Myosin Modulator Aficamten Inhibits Force by Altering Myosin's Biochemical Activity Without Changing Thick Filament Structure.

We investigated the effects of aficamten on cardiac muscle structure, biochemical activity, and contractile function. Aficamten does not structurally sequester myosin heads along the thick filament. It inhibits ATPase activity by decreasing myosin ATPase cycling kinetics, with the emergence of a super slow biochemical nucleotide turnover. This results in decreased force and calcium sensitivity without altering cross-bridge cycling. Our myofibril mechanical assay showed inhibition of force with accelerated relaxation. In engineered heart tissues, while mavacamten and aficamten inhibit cardiac twitch forces, mavacamten reduces the activation kinetics while both accelerate relaxation.