Mavacamten preserves length-dependent contractility and improves diastolic function in human engineered heart tissue

Abstract

Comprehensive functional characterization of cardiac tissue includes investigation of length and load dependence. Such measurements have been slow to develop in engineered heart tissues (EHTs), whose mechanical characterizations have been limited primarily to isometric and near-isometric behaviors. A more realistic assessment of myocardial function would include force-velocity curves to characterize power output and force-length loops mimicking the cardiac cycle to characterize work output. We developed a system that produces force-velocity curves and work loops in human EHTs using an adaptive iterative control scheme. We used human EHTs in this system to perform a detailed characterization of the cardiac β-myosin specific inhibitor, mavacamten. Consistent with the clinically proposed application of this drug to treat hypertrophic cardiomyopathy, our data support the premise that mavacamten improves diastolic function through reduction of diastolic stiffness and isometric relaxation time. Meanwhile, the effects of mavacamten on length- and load-dependent muscle performance were mixed. The drug attenuated the length-dependent response at small stretch values but showed normal length dependency at longer lengths. Peak power output of mavacamten-treated EHTs showed reduced power output as expected but also shifted peak power output to a lower load. Here, we demonstrate a robust method for the generation of isotonic contraction series and work loops in engineered heart tissues using an adaptive-iterative method. This approach reveals new features of mavacamten pharmacology, including previously unappreciated effects on intrinsic myosin dynamics and preservation of Frank-Starling behavior at longer muscle lengths.NEW & NOTEWORTHY We applied innovative methods to comprehensively characterize the length and load-dependent behaviors of engineered human cardiac muscle when treated with the cardiac β-myosin specific inhibitor mavacamten, a drug on the verge of clinical implementation for hypertrophic cardiomyopathy. We find mechanistic support for the role of mavacamten in improving diastolic function of cardiac tissue and note novel effects on work and power.

Keywords: engineered heart tissues; hypertrophic cardiomyopathy; mavacamten; power.

Figure 1.

Engineered heart tissue methods and mechanical testing setup. A: methods for fabrication engineered heart tissue. B: mechanical testing setup. C: workflow for iterative control method. ADC, analog-to-digital converter.

Figure 2.

Iterative control method workflow and characteristics, example force-velocity/power curves, and work-preload-afterload curves. A: iterative algorithm typically converged within 4 iterations to desired isotone. B: sequence for final measurement collecting showing beat-to-beat isometric and isotonic twitch, depending on length control command once algorithm converged. C: representative isotonic twitches and derived load-velocity and load-power curves.

Figure 3.

Isometric characteristics of baseline and mavacamten-treated engineered heart tissues (EHTs). A: representative isometric twitch of an EHT at 10% stretch under 1-Hz stimulus under baseline conditions, with 0.33 µM mavacamten treatment, and with 0.5 µM mavacamten treatment. B: time course of peak force (PF) and normalized tension-time integral (nTTI) as a function of mavacamten infusion time, demonstrating that steady state in isometric twitch is reached within 30 min. The exponential decay constant of PF and nTTI were identical (0.0016 ms⁻¹). C: PF was reduced on average by 40% with 0.33 µM mavacamten treatment (n = 5, paired t test, P = 0.0009) and by 85% with 0.50 µM mavacamten treatment (n = 7, paired t test, P = 0.0075). D: nTTI was reduced on average by 14% with 0.33 µM mavacamten (n = 5, paired t test, P = 0.0008) and by 30% with 0.5 µM mavacamten treatment (n = 7, paired t test, 0.0009). E: TTP (time to peak force) was reduced on average by 43 ms by 0.5 µM mavacamten treatment (n = 7, paired t test, P = 0.0021) and not affected by 0.33 µM mavacamten treatment (n = 5, paired t test). F: RT50 (time from peak to 50% relaxation) was reduced on average by 24% with 0.33 µM mavacamten treatment (n = 5, paired t test, P = 0.0002) and by 45% with 0.5 µM mavacamten treatment (n = 7, paired t test, P = 0.0003). **P < 0.01, ***P < 0.001.

