 1-sided 95% confidence interval (CI) was 0.7 ms (4 h postdose). Moxifloxacin resulted in a clear increase in mean ∆∆QTcF, with a peak value of 13.1 ms (90% CI: 11.71-14.57) at 3 hours; lower bound of the 1-sided 95% CI was > 5 ms at all of the 3 prespecified time points. Based on a concentration-QTc analysis, an effect on ∆∆QTcF exceeding 10 ms can be excluded up to OM plasma concentrations of ~800 ng/mL. There were no serious or treatment-emergent adverse events leading to discontinuation from the study.

CONCLUSION

OM does not have a clinically relevant effect on the studied ECG parameters.

Relative Bioavailability of Omecamtiv Mecarbil Pediatric Minitablet Formulations in Healthy Adult Subjects

BACKGROUND AND OBJECTIVE

Omecamtiv mecarbil (OM) is a cardiac myosin activator under clinical development for the treatment of heart failure. Two modified-release (MR) novel OM minitablet formulations were developed to support the planned investigation of chronic heart failure in pediatric patients. The primary objective of this study was to determine the bioavailability of the minitablets relative to the adult matrix MR formulation tablets.

METHODS

In a randomized, 5-period, crossover study, 20 healthy subjects received each of the following treatments orally: one 25-mg adult matrix MR tablet, 25 1-mg slow-release minitablets, 25 1-mg fast-release minitablets, six 1-mg slow-release minitablets, or six 1-mg fast-release minitablets after an overnight fast of at least 10 h with a minimum washout of 7 days between treatments. Blood samples were collected for up to 168 h. OM pharmacokinetic parameters were estimated using non-compartmental methods.

RESULTS

When OM was administered as 25 1-mg OM slow-release minitablets, AUC_last, AUC_inf, and C_max were 0.998-, 1.00-, and 1.29-fold of a single 25-mg OM matrix MR tablet, respectively. When OM was administered as 25 1-mg OM fast-release minitablets, AUC_last, AUC_inf, and C_max were 1.26-, 1.25-, and 2.21-fold of a single 25-mg OM matrix MR tablet, respectively. The slow- and fast-release minitablets display approximately dose-proportional pharmacokinetics. There were no serious adverse events or treatment-emergent adverse events leading to discontinuation from the study.

CONCLUSIONS

Relative bioavailability of slow-release minitablets was demonstrated to be similar to the adult matrix MR formulation.

Effect of Omecamtiv Mecarbil on the Pharmacokinetics of Metformin, a Probe Substrate for MATE1/MATE2-K, in Healthy Subjects

BACKGROUND AND OBJECTIVE

Omecamtiv mecarbil (OM) is a novel cardiac myosin activator in development for the treatment of heart failure with reduced ejection fraction. The objective of this study was to evaluate the potential for OM to affect the pharmacokinetics of metformin.

METHODS

This was an open-label, fixed-sequence study in 14 healthy subjects. On Day 1, subjects received an 850 mg oral dose of metformin. From Days 4 to 9, subjects received twice-daily 25 mg oral doses of OM tablets. On Day 10, subjects received an 850 mg oral dose of metformin and a single 25 mg tablet of OM. Blood and urine samples were collected up to 36 h post-dose following administration of metformin on Days 1 and 10 to characterize concentrations of metformin in plasma and urine.

RESULTS

The ratios of the geometric least square means of metformin coadministered with OM compared to metformin alone were 98.7%, 99.3%, and 110.2% for AUC_inf, AUC_last, and C_max, respectively. The mean renal clearance of metformin was similar following metformin administered alone (34.2 L/h) compared to metformin coadministered with OM (32.9 L/h). All adverse events were mild in severity and resolved prior to the end of the study. No serious adverse events or treatment-emergent adverse events led to discontinuation from the study.

CONCLUSIONS

There was no clinically relevant effect of OM on the pharmacokinetics of metformin in healthy subjects.

Myosin Modulators: The New Era of Medical Therapy for Systolic Heart Failure and Hypertrophic Cardiomyopathy

A new uprising pharmacological treatment for systolic heart failure and hypertrophic cardiomyopathy demonstrates very promising results the last years. Myosin modulators have already been tested in numerous studies. Myosin inhibitor (mavacamten) and myosin stimulator, (omecamtiv mecarbil) exhibit their effect by improving clinical outcomes, as well as reducing morbidity and mortality. More studies are however required for further evaluation and particularly effect on sarcomeric level. Side effects of both pharmacological agents have been described and should always be close monitored. Cardiopulmonary exercise test has a pivotal role by means of assessing treatment efficacy.

Endogenous Cardiotonics: Search And Problems

Medicinal preparations currently used for the treatment of patients with chronic cardiac failure involve those that reduce the heart load (vasodilators, diuretics, beta-blockers, and angiotensin-converting enzyme (ACE) inhibitors). Cardiotonic drugs with the cAMP-dependent mechanism are unsuitable for long-term administration due to the intensification of metabolic processes and an increase in the oxygen demand of the myocardium and all tissues of the body. For many years, digoxin has remained the only preparation enhancing the efficiency of myocardial performance. The detection of digoxin and ouabain in intact animals has initiated a search for other compounds with cardiotonic activity. The review summarizes current data on the effect exerted on the heart performance by endogenous compounds, from simple, such as NO and CO, to steroids, fatty acids, polypeptides, and proteins. Controversial questions and problems with the introduction of scientific achievements into clinical practice are discussed. The results obtained by the authors and their colleagues after many years of studies on the cardiotropic properties of serum lipoproteins are also reported. The experimentally established cardiotonic activity of apoprotein A-1, which is accompanied by a decrease in the relative consumption of oxygen, maybe of great interest.

