ABSTRACT
Therapy with cyclosporin A (CsA) for immunosuppression after organ transplantation requires monitoring of its levels in blood owing to the narrow therapeutic index of the drug and to the high inter-individual variability of the drug absorption and metabolism. We describe the preparation of CsA labelled with stable isotopes ((13)C and (2)H) with an isotopic enrichment of about 99% using labelled glucose and its use as internal standard for quantification of CsA blood levels by isotope dilution/electrospray ionization mass spectrometry. The method was found to be linear in the tested range (1-1000 ng) with and without the matrix. The accuracy of the bracketting calibration curves prepared using 100 ng ml(-1) labelled CsA was within +/-1.7% (bias). The results confirmed the usefulness of the procedure as a reference method for the external quality assessment of the field methods for the evaluation of CsA blood concentration, the imprecision (relative standard deviation) and accuracy (bias) being <2%.