Simultaneous Quantification of Aromatase Inhibitors and Estrogens in Postmenopausal Breast Cancer Patients.

CONTEXT: Currently there are no assays that can simultaneously quantify serum levels of the third generation aromatase inhibitors (AI); letrozole, anastrozole, and exemestane, and the ultralow levels of estrogens in postmenopausal breast cancer patients on AI treatment. Such measurements may be pivotal for the determination of optimal and individualized treatment regimens. We aimed at developing an LC-MS/MS method for simultaneous assessment of letrozole, anastrozole, exemestane and 17-hydroxy-exemestane as well as subpicomolar levels of estradiol and estrone.

METHODS: Internal standards, calibrators, serum samples and quality controls were in fully automated steps transferred to a deepwell plate for a two-step liquid-liquid-extraction. The extracts were reconstituted and analytes were separated chromatographically using two serially coupled columns, then subject to MS/MS in ESI mode. The method was thoroughly validated and is traceable to two accredited estrogen-methods.

RESULTS: Measurement range for estrone and estradiol was 0.2 - 12 000 pmol/L and 0.8 - 13 000 pmol/L, and covered the expected therapeutic range for the AIs. All analytes had precision ≤ 13 %, and accuracies within 100 ± 8 %. As proof-of-concept, AI and estrogen levels were determined in sera from postmenopausal breast cancer patients under treatment.

CONCLUSIONS: We present here an assay suitable for the simultaneous measurement of serum levels of all third generation AIs and ultralow levels of estrogens, providing a powerful new tool to study drug efficacy and compliance. The method is highly valuable for postmenopausal patients whose pretreatment estradiol levels are below threshold of detection for most routine assays, but still require suppression.