CYCLOSPORINE INHIBITION OF CALCINEURIN ACTIVITY IN HUMAN-LEUKOCYTES IN-VIVO IS RAPIDLY REVERSIBLE

Despite increasing information about the mechanism of action of cyclos porine A (CsA), lithe is known about the way lymphocytes recover from CsA, Recovery is central to understanding the pharmacodynamics of CsA in vivo, We studied the recovery of calcineurin phosphatase (CN) activ ity in CsA-treated cells, Single dose kinetics in renal transplant pat ients showed that inhibition of CN activity in PBL increased and fell concomitant with CsA blood levels, In vitro, control PBL treated with CsA 100 mu g/l, washed, and resuspended in CsA-free medium showed litt Ie recovery (0-20%) after 24 h, Erythrocytes or anti-CsA Ab added to t he recovery medium increased recovery to 50% within 4 h, Similar recov ery was seen in the ability of cells to produce IFN-gamma after OKT3 s timulation, Recovery of CN activity was associated with the efflux of [H-3]CsA, was not blocked by cycloheximide and was temperature sensiti ve, A cell line with high expression of surface P glycoprotein (PGP), showed rapid recovery, However, PGP blockade did not prevent recovery in PBL, indicating a different PGP-independent mechanism. In PBL, reco very from CsA is slow and limited in vitro, but rapid in vivo, where C sA equilibrates among a complex set of extralymphocytic binding sites.