Development of a rapid LC-MS/MS assay to determine letrozole in human plasma and its application in a pharmacokinetic study after oral administration

Abstract

Abstract Letrozole is one of the third generation aromatase inhibitors. It is suitable for the treatment of postmenopausal patients with advanced breast cancer and early treatment of breast cancer. It is necessary to develop a rapid, reliable, selective and sensitive LC–MS/MS assay to determine letrozole in human plasma to evaluate the clinical efficacy and adverse reactions with clinical pharmacokinetic and therapeutic drug monitoring. Separation was carried out on a Kromasil-C18 column using acetonitrile-water (55: 45, v/v) as mobile phase. Detection was carried out by multiple reaction monitoring on a 3200Qtrap mass spectrometry. The method needed one-step protein precipitation procedure only, and the cycle time was 2.5 min allowing 500–550 samples per day. It was linear within 0.30–50.00 ng/mL for plasma with the limit of detection (LOD) of 0.030 ng/mL. The intra- and inter-day RSD were 5.51–8.63%, 2.28–9.95% and the RE was 0.18–1.65%. The recovery rates of letrozole and internal standard for plasma were 89.30–98.55%. Letrozole was stable under all the conditions in the study. The method was sensitive enough to quantitate letrozole over a period of 288 h after giving a single oral dose of 2.5 mg–24 Chinese healthy volunteers. The absorption of letrozole was rapid with small individual difference, the tissue distribution of letrozole was more than that in blood, and the clearance was slow. Letrozole was similar to three-compartment model in vivo. Due to metabolism and excretion, the AUCs of letrozole varied greatly among individuals.