Tacrolimus (FK506) is a 23-membered macrolide lactone, it reduces peptidyl-prolyl isomerase activity in T cells by binding to the immunophilin FKBP12 (FK506 binding protein) creating a new complex.
FK-506 and cyclosporin A block translocation of the cytoplasmic component without affecting synthesis of the nuclear subunit in T lymphocytes.  FK-506 prevents T-cell proliferation by inhibiting a Ca(2+)-dependent event required for induction of interleukin-2 transcription.  FK 506 binds to distinct families of intracellular proteins (immunophilins) termed cyclophilins and FK 506-binding proteins (FKBPs). FK-506 specifically inhibits cellular calcineurin at drug concentrations that inhibit interleukin 2 production in activated T cells.  FK-506 and CsA exert nearly identical biological effects in cells by inhibiting the same subset of early calcium-associated events involved in lymphokine expression, apoptosis, and degranulation. FK-506 binds to a family of intracellular receptors termed the FK-506 binding proteins (FKBPs). FK-506 results in increase in the paw and tail withdrawal threshold as revealed by behavioral pain assessment in rats against hyperalgesic and allodynic stimuli. FK-506 also leads to a decrease in the serum nitrate and thiobarbituric acid reactive substance (TBARS) levels along with reduction in tissue myeloperoxidase (MPO) and total calcium levels, whereas, rise in tissue reduced glutathione levels in rats. FK-506 ameliorates the increase in the neuronal edema and axonal degeneration in rats with ischemia reperfusion (I/R).