Identification and functional study of genetic polymorphisms in cyclic nucleotide phosphodiesterase 3A (PDE3A)

Phosphodiesterase 3A (PDE3A) is an enzyme that plays an important role in the regulation of cyclic adenosine monophosphate (cAMP)-mediated intracellular signaling in cardiac myocytes and platelets. PDE3A hydrolyzes cAMP, which results in a decrease in intracellular cAMP levels and leads to platelet activation. Whole-exome sequencing of 50 DNA samples from a healthy Korean population revealed a total of 13 single nucleotide polymorphisms including five missense variants, D12N, Y497C, H504Q, C707R, and A980V. Recombinant proteins for the five variants of PDE3A (and wild-type protein) were expressed in a FreeStyle 293 expression system with site-directed mutagenesis. The expression of the recombinant PDE3A proteins was confirmed with Western blotting. Catalytic activity of the PDE3A missense variants and wild-type enzyme was measured with a PDE-based assay. Effects of the missense variants on the inhibition of PDE3A activity by cilostazol were also investigated. All variant proteins showed reduced activity (33-53%; p < .0001) compared to the wild-type protein. In addition, PDE3A activity was inhibited by cilostazol in a dose-dependent manner and was further suppressed in the missense variants. Specifically, the PDE3A Y497C showed significantly reduced activity, consistent with the predictions of in silico analyses. The present study provides evidence that individuals carrying the PDE3A Y497C variant may have lower enzyme activity for cAMP hydrolysis, which could cause interindividual variation in cAMP-mediated physiological functions.

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.

Genetic, clinical, molecular, and pathogenic aspects of the South Asian-specific polymorphic MYBPC3Δ25bp variant

Hypertrophic cardiomyopathy (HCM) is a cardiac genetic disease characterized by ventricular enlargement, diastolic dysfunction, and increased risk for sudden cardiac death. Sarcomeric genetic defects are the predominant known cause of HCM. In particular, mutations in the myosin-binding protein C gene (MYBPC3) are associated with ~ 40% of all HCM cases in which a genetic basis has been established. A decade ago, our group reported a 25-base pair deletion in intron 32 of MYBPC3 (MYBPC3Δ25bp) that is uniquely prevalent in South Asians and is associated with autosomal dominant cardiomyopathy. Although our studies suggest that this deletion results in left ventricular dysfunction, cardiomyopathies, and heart failure, the precise mechanism by which this variant predisposes to heart disease remains unclear. Increasingly appreciated, however, is the contribution of secondary risk factors, additional mutations, and lifestyle choices in augmenting or modifying the HCM phenotype in MYBPC3Δ25bp carriers. Therefore, the goal of this review article is to summarize the current research dedicated to understanding the molecular pathophysiology of HCM in South Asians with the MYBPC3Δ25bp variant. An emphasis is to review the latest techniques currently applied to explore the MYBPC3Δ25bp pathogenesis and to provide a foundation for developing new diagnostic strategies and advances in therapeutics.

Effects of danicamtiv, a novel cardiac myosin activator, in heart failure with reduced ejection fraction: experimental data and clinical results from a phase 2a trial

AIMS

Both left ventricular (LV) and left atrial (LA) dysfunction and remodelling contribute to adverse outcomes in heart failure with reduced ejection fraction (HFrEF). Danicamtiv is a novel, cardiac myosin activator that enhances cardiomyocyte contraction.

METHODS AND RESULTS

We studied the effects of danicamtiv on LV and LA function in non-clinical studies (ex vivo: skinned muscle fibres and myofibrils; in vivo: dogs with heart failure) and in a randomized, double-blind, single- and multiple-dose phase 2a trial in patients with stable HFrEF (placebo, n = 10; danicamtiv, n = 30; 50-100 mg twice daily for 7 days). Danicamtiv increased ATPase activity and calcium sensitivity in LV and LA myofibrils/muscle fibres. In dogs with heart failure, danicamtiv improved LV stroke volume (+10.6 mL, P < 0.05) and LA emptying fraction (+10.7%, P < 0.05). In patients with HFrEF (mean age 60 years, 25% women, ischaemic heart disease 48%, mean LV ejection fraction 32%), treatment-emergent adverse events, mostly mild, were reported in 17 patients (57%) receiving danicamtiv and 4 patients (40%) receiving placebo. Danicamtiv (at plasma concentrations ≥2000 ng/mL) increased stroke volume (up to +7.8 mL, P < 0.01), improved global longitudinal (up to -1.0%, P < 0.05) and circumferential strain (up to -3.3%, P < 0.01), decreased LA minimal volume index (up to -2.4 mL/m² , P < 0.01) and increased LA function index (up to 6.1, P < 0.01), when compared with placebo.

CONCLUSIONS

Danicamtiv was well tolerated and improved LV systolic function in patients with HFrEF. A marked improvement in LA volume and function was also observed in patients with HFrEF, consistent with pre-clinical findings of direct activation of LA contractility.

Integration of mechanical conditioning into a high throughput contractility assay for cardiac safety assessment

INDUCTION

Despite increasing acceptance of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) in safety pharmacology, controversy remains about the physiological relevance of existing in vitro models for their mechanical testing. We hypothesize that existing signs of immaturity of the cell models result from an improper mechanical environment. With the presented study, we aimed at validating the newly developed FLEXcyte96 technology with respect to physiological responses of hiPSC-CMs to pharmacological compounds with known inotropic and/or cardiotoxic effects.

