RECOVERY OF BLOOD MONONUCLEAR CELL CALCINEURIN ACTIVITY SEGREGATES 2 POPULATIONS OF RENAL-TRANSPLANT PATIENTS WITH DIFFERENT SENSITIVITIES TO CYCLOSPORINE INHIBITION

In vitro studies have shown that the immunosuppressive property of cyc losporine (CsA) depends on its ability to inhibit the phosphatase acti vity of calcineurin, a critical enzyme for T cell activation. Here we sought to investigate whether measurement of calcineurin activity in p eripheral blood mononuclear cells (PBMC) from 30 renal transplant pati ents given CsA as a part of their immunosuppressive regimen would help in optimizing CsA therapy. We first documented that in PBMC from thes e patients complete inhibition of calcineurin phosphatase activity by in vitro addition of CsA occurs at concentrations that are easily achi eved in vivo for a dose as low as 3 mg/kg/day orally, which correspond s to trough CsA blood levels of 100-150 ng/ml. However, ex vivo, at a blood CsA trough level of 250 ng/ml, calcineurin activity in PBMC was only inhibited from 40% to 70% as compared with controls. Patients on higher doses of CsA had a further inhibition of baseline calcineurin a ctivity, although a complete suppression was never reached. A signific ant correlation was found between trough CsA concentration and the bas al calcineurin activity (r=0.48; P=0.0085). To clarify the relationshi p between the daily exposure of patients to CsA and changes in the enz yme activity of calcineurin, we then correlated the pharmacokinetic pr ofile of CsA in these patients with different CsA dosing (<4, 4-6, >6- 8, >8 mg/kg/day) with the profile of calcineurin activity at different intervals from dosing. Each of the above CsA doses suddenly reduced c alcineurin activity, with a nadir at 2 hr after maximum blood concentr ation. The degree of the inhibition was not a function of peak CsA blo od levels. In all patients, CsA blood level returned to basal values 1 0 hr after dosing. By contrast, only in 50-70% of patients (depending on the dose) did calcineurin activity return to baseline at the same t ime point after dosing. In summary we have shown that (1) inhibition o f calcineurin activity measured ex vivo in PBMC taken from CsA-treated transplanted recipients reflects the blood CsA trough level; (2) afte r CsA the time-course of inhibition of enzyme activity is relatively i ndependent from CsA pharmacokinetics; (3) the rate of recovery of calc ineurin activity 10 hr after CsA dosing segregates two populations of transplanted recipients -- one with complete recovery of the enzyme ac tivity and another that never returns to the baseline calcineurin leve l.