A short-term neoadjuvant pilot study comparing exemestane and letrozole in postmenopausal women with estrogen receptor-positive HER2-negative breast cancer.

Abstract

591 Background: Current approaches of neoadjuvant endocrine therapy (NAET) are focusing on biomarker analysis of the diagnostic specimens. One of the important things in neoadjuvant treatment is to determine the in-vivo responsiveness to the agent. Little is known regarding biomarker changes in specific agents and the association with biomarker changes and treatment duration. A pilot study was conducted to investigate the differences of biomarker changes in short-term NAET (letrozole vs. exemestane) and also the difference in treatment durations of each AI. Methods: Totally 120 samples from 60 postmenopausal women with ER positive HER2 negative stage 1-3 breast cancers in NAET comparing letrozole (LET) and exemestane (EXE) were analyzed with pre- and post-treatment mRNA expression of ERα, ERβ, PR, HER2, Ki67, PIK3CA, and MAPK using QuantiGene Assay. Thirty-one patients took letrozole 2.5mg daily and 29 did exemestane 25mg daily until the surgery (median duration 22 days). Statistical tests were two-sided. Results: Clinical characteristics were well balanced between the two treatment arms. Median % changes of Ki67 were -58%, ERα -44%, ERβ +93%, and PR -69% in LET and Ki67 -59%, ERα -30%, ERβ +135%, PR -72% in EXE. There were no significant differences of these marker changes between two agents. Dividing into 4 groups in treatment duration, 10-14 days (q1), 15-21 days (q2), 22-28 days (q3), and over 29 days (q4), LET had 2 peaks in Ki67 decreases (-69% (q1), -28% (q2), -78% (q3), -59% (q4)), whereas EXE had a peak in q2 (-62%, -77%, -49%, -26%). Ki67 increasing cases including primary resistance and tachyphylaxis were 5 (16%) in LET (3 in q2 and 2 in q4) and 3 (10%) in EXE (one of each in q2-4). All 8 cases had increases of ERβ, PIK3CA, and MAPK. Conclusions: Although both LET and EXE for short duration had significantly decreased Ki67 mRNA level irrespective of treatment duration, no significant differences was found between two agents. Tumor resistance with AIs may be associated with the other pathways.