Monitoring serum estradiol levels in breast cancer patients during extended adjuvant letrozole treatment after five years of tamoxifen: a prospective trial.

Purpose: To analyze whether monitoring serum estradiol (E2) levels using a highly sensitive and specific liquid chromatography tandem mass spectrometry (LC-MS/MS) method may identify patients with AI failure with E2 levels below the lower limit of quantification (LLOQ) after schwitching from tamoxifen to letrozole.
Methods: In a prospective study of breast cancer patients switching to letrozole treatment after previous tamoxifen, plasma estrogen levels were measured at baseline and after 3- and 12-months using LC-MS/MS.
Results: Forty-six patients were classified postmenopausal and entered into the final analysis. Thirty-nine (85%) patients had three- and 12-month E2 concentrations below the LLOQ (5 pmol/L). In the seven patients classified as AI-failures during letrozole treatment, serum E2-MS level rose above 5 pmol/L at 3 months with a mean E2-MS 77.5 pmol/L or 12 months with a mean E2-MS 21 pmol/L. None of the baseline variables i.e., age at diagnosis, age at study entry, age at menarche, BMI, endometrial thickness, total ovarian volume, baseline FSH, E2-IA, or E2-MS were significantly associated with the risk of AI failure in logistic regression. E2 levels at baseline measured by E2-IA did not significantly correlate to the levels measured by E2-MS.
Conclusions: There is a relatively high risk of inadequate estrogen suppression in patients who switch from tamoxifen treatment to AIs. The use of sensitive and specific assays, such as LC-MS/MS methods, to monitor estrogen levels during AI treatment is essential to minimize the risk of a proceeding inefficient endocrine therapy.