Mycalolide-B is a marine sponge-derived compound that functions as a selective inhibitor of actomyosin ATPase. It effectively hinders ATP-induced contraction and Mg2+-ATPase activity in the absence of Ca2+.

Mycalolide-B, a macrolide antibiotic derived from a marine sponge, presents a potent cytotoxicity against L1210 murine leukemia cells with an IC 50 value ranging from 10-50 nM, attributed to its tris-oxazole structure. This compound effectively suppresses smooth muscle contractions and Ca2+-induced contractions in permeabilized smooth muscles without altering cytosolic Ca2+ levels or myosin light chain phosphorylation. Additionally, Mycalolide-B impedes superprecipitation and Mg2+-ATPase activity stimulated by Ca2+, alongside inhibiting ATP-induced contraction and Mg2+-ATPase activity in the absence of Ca2+[1].