METHODS

hiPSC-CMs were cultured in a 96-well format on hyperelastic silicone membranes imitating their native mechanical environment. Cardiomyocyte contractility was measured contact-free by application of capacitive displacement sensing of the cell-membrane biohybrids. Acute effects of positive inotropic compounds with distinct mechanisms of action were examined. Additionally, cardiotoxic effects of tyrosine kinase inhibitors and anthracyclines were repetitively examined during repeated exposure to drug concentrations for up to 5 days.

RESULTS

hiPSC-CMs grown on biomimetic membranes displayed increased contractility responses to isoproterenol, S-Bay K8644 and omecamtiv mecarbil without the need for additional stimulation. Tyrosine kinase inhibitor erlotinib, vandetanib, nilotinib, gefitinib, A-674563 as well as anthracycline idarubicin showed the expected cardiotoxic effects, including negative inotropy and induction of proarrhythmic events.

DISCUSSION

We conclude that the FLEXcyte 96 system is a reliable high throughput tool for invitro cardiac contractility research, providing the user with data obtained under physiological conditions which resemble the native environment of human heart tissue. We showed that the results obtained for both acute and sub-chronic compound administration are consistent with the respective physiological responses in humans.

Inotropic assessment in engineered 3D cardiac tissues using human induced pluripotent stem cell-derived cardiomyocytes in the BiowireTM II platform

To develop therapeutics for cardiovascular disease, especially heart failure, translational models for assessing cardiac contractility are necessary for preclinical target validation and lead optimization. The availability of stem cell-derived cardiomyocytes (SC-CM) has generated a great opportunity in developing new in-vitro models for assessing cardiac contractility. However, the immature phenotype of SC-CM is a well-recognized limitation in inotropic evaluation, especially regarding the lack of or diminished positive inotropic response to β-adrenergic agonists. Recent development of 3D engineered cardiac tissues (ECTs) using human induced pluripotent stem cell derived-cardiomyocytes (hiPSC-CM) in the Biowire^TM II platform has shown improved maturation. To evaluate their suitability to detect drug-induced changes in cardiac contractility, positive inotropes with diverse mechanisms, including β-adrenergic agonists, PDE3 inhibitors, Ca²⁺-sensitizers, myosin and troponin activators, and an apelin receptor agonist, were tested blindly. A total of 8 compounds were evaluated, including dobutamine, milrinone, pimobendan, levosimendan, omecamtiv mecarbil, AMG1, AMG2, and pyr-apelin-13. Contractility was evaluated by analyzing the amplitude, velocity and duration of contraction and relaxation. All tested agents, except pyr-apelin-13, increased contractility by increasing the amplitude of contraction and velocity. In addition, myosin and troponin activators increase contraction duration. These results indicate that ECTs generated in the Biowire^TM II platform can identify inotropes with different mechanisms and provides a human-based in-vitro model for evaluating potential therapeutics.

Pathogenesis and pathophysiology of heart failure with reduced ejection fraction: translation to human studies

Heart failure represents the end result of different pathophysiologic processes, which culminate in functional impairment. Regardless of its aetiology, the presentation of heart failure usually involves symptoms of pump failure and congestion, which forms the basis for clinical diagnosis. Pathophysiologic descriptions of heart failure with reduced ejection fraction (HFrEF) are being established. Most commonly, HFrEF is centred on a reactive model where a significant initial insult leads to reduced cardiac output, further triggering a cascade of maladaptive processes. Predisposing factors include myocardial injury of any cause, chronically abnormal loading due to hypertension, valvular disease, or tachyarrhythmias. The pathophysiologic processes behind remodelling in heart failure are complex and reflect systemic neurohormonal activation, peripheral vascular effects and localised changes affecting the cardiac substrate. These abnormalities have been the subject of intense research. Much of the translational successes in HFrEF have come from targeting neurohormonal responses to reduced cardiac output, with blockade of the renin-angiotensin-aldosterone system (RAAS) and beta-adrenergic blockade being particularly fruitful. However, mortality and morbidity associated with heart failure remains high. Although systemic neurohormonal blockade slows disease progression, localised ventricular remodelling still adversely affects contractile function. Novel therapy targeted at improving cardiac contractile mechanics in HFrEF hold the promise of alleviating heart failure at its source, yet so far none has found success. Nevertheless, there are increasing calls for a proximal, 'cardiocentric' approach to therapy. In this review, we examine HFrEF therapy aimed at improving cardiac function with a focus on recent trials and emerging targets.

Inhalation of printer-emitted particles impairs cardiac conduction, hemodynamics, and autonomic regulation and induces arrhythmia and electrical remodeling in rats

BACKGROUND

Using engineered nanomaterial-based toners, laser printers generate aerosols with alarming levels of nanoparticles that bear high bioactivity and potential health risks. Yet, the cardiac impacts of printer-emitted particles (PEPs) are unknown. Inhalation of particulate matter (PM) promotes cardiovascular morbidity and mortality, and ultra-fine particulates (< 0.1 μm aerodynamic diameter) may bear toxicity unique from larger particles. Toxicological studies suggest that PM impairs left ventricular (LV) performance; however, such investigations have heretofore required animal restraint, anesthesia, or ex vivo preparations that can confound physiologic endpoints and/or prohibit LV mechanical assessments during exposure. To assess the acute and chronic effects of PEPs on cardiac physiology, male Sprague Dawley rats were exposed to PEPs (21 days, 5 h/day) while monitoring LV pressure (LVP) and electrocardiogram (ECG) via conscious telemetry, analyzing LVP and heart rate variability (HRV) in four-day increments from exposure days 1 to 21, as well as ECG and baroreflex sensitivity. At 2, 35, and 70 days after PEPs exposure ceased, rats received stress tests.

