Abstract P5-12-02: Effect of 5 vs 2.5 mg/day letrozole on residual estrogen levels in post-menopausal women with high BMI - A prospective crossover study
Authors: Cescon DW , Ennis M , Pritchard KI , Townsley C , Warr D , Elser C , Rao L , Stambolic V , Sridhar S , Goodwin PJ 
Published: May 2016

Background: Some studies have suggested that women with high BMI have less benefit from aromatase inhibitors (AI) vs. tamoxifen as adjuvant treatment for early breast cancer. One possible mechanism for this observation is that complete suppression of estrogen is not achieved in these women with the standard flat dose of AI. We evaluated whether a doubling of letrozole to 5 mg/day for 4 weeks affected residual estrogen levels in this population.

Methods: Post-menopausal women with early breast cancer and BMI&gt;25 already taking adjuvant letrozole for at least 3 months were recruited from medical oncology clinics at 4 sites in Toronto, Canada. Fasting blood samples were collected 24 hours following the last dose at baseline (routine use of own letrozole), after 28 days of monitored adherence to a provided supply of letrozole (Femara) 2.5 mg/day (Part A), and after an additional 28 days of letrozole (Femara) 5 mg/day (Part B). Symptom/quality of life questionnaires were completed at the same timepoints. Estradiol and estrone were measured using a high sensitivity liquid chromatography-tandem mass spectrometry assay. One interim analysis for futility and efficacy was planned after 31 eligible patients had completed the study, using estradiol and O'Brien-Fleming boundaries with an inner wedge.

Results: 36 patients were enrolled and started on study, and 31 eligible patients completed Parts A and B. The 5 non-completers withdrew because of adverse events (n=4, unlikely related to drug) or withdrawal of consent (n=1). Median age was 62 (range 48 to 77) and BMI 28.3 kg/m2 (Range 25.2 to 42.2 kg/m2). One patient had non-postmenopausal estrogen levels at Day 29 and Day 57 and one patient's blood assay was unsuccessful; both were excluded from further analyses. The predetermined stopping rule for futility was met. Estradiol levels (mean±standard deviation) changed from 2.68±0.40 pg/mL at baseline to 2.67±0.59 pg/mL at Day 29 to 2.70±0.53 pg/mL at Day 57. Mean change from Day 29 to Day 57 was 0.03±0.48 pg/mL (95% confidence interval -0.15 to 0.21 pg/mL). Four patients reported new or increased arthralgias (to NCI CTCAE Grade 2 or 3) while taking letrozole 5 mg/day in Part B. There was no association between changes in estradiol levels and either study non-completion or the development of arthralgias. Estrone results were similar.

Conclusion: Increasing letrozole from 2.5 to 5 mg/day did not further suppress estrogen levels in women with BMI&gt;25. It is unlikely that letrozole dosing tailored to body size would improve clinical outcomes. The letrozole 5 mg/day intervention was terminated based on the results of the interim analysis for futility.

Citation Format: Cescon DW, Ennis M, Pritchard KI, Townsley C, Warr D, Elser C, Rao L, Stambolic V, Sridhar S, Goodwin PJ. Effect of 5 vs 2.5 mg/day letrozole on residual estrogen levels in post-menopausal women with high BMI - A prospective crossover study. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P5-12-02.