Background: In animal models of hormone receptor positive (HR+) breast cancer, acquired resistance to continued letrozole was shown to be reversed by estrogen-induced apoptosis. We hypothesized that the rise in estrogen levels during short treatment interruptions would resensitize breast cancer cells to letrozole and improve treatment outcome. SOLE tested the hypothesis that 3 mos treatment-free intervals during extended adjuvant therapy will improve disease-free survival (DFS). We previously reported the primary endpoint after 60 mos median follow-up: extended intermittent letrozole did not improve DFS vs extended continuous letrozole. However, only 9% of pts had breast cancer events, justifying updating the analysis with longer follow-up. The dynamic of recovery of estrogen levels after stopping letrozole therapy has not been previously reported.

Methods: SOLE enrolled 4884 postmenopausal women with HR+ lymph node-positive BC who had completed 4-6 yrs of adjuvant endocrine therapy (19% SERM, 43% AI, 38% both; stratification factor). Pts were randomized to an additional 5 yrs continuous letrozole (2.5 mg daily; n=2441) vs 5 yrs intermittent letrozole (taken for the first 9 mos of yrs 1-4, and 12 mos in yr 5; n=2443). We report the final analysis of the SOLE trial after 84 mos median follow-up. In SOLE-EST, levels of estradiol (E2), estrone (E1) and estrone sulphate (E1S) at 0, 9, 10.5 and 12 mos after randomization were determined using a highly sensitive assay in a subgroup of 90 evaluable patients (21 in the continuous and 69 in the intermittent group).

Results: There were 923 DFS events. 7 yr DFS was 81.5% in both groups. More pts had distant metastases in the continuous group (8.7% vs 7.5%) while second (non-breast) malignancies were more frequent in the intermittent group (5.5% vs 4.7%). Similar outcomes were observed for breast cancer-free interval (BCFI) (88.6% vs 88.0%), distant recurrence-free interval (DRFI) (91.6% vs 90.4%), and overall survival (OS) (90.6% vs 89.6%) for pts assigned intermittent vs continuous letrozole. In the intermittent group, median E2, E1 and E1S levels more than doubled compared with levels at 9 mos after randomization in the first 6 weeks after stopping letrozole during the treatment free interval while levels were stable for the 21 pts tested in the continuous group.

Conclusions: Among postmenopausal women with HR+ BC, extended intermittent letrozole did not improve DFS vs continuous letrozole. Similar outcome was consistently observed for BCFI, DRFI and OS. The SOLE-EST substudy indicates an important increase in estrogen levels as soon as 6 weeks after stopping letrozole therapy in the intermittent group. Further investigation of prior exposure to aromatase inhibitors in relation with outcome and with E2, E1 and E1S levels in SOLE-EST are underway.

Citation Format: Guy Jerusalem, Subrina Farah, Jacquie Chirgwin, Stefan Aebi, Per Karlsson, Patrick Neven, Erika Hitre, Marie-Pascale Graas, Edda Simoncini, Claus Kamby, Alastair Thompson, Sibylle Loibl, Joaquín Gavilá, Katsumasa Kuroi, Christian Marth, Bettina Müller, Seamus O'Reilly, Andrea Gombos, Thomas Ruhstaller, Harold Burstein, Manuela Rabaglio, Barbara Ruepp, Giuseppe Viale, Richard D Gelber, Alan S Coates, Angelo Di Leo, Aron Goldhirsch, Meredith Regan, Marco Colleoni. SOLE (study of letrozole extension), a phase 3 randomized clinical trial of continuous vs intermittent letrozole in postmenopausal women who have received 4-6 years of adjuvant endocrine therapy for lymph node-positive, early breast cancer (BC): Final analysis and sole estrogen substudy (SOLE-EST) [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-12-01.