Probing the mechanism of cardiovascular drugs using a covalent levosimendan analog.

One approach to improve contraction in the failing heart is the administration of calcium (Ca(2+)) sensitizers. Although it is known that levosimendan and other sensitizers bind to troponin C (cTnC), their in vivo mechanism is not fully understood. Based on levosimendan, we designed a covalent Ca(2+) sensitizer (i9) that targets C84 of cTnC and exchanged this complex into cardiac muscle. The NMR structure of the covalent complex showed that i9 binds deep in the hydrophobic pocket of cTnC. Despite slightly reducing troponin I affinity, i9 enhanced the Ca(2+) sensitivity of cardiac muscle. We conclude that i9 enhances Ca(2+) sensitivity by stabilizing the open conformation of cTnC. These findings provide new insights into the in vivo mechanism of Ca(2+) sensitization and demonstrate that directly targeting cTnC has significant potential in cardiovascular therapy.