RESULTS

On day 21 of exposure, PEPs significantly (P < 0.05 vs. Air) increased LV end systolic pressure (LVESP, + 18 mmHg) and rate-pressure-product (+ 19%), and decreased HRV indicating sympathetic dominance (root means squared of successive differences [RMSSD], - 21%). Overall, PEPs decreased LV ejection time (- 9%), relaxation time (- 3%), tau (- 5%), RMSSD (- 21%), and P-wave duration (- 9%). PEPs increased QTc interval (+ 5%) and low:high frequency HRV (+ 24%; all P < 0.05 vs. Air), while tending to decrease baroreflex sensitivity and contractility index (- 15% and - 3%, P < 0.10 vs. Air). Relative to Air, at both 2 and 35 days after PEPs, ventricular arrhythmias increased, and at 70 days post-exposure LVESP increased. PEPs impaired ventricular repolarization at 2 and 35 days post-exposure, but only during stress tests. At 72 days post-exposure, PEPs increased urinary dopamine 5-fold and protein expression of ventricular repolarizing channels, Kv1.5, Kv4.2, and Kv7.1, by 50%.

CONCLUSIONS

Our findings suggest exposure to PEPs increases cardiovascular risk by augmenting sympathetic influence, impairing ventricular performance and repolarization, and inducing hypertension and arrhythmia. PEPs may present significant health risks through adverse cardiovascular effects, especially in occupational settings, among susceptible individuals, and with long-term exposure.

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Inotropes in Acute Heart Failure: From Guidelines to Practical Use: Therapeutic Options and Clinical Practice

Inotropes are pharmacological agents that are indicated for the treatment of patients presenting with acute heart failure (AHF) with concomitant hypoperfusion due to decreased cardiac output. They are usually administered for a short period during the initial management of AHF until haemodynamic stabilisation and restoration of peripheral perfusion occur. They can be used for longer periods to support patients as a bridge to a more definite treatment, such as transplant of left ventricular assist devices, or as part of a palliative care regimen. The currently available inotropic agents in clinical practice fall into three main categories: beta-agonists, phosphodiesterase III inhibitors and calcium sensitisers. However, due to the well-documented potential for adverse events and their association with increased long-term mortality, physicians should be aware of the indications and dosing strategies suitable for different types of patients. Novel inotropes that use alternative intracellular pathways are under investigation, in an effort to minimise the drawbacks that conventional inotropes exhibit.

Design and synthesis of sulfonamidophenylethylamides as novel cardiac myosin activator

The sulfonamidophenylethylamide analogues were explored for finding novel and potent cardiac myosin activators. Among them, N-(4-(N,N-dimethylsulfamoyl)phenethyl-N-methyl-5-phenylpentanamide (13, CMA at 10 µM = 48.5%; FS = 26.21%; EF = 15.28%) and its isomer, 4-(4-(N,N-dimethylsulfamoyl)phenyl-N-methyl-N-(3-phenylpropyl)butanamide (27, CMA at 10 µM = 55.0%; FS = 24.69%; EF = 14.08%) proved to be efficient cardiac myosin activators both in in vitro and in vivo studies. Compounds 13 (88.2 + 3.1% at 5 µM) and 27 (46.5 + 2.8% at 5 µM) showed positive inotropic effect in isolated rat ventricular myocytes. The potent compounds 13 and 27 were highly selective for cardiac myosin over skeletal and smooth muscle myosin, and therefore these potent and selective amide derivatives could be considered a new class of cardiac myosin activators for the treatment of systolic heart failure.

The energy-saving effect of a new myosin activator, omecamtiv mecarbil, on LV mechanoenergetics in rat hearts with blood-perfused isovolumic contraction model

A novel myosin activator, omecamtiv mecarbil (OM), is a cardiac inotropic agent with a unique new mechanism of action, which is thought to arise from an increase in the transition rate of myosin into the actin-bound force-generating state without increasing calcium (Ca²⁺) transient. There remains, however, considerable controversy about the effects of OM on cardiac contractility and energy expenditure. In the present study, we investigated the effects of OM on left ventricular (LV) mechanical work and energetics, i.e., mechanoenergetics in rat normal hearts (CTL) and failing hearts induced by chronic administration of isoproterenol (1.2 mg/kg/day) for 4 weeks (ISO-HF). We analyzed the LV end-systolic pressure-volume relation (ESPVR) and the linear relation between the myocardial oxygen consumption per beat (VO₂) and systolic pressure-volume area (PVA; a total mechanical energy per beat) in isovolumically contracting rat hearts at 240- or 300-bpm pacing in the absence or presence of OM. OM did not change the ESPVR in CTL and ISO-HF. OM, however, significantly decreased the slope of VO₂-PVA relationship in both CTL and ISO-HF, and significantly increased the mean VO₂ intercept without changes in basal metabolism in ISO-HF. These results suggested that OM improved the oxygen cost of PVA (contractile efficiency) with the unchanged LV contractility in both CTL and ISO-HF but increased VO₂ for Ca²⁺ handling in excitation-contraction (E-C) coupling in ISO-HF. We concluded that OM improves contractile efficiency in normal and failing hearts but increases O₂ consumption of Ca²⁺ handling in failing hearts in isovolumically contracting rat model.

