Abstract P5-11-09: A Validated Analytical Method for Monitoring the Plasma Levels of Tamoxifen, Anastrozole and Letrozole in Patients Undergoing Endocrine Therapy
Authors: Schubert B , Beer B , Oberguggenberger A , Meraner V , Holzner B , Hubalek M , Oberacher H. 
Published: April 2011

Abstract
Background: Clinical data have repeatedly shown that tamoxifen as well as anastrazole and letrozole significantly increase the overall survival among breast cancer patients. However, in several cases the use of these drugs is limited by side effects, whose appearance are described to impair the adherence of a patient to endocrine treatment. Frequently, adherence is rated based on patient self-reports. One competent approach to obtain impartial information about patient adherence and its clinical relevance is based on the determination of plasma drug concentrations. The individual steady state plasma level represents an objective measure that may serve as an important comparator to check the patient-reported adherence status in clinical studies. In addition, it may also reveal useful information for the treating physician regarding concentration dependent side effects or therapeutic failure. We have developed and validated a liquid chromatography-tandem mass spectrometry (LC/MS/MS) method for the simultaneous analysis of tamoxifen, anastrazole and letrozole in human plasma. The method was applied in the PRO-BETh study to monitor 320 breast cancer patients undergoing endocrine therapy.
Material and methods: Blood plasma samples were collected from 320 patients undergoing endocrine breast cancer therapy and stored at -20° C. To prepare a sample for LC/MS/MS analysis, 1 ml plasma was treated with a solid phase extraction procedure using a cation mixed-mode polymeric sorbent phase (Strata-X-C cartridges, Phenomenex, CA). Chromatographic separation was accomplished on a reversed-phase column (200 mm x 0.5 mm, Eurosphere-C18, 5 μm, Knauer, Berlin) by using a gradient of acetone in an aqueous hexafluorobutyric acid solution. Mass spectrometric detection was performed on a quadrupole-quadrupole-linear ion trap instrument (Q Trap 3200, Applied Biosystems, Foster City, CA). Results: We have developed a fully validated method for the simultaneous quantitative analysis of tamoxifen, anastrozole and letrozole in human plasma. Validation was accomplished for a concentration range of 25-500 ng/ml for tamoxifen, 10-200 ng/ml for endoxifen, 5-200 ng/ml for anastrozol and 10-300 ng/ml for letrozole. The applicability of the method was demonstrated in the context of the PRO-BETh study, by analyzing plasma samples of 320 patients undergoing endocrine breast cancer therapy. The observed plasma levels showed a high inter-patient variability with measured values between 26-307 ng/ml (mean 125 ng/ml) regarding tamoxifen, 17-301 ng/ml (mean 107 ng/ml) regarding letrozole and 6-102 ng/ml (mean 37 ng/ml) regarding anastrozole. Eight samples did not contain a quantifiable amount of drug, indicating longer abstinence of the corresponding patients regarding endocrine therapy. Conclusions: The developed method represents a reliable and convenient tool for the quantitative analysis of tamoxifen, anastrozol and letrozole in human plasma. The method is dedicated to drug monitoring which is an important part of adherence rating. As exemplified in the context of the PRO-BETh study, the measured steady state plasma levels represent objective measures that serve as important comparators to check the patient-reported adherence status.
Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P5-11-09.