Calmidazolium chloride (R 24571) is a calmodulin (CaMK) antagonist, antagonizing CaM-dependent phosphodiesterase and calmodulin-induced activation of erythrocyte Ca2+-transporting ATPase with IC50s of 0.15 and 0.35 μM, respectively. Also in anti-cancer research. Calmidazolium binds to CaMK with a Kd of 3 nM.