Sensitive and specific quantification of sirolimus (rapamycin) and its metabolites in blood of kidney graft recipients by HPLC/electrospray-mass spectrometry

Sirolimus (rapamycin) has a macrolide structure and is under clinical investigation as an immunosuppressant after organ transplantation. An HPLC/mass spectrometry assay to quantify sirolimus in blood was developed. 28-O-Acetyl sirolimus was used as internal standard. Blood samples were extracted with C18 columns. The extracts were injected into an HPLC system and isocratically eluted with methanol/1% formic acid (90/10 by vol) from a 150 X 4 mm C18 analytical column. The HPLC system was connected to a triple-stage quadrupole mass spectrometer with an electrospray interface and positive ions were detected. The limit of quantification in 1 mL of blood was 0.25 microgram/L and the calibration curve in blood was linear up to 250 microgram/L. The recovery from blood was 88 +/- 26% and interassay variation at 1 microgram/L was 19% and at 15 microgram/L 9.3%. Hydroxy, dihydroxy, demethyl, and didemethyl sirolimus as well as sirolimus were detected in blood of kidney graft patients.