Measurement of whole blood sirolimus by an HPLC assay using solid-phase extraction and UV detection

The authors developed an HPLC assay for determining blood sirolimus concentration using a relatively simple solid-phase extraction and UV detection. The retention times of sirolimus and the internal standard, 32-desmethoxyrapamycin, are 8.7 and 9.3 minutes, respectively. The assay possesses linearity up to 200 ng/mL, sensitivity to 2.0 ng/mL, and day-to-day reproducibility of 8.8, 9.8, 6.1, and 6.4% at sirolimus concentrations of 6, 10, 20, and 30 ng/mL, respectively. A patient correlation study using this HPLC method and an established LC/MS/MS assay revealed a slope of 0.982 and intercept of -0.021 ng/mL and a correlation coefficient of 0.99 (n = 37). Of the 31 different drugs tested none interfered with the measurement of the drug of interest, and a recovery study gave an overall mean recovery of 101.8%. The authors conclude that the method described here is suited for the therapeutic monitoring of blood sirolimus concentration.