-Verticilide B1 inhibits type 2 ryanodine receptor channels and is antiarrhythmic in Casq2-/- mice.

Unlabelled: Ca leak from cardiac ryanodine receptor (RyR2) is an established mechanism of sudden cardiac death (SCD), whereby dysregulated Ca handling causes ventricular arrhythmias. We previously discovered the RyR2-selective inhibitor (+)-verticilide ( -1), a 24-membered cyclooligomeric depsipeptide that is the enantiomeric form of a natural product ( -(-)-verticilide). Here, we examined its 18-membered ring-size oligomer ( -verticilide B1; " -B1") in single RyR2 channel assays, [ H]ryanodine binding assays, and in cardiomyocytes and mice, a gene-targeted model of SCD. -B1 inhibited RyR2 single-channels and [ H]ryanodine binding with low micromolar potency, and RyR2-mediated spontaneous Ca release in Casq2-/- cardiomyocytes with sub-micromolar potency. -B1 was a partial RyR2 inhibitor, with maximal inhibitory efficacy of less than 50%. -B1 was stable in plasma, with a peak plasma concentration of 1460 ng/ml at 10 min and half-life of 45 min after intraperitoneal administration of 3 mg/kg in mice. Both 3 mg/kg and 30 mg/kg -B1 significantly reduced catecholamine-induced ventricular arrhythmia in Casq2-/- mice. Hence, we have identified a novel chemical entity - -B1 - that preserves the mechanism of action of a hit compound and shows therapeutic efficacy. These findings strengthen RyR2 as an antiarrhythmic drug target and highlight the potential of investigating the mirror-image isomers of natural products to discover new therapeutics.

Significance Statement: The cardiac ryanodine receptor (RyR2) is an untapped target in the stagnant field of antiarrhythmic drug development. We have confirmed RyR2 as an antiarrhythmic target in a mouse model of sudden cardiac death and shown the therapeutic efficacy of a second enantiomeric natural product.