Synthesis and Biophysical Characterization of Fingolimod Derivatives as Cardiac Troponin Antagonists

Calcium binding to cardiac troponin C (cTnC) in the thin filaments acts as a trigger for cardiac muscle contraction. The N-lobe of cTnC (NcTnC) undergoes a conformational change in the presence of calcium that allows for interaction with the switch region of cardiac troponin I (cTnI_SP), releasing its inhibitory effect on the thin filament structure. The small molecule fingolimod inhibits cTnC-cTnI_SP interactions via electrostatic repulsion between its positively charged tail and positively charged residues in cTnI_SP and acts as a calcium desensitizer of the contractile myofilaments. Here we investigate the structure-activity relationship of the fingolimod hydrophobic headgroup and show that increasing the alkyl chain length increases both its affinity for NcTnC and its inhibitory effect on the NcTnC-cTnI_SP interaction and that decreasing flexibility completely abolishes these effects. Strikingly, the longer derivatives have no effect on the calcium affinity of cTnC, suggesting that they act as better inhibitors.