Cardiac Resynchronisation Therapy and Cellular Bioenergetics: Effects Beyond Chamber Mechanics

Cardiac resynchronisation therapy is a cornerstone in the treatment of advanced dyssynchronous heart failure. However, despite its widespread clinical application, precise mechanisms through which it exerts its beneficial effects remain elusive. Several studies have pointed to a metabolic component suggesting that, both in concert with alterations in chamber mechanics and independently of them, resynchronisation reverses detrimental changes to cellular metabolism, increasing energy efficiency and metabolic reserve. These actions could partially account for the existence of responders that improve functionally but not echocardiographically. This article will attempt to summarise key components of cardiomyocyte metabolism in health and heart failure, with a focus on the dyssynchronous variant. Both chamber mechanics-related and -unrelated pathways of resynchronisation effects on bioenergetics – stemming from the ultramicroscopic level – and a possible common underlying mechanism relating mechanosensing to metabolism through the cytoskeleton will be presented. Improved insights regarding the cellular and molecular effects of resynchronisation on bioenergetics will promote our understanding of non-response, optimal device programming and lead to better patient care.

Lost in translation: Interpreting cardiac muscle mechanics data in clinical practice

Current inotropic therapies improve systolic function in heart failure patients but also elicit undesirable side effects such as arrhythmias and increased intracellular Ca2+ transients. In order to maintain myocyte Ca2+ homeostasis, the increased cytosolic Ca2+ needs to be actively transported back to sarcoplasmic reticulum leading to depleted ATP reserves. Thus, an emerging approach is to design sarcomere-based treatments to correct impaired contractility via a direct and allosteric modulation of myosin's intrinsic force-generating behavior -a concept that potentially avoids the "off-target" effects. To achieve this goal, various biophysical approaches are utilized to investigate the mechanistic impact of sarcomeric modulators but information derived from diverse approaches is not fully integrated into therapeutic applications. This is in part due to the lack of information that provides a coherent connecting link between biophysical data to in vivo function. Hence, our ability to clearly discern the drug-mediated impact on whole-heart function is diminished. Reducing this translational barrier can significantly accelerate clinical progress related to sarcomere-based therapies by optimizing drug-dosing and treatment duration protocols based on information obtained from biophysical studies. Therefore, we attempt to link biophysical mechanical measurements obtained in isolated cardiac muscle and in vivo contractile function.

Impact of the Myosin Modulator Mavacamten on Force Generation and Cross-Bridge Behavior in a Murine Model of Hypercontractility

Background Recent studies suggest that mavacamten (Myk461), a small myosin-binding molecule, decreases hypercontractility in myocardium expressing hypertrophic cardiomyopathy-causing missense mutations in myosin heavy chain. However, the predominant feature of most mutations in cardiac myosin binding protein-C ( cMyBPC ) that cause hypertrophic cardiomyopathy is reduced total cMyBPC expression, and the impact of Myk461 on cMyBPC -deficient myocardium is currently unknown. Methods and Results We measured the impact of Myk461 on steady-state and dynamic cross-bridge ( XB ) behavior in detergent-skinned mouse wild-type myocardium and myocardium lacking cMyBPC (knockout (KO)). KO myocardium exhibited hypercontractile XB behavior as indicated by significant accelerations in rates of XB detachment (krel) and recruitment (kdf) at submaximal Ca2+ activations. Incubation of KO and wild-type myocardium with Myk461 resulted in a dose-dependent force depression, and this impact was more pronounced at low Ca2+ activations. Interestingly, Myk461-induced force depressions were less pronounced in KO myocardium, especially at low Ca2+ activations, which may be because of increased acto-myosin XB formation and potential disruption of super-relaxed XB s in KO myocardium. Additionally, Myk461 slowed krel in KO myocardium but not in wild-type myocardium, indicating increased XB " on" time. Furthermore, the greater degree of Myk461-induced slowing in kdf and reduction in XB recruitment magnitude in KO myocardium normalized the XB behavior back to wild-type levels. Conclusions This is the first study to demonstrate that Myk461-induced force depressions are modulated by cMyBPC expression levels in the sarcomere, and emphasizes that clinical use of Myk461 may need to be optimized based on the molecular trigger that underlies the hypertrophic cardiomyopathy phenotype.

Current Status of Inotropes in Heart Failure

Inotropes are medications that improve the contractility of the heart and are used in patients with low cardiac output or evidence of end-organ dysfunction. Since their initial discovery, inotropes have held promise in alleviating symptoms and potentially increasing longevity in such patients. Decades of intensive study have further elucidated the benefits and risks of using inotropes. In this article, the authors discuss the history of inotropes, their indications, mechanism of action, and current guidelines pertaining to their use in heart failure. The authors provide insight into their appropriate use and related shortcomings and the practical aspects of inotrope use.

Exploration of diphenylalkyloxadiazoles as novel cardiac myosin activator

To explore novel cardiac myosin activator, a series of diphenylalkyl substituted 1,3,4-oxadiazoles and 1,2,4-oxadiazoles have been prepared and tested for cardiac myosin ATPase activation in vitro. In all cases, three carbon spacer between the oxadiazole core and one of the phenyl ring was considered crucial. In case of 1,3,4-oxadiazole, zero to two carbon spacer between oxadiazole core and other phenyl ring are favorable. Phenyl ring can be replaced by cyclohexyl moiety. In case of 1,2,4-oxadiazole, zero or one carbon spacer between the oxadiazole and other phenyl ring are favorable. Introduction of hydrogen bonding donor (NH) group at the 2^nd position of the 1,3,4-oxadiazole enhances the activity. Substitutions on either of the phenyl rings or change of phenyl ring to other heterocycle are not tolerated for both the oxadiazoles. The prepared oxadiazoles showed selective activation for cardiac muscle over smooth and skeleton muscles.

