Transient Receptor Potential Melastatin 7 (TRPM7) Ion Channel Inhibitors: Preliminary SAR and Conformational Studies of Xenicane Diterpenoids from the Hawaiian Soft Coral .

Waixenicin A, a xenicane diterpene from the octocoral , is a selective, potent inhibitor of the TRPM7 ion channel. To study the structure-activity relationship (SAR) of waixenicin A, we isolated and assayed related diterpenes from . In addition to known waixenicins A () and B (), we purified six xenicane diterpenes, 7,8-epoxywaixenicins A () and B (), 12-deacetylwaixenicin A (), waixenicin E (), waixenicin F (), and 20-acetoxyxeniafaraunol B (). We elucidated the structures of - by NMR and MS analyses. Compounds , , , , and inhibited TRPM7 activity in a cell-based assay, while , , and were inactive. A preliminary SAR emerged showing that alterations to the nine-membered ring of did not reduce activity, while the 12-acetoxy group, in combination with the dihydropyran, appears to be necessary for TRPM7 inhibition. The bioactive compounds are proposed to be latent electrophiles by formation of a conjugated oxocarbenium ion intermediate. Whole-cell patch-clamp experiments demonstrated that waixenicin A inhibition is irreversible, consistent with a covalent inhibitor, and showed nanomolar potency for waixenicin B (). Conformational analysis (DFT) of , , , and revealed insights into the conformation of waixenicin A and congeners and provided information regarding the stabilization of the proposed pharmacophore.