A liquid chromatography-tandemmass spectrometry method for the simultaneous determination of exemestane and its metabolite 17-dihydroexemestane in human plasma

A simple and sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) method has been developed and validated for the quantitation of exemestane (Exe) and its main metabolite 17-dihydroexemestane (DhExe) in human plasma. The analytes were extracted by protein precipitation with acetonitrile, containing stable ¹³C-labelled Exe ( ¹³C ₃-Exe) as internal standard, and measured by LC-MS/MS. The best chromatographic separationof the analytes from the interferences was achieved by using a Phenyl column operating under isocratic regime conditions. The total chromatographic runtime was 5.0 min and the elution of Exe and DhExe occurred at 2.5 min and 2.9 min, respectively. Quantitation was performed by employing the positive electrospray ionization (ESI) technique and multiple reaction monitoring mode (MRM). The monitored precursor to product-ion transitions for Exe, DhExe and ¹³C ₃-Exe internal standard were m/z 297.0 → 120.8, m/z 299.1 → 134.9 and m/z 300.0 → 123.2, respectively. The lower limit of quantitation (LLOQ) was 0.1 ng/ml for DhExe and 0.2 ng/ml for Exe. The method was linear up to 36-51 ng/ml with r ² ≥ 0.998. The intra- and inter-assay precision were ≤7.7% and 5.1% for Exe and ≤8.1 and 4.9% for DhExe while deviations from nominal values were in the 1.5-13.2% and -9.0-5.8% ranges for Exe and DhExe, respectively. The analytical method resulted robust and suitable for pharmacokinetic monitoring of Exe and its main metabolite during adjuvant therapy in patients with breast cancer.