Time resolved depression of isometric force by Mavacamten in single myofibrils from rabbit psoas and human cardiac muscle

Mavacamtem (MYK-641, Axon Medchem BV) is a promising small molecule designed to act as allosteric inhibitor of sarcomeric myosins and presently used in preclinical/clinical trials for HCM treatment (Anderson et al., 2018)). Studies of the effects of ligands on the force generation mechanism in intact or skinned striated muscle fibres are complicated by diffusional barriers. These limitations can be overcome by the use of single myofibrils submitted to perturbations of the contractile environment by sudden solution changes (Tesi et al., 2000). Here, single myofibrils or thin bundles of myofibrils from rabbit fast skeletal muscle (psoas) and human donor ventricle (frozen biopsies) have been used to study the effects of μmolar Mavacamtem on maximal isometric force. Both myofibril types were mounted in relaxing solution (pCa 9; [Pi] ~ 200 μM, 15 °C,) and then fully activated (pCa 3.5) by fast alternation of perfusing fluxes. Myofibrils were then suddenly moved to and from a second flux of activating solution containing selected concentrations of Mavacamtem (“jumps”; solution change ~ 10 ms). Relaxation of force was achieved by returning myofibrils to the relaxing solution. When submitted to Mavacamten jumps, both myofibril types responded with a rapid, relaxation-like force drop. The effect was fully reversible but with significantly slower kinetics than that of force development. Dose–response curves confirmed a higher sensitivity of cardiac muscle (IC50 ~ 0.5 μM) compared to fast skeletal (IC50 ~ 5 μM), as previously reported for pCa50-activated ATPase of the same myofibrillar systems (Kawas et al., 2017). Interestingly, Mavacamtem also decreased the rate of force development, in agreement with the reported inhibition of Pi release rate but also with a possible effect on the regulation state mediated by the availability of strongly actin binding heads. SilicoFCM grant agreement n. 777204.