Perspectives of a myosin motor activator agent with increased selectivity

Clinical treatment of heart failure is still not fully solved. A novel class of agents, the myosin motor activators, acts directly on cardiac myosin resulting in an increased force generation and prolongation of contraction. Omecamtiv mecarbil, the lead molecule of this group, is now in human phase 3 displaying promising clinical performance. However, omecamtiv mecarbil is not selective to myosin, because it readily binds to and activates cardiac ryanodine receptors (RyR-2), an effect that may cause complications in case of overdose. In this study, in silico analysis was performed to investigate the docking of omecamtiv mecarbil and other structural analogues to cardiac myosin heavy chain and RyR-2 to select the structure that has a higher selectivity to myosin over RyR-2. In silico docking studies revealed that omecamtiv mecarbil has comparable affinity to myosin and RyR-2: the respective K_d values are 0.60 and 0.87 μmol/L. Another compound, CK-1032100, has much lower affinity to RyR-2 than omecamtiv mecarbil, while it still has a moderate affinity to myosin. It was concluded that further research starting from the chemical structure of CK-1032100 may result a better myosin activator burdened probably less by the RyR-2 binding side effect. It also is possible, however, that the selectivity of omecamtiv mecarbil to myosin over RyR-2 cannot be substantially improved, because similar moieties seem to be responsible for the high affinity to both myosin and RyR-2.

Cardiac Myosin Activation with Gene Therapy Produces Sustained Inotropic Effects and May Treat Heart Failure with Reduced Ejection Fraction

Chronic inotropic therapy is effective for the treatment of heart failure with reduced ejection fraction, but has been limited by adverse long-term safety profiles, development of tolerance, and the need for chronic parenteral administration. A safe and convenient therapeutic agent that produces sustained inotropic effects could improve symptoms, functional capacity, and quality of life. Small amounts of 2-deoxy-adenosine triphosphate (dATP) activate cardiac myosin leading to enhanced contractility in normal and failing heart muscle. Cardiac myosin activation triggers faster myosin crossbridge cycling with greater force generation during each contraction. This paper describes the rationale and results of a translational medicine effort to increase dATP levels using a gene therapy strategy to deliver and upregulate ribonucleotide reductase (R1R2), the enzyme responsible for dATP synthesis, selectively in cardiomyocytes. In small and large animal models of heart failure, a single dose of this gene therapy has led to sustained inotropic effects with a benign safety profile. Further animal studies are appropriate with the goal of testing this agent in patients with heart failure.

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.

Cardiovascular Pharmacology: An Update and Anesthesia Considerations

Cardiovascular disease remains a leading cause of morbidity and mortality worldwide. The development of therapeutic agents for the treatment of cardiovascular diseases has always been a priority because of the huge potential market for these drugs. These medications should be part of the anesthesiologist's armamentarium because the typical surgical patient is older and has more comorbidities than in the past. This article reviews commonly used cardiovascular medications that are important in managing patients with unstable hemodynamics.

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.

The fibrosis-cell death axis in heart failure

Cardiac stress can induce morphological, structural and functional changes of the heart, referred to as cardiac remodeling. Myocardial infarction or sustained overload as a result of pathological causes such as hypertension or valve insufficiency may result in progressive remodeling and finally lead to heart failure (HF). Whereas pathological and physiological (exercise, pregnancy) overload both stimulate cardiomyocyte growth (hypertrophy), only pathological remodeling is characterized by increased deposition of extracellular matrix proteins, termed fibrosis, and loss of cardiomyocytes by necrosis, apoptosis and/or phagocytosis. HF is strongly associated with age, and cardiomyocyte loss and fibrosis are typical signs of the aging heart. Fibrosis results in stiffening of the heart, conductivity problems and reduced oxygen diffusion, and is associated with diminished ventricular function and arrhythmias. As a consequence, the workload of cardiomyocytes in the fibrotic heart is further augmented, whereas the physiological environment is becoming less favorable. This causes additional cardiomyocyte death and replacement of lost cardiomyocytes by fibrotic material, generating a vicious cycle of further decline of cardiac function. Breaking this fibrosis-cell death axis could halt further pathological and age-related cardiac regression and potentially reverse remodeling. In this review, we will describe the interaction between cardiac fibrosis, cardiomyocyte hypertrophy and cell death, and discuss potential strategies for tackling progressive cardiac remodeling and HF.

Translation of Cardiac Myosin Activation with 2-deoxy-ATP to Treat Heart Failure via an Experimental Ribonucleotide Reductase-Based Gene Therapy

Despite recent advances, chronic heart failure remains a significant and growing unmet medical need, reaching epidemic proportions carrying substantial morbidity, mortality, and costs. A safe and convenient therapeutic agent that produces sustained inotropic effects could ameliorate symptoms and improve functional capacity and quality of life. The authors discovered that small amounts of 2-deoxy-ATP (dATP) activate cardiac myosin leading to enhanced contractility in normal and failing heart muscle. Cardiac myosin activation triggers faster myosin cross-bridge cycling with greater force generation during each contraction. They describe the rationale and results of a translational medicine effort to increase dATP levels using a gene therapy strategy that up-regulates ribonucleotide reductase, the rate-limiting enzyme for dATP synthesis, selectively in cardiomyocytes. In small and large animal models of heart failure, a single dose of this gene therapy has led to sustained inotropic effects with no toxicity or safety concerns identified to date. Further animal studies are being conducted with the goal of testing this agent in patients with heart failure.

Novel Therapies for Heart Failure - Where Do They Stand?

