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 (cTnISP), releasing its inhibitory effect on the thin filament structure. The small molecule fingolimod inhibits cTnC–cTnISP interactions via electrostatic repulsion between its positively charged tail and positively charged residues in cTnISP 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–cTnISP 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.