Effects of mavacamten on Ca²⁺ sensitivity of contraction as sarcomere length varied in human myocardium

Abstract

Background and Purpose: Heart failure can reflect impaired contractile function at the myofilament level. In healthy hearts, myofilaments become more sensitive to Ca 2+ as cells are stretched. This represents a fundamental property of the myocardium that contributes to the Frank-Starling response, although the molecular mechanisms underlying the effect remain unclear. Mavacamten, which binds to myosin, is under investigation as a potential therapy for heart disease. We investigated how mavacamten affects the sarcomere-length dependence of Ca 2+-sensitive isometric contraction to determine how mavacamten might modulate the Frank-Starling mechanism. Experimental Approach: Multicellular preparations from the left ventricular-free wall of hearts from organ donors were chemically permeabilized and Ca 2+ activated in the presence or absence of 0.5-μM mavacamten at 1.9 or 2.3-μm sarcomere length (37 C). Isometric force and frequency-dependent viscoelastic myocardial stiffness measurements were made. Key Results: At both sarcomere lengths, mavacamten reduced maximal force and Ca 2+ sensitivity of contraction. In the presence and absence of mavacamten, Ca 2+ sensitivity of force increased as sarcomere length increased. This suggests that the length-dependent activation response was maintained in human myocardium, even though mavacamten reduced Ca 2+ sensitivity. There were subtle effects of mavacamten reducing force values under relaxed conditions (pCa 8.0), as well as slowing myosin cross-bridge recruitment and speeding cross-bridge detachment under maximally activated conditions (pCa 4.5). Conclusion and Implications: Mavacamten did not eliminate sarcomere lengthdependent increases in the Ca 2+ sensitivity of contraction in myocardial strips from organ donors at physiological temperature. Drugs that modulate myofilament function may be useful therapies for cardiomyopathies. K E Y W O R D S cardiac muscle mechanics, human myosin, mavacamten, sarcomere length Abbreviations: (Pi), inorganic phosphate; (pCa), −log 10 [Ca 2+ ]; (F pas), passive stress under relaxed conditions; (F act), maximal Ca 2+-activated stress; (pCa 50), free Ca 2+ concentration required to develop half the maximum Ca 2+-activated stress; (nH), Hill coefficient.

Mavacamten has a differential impact on force generation in myofibrils from rabbit psoas and human cardiac muscle

“…This result supports the conclusion of Tanner and co-workers ( Awinda et al, 2020 ) who attributed the MAVA-dependent decrease of passive force of skinned human ventricular strips at 37°C to the decrease of a myosin-based contribution to thin-filament activation. MAVA, instead, did not affect the passive properties of skeletal and cardiac myofilaments in a wide range of sarcomere lengths, confirming previous observations about the lack of effect of 2,3-butanedione monoxime (BDM) on the sarcomere length–resting tension relation of isolated myofibrils ( Scellini et al, 2017 ).…”

“…Present results represent a significant advancement of knowledge on the impact of MAVA on human cardiac sarcomeres. What is known about MAVA effects in human myocardium is not much and has been obtained in freely shortening isolated paced iPSC-CMs compared with isogenic WT iPSC-CMs ( Toepfer et al, 2020 ) or skinned ventricular strips ( Anderson et al, 2018 ; Awinda et al, 2020 ). In these latter studies, the inhibitory action of MAVA on force was estimated at only one concentration of the drug (very high in the former study [50 µM] and approximately the IC 50 in the latter one [0.5 µM]).…”

“…This indicates that in the usual conditions of myofibril experiments (15°C, 5 mM ATP, presence of ATP regenerating system and no ADP), Ca 2+ -independent force generation is not significant, at variance with what may occur in the intact tissue. In multicellular cardiac preparations at physiological temperature and [Mg 2+ ], a small but significant degree of Ca 2+ -independent tension is present ( Sequeira et al, 2015 ) that may account for the reported inhibitory effect of MAVA on cardiac passive properties at 37°C ( Awinda et al, 2020 ). This effect of the drug may be able to counteract the basal sarcomere activation in the virtual absence of Ca 2+ that, together with the increase in myofilament Ca 2+ sensitivity, often contributes to the diastolic dysfunction described in human HCM ( Sequeira et al, 2015 ; Tardiff et al, 2015 ; Ferrantini et al, 2017 ).…”

“…When tested on myocardial function, MAVA has been proved to similarly reduce in vitro contractility in control and HCM animals ( Green et al, 2016 ; Anderson et al, 2018 ; Mamidi et al, 2018 ) as well as relieve left ventricular outflow tract obstruction in HCM feline hearts ( Stern et al, 2016 ) and suppress the development of the hypertrophic phenotype in HCM mouse models. Recently, it has been shown that MAVA decreases the Ca 2+ sensitivity in both human and mouse ventricle ( Awinda et al, 2020 ; Awinda et al, 2021 ) and rescues the increase in Ca 2+ sensitivity caused by thin-filament HCM mutations in mouse models ( Sparrow et al, 2019 ). These observations, indicating a window of opportunities in which restoration of physiological sarcomere performance may prevent cardiac remodeling, are the basis for the present use of MAVA in preclinical/clinical trials for HCM treatment, as confirmed by the very promising results of a phase 2 open-label trials ( Heitner et al, 2019 ) and a recently completed multicenter placebo-controlled randomized phase 3 trial (EXPLORER-HCM; Olivotto et al, 2020 ).…”

“…Despite the advancing of clinical studies, very little is known of MAVA action on the mechanics of contraction of fast skeletal and slow/cardiac muscles. The study of MAVA effects in human cardiac tissue has only very recently started to go beyond a very preliminary stage with direct measurements of the drug action at physiological temperature (37°C) in control permeabilized human left ventricular strips at different Ca 2+ activation levels and sarcomere lengths ( Awinda et al, 2020 ). Results of this work showed that low doses of MAVA decrease maximal force development and work production as well as Ca 2+ sensitivity of human myocardium but preserve the length-dependent activation mechanism.…”