Despite advances in therapy, patients with heart failure (HF) continue to experience unacceptably high rates of hospitalization and death, as well as poor quality of life. As a consequence, there is an urgent need for new treatments that can improve the clinical course of the growing worldwide population of HF patients. Serelaxin and ularatide, both based on naturally occurring peptides, have potent vasodilatory as well as other effects on the heart and kidneys. For both agents, phase 3 studies that are designed to determine whether they improve outcomes in patients with acute HF have completed enrollment. TRV027, a biased ligand for the type 1 angiotensin receptor with effects that extend beyond traditional angiotensin-receptor blockers is also being studied in the acute HF population. Omecamtiv mecarbil, an inotropic agent that improves myocardial contractility by a novel mechanism, and vericiguat, a drug that stimulates soluble guanylate cyclase, are both being developed to treat patients with chronic HF. Finally, despite the negative results of the CUPID study, gene transfer therapy continues to be explored as a means of improving the function of the failing heart. The basis for the use of these drugs and their current status in clinical trials are discussed. (Circ J 2016; 80: 1882-1891).

Omecamtiv Mecarbil, a Cardiac Myosin Activator, Increases Ca2+ Sensitivity in Myofilaments With a Dilated Cardiomyopathy Mutant Tropomyosin E54K

Apart from transplant, there are no satisfactory therapies for the severe depression in contractility in familial dilated cardiomyopathy (DCM). Current heart failure treatments that act by increasing contractility involve signaling cascades that alter calcium homeostasis and induce arrhythmias. Omecamtiv mecarbil is a promising new inotropic agent developed for heart failure that may circumvent such limitations. Omecamtiv is a direct cardiac myosin activator that promotes and prolongs the strong myosin-actin binding conformation to increase the duration of systolic elastance. We tested the effect of omecamtiv on Ca(2+) sensitivity of myofilaments of a DCM mouse model containing a tropomyosin E54K mutation. We compared tension and ATPase activity of detergent-extracted myofilaments with and without treatment with 316 nM omecamtiv at varying pCa values. When transgenic myofilaments were treated with omecamtiv, the pCa50 for activation of tension increased from 5.70 ± 0.02 to 5.82 ± 0.02 and ATPase activity increased from 5.73 ± 0.06 to 6.07 ± 0.04. This significant leftward shift restored Ca(2+) sensitivity to levels no longer significantly different from controls. Proteomic studies lacked changes in sarcomeric protein phosphorylation. Our data demonstrate that omecamtiv can potentially augment cardiac contractility in DCM by increasing Ca(2+) sensitivity. The use of direct myosin activators addresses functional defects without incurring the adverse side effects of Ca(2+)-dependent treatments.

A single-chain derivative of the relaxin hormone is a functionally selective agonist of the G protein-coupled receptor, RXFP1

A single-chain derivative of the relaxin hormone ameliorates fibrosis without side-effects.

Human gene-2 relaxin (H2 relaxin) is a pleiotropic hormone with powerful vasodilatory and anti-fibrotic properties which has led to its clinical evaluation and provisional FDA approval as a treatment for acute heart failure. The diverse effects of H2 relaxin are mediated via its cognate G protein coupled-receptor (GPCR), Relaxin Family Peptide Receptor (RXFP1), leading to stimulation of a combination of cell signalling pathways that includes cyclic adenosine monophosphate (cAMP) and extracellular-signal-regulated kinases (ERK)1/2. However, its complex two-chain (A and B), disulfide-rich insulin-like structure is a limitation to its facile preparation, availability and affordability. Furthermore, its strong activation of cAMP signaling is likely responsible for reported detrimental tumor-promoting actions that may preclude long-term use of this drug for treating human disease. Here we report the design and synthesis of a H2 relaxin B-chain-only analogue, B7-33, which was shown to bind to RXFP1 and preferentially activate the pERK pathway over cAMP in cells that endogenously expressed RXFP1. Thus, B7-33 represents the first functionally selective agonist of the complex GPCR, RXFP1. Importantly, this small peptide agonist prevented or reversed organ fibrosis and dysfunction in three pre-clinical rodent models of heart or lung disease with similar potency to H2 relaxin. The molecular mechanism behind the strong anti-fibrotic actions of B7-33 involved its activation of RXFP1-angiotensin II type 2 receptor heterodimers that induced selective downstream signaling of pERK1/2 and the collagen-degrading enzyme, matrix metalloproteinase (MMP)-2. Furthermore, in contrast to H2 relaxin, B7-33 did not promote prostate tumor growth in vivo. Our results represent the first known example of the minimisation of a two-chain cyclic insulin-like peptide to a single-chain linear peptide that retains potent beneficial agonistic effects.

The myosin activator omecamtiv mecarbil: a promising new inotropic agent

Heart failure became a leading cause of mortality in the past few decades with a progressively increasing prevalence. Its current therapy is restricted largely to the suppression of the sympathetic activity and the renin-angiotensin system in combination with diuretics. This restrictive strategy is due to the potential long-term adverse effects of inotropic agents despite their effective influence on cardiac function when employed for short durations. Positive inotropes include inhibitors of the Na⁺/K⁺ pump, β-receptor agonists, and phosphodiesterase inhibitors. Theoretically, Ca²⁺ sensitizers may also increase cardiac contractility without resulting in Ca²⁺ overload; nevertheless, their mechanism of action is frequently complicated by other pleiotropic effects. Recently, a new positive inotropic agent, the myosin activator omecamtiv mecarbil, has been developed. Omecamtiv mecarbil binds directly to β-myosin heavy chain and enhances cardiac contractility by increasing the number of the active force-generating cross-bridges, presumably without major off-target effects. This review focuses on recent in vivo and in vitro results obtained with omecamtiv mecarbil, and discusses its mechanism of action at a molecular level. Based on clinical data, omecamtiv mecarbil is a promising new tool in the treatment of systolic heart failure.

