The new immunosuppressant, isogarcinol, binds directly to its target enzyme calcineurin, unlike cyclosporin A and tacrolimus

• Enzyme kinetic analysis showed that inhibition of calcineurin by isogarcinol was competitive.
• Isothermal titration calorimetry, fluorescence spectroscopy and molecular docking indicated that isogarcinol bound directly to calcineurin in vitro.
• In summary, isogarcinol binds directly to calcineurin in vitro, unlike the classical calcineurin inhibitors cyclosporin A and tacrolimus.

Isogarcinol, a bioactive polyisoprenylated benzophenone derivative isolated from Garcinia mangostana L., has been shown previously to exert a strong inhibitory effect on calcineurin and is thus a potential oral, low-toxicity immunomodulatory drug. In the present study, enzyme kinetic analysis showed that inhibition of calcineurin by isogarcinol was competitive. Fluorescence spectroscopy indicated that isogarcinol bound to calcineurin. Isothermal titration calorimetry showed that binding was mainly driven by enthalpy, and was exothermic because the enthalpy change exceeded the entropy reduction. The interaction force is either hydrogen bonding or Van der Waals forces. Fluorescence resonance energy transfer and molecular docking experiments indicated that there were two potential binding sites for isogarcinol in the catalytic domain of calcineurin. In summary, isogarcinol binds directly to calcineurin in vitro, unlike the classical calcineurin inhibitors cyclosporin A and tacrolimus.