Figure 4.

Force-length characteristics of baseline and mavacamten-treated engineered heart tissues (EHTs). A: representative trace of full-force recording of an EHT undergoing constant slow linear stretch from 0.95 to 1.10 stretch while being stimulated at 1 Hz. B: representative diastolic force-length trace of an EHT from 0.96 to 1.10 stretch ratio at 1-Hz stimulus with and without 0.33 µM mavacamten acute treatment. C: representative diastolic force-length trace of an EHT from 0.95 to 1.10 stretch ratio at 1-Hz stimulus with and without 0.5 µM mavacamten acute treatment. D: 0.33 µM mavacamten significantly altered the effect of stretch on diastolic force (n = 5, 2-way ANOVA, repeated measures by stretch and by drug treatment, interaction P < 0.0001). E: 0.50 µM mavacamten significantly altered the effect of stretch on diastolic force (n = 7, 2-way ANOVA, repeated measures by stretch and by drug treatment, interaction P = 0.0010). F: representative active force-length trace of an EHT from 0.96 to 1.10 stretch ratio at 1-Hz stimulus with and without 0.33 µM mavacamten acute treatment. G: representative active force-length trace of an EHT from 0.95 to 1.10 stretch ratio at 1-Hz stimulus with and without 0.50 µM mavacamten acute treatment. H: 0.33 µM mavacamten significantly altered the effect of stretch on the Frank-Starling active force relationship (n = 5, 2-way ANOVA, repeated measures by stretch and by drug treatment, interaction P < 0.0001). I: 0.50 µM mavacamten significantly altered the effect of stretch on the Frank-Starling active force relationship (n = 7, 2-way ANOVA, repeated measures by stretch and by drug treatment, interaction P < 0.0001). J: mavacamten at 0.50 µM reduced the slope of the Frank-Starling active force relation at lower stretch (n = 7, paired t test, P < 0.0001 at 0.96 stretch ratio). *P < 0.05, ***P < 0.001, and ****P < 0.0001.

Figure 5.

Load-velocity and power characteristics of baseline and mavacamten treated engineered heart tissues (EHTs). A: representative isotonic twitches of an EHT at 1 Hz and 10% stretch with and without 0.5 µM mavacamten acute treatment. Note that the force records have been normalized to the peak isometric force under the respective conditions. Shading of each force response matches its corresponding length transient shown on the axis immediately below. B: representative shortening velocity and power versus afterload measurements of an EHT at 1 Hz and 10% stretch with and without 0.5 µM mavacamten acute treatment. C: mavacamten reduced maximum shortening velocity (V_max) by 1.9 mm/s in EHTs (n = 7, paired t test, P = 0.0094). D: mavacamten reduced peak normalized power output (P_max) of EHTs by 0.47 mm/s (n = 7, paired t test, P = 0.0077). E: mavacamten reduced optimal afterload (F_opt) by ∼4% in EHTs (n = 7, paired t test, P = 0.0066). **P < 0.01.

Figure 6.

Work loop characteristics of baseline and mavacamten-treated engineered heart tissues (EHTs). A: representative work loops at a preload of 6.6 mm in an EHT before and after treatment with 0.5 µM acute mavacamten. B: mavacamten modifies the dependence of work on afterload in EHTs (n = 7, 2-way ANOVA, repeated measures by stretch and by drug treatment, interaction P = 0.0032). C: representative work loops of an EHT at different preloads before and after 0.5 µM acute mavacamten treatment. D: representative linear fits and measurements of end-systolic force and corresponding end-systolic length of an EHT before and after mavacamten treatment. E: mavacamten decreases the slope of the end-systolic force-length relation in EHTs (n = 6, paired t test, P = 0.0082). **P < 0.01.