Development and validation of a sensitive liquid chromatography/tandem mass spectrometry method for the determination of exemestane in human plasma

Exemestane, irreversible steroidal aromatase inhibitor, acts as a false substrate for aromatase enzyme and significantly lowers circulating estrogen concentrations in postmenopausal women with hormone-sensitive breast cancer. A sensitive bioanalytical method was developed and validated to study pharmacokinetics of exemestane. The method was based on liquid–liquid extraction of exemestane with methyl t-butyl ether followed by reversed-phase liquid chromatography. Positive electrospray ionization tandem mass spectrometry in multiple reaction monitoring mode was applied for detection of exemestane. Anastrozole was used as internal standard. Calibration curve, fitted to 1/x2 weighted linear regression model, was linear in the range of 0.1–40.0 ng/mL. Intra-run precision and accuracy were 1.80–3.17% and 103.4–111.5%, respectively. Inter-run precision and accuracy measured within 3 days were 3.37–4.19% and 101.8–109.6%, respectively. Extraction recoveries of exemestane and internal standard were 79.7–86.2% and 82.9–83.6%, respectively. The method was fully validated and may be applied to pharmacokinetic studies in humans after a single dose administration of 25 mg exemestane tablets.