Various Themes of Myosin Regulation

Members of the myosin superfamily are actin-based molecular motors that are indispensable for cellular homeostasis. The vast functional and structural diversity of myosins accounts for the variety and complexity of the underlying allosteric regulatory mechanisms that determine the activation or inhibition of myosin motor activity and enable precise timing and spatial aspects of myosin function at the cellular level. This review focuses on the molecular basis of posttranslational regulation of eukaryotic myosins from different classes across species by allosteric intrinsic and extrinsic effectors. First, we highlight the impact of heavy and light chain phosphorylation. Second, we outline intramolecular regulatory mechanisms such as autoinhibition and subsequent activation. Third, we discuss diverse extramolecular allosteric mechanisms ranging from actin-linked regulatory mechanisms to myosin:cargo interactions. At last, we briefly outline the allosteric regulation of myosins with synthetic compounds.

Omecamtiv mecarbil: a new cardiac myosin activator for the treatment of heart failure

INTRODUCTION

Current available inotropic agents increase cardiac contractility, but are associated with myocardial ischemia, arrhythmias, and mortality. A novel selective cardiac myosin activator, omecamtiv mecarbil (CK-1827452/ AMG-423) is a small molecule that activates the sarcomere proteins directly, resulting in prolonged systolic ejection time and increased cardiac contractility.

AREAS COVERED

This paper discusses the chemistry, pharmacokinetics, clinical efficacy and safety of omecamtiv mecarbil. Omecamtiv mecarbil represents a novel therapeutic approach to directly improve cardiac function and is therefore proposed as a potential new treatment of patients with systolic heart failure. The authors review results of previous studies investigating the effect of omecamtiv mecarbil in heart failure animal models, healthy volunteers, and patients with acute and chronic systolic heart failure.

EXPERT OPINION

Results of phase I and phase II studies demonstrate that omecamtiv mecarbil is safe and well tolerated both as an intravenous and oral formulation. In healthy volunteers and chronic systolic heart failure patients, administration of omecamtiv mecarbil resulted in a concentration-dependent increase of left ventricular ejection time, ejection fraction, fractional shortening, and stroke volume. The first results of a double-blind, randomized, placebo-controlled phase IIb dose-finding study with the oral formulation of omecamtiv mecarbil demonstrated beneficial effects on cardiac function and N-terminal pro-brain natriuretic peptide levels. This study will provide essential dosing information for the requisite phase III trials which will investigate whether the beneficial effects of omecamtiv mecarbil translate into improved clinical outcomes.

Use of Inotropic Agents in Treatment of Systolic Heart Failure

The most common use of inotropes is among hospitalized patients with acute decompensated heart failure, with reduced left ventricular ejection fraction and with signs of end-organ dysfunction in the setting of a low cardiac output. Inotropes can be used in patients with severe systolic heart failure awaiting heart transplant to maintain hemodynamic stability or as a bridge to decision. In cases where patients are unable to be weaned off inotropes, these agents can be used until a definite or escalated supportive therapy is planned, which can include coronary revascularization or mechanical circulatory support (intra-aortic balloon pump, extracorporeal membrane oxygenation, impella, left ventricular assist device, etc.). Use of inotropic drugs is associated with risks and adverse events. This review will discuss the use of the inotropes digoxin, dopamine, dobutamine, norepinephrine, milrinone, levosimendan, and omecamtiv mecarbil. Long-term inotropic therapy should be offered in selected patients. A detailed conversation with the patient and family shall be held, including a discussion on the risks and benefits of use of inotropes. Chronic heart failure patients awaiting heart transplants are candidates for intravenous inotropic support until the donor heart becomes available. This helps to maintain hemodynamic stability and keep the fluid status and pulmonary pressures optimized prior to the surgery. On the other hand, in patients with severe heart failure who are not candidates for advanced heart failure therapies, such as transplant and mechanical circulatory support, inotropic agents can be used for palliative therapy. Inotropes can help reduce frequency of hospitalizations and improve symptoms in these patients.

Fluorescence spectra of cardiac myosin and in vivo experiment: studies on daunorubicin-induced cardiotoxicity

OBJECTIVES

The objective of this study was to investigate the interaction of daunorubicin (DNR) and cardiac myosin (CM) and the changes in mice hearts to exhibit DNR-induced cardiotoxicity.

MATERIALS AND METHODS

The interaction between DNR and CM was expressed using fluorescence quenching at pH 4.0-9.0 and 15-37 °C. DNR-induced cardiotoxicity was studied using in vivo experiment. Forty groups mice were used control group in which mice were treated with DNR orally, and three DNR-treated groups in which mice were injected intraperitoneally with DNR at seven bolus doses of 2.0, 4.0, and 6.0 mg/kg body weight, respectively. Heart indices and myocardial enzyme levels were obtained by histopathological and biochemical analysis.

RESULTS

The fluorescence quenching mechanism of DNR-CM complex was observed to be a static procedure at 20 °C (pH 7.4), and weakly acidic environment (pH 4.0-6.0) or higher temperature (30-37 °C) promoted the interaction between DNR and CM, causing variations in conformation and normal physiological functions of CM. Thermodynamic studies demonstrated that the binding of DNR to CM was a spontaneous process driven by entropy. It also indicated that hydrophobic interaction and hydrogen bonds may play essential roles in the combination of DNR with CM. In addition, 4.0-6.0 mg/kg DNR-treated mice exhibited obvious histopathological lesion, increase in myocardial enzyme level, and reductions in blood cell count.

CONCLUSION

Our results are valuable for better understanding the particular mode of DNR-CM interaction, and are important to have a deeper insight into the DNR-induced cardiotoxicity.