Stability of cyclosporin A in human serum

Cyclosporin A (CyA), a new immunosuppressive drug, is used for the prevention of organ graft rejection in man. The main side effect of CyA is nephrotoxicity which is usually reversible when the drug is stopped. Nephrotoxicity may be avoided in patients by careful monitoring of serum CyA concentration by radioimmunoassay, in addition to the routine biochemical tests of renal function. There is a good correlation between trough (12 hour) serum concentrations of CyA and plasma creatinine. Cyclosporin A was stable in serum at room temperature for up to seven days and on storage at -20 degrees C for up to five months. Delayed separation of blood stored at room temperature led to increased serum CyA concentrations while blood stored at 4 degrees C before separation was much more stable. Blood samples should be separated as quickly as possible to avoid the release of CyA from the cellular fraction into the serum. Haemolysed samples should be avoided. CyA is sufficiently stable at room temperature for serum samples to be sent in the first class post to a control laboratory for CyA assay, after patients are discharged from the transplant centre. Control and patient serum samples should be stored for a maximum of five months at -20 degrees C prior to assay for CyA.