Stability of tacrolimus (FK 506) and cyclosporin G in whole blood

The stability of two new immunosuppressants, tacrolimus and cyclosporin G (CsG), was evaluated in whole blood following incubation of patient specimens at ambient temperature or 4 degrees C for time periods of 2 (48 h) and 7 days (168 h) after collection. No decrease in CsG concentrations was noted over a 7-day period for specimens stored at either ambient temperature or 4 degrees C. For tacrolimus, the concentrations of drug in whole blood stored at ambient temperature for 7 days had a slight downward trend. At 7 days, the ratio of tacrolimus concentrations to day zero concentrations ranged from 0.86 to 1.15, with a mean decrease of 5%. However, these changes are within the published precision characteristics of the tacrolimus immunoassay. Evaluation of a smaller group of whole blood specimens provided evidence of adequate stability of these two drugs over a 13-day period.